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  • 1.
    Aikebaier, Faluke
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Effects of short-range electron-electron interactions in doped graphene2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 15, artikel-id 155420Artikel i tidskrift (Refereegranskat)
    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.

  • 2.
    Azimi Mousolou, Vahid
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Sjöqvist, Erik
    Uppsala University, Department of Chemistry.
    Non-Abelian off-diagonal geometric phases in nano-engineered four-qubit systems2013Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 103, nr 6, s. 60011-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The concept of off-diagonal geometric phase (GP) has been introduced in order to recover interference information about the geometry of quantal evolution where the standard GPs are not well-defined. In this Letter, we propose a physical setting for realizing non-Abelian off-diagonal GPs. The proposed non-Abelian off-diagonal GPs 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 GP.

  • 3.
    Azimi Mousolou, Vahid
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Sjöqvist, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Quantum Chemistry) (Quantum information theory.
    Non-Abelian quantum holonomy of hydrogen-like atoms2011Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, ISSN 1050-2947, Vol. 84, nr 3, s. Article ID: 032111-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the Uhlmann holonomy [Rep. Math. Phys. 24, 229 (1986)] of quantum states for hydrogen-like atoms, where the intrinsic spin and orbital angular momentum are coupled by the spin-orbit interaction and subject to a slowly varying magnetic field. We show that the holonomy for the orbital angular momentum and spin subsystems is non-Abelian, while the holonomy of the whole system is Abelian. Quantum entanglement in the states of the whole system is crucially related to the non-Abelian gauge structure of the subsystems. We analyze the phase of the Wilson loop variable associated with the Uhlmann holonomy, and find a relation between the phase of the whole system with corresponding marginal phases. Based on the result for the model system we provide evidence that the phase of the Wilson loop variable and the mixed-state geometric phase [Phys. Rev. Lett. 85, 2845 (2000)] are in general inequivalent.

  • 4.
    Azimi Mousolou, Vahid
    et al.
    University of Isfahan, Iran.
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Sjöqvist, Erik
    Uppsala University.
    Spin-electric Berry phase shift in triangular molecular magnets2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 23, artikel-id 235423Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose a Berry phase effect on the chiral degrees of freedom of a triangular magnetic molecule. The phase is induced by adiabatically varying an external electric field in the plane of the molecule via a spin-electric coupling mechanism present in these frustrated magnetic molecules. The Berry phase effect depends on spin-orbit interaction splitting and on the electric dipole moment. By varying the amplitude of the applied electric field, the Berry phase difference between the two spin states can take any arbitrary value between zero and π, which can be measured as a phase shift between the two chiral states by using spin-echo techniques. Our result can be used to realize an electric-field-induced geometric phase-shift gate acting on a chiral qubit encoded in the ground-state manifold of the triangular magnetic molecule.

  • 5.
    Azimi Mousolou, Vahid
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    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 transition2013Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, nr 1, s. Article ID: 012310-Artikel i tidskrift (Refereegranskat)
    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.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    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 MagnetsManuskript (preprint) (Övrigt vetenskapligt)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Sjöqvist, Erik
    Uppsala University ; National University of Singapore, Singapore.
    Universal Non-adiabatic Holonomic Gates in Quantum Dots and Single-Molecule Magnets2014Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, artikel-id 013029Artikel i tidskrift (Refereegranskat)
    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. Benza, V G
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Strini, G
    Landau-Zener quantum tunneling in disordered nanomagnets2004Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, nr 18, s. 1844261-9-184426-9Artikel i tidskrift (Refereegranskat)
  • 9.
    Borschel, Christian
    et al.
    Jena Universty, Germany .
    Messing, Maria
    Lund University.
    Borgstrom, Magnus T.
    Lund University.
    Paschoal, Waldomiro Jr.
    Wallentin, Jesper
    Lund University.
    Kumar, Sandeep
    Lund University.
    Mergenthaler, Kilian
    Lund University.
    Deppert, Knut
    Lund University.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Pettersson, Håkan
    Halmstad University.
    Samuelson, Lars
    Lund University.
    Ronning, Carsten
    Jena University, Germany.
    A New Route toward Semiconductor Nanospintronics: Highly Mn-Doped GaAs Nanowires Realized by Ion-Implantation under Dynamic Annealing Conditions2011Ingår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, nr 9, s. 3935-3940Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on highly Mn-doped GaAs nanowires (NWs) of high crystalline quality fabricated by ion beam implantation, a technique that allows doping concentrations beyond the equilibrium solubility limit. We studied two approaches for the preparation of Mn-doped GaAs NWs: First, ion implantation at room temperature with subsequent annealing resulted in polycrystalline NWs and phase segregation of MnAs and GaAs. The second approach was ion implantation at elevated temperatures. In this case, the single-crystallinity of the GaAs NWs was maintained, and crystalline, highly Mn-doped GaAs NWs were obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV). Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our findings suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic material system.

