8,204 research outputs found
Emergence of massless Dirac fermions in graphene's Hofstadter butterfly at switches of the quantum Hall phase connectivity
The fractal spectrum of magnetic minibands (Hofstadter butterfly), induced by
the moir\'e super- lattice of graphene on an hexagonal crystal substrate, is
known to exhibit gapped Dirac cones. We show that the gap can be closed by
slightly misaligning the substrate, producing a hierarchy of conical
singularities (Dirac points) in the band structure at rational values Phi =
(p/q)(h/e) of the magnetic flux per supercell. Each Dirac point signals a
switch of the topological quantum number in the connected component of the
quantum Hall phase diagram. Model calculations reveal the scale invariant
conductivity sigma = 2qe^2 / pi h and Klein tunneling associated with massless
Dirac fermions at these connectivity switches.Comment: 4 pages, 6 figures + appendix (3 pages, 1 figure
Andreev reflection from a topological superconductor with chiral symmetry
It was pointed out by Tewari and Sau that chiral symmetry (H -> -H if e
h) of the Hamiltonian of electron-hole (e-h) excitations in an N-mode
superconducting wire is associated with a topological quantum number
Q\in\mathbb{Z} (symmetry class BDI). Here we show that Q=Tr(r_{he}) equals the
trace of the matrix of Andreev reflection amplitudes, providing a link with the
electrical conductance G. We derive G=(2e^2/h)|Q| for |Q|=N,N-1, and more
generally provide a Q-dependent upper and lower bound on G. We calculate the
probability distribution P(G) for chaotic scattering, in the circular ensemble
of random-matrix theory, to obtain the Q-dependence of weak localization and
mesoscopic conductance fluctuations. We investigate the effects of chiral
symmetry breaking by spin-orbit coupling of the transverse momentum (causing a
class BDI-to-D crossover), in a model of a disordered semiconductor nanowire
with induced superconductivity. For wire widths less than the spin-orbit
coupling length, the conductance as a function of chemical potential can show a
sequence of 2e^2/h steps - insensitive to disorder.Comment: 10 pages, 5 figures. Corrected typo (missing square root) in
equations A13 and A1
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
Topologically protected charge transfer along the edge of a chiral -wave superconductor
The Majorana fermions propagating along the edge of a topological
superconductor with pairing deliver a shot noise power of
per eV of voltage bias. We calculate the full
counting statistics of the transferred charge and find that it becomes
trinomial in the low-temperature limit, distinct from the binomial statistics
of charge- transfer in a single-mode nanowire or charge- transfer
through a normal-superconductor interface. All even-order correlators of
current fluctuations have a universal quantized value, insensitive to disorder
and decoherence. These electrical signatures are experimentally accessible,
because they persist for temperatures and voltages large compared to the
Thouless energy.Comment: 5 pages, 4 figures. v3 [post-publication]: added an appendix on the
effect of a tunnel barrier at the normal-superconductor contac
Phase-locked magnetoconductance oscillations as a probe of Majorana edge states
We calculate the Andreev conductance of a superconducting ring interrupted by
a flux-biased Josephson junction, searching for electrical signatures of
circulating edge states. Two-dimensional pair potentials of spin-singlet d-wave
and spin-triplet p-wave symmetry support, respectively, (chiral) Dirac modes
and (chiral or helical) Majorana modes. These produce h/e-periodic
magnetoconductance oscillations of amplitude \simeq (e^{2}/h)N^{-1/2}, measured
via an N-mode point contact at the inner or outer perimeter of the grounded
ring. For Dirac modes the oscillations in the two contacts are independent,
while for an unpaired Majorana mode they are phase locked by a topological
phase transition at the Josephson junction.Comment: 10 pages, 6 figures. New appendix on the gauge invariant
discretization of the Bogoliubov-De Gennes equation. Accepted for publication
in PR
The Multilingual design of the EuroWordNet Database
This paper discusses the design of the EuroWordNet database, in which semantic databases like WordNet1.5 for several languages are combined via an inter-lingua. In this database, language-independent data is shared and language-specific properties are maintained as well. A special interface has been developed to compare the semantic configurations across languages and to track down differences. The pragmatic design of the database makes it possible to gather empirical evidence for a common cross-linguistic ontology. Abstract This paper discusses the design of the EuroWordNet database, in which semantic databases like WordNet1.5 for several languages are combined via a so-called interlingual -index. In this database, languageindependent data is shared and languagespecific properties are maintained as well. A special interface has been developed to compare the semantic configurations across languages and to track down differences. The pragmatic design of the database makes it possible..
Atomic Interferometer with Amplitude Gratings of Light and its Applications to Atom Based Tests of the Equivalence Principle
We have developed a matter wave interferometer based on the diffraction of
atoms from effective absorption gratings of light. In a setup with cold
rubidium atoms in an atomic fountain the interferometer has been used to carry
out tests of the equivalence principle on an atomic basis. The gravitational
acceleration of the two isotopes 85Rb and 87Rb was compared, yielding a
difference Dg/g =(1.2 +-1.7)x10^{-7}. We also perform a differential free fall
measurement of atoms in two different hyperfine states, and obtained a result
of Dg/g =(0.4 +-1.2)x10^{-7}.Comment: 4 Pages, 4 figures, accepted for Physical Review Letter
Bloch oscillations in an aperiodic one-dimensional potential
We study the dynamics of an electron subjected to a static uniform electric
field within a one-dimensional tight-binding model with a slowly varying
aperiodic potential. The unbiased model is known to support phases of localized
and extended one-electron states separated by two mobility edges. We show that
the electric field promotes sustained Bloch oscillations of an initial Gaussian
wave packet whose amplitude reflects the band width of extended states. The
frequency of these oscillations exhibit unique features, such as a sensitivity
to the initial wave packet position and a multimode structure for weak fields,
originating from the characteristics of the underlying aperiodic potential.Comment: 6 pages, 7 figure
- …