6,500 research outputs found
Investigating superconductor-insulator transition in thin films using drag resistance:Theoretical analysis of a proposed experiment
The magnetically driven superconductor-insulator transition in amorphous thin
films (e.g., InO, Ta) exhibits several mysterious phenomena, such as a putative
metallic phase and a huge magnetoresistance peak. Unfortunately, several
conflicting categories of theories, particularly quantum-vortex condensation,
and normal region percolation, explain key observations equally well. We
propose a new experimental setup, an amorphous thin-film bilayer, where a drag
resistance measurement would clarify the role quantum vortices play in the
transition, and hence decisively point to the correct picture. We provide a
thorough analysis of the device, which shows that the vortex paradigm gives
rise to a drag with an opposite sign and orders of magnitude larger than the
drag measured if competing paradigms apply.Comment: 5 pages, 2 figure
Particle-hole symmetric localization in optical lattices using time modulated random on-site potentials
The random hopping models exhibit many fascinating features, such as
diverging localization length and density of states as energy approaches the
bandcenter, due to its particle-hole symmetry. Nevertheless, such models are
yet to be realized experimentally because the particle-hole symmetry is easily
destroyed by diagonal disorder. Here we propose that a pure random hopping
model can be effectively realized in ultracold atoms by modulating a disordered
onsite potential in particular frequency ranges. This idea is motivated by the
recent development of the phenomena called "dynamical localization" or
"coherent destruction of tunneling". Investigating the application of this idea
in one dimension, we find that if the oscillation frequency of the disorder
potential is gradually increased from zero to infinity, one can tune a
non-interacting system from an Anderson insulator to a random hopping model
with diverging localization length at the band center, and eventually to a
uniform-hopping tight-binding model.Comment: 7 pages, 5 figure
Theoretical analysis of drag resistance in amorphous thin films exhibiting superconductor-insulator transitions
The magnetical field tuned superconductor-insulator transition in amorphous thin films, e.g., Ta and InO,
exhibits a range of yet unexplained curious phenomena, such as a putative low-resistance metallic phase
intervening the superconducting and the insulating phase, and a huge peak in the magnetoresistance at large
magnetic field. Qualitatively, the phenomena can be explained equally well within several significantly different
pictures, particularly the condensation of quantum vortex liquid, and the percolation of superconducting
islands embedded in normal region. Recently, we proposed and analyzed a distinct measurement in Y. Zou,
G. Refael, and J. Yoon, Phys. Rev. B 80, 180503 (2009) that should be able to decisively point to the correct
picture: a drag resistance measurement in an amorphous thin-film bilayer setup. Neglecting interlayer tunneling,
we found that the drag resistance within the vortex paradigm has opposite sign and is orders of magnitude
larger than that in competing paradigms. For example, two identical films as in G. Sambandamurthy, L. W.
Engel, A. Johansson, and D. Shahar, Phys. Rev. Lett. 92, 107005 _2004_ with 25 nm layer separation at
0.07 K would produce a drag resistance ~10^(−4) Ω according the vortex theory but only ~10^(−12) Ω for the
percolation theory. We provide details of our theoretical analysis of the drag resistance within both paradigms
and report some results as well
Effect of inhomogeneous coupling on BCS superconductors
We investigate the influence of inhomogeneity in the pairing coupling constant U(r-vector) on dirty BCS superconductors, focusing on Tc, the order parameter Delta(r-vector), and the energy gap Eg(r-vector). Within mean-field theory, we find that when the length scale of the inhomogeneity is comparable to or larger than the coherence length, the ratio 2Eg/Tc is significantly reduced from that of a homogeneous superconductor, while in the opposite limit, this ratio stays unmodified. In two dimensions, when strong phase fluctuations are included, the Kosterlitz-Thouless temperature TKT is also studied. We find that when the inhomogeneity length scale is much larger than the coherence length, 2Eg/TKT can be larger than the usual BCS value. We use our results to qualitatively explain recent experimental observation of a surprisingly low value of 2Eg/Tc in thin films
Reentrant phase transitions of higher-dimensional AdS black holes in dRGT massive gravity
We study the criticality and phase transition in the extended phase
space of anti-de Sitter (AdS) black holes in higher-dimensional de Rham,
Gabadadze and Tolley (dRGT) massive gravity, treating the cosmological constant
as pressure and the corresponding conjugate quantity is interpreted as
thermodynamic volume. Besides the usual small/large black hole phase
transitions, the interesting thermodynamic phenomena of reentrant phase
transitions (RPTs) are observed for black holes in all -dimensional
spacetime when the coupling coefficients of massive potential satisfy
some certain conditions.Comment: 14 pages, several references are added, v2: published in EPJ
Reentrant phase transitions and triple points of topological AdS black holes in Born-Infeld-massive gravity
Motivated by recent developments of black hole thermodynamics in de Rham,
Gabadadze and Tolley(dRGT) massive gravity, we study the critical behaviors of
four-dimensional topological Anti-de Sitter(AdS) black holes in the presence of
Born-Infeld nonlinear electrodynamics by treating the cosmological constant as
pressure and the corresponding conjugate quantity is interpreted as
thermodynamic volume. It shows that besides the Van der Waals-like SBH/LBH
phase transitions appears, the so-called reentrant phase transitions (RPTs) are
also observed when the coupling coefficients of massive potential and
Born-Infeld parameter satisfy some certain conditions.Comment: arXiv admin note: text overlap with arXiv:1612.08056; text overlap
with arXiv:1402.2837, arXiv:1306.5756 by other autho
Clausius-Clapeyron relations for first-order phase transitions in bilayer quantum Hall systems
A bilayer system of two-dimensional electron gases in a perpendicular magnetic field exhibits rich phenomena. At total filling factor ν_(tot)=1, as one increases the layer separation, the bilayer system goes from an interlayer-coherent exciton condensed state to an incoherent phase of, most likely, two decoupled composite-fermion Fermi liquids. Many questions still remain as to the nature of the transition between these two phases. Recent experiments have demonstrated that spin plays an important role in this transition. Assuming that there is a direct first-order transition between the spin-polarized interlayer-coherent quantum Hall state and spin partially polarized composite Fermi-liquid state, we calculate the phase boundary (d/l)_c as a function of parallel magnetic field, NMR/heat pulse, temperature, and density imbalance, and compare with experimental results. Remarkably good agreement is found between theory and various experiments
- …