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  • 1.
    Aikebaier, Faluke
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Effects of electron-electron interaction in pristine and doped graphene2014Independent thesis Advanced level (degree of Master (Two Years)), 30 credits / 45 HE creditsStudent thesis
    Abstract [en]

    The goal of this master thesis is to investigate the eect of electron-electron interaction on electronic properties of graphene that can be measured experimentally. A tight-binding model, which includes up to next-nearest-neighbor hopping, with parameters tted to density functional theory calculations, has been used to describe the electronic structure of graphene. The electron-electron interaction is described by the Hubbard model using a mean- eld approximation. Based on the analysis of dierent tight-binding models available in the literature, we conclude that a next-nearest-neighbor tight-binding model is in better agreement with density functional theory calculations, especially for the linear dispersion around the Dirac point. The Fermi velocity in this case is very close to the experimental value, which was measured by using a variety of techniques. Interaction-induced modi cations of the linear dispersion around the Dirac point have been obtained. Unlike the non-local Hartree-Fock calculations, which take into account the long-range electron-electron interaction and yield logarithmic corrections, in agreement with experiment, we found only linear modi cations of the Fermi velocity. The reasons why one cannot obtain logarithmic corrections using the mean- eld Hubbard model have been discussed in detail. The remaining part of the thesis is focused on calculations of the local density of states around a single substitutional impurity in graphene. This quantity can be directly compared to the results of the scanning tunneling microscopy in doped graphene. We compare explicitly non-interacting and interacting cases. In the latter case, we performed self-consistent calculations, and found that electron-electron interaction has a signi cant eect on the local density of states. Furthermore, the band gap at high-symmetry points of the Brillouin zone of a supercell, triggered by the impurity, is modi ed by interactions. We use a perturbative model to explain this eect and quantitative agreement with numerical results. In conclusion, it is expected that the long-range electron-electron nteraction is extremely strong and important in graphene. However, as this thesis has shown, interactions at the level of the Hubbard model and mean- eld approximation also introduce corrections to the electronic properties of graphene.

     

  • 2.
    Aikebaier, Faluke
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Effects of short-range electron-electron interactions in doped graphene2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 15, 155420Article in journal (Refereed)
    Abstract [en]

    We study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of substitutional impurities. Our computational approach is based on the π orbital tight-binding model for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. The finite impurity concentration is modeled using the supercell approach. We compare explicitly noninteracting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens up at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.

  • 3.
    Argyropoulou, Eftychia
    et al.
    National and Kapodistrian University of Athens, Greece.
    Ioannidis, Andreas D.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    The eigenvalue problem for a bianisotropic cavity2013In: PIERS proceedings 2013: Progress in Electromagnetics Research Symposium, Electromagnetics Acad , 2013, 858-862 p.Conference paper (Refereed)
    Abstract [en]

    We discuss the eigenvalue problem for a perfectly conducting bianisotropic cavity. We formulate the corresponding mathematical problem and we give a characterization of the eigenelements (non-zero eigenfrequencies and modes) via a perturbation argument involving the eigenelements of the hollow cavity.

  • 4.
    Azimi Mousolou, Vahid
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Quantum Holonomy for Many-Body Systems and Quantum Computation2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The research of this Ph. D. thesis is in the field of Quantum Computation and Quantum

    Information. A key problem in this field is the fragile nature of quantum states. This

    becomes increasingly acute when the number of quantum bits (qubits) grows in order to

    perform large quantum computations. It has been proposed that geometric (Berry) phases

    may be a useful tool to overcome this problem, because of the inherent robustness of such

    phases to random noise. In the thesis we investigate geometric phases and quantum

    holonomies (matrix-valued geometric phases) in many-body quantum systems, and elucidate

    the relationship between these phases and the quantum correlations present in the systems.

    An overall goal of the project is to assess the feasibility of using geometric phases and

    quantum holonomies to build robust quantum gates, and investigate their behavior when the

    size of a quantum system grows, thereby gaining insights into large-scale quantum

    computation.

    In a first project we study the Uhlmann holonomy of quantum states for hydrogen-like

    atoms. We try to get into a physical interpretation of this geometric concept by analyzing its

    relation with quantum correlations in the system, as well as by comparing it with different

    types of geometric phases such as the standard pure state geometric phase, Wilczek-Zee

    holonomy, Lévay geometric phase and mixed-state geometric phases. In a second project we

    establish a unifying connection between the geometric phase and the geometric measure of

    entanglement in a generic many-body system, which provides a universal approach to the

    study of quantum critical phenomena. This approach can be tested experimentally in an

    interferometry setup, where the geometric measure of entanglement yields the visibility of

    the interference fringes, whereas the geometric phase describes the phase shifts. In a third

    project we propose a scheme to implement universal non-adiabatic holonomic quantum

    gates, which can be realized in novel nano-engineered systems such as quantum dots,

    molecular magnets, optical lattices and topological insulators. In a fourth project we propose

    an experimentally feasible approach based on “orange slice” shaped paths to realize non-

    Abelian geometric phases, which can be used particularly for geometric manipulation of

    qubits. Finally, we provide a physical setting for realizing non-Abelian off-diagonal

    geometric phases. The proposed setting can be implemented in a cyclic chain of four qubits

    with controllable nearest-neighbor interactions. Our proposal seems to be within reach in

    various nano-engineered systems and therefore opens up for first experimental test of the

    non-Abelian off-diagonal geometric phase.

  • 5.
    Azimi Mousolou, Vahid
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Sjöqvist, Erik
    Department of Quantum Chemistry, Uppsala University, Box 518, Se-751 20 Uppsala, Sweden.
    Unifying geometric entanglement and geometric phase in a quantum phase transition2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, no 1, Article ID: 012310- p.Article in journal (Refereed)
    Abstract [en]

    Geometric measure of entanglement and geometric phase have recently been used to analyze quantum phase transition in the XY spin chain. We unify these two approaches by showing that the geometric entanglement and the geometric phase are respectively the real and imaginary parts of a complex-valued geometric entanglement, which can be investigated in typical quantum interferometry experiments. We argue that the singular behavior of the complex-valued geometric entanglement at a quantum critical point is a characteristic of any quantum phase transition, by showing that the underlying mechanism is the occurrence of level crossings associated with the underlying Hamiltonian.

