98 research outputs found
Side-jumps in the spin-Hall effect: construction of the Boltzmann collision integral
We present a systematic derivation of the side-jump contribution to the
spin-Hall current in systems without band structure spin-orbit interactions,
focusing on the construction of the collision integral for the Boltzmann
equation. Starting from the quantum Liouville equation for the density operator
we derive an equation describing the dynamics of the density matrix in the
first Born approximation and to first order in the driving electric field.
Elastic scattering requires conservation of the total energy, including the
spin-orbit interaction energy with the electric field: this results in a first
correction to the customary collision integral found in the Born approximation.
A second correction is due to the change in the carrier position during
collisions. It stems from the part of the density matrix off-diagonal in wave
vector. The two corrections to the collision integral add up and are
responsible for the total side-jump contribution to the spin-Hall current. The
spin-orbit-induced correction to the velocity operator also contains terms
diagonal and off-diagonal in momentum space, which together involve the total
force acting on the system. This force is explicitly shown to vanish (on the
average) in the steady state: thus the total contribution to the spin-Hall
current due to the additional terms in the velocity operator is zero.Comment: Added references, expanded discussion, revised introductio
Gilbert damping and spin Coulomb drag in a magnetized electron liquid with spin-orbit interaction
We present a microscopic calculation of the Gilbert damping constant for the
magnetization of a two-dimensional spin-polarized electron liquid in the
presence of intrinsic spin-orbit interaction. First we show that the Gilbert
constant can be expressed in terms of the auto-correlation function of the
spin-orbit induced torque. Then we specialize to the case of the Rashba
spin-orbit interaction and we show that the Gilbert constant in this model is
related to the spin-channel conductivity. This allows us to study the Gilbert
damping constant in different physical regimes, characterized by different
orderings of the relevant energy scales -- spin-orbit coupling, Zeeman
coupling, momentum relaxation rate, spin-momentum relaxation rate, spin
precession frequency -- and to discuss its behavior in various limits.
Particular attention is paid to electron-electron interaction effects,which
enter the spin conductivity and hence the Gilbert damping constant via the spin
Coulomb drag coefficient.Comment: 18 pages, 8 figure
Spin-dependent thermoelectric transport in HgTe/CdTe quantum wells
We analyze thermally induced spin and charge transport in HgTe/CdTe quantum
wells on the basis of the numerical non-equilibrium Green's function technique
in the linear response regime. In the topologically non-trivial regime, we find
a clear signature of the gap of the edge states due to their finite overlap
from opposite sample boundaries -- both in the charge Seebeck and spin Nernst
signal. We are able to fully understand the physical origin of the
thermoelectric transport signatures of edge and bulk states based on simple
analytical models. Interestingly, we derive that the spin Nernst signal is
related to the spin Hall conductance by a Mott-like relation which is exact to
all orders in the temperature difference between the warm and the cold
reservoir.Comment: 11 pages, 13 figures, submitted to PR
Topological Excitonic Superfluids in Three Dimensions
We study the equilibrium and non-equilibrium properties of topological
dipolar intersurface exciton condensates within time-reversal invariant
topological insulators in three spatial dimensions without a magnetic field. We
elucidate that, in order to correctly identify the proper pairing symmetry
within the condensate order parameter, the full three-dimensional Hamiltonian
must be considered. As a corollary, we demonstrate that only particles with
similar chirality play a significant role in condensate formation. Furthermore,
we find that the intersurface exciton condensation is not suppressed by the
interconnection of surfaces in three-dimensional topological insulators as the
intersurface polarizability vanishes in the condensed phase. This eliminates
the surface current flow leaving only intersurface current flow through the
bulk. We conclude by illustrating how the excitonic superfluidity may be
identified through an examination of the terminal currents above and below the
condensate critical current.Comment: 8 pages, 6 figure
Weak antilocalization in HgTe quantum wells and topological surface states: Massive versus massless Dirac fermions
HgTe quantum wells and surfaces of three-dimensional topological insulators
support Dirac fermions with a single-valley band dispersion. In the presence of
disorder they experience weak antilocalization, which has been observed in
recent transport experiments. In this work we conduct a comparative theoretical
study of the weak antilocalization in HgTe quantum wells and topological
surface states. The difference between these two single-valley systems comes
from a finite band gap (effective Dirac mass) in HgTe quantum wells in contrast
to gapless (massless) surface states in topological insulators. The finite
effective Dirac mass implies a broken internal symmetry, leading to suppression
of the weak antilocalization in HgTe quantum wells at times larger than certain
t_M, inversely proportional to the Dirac mass. This corresponds to the opening
of a relaxation gap 1/t_M in the Cooperon diffusion mode which we obtain from
the Bethe-Salpeter equation including relevant spin degrees of freedom. We
demonstrate that the relaxation gap exhibits an interesting nonmonotonic
dependence on both carrier density and band gap, vanishing at a certain
combination of these parameters. The weak-antilocalization conductivity
reflects this nonmonotonic behavior which is unique to HgTe QWs and absent for
topological surface states. On the other hand, the topological surface states
exhibit specific weak-antilocalization magnetoconductivity in a parallel
magnetic field due to their exponential decay in the bulk.Comment: 14 pages, 10 figures, version as publishe
Optical properties of metallic (III,Mn)V ferromagnetic semiconductors in the infrared to visible range
We report on a study of the ac conductivity and magneto-optical properties of
metallic ferromagnetic (III,Mn)V semiconductors in the infrared to visible
spectrum. Our analysis is based on the successful kinetic exchange model for
(III,Mn)V ferromagnetic semiconductors. We perform the calculations within the
Kubo formalism and treat the disorder effects pertubatively within the Born
approximation, valid for the metallic regime. We consider an eight-band
Kohn-Luttinger model (six valence bands plus two conduction bands) as well as a
ten-band model with additional dispersionless bands simulating
phenomenologically the upper-mid-gap states induced by antisite and
interstitial impurities. These models qualitatively account for
optical-absorption experiments and predict new features in the mid-infrared
Kerr angle and magnetic-circular-dichroism properties as a function of Mn
concentration and free carrier density.Comment: 10 pages, 7 figures, some typos correcte
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