Doping-induced symmetry change of superconducting order

Superconductivity in doped cuprate compounds can be associated with different microscopic mechanisms: Coulomb, phonon- and magnon-mediated pairing. These mechanisms simultaneously contribute into superconducting order and their competition determines the symmetry of the order parameter. The magnon-mediated pairing is strongly dependent on doping, therefore, one can expect a change of the symmetry of the order parameter under variation of doping that can result in phase transitions inside the superconducting state.     

Suppression of superconductivity in high-T c cuprates due to nonmagnetic impurities: Implications for the order parameter symmetry

We show explicitly that the broad histogram single-spin-flip random walk dynamics does not give correct microcanonical average even in one dimension. The dynamics violates the detailed balance condition by an amount proportional to the inverse system size. As a result, in distribution different configurations with the same energy can have different probabilities. We propose a modified dynamics which ensures detailed balance and the histogram obtained from this dynamics is exactly flat. The broad histogram equation relating the average number of potential moves to density of states is generally valid. Received 2 October 1998 and Received in final form 13 October 1998     

Time-Reversal symmetry violation and the structure of the superconducting order parameter of PrOs4Sb12

The antisymmetric two-electron wave functions corresponding to the symmetry of Cooper pairs are constructed for the symmetry group T _h of the PrOs₄Sb₁₂ superconductor and its subgroup D _2h . The dependence of the structure of these functions (the presence of point nodes) on the group of the wave vector and the multiplicity is investigated. The theoretical nodal structure obtained for the two-electron wave functions makes it possible to explain the experimentally observed structure of the superconducting order parameter of the PrOs₄Sb₁₂ compound (the presence of two phases with different nodal structures) by the time-reversal symmetry violation.     

Origin of intrinsic Josephson coupling in the cuprates and its relation to order parameter symmetry: An incoherent hopping model

Experiments on the cuprate superconductors demonstrate that these materials may be viewed as a stack of Josephson junctions along the direction normal to the CuO₂ planes (the c-axis). In this paper, we present a model which describes this intrinsic Josephson coupling in terms of incherent quasiparticle hopping along the c-axis arising from wave-function overlap, impurity-assisted hopping, and boson-assised hopping. We use this model to compute the magnitude and temperature T dependence of the resulting Josephson critical current j_c(T) for s- and d-wave superconductors. Contrary to other approaches, d-wave pairing in this model is compatible with an intrinsic Josephson effect at all hole concentrations and leads to j_c(T) T at low T. By parameterizing our theory with c-axis resistivity data from YBa₂Cu₃O_7−δ (YBCO), we estimate j_c(T) for optimally doped and underdoped members of this family. j_c(T) can be measured either directly or indirectly through microwave penetration depth experiments, and current measurements on Bi₂Sr₂CaCu₂O₈ and La_2−xSr_xCuO₄ are found to be consistent with s-wave pairing and the dominance of assisted hopping processes. The situation in YBCO is still unclear, but our estimates suggest that further experiments on this compound would be of great help in elucidating the validity of our model in general and the pairing symmetry in particular.     

Superconductor order parameter symmetry in UPd Al

The heavy fermion compound UPd₂Al₃ has attracted much interest on account of the coexistence of antiferromagnetism and superconductivity at temperatures below 2 K. The antiferromagnetic fluctuations provide, principally via inelastic neutron scattering, a window on the low frequency dynamics in this material. By an analysis of neutron scattering data, and taking into consideration results from other experimental probes, it is suggested which sheet(s) of the f-electron Fermi surface may play an active role in forming the superconducting state in UPd₂Al₃. The proposed scheme sheds new light on previously reported anomalies in this material. Received 16 July 1999     

高温超导体中的赝隙与超导位相涨落

高温超导体赝隙态与超导态之间的关系一直是研究的焦点.交流电导和能斯特（Nernst）效应测量相继探测到超导转变温度Tc0以上温区一定范围内存在磁通涡旋激发,利用力矩技术的磁化率测量则探测到超导涨落引起的弱抗磁性.这些发现都支持了高温超导体赝隙相中存在有限的超导序参量振幅和强烈的位相涨落的图像,说明Tc0处的相变是由库珀对之间长程位相关联的消失所驱动的.文章首先简短地介绍高温超导体的电子态相图和赝隙态,以及能斯特效应的原理和测量方法,然后对能斯特效应的测量结果作一评述性介绍,还讨论了相关的理论模型.     

Time reversal symmetry breaking in cuprates induced by the spiral spin order

We propose a new interpretation of the spontaneous time reversal symmetry breaking (TRSB) observed recently in a pseudogap state of cuprates (Kaminsky et al.). It is shown that the TRSB dichroism in an ARPES signal may be related to the local spin spiral structures in the system. It may be caused by a spin-orbit interaction and by spin polarization of electrons at various sections of the Fermi surface in the spiral state. The angular dependence of the dichroism signal is studied in a schematic KKR approximation. Tests are proposed to check the existence of the local spiral spin structure and to distinguish it from the TRSB state with microcurrents constructed by Varma.     