  • 6.
    Azimi Mousolou, Vahid
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Sjöqvist, Erik
    Department of Quantum Chemistry, Uppsala University, Box 518, Se-751 20 Uppsala, Sweden.
    Universal Non-adiabatic Holonomic Gates in Quantum Dots and Single-Molecule MagnetsManuscript (preprint) (Other academic)
    Abstract [en]

    Geometric manipulation of a quantum system offers a method for fast, universal, and robust quantum information processing. Here, we propose a scheme for universal all-geometric quantum computation using non-adiabatic quantum holonomies. We propose three different realizations of the scheme based on an unconventional use of quantum dot and single-molecule magnet devices,which offer promising scalability and robust efficiency.

  • 7.
    Azimi Mousolou, Vahid
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Sjöqvist, Erik
    Uppsala University ; National University of Singapore, Singapore.
    Universal Non-adiabatic Holonomic Gates in Quantum Dots and Single-Molecule Magnets2014In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, 013029Article in journal (Refereed)
    Abstract [en]

    Geometric manipulation of a quantum system offers a method for fast, universal, and robust quantum information processing. Here, we propose a scheme for universal all-geometric quantum computation using non-adiabatic quantum holonomies. We propose three different realizations of the scheme based on an unconventional use of quantum dot and single-molecule magnet devices,which offer promising scalability and robust efficiency.

  • 8.
    Bergvall, Anders
    et al.
    Chalmers University of Technology.
    Fogelström, Mikael
    Chalmers University of Technology.
    Holmqvist, Cecilia
    Universität Konstanz, Germany.
    Löfwander, Tomas
    Chalmers University of Technology.
    Basic theory of electron transport through molecular contacts2015In: Handbook of Single Molecule Electronics / [ed] K. Moth-Poulsen, Pan Stanford Publishing, 2015, 31-78 p.Chapter in book (Refereed)
  • 9.
    Bozkurt, M.
    et al.
    Photonics and Semiconductor Nanophysics, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, NL-5600 MB Eindhoven, The Netherlands.
    Mahani, Mohammad Reza
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Studer, P.
    London Center for NanoTechnology, 17-19 Gordon Street, WC1H 0AH, London, U.K..
    Tang, J.-M.
    5Department of Physics, University of New Hampshire, Durham, New Hampshire 03824-3520, USA.
    Schofield, S.
    London Center for NanoTechnology, 17-19 Gordon Street, WC1H 0AH, London, U.K..
    Curson, N.
    London Center for NanoTechnology, 17-19 Gordon Street, WC1H 0AH, London, U.K..
    Flatt’e, M.E.
    Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242-1479,U.S.A..
    Silov, A.Yu.
    Photonics and Semiconductor Nanophysics, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, NL-5600 MB Eindhoven, The Netherlands.
    Hirjibehedin, C. F.
    Department of Chemistry, UCL, London, WC1H 0AJ, United Kingdom.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Koenraad, P.M.
    Photonics and Semiconductor Nanophysics, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, NL-5600 MB Eindhoven, The Netherlands.
    Magnetic anisotropy of single Mn acceptors in GaAs in an external magnetic field2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, Article ID: 205203- p.Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of an external magnetic field on the physical properties of the acceptor hole statesassociated with single Mn acceptors placed near the (110) surface of GaAs. Cross-sectional scanning tunnelingmicroscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported bytheoretical calculations based on a tight-binding model of Mn acceptors in GaAs. For Mn acceptors on the (110)surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantlythe magnetic anisotropy landscape. However, the acceptor hole wave function is strongly localized around theMn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mnacceptors placed on deeper layers below the surface, the acceptor hole wave function is more delocalized andthe corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However, themagnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 Tcan hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis.We predict that substantially larger magnetic fields are required to observe a significant field dependence of thetunneling current for impurities located several layers below the GaAs surface.

  • 10.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    First-principle studies of tunneling transport in single-molecule magnets2010Conference paper (Refereed)
  • 11.
    Costanza, Gabriele
    et al.
    Lund University.
    Ioannidis, Andreas D.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Remarks on the mathematical solution of the hollow cavity eigenvalue problem2013In: Progress in Electromagnetics Research Symposium, Electromagnetics Acad. , 2013, 79-83 p.Conference paper (Refereed)
    Abstract [en]

    We discuss the eigenvalue problem for a perfectly conducting hollow cavity under a strict functional analytic point of view. We make use of a variant of the classical spectral theorem for compact selfadjoint operators and we pay extra attention on the null space of the Maxwell operator. We also discuss the corresponding inhomogeneous problem, where currents are present, even when they may depend on the fields.

  • 12.
    Filipović, Milena
    et al.
    Universität Konstanz, Germany.
    Holmqvist, Cecilia
    Universität Konstanz, Germany.
    Haupt, Federica
    RWTH Aachen, Germany.
    Belzig, Wolfgang
    Universität Konstanz, Germany.
    Spin transport and tunable Gilbert damping in a single-molecule magnet junction2013In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 87, 045426Article in journal (Refereed)
    Abstract [en]

    We study time-dependent electronic and spin transport through an electronic level connected to two leads and coupled with a single-molecule magnet via exchange interaction. The molecular spin is treated as a classical variable and precesses around an external magnetic field. We derive expressions for charge and spin currents by means of the Keldysh nonequilibrium Green's functions technique in linear order with respect to the time-dependent magnetic field created by this precession. The coupling between the electronic spins and the magnetization dynamics of the molecule creates inelastic tunneling processes which contribute to the spin currents. The inelastic spin currents, in turn, generate a spin-transfer torque acting on the molecular spin. This back-action includes a contribution to the Gilbert damping and a modification of the precession frequency. The Gilbert damping coefficient can be controlled by the bias and gate voltages or via the external magnetic field and has a nonmonotonic dependence on the tunneling rates.

  • 13.
    Gooth, Johannes
    et al.
    University of Hamburg, Germany ; IBM Research-Zurich, Switzerland.
    Zierold, Robert
    University of Hamburg, Germany.
    Sergelius, Philip
    University of Hamburg, Germany.
    Hamdou, Bacel
    University of Hamburg, Germany.
    Garcia, Javier
    IFW Dresden, Germany.
    Damm, Christine
    IFW Dresden, Germany.
    Rellinghaus, Bernd
    IFW Dresden, Germany.
    Pettersson, Håkan Jan
    Lund University ; Halmstad University.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Borg, Mattias
    IBM Research-Zurich, Switzerland.
    Nielsch, Kornelius
    University of Hamburg, Germany ; IFW Dresden, Germany.
    Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires2016In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 10, no 7, 7180-7188 p.Article in journal (Refereed)
    Abstract [en]

    Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.

