135 research outputs found
Surface states, Friedel oscillations, and spin accumulation in p-doped semiconductors
We consider a hole-doped semiconductor with a sharp boundary and study the
boundary spin accumulation in response to a charge current. First, we solve
exactly a single-particle quantum mechanics problem described by the isotropic
Luttinger model in half-space and construct an orthonormal basis for the
corresponding Hamiltonian. It is shown that the complete basis includes two
types of eigenstates. The first class of states contains conventional incident
and reflected waves, while the other class includes localized surface states.
Second, we consider a many-body system in the presence of a charge current
flowing parallel to the boundary. It is shown that the localized states
contribute to spin accumulation near the surface. We also show that the spin
density exhibits current-induced Friedel oscillations with three different
periods determined by the Fermi momenta of the light and heavy holes. We find
an exact asymptotic expression for the Friedel oscillations far from the
boundary. We also calculate numerically the spin density profile and compute
the total spin accumulation, which is defined as the integral of the spin
density in the direction perpendicular to the boundary. The total spin
accumulation is shown to fit very well the simple formula S ~(1 - m_L/m_H)^2,
where m_L and m_H are the light- and heavy-hole masses. The effects of disorder
are discussed. We estimate the spin relaxation time in the Luttinger model and
argue that spin physics cannot be described within the diffusion approximation.Comment: 22 pages, 8 color figure
Ordering of magnetic impurities and tunable electronic properties of topological insulators
We study collective behavior of magnetic adatoms randomly distributed on the
surface of a topological insulator. As a consequence of the spin-momentum
locking on the surface, the RKKY-type interactions of two adatom spins depend
on the direction of the vector connecting them, thus interactions of an
ensemble of adatoms are frustrated. We show that at low temperatures the
frustrated RKKY interactions give rise to two phases: an ordered ferromagnetic
phase with spins pointing perpendicular to the surface, and a disordered
spin-glass-like phase. The two phases are separated by a quantum phase
transition driven by the magnetic exchange anisotropy. Ferromagnetic ordering
occurs via a finite-temperature phase transition. The ordered phase breaks
time-reversal symmetry spontaneously, driving the surface states into a gapped
state, which exhibits an anomalous quantum Hall effect and provides a
realization of the parity anomaly. We find that the magnetic ordering is
suppressed by potential scattering. Our work indicates that controlled
deposition of magnetic impurities provides a way to modify the electronic
properties of topological insulators.Comment: 4+ pages, 2 figure
Mesoscopic mechanism of exchange interaction in magnetic multilayers
We discuss a mesoscopic mechanism of exchange interaction in
ferromagnet-normal metal-ferromagnet multilayers. We show that in the case when
the metal's thickness is larger than the electron mean free path, the relative
orientation of magnetizations in the ferromagnets is perpendicular. The
exchange energy between ferromagnets decays with the metal thickness as a power
law
Edge spin accumulation in a ballistic regime
We consider a mesoscopic {\it ballistic} structure with Rashba spin-orbit
splitting of the electron spectrum. The ballistic region is attached to the
leads with a voltage applied between them. We calculate the edge spin density
which appears in the presence of a charge current through the structure due to
the difference in populations of electrons coming from different leads.
Combined effect of the boundary scattering and spin precession leads to
oscillations of the edge polarization with the envelope function decaying as a
power law of the distance from the boundary. The problem is solved with the use
of scattering states. The simplicity of the method allows to gain an insight
into the underlaying physics. We clarify the role of the unitarity of
scattering for the problem of edge spin accumulation. In case of a straight
boundary it leads to exact cancellation of all long-wave oscillations of the
spin density. As a result, only the Friedel-like spin density oscillations with
the momentum 2k_F survive. However, this appears to be rather exceptional case.
