Influence of a Finite Tc on Enhanced Superconductivity Near the Metal/Insulator Transition

We adapted the BCS equation for T _c so that it applies to disordered conductors near the metal/insulator transition (MIT). The resulting expression for T _c as a function of the conductivity accounted quite well for several disparate classes of superconducting materials near the MIT [disordered metals, oxide conductors (including high-T _c superconductors), semiconducting materials, organic conductors, and C₆₀]. This function, however, was applicable only to situations for which T _c was zero in the pure limit—which was the case for the data chosen. This paper extends the model to the nonzero case and compares the predictions to the appropriate data.     

Specific heat and enthalpy of lattice disordering of LaF3 superionic crystals

Specific heat (C _p ) data obtained near the phase transition of LaF₃ are used to estimate the activation enthalpy for anion disordering, H _d. At the critical point T _c = 263 K, the concentration of disordered fluoride ions is n _c = 2.86 × 10²⁰ g⁻¹. In the dielectric phase of LaF₃ (T < T _c), H _d is 0.24–0.26 eV. In the superionic phase (T ≥ T _c), where the concentration of disordered fluoride ions exceeds n _c, the enthalpy of disordering drops to H _d ≃ 0.04 eV.     

The possibility of increasingTc in oxide high-Tc superconductor films: Interaction with elastic fields of defects

The influence onT _c of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-T_c superconductors is considered. The distribution function ofT _c in this case was obtained. This is the Lorentzian with center shifted to largerT _c values. The half width of theT _c distribution n (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT _c fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT ^(c) near the surface appears. The inequalityn>T ^(c)>T _c ^initial is fulfilled. ForT>T ^(c) the quasi-2D Josephson media takes place where a finite superconductive domain withT _c ^local >T ^(c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions.     

Thermal conductivity of high-Tc superconductors

This paper reviews existing data on the thermal conductivity of high-T _c superconductors. Included are discussions of pristine polycrystalline high-T _c ceramics, single crystal specimens, and high-T _c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high-T _c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high-T _c perovskites are discussed and implications for theories of high-T _c superconductivity are noted.     

Superconductivity and random disorder in the narrow and wide band limit

We investigate the effects of a finite electronic bandwidth in a disordered superconductor by considering the current tight binding Anderson model of disorder and a mean field BCS interaction. A general relation, between the transition temperatureT _c and the disorder averaged spectral density associated with the electron propagator between neighboring lattice sites is found which enables us to predict superconductivity in the disorder induced localized state. A reasonable interpolation, between weak and strong disorder, is investigated and it predicts a maximum bandwidth where superconductivity is destroyed. In this case,T _c as a function of the disorder is shown for a few values of the hopping constant. A brief discussion of the model validity range and its applications to disordered granular superconductors is presented.     

Dynamical Screening and Electron-Mediated Superconductivity in Intercalated Layered Metallochloronitrides

Metallochloronitrides are layered conductors that have recently been found to superconduct at critical temperatures up to T _c 26 K. It is known that the electron–phonon interaction is small in these systems, so that the conventional pairing mechanism through exchange of phonons is unable to provide for such high T _cs. We show that the dominant contribution to the pairing interaction in these materials comes from the exchange of low-energy collective modes, specific to layered conductors. The existence of such acoustic plasmons results from the incomplete screening of the Coulomb interaction perpendicular to the conducting planes which expresses itself in the dynamical nature of the screened Coulomb interaction.     

Resistivity and T c in disordered superconductors

The universal depression of the superconducting transition temperature T _cin disordered A-15 compounds is examined. It is found that their anomalous behavior can be explained by a simple model for the density of states, which is enhanced by disorder in some cases. The dramatic drop in T _cin constant-density-of-states A-15 compounds like Nb₃Ge or Nb₃Al at a critical value of the resistivity can be attributed to overdamping of acoustic plasmons, which decreases the electron pairing interaction despite relatively small changes in the density of states. Agreement for T _cand susceptibility with previous calculations is found if the position of the Fermi energy is near a peak. Possible experiments are proposed to check the above models.This work was supported in part by NSF Grant No. DMR77-13167.     

Upper Critical Field Measurements up to 60 T in Arsenic-Deficient LaO0.9F0.1FeAs1−δ : Pauli Limiting Behavior at High Fields vs. Improved Superconductivity at Low Fields

We report resistivity and upper critical field B _c2(T) data for As-deficient LaO_0.9F_0.1FeAs_1?δ in a wide temperature and field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T _c =29 K and a significantly enlarged slope dB _c2/dT=?5.4 T/K near T _c which contrasts with a flattening of B _c2(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behavior (PLB) with B _c2(0)≈63 T. We compare our results with B _c2(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B _c2(T) for applied fields H ∥ ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.     

