117 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
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
High-pressure effects on isotropic superconductivity in the iron-free layered pnictide superconductor BaPd2As2
While the layered 122 iron arsenide superconductors are highly anisotropic,
unconventional, and exhibit several forms of electronic orders that coexist or
compete with superconductivity in different regions of their phase diagrams, we
find in the absence of iron in the structure that the superconducting
characteristics of the end member BaPd2As2 are surprisingly conventional. Here
we report on complementary measurements of specific heat, magnetic
susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron
high pressure x-ray diffraction measurements supplemented with theoretical
calculations for BaPd2As2. Its superconducting properties are completely
isotropic as demonstrated by the critical fields, which do not depend on the
direction of the applied field. Under the application of high pressure, Tc is
linearly suppressed, which is the typical behavior of classical phonon-mediated
superconductors with some additional effect of a pressure-induced decrease in
the electronic density of states and the electron-phonon coupling parameters.
Structural changes in the layered BaPd2As2 have been studied by means of
angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we
observed pressure induced lattice distortions manifesting as the discontinuity
and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk
modulus is B0=40(6) GPa below 12 GPa and B0=142(3) GPa below 27.2 GPa
Screw dynamo in a time-dependent pipe flow
The kinematic dynamo problem is investigated for the flow of a conducting
fluid in a cylindrical, periodic tube with conducting walls. The methods used
are an eigenvalue analysis of the steady regime, and the three-dimensional
solution of the time-dependent induction equation. The configuration and
parameters considered here are close to those of a dynamo experiment planned in
Perm, which will use a torus-shaped channel. We find growth of an initial
magnetic field by more than 3 orders of magnitude. Marked field growth can be
obtained if the braking time is less than 0.2 s and only one diverter is used
in the channel. The structure of the seed field has a strong impact on the
field amplification factor. The generation properties can be improved by adding
ferromagnetic particles to the fluid in order to increase its relative
permeability,but this will not be necessary for the success of the dynamo
experiment. For higher magnetic Reynolds numbers, the nontrivial evolution of
different magnetic modes limits the value of simple `optimistic' and
`pessimistic' estimates.Comment: 10 pages, 12 figure
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.
Optical and thermodynamic properties of the high-temperature superconductor HgBa_2CuO_4+delta
In- and out-of-plane optical spectra and specific heat measurements for the
single layer cuprate superconductor Hg-1201 at optimal doping (Tc = 97 K) are
presented. Both the in-plane and out-of-plane superfluid density agree well
with a recently proposed scaling relation rho_{s}=sigma_{dc}T_{c}. It is shown
that there is a superconductivity induced increase of the in-plane low
frequency spectral weight which follows the trend found in underdoped and
optimally doped Bi-2212 and optimally doped Bi-2223. We observe an increase of
optical spectral weight which corresponds to a change in kinetic energy of
approximately 0.5 meV/Cu which is more than enough to explain the condensation
energy. The specific heat anomaly is 10 times smaller than in YBCO and 3 times
smaller than in Bi-2212. The shape of the anomaly is similar to the one
observed in YBCO showing that the superconducting transition is governed by
thermal fluctuations.Comment: 11 pages, 13 figure
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