The flux distribution and hysteresis losses in highT c superconductors including viscous forces on flux lines

By comparing the contributions of pinning and viscous forces to the restoring force on flux ines in type II and highT _c superconductors, it is shown that the flux flow in highT _c superconducto rs should play an important role in determining the magnetic flux distribution and hence the hyste resis losses in a.c. fields. Both quantities are calculated in the extreme case of very large viscous forces with respect to the pinning force. The magnetic field and frequency dependence of the losses are changed with respect to the results obtained from the critical state model. The theoretical results are qualitatively confirmed by a.c. susceptibility measurements at different magnetic field amplitudes and frequencies. The quantitative differences indicate that the flux flow effects in highT _c superconductors are by far not so strong as expected and supposed by some theories. The reasons for these discrepancies are discussed.     

Generalizing the Cooper-pair instability to doped Mott insulators

Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high-temperature superconducting mechanism.     

Surface states and tunneling spectroscopy of high-Tcsuperconductors

Recent studies of high- T_csuperconductors have clarified new aspects of tunneling spectroscopy. The unconventional pairing states, i.e. d-wave symmetry in these materials have been established through various measurements. Differently from isotropic s-wave superconductors, d-wave pairing states have an internal phase of the pair potential. The internal phase modifies the surface states due to the interference effect of the quasiparticles. Along these lines, a novel formula of tunneling spectroscopy has been presented that fully takes into account of the anisotropy of the pair potential. The most essential difference of this formula from conventional ones is that it suggests the phase-sensitive capability of tunneling spectroscopy. The formula suggests that the symmetry of the pair potential is determined by the orientational dependence measurements of tunneling spectroscopy. Along these lines, several experiments have been performed on high-T_c superconductors. The observation of the zero-bias conductance peaks (ZBCP) on Y Ba₂Cu₃O_{7 − δ}strongly suggests the d_{x² − y²}-wave pairing states of hole-doped high-T_c superconductors. On the other hand, the absence of ZBCP on (electron-doped)Nd_1.85Ce_0.15CuO_{4 − δ}indicates that the pair potential of this material is a nodeless state. In this paper, recent developments of tunneling spectroscopy for anisotropic superconductors are reviewed both on theoretical and experimental aspects.     

Advances in Carbon Nanomaterials: Science and Applications,by Nikos Tagmatarchis

It has recently been realized that there is more than one kind of superconductor, and we now recognize superconducting metals and alloys as being either ‘type-1’ or ‘type-2’. The ‘new’ type-2 superconductors can exist in a state which allows them to carry isrestanceless currents even in very strong magnetic fields, and they are used to construct very powerful electromagnets. This article describes the properties of type-2 superconductors, why very strong magnetic fields are required to drive them normal and why under certain circumstances they can carry large lossless currents.     

Universal behavior of the upper critical field in iron-based superconductors

The newly discovered iron-based high temperature superconductors have demonstrated rich physical properties. Here we give a brief review on the recent studies of the upper critical field and its anisotropy in a few typical series of the iron-based superconductors (FeSCs). In spite of their characters of a layered crystal structure, all the FeSCs possess an extremely large upper critical field and a weak anisotropy of superconductivity, being unique among the layered superconductors. These particular properties indicate potential applications of the FeSCs in the future. Based on the experimental facts of the FeSCs, we will discuss the possible mechanisms of pair breaking in high magnetic fields and its restrictions on the theoretical analysis of the superconducting pairing mechanisms.     

Theory of novel properties of Josephson effect in anisotropic superconductors

Recent studies on high- T_csuperconductors aroused our new interest on the Josephson effects in anisotropic superconductors. In contrast to the conventional s-wave superconductors, there are two additional intrinsic effects originating from an internal phase of the pair potential. One is the sign change of the Josephson current depending on the direction of the motion of the Cooper pair. The other is the formation of localized states near the insulator. By taking account of these two effects, a general formula for the Josephson current is presented. Calculated results predict several anomalous properties including a strong enhancement of the Josephson current at low temperature. The influences of the spatial dependence of the pair potential on the Josephson current are also clarified.     

Magnetic forces in thallium- and bismuth-based superconductors

Hysteretic force-separation relations for Tl, Bi and Y based superconductors and a magnet are compared. The magnitudes of the forces, the hysteresis, and the magnetic stiffness were all largest in the three-layer Tl₂Ba₂Ca₂Cu₃O_x compound, indicating strong flux pinning. The hysteresis in both two and three layer thallium compounds was sufficient to generate attractive forces as the superconductor and magnet were moved apart. The use of remanent fields to improve stability of levitated and suspended magnets is described.     

