Critical Temperature Superconductors Research Articles

Gap anisotropy and critical temperature of anisotropic superconductors.

We study the gap anisotropy and critical temperature of superconductors within Eliashberg theory. We consider a general anisotropy and spectral shape. In the attractive strong-coupling limit, we find a qualitatively new universal result: The gap becomes isotropic. This sets severe constraints on standard strong-coupling theories of high-${\mathit{T}}_{\mathit{c}}$ superconductors. In the weak-coupling limit, the gap is sensitive to anisotropy. We obtain exact results for the critical temperature both in the strong- and the weak-coupling limits. We propose a simple natural interpolation in the intermediate regime.

Critical temperature of superconductivity and X-ray crystallographic studies of Nb0.78Al0.17Ge0.05 of A15 structure

Compounds of niobium and aluminium and of niobium, aluminium and germanium which crystallize in the A15 structure were prepared by sintering and by floating-zone methods. Single crystals of up to 1 × 1 × 1 mm were obtained for the composition Nb0.78Al0.17Ge0.05. Transition temperatures to superconductivity ranged from midpoint values of 18.6 to 19.4 K over intervals of 3 K. Single-crystal X-ray diffraction measurements were made at 293 and at 11–12 K. Powder diffractometer data were obtained at 293 K. The structure was refined with both powder data (Rietveld technique) and single-crystal data. Photographic powder patterns were obtained from room temperature down to 11.7 K. No phase transition was observed. No evidence was found for deviations from the A15 structure.

Anisotropic screening of oxygen polarizabilities: a scenario to understand superconductivity in oxides

The highly anharmonic motion of apical oxygen in a double-well potential, related to its large unscreened polarizability, in an oxygen breathing mode configuration, enhances the electron-phonon coupling to produce high critical temperature superconductivity. It also explains the weakness of the isotope effect. Available experimental data, mainly from IR and neutron experiments, which are relevant to this mechanism are discussed.

Molecular materials III Molecular materials with increased superconducting and ferromagnetic critical temperatures

Research News/Molecular Materials III: Recent developments in the molecular approach to superconductivity, for example the work of Williams et al. in increasing the Tc to 12.8 K, and new work in the field of molecular ferromagnets, where the Tc has also been significantly increased are described.

Principles of application of high temperature superconductors to electromagnetic launch technology

A review is presented of advances in the performance of bulk high-temperature superconductors (HTSC) which permit conductor and magnet development at practical magnetic fields to be pursued for high-current applications such as electromagnetic launchers (EMLs). While early hopes for a superconductor critical temperature (T/sub c/) approaching room temperature have not been fulfilled, numerous HTSC with T/sub c/ between 60 K and 125 K exist which can be successfully processed. Some of these HTSC are well enough understood that small conductors and coils may be fabricated for operation near 20 K. Numerous physics, magnetic flux mechanics, materials processing, and structural support issues remain for resolution before large-scale coils made of HTSC can be operated at high energy storage density at temperature well above 20 K. Properties and materials processing of HTSC and their relation to EML applications technology are described. >

Highest Tc Continues to Increase Among Organic Superconductors

The first organic superconductor was discovered in 1980. Its critical temperature was less than 1 K. The highest critical temperature for superconductivity in an organic material crossed the 10-kelvin mark in 1988. That temperature has increased again: Researchers at Argonne National Laboratory now report synthesis of two new organic superconductors. The critical temperature for one is 11.6 K at ambient pressure; the second material becomes superconducting at 12.8 K under a pressure of 0.3 kbar. (See the figure on page 18.)

Superconductive critical temperature depression in the (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+δ (2223) phase by post-sinter quenching

Changes in T c resulting from variations in the post-sintering cooling rate, from liquid quenching to slow air cooling, were investigated for the 2223 phase in the Bi-Pb-Sr-Ca-Cu-O system with a nominal composition of Bi 1.6Pb 0.4Sr 2Ca 3Cu 4O x. Complex ac susceptibility measurements were used to identify the critical temperatures of the high-T c, 2223 and the low-T c, 2212 phases, as well as those of the bulk material. T c of the granular 2223 phase decreased from 107 K to 93 K after fast quenching. Similar treatment, however, raised T c of the 2212 phase from 63 K to 80 K. The opposing behaviors of these two superconducting phases, associated with variations in oxygen stoichiometry resulting from different cooling rates, and their effects on bulk T c values are discussed.

