Lattice and spin bipolarons in metal oxides and doped fullerenes

We extrapolate the BCS theory to the strong electron-phonon and (or) electron-spin fluctuation interaction and show that in the strong-coupling limit the ground state is a charged Bose liquid of lattice and (or) spin bipolarons. Kinetic and thermodynamic properties of charged bosons on a lattice in the normal and superconducting states are discussed, and some evidence for the model is given from NMR, neutron scattering, near-infrared absorption, Hall effect, resistivity, thermal conductivity, isotope effect, heat capacity, and critical magnetic fields of high-T _c oxides. The maximum attainableT _c is estimated to be in the region of the transition from the Fermi liquid to a charged Bose liquid (polaronic superconductivity). The proposed theory is not restricted by low dimensionality and might be applied to cubic oxides such as the old BaPbBiO and to alkali-doped C₆₀ as well.We thank D. Khmelnitskii, W. Liang, J. Loram, M. Pepper, E. Salje, and J. Wheattey for helpful stimulating discussions, and J. Cooper, A. Carrington, and A. Mackenzie for extensive experimental data and discussion. A. Bratkovsky has been instrumental in elaborating the temperature dependence of the infrared absorption and the electrical conductivity. One of us (A.S.A.) appreciates the financial support from the Leverhulme Trust.     

Phenomenological study of a single craze propagating in several glassy polymers over a wide range of temperatures

The single craze which appears at the crack tip in some amorphous polymers has been observed between — 100 and + 100° C in five different polymers. The results show that the single craze exists only between two temperatures: a certain critical temperatureT _c and the glass transition temperature. In the case of oriented block co-polymers the structure also affects the formation of the craze. Below the critical temperatureT _c, a multiple craze appears ahead of the crack and the fracture toughnessK _c increases.     

Inhomogeneous magnetic structure in clean magnetic superconductors

Clean magnetic superconductors are considered in which the phase transition into the ferromagnetic state takes place at a temperature ? in the absence of superconductivity, with ??T _c1, whereT _c1 is the superconducting critical temperature. The exchange and electromagnetic interactions of electrons and localized magnetic moments are taken into account, as well as magnetic anisotropy. We show that below the temperatureT _M≈? in the superconducting state the inhomogeneous magnetic structure of transverse one-dimensional domain type (DS phase) should occur at real values of the exchange interaction and anisotropy. In the DS phase gapless superconductivity is realized at temperatures sufficiently far fromT _M. Here the equilibrium direction of the magnetic structure wave vectorQ can be changed by applying a supercurrent across the sample. The behavior of this DS phase in an external magnetic field is also considered.     

A new look at the highT c superconductors using BCS-theory

Using the conventional phonon-exchange mechanism of superconductivity we have succeeded in reproducing the transition temperatureT _c of a large number of newly discovered highT _c superconductors by introducing a certain modification to the well-known BCS-formula forT _c .     

Effect of gap anisotropy in superconductors containing paramagnetic impurities with local states within the gap

The Shiba model for the paramagnetic impurities in a superconductor has been generalized to include the effect of gap anisotropy. The calculated properties are the transition temperatureT _c and the specific heat jump atT _c. The changes brought in these properties by anisotropy are significant.Work supported in part by the Natural Sciences and Engineering Research Council of Canada.     

Low-frequency kinetics of high-T c metal oxides

A microscopic explanation of the low-frequency kinetic properties of metal oxides is proposed. It is based on a strong electron-phonon interaction, which forms a charged Bose liquid of small bipolarons. The large value, the nonKorringa temperature dependence above T _c, and the absence below T _c of the coherent peak of the nuclear spin relaxation, as well as an unexpected coherent peak of the low frequency dynamic conductivity and the linear T-dependence of the resistivity are explained.     

Research and Prospects of Iron‐Based Superconductors

The discovery of a new superconductor, LaFeAsO_1?xF_x with a superconducting critical temperatureT,_c, of 26 K in 2008, has quickly renewed interest in the exploration of iron‐based superconductors. More than 70 new superconductors have been discovered within several months, with the highest T_c of up to 55 K being observed in the SmFeAsO_1?x compound. High T_cs have previously only been observed in cuprates; these new iron‐based superconductors have been added as second members of the high‐T_c family. The crystal structure of these compounds contains an almost 2D Fe–As layer formed by FeAs₄ tetrahedrons, which can be separated by an oxide or metal layer that provides extra electrons to the Fe–As layer, and the itinerant iron 3d electrons form an antiferromagnetic (AFM) order state in the undoped parent compounds at around 100–200 K. Superconductivity can be induced by carrier doping, which destroys the AFM ground state. In this Review, the most recent findings on and basic experimental facts about this class of high‐T_c materials will be presented, including the various superconducting structures, the synthesis methods, the physical properties of the parent compounds, the doping methods that could produce superconductivity, pressure effects, and the prospects for this new iron‐based high‐T_c family.     