Higgs phenomenon without symmetry breaking order parameter

We discuss symmetry-breaking order parameters, e.g. 〈?〉, in gauge theories with Higgs scalars, ?, in suitable gauges. We show that, typically, 〈?〉 = 0. A complete set of gauge-invariant, observable composite fields for such theories, local ones and ones localized near strings (paths) is constructed. We then examine the validity of standard perturbation theory, based on assuming that 〈?〉 ≠ 0, and reformulate it in terms of our gauge-invariant fields and without assuming that 〈?〉 ≠ 0. Finally, we classify classical field configurations with non-trivial topology (“defects”) in such theories and propose a defect-gas approach to predict their effects.     

Superfluid density and critical current density in superconducting cuprates with an extended d-wave pairing symmetry

The European Physical Journal B - We propose a novel dynamic link weight adjustment model, in which link weights on static network will be dynamically adjusted according to agents’ influence...     

高温超导体中的赝隙与超导位相涨落

高温超导体赝隙态与超导态之间的关系一直是研究的焦点.交流电导和能斯特（Nernst）效应测量相继探测到超导转变温度Tc0以上温区一定范围内存在磁通涡旋激发,利用力矩技术的磁化率测量则探测到超导涨落引起的弱抗磁性.这些发现都支持了高温超导体赝隙相中存在有限的超导序参量振幅和强烈的位相涨落的图像,说明Tc0处的相变是由库珀对之间长程位相关联的消失所驱动的.文章首先简短地介绍高温超导体的电子态相图和赝隙态,以及能斯特效应的原理和测量方法,然后对能斯特效应的测量结果作一评述性介绍,还讨论了相关的理论模型.     

Pseudogap and spin fluctuations in the normal state of the electron-doped cuprates

We present reliable many-body calculations for the t-t(')-t(')-U Hubbard model that explain in detail the results of recent angle-resolved photoemission experiments on electron-doped high-temperature superconductors. The origin of the pseudogap is traced to two-dimensional antiferromagnetic spin fluctuations whose calculated temperature-dependent correlation length also agrees with recent neutron scattering measurements. We make specific predictions for photoemission, for neutron scattering, and for the phase diagram.     

Giant fluctuations of superconducting order parameter in ferromagnet-superconductor single-electron transistors

Spin dependent transport in a ferromagnet-superconductor single-electron transistor is studied theoretically taking into account spin accumulation, spin relaxation, gap suppression, and charging effects. A strong dependence of the gap on the magnetic state of the electrodes is found, which gives rise to a magnetoresistance of up to 100%. We predict that fluctuations of the spin accumulation can play such an important role as to cause the island to fluctuate between the superconducting and normal states. Furthermore, the device exhibits a nearly complete gate-controlled spin-valve effect.     

Pair-density-wave pseudogap and superconducting states in cuprates

Bogoliubov quasiparticle interference and localized high-energy excitations observed in cuprates in nodal and antinodal regions of the momentum space, respectively, would lead to a conclusion that only the nodal region might give rise to superconductivity whereas the antinodal one might be associated with the pseudogap. We argue that both pseudogap and superconducting states arise exactly in the antinodal region with pronounced nesting feature of the Fermi contour as spatially inhomogeneous incoherent and coherent states of pairs with large momentum. The nodal region gives rise to conventional superconducting pairing with zero momentum which, together with the pairing with large momentum in the antinodal region, forms a biordered superconducting state in the whole of the Brillouin zone. This coherent state with complicated momentum dependence of the order parameter manifests itself as a pair-density wave that can exist without any driving insulating order. We believe that quasiparticle interference, other than observed in the nodal region, should be observable in the antinodal region as well.     

Relaxation of the superconducting order parameter under microwave irradiation

In superconducting aluminium we measured the influence of microwave irradiation on the time delay between the start of a supercritical current pulse and onset of the corresponding voltage response. We observed a shortening of the delay times due to the microwaves. This reflects the redistribution of the quasiparticles and the enhancement of the superconducting energy gap.     

Resonance tunneling in superconducting junctions with different order parameter symmetries

Resonance tunneling in superconducting junctions with electrode order parameters of s and d symmetry was studied. The Green function formalism was used to derive an equation for the resonance current in junctions of arbitrary dimensionality and order parameter symmetry. A universal equation for resonance super-current was obtained for junctions of arbitrary dimensionality with isotropic electrode order parameters. A numerical analysis of resonance current transport in junctions of various types was performed for the two-dimensional model.     

On electron pairing in superconducting cuprates

The superconducting properties of materials of layered structure containing copper and other metal oxides are compared with the expectations ofa recently proposed electron pairing model 1. The role of the oxygen content of samples is emphasized. Evidence is found showing that superconduction is originated only in presence of coupled layers of metal oxides holding unpaired electrons. Received 8 November 2000     

Nano-scale complexities in the superconducting cuprates

Underdoped cuprates are characterized by nano-scale complexity with strong spatial variation in the electronic properties, including superconductivity. It is often assumed that the stripe order underlies this spatial complexity, but the evidence of local stripe order in the superconducting phase is weak. We propose an alternative idea of electronically driven two-dimensional local order that leads to phase separation in the reciprocal space, which could be the basis for two-component superconductivity.