65 research outputs found
Quantum Spin Hall Effect and Enhanced Magnetic Response by Spin-Orbit Coupling
We show that the spin Hall conductivity in insulators is related with a
magnetic susceptibility representing the strength of the spin-orbit coupling.
We use this relationship as a guiding principle to search real materials
showing quantum spin Hall effect. As a result, we theoretically predict that
bismuth will show the quantum spin Hall effect, both by calculating the helical
edge states, and by showing the non-triviality of the Z_2 topological number,
and propose possible experiments.Comment: 5 pages, 2 figures, accepted for publication in Phys. Rev. Let
Bi(111) thin film with insulating interior but metallic surfaces
The electrical conductance of molecular beam epitaxial Bi on BaF2(111) was
measured as a function of both film thickness (4-540 nm) and temperature (5-300
K). Unlike bulk Bi as a prototype semimetal, the Bi thin films up to 90 nm are
found to be insulating in the interiors but metallic on the surfaces. This
result has not only resolved unambiguously the long controversy about the
existence of semimetal-semiconductor transition in Bi thin film but also
provided a straightforward interpretation for the long-puzzled temperature
dependence of the resistivity of Bi thin films, which in turn might suggest
some potential applications in spintronics
Mode Coupling in Quantized High Quality Films
The effect of coupling of quantized modes on transport and localization in
ultrathin films with quantum size effect (QSE) is discussed. The emphasis is on
comparison of films with Gaussian, exponential, and power-law long-range
behavior of the correlation function of surface, thickness, or bulk
fluctuations. For small-size inhomogeneities, the mode coupling is the same for
inhomogeneities of all types and the transport coefficients behave in the same
way. The mode coupling becomes extremely sensitive to the correlators for
large-size inhomogeneities leading to the drastically distinct behavior of the
transport coefficients. In high-quality films there is a noticeable difference
between the QSE patterns for films with bulk and surface inhomogeneities which
explains why the recently predicted new type of QSE with large oscillations of
the transport coefficients can be observed mostly in films with surface-driven
relaxation. In such films with surface-dominated scattering the higher modes
contribute to the transport only as a result of opening of the corresponding
mode coupling channels and appear one by one. Mode coupling also explains a
much higher transport contribution from the higher modes than it is commonly
believed. Possible correlations between the inhomogeneities from the opposite
walls provide, because of their oscillating response to the mode quantum
numbers, a unique insight into the mode coupling. The presence of
inhomogeneities of several sizes leads not to a mechanical mixture of QSE
patterns, but to the overall shifting and smoothing of the oscillations. The
results can lead to new, non-destructive ways of analysis of the buried
interfaces and to study of inhomogeneities on the scales which are inaccessible
for scanning techniques
Optical conductivity of metal nanofilms and nanowires: The rectangular-box model
The conductivity tensor is introduced for the low-dimensional electron
systems. Within the particle-in-a-box model and the diagonal response
approximation, components of the conductivity tensor for a quasi-homogeneous
ultrathin metal film and wire are calculated under the assumption (where is the characteristic small dimension of the
system, is the Fermi wavelength for bulk metal). We find the
transmittance of ultrathin films and compare these results with available
experimental data. The analytical estimations for the size dependence of the
Fermi level are presented, and the oscillations of the Fermi energy in
ultrathin films and wires are computed. Our results demonstrate the strong size
and frequency dependences of the real and imaginary parts of the conductivity
components in the infrared range. A sharp distinction of the results for Au and
Pb is observed and explained by the difference in the relaxation time of these
metals.Comment: 13 pages, 8 figure
Phase transition between the quantum spin Hall and insulator phases in 3D: emergence of a topological gapless phase
Phase transitions between the quantum spin Hall and the insulator phases in
three dimensions are studied. We find that in inversion-asymmetric systems
there appears a gapless phase between the quantum spin Hall and insulator
phases in three dimensions, which is in contrast with the two-dimensional case.
