Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"

By using high magnetic field data to estimate the background conductivity, Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522 (2011)] experimental evidence that the in-plane paraconductivity in cuprates is almost independent of doping. In this Comment we also show that, in contrast with their claims, these useful data may be explained at a quantitative level in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors, extended by Carballeira and coworkers to high reduced-temperatures by introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When combined, these two conclusions further suggest that the paraconductivity in cuprates is conventional, i.e., associated with fluctuating superconducting pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur

On the energy saved by interlayer interactions in the superconducting state of cuprates

A Ginzburg-Landau-like functional is proposed reproducing the main low-energy features of various possible high-Tc superconducting mechanisms involving energy savings due to interlayer interactions. The functional may be used to relate these savings to experimental quantities. Two examples are given, involving the mean-field specific heat jump at Tc and the superconducting fluctuations above Tc. Comparison with existing data suggests, e.g., that the increase of Tc due to the so-called interlayer tunneling (ILT) mechanism of interlayer kinetic-energy savings is negligible in optimally-doped Bi-2212.Comment: 12 pages, no figures. Version history: 21-aug-2003, first version (available on http://arxiv.org/abs/cond-mat/0308423v1); 15-jan-2004, update to match Europhys. Lett. publication (minor grammar changes, updates in bibliography - e.g., refs. 5 and 26

Entanglement entropy in collective models

We discuss the behavior of the entanglement entropy of the ground state in various collective systems. Results for general quadratic two-mode boson models are given, yielding the relation between quantum phase transitions of the system (signaled by a divergence of the entanglement entropy) and the excitation energies. Such systems naturally arise when expanding collective spin Hamiltonians at leading order via the Holstein-Primakoff mapping. In a second step, we analyze several such models (the Dicke model, the two-level BCS model, the Lieb-Mattis model and the Lipkin-Meshkov-Glick model) and investigate the properties of the entanglement entropy in the whole parameter range. We show that when the system contains gapless excitations the entanglement entropy of the ground state diverges with increasing system size. We derive and classify the scaling behaviors that can be met.Comment: 11 pages, 7 figure

Pairing of Cooper Pairs in a Fully Frustrated Josephson Junction Chain

We study a one-dimensional Josephson junction chain embedded in a magnetic field. We show that when the magnetic flux per elementary loop equals half the superconducting flux quantum $\phi_0=h/2e$, a local \nbZ_2 symmetry arises. This symmetry is responsible for a nematic Luttinger liquid state associated to bound states of Cooper pairs. We analyze the phase diagram and we discuss some experimental possibilities to observe this exotic phase.Comment: 4 pages, 4 EPS figure

Boundary quantum critical phenomena with entanglement renormalization

We extend the formalism of entanglement renormalization to the study of boundary critical phenomena. The multi-scale entanglement renormalization ansatz (MERA), in its scale invariant version, offers a very compact approximation to quantum critical ground states. Here we show that, by adding a boundary to the scale invariant MERA, an accurate approximation to the critical ground state of an infinite chain with a boundary is obtained, from which one can extract boundary scaling operators and their scaling dimensions. Our construction, valid for arbitrary critical systems, produces an effective chain with explicit separation of energy scales that relates to Wilson's RG formulation of the Kondo problem. We test the approach by studying the quantum critical Ising model with free and fixed boundary conditions.Comment: 8 pages, 12 figures, for a related work see arXiv:0912.289

On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state

We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, Tc, is dominated by the uncertainty principle which, in particular, leads to a well-defined temperature, T^C, above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced-temperature, ln(T^C/Tc), is estimated to be around 0.55, i.e., above T^C \approx 1.7Tc coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different low- and high-Tc superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of ln(T^C/Tc).Comment: LaTeX, 10 pages, 3 figures, as published in Europhysics Letter

Observation of enhanced transmission for s-polarized light through a subwavelength slit

Enhanced optical transmission (EOT) through subwavelength apertures is usually obtained for p-polarized light. The present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. Transmission measurements are compared with a coupled mode model and show good qualitative agreement. Adding a waveguide can improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.Comment: 11 pages, 3 figures, submitted to Applied Physics Letter

Dynamics of entanglement of bosonic modes on symmetric graphs

We investigate the dynamics of an initially disentangled Gaussian state on a general finite symmetric graph. As concrete examples we obtain properties of this dynamics on mean field graphs of arbitrary sizes. In the same way that chains can be used for transmitting entanglement by their natural dynamics, these graphs can be used to store entanglement. We also consider two kinds of regular polyhedron which show interesting features of entanglement sharing.Comment: 14 pages, 11 figures, Accepted for publication in Physics Letters

How to escape Aharonov-Bohm cages ?

We study the effect of disorder and interactions on a recently proposed magnetic field induced localization mechanism. We show that both partially destroy the extreme confinement of the excitations occuring in the pure case and give rise to unusual behavior. We also point out the role of the edge states that allows for a propagation of the electrons in these systems.Comment: 22 pages, 20 EPS figure

Interaction induced delocalisation for two particles in a periodic potential

We consider two interacting particles evolving in a one-dimensional periodic structure embedded in a magnetic field. We show that the strong localization induced by the magnetic field for particular values of the flux per unit cell is destroyed as soon as the particles interact. We study the spectral and the dynamical aspects of this transition.Comment: 4 pages, 5 EPS figures, minor misprints correcte