arXiv subjects

J. Barnaś

Publications and source records attributed to J. Barnaś.

29 records · Page 2Linked to original sources

arXiv2015

Thermoelectric and thermospin transport in a ballistic junction of graphene

We consider theoretically a wide graphene ribbon, that on both ends is attached to electronic reservoirs which generally have different temperatures. The graphene ribbon is assumed to be deposited on a substrate, that leads to a spin-orbit coupling of Rashba type. We calculate the thermally induced charge current in the ballistic transport regime as well as the thermoelectric voltage (Seebeck effect). Apart from this, we also consider thermally induced spin current and spin polarization of the graphene ribbon. The spin currents are shown to have generally two components; one parallel to the temperature gradient and the other one perpendicular to this gradient. The latter corresponds to the spin current due to the spin Nernst effect. Additionally, we also consider the heat current between the reservoirs due to transfer of electrons.

cond-mat.mes-hall↗

arXiv2015

Enhanced photogalvanic effect in graphene due to Rashba spin-orbit coupling

We analyze theoretically optical generation of a spin-polarized charge current (photogalvanic effect) and spin polarization in graphene with Rashba spin-orbit coupling. An external magnetic field is applied in the graphene plane, which plays a crucial role in the mechanism of current generation. We predict a highly efficient resonant-like photogalvanic effect in a narrow frequency range which is determined by the magnetic field. A relatively less efficient photogalvanic effect appears in a broader frequency range, determined by the electron concentration and spin-orbit coupling strength.

cond-mat.mes-hall↗

arXiv2014

Thermoelectric effect enhanced by the resonant states in graphene

Thermoelectric effects in graphene are considered theoretically with particular attention paid to the role of impurities. Using the T -matrix method we calculate the impurity resonant states and the momentum relaxation time due to scattering on impurities. The Boltzmann kinetic equation is used to determine the thermoelectric coefficients. It is shown that the resonant impurity states near the Fermi level give rise to a resonant enhancement of the Seebeck coefficient and of the figure of merit $ZT$ . The Wiedemann-Franz ratio deviates from that known for ordinary metals, where this ratio is constant and equal to the Lorentz number. This deviation appears for small chemical potentials and in the vicinity of the resonant states. In the limit of a constant relaxation time, this ratio has been calculated analytically for $μ=0$.

cond-mat.mes-hall↗

arXiv2014

Optical spin injection in graphene with Rashba spin-orbit interaction

We calculate the efficiency of infrared optical spin injection in single-layer graphene with Rashba spin-orbit coupling and for in-plane magnetic field. The injection rate in the photon frequency range corresponding to the Rashba splitting is shown to be proportional to the ratio of the Zeeman and Rashba splittings. As a result, large spin polarization can be controllably achieved for experimentally available values of the spin-orbit coupling and in magnetic fields below 10 Tesla.

cond-mat.mes-hall↗

arXiv2014

Enhanced thermoelectric efficiency in ferromagnetic silicene nanoribbons asymmetrically terminated with hydrogen atoms

Using ab-initio methods we calculate thermoelectric and spin thermoelectric properties of silicene nanoribbons with bare, mono-hydrogenated and di-hydrogenated edges. Asymmetric structures, in which one edge is either bare or di-hydrogenated while the other edge is mono-hydrogenated (0H-1H and 2H-1H nanoribbons) have ferromagnetic ground state and display remarkable conventional and spin thermoelectric properties. Strong enhancement of the thermoelectric efficiency, both conventional and spin ones, results from a very specific band structure of such nanoribbons, where one spin channel is blocked due to an energy gap while the other spin channel is highly conducting. In turn, 0H-2H and 2H-2H nanoribbons (with one edge being either bare or di-hydrogenated and the other edge being di-hydrogenated) are antiferromagnetic in the ground state. Accordingly, the corresponding spin channels are equivalent, and only conventional thermoelectric effects can occur in these nanoribbons.

