2,237 research outputs found
Theoretical study of X-ray absorption of three-dimensional topological insulator
X-ray absorption edge singularity which is usually relevant for metals is
studied for the prototype topological insulator .
The generalized integral equation of Nozi\`eres and Dominicis type for X-ray
edge singularity is derived and solved. The spin texture of surfaces states
causes a component of singularity dependent on the helicity of the spin
texture. It also yields another component for which the singularity from
excitonic processes is absent.Comment: RevTeX 4.1. 4 pages, no figur
The thermopower as a fingerprint of the Kondo breakdown quantum critical point
We propose that the thermoelectric power distinguishes two competing
scenarios for quantum phase transitions in heavy fermions : the
spin-density-wave (SDW) theory and breakdown of the Kondo effect. In the Kondo
breakdown scenario, the Seebeck coefficient turns out to collapse from the
temperature scale , associated with quantum fluctuations of the Fermi
surface reconfiguration. This feature differs radically from the physics of the
SDW theory, where no reconstruction of the Fermi surface occurs, and can be
considered as the hallmark of the Kondo breakdown theory. We test these ideas,
upon experimental results for YbRhSi
Spinon-Holon binding in model
Using a phenomenological model, we discuss the consequences of spinon-holon
binding in the U(1) slave-boson approach to model. Within a small
( hole concentration) expansion, we show that spinon-holon binding produces
a pseudo-gap normal state with a segmented Fermi surface and the
superconducting state is formed by opening an "additional" d-wave gap on the
segmented Fermi surface. The d-wave gap merge with the pseudo-gap smoothly as
temperature . The quasi-particles in the superconducting state are
coupled to external electromagnetic field with a coupling constant of order
where , depending on the strength of the
effective spinon-holon binding potential.Comment: 9 pages, 3 figure
Boundary-induced violation of the Dirac fermion parity and its signatures in local and global tunneling spectra of graphene
Extended defects in graphene, such as linear edges, break the translational
invariance and can also have an impact on the symmetries specific to massless
Dirac-like quasiparticles in this material. The paper examines the consequences
of a broken Dirac fermion parity in the framework of the effective boundary
conditions varying from the Berry-Mondragon mass confinement to a zigzag edge.
The parity breaking reflects the structural sublattice asymmetry of zigzag-type
edges and is closely related to the previously predicted time-reversal
symmetric edge states. We calculate the local and global densities of the edge
states and show that they carry a specific polarization, resembling, to some
extent, that of spin-polarized materials. The lack of the parity leads to a
nonanalytical particle-hole asymmetry in the edge-state properties. We use our
findings to interpret recently observed tunneling spectra in zigzag-terminated
graphene. We also propose a graphene-based tunneling device where the
particle-hole asymmetric edge states result in a strongly nonlinear
conductance-voltage characteristics, which could be used to manipulate the
tunneling transport.Comment: 8 pages, 5 figures, to be published in Phys. Rev.
Localized states due to expulsion of resonant impurity levels from the continuum in bilayer graphene
Anderson impurity problem is considered for a graphene bilayer subject to a
gap-opening bias. In-gap localized states are produced even when the impurity
level overlaps with the continuum of band electrons. The effect depends
strongly on the polarity of the applied bias as long as hybridization with the
impurity occurs within a single layer. For an impurity level inside the
conduction band a positive bias creates the new localized in-gap state. A
negative bias does not produce the same result and leads to a simple broadening
of the impurity level. The implications for transport are discussed including a
possibility of gate-controlled Kondo effect.Comment: 5 pages, 2 figure
Tunneling spectra of layered strongly correlated d-wave superconductors
Tunneling conductance experiments on cuprate superconductors exhibit a large
diversity of spectra that appear in different nano-sized regions of
inhomogeneous samples. In this letter, we use a mean-field approach to the
tt't''J model in order to address the features in these spectra that deviate
from the BCS paradigm, namely, the bias sign asymmetry at high bias, the
generic lack of evidence for the Van Hove singularity, and the occasional
absence of coherence peaks. We conclude that these features can be reproduced
in homogeneous layered d-wave superconductors solely due to a proximate Mott
insulating transition. We also establish the connection between the above
tunneling spectral features and the strong renormalization of the electron
dispersion around (0,pi) and (pi,0) and the momentum space anisotropy of
electronic states observed in ARPES experiments.Comment: 4 pages, 3 figures. Added comment on the role of sample
inhomogeneity. Published version. Homepage http://dao.mit.edu/~wen
Decoherence in quantum dots due to real and virtual transitions: a non-perturbative calculation
We investigate theoretically acoustic phonon induced decoherence in quantum
dots. We calculate the dephasing of fundamental (interband or intraband)
optical transitions due to real and virtual transitions with higher energy
levels. Up to two acoustic phonon processes (absorption and/or emission) are
taken into account simultaneously in a non-perturbative manner. An analytic
expression of acoustic phonon induced broadening is given as a function of the
electron-phonon matrix elements and is physically interpreted. The theory is
applied to the dephasing of intersublevel transitions in self-assembled quantum
dots.Comment: 8 pages, 4 figure
X-ray edge singularity of bilayer graphene
The X-ray edge singularity of bilayer graphene is studied by generalizing the
path integral approach based on local action which was employed for monolayer
graphene. In sharp contrast to the case of monolayer graphene, the bilayer
graphene is found to exhibit the edge singularity even at half-filling and its
characteristics are determined by interlayer coupling. At finite bias the
singular behaviors sensitively depend on the relative magnitude of fermi energy
and applied bias, which is due to the peculiar shape of energy band at finite
bias.Comment: RevTeX 4.1, 4 pages. No figur
Polarons and Molecules in a Two-Dimensional Fermi Gas
We study an impurity atom in a two-dimensional Fermi gas using variational
wave functions for (i) an impurity dressed by particle-hole excitations
(polaron) and (ii) a dimer consisting of the impurity and a majority atom. In
contrast to three dimensions, where similar calculations predict a sharp
transition to a dimer state with increasing interspecies attraction, we show
that the polaron ansatz always gives a lower energy. However, the exact
solution for a heavy impurity reveals that both a two-body bound state and
distortions of the Fermi sea are crucial. This reflects the importance of
particle-hole pairs in lower dimensions and makes simple variational
calculations unreliable. We show that the energy of an impurity gives important
information about its dressing cloud, for which both ans\"atze give inaccurate
results.Comment: 5 pages, 2 figures, minor change
Nonlinear photon transport in a semiconductor waveguide-cavity system containing a single quantum dot: Anharmonic cavity-QED regime
We present a semiconductor master equation technique to study the
input/output characteristics of coherent photon transport in a semiconductor
waveguide-cavity system containing a single quantum dot. We use this approach
to investigate the effects of photon propagation and anharmonic cavity-QED for
various dot-cavity interaction strengths, including weakly-coupled,
intermediately-coupled, and strongly-coupled regimes. We demonstrate that for
mean photon numbers much less than 0.1, the commonly adopted weak excitation
(single quantum) approximation breaks down, even in the weak coupling regime.
As a measure of the anharmonic multiphoton-correlations, we compute the Fano
factor and the correlation error associated with making a semiclassical
approximation. We also explore the role of electron--acoustic-phonon scattering
and find that phonon-mediated scattering plays a qualitatively important role
on the light propagation characteristics. As an application of the theory, we
simulate a conditional phase gate at a phonon bath temperature of K in the
strong coupling regime.Comment: To appear in PR
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