NMR study of magnetic moments arising in superconducting cuprates under electron/neutron irradiation

The measurements of direct-current magnetic susceptibility and nuclear magnetic resonance (NMR) were performed in the superconducting cuprates YBa₂Cu₃O_6.9 (⁸⁹Y,⁶³Cu), Tl₂Ba₂CaCu₂O₈ (¹⁷O,²⁰⁵Tl), irradiated by fast neutrons (E _n = 1 MeV) and electrons (E _e = 5 MeV). The influence of the radiation-induced structural disorder on the spin susceptibility and magnetic state of atoms in the CuO₂ layer is considered. The possible effect onT _c of the spin-exchange scattering of carriers by localized moments arising under irradiation was estimated following the Abrikosov-Gorkov approach and its contribution was found to be too small in comparison with the real suppression ofT _c under irradiation.     

Staggered flux vortices and the superconducting transition in the layered cuprates

We propose an effective model for the superconducting transition in the high-T(c) cuprates motivated by the SU(2) gauge theory approach. In addition to variations of the superconducting phase we allow for local admixture of staggered flux order. This leads to an unbinding transition of vortices with a staggered flux core that are energetically preferable to conventional vortices. Based on parameter estimates for the two-dimensional t-J model we argue that the staggered flux vortices provide a way to understand a phase with a moderate density of mobile vortices over a large temperature range above T(c) that yet exhibits otherwise normal transport properties. This picture is consistent with the large Nernst signal observed in this region.     

Metal-insulator crossover in superconducting cuprates in strong magnetic fields

The metal-insulator crossover of the in-plane resistivity upon temperature decrease, recently observed in several classes of cuprate superconductors, when a strong magnetic field suppresses the superconductivity, is explained using the U(1)xSU(2) Chern-Simons gauge field theory. The origin of this crossover is the same as that for a similar phenomenon observed in heavily underdoped cuprates without magnetic field. It is due to the interplay between the diffusive motion of the charge carriers and the "peculiar" localization effect due to short-range antiferromagnetic order. We also calculate the in-plane transverse magnetoresistance which is in fairly good agreement with available experimental data.     

Nuclear magnetic resonance study of the electron-doped high-temperature superconducting cuprates

Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr_2−xCe_xCuO₄ and Sr_0.9La_0.1CuO₂ that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a d_x²−y² symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin–lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y_1−xCa_xBa₂Cu₃O_7−δ for a hole concentration range of 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.     

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.     

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     

On the phase transition of superconducting cuprates in zero and non-zero magnetic fields

We present the results of measurements of the magnetizationM(T) and the specific heatC _p (T) of YBa₂Cu₃O_6.9 and Bi₂Sr₂CaCu₂O₈ in the vicinity of their respective critical temperature and in magnetic fields between 0 and 5 T. From these data we calculate the temperature- and magnetic field derivatives ofM andC _p . Both these measured and calculated quantities are used for testing theoretical predictions concerning the universality class of the transitions and to verify the influence of fluctuations accompanying the transitions.     

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.     

Interacting random Dirac fermions in superconducting cuprates

We study the effects of quasiparticle interactions on disorder-induced localization of Dirac-like nodal excitations in superconducting high- Tc cuprates. As suggested by the experimental angle-resolved photoemission spectroscopy and terahertz conductivity data in Bi2Sr2CaCu2O(8+delta), we focus on the interactions mediated by the order parameter fluctuations near an incipient second pairing transition d --> d + is(id'). We find interaction corrections to the density of states, specific heat, and conductivity as well as phase and energy relaxation rates and assess the applicability of the recent localization scenarios for noninteracting random Dirac fermions to the cuprates.     

Dual features of magnetic susceptibility in superconducting cuprates: a comparison to inelastic neutron scattering

Starting from the generalized t−J−G model Hamiltonian, we analyze the spin response in the superconducting cuprates taking into account both local and itinerant spin components which are coupled to each other self-consistently. We demonstrate that derived expression reproduces the basic observations of neutron scattering data in YBa₂Cu₃O_6+y compounds near the optimal doping level.     

Anomalous magnetic and superconducting properties in a Ru-based double perovskite

We have studied the double perovskite [1] structure Sr₂Y(Ru_1-x Cu_x)O₆ system. The parent compound is an antiferromagnetic insulator with Neel temperature ～ 26 K. Partially substituted the Ru ion by Cu the compounds increase their conductivity drastically and eventually become superconducting. More intriguingly is the observation of the coexistence of superconductivity and magnetic ordering. The superconducting transition temperature T _c and the magnetic ordering temperature T _m are of the same order. The observed magnetic structure and superconductivity of these compounds can be understood in terms of a plausible theoretical model based on the double exchange idea.     

Spin gaps and spin dynamics in superconducting cuprates

Spin gap effects, consisting of a declining uniform susceptibility and spin paramagnetic NMR shift at low temperaturesin the normal state and associatedT ₁ behavior, are discussed and documented in several cuprate superconductors. Dynamic spin magnetism in these systems is further reviewed in the light of mean-field models, where we note that detailed results from the model by Millis, Monien, and Pines are not borne out in recent neutron data on YBa₂Cu₃O_6.92.T ₁ data on¹⁷O in La_1.85Sr_0.15CuO₄ are presented, showing consistency with neutron dynamic susceptibility data forT≧80 K, but exhibiting a strong spin gap character below 80 K which is not present in the neutron data. Data for Zn-doped YBCO withT _c≈60 K are also presented, showing strong RKKY broadening from localized moments in the planes, but no spin gap effect such as that found in theT _c=60 K oxygen-deficient phase.     

On the superconducting gap dispersion in hole-doped cuprates

Solutions of the equation for the superconducting gap including superexchange, spin–fluctuation, plasmon, and phonon pairing mechanisms are obtained. Solutions of the Bardeen–Cooper–Schrieffer equation are approximated by the expression Δ_k = Δ₀(B cos(2?) + (1 ? B)cos(6?) at a carrier concentration close to optimal. It is found that the dependence proportional to cos(6?) is due to the spin–fluctuation and phononmediated interactions.     

Electron spin-lattice relaxation in layered cuprates and fullerides

A new technique of measuring extremely short electron spin-lattice relaxation times T₁～10^?6–10^?10 sec is developed and applied to study some oxide high-T_c materials, related compounds and fullerides RbC₆₀.     

Temperature dependence of the superconducting gap in high-Tc cuprates

It is proposed that the temperature dependence of the superconducting gap Delta(T) in high-T(c) cuprates can be predicted just from the knowledge of Delta(0) and the critical temperature T(c), and, in particular, Delta(0)/T(c)>4 implies that Delta(T(c)) not equal 0, while Delta(0)/T(c)相似文献   

Asymmetric superconducting gap and DDW state in high-Tc cuprates

The asymmetric gap superconductivity is considered in orthorhombic high T_c cuprates. Recent experiments predict an anisotropy in the gap where |Δ(0,π)|> |Δ(π,0)| and the gap node deviates from the diagonal direction toward the k_x axis. The temperature dependencies of the specific heat and penetration depth along the a and b directions are calculated for the anisotropic gap superconductors. However, the anisotropy in the penetration depth can be consistent with the experimental observations only after the inclusion of the plane and chain coupling. The d-density wave (DDW) phase that explains the pseudogap has also been considered to study the phase diagrams of the cuprates.