  • 10.
    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
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    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 field2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, s. Article ID: 205203-Artikel i tidskrift (Refereegranskat)
    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.

  • 11.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Collective Excitations in Ferromagnetic Metal Nanoparticles2002Ingår i: American Institute of Physics, Vol. 47, No.2Artikel i tidskrift (Övrigt vetenskapligt)
  • 12.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Disorder and interaction-Induced Pairing in the addition spectra of quantum dots2000Ingår i: Physical Review Letters, Vol. 84Artikel i tidskrift (Refereegranskat)
  • 13.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    First-principle studies of tunneling transport in single-molecule magnets2010Konferensbidrag (Refereegranskat)
  • 14.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Magnetic anisotropy, hysteresis and quasiparticle states in ferromagnetic metal nanoparticles2002Övrigt (Övrigt vetenskapligt)
  • 15.
    Canali, Carlo M.
    et al.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Cehovin, Alexander
    H. MacDonald, Allan
    Chern Numbers for Spin Models of Transition Metal Nanomagnets2003Ingår i: Physical review letters, Vol. 91Artikel i tidskrift (Refereegranskat)
  • 16.
    Canali, Carlo M.
    et al.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    H. MacDonald, Allan
    Theory of tunneling spectroscopy in ferromagnetic nanoparticles2000Ingår i: Physical Review Letters, Vol. 85Artikel i tidskrift (Refereegranskat)
  • 17. Cehovin, Alexander
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    H. MacDonald, Allan
    Elementary Excitations of Ferromagnetic Metal Nanoparticles2003Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, nr 1, s. 144231-1442317Artikel i tidskrift (Refereegranskat)
  • 18. Cehovin, Alexander
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    H. MacDonald, Allan
    Magnetization orientation dependence of the quasiparticle and hysteresis in ferromagnetic metal nanoparticles2002Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, nr 9, s. 944301-9443015Artikel i tidskrift (Refereegranskat)
  • 19. Cehovin, Alexander
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    MacDonald, Allan
    Orbital and Spin Contributions to the g-Tensors in Metal Nanoparticles2004Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, nr 4, s. 454111-8-45411-8Artikel i tidskrift (Refereegranskat)
  • 20. Cehovin, Alexander
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    MacDonald, Allan H.
    Quantum physics of ferromagnetic metal nanoparticles in an external magnetic field2002Övrigt (Övrigt vetenskapligt)
  • 21.
    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
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Borg, Mattias
    IBM Research-Zurich, Switzerland.
    Nielsch, Kornelius
    University of Hamburg, Germany ; IFW Dresden, Germany.
    Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires2016Ingår i: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 10, nr 7, s. 7180-7188Artikel i tidskrift (Refereegranskat)
    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.

  • 22.
    Islam, Fhokrul
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Ab initio calculations of the magnetic properties of Mn impurities on GaAs (110) surfaces2012Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. B 85, s. Article ID: 155306-Artikel i tidskrift (Refereegranskat)
    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.

  • 23.
    Islam, Fhokrul
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, A.
    NORDITA, Stockholm.
    Balatsky, A.
    NORDITA, Stockholm.
    Mahatha, S. K.
    CNR, Italy.
    Carbone, C.
    CNR, Italy.
    Barla, A.
    CNR, Italy.
    Kokh, K. A.
    Novosibirsk State Univ, Russia.
    Tereshchenko, O. E.
    Novosibirsk State Univ, Russia.
    Jimenez, E.
    European Synchrotron Radiat Facil, France.
    Brookes, N. B.
    European Synchrotron Radiat Facil, France.
    Gargiani, P.
    ALBA Synchrotron Light Source, Spain.
    Valvidares, M.
    ALBA Synchrotron Light Source, Spain.
    Schatz, S.
    Univ Wurzburg, Germany.
    Peixoto, T. R. F.
    Univ Wurzburg, Germany.
    Bentmann, H.
    Univ Wurzburg, Germany.
    Reinert, F.
    Univ Wurzburg, Germany.
    Jung, J.
    Univ Wurzburg, Germany.
    Bathon, T.
    Univ Wurzburg, Germany.
    Fauth, K.
    Univ Wurzburg, Germany.
    Bode, M.
    Univ Wurzburg, Germany.
    Sessi, P.
    Univ Wurzburg, Germany.
    Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, nr 15, artikel-id 155429Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here, we report on a detailed and systematic investigation of transition metal (TM) doped Sb2Te3. By combining density functional theory calculations with complementary experimental techniques, i.e., scanning tunneling microscopy, resonant photoemission, and x-raymagnetic circular dichroism, we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM doped topological insulators. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V-and Fe-doped Sb2Te3 display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity dependent and can vary from in plane to out of plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM doped Sb2Te3 in the ferromagnetic state.

