Electron transport and anisotropy of the upper critical magnetic field in a Ba0.68K0.32Fe2As2 single crystals

Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite for high-Tc superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, Hc2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of the electrical resistivity, \Gamma(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single crystals in zero magnetic field and for Ba0.68K0.32Fe2As2 as well in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the \Gamma point in reciprocal space. From our data, we construct an H-T phase diagram for the inter-plane (H || c) and in-plane (H || ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data for underdoped 122 FeAs compounds, we find that Hc2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, {\gamma} = Habc2/Hcc2, is about 2.2 at Tc. For both field orientations we find a concave curvature of the Hc2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism we perfectly can describe Hc2(T) and its anisotropy.Comment: 7 pages, 3 figure

Superconductivity at 5.4 K in β\beta-Bi2_2Pd

We investigate bulk superconductivity in a high-quality single crystal of Bi$_2$Pd ($\beta$-Bi$_2$Pd, space group; I4/mmm) at temperatures less than 5.4 K by exploring its electrical resistivity, magnetic susceptibility, and specific heat. The temperature dependence of the electrical resistivity shows convex-upward behaviors at temperatures greater than 40-50 K, which can be explained by a parallel-resistor model. In addition, we demonstrate that this material is a multiple-band/multiple-gap superconductor based on the temperature dependences of the specific heat and the upper critical field.Comment: 4 pages, 3 figure

A Rapid Scanning Inspection Method for Insulated Ferromagnetic Tubing

Until the present there has been no effective way to rapidly scan thermally insulated refinery or process piping for corrosion or thin wall. Such defects, if left unattended, can lead to wasteful losses of time, energy and money. To date the most common means of locating such defects has been random spot measurement of suspect regions using ultrasonic or radiographic techniques. The obvious weakness of such an approach is the low probability of selecting the region of most severe corrosion in a long expanse of insulated piping. PA Incorporated has developed an electromagnetic inspection device which rapidly scans nearly 100 percent of an insulated pipeline (flanges and tees excluded) and provides a quantitative (2 percent) measurement of average wall thickness and qualitative evaluation of external and internal corrosion. The value of the device lies in its ability to quickly locate major problem areas through thermal insulation which can then be examined in more detail by removing insulation and using other techniques. This paper describes the device, the inspection results to date, and the unique advantages of this new inspection tool

THERMAL EXPANSlON OF V3Si

Des mesures dilatométriques effectuées entre 4,2 K et 293 K sur cinq echantillons de V3Si monocristallins n'ont pas permis de confirmer le coefficient de dilatation négatif extraordinaire observé juste en dessus de Tc dans des échantillons polycristallins par Smith et al. Dans les deux échantillons présentant la transformation martensitique l'anomalie dilatométrique associée à la transformation est observée au moins jusque 60 K.Measurements of the thermal expansion coefficient of five single crystals of V3Si in the temperature range 4.2 - 293 K fail to reveal the unusual negative values just above Tc reported by Smith et al. for polycrystalline samples. In two of the samples exhibiting a lattice transformation at low temperature associated anomalies are found to extend up to at least 60 K

Exponential temperature dependence of the electrical resistivity of V3Si

URL:http://link.aps.org/doi/10.1103/PhysRevB.13.5199 DOI:10.1103/PhysRevB.13.5199We report the results and interpretation of precision measurements of the electrical resistivity of V3Si in the temperature range Tc≤T≤77 K. Four samples of a wide range in residual resistance ratio (RRR) and with 15.1≤Tc≤16.8 K have been investigated. We find that the resistivity of all the samples is well described by a temperature dependence of the form ρ(T)=ρ0+bTn+dexp(-T0/T), where n falls within the range 1≤n≤2. The parameter d is sensitive to the RRR of the sample, whereas b and T0 are relatively insensitive. The characteristic temperature T0≈175 K is essentially independent of the choice of n within the stated range. A similar exponential term in ρ(T) of Nb3Sn with T0≈85 K has previously been identified by Woodard and Cody. Nothing that in both V3Si and Nb3Sn the value of T0 corresponds to the energy of [100] TA phonons near the zone boundary, we argue that the exponential term is due either to phonon-assisted interband scattering or intraband umklapp scattering. The reasons for the scattering effectiveness of this phonon will be discussed in light of recent band-structure calculations by Mattheiss and previous band models proposed to explain the anomalous normal-state properties of A-15 compounds. The nonexponential term in the resistivity is more difficult to characterize empirically and its origin is correspondingly more uncertain. We suggest it arises from intraband electron-electron scattering. The temperature dependence of the resistivity will be discussed with respect to anomalies observed in the low-temperature elastic constants, magnetic susceptibility, and specific heat of V3Si.Supported by the Army Research Office (Durham) under Grant DA-AROD-31-124-73-G158