Critical Field Measurements Research Articles

Superconducting and Normal State Properties of the A3C60 Compounds

The A 3 C 60 compounds, where A=alkali atom, form fcc -lattices (cryolite structure) and are highly ionic [ A +]3·[ C 60]3−. The C 60 icosahedra are located in sites of local cubic symmetry thereby preserving the degeneracy of the t 1u orbitals, allowing for the formation of a narrow half filled band of a width comparable to or smaller than the various molecular excitation energies. The T c - s of the dozen or so compounds already synthesized span the range from 2–33 K; the variation of T c with pressure and from material to material suggests that the attraction responsible for the Cooper pair formation is a local property of the C 60 molecules, so that variations of the density-of-state ρ(ε F ) at the Fermi level (i.e. bandwidth) determine T c . The values of the superconducting parameters, λ L and ξ o , determined from critical field and µSR measurements, favor a local pairing picture, but are at best only marginally supportive for the expected density-ofstate variations. The so far available 13 C nuclear relaxation, susceptibility and ESR measurements in the normal state manifest several features that are more related to the complex correlated nature of the C 60 molecules than to the freeelectron band effects of the simple lattice they are arranged in.

Influence of 5f electrons on transport properties in uranium-based metallic glasses

In order to study the influence of 5f electrons on the superconducting and the magnetic properties of uranium glasses, X-ray, upper critical magnetic field and susceptibility measurements were performed on three binary metallic glass systems: U-Fe, U-Co and U-Ni. For each alloy system a continuous range of uranium-rich compositions prepared by splat cooling has been examined. It is found that these samples have ferromagnetic inclusions which may be due either to ferromagnetic impurities or to the formation of clusters with high transition metal densities. The superconducting transition temperature is reduced by strong spin fluctuations. Both the superconductivity and the spin fluctuation are due to the presence of the 5f electrons.

Low temperature measurements of Hc2 in HTSC using megagauss magnetic fields

Measurements of the upper critical fields in YBCO and BSCCO polycrystalline samples and YBCO epitaxial films were carried out at low temperatures in a pulsed ultrahigh magnetic field obtained in a magnetocumulative generator MC-1. The values of the critical fields at 4.2 K were found to be 168±26 T and 200±25 T for YBCO and BSCCO polycrystalline samples, respectively, and 110±10 T for YBCO films.

Superconducting and normal state properties of the A 3C 60 compounds

This report is a critical review of the measurements and their interpretations of the normal and superconducting state of the A 3C 60 compounds, where A = alkali atom. These compounds are highly ionic [A +] 3 · [C 60] 3− and form fcc lattices (cryolite structure) which locate the C 60 icosahedra in sites of local cubic symmetry, thereby preserving the degeneracy of the t lu orbitals, allowing for the formation of a narrow half-filled band of a width comparable to or smaller than the different molecular excitation energies. The T c -s of the more than a dozen compounds synthesized so far span the range 2–33 K; the variation of T c with pressure and from material to material is assessed as an empirical T c lattice parameter relation, suggestive that the attraction responsible for the Cooper pair formation is a local property of the C 60 molecules and variations of the density of state ρ( ϵ f ) at the Fermi level (i.e., band width) determine T c . The superconducting parameters, λ L and ξ 0 determined from critical field and μSR measurements, favoring a local pairing image, are marginally supportive of the expected density of state variations. The so far available 13C nuclear relaxation, susceptibility and ESR measurements in the normal state manifest several features more related to the complex correlated nature of the C 60 molecules than free electron band effects of the simple lattice they are arranged in. The paper emphasizes these unusual characteristics.

Low temperature direct measurements of H c2 in HTSC using megagauss magnetic fields

The measurements of upper critical fields in YBCO and BSCCO polycrystalline samples and YBCO epitaxial films were carried out at low temperatures in ultrahigh pulsed magnetic field obtained in magnetocumulative generator MC-1. The values of critical fields at 4.2 K defined by different methods were found to be 168±26 T and 200±25 T for YBCO and BSCCO polycrystalline samples, respectively, and 110±10 T for YBCO films.