  • 24.
    Islam, Fhokrul
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Nossa, Javier
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Pederson, Mark
    Naval Research Laboratory, USA.
    First-principles study of spin-electric coupling in a {Cu3} single molecular magnet2010Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, nr 15, s. article number-155446Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on a study of the electronic and magnetic properties of the triangular antiferromagnetic {Cu3} single-molecule magnet, based on spin-density-functional theory. Our calculations show that the low-energy magnetic properties are correctly described by an effective three-site spin s = 1/2 Heisenberg model, with an antiferromagnetic exchange coupling J approximate to 5 meV. The ground-state manifold of the model is composed of two degenerate spin S = 1/2 doublets of opposite chirality. Due to lack of inversion symmetry in the molecule these two states are coupled by an external electric field, even when spin-orbit interaction is absent. The spin-electric coupling can be viewed as originating from a modified exchange constant delta J induced by the electric field. We find that the calculated transition rate between the chiral states yields an effective electric dipole moment d = 3.38 x 10(-33) C m approximate to e10(-4)a, where a is the Cu separation. For external electric fields epsilon approximate to 10(8) V/m this value corresponds to a Rabi time tau approximate to 1 ns and to a delta J on the order of a few mu eV.

  • 25.
    Islam, Fhokrul
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE). Dept. of physics, Linnéuniversitetet.
    Pertsova, Anna
    Nordita, Sweden.
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization2019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, nr 15, s. 1-6, artikel-id 155401Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The quantum anomalous Hall effect (QAHE), characterized by dissipationless quantized edge transport, relies crucially on a nontrivial topology of the electronic bulk band structure and a robust ferromagnetic order that breaks time-reversal symmetry. Magnetically doped topological insulators (TIs) satisfy both these criteria, and are the most promising quantum materials for realizing the QAHE. Because the spin of the surface electrons aligns along the direction of the magnetic-impurity exchange field, only magnetic TIs with an out-of-plane magnetization are thought to open a gap at the Dirac point (DP) of the surface states, resulting in the QAHE. Using a continuum model supported by atomistic tight-binding and first-principles calculations of transition-metal doped Bi2Se3, we show that a surface-impurity potential generates an additional effective magnetic field which spin polarizes the surface electrons along the direction perpendicular to the surface. The predicted gap-opening mechanism results from the interplay of this additional field and the in-plane magnetization that shifts the position of the DP away from the Γ point. This effect is similar to the one originating from the hexagonal warping correction of the band structure but is one order of magnitude stronger. Our calculations show that in a doped TI with in-plane magnetization the impurity-potential-induced gap at the DP is comparable to the one opened by an out-of-plane magnetization.

  • 26.
    Kumar, Sandeep
    et al.
    Solid State Physics/The Nanometer Structure Consortium, Lund University.
    Paschoal Jr, Waldomiro
    Solid State Physics/The Nanometer Structure Consortium, Lund University; Dept. of Mathematics, Physics and Electrical Engineering, Halmstad University.
    Johannes, Andreas
    Institute for Solid State Physics, Friedrich-Schiller-University Jena.
    Jacobsson, Daniel
    Solid State Physics/The Nanometer Structure Consortium, Lund University.
    Borschel, Christian
    Institute for Solid State Physics, Friedrich-Schiller-University Jena.
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Wang, Chih-Han
    Institute of Physics, Academia Sinica, Taipei.
    Wu, Maw-Kuen
    Institute of Physics, Academia Sinica, Taipei; Department of Physics, National Donghwa University, Taiwan.
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Ronning, Carsten
    Institute for Solid State Physics, Friedrich-Schiller-University Jena.
    Samuelson, Lars
    Solid State Physics/The Nanometer Structure Consortium, Lund University.
    Pettersson, Håkan
    Solid State Physics/The Nanometer Structure Consortium, Lund University; Dept. of Mathematics, Physics and Electrical Engineering, Halmstad University.
    Magnetic polarons and large negative magnetoresistance in GaAs nanowires implanted with Mn ions2013Ingår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 13, nr 11, s. 5079-5084Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on low-temperature magnetotransport and SQUID measurements on heavily doped Mn-implanted GaAs nanowires. SQUID data recorded at low magnetic fields exhibit clear signs of the onset of a spin-glass phase with a transition temperature of about 16 K. Magnetotransport experiments reveal a corresponding peak in resistance at 16 K and a remarkably large negative magnetoresistance, reaching 40 % at 1.6 K and 8 T. The negative magnetoresistance decreases at elevated temperatures and vanishes at about 100 K. We interpret our transport data in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons forming a paramagnetic/spin-glass phase.