Magnetic ordering in57Fe-doped high-T c superconductors

The high-temperature superconductor, Tl2CaBa2(Cu1−xFex)2O8+δ (the 2122 compound), has been investigated by a number of techniques, including X-ray diffraction, resistance and ac susceptibility measurements, and Mossbauer spectroscopy. The procedures followed to make close to single-phase samples are described. The decrease in the critical temperature for superconductivity,Tc, is less than for the iron-doped 123 compounds. The Mossbauer spectra at 77 K and above consist of an asymmetric doublet. Below about 10 K magnetic hyperfine splitting occurs; relaxation effects are still present at 2.3 K. The spectra can be fitted with two overlapping patterns. Their origin is discussed: comparisons are made with other high-Tc superconductors.

The heat capacity and derived thermophysical properties of the high T C superconductor YBa2Cu3O7−δ from 5.3 to 350 K

The heat capacity of the perovskite high-TC superconductor YBa2Cu3O7−δ was measured from 5.3 to 350 K in an adiabatic calorimetric cryostat. A break in the heat-capacity curve, associated with the critical temperature for superconductivity was observed between 90.09 and 92.59 K. The transition temperature was identified as 91.44 K, and ΔCp,m was calculated to be 0.559R at that temperature. The lattice heat capacity was evaluated by means of the recently developed Komada/Westrum phonon distribution model and the apparent characteristic temperature ΘKW was calculated to be 107.7 K. The excess electronic heat capacity for the superconducting phase was evaluated and the energy gap was identified as 234. R K. Excess contribution, resulting from magnetic impurities, was noted below 20 K. Thermodynamic properties at selected temperatures are presented.

Soliton statistics for critical temperature of superconductivity in organic quasi-one-dimensional crystals.

We calculate microscopic parameters in the Davydov model on the basis of the bisoliton idea. We also prove that the displacement \ensuremath{\beta}(\ensuremath{\eta}) in Davydov theory satisfies the ${\mathrm{\ensuremath{\varphi}}}^{4}$ field form. Critical temperature is studied from two aspects: the statistics of lattice kinks and the exciton model considering thermal vibrations. The agreement between theoretical and experimental data for the critical temperature of organic superconductors supports Davydov's bisoliton theory.

Hydration behaviour and superconductivity of the rare earth intercalation compounds of 2HTaS 2

We have prepared the hydrated rare earth (RE) intercalation compounds of 2HTaS 2 (chemical formula RE x (H 2O) y TaS 2) by electrochemical techniques and report a systematic characterization of these substances by means of in situ dilatometry and X-ray investigations and by studying the critical temperature of superconductivity. Single phase compounds are obtained for 0.09 < x < 0.12. We have observed changes in the hydration shell arrangement as a function of the RE content x and as a function of the water vapor pressure. The compounds with the RE ions from La to Eu show a different hydration behaviour from the compounds with Gd to Lu. The superconducting properties are not influenced by the magnetic moment of the RE ion, but are sensitive to the water ion arrangement.

Epoxy‐Encapsulated Ceramic Superconductor Microelectrodes

A procedure is outlined for fabricating well‐behaved microelectrodes from ceramic pellets of and which involves systematic polishing of an epoxy‐encapsulated superconductor chip, under solution, to a potentiometric end point. Voltammetry of the resulting microelectrodes in acetonitrile is illustrated and compared to that arising from alternative superconductor electrode geometries. The microelectrodes have active electrode surface areas ranging from , as characterized electrochemically and microscopically. The results discussed herein are significant steps toward developing the methodology necessary to study the electrochemical response of high temperature superconductor phases at temperatures below their superconductor critical temperature.

High-Tc superconductors and weighted harmonic mean electronegativities

A new electronegativity scale based on Mulliken's concept is suggested which indicates systematic changes in the superconductive critical temperature (T{sub c}) with composition for some high-T{sub c} oxides. By this scale the optimum electronegativity seems to exist between 4.7 and 4.9 eV, which is a very narrow range compared to that for low-T{sub c} binary alloys of normal stoichiometry as well as A15 type crystal structure.

Structure and defect structure of recent high Tc superconductors

Defects in the new superconducting materials play an important role and the superconducting properties are often influenced by their presence. In this review we will not consider the “classical” 1-2-3 compounds since they will be treated in a separate contribution.The Bi2Sr2CanCun+1Ox compounds show a strong intergrowth of lamellae of each of the phases corresponding with different n-values. An experimental relationship has been established between the n-values and the critical temperature for superconductivity; n=0,1,2 corresponding respectively with Tc values of 20K, 80K and 110K. Each of these phases is moreover incommensurately modulated. The origin of this modulation is still not definitively elucidated but we have performed a number of experiments and substitutions in order to get more information on the nature of this distortion. Several models are compared; the most likely one being the presence of extra oxygen in the BiO layers and the subsequent relaxation of the atom positions.