  • 14.
    Gryglas-Borysiewicz, Marta
    et al.
    Warsaw University, Poland.
    Juszynski, Piotr
    Warsaw University, Poland.
    Kwiatkowski, Adam
    Warsaw University, Poland.
    Przybytek, Jacek
    Warsaw University, Poland.
    Sadowski, Janusz
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lund University ; Polish Acad Sci, Poland .
    Sawicki, Maciej
    Polish Acad Sci, Poland.
    Tokarczyk, Mateusz
    Warsaw University, Poland.
    Kowalski, Grzegorz
    Warsaw University, Poland.
    Dietl, Tomasz
    Polish Acad Sci, Poland ; University of Warsaw, Poland ; Tohoku University, Japan.
    Wasik, Dariusz
    Warsaw University, Poland.
    Hydrostatic-pressure-induced changes of magnetic anisotropy in (Ga, Mn) As thin films2017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 11, 115805Article in journal (Refereed)
    Abstract [en]

    The impact of hydrostatic pressure on magnetic anisotropy energies in (Ga, Mn) As thin films with in-plane and out-of-plane magnetic easy axes predefined by epitaxial strain was investigated. In both types of sample we observed a clear increase in both in-plane and out-of-plane anisotropy parameters with pressure. The out-of-plane anisotropy constant is well reproduced by the mean-field p-d Zener model; however, the changes in uniaxial anisotropy are much larger than expected in the Mn-Mn dimer scenario.

  • 15.
    Gustafsson, Alexander
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Electron transport in quantum point contacts: A theoretical study2011Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Electron transport in mesoscopic systems, such as quantum point contacts and Aharonov-Bohm rings are investigated numerically in a tight-binding language with a recursive Green's function algorithm. The simulation reveals among other things the quantized nature of the conductance in point contacts, the Hall conductance, the decreasing sensitivity to scattering impurities in a magnetic field, and the periodic magnetoconductance in an Aharonov-Bohm ring. Furthermore, the probability density distributions for some different setups are mapped, making the transmission coefficients, the quantum Hall effect, and the cyclotron radius visible, where the latter indicates the correspondance between quantum mechanics and classical physics on the mesoscopic scale.

  • 16.
    Gustafsson, Alexander
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Modeling of non-equilibrium scanning probe microscopy2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The work in this thesis is basically divided into two related but separate investigations.

    The first part treats simple chemical reactions of adsorbate molecules on metallic surfaces, induced by means of a scanning tunneling probe (STM). The investigation serves as a parameter free extension to existing theories. The theoretical framework is based on a combination of density functional theory (DFT) and non-equilibrium Green's functions (NEGF). Tunneling electrons that pass the adsorbate molecule are assumed to heat up the molecule, and excite vibrations that directly correspond to the reaction coordinate. The theory is demonstrated for an OD molecule adsorbed on a bridge site on a Cu(110) surface, and critically compared to the corresponding experimental results. Both reaction rates and pathways are deduced, opening up the understanding of energy transfer between different configurational geometries, and suggests a deeper insight, and ultimately a higher control of the behaviour of adsorbate molecules on surfaces.

    The second part describes a method to calculate STM images in the low bias regime in order to overcome the limitations of localized orbital DFT in the weak coupling limit, i.e., for large vacuum gaps between a tip and the adsorbate molecule. The theory is based on Bardeen's approach to tunneling, where the orbitals computed by DFT are used together with the single-particle Green's function formalism, to accurately describe the orbitals far away from the surface/tip. In particular, the theory successfully reproduces the experimentally well-observed characteristic dip in the tunneling current for a carbon monoxide (CO) molecule adsorbed on a Cu(111) surface. Constant height/current STM images provide direct comparisons to experiments, and from the developed method further insights into elastic tunneling are gained.

  • 17.
    Gustafsson, Alexander
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Ueba, Hiromu
    Toyama Univ, Japan.
    Paulsson, Magnus
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Theory of vibrationally assisted tunneling for hydroxyl monomer flipping on Cu(110)2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 16, Article ID: 165413- p.Article in journal (Refereed)
    Abstract [en]

    To describe vibrationally mediated configuration changes of adsorbates on surfaces we have developed a theory to calculate both reaction rates and pathways. The method uses the T-matrix to describe excitations of vibrational states by the electrons of the substrate, adsorbate, and tunneling electrons from a scanning tunneling probe. In addition to reaction rates, the theory also provides the reaction pathways by going beyond the harmonic approximation and using the full potential energy surface of the adsorbate which contains local minima corresponding to the adsorbates different configurations. To describe the theory, we reproduce the experimental results in [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)], where the hydrogen/deuterium atom of an adsorbed hydroxyl (OH/OD) exhibits back and forth flipping between two equivalent configurations on a Cu(110) surface at T = 6 K. We estimate the potential energy surface and the reaction barrier, similar to 160 meV, from DFT calculations. The calculated flipping processes arise from (i) at low bias, tunneling of the hydrogen through the barrier, (ii) intermediate bias, tunneling electrons excite the vibrations increasing the reaction rate although over the barrier processes are rare, and (iii) higher bias, overtone excitations increase the reaction rate further.

  • 18.
    Hakimi, Shirin
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Exact Diagonalization of Few-electron Quantum Dots2009Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    We consider a system of few electrons trapped in a two-dimensional circularquantum dot with harmonic confinement and in the presence of ahomogeneous magnetic field, with focus on the role of e-e interaction. Byperforming the exact diagonalization of the Hamiltonian in second quantization,the low-lying energy levels for spin polarized system are obtained. The singlet-triplet oscillation in the ground state of the two-electron system showing up inthe result is explained due to the role of Coulomb interaction. The splitting ofthe lowest Landau level is another effect of the e-e interaction, which is alsoobserved in the results.

  • 19. Holmqvist, Cecilia
    Spin-singlet and spin-triplet superconducting correlations in Josephson junctions2010In: The Linnaeus Summer School in Quantum Engineering, Hindås (2010), The Linnaeus Summer School in Quantum Engineering, Hindås (2010) , 2010Conference paper (Refereed)
  • 20.
    Holmqvist, Cecilia
    et al.
    Universität Konstanz, Germany.
    Belzig, W.
    Universität Konstanz, Germany.
    Fogelström, M.
    Chalmers University of Technology.
    Spin-precession-assisted supercurrent in a superconducting quantum point contact coupled to a single-molecule magnet2012In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 86, 054519Article in journal (Refereed)
    Abstract [en]

    The supercurrent through a quantum point contact coupled to a nanomagnet strongly depends on the dynamics of the nanomagnet's spin. We employ a fully microscopic model to calculate the transport properties of a junction coupled to a spin whose dynamics is modeled as Larmor precession brought about by an external magnetic field and find that the dynamics affects the charge and spin currents by inducing transitions between the continuum states outside the superconducting gap region and the Andreev levels. This redistribution of the quasiparticles leads to a nonequilibrium population of the Andreev levels and an enhancement of the supercurrent which is visible as a modified current-phase relation as well as a nonmonotonous critical current as function of temperature. The nonmonotonous behavior is accompanied by a corresponding change in spin-transfer torques acting on the precessing spin and leads to the possibility of using temperature as a means to tune the back-action on the spin.

