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Holmqvist, C., Belzig, W. & Fogelström, M. (2018). Non-equilibrium charge and spin transport in superconducting–ferromagnetic–superconducting point contacts. Philosophical Transactions. Series A: Mathematical, physical, and engineering science, 376(2125), Article ID 20150229.
Open this publication in new window or tab >>Non-equilibrium charge and spin transport in superconducting–ferromagnetic–superconducting point contacts
2018 (English)In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 376, no 2125, article id 20150229Article in journal (Refereed) Published
##### Abstract [en]

The conventional Josephson effect may be modified by introducing spin-active scattering in the interface layer of the junction. Here, we discuss a Josephson junction consisting of two s-wave superconducting leads coupled over a classical spin that precesses with the Larmor frequency due to an external magnetic field. This magnetically active interface results in a time-dependent boundary condition with different tunnelling amplitudes for spin-up and -down quasi- particles and where the precession produces spin-flip scattering processes. As a result, the Andreev states develop sidebands and a non-equilibrium population that depend on the details of the spin precession. The Andreev states carry a steady-state Josephson charge current and a time-dependent spin current, whose current–phase relations could be used to characterize the precessing spin. The spin current is supported by spin-triplet correlations induced by the spin precession and creates a feedback effect on the classical spin in the form of a torque that shifts the precession frequency. By applying a bias voltage, the Josephson frequency adds another complexity to the situation and may create resonances together with the Larmor frequency. These Shapiro resonances manifest as torques and, under suitable 2conditions, are able to reverse the direction of the classical spin in sub-nanosecond time. Another characteristic feature is the subharmonic gap structure in the DC charge current displaying an even–odd effect attributable to precession-assisted multiple Andreev reflections. This article is part of the theme issue ‘Andreev bound states’.

##### Place, publisher, year, edition, pages
The Royal Society Publishing, 2018
##### Keywords
superconductivity
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-78125 (URN)10.1098/rsta.2015.0229 (DOI)
Available from: 2018-10-02 Created: 2018-10-02 Last updated: 2019-07-09Bibliographically approved
Pournaghavi, N., Holmqvist, C., Pertsova, A. & Canali, C. M. (2018). Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes [Letter to the editor]. Physica Status Solidi. Rapid Research Letters, 12(11, Special Issue), Article ID 1800210.
Open this publication in new window or tab >>Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes
2018 (English)In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 12, no 11, Special Issue, article id 1800210Article in journal, Letter (Refereed) Published
##### Abstract [en]

Using the non-equilibrium Green’s function method and the Keldysh formalism, we study the effects of spin–orbit interactions and time-reversal symmetry breaking exchange fields on non-equilibrium quantum transport in graphene armchair nanoribbons. We identify signatures of the quantum spin Hall (QSH) and the quantum anomalous Hall (QAH) phases in nonequilibrium edge transport by calculating the spin-resolved real space charge density and local currents at the nanoribbon edges. We find that the QSH phase, which is realized in a system with intrinsic spin–orbit coupling, is characterized by chiral counter-propagating local spin currents summing up to a net charge flow with opposite spin polarization at the edges. In the QAH phase, emerging in the presence of Rashba spin–orbit coupling and a ferromagnetic exchange field, two chiral edge channels with opposite spins propagate in the same direction at each edge, generating an unpolarized charge current and a quantized Hall conductance $G=\frac{2e^{2}}{h}$ . Increasing the intrinsic spin–orbit coupling causes a transition from the QAH to the QSH phase, evinced by characteristic changes in the non-equilibrium edge transport. In contrast, an antiferromagnetic exchange field can coexist with a QSH phase, but can never induce a QAH phase due to a symmetry that combines time-reversal and sublattice translational symmetry.

##### Place, publisher, year, edition, pages
Wiley-Blackwell, 2018
##### Keywords
graphene nanoribbons, quantum anomalous Hall effect, quantum spin Hall effect, topological insulators
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-76947 (URN)10.1002/pssr.201800210 (DOI)000450130300007 ()2-s2.0-85050622980 (Scopus ID)
##### Funder
Carl Tryggers foundation , CTS 14:178Swedish Research Council, 621‐2014‐4785 Available from: 2018-07-19 Created: 2018-07-19 Last updated: 2019-08-29Bibliographically approved
Qaiumzadeh, A., Skarsvåg, H., Holmqvist, C. & Brataas, A. (2017). Spin Superfluidity in Biaxial Antiferromagnetic Insulators [Letter to the editor]. Physical Review Letters, 118, Article ID 137201.
Open this publication in new window or tab >>Spin Superfluidity in Biaxial Antiferromagnetic Insulators
2017 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, article id 137201Article in journal, Letter (Refereed) Published
##### Abstract [en]

Antiferromagnets may exhibit spin superfluidity since the dipole interaction is weak. We seek toestablish that this phenomenon occurs in insulators such as NiO, which is a good spin conductor accordingto previous studies. We investigate nonlocal spin transport in a planar antiferromagnetic insulator witha weak uniaxial anisotropy. The anisotropy hinders spin superfluidity by creating a substantial thresholdthat the current must overcome. Nevertheless, we show that applying a high magnetic field removes thisobstacle near the spin-flop transition of the antiferromagnet. Importantly, the spin superfluidity can thenpersist across many micrometers, even in dirty samples.

