191 research outputs found
Josephson effect in Graphene SNS Junction with a Single Localized Defect
Imperfections change essentially the electronic transport properties of
graphene. Motivated by a recent experiment reporting on the possible
application of graphene as junctions, we study transport properties in
graphene-based junctions with single localized defect. We solve the
Dirac-Bogoliubov-de-Gennes equation with a single localized defect
superconductor-normal(graphene)-superconductor (SNS) junction. We consider the
properties of tunneling conductance and Josephson current through an undoped
strip of graphene with heavily doped s-wave superconducting electrodes in the
dirty limit. We find that spectrum of Andreev bound states are modified in the
presence of single localized defect in the bulk and the minimum tunneling
conductance remains the same. The Josephson junction exhibits sign
oscillations.Comment: 5 pages, 4 figure
Phases and Density of States in a Generalized Su-Schrieffer-Heeger Model
Self-consistent solutions to a generalized Su-Schrieffer-Heeger model on a
2-dimensional square lattice are investigated. Away from half-filling,
spatially inhomogeneous phases are found. Those phases may have topological
structures on the flux order, large unit cell bond order, localized bipolarons,
or they are simply short-range ordered and glassy. They have an universal
feature of always possessing a gap at the Fermi level.Comment: 11 pages, 5 figure
On the Zero-Bias Conductance Peak in the Tunneling Spectroscopy
A generalized method of image, incorporated with the non-equilibrium
Keldysh-Green's function formalism, is employed to investigate the tunneling
spectroscopy of hybrid systems in the configuration of planar junction. In
particular, tunneling spectroscopies of several hybrid systems that exhibit
zero-bias conductance peaks (ZBCP) are examined. The well-known metal--d-wave
superconductor (ND) junction is first examined in detail. Both the evolution of
the ZBCP versus doping and the splitting of the ZBCP in magnetic fields are
computed in the framework of the slave-boson mean field theory. Further
extension of our method to analyze other states shows that states with
particle-hole pairing, such as d-density wave and graphene sheet, are all
equivalent to a simple 1D model, which at the same time also describes the
polyacetylene. We provide the criteria for the emergence of ZBCP. In
particular, broken reflection symmetry at the microscopic level is shown to be
a necessary condition for ZBCP to occur
Current transport in a superconducting superlattice system
We investigate the effect of the superlattice structure on the single
particle transport along the c-axis of high temperature superconductors. In
particular, superlattice systems that consists of metals/insulators and d-wave
superconductors (NS/IS superlattice) are considered. We find that for the NS
superlattice in the large mass anisotropy limit of the metal, the density of
state in the low energy section is bulk d-wave like except that the position of
the quasi-particle peak can be considerably smaller than the gap value, while
for the IS superlattice, the quasi-particle peak remains at the gap value. We
also calculate the nonlinear differential conductance in the planar junction
measurement. It is found that the width of the Andreev peak at zero-bias may be
affected strongly by the superlattice structure, specifically, it can be
considerably reduced due to the destructive interference of the Andreev
reflections from all the superconductors. Such a reduction in the width makes
the Andreev peak resonant-like and has been observed in a recent experiment.Comment: 21 pages, 14 figure
Transport in quantum wells in the presence of interface roughness
The effective Hamiltonian for two dimensional quantum wells with rough
interfaces is formally derived. Two new terms are generated. The first term is
identified to the local energy level fluctuations, which was introduced
phenomenologically in the literature for interface roughness scattering but is
now shown to be valid only for an infinite potential well or Hamiltonians with
one single length scale. The other term is shown to modulate the wavefunction
and cause fluctuations in the charge density. This will further reduce the
electron mobility to the magnitude that is close to the experimental result.Comment: 15 pages, 2 figures, a shorter version of this paper is to be
published in Phys. Rev.
Unconventional superconducting gap via spin fluctuations in iron-vacancy ordered AFeSe
Based on an effective 12-orbital tight-binding model, we examine the
superconducting states induced by the antiferromagnetic fluctuations for
iron-vacancy-ordered AFeSe. It is shown that due to the
broken reflection symmetry induced by the iron vacancies, new superconducting
states with symmetry emerge. In particular, we show that in the
symmetry, symmetric axes of the pairing momenta do not need to
coincide with axes of the unit cell. As a result, in addition to the magnitude
of the pairing gap, the relative orientation of the pairing wave function to
the lattice forms another degree of freedom for characterizing the
superconducting gap and can further help in gaining the condensation energy.
