262 research outputs found
Exponential Temperature Dependence of Penetration Depth in Single Crystal MgB_2
The temperature dependence of the London penetration depth,lambda(T), was
measured in both single crystal and polycrystalline MgB_2 samples by a
high-resolution, radio frequency technique. A clear exponential temperature
dependence of lambda(T) was observed at low temperature, indicating s-wave
pairing. A BCS fit to the lowest temperature data gives an in-plane energy gap
Delta of 2.6\pm0.2 meV (2\Delta/T_c=1.5\pm0.1), which is significantly smaller
than the standard BCS weak coupling value of 3.5. We find that the data are
best described by a two-gap model.Comment: Minor corrections added, as per published articl
Spins in the Vortices of a High Temperature Superconductor
Neutron scattering is used to characterise the magnetism of the vortices for
the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO(4)
(x=0.163) in an applied magnetic field. As temperature is reduced, low
frequency spin fluctuations first disappear with the loss of vortex mobility,
but then reappear. We find that the vortex state can be regarded as an
inhomogeneous mixture of a superconducting spin fluid and a material containing
a nearly ordered antiferromagnet. These experiments show that as for many other
properties of cuprate superconductors, the important underlying microscopic
forces are magnetic
Cooper pairing near charged black holes
We show that a quartic contact interaction between charged fermions can lead
to Cooper pairing and a superconducting instability in the background of a
charged asymptotically Anti-de Sitter black hole. For a massless fermion we
obtain the zero mode analytically and compute the dependence of the critical
temperature T_c on the charge of the fermion. The instability we find occurs at
charges above a critical value, where the fermion dispersion relation near the
Fermi surface is linear. The critical temperature goes to zero as the marginal
Fermi liquid is approached, together with the density of states at the Fermi
surface. Besides the charge, the critical temperature is controlled by a four
point function of a fermionic operator in the dual strongly coupled field
theory.Comment: 1+33 pages, 4 figure
Anisotropic Structure of the Order Parameter in FeSe0.45Te0.55 Revealed by Angle Resolved Specific Heat
The symmetry and structure of the superconducting gap in the Fe-based
superconductors are the central issue for understanding these novel materials.
So far the experimental data and theoretical models have been highly
controversial. Some experiments favor two or more constant or nearly-constant
gaps, others indicate strong anisotropy and yet others suggest gap zeros
("nodes"). Theoretical models also vary, suggesting that the absence or
presence of the nodes depends quantitatively on the model parameters. An
opinion that has gained substantial currency is that the gap structure, unlike
all other known superconductors, including cuprates, may be different in
different compounds within the same family. A unique method for addressing this
issue, one of the very few methods that are bulk and angle-resolved, calls for
measuring the electronic specific heat in a rotating magnetic field, as a
function of field orientation with respect to the crystallographic axes. In
this Communication we present the first such measurement for an Fe-based
high-Tc superconductor (FeBSC). We observed a fourfold oscillation of the
specific heat as a function of the in-plane magnetic field direction, which
allowed us to identify the locations of the gap minima (or nodes) on the Fermi
surface. Our results are consistent with the expectations of an extended s-wave
model with a significant gap anisotropy on the electron pockets and the gap
minima along the \Gamma M (or Fe-Fe bond) direction.Comment: 32 pages, 7 figure
Linear-T resistivity and change in Fermi surface at the pseudogap critical point of a high-Tc superconductor
A fundamental question of high-temperature superconductors is the nature of
the pseudogap phase which lies between the Mott insulator at zero doping and
the Fermi liquid at high doping p. Here we report on the behaviour of charge
carriers near the zero-temperature onset of that phase, namely at the critical
doping p* where the pseudogap temperature T* goes to zero, accessed by
investigating a material in which superconductivity can be fully suppressed by
a steady magnetic field. Just below p*, the normal-state resistivity and Hall
coefficient of La1.6-xNd0.4SrxCuO4 are found to rise simultaneously as the
temperature drops below T*, revealing a change in the Fermi surface with a
large associated drop in conductivity. At p*, the resistivity shows a linear
temperature dependence as T goes to zero, a typical signature of a quantum
critical point. These findings impose new constraints on the mechanisms
responsible for inelastic scattering and Fermi surface transformation in
theories of the pseudogap phase.Comment: 24 pages, 6 figures. Published in Nature Physics. Online at
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1109.htm
Diving into the vertical dimension of elasmobranch movement ecology
Knowledge of the three-dimensional movement patterns of elasmobranchs is vital to understand their ecological roles and exposure to anthropogenic pressures. To date, comparative studies among species at global scales have mostly focused on horizontal movements. Our study addresses the knowledge gap of vertical movements by compiling the first global synthesis of vertical habitat use by elasmobranchs from data obtained by deployment of 989 biotelemetry tags on 38 elasmobranch species. Elasmobranchs displayed high intra- and interspecific variability in vertical movement patterns. Substantial vertical overlap was observed for many epipelagic elasmobranchs, indicating an increased likelihood to display spatial overlap, biologically interact, and share similar risk to anthropogenic threats that vary on a vertical gradient. We highlight the critical next steps toward incorporating vertical movement into global management and monitoring strategies for elasmobranchs, emphasizing the need to address geographic and taxonomic biases in deployments and to concurrently consider both horizontal and vertical movements
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