113 research outputs found
Scattering theory of plasmon-assisted entanglement transfer and distillation
We analyse the quantum mechanical limits to the plasmon-assisted entanglement
transfer observed by E. Altewischer, M.P. van Exter, and J.P. Woerdman [Nature,
418, 304 (2002)]. The maximal violation S of Bell's inequality at the
photodetectors behind two linear media (such as the perforated metal films in
the experiment) can be described by two ratio's tau_1, tau_2 of
polarization-dependent transmission probabilities. A fully entangled incident
state is transferred without degradation for tau_1=tau_2, but a relatively
large mismatch of tau_1 and tau_2 can be tolerated with a small reduction of S.
We predict that fully entangled Bell pairs can be distilled out of partially
entangled radiation if tau_1 and tau_2 satisfy a pair of inequalities.Comment: 4 pages including 2 figures; two references added, plasmon model
include
Valley-isospin dependence of the quantum Hall effect in a graphene p-n junction
We calculate the conductance G of a bipolar junction in a graphene
nanoribbon, in the high-magnetic field regime where the Hall conductance in the
p-doped and n-doped regions is 2e^2/h. In the absence of intervalley
scattering, the result G=(e^2/h)(1-cos Phi) depends only on the angle Phi
between the valley isospins (= Bloch vectors representing the spinor of the
valley polarization) at the two opposite edges. This plateau in the conductance
versus Fermi energy is insensitive to electrostatic disorder, while it is
destabilized by the dispersionless edge state which may exist at a zigzag
boundary. A strain-induced vector potential shifts the conductance plateau up
or down by rotating the valley isospin.Comment: 5 pages, 6 figure
The geometric order of stripes and Luttinger liquids
It is argued that the electron stripes as found in correlated oxides have to
do with an unrecognized form of order. The manifestation of this order is the
robust property that the charge stripes are at the same time anti-phase
boundaries in the spin system. We demonstrate that the quantity which is
ordering is sublattice parity, referring to the geometric property of a
bipartite lattice that it can be subdivided in two sublattices in two different
ways. Re-interpreting standard results of one dimensional physics, we
demonstrate that the same order is responsible for the phenomenon of
spin-charge separation in strongly interacting one dimensional electron
systems. In fact, the stripe phases can be seen from this perspective as the
precise generalization of the Luttinger liquid to higher dimensions. Most of
this paper is devoted to a detailed exposition of the mean-field theory of
sublattice parity order in 2+1 dimensions. Although the quantum-dynamics of the
spin- and charge degrees of freedom is fully taken into account, a perfect
sublattice parity order is imposed. Due to novel order-out-of-disorder physics,
the sublattice parity order gives rise to full stripe order at long wavelength.
This adds further credibility to the notion that stripes find their origin in
the microscopic quantum fluctuations and it suggests a novel viewpoint on the
relationship between stripes and high Tc superconductivity.Comment: 29 pages, 14 figures, 1 tabl
Twisted Fermi surface of a thin-film Weyl semimetal
The Fermi surface of a conventional two-dimensional electron gas is
equivalent to a circle, up to smooth deformations that preserve the orientation
of the equi-energy contour. Here we show that a Weyl semimetal confined to a
thin film with an in-plane magnetization and broken spatial inversion symmetry
can have a topologically distinct Fermi surface that is twisted into a
\mbox{figure-8} opposite orientations are coupled at a crossing which is
protected up to an exponentially small gap. The twisted spectral response to a
perpendicular magnetic field is distinct from that of a deformed Fermi
circle, because the two lobes of a \mbox{figure-8} cyclotron orbit give
opposite contributions to the Aharonov-Bohm phase. The magnetic edge channels
come in two counterpropagating types, a wide channel of width and a narrow channel of width (with
the magnetic length and the momentum separation
of the Weyl points). Only one of the two is transmitted into a metallic
contact, providing unique magnetotransport signatures.Comment: V4: 10 pages, 14 figures. Added figure and discussion about
"uncrossing deformations" of oriented contours, plus minor corrections.
Published in NJ
Extended topological group structure due to average reflection symmetry
We extend the single-particle topological classification of insulators and
superconductors to include systems in which disorder preserves average
reflection symmetry. We show that the topological group structure of bulk
Hamiltonians and topological defects is exponentially extended when this
additional condition is met, and examine some of its physical consequences.
Those include localization-delocalization transitions between topological
phases with the same boundary conductance, as well as gapless topological
defects stabilized by average reflection symmetry.Comment: 8 pages, 5 figures, 1 table; improved section 4 "Extended topological
classification" incl. example of stacked QSH layer
How spin-orbit interaction can cause electronic shot noise
The shot noise in the electrical current through a ballistic chaotic quantum
dot with N-channel point contacts is suppressed for N --> infinity, because of
the transition from stochastic scattering of quantum wave packets to
deterministic dynamics of classical trajectories. The dynamics of the electron
spin remains quantum mechanical in this transition, and can affect the
electrical current via spin-orbit interaction. We explain how the role of the
channel number N in determining the shot noise is taken over by the ratio
l_{so}/lambda_F of spin precession length l_{so} and Fermi wave length
lambda_F, and present computer simulations in a two-dimensional billiard
geometry (Lyapunov exponent alpha, mean dwell time tau_{dwell}, point contact
width W) to demonstrate the scaling (lambda_F/l_{so})^{1/alpha tau_{dwell}} of
the shot noise in the regime lambda_F << l_{so} << W.Comment: 4 pages, 3 figure
Absence of a metallic phase in charge-neutral graphene with a random gap
It is known that fluctuations in the electrostatic potential allow for
metallic conduction (nonzero conductivity in the limit of an infinite system)
if the carriers form a single species of massless two-dimensional Dirac
fermions. A nonzero uniform mass opens up an excitation gap,
localizing all states at the Dirac point of charge neutrality. Here we
investigate numerically whether fluctuations in
the mass can have a similar effect as potential fluctuations, allowing for
metallic conduction at the Dirac point. Our negative conclusion confirms
earlier expectations, but does not support the recently predicted metallic
phase in a random-gap model of graphene.Comment: 3 pages, 3 figure
Bimodal conductance distribution of Kitaev edge modes in topological superconductors
A two-dimensional superconductor with spin-triplet p-wave pairing supports
chiral or helical Majorana edge modes with a quantized (length -independent)
thermal conductance. Sufficiently strong anisotropy removes both chirality and
helicity, doubling the conductance in the clean system and imposing a
super-Ohmic decay in the presence of disorder. We explain the
absence of localization in the framework of the Kitaev Hamiltonian, contrasting
the edge modes of the two-dimensional system with the one-dimensional Kitaev
chain. While the disordered Kitaev chain has a log-normal conductance
distribution peaked at an exponentially small value, the Kitaev edge has a
bimodal distribution with a second peak near the conductance quantum. Shot
noise provides an alternative, purely electrical method of detection of these
charge-neutral edge modes.Comment: 11 pages, 13 figure
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