  • 27. Liu, R
    et al.
    Pettersson, H
    Michalak, Lukasz
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Samuelson, Lars
    Probing Spin-relaxation Time in Ferromagnetic Single Electron Transistors2007Ingår i: Nano letters, Vol. 7, s. 81-85Artikel i tidskrift (Refereegranskat)
  • 28. Liu, R. S.
    et al.
    Yang, See-Hun
    Jiang, Xin
    Zhang, X. -G
    Rettner, Charles
    Gao, Li
    Topuria, Teya
    Rice, Philip M.
    Zhang, Weifeng
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Parkin, Stuart S. P.
    CoFe alloy as middle layer for strong spin dependent quantum well resonant tunneling in MgO double barrier magnetic tunnel junctions2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 2, s. 024411-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on spin dependent quantum well (QW) resonances in the CoFe alloy middle layer of CoFe/MgO/CoFe/MgO/CoFeB double barrier magnetic tunnel junctions. The dI/dV spectra reveal clear resonant peaks for the parallel magnetization configurations, which can be related to the existence of QW resonances obtained from first-principles calculations. We observe that the differential tunneling magnetoresistance (TMR) exhibits an oscillatory behavior as a function of voltage with a sign change as well as a pronounced TMR enhancement at resonant voltages at room temperature. The observation of strong QW resonances indicates that the CoFe film possesses a long majority spin mean-free path, and the substitutional disorder does not cause a significant increase of scattering. Both points are confirmed by first-principles electronic structure calculation. DOI: 10.1103/PhysRevB.87.024411

  • 29. Liu, Ruisheng
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Samuelson, Lars
    Pettersson, Håkan
    Magnetoresistance studies on Co/AlOX/Au and Co/AlOX/Ni/Au tunnel structures2008Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, s. Article ID: 203107-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on magnetoresistance (MR) studies on Co/AlOX/Au and Co/AlOX/Ni/Au magnetic tunnel junctions. In spite of the fact that the difference between the two samples is merely a 3 nm thick Ni layer, there is a sharp contrast in MR behavior indicating that the electronic structure at the interface between the ferromagnetic electrodes and the insulating barrier dominates the MR signal. The former sample exhibits a clear tunneling anisotropic MR (TAMR), with the characteristic correlation between resistance and current direction, in contrast to the latter sample which displays a conventional tunneling MR (TMR) dominated by the relative orientation between the magnetization directions of the two electrodes. In addition, the TAMR has a much stronger temperature dependence than the TMR, indicating a much faster drop-off of the tunneling density of states anisotropy than the tunneling electron spin polarization with increasing temperature. Finally, we propose a possible simple way to distinguish TAMR from normal TMR by measuring the resistance of the device at different angles of the external magnetic field.

  • 30. Liu, Ruisheng
    et al.
    Michalak, Lukasz
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Samuelson, Lars
    Pettersson, Håkan
    Tunneling Anisotropic Magnetoresistance in C0/AlOx/Au Tunnel Junctions,2008Ingår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 8, nr 3, s. 848-852Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We observe spin-valve-like effects in nanoscaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.