Origin of Superconductivity and Construction of Doping Curves for Cuprates on an Elasticity-Exchange Competition Model

Superconducting T _c is modeled over the doping range for the three prototype cuprate families (with respect to apical coordination). The model is based on a competition of elastic and exchange energies (ExEl model). Data for these energies are taken from direct observation such as T _c and the pseudo-gap T _p. An additional penalty for charge concentration in pairs is introduced for the low pair concentration region. These energies are internally calibrated. The rather different observed curve shapes and quantitative features in the doping curves of different materials can be related to the relative magnitude of T _c and T _p. Both linear and parabolic curve shapes near the T _c optimum can be explained on the ExEl model. The basis of the relevant phenomenology is indicated in bond ordering effects, for which new concepts, e.g. for resonating pair kernels, are developed. Accordingly, stripes of single holes are dimerized into pair strands through super-exchange. Conclusions are drawn about the limits of phenomenology for cuprates and other high T _c materials.     

Organic Superconductors: When Correlations and Magnetism Walk in

This survey provides a brief account for the start of organic superconductivity motivated by the quest for high T _c superconductors and its development since the 1980s. Besides superconductivity found in 1D organics in 1980, progresses in this field of research have contributed to better understand the physics of low-dimensional conductors highlighted by the wealth of new remarkable properties. Correlations conspire to govern the low-temperature properties of the metallic phase. The contribution of antiferromagnetic fluctuations to the interchain Cooper pairing proposed by the theory is borne out by experimental investigations and supports superconductivity emerging from a non-Fermi-liquid background. Quasi-one-dimensional organic superconductors can therefore be considered as simple prototype systems for the more complex high T _c materials.     

The effect of composition on Tc in some high Tc A-15 structure super conductors

We discuss the effect of composition on T_c for Nb₃X where X = Ge, Ga, Al, Sn and Pt on the Nb rich side of stoichiometry. The concentration dependence (average slope) of T_c for all the materials is between ?1.3 and ?2.6 K per atomic % Nb. To within the accuracy of the data no systematic variation is found between the slopes of T_c and the highest values of T_c observed in these systems. The highest observed T_c in each system occurs at or near the stoichiometric composition. Comparison with radiation damage results suggests that the disorder introduced by deviations from stoichiometry is a major cause of the depression of T_c.     

Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-Tc Cuprates

We argue that the collective mode as observed in angle resolved photoemission spectroscopy (ARPES) on a large class of cuprates can be associated with dynamic incommensurate CDW fluctuations present in these materials. This scenario is substantiated by a comparison of calculated spectra with experimental ARPES data where we obtain a mode frequency which decreases towards optimal doping thus strongly supporting the existence of a quantum critical point around this concentration. Moreover we extract the temperature dependence of the associated bosonic spectrum from ARPES data where it turns out that there is a continuous evolution from mode-type behavior below T _c to a marginal Fermi liquid structure well above T _c.     

Chemical (Sr,Co)-doping effect on critical current density for Dy123 melt-solidified bulks

The dilute impurity doping for CuO-chain site was found to largely improve the critical current properties of both Y123 single crystals and Y123 melt-solidified bulks in our previous study. In addition, dilute Sr-doping to Ba site is also effective for enhancement of J_c without serious decrease in T_c as in the case of dilute Zn doping for CuO₂-plane site. In the present study, we have attempted further enhancement of flux pinning force of the Dy123 melt-solidified bulk, which is essentially more promising materials than Y123, by impurity doping for Cu site in the CuO-chain and for Ba site. Although the (Sr,Co)-co-doped Dy123 melt-solidified bulks showed systematically suppressed T_c with their doping levels, high T_c's well exceeding 90 K were maintained. All the Sr-doped or Co-doped Dy123 bulks exhibited higher J_c than the undoped one. More improved J_c properties were achieved by (Sr,Co)-co-doping, suggesting that pinning potential at local regions around doped impurities become deeper by Co-doping, resulting in stronger point-defect-like pinning sites. In addition, more detailed studies on the dilute Sr-doping and the dilute (Sr,Co)-Co-doping were carried out for Y123 single crystals in order to clarify the difference between their doping effects on the J_c properties. The vortex transition field, H^*, to the disordered state in the M–H loops for the Co-doped Y123 single crystal was located at the lower field than that of the Sr-doped one, meaning that the Co ions strongly affect the vortex system compared with the Sr ions.     