  • 21.
    Holmqvist, Cecilia
    et al.
    Université Joseph Fourier, France ; Chalmers University of Technology.
    Feinberg, D.
    Université Joseph Fourier, France.
    Zazunov, A.
    Université Joseph Fourier, France ; Université de la Méditerranée, France.
    Emergence of a negative charging energy in a metallic dot capacitively coupled to a superconducting island2008In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 77, 054517Article in journal (Refereed)
  • 22.
    Holmqvist, Cecilia
    et al.
    Universit ̈ at Konstanz, Germany.
    Fogelström, M.
    Chalmers University of Technology.
    Belzig, W.
    Universit ̈ at Konstanz, Germany.
    Spin-polarized Shapiro steps and spin-precession-assisted multiple Andreev reflection2014In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 90, 014516Article in journal (Refereed)
    Abstract [en]

    We investigate the charge and spin transport of a voltage-biased superconducting point contact coupled toa nanomagnet. The magnetization of the nanomagnet is assumed to precess with the Larmor frequencyωLwhen exposed to ferromagnetic resonance conditions. The Larmor precession locally breaks the spin-rotationsymmetry of the quasiparticle scattering and generates spin-polarized Shapiro steps for commensurate Josephsonand Larmor frequencies that lead to magnetization reversal. This interplay between the ac Josephson current andthe magnetization dynamics occurs at voltages|V|=ωL/2enforn=1,2,..., and the subharmonic steps withn>1 are a consequence of multiple Andreev reflection (MAR). Moreover, the spin-precession-assisted MARgenerates quasiparticle scattering amplitudes that, due to interference, lead to current-voltage characteristics ofthe dc charge and spin currents with subharmonic gap structures displaying an even-odd effect.

  • 23.
    Holmqvist, Cecilia
    et al.
    Chalmers University of Technology.
    Teber, S.
    Université Pierre et Marie Curie, France.
    Fogelström, M.
    Chalmers University of Technology.
    Nonequilibrium effects in a Josephson junction coupled to a precessing spin2011In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 83, 104521Article in journal (Refereed)
    Abstract [en]

    We present a theoretical study of a Josephson junction consisting of two s-wave superconducting leads coupled over a classical spin. When an external magnetic field is applied, the classical spin will precess with the Larmor frequency. This magnetically active interface results in a time-dependent boundary condition with different tunneling amplitudes for spin-up and spin-down quasiparticles and where the precession produces spin-flip scattering processes. We show that as a result, the Andreev states develop sidebands and a nonequilibrium population which depend on the precession frequency and the angle between the classical spin and the external magnetic field. The Andreev states lead to a steady-state Josephson current whose current-phase relation could be used for characterizing the precessing spin. In addition to the charge transport, a magnetization current is also generated. This spin current is time dependent and its polarization axis rotates with the same precession frequency as the classical spin.

  • 24.
    Holmqvist, Cecilia
    et al.
    Chalmers University of Technology ; CNRS and Université Joseph Fourier, France.
    Teber, S.
    CNRS and Université Joseph Fourier, France ; Universites Paris 6 et 7, France.
    Houzet, M .
    INAC/SPSMS, France.
    Feinberg, D .
    CNRS and Université Joseph Fourier, France.
    Fogelström, M.
    Chalmers University of Technology.
    Josephson current through a precessing spin2009In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 150, 022027Article in journal (Refereed)
    Abstract [en]

    A study of the dc Josephson current between two superconducting leads in thepresence of a precessing classical spin is presented. The precession gives rise to a time-dependenttunnel potential which not only creates different tunneling probabilities for spin-up and spin-down quasiparticles, but also introduces a time-dependent spin-flip term. In particular, westudy the effects of the spin-flip term alone on the Josephson current between two spin-singletsuperconductors as a function of precession frequency and junction transparency. The systemdisplays a steady-state solution although the magnitude and nature of the current is indeedaffected by the precession frequency of the classical spin.

  • 25.
    Islam, Fhokrul
    Virginia Commonwealth University, USA.
    Stable magnetic order and charge induced rotation of magnetizationin nano-clusters2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 152409Article in journal (Refereed)
    Abstract [en]

    Efficient control of magnetic anisotropy and the orientation of magnetization are of centralimportance for the application of nanoparticles in spintronics. Conventionally, magnetization iscontrolled directly by an external magnetic field or by an electric field via spin-orbit coupling.Here, we demonstrate a different approach to control magnetization in small clusters. We firstshow that the low magnetic anisotropy of a Co5 cluster can be substantially enhanced by attachingbenzene molecules due to the mixing between p states of C and the d states of Co sites. We thenshow that the direction of magnetization vector of Co5 sandwiched between two benzene moleculesrotates by 90 when an electron is added or removed from the system. An experimental set up torealize such effect is also suggested.

  • 26.
    Islam, Fhokrul
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Ab initio calculations of the magnetic properties of Mn impurities on GaAs (110) surfaces2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. B 85, Article ID: 155306- p.Article in journal (Refereed)
    Abstract [en]

    We present a computational study of individual and pairs of substitutional Mn impurities on the (110) surface of GaAs samples based on density functional theory. We focus on the anisotropy properties of these magnetic centers and their dependence on on-site correlations, spin-orbit interaction, and surface-induced symmetry-breaking effects. For a Mn impurity on the surface, the associated acceptor-hole wave function tends to be more localized around the Mn than for an impurity in bulk GaAs. The magnetic anisotropy energy for isolated Mn impurities is of the order of 1 meV, and can be related to the anisotropy of the orbital magnetic moment of the Mn acceptor hole. Typically Mn pairs have their spin magnetic moments parallel aligned, with an exchange energy that strongly depends on the pair orientation on the surface. The spin magnetic moment and exchange energies for these magnetic entities are not significantly modified by the spin-orbit interaction, but are more sensitive to on-site correlations. Correlations in general reduce the magnetic anisotropy for most of the ferromagnetic Mn pairs.