Current-carrying states in superconducting channels with nonhomogeneities

The stability of a current-carrying normal state in a superconducting channel with nonhomogeneities is considered at Tc. A value of the current is found below which the normal state becomes absolutely unstable with respect to nucleation and growth of a superconducting domain. Furthermore, the superconducting state appearing near a nonhomogeneity characterized by a critical temperatureT _c1 higher than the critical temperatureT _c of the main material is studied for the case ofT>T _c .     

What does instrinsic Josephson coupling say about the pairing symmetry in the cuprates?

Measurements of the c-axis properties of the cuprate superconductors show anomalous behavior in both normal and superconducting states. In particular, there is strong evidence that pairs of CuO₂ planes in neighboring unit cells act as Josephson junctions below the critical temperatureT _c. We present a theory based on incoherent transport along the c-axis which naturally reproduces the anisotropic normal-state resistivity and the superconducting-state Josephson coupling. Applying this theory to YBa₂Cu₃O₇- (YBCO), we make quantitative predictions for the strength and temperature dependence of the Josephson coupling as well as the variation ofT _c with disorder. Beyond the expected low-temperature behavior, the Josephson critical current does not make a clean separation between s- and d-wave superconductors, but the disorder-inducedT _c variation does. Further experimental and theoretical work along these lines may therefore help determine the order parameter symmetry in the cuprates.     

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.     

Resistivity anomalies for ferromagnetic metals at curie points. II. Short-range magnetization fluctuations

Using the itinerant model of magnetic moment in ferromagnetic metals, we have investigated the resistivity anomalies near the Curie temperatureT _c . In this paper, the short-range magnetization fluctuations have been taken into account in calculating the spin correlation function in the paramagnetic region. The mean free path is considered to be comparable with the correlation length and to be temperature dependent nearT _c . A critical exponent is introduced for the mean free path and its value is established to be not smaller than 1/2. The resistivity is found to be continuous throughT _c . The temperature derivative of the resistivity is found to be divergent linearly and positively near and aboveT _c . These results confirm the phase transition nature of the resistivity anomalies of the ferromagnetic metals nearT _c and show that the short-range order must also exist in the itinerant model of the magnetic electrons.     

c-axis phonons and the enhancement of pairing correlations in high-temperature superconductors

We consider the two-dimensional Hubbard model including electron-phonon interaction. Strong local correlations (U limit) are taken into account within the mean-field approximation for auxiliary boson fields. Phonon-assisted transitions between intraand interlayer states are introduced as the source of coupling between two-dimensional CuO₂ layers. This type of processes effectively leads to the nonlinear (quadratic) interaction of intralayer electrons withc-axis phonons. We construct the Eliashberg equations for the resulting Hamiltonian and evaluate the superconducting transition temperatureT _c. Our model calculation demonstrates that a pronounced enhancement ofT _c in thed-wave channel is possible. The largest enhancement ofT _c tends to take place for small hole concentrations. This means that the coupling toc-axis phonons could compete with two-dimensional correlations responsible for the onset of antiferromagnetic order. It is remarkable that the two-dimensional features in the normal state are hardly affected by this specific interlayer interaction. Therefore,c-axis two-phonon-mediated interlayer coupling can cooperate with interlayer pair tunneling and substantially contribute to an increased pairing.     

On the absence of bipolarons in manganese perovskites

The paramagnetic phase of La_0.65Ca_0.35MnO₃ and La_0.65Pb_0.35MnO₃ powders was investigated by electron spin resonance. The resonance spectra, for a range of temperatures, was used to examine the presence of polarons and bipolarons. The resonance lines are compatible with the presence of polarons (g≈2), but the characteristic high-spin bipolaron lines (g≈4) are absent. The results are inconsistent with the presence of high-spin bipolarons. The evidence with respect to low-spin bipolarons is less conclusive, although the temperature dependence of the resonance line parameters speaks against the presence of low-spin bipolarons.     