Strong 2d spin correlation induced o-pair superconductivity in the low temperature region

It is shown that the strong correlated two-dimensional CuO 2 layer could show O-pair superconductivity due to strong spin interaction between Cu and O holes. The ground state and the excitation state of the system at finite temperature are studied. We find that the superconductivity may be destroyed by the CuO singlet pairs exciting to finite momentum state. The critical temperature of superconductivity is calculated and the results are consistent with experiments.

Critical temperature of low-dimensional high-T c superconductors

We generalized the Coherent Potential Approximation (CPA) method for the calculation of the critical temperature of superconductors of lower dimensionality. Two dimensional (2d) and one-dimensional (1d) models have been considered. The critical temperature is calculated as function of the electronic concentration and of the position of the Fermi level. The critical temperature can be expressed with either of these two parameters. We also calculate the dependence ofTc on an additional parameter which is the measure of the interplane coupling.

The effect of oxygen ordering through controlled exposure to oxygen on the superconductive properties of LaBa2Cu3Oy

In an effort to clarify the role of controlled oxygen exposure in the synthesis of LaBa2Cu3Oy samples which achieve zero resistivity above 90 K, we have prepared samples of La(1+x)Ba(2−x)Cu3Oy with x approximately equal to 0 and with y varying between 6.22 and 7.00. We find that for large values of y, 6.88<y<7.00, the value of the superconductive critical temperature is strongly dependent on synthetic conditions. Synthetic conditions promoting orthorhombic crystal structures consistent with oxygen ordering in the basal planes of the crystal produce materials with the highest values of the zero-resistance temperature.

The critical temperature of superconductors with a cylinder symmetrical Fermi surface

This paper presents model calculations of the critical temperature for two high temperature superconductors namely La-Sr-Cu-O and Y-Ba-Cu-O on the basis of a BCS-like effective electron-electron interaction and a cylinder symmetric Fermi surface. It is shown that such a model is in better agreement to known experimental data of the normal state properties than a simple Sommerfeld model approach. Critical temperatures of 39 K and 92 K respectively can be achieved within this model under the assumption of a coupling to existing high energy phonons (∼ 60 meV) using a rather moderate mass enhancement factor λ∼1. This is in agreement with the observed ‘weak-coupling’ behaviour of these new superconductors.

Critical temperatures of superconductivity and neutron diffraction studies at 293 and at 10 K of molybdenum–iridium single crystals of A15 structure

A crystal of dimensions 2.5 × 2.5 × 1.8 mm and of composition Mo3.25Ir was examined by neutron diffraction at 293 and 10 K. It has the A15 structure for which the space group is Pm3n and the ideal composition would be Mo3Ir. Least-squares refinements gave R(F) values of 0.013 and 0.017 for the 293 and the 10 K data, respectively. No structural phase transition was observed at 10 K. The lattice constants found were a = 4.967 (2) Å at 293 K and 4.958 (2) Å at 10 K. The transition temperature to superconductivity of the crystal was measured by an inductive method. An onset of superconductivity was found at 9.25 (5) K and a main transition at 9.10 (5) K. Another, approximately cylindrical, sample of volume 117 mm3 showed an onset at 8.6 (1) K and a main transition at 8.0 (1) K. About two thirds of this sample was a single-crystal.

Coexistence of superconductivity and antiferromagnetism in the Hubbard model: A phase diagram for finite temperature

Abstract The normal paramagnetic, superconducting, antiferromagnetic and simultaneously superconducting and antiferromagnetic states of correlated electrons in the nearly half-filled band have been described by the mean field approach. The Hubbard model has been used as a starting point. A general form of the spin density wave (SDW) order parameter has been proposed. The possibility of coexistence of SDW and superconducting (SC) phases has been confirmed by comparison of free energies. It has been proved that near half-filling superconductivity is suppressed by antiferromagnetism. A finite temperature phase diagram has been presented. It has been shown that SDW cannot enhance the critical temperature of superconductivity. The possible relevance of these results in copper oxides, especially to those where this coexistence was observed, has been discussed.