  • 27.
    Islam, Fhokrul
    et al.
    Virginia Commonwealth University, USA.
    Khanna, Shiv
    Virginia Commonwealth University, USA.
    On the enhancement of magnetic anisotropy in cobalt clusters via non-magnetic doping2014In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 125303Article in journal (Refereed)
    Abstract [en]

    We show that the magnetic anisotropy energy (MAE) in cobalt clusters can be significantlyenhanced by doping them with group IV elements. Our firstprincipleselectronic structurecalculations show that Co4C2 and Co12C4 clusters have MAEs of 25 K and 61 K, respectively. The large MAE is due to controlled mixing between Co dandC pstatesand can be furthertuned by replacing C by Si. Larger assemblies of such primitive units are shown to be stablewith MAEs exceeding 100 K in units as small as 1.2 nm, in agreement with the recentobservation of large coercivity. These results may pave the way for the use of nanoclustersinhigh density magnetic memory devices for spintronics applications

  • 28.
    Kitaguchi, Yuya
    et al.
    Kyoto University, Japan.
    Habuka, Satoru
    Kyoto University, Japan.
    Okuyama, Hiroshi
    Kyoto University, Japan.
    Hatta, Shinichiro
    Kyoto University, Japan.
    Aruga, Tetsuya
    Kyoto University, Japan.
    Frederiksen, Thomas
    Donostia International Physics Center (DIPC), Spain ; IKERBASQUE, Basque Foundation for Science, Spain.
    Paulsson, Magnus
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Ueba, Hiromu
    University of Toyama, Japan.
    Controlled switching of single-molecule junctions by mechanical motion of a phenyl ring2015In: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 6, 2088-2095 p.Article in journal (Refereed)
    Abstract [en]

    Mechanical methods for single-molecule control have potential for wide application in nanodevices and machines. Here we demonstrate the operation of a single-molecule switch made functional by the motion of a phenyl ring, analogous to the lever in a conventional toggle switch. The switch can be actuated by dual triggers, either by a voltage pulse or by displacement of the electrode, and electronic manipulation of the ring by chemical substitution enables rational control of the on-state conductance. Owing to its simple mechanics, structural robustness, and chemical accessibility, we propose that phenyl rings are promising components in mechanical molecular devices.

  • 29.
    Lindberg, Vanja
    et al.
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering. Fysik.
    Petersson, Tomas
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering. Fysik.
    Hellsing, Bo
    Quantum size effects of CO reactivity on metallic quantum dots2006In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 600, no 1, 6-14 p.Article in journal (Refereed)
    Abstract [en]

    We study the reactivity of a metallic quantum dot when exposed to a gas phase CO molecule. First, we perform a Newns–Anderson model calculation in which the valence electrons of the quantum dot are confined by a finite potential well and the molecule is characterized by its lowest unoccupied molecular orbital in the gas phase. A pronounced quantum size effect regarding the charge transfer between the quantum dot and molecule is observed. We then perform a first-principles calculation for a selected size interval. The quantum dot is described within the jellium model and the molecule by pseudopotentials. Our results show that the charge transfer between the quantum dot and the molecule depends critically on the size of the quantum dot, and that this dependence is intimately connected with the electronic structure. The key factor for charge transfer is the presence of states with the symmetry of the chemically active molecular orbital at the Fermi level.

  • 30.
    Magnuson, M.
    et al.
    Uppsala University, Sweden.
    Duda, L-C
    Uppsala University, Sweden.
    Butorin, S. M.
    Uppsala University, Sweden.
    Kuiper, Pieter
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering. Fysikavdelningen.
    Nordgren, J.
    Uppsala University, Sweden.
    Large magnetic circular dichroism in resonant inelastic x-ray scattering at the Mn L-edge of Mn-Zn ferrite2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 17, 172409Article in journal (Refereed)
    Abstract [en]

    We report resonant inelastic x-ray scattering (RIXS) excited by circularly polarized x rays on Mn-Zn ferrite at the Mn L2,3 resonances. We demonstrate that crystal-field excitations, as expected for localized systems, dominate the RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms of spin-resolved partial density of states, which has been the standard approach for RIXS dichroism. We observe large dichroic RIXS at the L2 resonance which we attribute to the absence of metallic core hole screening in the insulating Mn ferrite. On the other hand, reduced L3-RIXS dichroism is interpreted as an effect of longer scattering time that enables spin-lattice core hole relaxation via magnons and phonons occurring on a femtosecond time scale.

  • 31.
    Mahani, Mohammad Reza
    KNT University of Technology.
    Influence of in-Plane Magnetic Field on Spin Polarization in the Presence of the Oft-neglected k3-Dresselhaus Spin-Orbit Coupling2008In: Physics Letters A, ISSN 0375-9601, Vol. 372, no 38, 6022-6025 p.Article in journal (Refereed)
    Abstract [en]

    The influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k3k3-Dresselhaus spin–orbit coupling was investigated. The k3k3-Dresselhaus term can produce a limited spin polarization. The in-plane magnetic field plays a great role in the tunneling process. It can generate the perfect spin polarization of the electrons and the ideal transmission coefficient for spin up and down simultaneously. In energy scale, complete separation between spin up and down resonance was obtained by a relatively higher in-plane magnetic field while a comparatively lower in-plane magnetic field vanishes the spin separation. On the other hand, the spin relaxation can be suppressed by compensating the oft-neglected k3k3-Dresselhaus spin orbit coupling using a relatively lower in-plane magnetic field.

  • 32.
    Mahani, Mohammad Reza
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Magnetic solotronics near the surface of a semiconductor and a topological insulator2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Technology where a solitary dopant acts as the active component of an opto-electronic device is an emerging  field known as solotronics, and bears the promise to revolutionize the way in which information is stored, processed and transmitted. Magnetic doped semiconductors and in particular (Ga, Mn)As, the archetype of dilute magnetic semiconductors, and topological insulators (TIs), a new phase of quantum matter with unconventional characteristics, are two classes of quantum materials that have the potential to advance spin-electronics technology. The quest to understand and control, at the atomic level, how a few magnetic atoms precisely positioned in a complex environment respond to external stimuli, is the red thread that connects these two quantum materials in the research presented here.

