29 research outputs found
Spin-current quantization in a quantum point contact with spin-orbit interaction
We develop a realistic and analytically tractable model to describe the spin
current which arises in a quantum point contact (QPC) with spin-orbit
interaction (SOI) upon a small voltage is applied. In the model, the QPC is
considered as a saddle point of two-dimensional potential landscape. The SOI
acts within a finite region and is absent deep in the reservoirs. The SOI
strength is not supposed to be strong. It is shown that the spin polarization
appears in the third order of the perturbation theory as a result of definite
combinations of electron transitions. They include two intersubband transitions
to nearest subbands and one intrasubband transition. The spin current is
proportional to the cube of the SOI strength and strongly depends on geometric
parameters of the saddle point. The spin is polarized in the plane of the QPC
and directed normally to the electron current if the SOI is of Rashba type. As
a function of the saddle-point potential (i.e., the height of the QPC barrier),
the spin conductance and especially the spin polarization have characteristic
features (specifically, peaks) correlated with the charge conductance
quantization steps. The peak shape depends on the length of the region where
the SOI acts. In QPCs with sharp potential landscape, this picture is distorted
by interference processes.Comment: 10 pages, 5 figures, to appear in Phys. Rev.
Dynamics of one-dimensional electrons with broken spin-charge separation
Spin-charge separation is known to be broken in many physically interesting
one-dimensional (1D) and quasi-1D systems with spin-orbit interaction because
of which spin and charge degrees of freedom are mixed in collective
excitations. Mixed spin-charge modes carry an electric charge and therefore can
be investigated by electrical means. We explore this possibility by studying
the dynamic conductance of a 1D electron system with image-potential-induced
spin-orbit interaction. The real part of the admittance reveals an oscillatory
behavior versus frequency that reflects the collective excitation resonances
for both modes at their respective transit frequencies. By analyzing the
frequency dependence of the conductance the mode velocities can be found and
their spin-charge structure can be determined quantitatively.Comment: 4 pages, 2 figures. updated to the published versio
Edge states in two-dimensional electron gas with heterogeneous spin-orbit interaction
We show that edge states similar to those known for topological insulators
exist in two-dimensional electron system with one-band spectrum in the presence
of heterogeneous spin-orbit interaction (SOI). These states appear at
boundaries between regions with the SOIs of different kind or between the
regions with the SOI and without it. Depending on the system parameters they
can appear in an energy range lying both in the forbidden and conduction bands
of bulk states. The edge states have chiral spin texture and carry a spin
current under the equilibrium. We study also the size quantization of the edge
states in a strip structure with two boundaries to find an unusual dependence
of the quantization energy on the strip width.Comment: 5 pages, 4 figures, submitte
Electronic states induced by nonmagnetic defects in two-dimensional topological insulators
We study in-gap electronic states induced by a nonmagnetic defect with
short-range potential in two-dimensional topological insulators and trace their
evolution as the distance between the defect and the boundary changes. The
defect located far from the boundary is found to produce two bound states
independently of the sign of its potential. The states are classified as
electronlike and holelike. Each of these states can have two types of the
spatial distribution of the electron density. The first-type states have a
maximum of the density in the center and the second-type ones have a minimum.
When the defect is coupled with the boundary, the bound states are transformed
correspondingly into resonances of two types and take up the form of the edge
states flowing around the defect. Under certain conditions, two resonances
interfere giving rise to the formation of a bound state embedded into the
continuum spectrum of the edge states flowing around the defect. We calculate
the spatial distribution of the electron density in the edge states flowing
around the defect and estimate the charge accumulated near the defect. The
current density field of the edge states flowing around the defect contains two
components one of which flows around the defect and the other circulates around
it.Comment: 11 pages, 7 figure
Interface states in two-dimensional electron systems with spin-orbital interaction
Interface states at a boundary between regions with different spin-orbit
interactions (SOIs) in two-dimensional (2D) electron systems are investigated
within the one-band effective mass method with generalized boundary conditions
for envelope functions. We have found that the interface states unexpectedly
exist even if the effective interface potential equals zero. Depending on the
system parameters, the energy of these states can lie in either or both
forbidden and conduction bands of bulk states. The interface states have chiral
spin texture similar to that of the edge states in 2D topological insulators.