Theoretical study ofμ + site in YBa2Cu3O x site in YBa2Cu3O x

The muon is a useful probe of magnetic fields in superconductors, but knowing the field seen by the muon is often of limited value until we know where the muon is in the crystal lattice. Here we employ two independent theoretical methods to search for candidate muon sites:the potential energy field method, which seeks the minimum of the electrostatic potential of theμ ⁺, and themagnetic dipolar field method, which compares the calculated magnetic field (due to host electronic or nuclear dipolar fields) with the observed local fields at the muon. Work supported by Canadian NRC and NSERC.     

On the penetration of the magnetic field into a superconductor

It is known that in superconductors the exponential decay of the magnetic field is an approximation, which breaks down if the dimension of a Cooper pair ξ _f is of the order or smaller than the London penetration depth δ. The appearance of a nonlocal relation between current and field yields deviations from the exponential decay especially a sign reversal of the field at a certain distance. This sign reversal is connected with a change: of the surface energy in superconductors and of the structure of fluxoids together with their interaction. In this paper we present results on the decay of magnetic field which is calculated from the exact BCS-integral-kernel for weak fields. As a result, the nonlocal effects in the framework of BCS-theory can be described in good approximation by the ratio of the London penetration depth δ(T, l) and the dimension of Cooper pairs ξ _f (T, l). The evaluations show, that one has still sign reversal, i.e. large nonlocal effects, in Type II superconductors with a κ(T _c )?,1.6. It should be mentioned that the limit κ?1.6 coincides roughly with the experimentally observed region of attraction of fluxoids. In addition results on the penetration depths are summarized.     

Transport properties between unconventional superconductors and ferromagnets

The properties of spin-polarized tunneling between ferromagnets and d-wave superconductors are studied based on a scattering theory. Conductance spectra are given for arbitrary barrier height cases, and the effects of anisotropy in the pair potential are analyzed. It is shown that the polarization can be well estimated from the heights of zero-bias peak in the case of d-wave superconductors.     

Quantum Stabilization of Moduli in a Slice of AdS6 Compactified on S 1

We review the effective potential due to massive bulk scalar fields in higher-dimensional warped brane models found in Flachi et al. (Quantum stabilization of moduli in higher dimensional brane models, arXiv:hep-th/0301, 2003) specializing it to a slice of AdS₆ compactified on the circle. This model contains two moduli that parametrize the interbrane distance and the size of S ¹, or equivalently the positions of the two branes. Their values determine the Planck/EW hierarchy, in a combination of large volume and redshift effects. It is found that the observed hierarchy is compatible with both moduli stabilized by the Casimir forces without fine-tuning (except for the one needed to match the cosmological constant). This contrasts with the Randall—Sundrum model, where gauge fields in the bulk are needed.     

Connecting cluster dynamics and protein folding

The relaxation dynamics of clusters can be interpreted in terms of the topographies of their potential surfaces. Systems with short-range potentials have sawtooth-like potential surfaces with small drops in energy from one local minimum to the next and few-body motions as the clusters move from one minimum to another; such systems readily take on amorphous structures. These are called “glass-formers". Systems with long-range forces have potentials whose topographies are like rough staircases, with some large drops in energy from one minimum to the next; their well-to-well passages involve very collective motions and such systems are excellent structure-seekers. They find their way to well-ordered, highly selective structures under almost all circumstances. These characteristics generalize to describe the potential surfaces and folding behavior of polypeptides and proteins. The forces are effective long-range forces due to the polymer chain. Staircase topographies emerge both from direct sampling of potential surfaces and from the inversion of the kinetics generated by a much more aaabstract topological model, from which folding pathways can be inferred. Received 4 December 2000     

On-site repulsion as the source of pairing in carbon nanotubes and intercalated graphite

We show that different non-conventional superconductors have one fundamental feature in common: pair eigenstates of the Hamiltonian are repulsion-free, the W = 0 pairs. In extended Hubbard models, pairing can occur for reasonable parameter values. For (N, N) nanotubes the binding energy of the pair depends strongly on the filling and decreases towards a reduced but nonzero value for the graphite sheet N → ∞. Received 13 July 2002 Published online 29 November 2002     

Seebeck-effect in the mixed state of Y−Ba−Cu−O

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of ac-axis oriented expitaxial film of Y–Ba–Cu–O. In contrast to conventional superconductors we find a large Seebeck-coefficientS, which is comparable in magnitude to the Nernst-effect. The broadening of the super-conducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to normal quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude ofS. It is argued that such a quasiparticle contribution to the dissipation is large in the high-T _c superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.     

