521 research outputs found

### Coulomb Drag and Spin Coulomb Drag in the presence of Spin-orbit Coupling

Employing diagrammatic perturbation theory, we calculate the (charge) Coulomb
drag resistivity $\rho_D$ and spin Coulomb drag resistivity
$\rho_{\uparrow\downarrow}$ in the presence of Rashba spin-orbit coupling.
Analytical expressions for $\rho_D$ and $\rho_{\uparrow\downarrow}$ are
derived, and it is found that spin-orbit interaction produces a small
enhancement to $\rho_D$ and $\rho_{\uparrow\downarrow}$ in the ballistic regime
while $\rho_D$ is unchanged in the diffusive regime. This enhancement in the
ballistic regime is attributed to the enhancement of the nonlinear
susceptibility (i.e. current produced through the rectification of the thermal
electric potential fluctuations in the passive layer) while the lack of
enhancement in the diffusive regime is due to the suppression by disorder.Comment: 8 pages, 2 figure

### Magneto-optical and Magneto-electric Effects of Topological Insulators in Quantizing Magnetic Fields

We develop a theory of the magneto-optical and magneto-electric properties of
a topological insulator thin film in the presence of a quantizing external
magnetic field. We find that low-frequency magneto-optical properties depend
only on the sum of top and bottom surface Dirac-cone filling factors
$\nu_{\mathrm{T}}$ and $\nu_{\mathrm{B}}$, whereas the low-frequency
magneto-electric response depends only on the difference. The Faraday rotation
is quantized in integer multiples of the fine structure constant and the Kerr
effect exhibits a $\pi/2$ rotation. Strongly enhanced cyclotron-resonance
features appear at higher frequencies that are sensitive to the filling factors
of both surfaces. When the product of the bulk conductivity and the film
thickness in $e^2/h$ units is small compared to $\alpha$, magneto-optical
properties are only weakly dependent on accidental doping in the interior of
the film.Comment: 4 page

### Giant Magneto-optical Kerr Effect and Universal Faraday Effect in Thin-film Topological Insulators

Topological insulators can exhibit strong magnetoelectric effects when their
time-reversal symmetry is broken. In this Letter we consider the
magneto-optical Kerr and Faraday effects of a topological insulator thin film
weakly exchange-coupled to a ferromagnet. We find that its Faraday rotation has
a universal value at low-frequencies, $\theta_{\mathrm{F}} =
\mathrm{tan}^{-1}\,\alpha$ where $\alpha$ is the vacuum fine structure
constant, and that it has a giant Kerr rotation $\theta_{\mathrm{K}} = \pi/2$.
These properties follow from a delicate interplay between thin-film cavity
confinement and the surface Hall conductivity of a topological insulator's
helical quasiparticles.Comment: 5 pages, 4 figure

### Energy Relaxation of Hot Dirac Fermions in Graphene

We develop a theory for the energy relaxation of hot Dirac fermions in
graphene. We obtain a generic expression for the energy relaxation rate due to
electron-phonon interaction and calculate the power loss due to both optical
and acoustic phonon emission as a function of electron temperature
$T_{\mathrm{e}}$ and density $n$. We find an intrinsic power loss weakly
dependent on carrier density and non-vanishing at the Dirac point $n = 0$,
originating from interband electron-optical phonon scattering by the intrinsic
electrons in the graphene valence band. We obtain the total power loss per
carrier $\sim 10^{-12} - 10^{-7} \mathrm{W}$ within the range of electron
temperatures $\sim 20 - 1000 \mathrm{K}$. We find optical (acoustic) phonon
emission to dominate the energy loss for $T_{\mathrm{e}} > (<) 200-300
\mathrm{K}$ in the density range $n = 10^{11}-10^{13} \mathrm{cm}^{-2}$.Comment: 5 page

### Ballistic Hot Electron Transport in Graphene

We theoretically study the inelastic scattering rate and the carrier mean
free path for energetic hot electrons in graphene, including both
electron-electron and electron-phonon interactions. Taking account of optical
phonon emission and electron-electron scattering, we find that the inelastic
scattering time $\tau \sim 10^{-2}-10^{-1} \mathrm{ps}$ and the mean free path
$l \sim 10-10^2 \mathrm{nm}$ for electron densities $n = 10^{12}-10^{13}
\mathrm{cm}^{-2}$. In particular, we find that the mean free path exhibits a
finite jump at the phonon energy $200 \mathrm{meV}$ due to electron-phonon
interaction. Our results are directly applicable to device structures where
ballistic transport is relevant with inelastic scattering dominating over
elastic scattering.Comment: 4 page

### Spin Accumulation in the Extrinsic Spin Hall Effect

The drift-diffusion formalism for spin-polarized carrier transport in
semiconductors is generalized to include spin-orbit coupling. The theory is
applied to treat the extrinsic spin Hall effect using realistic boundary
conditions. It is shown that carrier and spin diffusion lengths are modified by
the presence of spin-orbit coupling and that spin accumulation due to the
extrinsic spin Hall effect is strongly and qualitatively influenced by boundary
conditions. Analytical formulas for the spin-dependent carrier recombination
rates and inhomogeneous spin densities and currents are presented.Comment: 5 pages, 3 figure

### Two-Dimensional Topological Insulator State and Topological Phase Transition in Bilayer Graphene

We show that gated bilayer graphene hosts a strong topological insulator (TI)
phase in the presence of Rashba spin-orbit (SO) coupling. We find that gated
bilayer graphene under preserved time-reversal symmetry is a quantum valley
Hall insulator for small Rashba SO coupling $\lambda_{\mathrm{R}}$, and
transitions to a strong TI when $\lambda_{\mathrm{R}} > \sqrt{U^2+t_\bot^2}$,
where $U$ and $t_\bot$ are respectively the interlayer potential and tunneling
energy. Different from a conventional quantum spin Hall state, the edge modes
of our strong TI phase exhibit both spin and valley filtering, and thus share
the properties of both quantum spin Hall and quantum valley Hall insulators.
The strong TI phase remains robust in the presence of weak graphene intrinsic
SO coupling.Comment: 5 pages and 4 figure

- β¦