    The goal of the thesis is in part to elucidate the properties of transition metal (TM) impurities near the surface of GaAs semiconductors with focus on their response to local magnetic and electric fields, as well as to investigate the real-time dynamics of their localized spins. Our theoretical analysis, based on density functional theory (DFT) and using tight-binding (TB) models, addresses the mid-gap electronic structure, the local density of states (LDOS) and the magnetic anisotropy energy of individual Mn and Fe impurities near the (110) surface of GaAs. We investigate the effect of a magnetic field on the Mn acceptor LDOS measured in cross-sectional scanning tunneling microscopy, and provide an explanation of why the experimental LDOS images depend weakly on the field direction despite the strongly anisotropic nature of the Mn acceptor wavefunction. We also investigate the effects of a local electrostatic field generated by nearby charged As vacancies, on individual and pairs of ferromagnetically coupled magnetic dopants near the surface of GaAs, providing a means to control electrically the exchange interaction of Mn pairs. Finally, using the mixed quantum-classical scheme for spin dynamics, we calculate explicitly the time evolution of the Mn spin and its bound acceptor, and analyze the dynamic interaction between pairs of ferromagnetically coupled magnetic impurities in a nanoscaled semiconductor.

    The second part of the thesis deals with the theoretical investigation of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi2Se3 TI, using an approach that combines DFT and TB calculations. Our analysis clarifies the crucial role played by the spatial overlap and the quasi-resonant coupling between the Mn-acceptor and the topological surface states inside the Bi2Se3 band gap, in the opening of a gap at the Dirac point. Strong electronic correlations are also found to contribute significantly to the mechanism leading to the gap, since they control the hybridization between the p orbitals of nearest-neighbor Se atoms and the acceptor spin-polarization. Our results explain the effects of inversion-symmetry and time-reversal symmetry breaking on the electronic states in the vicinity of the Dirac point, and contribute to clarifying the origin of surface-ferromagnetism in TIs. The promising potential of magnetic-doped TIs accentuates the importance of our contribution to the understanding of the interplay between magnetic order and topological protected surface states.

  • 33.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    MacDonald, A.H.
    Anisotropy energy and local density of states of Mn acceptor states near the (110) surface of GaAs in the presence of an external magnetic field2011Conference paper (Refereed)
  • 34.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    MacDonald, A.H.
    University of Texas at Austin.
    Effect of As vacancies on the binding energy and exchange splitting of Mn impurities on a GaAs surface2012In: Bulletin of the American Physical Society, APS March Meeting 2012,  Volume 57, Number 1, 2012, L14.00002- p.Conference paper (Other academic)
    Abstract [en]

    State-of-the-art STM spectroscopy is nowadays able to manipulate and probe the magnetic properties of individual magnetic impurities located near the surface of a semiconductor. A recent advance of these technique employs the electric field generated by a As vacancy in GaAs to affect the environment surrounding substitutional Mn impurities in the host material [1]. Here we calculate the binding energy of a single Mn dopant in the presence of nearby As vacancies, by using a recently-introduced tight-binding method [2] that is able to capture the salient features of Mn impurities near the (110) GaAs surface. The As vacancies, modeled by the repulsive potential they produce, are expected to decrease the acceptor binding energy in agreement with experiment [1]. Within this theoretical model, we investigate the possible enhancement of the exchange splitting for a pair of ferromagnetically ordered Mn impurities, observed experimentally when As vacancies are present [3]. We also calculate the response of the Mn-impurity---As-vacancy complex to an external magnetic field. \\[4pt] [1] H. Lee and J. A. Gupta, Science, 1807-1810, (2010). \\[0pt] [2] T. O. Strandberg, C. M. Canali, A. H. MacDonald, Phys. Rev. B 80, 024425, (2009). \\[0pt] [3] J.A. Gupta, private communication.

  • 35.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    MacDonald, A.H.
    University of Texas at Austin.
    Effect of magnetic field on the local density of states of Mn acceptor magnets in GaAs2011In: Bulletin of the American Physical Society, Volume 56, Number 1: APS March Meeting 2011, 2011, W15.00002- p.Conference paper (Other academic)
    Abstract [en]

    Advances in atomic manipulation, real-space imagining and spectroscopic power of STM techniques have recently made it possible to investigate the local electronic properties of a few substitutional Mn impurities inserted in the GaAs surfaces [1]. Theoretical work [2] predicts that the local density of states in the vicinity of the Mn impurities should depend strongly on the direction of the Mn magnetic moment. In contrast, recent STM experiments [3] from several groups find a negligible dependence of the tunneling LDOS on the magnetic field direction for applied fields up to 7 T. Based on tight- binding calculations we interpret these findings by arguing that large LDOS signals require large angle moment rotations, and that the strength of the magnetic field used in present experiments is not strong enough to substantially modify the magnetic anisotropy landscape of Mn impurities near the GaAs surface.\\[4pt] [1] D. Kitchen et al., Nature, 442, 436 (2006); J. K. Garleff et al., Phys. Rev. B 82, 035303 (2010).\\[0pt] [2] T. O. Strandberg, C. M. Canali, and A. H. MacDonald, Phys. Rev. B 80, 024425 (2009). [3] P. M. Koenraad, Private Communication.

  • 36.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Engineering.
    MacDonald, A.H.
    Local manipulation of the magnetic properties of Mn impurities on a GaAs surface by As vacancies2012Conference paper (Refereed)
  • 37.
    Mahani, Mohammad Reza
    et al.
    KNT University of Technology.
    Faizabadi, E.
    Efficient Spin Filtering in a Disordered Semiconductor Superlattice in the Presence of Dresselhaus Spin-Orbit coupling2008In: Physics Letters A, ISSN 0375-9601, Vol. 372, no 11, 1926-1929 p.Article in journal (Refereed)
    Abstract [en]

    The influence of the Dresselhaus spin–orbit coupling on spin polarization by tunneling through a disordered semiconductor superlattice was investigated. The Dresselhaus spin–orbit coupling causes the spin polarization of the electron due to transmission possibilities difference between spin up and spin down electrons. The electron tunneling through a zinc-blende semiconductor superlattice with InAs and GaAs layers and two variable distance InxGa(1−x)As impurity layers was studied. One hundred percent spin polarization was obtained by optimizing the distance between two impurity layers and impurity percent in disordered layers in the presence of Dresselhaus spin–orbit coupling. In addition, the electron transmission probability through the mentioned superlattice is too much near to one and an efficient spin filtering was recommended.

