127 research outputs found
Evidence of topological superconductivity in planar Josephson junctions
Majorana zero modes are quasiparticle states localized at the boundaries of
topological superconductors that are expected to be ideal building blocks for
fault-tolerant quantum computing. Several observations of zero-bias conductance
peaks measured in tunneling spectroscopy above a critical magnetic field have
been reported as experimental indications of Majorana zero modes in
superconductor/semiconductor nanowires. On the other hand, two dimensional
systems offer the alternative approach to confine Ma jorana channels within
planar Josephson junctions, in which the phase difference {\phi} between the
superconducting leads represents an additional tuning knob predicted to drive
the system into the topological phase at lower magnetic fields. Here, we report
the observation of phase-dependent zero-bias conductance peaks measured by
tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al
heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces
the critical field at which the zero-bias peak appears, with respect to {\phi}
= 0. The phase and magnetic field dependence of the zero-energy states is
consistent with a model of Majorana zero modes in finite-size Josephson
junctions. Besides providing experimental evidence of phase-tuned topological
superconductivity, our devices are compatible with superconducting quantum
electrodynamics architectures and scalable to complex geometries needed for
topological quantum computing.Comment: main text and extended dat
Critical conductance of a one-dimensional doped Mott insulator
We consider the two-terminal conductance of a one-dimensional Mott insulator
undergoing the commensurate-incommensurate quantum phase transition to a
conducting state. We treat the leads as Luttinger liquids. At a specific value
of compressibility of the leads, corresponding to the Luther-Emery point, the
conductance can be described in terms of the free propagation of
non-interacting fermions with charge e/\sqrt{2}. At that point, the temperature
dependence of the conductance across the quantum phase transition is described
by a Fermi function. The deviation from the Luther-Emery point in the leads
changes the temperature dependence qualitatively. In the metallic state, the
low-temperature conductance is determined by the properties of the leads, and
is described by the conventional Luttinger liquid theory. In the insulating
state, conductance occurs via activation of e/\sqrt{2} charges, and is
independent of the Luttinger liquid compressibility.Comment: 13 pages, 3 figures. Published versio
Spin orbit effects in a GaAs quantum dot in a parallel magnetic field
We analyze the effects of spin-orbit coupling on fluctuations of the
conductance of a quantum dot fabricated in a GaAs heterostructure. We argue
that spin-orbit effects may become important in the presence of a large
parallel magnetic field B_{||}, even if they are negligble for B_{||}=0. This
should be manifest in the level repulsion of a closed dot, and in reduced
conductance fluctuations in dots with a small number of open channels in each
lead, for large B_{||}. Our picture is consistent with the experimental
observations of Folk et al.Comment: 5 page
The Evolution of Quasiparticle Charge in the Fractional Quantum Hall Regime
The charge of quasiparticles in a fractional quantum Hall (FQH) liquid,
tunneling through a partly reflecting constriction with transmission t, was
determined via shot noise measurements. In the nu=1/3 FQH state, a charge
smoothly evolving from e*=e/3 for t=1 to e*=e for t<<1 was determined, agreeing
with chiral Luttinger liquid theory. In the nu=2/5 FQH state the quasiparticle
charge evolves smoothly from e*=e/5 at t=1 to a maximum charge less than e*=e/3
at t<<1. Thus it appears that quasiparticles with an approximate charge e/5
pass a barrier they see as almost opaque.Comment: 4 pages, Correct figure 3 and caption include
An accurate effective action for `baby' to `adult' skyrmions
Starting with a Chern-Simons theory, we derive an effective action for
interacting quantum Hall skyrmions that takes into account both large-distance
physics and short-distance details as well. We numerically calculate the
classical static skyrmion profile from this action and find excellent agreement
with other, microscopic calculations over a wide range of skyrmion sizes
including the experimentally relevant one. This implies that the essential
physics of this regime might be captured by a continuum classical model rather
than resorting to more microscopic approaches. We also show that the skyrmion
energy closely follows the formula suggested earlier by Sondhi et al. for a
broad parameter range of interest as well.Comment: 13 pages (Revtex) + 3 ps-figure
Coulomb drag in compressible quantum Hall states
We consider the Coulomb drag between two layers of two-dimensional electronic
gases subject to a strong magnetic field. We first focus on the case in which
the electronic density is such that the Landau level filling fraction in
each layer is at, or close to, . Discussing the coupling between the
layers in purely electronic terms, we show that the unique dependence of the
longitudinal conductivity on wave-vector, observed in surface acoustic waves
experiments, leads to a very slow decay of density fluctuations. Consequently,
it has a crucial effect on the Coulomb drag, as manifested in the
transresistivity . We find that the transresistivity is very large
compared to its typical values at zero magnetic field, and that its temperature
dependence is unique -- . For filling factors at or
close to and the transresistivity has the same -dependence, and
is larger than at . We calculate for the case and
propose that it might shed light on the spin polarization of electrons at
. We compare our results to recent calculations of at
where a composite fermion approach was used and a
-dependence was obtained. We conclude that what appears in the
composite fermion language to be drag induced by Chern-Simons interaction is,
physically, electronic Coulomb drag.Comment: 11 pages, REVTeX with two Postscript figure
Singularities in the Fermi liquid description of a partially filled Landau level and the energy gaps of fractional quantum Hall states
We consider a two dimensional electron system in an external magnetic field
at and near an even denominator Landau level filling fraction. Using a
fermionic Chern--Simons approach we study the description of the system's low
energy excitations within an extension of Landau's Fermi liquid theory. We
calculate perturbatively the effective mass and the quasi--particle interaction
function characterizing this description. We find that at an even denominator
filling fraction the fermion's effective mass diverges logarithmically at the
Fermi level, and argue that this divergence allows for an {\it exact}
calculation of the energy gaps of the fractional quantized Hall states
asymptotically approaching these filling fractions. We find that the
quasi--particle interaction function approaches a delta function. This singular
behavior leads to a cancelation of the diverging effective mass from the long
wavelength low frequency linear response functions at even denominator filling
fractions.Comment: 46 pages, RevTeX, 5 figures included in a uuencoded postscript file.
Minor revisions relative to the original version. The paper will be published
in the Physical Review B, and can be retrieved from the World Wide Web, in
http://cmtw.harvard.edu/~ster
Dissipationless transport in low density bilayer systems
In a bilayer electronic system the layer index may be viewed as the
z-component of an isospin-1/2. An XY isospin-ordered ferromagnetic phase was
observed in quantum Hall systems and is predicted to exist at zero magnetic
field at low density. This phase is a superfluid for opposite currents in the
two layers. At B=0 the system is gapless but superfluidity is not destroyed by
weak disorder. In the quantum Hall case, weak disorder generates a random gauge
field which probably does not destroy superfluidity. Experimental signatures
include Coulomb drag and collective mode measurements.Comment: 4 pages, no figures, submitted to Phys. Rev. Let
Plasmas and Controlled Nuclear Fusion
Contains reports on four research project.U. S. Atomic Energy Commission (Contract AT(11-1)-3070
Plasma Dynamics
Contains reports on two research projects.U. S. Energy Research and Development Administration (Contract E(l1-1)-3070)National Science Foundation (Grant ENG75-06242
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