123 research outputs found
Majorana fermions manifested as interface-states in semiconductor hybrid structures
Motivated by recent proposals for the generation of Majorana fermions in
semiconducting hybrid structures, we examine possible experimental fingerprints
of such excitations. Whereas previous works mainly have focused on zero-energy
states in vortex cores in this context, we demonstrate analytically an
alternative route to detection of Majorana excitations in semiconducting hybrid
structures: interface-bound states that may be probed directly via conductance
spectroscopy or STM-measurements. We estimate the necessary experimental
parameters required for observation of our predictions.Comment: 4 pages, 2 figures
Tunneling conductance in - and d-wave superconductor-graphene junctions: Extended Blonder-Tinkham-Klapwijk formalism
We investigate the conductance spectra of a normal/superconductor graphene
junction using the extended Blonder-Tinkham-Klapwijk formalism, considering
pairing potentials that are both conventional (isotropic s-wave) and
unconventional (anisotropic d-wave). In particular, we study the full crossover
from normal to specular Andreev reflection without restricting ourselves to
special limits and approximations, thus expanding results obtained in previous
work. In addition, we investigate in detail how the conductance spectra are
affected if it is possible to induce an unconventional pairing symmetry in
graphene, for instance a d-wave order parameter. We also discuss the recently
reported conductance-oscillations that take place in normal/superconductor
graphene junctions, providing both analytical and numerical results.Comment: 14 pages, 16 figures. High-resolution figures available in the
published versio
Phase transitions in a three dimensional lattice London superconductor
We consider a three-dimensional lattice superconductor in
the London limit, with two individually conserved condensates. The problem,
generically, has two types of intercomponent interactions of different
characters. First, the condensates are interacting via a minimal coupling to
the same fluctuating gauge field. A second type of coupling is the direct
dissipationless drag represented by a local intercomponent current-current
coupling term in the free energy functional. The interplay between these two
types of interactions produces a number of physical effects not present in
previously investigated models with only one kind of
intercomponent interaction. In this work, we present a study of the phase
diagram of a superconductor which includes both of these
interactions. We study phase transitions and two types of competing paired
phases which occur in this general model: (i) a metallic superfluid phase
(where there is order only in the gauge invariant phase difference of the order
parameters), (ii) a composite superconducting phase where there is order in the
phase sum of the order parameters which has many properties of a
single-component superconductor but with a doubled value of electric charge. We
investigate the phase diagram with particular focus on what we call "preemptive
phase transitions". These are phase transitions {\it unique to multicomponent
condensates with competing topological objects}. A sudden proliferation of one
kind of topological defects may come about due to a fluctuating background of
topological defects in other sectors of the theory.Comment: 17 pages, 6 figures. Submitted to Physical Review
Observation of a metallic superfluid in a numerical experiment
We report the observation, in Monte Carlo simulations, of a novel type of
quantum ordered state: {\it the metallic superfluid}. The metallic superfluid
features ohmic resistance to counter-flows of protons and electrons, while
featuring dissipationless co-flows of electrons and protons. One of the
candidates for a physical realization of this remarkable state of matter is
hydrogen or its isotopes under high compression. This adds another potential
candidate to the presently known quantum dissipationless states, namely
superconductors, superfluid liquids and vapours, and supersolids.Comment: 4 pages, 2 figures. Accepted for publication in Phys. Rev. Let
Josephson effect in thin-film superconductor/insulator/superconductor junctions with misaligned in-plane magnetic fields
We study a tunnel junction consisting of two thin-film s-wave superconductors
separated by a thin, insulating barrier in the presence of misaligned in-plane
exchange fields. We find an interesting interplay between the superconducting
phase difference and the relative orientation of the exchange fields,
manifested in the Josephson current across the junction. Specifically, this may
be written , where
I_0 and I_m are constants, and is the relative orientation of the
exchange fields while is the superconducting phase difference.
Similar results have recently been obtained in other S/I/S junctions coexisting
with helimagnetic or ferromagnetic order. We calculate the superconducting
order parameter self-consistently, and investigate quantitatively the effect
which the misaligned exchange fields constitute on the Josephson current, to
see if I_m may have an appreciable effect on the Josephson current. It is found
that I_0 and I_m become comparable in magnitude at sufficiently low
temperatures and fields close to the critical value, in agreement with previous
work. From our analytical results, it then follows that the Josephson current
in the present system may be controlled in a well-defined manner by a rotation
of the exchange fields on both sides of the junction. We discuss a possible
experimental realization of this proposition.Comment: 8 pages, 8 figures. Accepted for publication in Phys. Rev.
Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films
We have patterned a hexagonal array of nano-scale holes into a series of
ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K
5 K. These regular perforations give the films a phase-sensitive
periodic response to an applied magnetic field. By measuring this response in
their resistive transitions, , we are able to distinguish regimes in
which fluctuations of the amplitude, both the amplitude and phase, and the
phase of the superconducting order parameter dominate the transport. The
portion of dominated by amplitude fluctuations is larger in lower
films and thus, grows with proximity to the superconductor to
insulator transition.Comment: Revised title, abstract, text, figure
A superconductor to superfluid phase transition in liquid metallic hydrogen
Although hydrogen is the simplest of atoms, it does not form the simplest of
solids or liquids. Quantum effects in these phases are considerable (a
consequence of the light proton mass) and they have a demonstrable and often
puzzling influence on many physical properties, including spatial order. To
date, the structure of dense hydrogen remains experimentally elusive. Recent
studies of the melting curve of hydrogen indicate that at high (but
experimentally accessible) pressures, compressed hydrogen will adopt a liquid
state, even at low temperatures. In reaching this phase, hydrogen is also
projected to pass through an insulator-to-metal transition. This raises the
possibility of new state of matter: a near ground-state liquid metal, and its
ordered states in the quantum domain. Ordered quantum fluids are traditionally
categorized as superconductors or superfluids; these respective systems feature
dissipationless electrical currents or mass flow. Here we report an analysis
based on topological arguments of the projected phase of liquid metallic
hydrogen, finding that it may represent a new type of ordered quantum fluid.
Specifically, we show that liquid metallic hydrogen cannot be categorized
exclusively as a superconductor or superfluid. We predict that, in the presence
of a magnetic field, liquid metallic hydrogen will exhibit several phase
transitions to ordered states, ranging from superconductors to superfluids.Comment: for a related paper see cond-mat/0410425. A correction to the front
page caption appeared in Oct 14 issue of Nature:
http://www.nature.com/nature/links/041014/041014-11.htm
Strong diamagnetic response and specific heat anomaly above T_c in underdoped La_(2-x)Sr_xCuO_4
By measuring AC susceptibility using a very low amplitude of the AC field (<1
mG) it is shown that underdoped samples of La_(2-x)Sr_xCuO_4 (LASCO), are
diamagnetic in a temperature region above T_c up to a temperature T^*. This
behavior is only observed with AC fields along the c-axis whereas for fields in
the ab-plane no diamagnetism above Tc was detected. The diamagnetism is almost
frequency independent in the frequency range 0.1-10 kHz. At T* a broad step
anomaly in the specific heat is inferred through measurements of the elastic
constant c33. We suggest that the observed diamagnetism and the anomaly in the
elastic constant are associated with the existence of phase incoherent Cooper
pairs between Tc and T*.Comment: 5 pages 7 figures, to appear in Phys. rev
Paramagnetic liposomes as thermosensitive probes for MRI-guided thermal treatment: In vitro feasibility studies
In this work the potential of thermosensitive paramagnetic liposomes for in vitro temperature monitoring during radiofrequency heating has been assessed. Two thermosensitive liposome formulations with different phase-transition properties were investigated. Temperature-dependent spin-lattice (T 1) relaxivity measurements were performed at 0.24 T. Magnetic resonance imaging was performed at 2 T in liposome-containing phantom models and T 1 relaxation rates (R 1) were quantified as a function of temperature. Independent temperature measurements were performed using both thermocouple and magnetic-resonance-based methods (proton resonance frequency and diffusion-based thermometry). The relaxometric measurements showed that the T 1 relaxivity increased from low values (about 0.3 s -1mM -1 at 35 °C) to about 4 s -1mM -1 when the temperature approached and exceeded the phase-transition temperature (T c) of the liposome preparations. These data correlated well to the imaging data where an increased signal intensity was observed on T 1-weighted images at temperatures above T c. The derived R 1 maps reflected the measured liposomal temperature sensitivity and temperature quantification was possible on the basis of the measured linear temperature versus R 1 correlation in the transition range of the liposomes. The studies have therefore shown that thermosensitive paramagnetic liposomes exhibit the required temperature sensitivity to allow for an accurate mapping of the temperature changes in an in vitro imaging model. © 2008 Springer-Verlag
Effects of boundaries and density inhomogeneity on states of vortex matter in Bose--Einstein condensates at finite temperature
Most of the literature on quantum vortices predicting various states of
vortex matter in three dimensions at finite temperatures in quantum fluids is
based on an assumption of an extended and homogeneous system. It is well known
not to be the case in actual Bose--Einstein condensates in traps which are
finite systems with nonuniform density. This raises the question to what extent
one can speak of different aggregate states of vortex matter (vortex lattices,
liquids and tensionless vortex tangle) in these system. To address this point,
in the present work we focus on the finite-size, boundaries and density
inhomogeneity effects on thermal vortex matter in a Bose--Einstein condensate.
To this end we perform Monte Carlo simulations on a model system describing
trapped Bose--Einstein condensates. Throughout the paper, we draw on analogies
with results for vortex matter obtained for extended systems. This work
suggests that finiteness and intrinsic inhomogeneity of the system not
withstanding, one nonetheless can approximately invoke the notion of distinct
aggregate states of vortex matter realized at certain length scales. This might
be helpful, in particular in search of possible new states of vortex matter in
Bose--Einstein condensates with multiple components and different symmetries.Comment: 15 pages, 13 figures. Submitted to Physical Review A. High resolution
pictures will be available in published versio
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