  • 38.
    Mahani, Mohammad Reza
    et al.
    KNT University of Technology.
    Faizabadi, Edris
    Particular Nanowire Superlattice as a Spin Filter2009In: Physics Letters A, ISSN 0375-9601, Vol. 373, no 43, 3994-3996 p.Article in journal (Refereed)
    Abstract [en]

    A nanowire superlattice of InAs and GaAs layers with In0.47Ga0.53As as the impure layers is proposed. The oft-neglected k3k3 Dresselhaus spin–orbit coupling causes the spin polarization of the electron but often can produce a limited spin polarization. In this nanowire superlattice, Dresselhaus term produce complete spin filtering by optimizing the distance between the In0.47Ga0.53As layers and the Indium (In) in the impure layers. The proposed structure is an optimized nanowire superlattice that can efficiently filter any component of electron spins according to its energy. In fact, this nanowire superlattice is an energy dependent spin filter structure.

  • 39.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Islam, Fhokrul
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    MacDonald, A.H.
    As vacancies in MnGaAs: tight binding and first-principles studies2012Conference paper (Refereed)
  • 40.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Islam, Fhokrul
    Virginia Commonwealth Univ, Richmond, VA 23284 USA.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 16, Article ID: 165408- p.Article in journal (Refereed)
    Abstract [en]

    Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically theelectronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+](0) plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity d electrons are in good agreement with approaches that treat the spin ofthe impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+](0) and the ionized [Fe2+](-)states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of d orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state [Fe2+, h](0), the magnetic-anisotropy dependence on the impurity sublayer resembles the case of [Mn2+, h](0). In contrast, for [Fe3+](0) electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e. g., by an external magnetic field, with detectable consequences in the local density of states probed by STM.

  • 41.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    MacDonald, A. H.
    University of Texas at Austin, USA.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Electric manipulation of the Mn-acceptor binding energy and the Mn-Mn exchange interaction on the GaAs (110) surface by nearby As vacancies2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 4, 045304Article in journal (Refereed)
    Abstract [en]

    We investigate theoretically the effect of nearby As (arsenic) vacancies on the magnetic properties of substitutional Mn (manganese) impurities on the GaAs (110) surface, using a microscopic tight-binding model which captures the salient features of the electronic structure of both types of defects in GaAs. The calculations show that the binding energy of the Mn-acceptor is essentially unaffected by the presence of a neutral As vacancy, even at the shortest possible VAs--Mn separation. On the other hand, in contrast to a simple tip-induced-band-bending theory and in agreement with experiment, for a positively charged As vacancy the Mn-acceptor binding energy is significantly reduced as the As vacancy is brought closer to the Mn impurity. For two Mn impurities aligned ferromagnetically, we find that nearby charged As vacancies enhance the energy level splitting of the associated coupled acceptor levels, leading to an increase of the effective exchange interaction. Neutral vacancies leave the exchange splitting unchanged. Since it is experimentally possible to switch reversibly between the two charge states of the vacancy, such a local electric manipulation of the magnetic dopants could result in an efficient real-time control of their exchange interaction.

  • 42.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Spin dynamics of Mn impurities and their bound acceptors in GaAs2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 24, 245406Article in journal (Refereed)
    Abstract [en]

    We present results of tight-binding spin-dynamics simulations of individual and pairs of substitutionalMn impurities in GaAs. Our approach is based on the mixed quantum-classical schemefor spin dynamics, with coupled equations of motions for the quantum subsystem, representing thehost, and the localized spins of magnetic dopants, which are treated classically. In the case ofa single Mn impurity, we calculate explicitly the time evolution of the Mn spin and the spins ofnearest-neighbors As atoms, where the acceptor (hole) state introduced by the Mn dopant resides.We relate the characteristic frequencies in the dynamical spectra to the two dominant energy scalesof the system, namely the spin-orbit interaction strength and the value of the p-d exchange couplingbetween the impurity spin and the host carriers. For a pair of Mn impurities, we find signaturesof the indirect (carrier-mediated) exchange interaction in the time evolution of the impurity spins.Finally, we examine temporal correlations between the two Mn spins and their dependence on theexchange coupling and spin-orbit interaction strength, as well as on the initial spin-configuration andseparation between the impurities. Our results provide insight into the dynamic interaction betweenlocalized magnetic impurities in a nano-scaled magnetic-semiconductor sample, in the extremelydilute(solotronics) regime.

  • 43.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Time-Dependent Spin Dynamics of Few Transition Metal Impurities in a Semiconductor Host2014In: 2014 MRS Spring Meeting and Exhibit, April 21-25, San Francisco, 2014Conference paper (Refereed)
    Abstract [en]

    Recently, remarkable progress has been achieved in describing electronic and magnetic properties of individual dopants in semiconductors, both experimentally [1] and theoretically [2, 3], offering exciting prospects for applications in future electronic devices. In view of potential novel applications, which involve communication between individual magnetic dopants, mediated by the electronic carriers of the host, the focus of this research field has been shifting towards fundamental understanding and control of spin dynamics of these atomic-scale magnetic centers. Importantly, the development of time-resolved spectroscopic techniques has opened up the possibility to probe the dynamics of single spin impurities experimentally [4]. These advances pose new challenges for theory, calling for a fully microscopic time-dependent description of spin dynamics of individual impurities in the solid states environment.We present results of theoretical investigations of real-time spin dynamics of individual and pairs of transition metal (Mn) impurities in GaAs. Our approach combines the microscopic tight-binding description of substitutional dopants in semicondutors [3] with the time-dependent scheme for simulations of spin dynamics [5], based on the numerical integration of equations of motion for the coupled system of the itinerant electronic degrees of freedom of the host and the localized impurity spins. We study the spin dynamics of impurities in finite clusters containing up to hundred atoms, over time scales of a few hundred femtoseconds. In particular, we calculate explicitly the time-evolution of the impurity spins and electrons of the host upon weak external perturbations. From the Fourier spectra of the time-dependent spin trajectories, we identify the energy scales associated with intrinsic interactions of the system, namely the spin-orbit interaction and the exchange interaction between the impurity spins and the spins of the nearest-neighbor atoms of the host. Furthermore, we investigate the effective dynamical coupling between the spins of two spatially separated Mn impurities, mediated by the host carriers. We find signatures of ferromagnetic coupling between the impurities in the time-evolution of their spins. Finally, we propose a scheme for investigating the spin relaxation of Mn dopants in GaAs, by extending the time-dependent approach for spin dynamics in an isolated conservative system to the case of an open system, with dephasing mechanisms included as an effective interaction between the system and an external bath [5].

