1,131 research outputs found
Bulk metals with helical surface states
In the flurry of experiments looking for topological insulator materials, it
has been recently discovered that some bulk metals very close to topological
insulator electronic states, support the same topological surface states that
are the defining characteristic of the topological insulator. First observed in
spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the
helical surface states in the metallic systems appear to be robust to at least
mild disorder. We present here a theoretical investigation of the nature of
these "helical metals" - bulk metals with helical surface states. We explore
how the surface and bulk states can mix, in both clean and disordered systems.
Using the Fano model, we discover that in a clean system, the helical surface
states are \emph{not} simply absorbed by hybridization with a non-topological
parasitic metallic band. Instead, they are pushed away from overlapping in
momentum and energy with the bulk states, leaving behind a finite-lifetime
surface resonance in the bulk energy band. Furthermore, the hybridization may
lead in some cases to multiplied surface state bands, in all cases retaining
the helical characteristic. Weak disorder leads to very similar effects -
surface states are pushed away from the energy bandwidth of the bulk, leaving
behind a finite-lifetime surface resonance in place of the original surface
states
Near zero modes in condensate phases of the Dirac theory on the honeycomb lattice
We investigate a number of fermionic condensate phases on the honeycomb
lattice, to determine whether topological defects (vortices and edges) in these
phases can support bound states with zero energy. We argue that topological
zero modes bound to vortices and at edges are not only connected, but should in
fact be \emph{identified}. Recently, it has been shown that the simplest s-wave
superconducting state for the Dirac fermion approximation of the honeycomb
lattice at precisely half filling, supports zero modes inside the cores of
vortices (P. Ghaemi and F. Wilczek, 2007). We find that within the continuum
Dirac theory the zero modes are not unique neither to this phase, nor to half
filling. In addition, we find the \emph{exact} wavefunctions for vortex bound
zero modes, as well as the complete edge state spectrum of the phases we
discuss. The zero modes in all the phases we examine have even-numbered
degeneracy, and as such pairs of any Majorana modes are simply equivalent to
one ordinary fermion. As a result, contrary to bound state zero modes in superconductors, vortices here do \emph{not} exhibit non-Abelian exchange
statistics. The zero modes in the pure Dirac theory are seemingly topologically
protected by the effective low energy symmetry of the theory, yet on the
original honeycomb lattice model these zero modes are split, by explicit
breaking of the effective low energy symmetry.Comment: Final version including numerics, accepted for publication in PR
Non-adiabatic pumping in an oscillating-piston model
We consider the prototypical "piston pump" operating on a ring, where a
circulating current is induced by means of an AC driving. This can be regarded
as a generalized Fermi-Ulam model, incorporating a finite-height moving wall
(piston) and non trivial topology (ring). The amount of particles transported
per cycle is determined by a layered structure of phase-space. Each layer is
characterized by a different drift velocity. We discuss the differences
compared with the adiabatic and Boltzmann pictures, and highlight the
significance of the "diabatic" contribution that might lead to a
counter-stirring effect.Comment: 6 pages, 4 figures, improved versio
A thermodynamic measure of the Magneto-electric coupling in the 3D topological insulator
We show that the magneto-electric coupling in 3D (strong) topological
insulators is related to a second derivative of the bulk magnetization. The
formula we derive is the non-linear response analog of the Streda formula for
Hall conductivity (P. Streda, J. Phys. C: Solid State Physics, 15, 22 (1982)),
which relates the Hall conductivity to the derivative of the magnetization with
respect to chemical potential. Our finding allows one to extract the
magneto-electric coefficient by measuring the magnetization, while varying the
chemical potential and one more perturbing field. Such an experimental setup
could circumvent many of the current difficulties with measuring the
magneto-electric response in 3D topological insulators. The relation we find
also makes transparent the effect of disorder on the magneto-electric response,
which occurs only through the density of states, and has no effect when the
system is gapped
Coin Tossing as a Billiard Problem
We demonstrate that the free motion of any two-dimensional rigid body
colliding elastically with two parallel, flat walls is equivalent to a billiard
system. Using this equivalence, we analyze the integrable and chaotic
properties of this new class of billiards. This provides a demonstration that
coin tossing, the prototypical example of an independent random process, is a
completely chaotic (Bernoulli) problem. The related question of which billiard
geometries can be represented as rigid body systems is examined.Comment: 16 pages, LaTe
Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAlGaO: Lattice expansion and site disorder
The spinels CoBO with magnetic Co ions on the diamond lattice
A site can be frustrated because of competing near-neighbor () and
next-near neighbor () interactions. Here we describe attempts to tune the
relative strengths of these interactions by substitution on the non-magnetic
B-site. The system we employ is CoAlGaO, where Al is
systematically replaced by the larger Ga, ostensibly on the B site. As
expected, Ga substitution expands the lattice, resulting in Co atoms on the
A-site being pushed further from one other and thereby weakening magnetic
interactions. In addition, Ga distributes between the B and the A site in a
concentration dependent manner displacing an increasing amount of Co from the A
site with increasing . This increased inversion, which is confirmed by
neutron diffraction studies carried out at room temperature, affects magnetic
ordering very significantly, and changes the nature of the ground state.
Modeling of the magnetic coupling illustrates the complexity that arises from
the cation site disorder.Comment: 9 pages, 10 figure
Measurement of the Total (p,Pi) Cross Sections Through Residual Activity
Supported by the National Science Foundation and Indiana Universit
Nonequilibrium Josephson current in ballistic multiterminal SNS-junctions
We study the nonequilibrium Josephson current in a long two-dimensional
ballistic SNS-junction with a normal reservoir coupled to the normal part of
the junction. The current for a given superconducting phase difference
oscillates as a function of voltage applied between the normal reservoir and
the SNS-junction. The period of the oscillations is , with
the length of the junction, and the amplitude of the oscillations decays as
for and zero temperature. The critical
current shows a similar oscillating, decaying behavior as a function of
voltage, changing sign every oscillation. Normal specular or diffusive
scattering at the NS-interfaces does not qualitatively change the picture.Comment: Proceeding of MS2000, to appear in Physica
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