  • 31.
    Liu, Ruisheng
    et al.
    Lund University, Solid State Physics.
    Pettersson, H.
    Lund University, Solid State Physics.
    Suyatin, D.
    Lund University, Solid State Physics.
    Michalak, L.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Mahani, Mohammad Reza
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Samuelson, L.
    Lund University, Solid State Physics.
    Nano-scaled Ferromagnetic Single-electron transistors2007Ingår i: 2007 7TH IEEE CONFERENCE ON NANOTECHNOLOGY, 2007, s. 420-423Konferensbidrag (Refereegranskat)
  • 32. Liu, Ruisheng
    et al.
    Pettersson, Håkan
    Michalak, Lukasz
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Samuelson, Lars
    Suyatin, Dmitri
    Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors2007Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, nr 12Artikel i tidskrift (Refereegranskat)
  • 33. MacDonald, Allan
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Qunatum description of ferromagnetic metal nanoparticles2001Ingår i: Solid State Communications, Vol. 119Artikel i tidskrift (Refereegranskat)
  • 34. MacDonald, Allan
    et al.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Thermoinduced magnetization in nanoparticles of antiferromagnetic materials2005Ingår i: Physical review letters, Vol. 94 (8)Artikel i tidskrift (Refereegranskat)
  • 35.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    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 field2011Konferensbidrag (Refereegranskat)
  • 36.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    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 surface2012Ingår i: Bulletin of the American Physical Society, APS March Meeting 2012,  Volume 57, Number 1, 2012, s. L14.00002-Konferensbidrag (Övrigt vetenskapligt)
    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.

  • 37.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    MacDonald, A.H.
    University of Texas at Austin.
    Effect of magnetic field on the local density of states of Mn acceptor magnets in GaAs2011Ingår i: Bulletin of the American Physical Society, Volume 56, Number 1: APS March Meeting 2011, 2011, s. W15.00002-Konferensbidrag (Övrigt vetenskapligt)
    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.

  • 38.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för teknik, TEK.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för teknik, TEK.
    MacDonald, A.H.
    Local manipulation of the magnetic properties of Mn impurities on a GaAs surface by As vacancies2012Konferensbidrag (Refereegranskat)
  • 39.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Islam, Fhokrul
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    MacDonald, A.H.
    As vacancies in MnGaAs: tight binding and first-principles studies2012Konferensbidrag (Refereegranskat)
  • 40.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Islam, Fhokrul
    Virginia Commonwealth Univ, Richmond, VA 23284 USA.
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, nr 16, s. Article ID: 165408-Artikel i tidskrift (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    MacDonald, A. H.
    University of Texas at Austin, USA.
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Electric manipulation of the Mn-acceptor binding energy and the Mn-Mn exchange interaction on the GaAs (110) surface by nearby As vacancies2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 4, artikel-id 045304Artikel i tidskrift (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Spin dynamics of Mn impurities and their bound acceptors in GaAs2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 24, artikel-id 245406Artikel i tidskrift (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Time-Dependent Spin Dynamics of Few Transition Metal Impurities in a Semiconductor Host2014Ingår i: 2014 MRS Spring Meeting and Exhibit, April 21-25, San Francisco, 2014Konferensbidrag (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Trend of the magnetic anisotropy for individual Mn dopants near the (110) GaAs surface2014Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, nr 39, s. Article ID: 394006-Artikel i tidskrift (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Islam, Fhokrul
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    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 insulators2013Ingår i: MRS Online Proceedings Library/Volume 1564/2013, Materials Research Society, 2013Konferensbidrag (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Islam, Fhokrul
    Virginia Commonwealth University, USA.
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator2014Ingår i: MAR14 Meeting of The American Physical Society, Denver, Colorado: American Physical Society , 2014Konferensbidrag (Refereegranskat)
  • 47.
    Mahani, Mohammad Reza
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Islam, Fhokrul
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, s. Article ID: 195441-Artikel i tidskrift (Refereegranskat)
    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.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Pertsova, Anna
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Islam, Fhokrul
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Canali, Carlo M.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    The role of d levels of substitutional magnetic impurities at the (110) GaAs surface2013Konferensbidrag (Övrigt vetenskapligt)
    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.
    Michalak, Lukasz
    et al.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Canali, Carlo M.
    Högskolan i Kalmar, Naturvetenskapliga institutionen.
    Benza, V G
    Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles2006Ingår i: Physical review letters, Vol. 97 (9)Artikel i tidskrift (Refereegranskat)
  • 50.
    Michalak, Lukasz
    et al.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Canali, Carlo M.
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Pederson, Mark R.
    Navy Research Laboratory, Washington DC, (U.S.A).
    Paulsson, Magnus
    Linnéuniversitetet, Fakultetsnämnden för naturvetenskap och teknik, Institutionen för datavetenskap, fysik och matematik, DFM.
    Benza, V.G.
    Universitá dell'Insubria, Como (Italy).
    Theory of tunneling spectroscopy in a Mn12 single-electron transistor by DFT methods2010Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 104, nr 1, s. 017202-017205Artikel i tidskrift (Refereegranskat)
12 1 - 50 av 78
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