143 research outputs found
Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe2As2
We report specific-heat and resistivity experiments performed in parallel in
a Bridgman-type of pressure cell in order to investigate the nature of
pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The
presence of a pronounced specific-heat anomaly at Tc reveals a bulk nature of
the superconducting state. The thermodynamic transition temperature differs
dramatically from the onset of the resistive transition. Our data indicates
that superconductivity occurs in the vicinity of a crystallographic phase
transition. We discuss the discrepancy between the two methods as caused by
strain-induced superconducting precursors formed above the bulk thermodynamic
transition due to the vicinity of the structural instability
The Fulde-Ferrell-Larkin-Ovchinnikov State in the Organic Superconductor k-(BEDT-TTF)2Cu(NCS)2 as Observed in Magnetic Torque Experiments
We present magnetic-torque experiments on the organic superconductor
k-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D
superconducting layers. The experiments show a crossover from a second-order to
a first-order transition when the upper critical field reaches 21 T. Beyond
this field, which we interpret as the Pauli limit for superconductivity, the
upper critical field line shows a pro-nounced upturn and a phase transition
line separates the superconducting state into a low- and a high-field phase. We
interpret the data in the framework of a Fulde-Ferrell-Larkin-Ovchinnikov
state.Comment: 2 pages, 1 figur
Timber Felling Time, Costs, and Productivity in Arkansas
Sixteen stands were harvested by either clearcut, shelterwood, group selection, or single-tree selection methods. Harvest productivity was evaluated in four consecutive years (1991 through 1994). Three of the stands had uneven-aged structure, the other 13 were typical, mature, even-aged stands. Harvest intensity (proportion of basal area removed) ranged from 0.27 to 1.00. Logging contractors used one to three sawyers with production chain saws to fell trees on all 16 tracts. There was no statistical difference in production rate between sawyers on the same stand. Harvested sites were similar in slope, average diameter at breast height (DBH) and pre-harvest number of stems by two inch diameter class. Total felling time (including walk, acquire, fell, and limb-top times) was inversely related to harvesting intensity and directly related to stem DBH. Factors affecting total felling time (in decreasing order of importance) were DBH of harvested stems, intertree distance, and harvest intensity. Felling productivity (100 cubic feet/hour) was found to be highest under high intensity harvests oflarge trees and lowest under low intensity harvests of small trees. Productivity was more sensitive to stem diameter than harvest intensity. Felling cost was shown to have an inverse relationship with felling productivity
Specific heat and magnetization of a ZrB12 single crystal: characterization of a type II/1 superconductor
We measured the specific heat, the magnetization, and the magnetoresistance
of a single crystal of ZrB12, which is superconducting below Tc ~ 6 K. The
specific heat in zero field shows a BCS-type superconducting transition. The
normal- to superconducting-state transition changes from first order (with a
latent heat) to second order (without latent heat) with increasing magnetic
field, indicating that the pure compound is a low-kappa, type-II/1
superconductor in the classification of Auer and Ullmaier [J. Auer and H.
Ullmaier, Phys. Rev.B 7, 136 (1973)]. This behavior is confirmed by
magnetization measurements. The H-T phase diagram based on specific-heat and
magnetization data yields Hc2(0) =550 G for the bulk upper critical field,
whereas the critical field defined by vanishing resistance is a surface
critical field Hc3(0) ~ 1000 G.Comment: 17 pages, 8 figures, submitted to PR
Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6
The superconductor YB6 has the second highest critical temperature Tc among
the boride family MBn. We report measurements of the specific heat,
resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K,
using a single crystal with Tc = 7.2 K. The superconducting gap is
characteristic of medium-strong coupling. The specific heat, resistivity and
expansivity curves are deconvolved to yield approximations of the phonon
density of states, the spectral electron-phonon scattering function and the
phonon density of states weighted by the frequency-dependent Grueneisen
parameter respectively. Lattice vibrations extend to high frequencies >100 meV,
but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of
yttrium ions in oversized boron cages, appears to provide most of the
superconducting coupling and gives rise to an unusual temperature behavior of
several observable quantities. A surface critical field Hc3 is also observed.Comment: 29 pages, 5 tables, 17 figures. Accepted for publication in Phys.
Rev.
Multi-band Superconductivity in the Chevrel Phases SnMo6S8 and PbMo6S8
Sub-Kelvin scanning tunnelling spectroscopy in the Chevrel Phases SnMo6S8 and
PbMo6S8 reveals two distinct superconducting gaps with Delta_1 = 3 meV, Delta_2
~ 1.0 meV and Delta_1 = 3.1 meV, Delta_2 ~ 1.4 meV respectively. The gap
distribution is strongly anisotropic, with Delta_2 predominantly seen when
scanning across unit-cell steps on the (001) sample surface. The spectra are
well-fitted by an anisotropic two-band BCS s-wave gap function. Our
spectroscopic data are confirmed by electronic heat capacity measurements which
also provide evidence for a twin-gap scenario.Comment: 5 pages, 4 figure
Muon-Spin Rotation Study of the Ternary Noncentrosymmetric Superconductors Li2Pd x Pt3− x B
We investigated the superconducting state of the noncentrosymmetric superconductors Li2Pd x Pt3−x B with superconducting transition temperature T c=5.16(8)K (x=2.25), 3.56(8)K (x=1.5) and 2.60K (x=0) by means of muon-spin rotation (μSR) and specific heat experiments. The μSR relaxation rate σ sc was found to be constant at low temperatures for all the compounds. Data taken at different magnetic fields show that the magnetic penetration depth λ is field-independent for Li2Pd2.25Pt0.75B and Li2Pt3B. The electronic contribution to the specific heat measured in Li2Pd1.5Pt1.5B and Li2Pt3B increases exponentially at the lowest temperatures. These features suggest that the whole family of Li2Pd x Pt3−x B comprises single-gap s-wave superconductors across the entire doping regim
Superconducting transitions of intrinsic arrays of weakly coupled one-dimensional superconducting chains: the case of the extreme quasi-1D superconductor Tl(2)Mo(6)Se(6)
International audienceTl(2)Mo(6)Se(6) represents a model system for quasi-one-dimensional (quasi-1D) superconductors. We investigate its superconducting transition in detail by means of electrical transport experiments on high-quality single crystalline samples with onset T(c) = 6.8 K. Our measurements indicate a highly complex superconducting transition that occurs in different stages, with a characteristic bump in the resistivity and distinct plateau structures in the supercurrent gap imaged by V-I curves. We interpret these features as fingerprints of the gradual establishment of global phase coherence in an array of weakly coupled parallel 1D superconducting bundles. In this way, we demonstrate that superconducting Tl(2)Mo(6)Se(6) behaves like an intrinsic array of proximity or Josephson junctions, undergoing a complex superconducting phase-ordering transition at 4.5 K that shows many similarities to the Berezinskii-Kosterlitz-Thouless transition
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