New effects in highT c superconductors

We show that a proper interpretation of the critical field measurements can distinguish spin fluctuation mechanism from others in the context of the pairing hypothesis. We can predict the existence of a new mode in the mixed state of a superconductor.

Specific heat of amorphous AuxSn100-x at low temperatures-electrons and low-energy excitations

Measurements of the specific heat of amorphous AuxSn100-x films (27<or=x<or=74) between 0.35 K and 6 K in the superconducting as well as in the normal state are reported. In addition the temperature dependence of the upper critical field of these films is determined. In the superconducting state the specific heat shows a contribution proportional to the temperature which is strongly composition dependent and largest for x=74. In the author's opinion this contribution is caused by low-energy excitations well established in amorphous solids. Taking into account this contribution the specific heat in the normal state yields the electronic density of states at the Fermi energy in agreement with the data taken from critical field measurements. Using McMillan's formula the calculated bare density of states agrees with the free electron model over the whole concentration range. This result is surprising because recent Hall-effect and photoelectron-spectroscopy measurements indicate strong deviations from the free electron model, especially for samples with high Au concentrations.

Infrared Studies of the Superconducting Energy Gap and Normal State Dynamics of Y1Ba2Cu3O7 and Ba0.6K0.4BiO3

The discovery by Bednörz and Müller of superconductivity in La20xBaxCuO4 initiated an intense and fruitful search for additional perovskite-based superconducting compounds. Following the discovery and characterization of a number of important new compounds, and the development of techniques to produce them in pure crystalline form, it becomes relevant to carefully measure the fundamental physical properties of well-characterized samples of these new materials. Ultimately the goal of this work is to illuminate the basic physics of these novel materials.Traditionally many techniques, including critical field measurements, specific heat, penetration depth, magnetic resonance, and tunneling and infrared spectroscopies, have been useful for the study of superconducting elements and compounds. This article discusses the use of infrared measurements to study both cubic BaBiO3 based compounds and layered cuprates. We emphasize the work on the (ideally) stochiometric compound Y1Ba2Cu3O7, because its infrared properties have been obtained by a number of groups with good reproducibility. Infrared measurements are useful both in terms of their traditional role as a probe of the superconducting energy gap and for their ability to probe the dynamics of the normal state by measuring the conductivity vs. frequency in the crucial infrared range (~1 meV to 2 eV).

ChemInform Abstract: Flux Creep and Related Phenomena in High Temperature Superconductors

Abstract Irreversible features, which dominate the magnetic data in high temperature superconductors, are reviewed. Special attention is given to two phenomena: (i) The temperature and field dependence of the relaxation rate and its implication on measurement of critical fields. (ii) The field dependence of the magnetization which exhibits scaling features below the irreversibility line. The discrepancy in the barrier heights from magnetic relaxation and from resistivity measurements is discussed.

Broken symmetry and unconventional superconductivity in uniaxial crystals

We show that the coupling of a two-dimensional order parameter to a field that breaks the symmetry of a uniaxial crystal splits the superconducting transition, leading to two superconducting phases in zero field. The high-temperature superconducting phase is one-dimensional and exhibits the broken symmetry of the symmetry breaking field (SBF) while the two-dimensional low temperature phase spontaneously breaks time-reversal symmetry. We calculate the specific heat jumps at both transitions. We show that H c1( T), a probe of the zero field phase, displays an abrupt change in slope at the second transition. Moreover, a kink in H c2( T) occurs for fields in the basal plane reflecting a finite field phase transition. For an SBF fixed to the lattice, the symmetry of H c1 and H c2 is broken in the basal plane. The observation of all three of these features, whether intrinsic or induced by external means, on the same crystal with uniaxial symmetry would provide convincing evidence of unconventional superconductivity. Our results are in qualitative agreement with recent measurements on different samples of the heavy electron system UPt 3. We find that sizeable strong-coupling corrections are needed to explain the magnitudes of the heat capacity jumps and the splitting of the transition in some samples. Comparison of the discontinuity in the slope of H c2( T) with the data of Taillefer et al. (on a different UPt 3 crystal) leads to SBF couplings in qualitative agreement with those inferred from the heat capacity data. More recently, a kink in H c1( T) has also been observed. A quantitative comparison with specific heat and critical field measurements on the same sample would provide a stringent test of this model for UPt 3.