##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63795 (URN)10.1103/PhysRevLett.118.137201 (DOI)
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-05-15Bibliographically approved
Xu, F., Holmqvist, C., Rastelli, G. & Belzig, W. (2016). Dynamical Coulomb blockade theory of plasmon-mediated light emission from a tunnel junction. Physical Review B, 94(24), Article ID 245111.
Open this publication in new window or tab >>Dynamical Coulomb blockade theory of plasmon-mediated light emission from a tunnel junction
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 24, article id 245111Article in journal (Refereed) Published
##### Abstract [en]

Inelastic tunneling of electrons can generate the emission of photons with energies intuitively limited by the applied bias voltage. However, experiments indicate that more complex processes involving the interaction of electrons with plasmon polaritons lead to photon emission with overbias energies. We recently proposed a model of this observation in Phys. Rev. Lett. 113, 066801 (2014), in analogy to the dynamical Coulomb blockade, originally developed for treating the electromagnetic environment in mesoscopic circuits. This model describes the correlated tunneling of two electrons interacting with a local plasmon-polariton mode, represented by a resonant circuit, and shows that the overbias emission is due to the non-Gaussian fluctuations. Here we extend our model to study the overbias emission at finite temperature. We find that the thermal smearing strongly masks the overbias emission. Hence, the detection of the correlated tunneling processes requires temperatures k(B)T much lower than the bias energy eV and the plasmon energy h omega(0), a condition which is fortunately realized experimentally.

##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-59578 (URN)10.1103/PhysRevB.94.245111 (DOI)000389503400005 ()2-s2.0-85008485780 (Scopus ID)
Available from: 2017-01-03 Created: 2017-01-03 Last updated: 2018-05-17Bibliographically approved
Bergvall, A., Fogelström, M., Holmqvist, C. & Löfwander, T. (2015). Basic theory of electron transport through molecular contacts. In: K. Moth-Poulsen (Ed.), Handbook of Single Molecule Electronics: (pp. 31-78). Pan Stanford Publishing
Open this publication in new window or tab >>Basic theory of electron transport through molecular contacts
2015 (English)In: Handbook of Single Molecule Electronics / [ed] K. Moth-Poulsen, Pan Stanford Publishing, 2015, p. 31-78Chapter in book (Refereed)
##### Place, publisher, year, edition, pages
Pan Stanford Publishing, 2015
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63815 (URN)10.1201/b18763-4 (DOI)978-981-4463-38-6 (ISBN)978-981-4463-39-3 (ISBN)
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-05-15Bibliographically approved
Xu, F., Holmqvist, C. & Belzig, W. (2015). Over-bias light emission due to higher order quantum noise of a tunnel junction. In: 2015 International Conference on Noise and Fluctuations (ICNF): . Paper presented at 2015 International Conference on Noise and Fluctuations (ICNF), 2-6 June 2015, Xian, China. IEEE conference proceedings
Open this publication in new window or tab >>Over-bias light emission due to higher order quantum noise of a tunnel junction
2015 (English)In: 2015 International Conference on Noise and Fluctuations (ICNF), IEEE conference proceedings, 2015Conference paper, Published paper (Refereed)
##### Abstract [en]

Understanding tunneling from an atomically sharp tip to a metallic surface requires to account for interactions on a nanoscopic scale. Inelastic tunneling of electrons generates emission of photons, whose energies intuitively should be limited by the applied bias voltage. However, experiments [Phys. Rev. Lett. 102, 057401 (2009)] indicate that more complex processes involving the interaction of electrons with plasmon polaritons lead to photon emission characterized by over-bias energies. We propose a model of this observation in analogy to the dynamical Coulomb blockade, originally developed for treating the electronic environment in mesoscopic circuits. We explain the experimental finding quantitatively by the correlated tunneling of two electrons interacting with an LRC circuit modeling the local plasmon-polariton mode. To explain the over-bias emission, the non-Gaussian statistics of the tunneling dynamics of the electrons is essential.