In general, the smooth spin oscillations with the spin precession length
recover, as it happens, e.g., for the wiggly boundary. We demonstrate also,
that there is no relation between the spin current in the bulk, which is zero
in the considered case, and the edge spin accumulation.Comment: Latex, 6 pages, 2 fig
Point contact spectroscopy of hopping transport: effects of a magnetic field
The conductance of a point contact between two hopping insulators is expected
to be dominated by the individual localized states in its vicinity. Here we
study the additional effects due to an external magnetic field. Combined with
the measured conductance, the measured magnetoresistance provides detailed
information on these states (e.g. their localization length, the energy
difference and the hopping distance between them). We also calculate the
statistics of this magnetoresistance, which can be collected by changing the
gate voltage in a single device. Since the conductance is dominated by the
quantum interference of particular mesoscopic structures near the point
contact, it is predicted to exhibit Aharonov-Bohm oscillations, which yield
information on the geometry of these structures. These oscillations also depend
on local spin accumulation and correlations, which can be modified by the
external field. Finally, we also estimate the mesoscopic Hall voltage due to
these structures.Comment: 7 pages, 5 figur
Transmission through a n interacting quantum dot in the Coulomb blockade regime
The influence of electron-electron (e-e) interactions on the transmission
through a quantum dot is investigated numerically for the Coulomb blockade
regime. For vanishing magnetic fields, the conductance peak height statistics
is found to be independent of the interactions strength. It is identical to the
statistics predicted by constant interaction single electron random matrix
theory and agrees well with recent experiments. However, in contrast to these
random matrix theories, our calculations reproduces the reduced sensitivity to
magnetic flux observed in many experiments. The relevant physics is traced to
the short range Coulomb correlations providing thus a unified explanation for
the transmission statistics as well as for the large conductance peak spacing
fluctuations observed in other experiments.Comment: Final version as publishe
Concentration dependence of the transition temperature in metallic spin glasses
The dependence of the transition temperature in terms of the
concentration of magnetic impurities in spin glasses is explained on the
basis of a screened RKKY interaction. The two observed power laws, at
low and for intermediate , are described in a unified
approach.Comment: 4 page
Disorder-quenched Kondo effect in mesosocopic electronic systems
Nonmagnetic disorder is shown to quench the screening of magnetic moments in
metals, the Kondo effect. The probability that a magnetic moment remains free
down to zero temperature is found to increase with disorder strength.
Experimental consequences for disordered metals are studied. In particular, it
is shown that the presence of magnetic impurities with a small Kondo
temperature enhances the electron's dephasing rate at low temperatures in
comparison to the clean metal case. It is furthermore proven that the width of
the distribution of Kondo temperatures remains finite in the thermodynamic
(infinite volume) limit due to wave function correlations within an energy
interval of order , where is the elastic scattering time. When
time-reversal symmetry is broken either by applying a magnetic field or by
increasing the concentration of magnetic impurities, the distribution of Kondo
temperatures becomes narrower.Comment: 17 pages, 7 figures, new results on Kondo effect in quasi-1D wires
added, 6 Refs. adde
Zero-Field Satellites of a Zero-Bias Anomaly
Spin-orbit (SO) splitting, , of the electron Fermi surface
in two-dimensional systems manifests itself in the interaction-induced
corrections to the tunneling density of states, . Namely, in
the case of a smooth disorder, it gives rise to the satellites of a zero-bias
anomaly at energies . Zeeman splitting, , in a weak parallel magnetic field causes a narrow {\em plateau} of
a width at the top of each sharp satellite peak.
As exceeds , the SO satellites cross over to the
conventional narrow maxima at with SO-induced
plateaus at the tops.Comment: 7 pages including 2 figure
Interaction effects in 2D electron gas in a random magnetic field: Implications for composite fermions and quantum critical point
We consider a clean two-dimensional interacting electron gas subject to a
random perpendicular magnetic field, h({\bf r}). The field is nonquantizing, in
the sense, that {\cal N}_h-a typical flux into the area \lambda_{\text{\tiny
F}}^2 in the units of the flux quantum (\lambda_{\text{\tiny F}} is the de
Broglie wavelength) is small, {\cal N}_h\ll 1. If the spacial scale, \xi, of
change of h({\bf r}) is much larger than \lambda_{\text{\tiny F}}, the
electrons move along semiclassical trajectories. We demonstrate that a weak
field-induced curving of the trajectories affects the interaction-induced
electron lifetime in a singular fashion: it gives rise to the correction to the
lifetime with a very sharp energy dependence. The correction persists within
the interval \omega \sim \omega_0= E_{\text{\tiny F}}{\cal N}_h^{2/3} much
smaller than the Fermi energy, E_{\text{\tiny F}}. It emerges in the third
order in the interaction strength; the underlying physics is that a small phase
volume \sim (\omega/E_{\text{\tiny F}})^{1/2} for scattering processes,
involving {\em two} electron-hole pairs, is suppressed by curving. Even more
surprising effect that we find is that {\em disorder-averaged} interaction
correction to the density of states, \delta\nu(\omega), exhibits {\em
oscillatory} behavior, periodic in \bigl(\omega/\omega_0\bigr)^{3/2}. In our
calculations of interaction corrections random field is incorporated via the
phases of the Green functions in the coordinate space. We discuss the relevance
of the new low-energy scale for realizations of a smooth random field in
composite fermions and in disordered phase of spin-fermion model of
ferromagnetic quantum criticality.Comment: 32 pages, 15 figures, Revte
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