  • 44.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Trend of the magnetic anisotropy for individual Mn dopants near the (110) GaAs surface2014In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 39, Article ID: 394006- p.Article in journal (Refereed)
    Abstract [en]

    Using a microscopic finite-cluster tight-binding model, we investigate the trend of the magnetic anisotropy energy as a function of the cluster size for an individual Mn impurity positioned in the vicinity of the (1 1 0) GaAs surface. We present results of calculations for large cluster sizes containing approximately 104 atoms, which have not been investigated so far. Our calculations demonstrate that the anisotropy energy of a Mn dopant in bulk GaAs, found to be non-zero in previous tight-binding calculations, is purely a finite size effect that vanishes with inverse cluster size. In contrast to this, we find that the splitting of the three in-gap Mn acceptor energy levels converges to a finite value in the limit of the infinite cluster size. For a Mn in bulk GaAs this feature is related to the nature of the mean-field treatment of the coupling between the impurity and its nearest neighbor atoms. We also calculate the trend of the anisotropy energy in the sublayers as the Mn dopant is moved away from the surface towards the center of the cluster. Here the use of large cluster sizes allows us to position the impurity in deeper sublayers below the surface, compared to previous calculations. In particular, we show that the anisotropy energy increases up to the fifth sublayer and then decreases as the impurity is moved further away from the surface, approaching its bulk value. The present study provides important insights for experimental control and manipulation of the electronic and magnetic properties of individual Mn dopants at the semiconductor surface by means of advanced scanning tunneling microscopy techniques.

  • 45.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Islam, Fhokrul
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    MacDonald, A. H.
    Department of Physics, University of Texas at Austin, U.S.A.
    Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators2013In: MRS Online Proceedings Library/Volume 1564/2013, Materials Research Society, 2013Conference paper (Refereed)
    Abstract [en]

    We present results of theoretical studies of transition metal dopants in GaAs, based on microscopic tight-binding model and ab-initio calculations. We focus in particular on how the vicinity of surface affects the properties of the hole-acceptor state, its magnetic anisotropy and its magnetic coupling to the magnetic dopant.  In agreement with STM experiments, Mn substitutional dopants on the (110) GaAs surface give rise to a deep acceptor state, whose wavefunction is localized around the Mn center. We discuss a refinement of the theory that introduces explicitly the d-levels for the TM dopant. The explicit inclusion of d-levels is particularly important for addressing recent STM experiments on substitutional Fe in GaAs. In the second part of the paper we discuss an analogous investigation of single dopants in Bi2Se3 three-dimensional topological insulators, focusing in particular on how substitutional impurities positioned on the surface affect the electronic structure in the gap.  We present explicit results for BiSe antisite defects and compare with STM experiments.

  • 46.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Islam, Fhokrul
    Virginia Commonwealth University, USA.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator2014In: MAR14 Meeting of The American Physical Society, Denver, Colorado: American Physical Society , 2014Conference paper (Refereed)
  • 47.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Islam, Fhokrul
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, Article ID: 195441- p.Article in journal (Refereed)
    Abstract [en]

    We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi$_2$Se$_3$ topological insulator. Combining \textit{ab initio} calculations with microscopic tight-binding modeling, we identify the effects of inversion-symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in ${+2}$ valence state and introduces an acceptor in the bulk band gap. The Mn-acceptor has predominantly $p$--character, and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn $d$--levels and the $p$--levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasi-resonant coupling and the strong real-space overlap between the spin-chiral surface states and the mid-gap spin-polarized Mn-acceptor states.

  • 48.
    Mahani, Mohammad Reza
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Islam, Fhokrul
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    The role of d levels of substitutional magnetic impurities at the (110) GaAs surface2013Conference paper (Other academic)
    Abstract [en]

    The study of the spin of individual transition-metal dopants in a semiconductor host is an emergent field known as magnetic solotronics, bearing exciting prospects for novel spintronics devices at the atomic scale. Advances in different STM based techniques allowed experimentalists to investigate substitutional dopants at a semiconductor surface with unprecedented accuracy and degree of details [1]. Theoretical studies based both on microscopic tight-binding (TB) models and DFT techniques have contributed in elucidating the experimental findings. In particular, for the case of Mn dopants on the (110) GaAs surface, TB models [2] have provided a quantitative description of the properties of the associated acceptor states. Most of these TB calculations ignore dealing explicitly with the Mn d-levels and treat the associated magnetic moment as a classical vector. However recent STM experiments [3] involving other TM impurities, such as Fe, reveal topographic features that might be related to electronic transitions within the d-level shell of the dopant. In this work we have included explicitly the d levels in the Hamiltonian. The parameters of the model have been extracted from DFT calculations. We have investigated the role that d levels play on the properties of the acceptor states of the doped GaAs(110) surface, and analyzed their implications for STM spectroscopy.

  • 49.
    Medel, Victor M.
    et al.
    Virginia Commonwealth University, USA.
    Reveles, Ulises
    Virginia Commonwealth University, USA.
    Islam, Fhokrul
    Virginia Commonwealth University, USA.
    Khanna, Shiv
    Virginia Commonwealth University, USA.
    Robust Magnetic Moments on Impurities in Metallic Clusters: Localized Magnetic States in Superatoms2013In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 117, no 20, 4297-4303 p.Article in journal (Refereed)
    Abstract [en]

    Introducing magnetic impurities into clusters of simplemetals can create localized states for higher angular momentum quantumnumbers (l = 2 or 3) that can breed magnetism analogous to that invirtual bound states in metallic hosts, offering a new recipe for magneticsuperatoms. In this work, we demonstrate that MnCan clusterscontaining 6−15 Ca atoms show a spin magnetic moment of 5.0 μBirrespective of the cluster size. Theoretical analysis reveals that the Mn dstates hybridize only partially with superatomic states and introduce extramajority and minority d states, largely localized at the Mn site, with alarge gap. Successive addition of Ca atoms introduces superatomic statesof varying angular momentum that are embedded in this gap, allowingcontrol over the stability of the motifs without altering the moment. Assemblies of such clusters can offer novel electronic features due to theformation of localized magnetic “quasibound states” in a confined nearlyfree electron gas.

  • 50.
    Michalak, Lukasz
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Pederson, Mark R.
    Navy Research Laboratory, Washington DC, (U.S.A).
    Paulsson, Magnus
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Benza, V.G.
    Universitá dell'Insubria, Como (Italy).
    Theory of tunneling spectroscopy in a Mn12 single-electron transistor by DFT methods2010In: Physical Review Letters, ISSN 0031-9007, Vol. 104, no 1, 017202-017205 p.Article in journal (Refereed)
12 1 - 50 of 82
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