What can be learned from the normal state resistivity?
Abstract
A new comprehensive procedure for estimating the electron-phonon coupling constant λ from the high temperature resistivity ϱ(T) of the so-called “Bad Actor” superconductors has been developed. The procedure is applied to Nb3Sn and V3Si for which we find λ(T≈Tc) = 2.3±0.2 and λ(T≈Tc)=2.1±0.2 respectively.
At low temperatures (Tc<T<0.1θDebye) a new universal transition in the resistivity of strongly coupled superconductors has been discovered experimentally. This effect opens up an entirely new way of estimating λ.
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Pinning energy and irreversibility line in superconducting GdSr <inf>2</inf>RuCu<inf>2</inf>O<inf>8</inf>
2004, Physica C: Superconductivity and its ApplicationsTransport properties of superconducting polycrystalline GdSr2RuCu2O8 have been studied by resistivity versus temperature measurements in external magnetic fields up to 8 T. The analysis of the experimental data shows that an activated pinning mechanism is present with low values of the pinning energy and that an irreversibility line (IL) can be identified as the limit of dissipation due to a two-dimensional (2D) vortex motion. Comparing these results with those commonly observed in the case of other high temperature superconductors, a similarity with the 2D vortex motion for BSCCO is observed. This fact has been discussed considering the possible role played by the magnetic Ru ions on the superconducting properties of the compound.
Properties of ErNi<inf>2</inf>B<inf>2</inf>C superconducting thin films
1999, Physica C: Superconductivity and its ApplicationsMagnetic rare-earth (RE) borocarbide superconductors (RE-Ni2B2C) exhibit a wide variety of phenomena due to the interplay between their superconducting and magnetic properties. Recently, high quality thin films of the `non-magnetic' compound YNi2B2C were successfully obtained. In the present report, the `in-situ' deposition of ErNi2B2C thin films by planar magnetron sputtering is presented together with a comprehensive set of characterizations, including d.c. resistivity, critical current, magnetization, critical fields and quasiparticle density of states by low temperature scanning tunnel microscope (STM). Experimental data are compared with currently available results on single crystals.
Characteristic features of the exotic superconductors
1998, Physics ReportThe exotic superconductors of this survey are those defined by Uemura and co-workers — the materials which approximately satisfy an empirical relation Tc δ λL−2, where λL is the London penetration depth. As superconductors these materials are strange in many respects — in their chemistry and crystal structures, in many of their electronic and superconducting properties, and in their often conspicuously high transition temperatures. This category includes all of the presently known high-temperature superconductors. We now examine their unusual features in considerable detail, to sort out the features which are apparently universal (such as strong type-II behavior and high resistivity), or which are non-universal but common enough to be considered typical for this class (such as gap nodes). Several characteristics of the crystal chemistry are identified. Although fragments of this program have been reported often, this is the first attempt at a comprehensive examination. There appears to be a quasi-continuum of electronic behaviors, ranging from strongly exotic down to barely exotic cases. The location of a material within this continuum may thus depend on the relative strength of some “new” mechanism, as compared to the conventional phonon mechanism. In various places this survey is broadened to include other “strange formula” superconductors, since many other materials share the typical crystal-chemistry features of the exotic materials.
Transport and tunneling measurements in superconducting YNi<inf>2</inf>B<inf>2</inf>C
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Electrical and optical properties of silicide single crystals and thin films
1993, Materials Science ReportsElectrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ(T) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Krönig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of the experimental spectra is based on the calculated electronic structures or optical functions. Moreover, it is shown how the optical studies contribute to assess definitively the semiconducting character of some disilicides. Specific-heat measurements on single crystals between 0.1 and 8 K are reported. The Debye temperature and the density of electronics states at the Fermi surface are deduced from the lattice and electronic contributions, respectively. Some silicides have been found superconductors with small electron—phonon coupling constants. Emphasis is given to the comparison between the properties deduced from these studies and those obtained from the analysis of electrical transport data. The final part of this review is devoted to the calculation of some microscopic physical quantities, as for example the electron mean free path, the charge-carrier density, the Fermi velocity. The parameters of the best fit to the experimental resistivity curves, the free-carrier parameters obtained from infrared spectra and the density of electronic states at the Fermi surface determined from specific-heat measurements were used in such evaluations.
YBa<inf>2</inf>Cu<inf>4</inf>O<inf>8</inf>: A strong coupling high-T<inf>c</inf> superconductor?
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