cond-mat.mtrl-sci↗

arXiv2014

Spin effects in thermoelectric properties of Al and P doped zigzag silicene nanoribbons

Electric and thermoelectric properties of silicene nanoribbons doped with Al and P impurity atoms are investigated theoretically for both antiparallel and parallel orientations of the edge magnetic moments. In the former case, appropriately arranged impurities can lead to a net magnetic moment and thus also to spin thermoelectric effects. In the latter case, in turn, spin thermoelectric effects also occur in the absence of impurities. Numerical results based on ab-initio calculations show that the spin thermopower can be considerably enhanced by the impurities.

cond-mat.mtrl-sci↗

arXiv2013

Current-induced spin polarization in graphene due to Rashba spin-orbit interaction

Spin polarization induced by an external electric field in graphene is considered theoretically in the linear response regime. The graphene is assumed to be deposited on a substrate which leads to the spin-orbit interaction of Rashba type. The induced spin polarization is shown to be in the graphene plane and perpendicular to the electric field. However, the spin polarization changes sign when the Fermi level, whose position can be controlled by an external gate voltage, crosses the Dirac points.

cond-mat.mes-hall↗

arXiv2013

Spin Hall effect in AA-stacked bilayer graphene

Intrinsic spin Hall effect in the AA-stacked bilayer graphene is studied theoretically. The low-energy electronic spectrum for states in the vicinity of the Dirac points is obtained from the corresponding $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian. The spin Hall conductivity in the linear response regime is determined within the Green function formalism. Conditions for the existence of spin Hall insulator phase are also analyzed, and it is shown that the spin Hall insulator phase can exist for a sufficiently large spin-orbit coupling, which opens a gap in the spectrum. The electric field perpendicular to the graphene plane leads then to reduction of the gap width and suppression of the spin Hall insulator phase. The low temperature spin Nernst effect is also calculated from the zero temperature spin Hall conductivity.

cond-mat.mes-hall↗

arXiv2013

Thermoelectric effects in silicene nanoribbons

Transport and thermoelectric coefficients (including also spin thermopower) of silicene nanoribbons with zigzag edges are investigated by {\it ab-initio} numerical methods. Local spin density of such nanoribbons reveals edge magnetism. Like in graphene, one finds antiferromagnetic and ferromagnetic ordering, with spin polarization at one edge antiparallel or parallel to that at the other edge, respectively. Thermoelectric properties, especially the Seebeck coefficient, significantly depend on the electronic band structure and are enhanced when the Fermi level is in the energy gap. However, these thermoelectric properties are significantly reduced when the phonon contribution to the heat conductance is included. This phonon contribution has been calculated numerically by two different methods. Transition from antiferromagnetic to ferromagnetic states leads to a large magnetoresistance as well as to a considerable magnetothermopower. Thermoelectric parameters in the antiparallel configuration, when spin polarization in the left part of the nanoribbon is opposite to that in the right part, are also analyzed.

cond-mat.mtrl-sci↗

arXiv2013

Thermally-induced spin polarization of a two dimensional electron gas

Spin polarization of a two-dimensional electron gas with Rashba spin-orbit interaction, induced by a thermo-current, is considered theoretically. It is shown that a temperature gradient gives rise to an in-plane spin polarization of the electron gas, which is normal to the temperature gradient. The low-temperature spin polarization changes sign when the Fermi level crosses bottom edge of the upper electronic subband. We also compare the results with spin polarization induced by an external electric field (current).

cond-mat.mes-hall↗

arXiv2004

NRG study of the Kondo effect in the presence of itinerant-electron ferromagnetism

The Kondo effect in quantum dots (QDs) - artificial magnetic impurities - attached to ferromagnetic leads is studied with the numerical renormalization group (NRG) method. It is shown that the QD level is spin-split due to presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally-screened state, describable by Fermi liquid theory and a generalized Friedel sum rule, and transport in the unitary limit is not spin dependent.

cond-mat.mes-hall↗