Short-range order and melting anomalies in thin films

This paper presents electrical resistivity and superconductivity studies for the B1 (NaCl) structure oxides of titanium, vanadium, and niobium. In samples of nominal composition TiO _x , VO _x , and NbO _x ,x was varied from 0.8 to 1.2. It was found that all three of these oxides exhibit room-temperature electrical resistivities characteristic of metallic behavior. With decreasing temperature, the resistivity drops steeply in the case of NbO _x , but remains steady or rises somewhat in the case of TiO _x and VO _x , depending on the exact value ofx. It is suggested that in TiO _x and VO _x there is a large resistivity component due to scattering of carriers by disordered vacancies. Superconductivity was observed in NbO _x (T _c=1.38 K,x=1.0) and TiO _x (T _c=1.0 K,x=1.07). In the latter case the material showed a well-defined maximum ofT _c as a function of composition, withT _c<0.05 K forx<0.85 andx>1.20. Several VO _x samples remained normal to 0.07 K.     

Two-parameter fracture criterion (Kρ,c-Tef,c) based on notch fracture mechanics

A two-parameter fracture criterion has been proposed to predict fracture conditions of notched components. This criterion includes the critical notch stress intensity factor K _ρ,c , which represents fracture toughness of a material with a notch of radius ρ, and the effective T-stress. The effective T-stress T _ef has been estimated as the average value of the T-stress distribution in the region ahead of the notch tip at the effective distance X _ef . These parameters were derived from the volumetric method of notch fracture mechanics. The results of numerical T _ef,c -stress estimation are compared to the T _ef,c -stress results obtained from experimental analysis. The material failure curve or master curve K _ρ,c = f(T _ef,c ) has been established as a result of the notched specimen tests. A large T _ef,c range was covered from −0.80 σ _Y to +0.19 σ _Y using SENT, CT, RT (roman tile) and DCB specimens. It was shown that the notch fracture toughness is a linear decreasing function of the T _ef,c -stress. The use of the material failure curve to predict fracture conditions was demonstrated on gas pipes with the surface notch.     

Effect of superconducting fluctuations on the nmr relaxation rate of high-Tc superconductors

The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T ₁, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T _c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T₁ atT _c and briefly discuss possible reasons why this peak is not observed.     

Unusual behavior of superconductors near the tricritical Lifshitz point

The properties of superconductors near the tricritical Lifshitz(TL) point are studied. Unusual behavior ofH _c2(T) near the TL point in three-dimensional (3D) systems is found, as well as an oscillatory character ofH _c2(T) in quasi-2D systems. The oscillatory character of the critical currentj _c of the S-S-S junction as a function of thickness and temperature is also studied. Predictions are compared to experimental data in such quasi-2D systems as T_aS₂(pyridine).     

High-Tc superconductivity and electronic band structure

We review the main results of the van Hove scenario applied to superconducting cuprates. It is based on the assumption that in these materials, the Fermi level lies near a singularity in the density of states (DOS). This hypothesis has recently been confirmed experimentally. We show that this model explains many properties of the high-T _c superconductors. We show that an anaogous model with a peak in the DOS may also be applied to the superconducting doped fullerenes. A general feature of the model is a very short coherence length.     

Anisotropy of the generalized Ginzburg-Landau parameters x 1 and x 2 in clean,cubic, type II superconductors

From the nonlocal GLAG theory expressions are derived for x₁ and x₂ of a clean, cubic crystal with a general Fermi surface. In the region of weak nonlocality near T _cthe correction terms in H _c2, x₁, and x₂ due to the nonspherical Fermi surface are expressed by two dimensionless Fermi surface averages. For highly purified Nb single crystals and polycrystalline Nb samples, x₁ and x₂ were determined from magnetization measurements in the temperature range (0.3–0.95)T _c. The experimental data show that the orientation dependence in x₂ is twice as large as in x₁, in agreement with the theoretical result near T _c. The two Fermi surface averages that characterize the anisotropy of H _c2, x₁, and x₂ near T _care estimated from experimental data for Nb and V.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

Abstract

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1?xTe_x) and Fe(Se_1?xS_x). In the case of Fe(Se_1?xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1?xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1?xCo_x)AsF. The T_c of Fe(Se_1?xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.