Existence of this gapless phase stems from a topological nature of gapless
points (diabolical points) in three dimensions, but not in two dimensions.Comment: 16 pages, 5 figure
Novel Edge Excitations of Two-dimensional Electron Liquid in a Magnetic Field
We investigate the low-energy spectrum of excitations of a compressible
electron liquid in a strong magnetic field. These excitations are localized at
the periphery of the system. The analysis of a realistic model of a smooth edge
yields new branches of acoustic excitation spectrum in addition to the well
known edge magnetoplasmon mode. The velocities are found and the observability
conditions are established for the new modes.Comment: 9 pages + 2 figures by request preprint TPI-MINN-93/59-
Observation of three-dimensional behavior in surface states of bismuth nanowires and the evidence for bulk Bi charge fractionalization
Whereas bulk bismuth supports very-high mobility, light, Dirac electrons and
holes in its interior, its boundaries support a layer of heavy electrons in
surface states formed by spin orbit interaction in the presence of the surface
electric field. Small diameter d trigonal Bi nanowires (30 nm < d < 200 nm)
were studied via magnetotransport at low temperatures and for fields up to 14 T
in order to investigate the role of surfaces in electronic transport. A
two-dimensional behavior was expected for surface charges; however we found
instead a three-dimensional behavior, with a rich spectrum of Landau levels in
a nearly spherical Fermi surface. This is associated with the long penetration
length of surface states of trigonal wires. The prospect of the participation
of surface transport and surface-induced relaxation of bulk carriers in the
electronic properties of macroscopic samples is evaluated. We show that recent
observations of magnetoquantum peaks in the Nernst thermopower coefficient,
attributed to two-dimensional electron gas charge fractionalization, can be
more plausibly interpreted in terms of these surface states.Comment: 14 pages, 3 figure
Nanostratification of optical excitation in self-interacting 1D arrays
The major assumption of the Lorentz-Lorenz theory about uniformity of local
fields and atomic polarization in dense material does not hold in finite groups
of atoms, as we reported earlier [A. E. Kaplan and S. N. Volkov, Phys. Rev.
Lett., v. 101, 133902 (2008)]. The uniformity is broken at sub-wavelength
scale, where the system may exhibit strong stratification of local field and
dipole polarization, with the strata period being much shorter than the
incident wavelength. In this paper, we further develop and advance that theory
for the most fundamental case of one-dimensional arrays, and study nanoscale
excitation of so called "locsitons" and their standing waves (strata) that
result in size-related resonances and related large field enhancement in finite
arrays of atoms. The locsitons may have a whole spectrum of spatial
frequencies, ranging from long waves, to an extent reminiscent of ferromagnetic
domains, -- to super-short waves, with neighboring atoms alternating their
polarizations, which are reminiscent of antiferromagnetic spin patterns. Of
great interest is the new kind of "hybrid" modes of excitation, greatly
departing from any magnetic analogies. We also study differences between
Ising-like near-neighbor approximation and the case where each atom interacts
with all other atoms in the array. We find an infinite number of "exponential
eigenmodes" in the lossless system in the latter case. At certain "magic"
numbers of atoms in the array, the system may exhibit self-induced (but linear
in the field) cancellation of resonant local-field suppression. We also studied
nonlinear modes of locsitons and found optical bistability and hysteresis in an
infinite array for the simplest modes.Comment: 39 pages, 5 figures; v2: Added the Conclusions section, corrected a
typo in Eq. (5.3), corrected minor stylistic and grammatical imperfection
Far-infrared edge modes in quantum dots
We have investigated edge modes of different multipolarity sustained by
quantum dots submitted to external magnetic fields. We present a microscopic
description based on a variational solution of the equation of motion for any
axially symmetric confining potential and multipole mode. Numerical results for
dots with different number of electrons whose ground-state is described within
a local Current Density Functional Theory are discussed. Two sum rules, which
are exact within this theory, are derived. In the limit of a large neutral dot
at B=0, we have shown that the classical hydrodynamic dispersion law for edge
waves \omega(q) \sim \sqrt{q \ln (q_0/q)} holds when quantum and finite size
effects are taken into account.Comment: We have changed some figures as well as a part of the tex
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