Role of bounding layers in high-temperature superconductors: Calculation ofT c

A simple model of inter- and intra-layer couplings based on a BCS theory of layered superconductors is used to calculate the transition temperatureT _c of a number of layered systems. The role of the bounding layer in high-T _c oxide superconductors is investigated. Both the mechanisms of direct hopping and pair tunneling between active layers are found to play important roles in governing the behavior ofT _c . We give predictions for the dependence of the superconducting transition temperature on layer parameters.     

Magnetic susceptibility of superconducting granular aluminum

Measurements have been made of the magnetic susceptibility of samples of superconducting granular aluminum with different metal volume fractions. The penetration depth, determined from the results below the transition temperatureT _c , is found to diverge at the volume fraction where the discontinuity atT _c in heat capacity measurements disappears. The results near and aboveT _c have been interpreted in terms of both fluctuation and percolation models as showing effects of grain clustering. Possible evidence for the Kosterlitz-Thouless transition is discussed.This work was supported by the National Science Foundation through grants DMR-76-22967 and DMR-78-24213.     

The superconductivity and magnetic susceptibility of some zirconium-transition-metal compounds; evidence for an anticorrelation

The properties of the superconducting transition temperatureT _cof the CuAl ₂ -type compounds of Zr ₂ Co, Zr ₂ Ni, Zr ₂ Rh, Zr ₂ Fe, and Zr ₂ Ir are discussed with respect to effects due to alloying and heat treatment. Dilute pseudobinary alloys of transition-metal elements with Zr ₂ Rh (T _c=11.3 K) produced lower transition temperatures suggesting that the valence-electron concentration of 5.67 electrons/atom gives a maximum inT _c for this crystal type. Results for Zr ₂ Co (T _c=5.0 K) and Zr ₂ Ir (T _c=7.3 K) show that their transition temperatures are raised somewhat when the electron concentration is increased by alloying. A peak is seen near 5.72 electrons/atom. A peak in the room-temperature magnetic susceptibility in the Zr ₂ Co-Zr ₂ Ni system occurs near the same electron concentration as the peak inT _c. Similar susceptibility behavior takes place in the Zr ₂ Rh-Zr ₂ Ni system, however, with no peak inT _c. Results of susceptibility measurements on Zr ₂ Ir and its isomorphic alloys indicate correlation betweenT _c and susceptibility. An anticorrelation occurs for Zr ₂ Co alloys suggesting the presence of Coulomb interactions. A sharp symmetric drop inT _c near the stoichiometric composition as well as a marked decrease of the transition temperature with a low-temperature (600 C) anneal indicate that the coulomb interactions are sensitive to crystalline order. The lattice parameters of Zr ₂ Ir are reported as a=6.508 Å andc=5.721 Å. The superconductivity of Zr ₃ Co (T _c=3.9 K) and Zr ₃ Ir (T _c=2.13 K) is reported.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR grant number AF-AFOSR-631-67-A.     

Lattice instability in the normal state of La2?xSrxCuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Calculation of band structure and superconductivity in the simple cubic phase of phosphorus

We report a calculation of the band structure and superconductivity of phosphorus in the simple cubic phase under pressure. The effect of pressure on the band structure is obtained by means of the linear muffin-tin orbital method. The superconducting transition temperature(T _c) is calculated using the Allen-Dynes formula. It is found that the value ofT _c increases continuously with pressure from 110 kbar up to 210 kbar and then decreases. The change in the slope ofT _c is associated with the appearance of a new piece of Fermi surface. The calculated values ofT _c are compared with the available experimental data.     

Excess electrical conductivity due to fluctuations of quench-condensed superconducting Nb films

The resistive transition of superconducting Nb films evaporated onto a liquid-He-cooled substrate has been investigated, and shows a rounding characteristic of the thermal fluctuations. Transition curves have been measured above and below the superconducting transition temperatureT _c in the absence of a magnetic field. The excess electrical conductivity due to fluctuations aboveT _c of quench-condensed Nb films is well described in terms of the sum of the contributions from the Aslamazov-Larkin term and from the Maki-Thompson term, in contradistinction to the case of quench-condensed Bi, Ga, and Pb films. This suggests that thermal phonons play an important role in the contribution of the Maki term.