403 research outputs found
Phase separation from electron confinement at oxide interfaces
Oxide heterostructures are of great interest both for fundamental and
applicative reasons. In particular the two-dimensional electron gas at the
LaAlO/SrTiO or LaTiO/SrTiO interfaces displays many different
physical properties and functionalities. However there are clear indications
that the interface electronic state is strongly inhomogeneous and therefore it
is crucially relevant to investigate possible intrinsic electronic mechanisms
underlying this inhomogeneity. Here the electrostatic potential confining the
electron gas at the interface is calculated self-consistently, finding that the
electron confinement at the interface may induce phase separation, to avoid a
thermodynamically unstable state with a negative compressibility. This provides
a generic robust and intrinsic mechanism for the experimentally observed
inhomogeneous character of these interfaces.Comment: 8 pages and 4 figure
Optical excitation of phase modes in strongly disordered superconductors
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the
superconducting (SC) order parameter and for clean systems are optically
inactive. Here we show that for strongly disordered superconductors phase modes
acquire a dipole moment and appear as a subgap spectral feature in the optical
conductivity. This finding is obtained with both a gauge-invariant random-phase
approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as
with a prototypical bosonic model for disordered superconductors. In the
strongly disordered regime, where the system displays an effective granularity
of the SC properties, the optically active dipoles are linked to the isolated
SC islands, offering a new perspective for realizing microwave optical devices
Inhomogeneous multi-carrier superconductivity at LaXO3/SrTiO3 (X=Al or Ti) oxide interfaces
Several experiments reveal the inhomogeneous character of the superconducting
state that occurs when the carrier density of the two-dimensional electron gas
formed at the LaXO3/SrTiO3 (X=Al or Ti) interface is tuned above a threshold
value by means of gating. Re-analyzing previous measurements, that highlight
the presence of two kinds of carriers, with low and high mobility, we shall
provide a description of multi-carrier magneto-transport in an inhomogeneous
two-dimensional electron gas, gaining insight into the properties of the
physics of the systems under investigation. We shall then show that the
measured resistance, superfluid density, and tunneling spectra result from the
percolative connection of superconducting "puddles" with randomly distributed
critical temperatures, embedded in a weakly localizing metallic matrix. We
shall also show that this scenario is consistent with the characteristics of
the superconductor-to-metal transition driven by a magnetic field. A
multi-carrier description of the superconducting state, within a weak-coupling
BCS-like model, will be finally discussed.Comment: 12 pages 10 figure
Explicit Evidence Systems with Common Knowledge
Justification logics are epistemic logics that explicitly include
justifications for the agents' knowledge. We develop a multi-agent
justification logic with evidence terms for individual agents as well as for
common knowledge. We define a Kripke-style semantics that is similar to
Fitting's semantics for the Logic of Proofs LP. We show the soundness,
completeness, and finite model property of our multi-agent justification logic
with respect to this Kripke-style semantics. We demonstrate that our logic is a
conservative extension of Yavorskaya's minimal bimodal explicit evidence logic,
which is a two-agent version of LP. We discuss the relationship of our logic to
the multi-agent modal logic S4 with common knowledge. Finally, we give a brief
analysis of the coordinated attack problem in the newly developed language of
our logic
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
Enhanced bioavailability of zeaxanthin in a milk-based formulation of wolfberry (Gou Qi Zi; Fructus barbarum L.)
Author name used in this publication: Wai Y. Chung2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe
Universal scaling of the order-parameter distribution in strongly disordered superconductors
We investigate theoretically and experimentally the statistical properties of
the inhomogeneous order-parameter distribution (OPD) at the verge of the
superconductor-insulator transition (SIT). We find within two prototype
fermionic and bosonic models for disordered superconductors that one can
identify a universal rescaling of the OPD. By performing scanning-tunneling
microscopy experiments in three samples of NbN with increasing disorder we show
that such a rescaling describes also with an excellent accuracy the
experimental data. These results can provide a breakthrough in our
understanding of the SIT.Comment: 11 pages, 8 figures, revised version submitted to PR
Marginal Pinning of Quenched Random Polymers
An elastic string embedded in 3D space and subject to a short-range
correlated random potential exhibits marginal pinning at high temperatures,
with the pinning length becoming exponentially sensitive to
temperature. Using a functional renormalization group (FRG) approach we find
, with the
depinning temperature. A slow decay of disorder correlations as it appears in
the problem of flux line pinning in superconductors modifies this result, .Comment: 4 pages, RevTeX, 1 figure inserte
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