Critical magnetic field of disordered Zr-Cu alloys: Density of states and spin-orbit scattering time.

The critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ is measured for disordered ${\mathrm{Zr}}_{\mathit{x}}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ for 0.36x0.65 in magnetic fields up to 3.5 T and temperatures down to 30 mK. Werthamer-Helfand-Hohenberg theory is found to describe the data well. The density of states N(0) and the spin-orbit relaxation time ${\mathrm{\ensuremath{\tau}}}_{\mathrm{so}}$ are determined by fitting to the whole critical-field curve. We investigate how accurately these parameters can be obtained from critical-field measurements. ${\mathrm{\ensuremath{\tau}}}_{\mathrm{so}}^{\mathrm{\ensuremath{-}}1}$ is found to depend weakly on the Zr ionic mass. It is suggested that accurate critical-field data can give supplementing information in the study of weak localization and interaction effects. Evidence is found for some enhancement of N(0) obtained from the critical field as compared to results from the specific heat, thus confirming an earlier conjecture by Poon. Comparison with several published results suggests that this enhancement is small and only about 5--10 %.

Preparation process, crystal structure, and physical properties of the 110-K single-phase Pb-Bi-Sr-Ca-Cu-O superconductor.

The high-${\mathit{T}}_{\mathit{c}}$ (110-K) superconducting phase in the Pb-Bi-Sr-Ca-Cu-O has been isolated and its structural and physical properties investigated. Transmission-electron-microscopy results show that the structure belongs to a monoclinic system that is different from reports by other groups and in agreement with our x-ray-diffraction analysis. The refined crystal data are a=3.779 \AA{}, b=3.834 \AA{}, c=37.150 \AA{}, \ensuremath{\alpha}=\ensuremath{\beta}=90\ifmmode^\circ\else\textdegree\fi{}, \ensuremath{\gamma}=90\ifmmode^\circ\else\textdegree\fi{} 51', V=538.5 A${\mathrm{\r{}}}^{3}$, space group I112. Based on the resistivity, magnetization, critical-field, and specific-heat measurements, various parameters characterizing the monophase superconductors were determined or derived. Some of them are listed as follows: ${\mathit{T}}_{\mathit{c}\mathit{e}}$ (endpoint)=107 K, ${\mathit{T}}_{\mathit{c}\mathit{m}}$ (midpoint)=110 K, \ensuremath{\Delta}C (specific heat jump at 107 K)=5.3 J/mol K, \ensuremath{\gamma} (the Sommerfeld constant) =34.6 mJ/mol ${\mathrm{K}}^{2}$, FTHETA (Debye temperature)=418 K, ${\mathit{H}}_{\mathit{c}1}$(0)=900 G, ${\mathit{H}}_{\mathit{c}2}$(0)=184 T, and ${\mathit{J}}_{\mathit{c}}$(1.5 K, 0 T)=4.8\ifmmode\times\else\texttimes\fi{}${10}^{4}$ A/${\mathrm{cm}}^{2}$, etc. Morphology, thermal analysis, and pressure effects of the sample are also mentioned.

Flux creep and related phenomena in high temperature superconductors

Irreversible features, which dominate the magnetic data in high temperature superconductors, are reviewed. Special attention is given to two phenomena: (i) The temperature and field dependence of the relaxation rate and its implication on measurement of critical fields. (ii) The field dependence of the magnetization which exhibits scaling features below the irreversibility line. The discrepancy in the barrier heights from magnetic relaxation and from resistivity measurements is discussed.

