134 research outputs found
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
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
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. Š 2008 Springer-Verlag
Strong Pinning and Plastic Deformations of the Vortex Lattice
We investigate numerically the dynamically generated plastic deformations of
a 3D vortex lattice (VL) driven through a disorder potential with isolated,
strong pinning centers (point-like or extended along the field direction). We
find that the VL exhibits a very peculiar dynamical behavior in the plastic
flow regime, in particular, topological excitations consisting of three or four
entangled vortices are formed. We determine the critical current density
and the activation energy for depinning in the presence of a finite
density of strong pinning centers.Comment: 12 pages, TeX type, Postscript figure
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Electronic Properties of Chlorine, Methyl, and Chloromethyl as Substituents to the Ethylene Group-Viewed from the Core of Carbon
âSubstituent effectsâ is an important and useful concept in organic chemistry. Although there are many approaches to parametrizing the electronic and steric effects of substituents, the physical basis for the parameters is often unclear. The purpose of the present work is to explore the properties of chemical shifts in carbon 1s energies as a well-defined basis for characterizing substituents to an ethylene CâC moiety. To this end, high-resolution carbon 1s photoelectron spectra of six chloro-substituted ethenes and seven chloro-substituted propenes have been measured in the gas phase. Site-specific adiabatic ionization energies have been determined from the spectra using theoretical ab initio calculations to predict the vibrational structures. For two molecules, 3-chloropropene and 2,3-dichloropropene, the spectral analyses give quantitative results for the conformer populations. The observed shifts have been analyzed in terms of initial-state (potential) and relaxation effects, and charge relaxation has also been analyzed by means of natural resonance theory. On the basis of core-level spectroscopy and models, chlorine, methyl, and chloromethyl have been characterized in terms of their effect on the carbon to which they are attached (Îą site) as well as the neighboring sp² carbon (β site). The derived spectroscopic substituent parameters are characterized by both inductive (electronegativity) effects and the ability of each substituent to engage in electron delocalization via the Ď system. Moreover, the adopted approach is extended to include substituentâsubstituent interaction parameters
Conformation of 1,4-dihydropyridine â planar or boat-like?
AbstractThe geometry of the 1,4-dihydropyridine molecule was completely optimized employing three different ab initio basis sets (6â31 G*, 4â31 G, STOâ3G). The most reliable 6â31G* basis set provides a very flat boat conformation which may easily undergo defolding to a planar ring arrangement. This result is discussed with respect to enzymatic redox cofactors and the pharmacological activity of dihydropyridine calcium antagonists
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