##### Place, publisher, year, edition, pages
IEEE conference proceedings, 2015
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63813 (URN)10.1109/ICNF.2015.7288573 (DOI)978-1-4673-8336-3 (ISBN)978-1-4673-8335-6 (ISBN)
##### Conference
2015 International Conference on Noise and Fluctuations (ICNF), 2-6 June 2015, Xian, China
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-05-15Bibliographically approved
Skarsvåg, H., Holmqvist, C. & Brataas, A. (2015). Spin Superfluidity and Long-Range Transport in Thin-Film Ferromagnets. Physical Review Letters, 115, Article ID 237201.
Open this publication in new window or tab >>Spin Superfluidity and Long-Range Transport in Thin-Film Ferromagnets
2015 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, article id 237201Article in journal (Refereed) Published
##### Abstract [en]

In ferromagnets, magnons may condense into a single quantum state. Analogous to superconductors,this quantum state may support transport without dissipation. Recent works suggest that longitudinal spintransport through a thin-film ferromagnet is an example of spin superfluidity. Although intriguing, thistantalizing picture ignores long-range dipole interactions; here, we demonstrate that such interactionsdramatically affect spin transport. In single-film ferromagnets,“spin superfluidity”only exists at lengthscales (a few hundred nanometers in yttrium iron garnet) somewhat larger than the exchange length. Overlonger distances, dipolar interactions destroy spin superfluidity. Nevertheless, we predict the reemergenceof spin superfluidity in trilayer ferromagnet-normal metal-ferromagnet films that are∼1μm in size. Suchsystems also exhibit other types of long-range spin transport in samples that are several micrometers in size.

##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63796 (URN)10.1103/PhysRevLett.115.237201 (DOI)
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2018-05-17Bibliographically approved
Xu, F., Holmqvist, C. & Belzig, W. (2014). Overbias Light Emission due to Higher-Order Quantum Noise in a Tunnel Junction. Physical Review Letters, 113, Article ID 066801.
Open this publication in new window or tab >>Overbias Light Emission due to Higher-Order Quantum Noise in a Tunnel Junction
2014 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, article id 066801Article in journal (Refereed) Published
##### Abstract [en]

Understanding tunneling from an atomically sharp tip to a metallic surface requires us to account for interactions on a nanoscopic scale. Inelastic tunneling of electrons generates emission of photons, whose energies intuitively should be limited by the applied bias voltage. However, experiments [G. Schull et al., Phys. Rev. Lett. 102, 057401 (2009)] indicate that more complex processes involving the interaction of electrons with plasmon polaritons lead to photon emission characterized by overbias energies. We propose a model of this observation in analogy to the dynamical Coulomb blockade, originally developed for treating the electronic environment in mesoscopic circuits. We explain the experimental finding quantitatively by the correlated tunneling of two electrons interacting with a LRC circuit modeling the local plasmon-polariton mode. To explain the overbias emission, the non-Gaussian statistics of the tunneling dynamics of the electrons is essential.

##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63797 (URN)10.1103/PhysRevLett.113.066801 (DOI)
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-05-15Bibliographically approved
Holmqvist, C., Fogelström, M. & Belzig, W. (2014). Spin-polarized Shapiro steps and spin-precession-assisted multiple Andreev reflection. Physical Review B, 90, Article ID 014516.
Open this publication in new window or tab >>Spin-polarized Shapiro steps and spin-precession-assisted multiple Andreev reflection
2014 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 90, article id 014516Article in journal (Refereed) Published
##### 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.

##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63798 (URN)10.1103/PhysRevB.90.014516 (DOI)
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-11-29Bibliographically approved
Stadler, P., Holmqvist, C. & Belzig, W. (2013). Josephson current through a quantum dot coupled to a molecular magnet. Physical Review B, 88, Article ID 104512.
Open this publication in new window or tab >>Josephson current through a quantum dot coupled to a molecular magnet
2013 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 88, article id 104512Article in journal (Refereed) Published
##### Abstract [en]

Josephson currents are carried by sharp Andreev states within the superconducting energy gap. We theoretically study the electronic transport of a magnetically tunable nanoscale junction consisting of a quantum dot connected to two superconducting leads and coupled to the spin of a molecular magnet. The exchange interaction between the molecular magnet and the quantum dot modifies the Andreev states due to a spin-dependent renormalization of the quantum dot's energy level and the induction of spin flips. A magnetic field applied to the central region of the quantum dot and the molecular magnet further tunes the Josephson current and starts a precession of the molecular magnet's spin. We use a nonequilibrium Green's function approach to evaluate the transport properties of the junction. Our calculations reveal that the energy level of the dot, the magnetic field, and the exchange interaction between the molecular magnet and the electrons occupying the energy level of the quantum dot can trigger transitions from a 0 to a π state of the Josephson junction. The redistribution of the occupied states induced by the magnetic field strongly modifies the current-phase relation. The critical current exhibits a sharp increase as a function of either the energy level of the dot, the magnetic field, or the exchange interaction.

##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-63803 (URN)10.1103/PhysRevB.88.104512 (DOI)
Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-11-29Bibliographically approved
##### Identifiers
ORCID iD: orcid.org/0000-0001-5551-8980

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