A new method of synthesizing high-Tc superconducting materials

A new sol-gel method for the preparation of the ceramic superconductors has been developed based upon the formation of complexes of the metal cations by Ethylene Diamine Tetraacetic Acid (EDTA). The materials have been characterized by transport, magnetisation, critical field, XRD and SEM measurements. Screen printed thick films made out of the powders have been fabricated and characterized.

Superconducting phases of heavy fermion superconductors

Recent experiments, particularly ultrasonic absorption, specific heat, and upper critical field measurements, have gone a long way toward identifying the superconducting phases of UPt3 and URu2Si2. There is now no doubt that these are unconventional superconductors, and very strong evidence that they belong to the E1 representation. The Ginzburg-Landau theory of these exotic phases in the presence of a magnetic field is discussed, and compared with experiment.

Evidence for spin fluctuations in vanadium from a tunneling study of Fermi-liquid effects.

The Fermi-liquid parameter ${G}_{0}$ is found to be close to zero in vanadium, providing new experimental evidence for the importance of spin fluctuations in this material. Al/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/V and V/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/Fe tunnel junctions were used to observe the renormalization caused by Fermi-liquid interactions of the spin splitting of the vanadium's superconducting density of states in a magnetic field. This was done by fitting the theory of Rainer to the measured dynamic tunneling conductance. Excellent agreement was found with this theory, and the results were consistent with critical field measurements made on the same vanadium films. The spin-orbit scattering rate ${b}_{\mathrm{so}}$ was determined to be 0.07\ifmmode\pm\else\textpm\fi{}0.03.

Observation of large-scale inhomogeneities in a granular metal

Abstract Thick films of granular aluminium dispersed in a tellurium-rich matrix have been formed by co-deposition of the pure elements. Correlation of Rutherford backscattering and superconducting critical-field measurements demonstrates that the films are subdivided into compositionally and electrically distinct layers typically 400–600 A thick extending across large areas of the specimens. Comparison with the superconducting properties of granular Al: Al2O3 supports earlier suggestions that similar layering is present in that system.

Tunneling study of Fermi-liquid effects in amorphous gallium.

The Fermi-liquid parameter ${G}_{0}$ has been determined in amorphous gallium. Al/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/a-Ga tunnel junctions were used to observe the renormalization caused by Fermi-liquid interactions of the spin splitting of the a-Ga superconducting density of states in a magnetic field. This was done by fitting the dynamic tunneling conductance to the theory of Rainer. Excellent agreement was found with this theory, and the results were consistent with critical-field measurements made on the same films. A large renormalization was observed (${G}_{0}$=0.81\ifmmode\pm\else\textpm\fi{}0.14), as expected for this strongly coupled material. Correlation effects were also seen. The spin-orbit scattering rate ${b}_{\mathrm{SO}}$ was found to be 0.18\ifmmode\pm\else\textpm\fi{}0.03.

Upper critical field measurements in high-T c superconducting oxides

Using a pulsed quasi-static magnetic field we measured the magnetoresistance of high- T c superconducting oxides and we deduced the values of the upper critical field near T c. The influence of oxygen stoichiometry is discussed.

Transport properties of κ-(BEDT-TTF) 2Cu(NCS) 2; HC2, Its anisotropy and their pressure dependence

Electronic properties of κ-(BEDT-TTF) 2Cu(NCS) 2 are surveyed through measurements of the upper critical fields ( H c2), its anisotropy and their pressure dependence. The value of H c2 parallel to the plane (H c2//) at 1.8 K is ∼ 20 T, which is almost the same as the Pauli limit value. The anisotropy of H c2/// H c2⊥ is 19:1 at ambient pressure which reproduced Oshima's result. However, the anisostropy of H c2/// H c2⊥ increased with increasing pressure. These properties are discussed comparing the Fermi surface study and the pressure dependence on T c.