However, their energy spectrum is more sensitive to the interfacial potential,
the largest effect being produced by the spin-dependent component of the
interfacial potential. We have also studied the size quantization of the
interface states in a strip of 2D electron gas with SOI and found an unusual
(non-monotonic) dependence of the quantization energy on the strip width.Comment: 16 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1011.368
Van Hove scenario of anisotropic transport in a two-dimensional spin-orbit coupled electron gas in an in-plane magnetic field
We study electronic transport in two-dimensional spin-orbit coupled electron
gas subjected to an in-plane magnetic field. The interplay of the spin-orbit
interaction and the magnetic field leads to the Van Hove singularity of the
density of states and strong anisotropy of Fermi contours. We develop a method
that allows one to exactly calculate the nonequilibrium distribution function
for these conditions within the framework of the semiclassical Boltzmann
equation without using the scattering time approximation. The method is applied
to calculate the conductivity tensor and the tensor of spin polarization
induced by the electric field (Aronov-Lyanda-Geller-Edelstein effect). It is
found that both the conductivity and the spin polarization have a sharp
singularity as functions of the Fermi level or magnetic field, which occurs
when the Fermi level passes through the Van Hove singularity. In addition, the
transport anisotropy dramatically changes near the singularity.Comment: 8 pages, 6 figure
Non-magnetic defects in the bulk of two-dimensional topological insulators
We found that non-magnetic defects in two-dimensional topological insulators
induce bound states of two kinds for each spin orientation: electron- and
hole-like states. Depending on the sign of the defect potential these states
can be also of two kinds with different distribution of the electron density.
The density has a maximum or minimum in the center. A surprising effect caused
by the topological order is a singular dependence of the bound-state energy on
the defect potential.Comment: 4 pages, 2 figures, to be published in Physica Status Solidi RR
Helical bound states in the continuum of the edge states in two dimensional topological insulators
We study bound states embedded into the continuum of edge states in
two-dimensional topological insulators. These states emerge in the presence of
a short-range potential of a structural defect coupled to the boundary. In this
case the edge states flow around the defect and have two resonances in the
local density of states. The bound state in continuum (BIC) arises due to an
interference of the resonances when they are close to the degeneracy. We find
the condition under which the BIC appears, study the spacial distribution of
the electron density, and show that the BIC has a helical structure with an
electron current circulating around the defect.Comment: 6 pages, 3 figures, to be published in Physics Letters A, typos
correcte
Spin current in an electron waveguide tunnel-coupled to topological insulator
We show that electron tunneling from edge states in two-dimensional
topological insulator into a parallel electron waveguide leads to the
appearance of spin-polarized current in the waveguide. The spin polarization
can be very close to unity and the electron current passing through the
tunnel contact splits in the waveguide into two branches flowing from the
contact. The polarization essentially depends on the electron scattering by the
contact and the electron-electron interaction in the one-dimensional edge
states. The electron-electron interaction is treated within the Luttinger
liquid model. The main effect of the interaction stems from the renormalization
of the electron velocity, due to which the polarization increases with the
interaction strength. Electron scattering by the contact leads to a decrease in
. A specific effect occurs when the bottom of the subbands in the waveguide
crosses the Dirac point of the spectrum of edge states when changing the
voltage or chemical potential. This leads to changing the direction of the spin
current.Comment: 11 pages, 5 figures, accepted in J. Phys.: Condens. Matte
Metastable and spin-polarized states in electron systems with localized electron-electron interaction
We study the formation of spontaneous spin polarization in inhomogeneous
electron systems with pair interaction localized in a small region that is not
separated by a barrier from surrounding gas of non-interacting electrons. Such
a system is interesting as a minimal model of a quantum point contact, in which
the electron-electron interaction is strong in a small constriction coupled to
electron reservoirs without barriers. Based on the analysis of the grand
potential within the self-consistent field approximation, we find that the
formation of the polarized state strongly differs from the Bloch or Stoner
transition in homogeneous interacting systems. The main difference is that a
metastable state appears in the critical point in addition to the globally
stable state, so that when the interaction parameter exceeds a critical value,
two states coexist. One state has spin polarization and the other is
unpolarized. Another feature is that the spin polarization increases
continuously with the interaction parameter and has a square-root singularity
in the critical point. We study the critical conditions and the grand
potentials of the polarized and unpolarized states for one-dimensional and
two-dimensional models in the case of extremely small size of the interaction
region.Comment: 10 pages, 7 figures, typos corrected. Published versio