Ground state of spin liquids,hole-hole attraction and high-T csuperconductivity

We develop a notion of high-T _csuperconductivity, which considers that phenomenon as a general property of close to half-filled bands of strongly correlated electrons confined to low-dimensional orbital lattices. Based on Andersons suggestion [1] we initially investigate the Cud electron system of the undoped mother substances of the high-T _c superconductors in the framework of the spin 1/2 Heisenberg model. We derive a new representation of the corresponding Hamiltonian in terms of triplet quasi-particles. The triplet representation leads to simple physical pictures for the spin liquid state and to a hierarchy of nearly exact variational wave functions for the ground state of the linear chain. These wave functions are employed as vacuum states for doping with holes. Holes are shown to form a conduction band embedded in the Hubbard-Mott insulating phase of strongly correlated electrons. The chemical binding forces among these localized electrons entail a strong attractive pair potential acting between the mobile defects. The generality of the hole-hole attraction and its independence of a strict localization of the electrons in the Hubbard-Mott phase is demonstrated by nearly exact solutions of the Hubbard and Pariser-Parr-Pople models of small regular electron chains at various degrees of electron correlation. It is suggested that this exchange-driven attraction leads to an instability of the free hole gas towards Cooper pair formation.     

Calculation of electric hyperfine interaction parameters in solids

A theoretical approach for the calculation of hyperfine parameters in solids from first principles is presented. These calculations are based on the full potential linearized-augmented-plane-wave (LAPW) band structure method, which is currently one of the most accurate schemes to determine the electronic structure in ordered solids. Exchange and correlation is treated within density functional theory using the generalized gradient approximation. Once the electron density is calculated self-consistently with high accuracy, quantities like electric field gradients (EFG), isomer shifts or hyperfine fields can easily be obtained from this density without further approximations. Using this approach we have studied various systems including metals, insulators, ionic compounds or the highT _c superconductors. In general we find good agreement between theory and experiment, which proves that our method is very accurate. Having these results in mind we are confident that this method is accurate enough to determine the value of the nuclear quadrupole momentQ, provided experimental measurements of the quadrupole coupling constant are available. This procedure is demonstrated forQ of⁷⁷Se and¹⁰⁰Rh, two nuclei recently used in PAC measurements. An extensive study of EFGs at Fe sites in various Fe-compounds has been performed leading to a very reliable quadrupole moment ofQ(⁵⁷Fe)=0.16 b, a value twice as large as that deduced from recent HF calculations but back to older estimates.     

Bipolaronic superconductivity in antiferromagnets

As a model of the cuprate superconductors, we have studied thep hole motion in a planar antiferromagnetic (AFM) background and ac-axis boson field. The indirect coupling between thed spins through thep holes is considered. In a range of the hole concentration, the indirect Cu–Cu interaction enhances the planar AFM coupling though it destroys the weakc-axis AFM order. At higher concentrations, the compensation of thed spins by thep holes occurs. For the strongp-d exchange coupling, thep holes can pair to form small magnetic bipolarons in the enhanced planar AFM background. The in-plane motion of the bipolarons is independent of thec-axis motion assisted by bosons. The superconducting properties of the cuprate superconductors are determined by a 2+1 dimensional bipolaron Hamiltonian. The results obtained from our model are consistent with the observations on the cuprate superconductors.     

Pair potential for a weak-coupling,single-crystal superconductor

In the standard treatment of weak-coupling superconductors, the pair potential enters as a nondiagonal but local potential. It is shown that for a pure, weak-coupling, bulk, single-crystal superconductor the quasiparticle wave functions are then Bloch functions and the self-consistent pair potential is periodic in space with the periodicity of the crystal structure. The relation between this periodic character of the pair potential and energy gap structure is discussed, and it is pointed out that energy gap structure of the general character expected appears to have been observed. The validity of a local pair potential is also briefly considered.Supported in part by the National Science FoundationA preliminary account of this work is abstracted in Bull. Am. Phys. Soc.19, 277 (1974)     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

Anomalous Hall effect in the mixed state of high-temperature superconductors

The Hall effect in the mixed state of high-T_c superconductors (HTSC) is of an anomalous nature: near the transition there is a range of temperatures and of magnetic fields where the sign of the Hall effect is opposite to that in the normal state. The universality of the phenomenon in question is indicative of its connection with some general properties of the mixed state of type-II superconductors, namely, with peculiarities of motion of magnetic flux vortex lines (vortices) in these superconductors. This work puts forward a model accounting for a number of vortex motion specific features and providing a possibility to obtain the characteristics of the anomalous Hall effect.

The work is based on the phenomenologically generalized results of Bardeen-Stephen and Nozieres-Vinen, supplemented with an allowance for a new mechanism of vortex “friction” associated with Andreev electron reflection on the interface between the normal core and the superconducting periphery of a vortex. Within the framework of the model suggested, magnetic field (and temperature) dependences of the longitudinal and Hall resistances of a mixed state superconductor have been calculated at temperatures nearing T_c. At certain quite realistic parameters which define the forces acting on the vortices, there is a range of magnetic fields and temperatures where the sign of the Hall effect is opposite to that in the normal state. The lower limit of this range is the irreversibility line and the upper critical field.