172 research outputs found
The Signs of Quantum Dot-Lead Matrix Elements: The Effect on Transport vs. Spectral Properties
A small quantum dot coupled to two external leads is considered. Different
signs of the dot-leads coupling matrix elements give rise to qualitatively
different behavior of physical observables such as the conductance, the phase
of the transmission amplitude and the differential capacitance of the dot. For
certain relative signs the conductance may vanish at values of the gate
potential, where the spectral density is maximal. Zeroes of the conductance are
robust against increasing the dot-lead coupling. They are associated with
abrupt phase lapses in the transmission phase whose width vanishes as the
square of the temperature. We carefully distinguish between phase lapses of
and phase anti-lapses of .Comment: 10 pages, 10 figure
Non-Abelian topological insulators from an array of quantum wires
We suggest a construction of a large class of topological states using an
array of quantum wires. First, we show how to construct a Chern insulator using
an array of alternating wires that contain electrons and holes, correlated with
an alternating magnetic field. This is supported by semi-classical arguments
and a full quantum mechanical treatment of an analogous tight-binding model. We
then show how electron-electron interactions can stabilize fractional Chern
insulators (Abelian and non-Abelian). In particular, we construct a relatively
stable non-Abelian parafermion state. Our construction is
generalized to wires with alternating spin-orbit couplings, which give rise to
integer and fractional (Abelian and non-Abelian) topological insulators. The
states we construct are effectively two-dimensional, and are therefore less
sensitive to disorder than one-dimensional systems. The possibility of
experimental realization of our construction is addressed
Fluctuation persistent current in small superconducting rings
We extend previous theoretical studies of the contribution of fluctuating
Cooper pairs to the persistent current in superconducting rings subjected to a
magnetic field. For sufficiently small rings, in which the coherence length
exceeds the radius , mean field theory predicts the emergence of a
flux-tuned quantum critical point separating metallic and superconducting
phases near half-integer flux through the ring. For larger rings with , the transition temperature is periodically reduced, but superconductivity
prevails at very low temperatures. We calculate the fluctuation persistent
current in different regions of the metallic phase for both types of rings.
Particular attention is devoted to the interplay of the angular momentum modes
of the fluctuating order parameter field. We discuss the possibility of using a
combination of different pair-breaking mechanisms to simplify the observation
of the flux-tuned transition in rings with .Comment: 16 pages, 8 figure
Fermions and Bosons in Superconducting Amorphous Wires
We discuss the destruction of superconductivity in quasi-one-dimensional
systems due to the interplay between disorder and Coulomb repulsion. We argue
that to understand the behavior of the system one has to study both fermionic
and bosonic mechanisms of suppression of superconductivity. The former
describes reduction in the mean field critical temperature , while the
latter refers to thermal and quantum fluctuations in the order parameter. A
change in parameters such as wire width and disorder strength significantly
affects both mechanisms.Comment: To be published in "Electronic Correlations:From meso- to
nano-physics" Proceedings of the XXXVI Rencontres de Moriond, T. Martin, G.
Montambaux & J. Tran Tran Van Eds. (2001
From an array of quantum wires to three-dimensional fractional topological insulators
The coupled-wires approach has been shown to be useful in describing
two-dimensional strongly interacting topological phases. In this manuscript we
extend this approach to three-dimensions, and construct a model for a
fractional strong topological insulator. This topologically ordered phase has
an exotic gapless state on the surface, called a fractional Dirac liquid, which
cannot be described by the Dirac theory of free fermions. Like in
non-interacting strong topological insulators, the surface is protected by the
presence of time-reversal symmetry and charge conservation. We show that upon
breaking these symmetries, the gapped fractional Dirac liquid presents unique
features. In particular, the gapped phase that results from breaking
time-reversal symmetry has a halved fractional Hall conductance of the form
if the filling is . On the
other hand, if the surface is gapped by proximity coupling to an -wave
superconductor, we end up with an exotic topological superconductor. To reveal
the topological nature of this superconducting phase, we partition the surface
into two regions: one with broken time-reversal symmetry and another coupled to
a superconductor. We find a fractional Majorana mode, which cannot be described
by a free Majorana theory, on the boundary between the two regions. The density
of states associated with tunneling into this one-dimensional channel is
proportional to , in analogy to the edge of the corresponding
Laughlin state
Non-Fermi-Liquid in a modified single electron transistor
At low temperatures, a system built from a small droplet of electrons and a
larger, but still finite, droplet may display non-Fermi-liquid behavior.