Nonetheless, similar to other iron-based superconductors, the singlet ground
state is still dominated by s-wave or d-wave, which are nearly degenerate with
anisotropic gaps. Furthermore, s-wave and d-wave superconducting states are
separated by a quantum critical point controlled by the Hund's rule coupling
.Comment: 9 pages, 8 figure
Generalized method of image and the tunneling spectroscopy in high-Tc superconductors
A generalized method of image is developed to investigate the tunneling
spectrum from the metal into a class of states, with the tight-binding
dispersion fully included. The broken reflection symmetry is shown to be the
necessary condition for the appearance of the zero-bias conductance peak
(ZBCP). Applying this method to the d-wave superconductor yields results in
agreement with experiments regarding the splitting of ZBCPs in magnetic field.
Furthermore, a ZBCP is predicted for tunneling into the (110) direction of the
d-density wave state, providing a signature to look for in experiments
Electron scattering from a mesoscopic disk in Rashba system
Electrons with spin-orbit coupling moving in mesoscopic structures can often
exhibit local spin polarization. In this paper, we study the influence of the
Rashba coupling on the scattering of two-dimensional electrons from a circular
disk. It is observed that spin-polarized regions exist, even if the incident
electrons are unpolarized. In addition to the distributions of charge and spin
current in the near-field region, we also analyze the symmetry and the
differential cross-section of the scattering.Comment: 13 page, 4 figure
Continuum model description of thin film growth morphology
We examine the applicability of the continuum model to describe the surface
morphology of a hetero-growth system: compositionally-graded, relaxed GeSi
films on (001) Si substrates. Surface roughness versus lateral dimension was
analyzed for samples what were grown under different conditions. We find that
all samples belong to the same growth class, in which the surface roughness
scales linearly with lateral size at small scales and appears to saturate at
large scales. For length scales ranging from 1 nm to 100 m, the scaling
behavior can be described by a linear continuum model consisting of a surface
diffusion term and a Laplacian term. However, in-depth analysis on
non-universal amplitudes indicates the breaking of up-down symmetry, suggesting
the presence of non-linear terms in the microscopic model. We argue that the
leading non-linear term has the form of , but its
effect on scaling exponents will not be evident for length scales less than 1
mm. Therefore, the growth dynamics of this system is described by the
Kuramoto-Sivashinsky equation, consisting of the two linear terms plus , driven by a Gaussian noise. We also discuss the negative
coefficient in the Laplacian term as an instability mechanism responsible for
large scale film morphology on the final surface.Comment: 25 pages, revtex, 6 figures available upon request, to appear in
Phys. Rev.
Duality in topological superconductors and topological ferromagnetic insulators in a honeycomb lattice
The ground state of large Hubbard limit of a honeycomb lattice near
half-filling is known to be a singlet -wave superconductor. It is also
known that this superconductor exhibits a chiral pairing locally
at the Dirac cone, characterized by a topological invariant. By
constructing a dual transformation, we demonstrate that this
topological superconductor is equivalent to a collection of two topological
ferromagnetic insulators. As a result of the duality, the topology of the
electronic structures for a superconductor is controllable via the
change of the chemical potential by tuning the gate voltage. In particular,
instead of being always a chiral superconductor, we find that the
superconductor undergoes a topological phase transition from a chiral
superconductor to a quasi-helical superconductor as the gap amplitude or the
chemical potential decreases. The quasi-helical superconducting phase is found
to be characterized by a topological invariant in the pseudo-spin charge sector
with vanishing both the Chern number and the spin Chern number. We further
elucidate the topological phase transition by analyzing the relationship
between the topological invariant and the rotation symmetry. Due to the angular
momentum carried by the gap function and spin-orbit interactions, we show that
by placing superconductors in proximity to ferromagnets, varieties of
chiral superconducting phases characterized by higher Chern numbers can be
accessed, providing a new platform for hosting large numbers of Majorana modes
at edges.Comment: 12 pages, 6 figure
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