Stabilization of a multi-channel Kondo fixed point requires fine control of the
electrochemical potential in each droplet. The desired fine control can be
achieved by adjusting voltages on nearby gate electrodes. We study the
conditions for obtaining this type of non-Fermi-liquid behavior and discuss the
experimentally-observable consequences
Fractional Helical Liquids and Non-Abelian Anyons in Quantum Wires
We study one dimensional wires with spin-orbit coupling. We show that in the
presence of Zeeman field and strong electron-electron interaction a clean wire
may form fractional helical liquid states with phenomenology similar to
fractional quantum Hall liquids. Most notably, the wire's two terminal
conductance is predicted to show fractional quantized conductance plateaus at
low electron density. When the system is proximity-coupled to a superconductor,
fractional Majorana bound states may be stabilized. We discuss how disorder
destabilizes these fractional phases. Possible experimental realizations of
similar states in double wire systems are discussed.Comment: 8 pages, 4 figure
Memory Effects in the Electron Glass
We investigate theoretically the slow non-exponential relaxation dynamics of
the electron glass out of equilibrium, where a sudden change in carrier density
reveals interesting memory effects. The self-consistent model of the dynamics
of the occupation numbers in the system successfully recovers the general
behavior found in experiments. Our numerical analysis is consistent with both
the expected logarithmic relaxation and our understanding of how increasing
disorder or interaction slows down the relaxation process, thus yielding a
consistent picture of the electron glass. We also present a novel finite size
"domino" effect where the connection to the leads affects the relaxation
process of the electron glass in mesoscopic systems. This effect speeds up the
relaxation process, and even reverses the expected effect of interaction;
stronger interaction then leading to a faster relaxation.Comment: 5 pages, 5 figure
Realizing Topological Superconductivity with Superlattices
The realization of topological superconductors (SCs) in one or two dimensions
is a highly pursued goal. Prominent proposed realization schemes include
semiconductor/superconductor heterostructures and set stringent constraints on
the chemical potential of the system. However, the ability to keep the chemical
potential in the required range while in the presence of an adjacent SC and its
accompanied screening effects, is a great experimental challenge. In this work,
we study a SC lattice structure in which the SC is deposited periodically on a
one- or two-dimensional sample. We demonstrate that this realization platform
overcomes the challenge of controlling the chemical potential in the presence
of the superconductor's electrostatic screening. We show how Majorana bound
states emerge at the ends of a one-dimensional system proximity coupled to a
one-dimensional SC lattice, and move on to present a SC-lattice-based
realization of the two-dimensional px+ipy SC, hosting chiral Majorana modes at
its edges. In particular, we establish that even when assuming the worst case
of absolute screening, in which the chemical potential under the SC is
completely unaffected by the external gate potential, the topological phase can
be reached by tuning the chemical potential in the area not covered by the SC.
Finally, we briefly discuss possible effects of Coulomb blockade on the
properties of the system
Variable range hopping in the Coulomb glass
We use a mean-field (Hartree-like) approach to study the conductance of a
strongly localized electron system in two dimensions. We find a crossover
between a regime where Coulomb interactions modify the conductance
significantly to a regime where they are negligible. We show that under rather
general conditions the conduction obeys a scaling relation which we verify
using numerical simulations. The use of a Hartree self-consistent approach
gives a clear physical picture, and removes the ambiguity of the use of
single-particle tunneling density-of-states (DOS) in the calculation of the
conductance. Furthermore, the theory contains interaction-induced correlations
between the on site energy of the localized states and distances, as well as
finite temperature corrections of the DOS
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