Microscopic Bardeen-Cooper-Schrieffer formulation of the critical temperature of multilayer copper-oxide superconductors

We study superconductivity in multilayer copper oxides, in the frame of a realistic microscopic formulation. Solving the full temperature dependent BCS gap equations, we obtain a maximum in the transition temperature T_c for M = 3 or 4 CuO₂ layers in the unit cell for appropriate values of the interlayer tunneling (negative pair tunneling), and via the consideration of the doping imbalance between the inner and outer layers. This is the ubiquitous experimental result for Ca intercalated copper oxides, as opposed to other intercalating elements. Further, using a restricted set of parameters, we obtain an exact fit of T_c(M = 1–4) for five different Ca intercalated homologuous copper oxide families.     

Combinatorial synthesis and high-throughput characterization of copper-oxide superconductors

Fast synthesis and screening of materials are vital to the advance of materials science and are an essential component of the Materials Genome Initiative. Here we use copper-oxide superconductors as an example to demonstrate the power of integrating combinatorial molecular beam epitaxy synthesis with high-throughput electric transport measurements. Leveraging this method, we have generated a phase diagram with more than 800 compositions in order to unravel the doping dependence of interface superconductivity. In another application of the same method, we have studied the superconductorto-insulator quantum phase transition with unprecedented accuracy in tuning the chemical doping level.     

Application of organic and copper-oxide superconductors for EPR imaging

A new electron paramagnetic resonance (EPR) imaging method on the surface of a super-conducting finite-size plate is experimentally demonstrated. The single crystals of the layered organic charge-transfer saltk-(ET)₂Cu(NCS)₂ and copper oxide YBa₂Cu₃O_6.95 are applied to create field gradients up to 0.5 and 5 kG/cm respectively. Surface gradient mapping at different temperatures and de field excursions is carried out. Qualitative explanation for the gradient mechanism in terms of the inhomogeneous critical state under field cycling is presented. Reconstruction of one-dimensional EPR images for model samples is presented at low temperatures. Spatial resolution of the order of 1 μm is experimentally reached on a standard X-band continuous-wave EPR spectrometer.     

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     

铜氧化物高温超导体中的电子有序态

铜氧化物高温超导现象自30年前被发现以来,对现代凝聚态物理的发展产生了极其重要的影响,然而其微观机制至今依然是一个谜。近年来,多种实验手段的研究结果发现,在铜氧化物高温超导体中电子除了形成库珀对,还可能形成多种其他新奇的有序态,例如自旋有序态、电荷有序态以及库珀对密度波等。这些有序态的起源及其与赝能隙态和超导态的关联对于理解高温超导机理可能具有重要的意义。文章将主要从实验的角度对铜氧化物超导体中的电子有序态做一个概述。     

Spin triplet pairing in high temperature superconductors

Various superconductivity mechanisms show that the electron pair is in spin singlet (S=0) belowTc. In contrast with these, our EPR measurements show that the electron pair is in spin triplet (S=1) rather than the spin singlet. Since an intense zero field absorption was observed in the high temperature superconductors of YBa₂Cu₃O_7–y , and YBa₂Cu₃O_7–y F₁, which violated various spin-pairing singlet superconductivity mechanisms.     

Coherence transition in granular high temperature superconductors

We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa₂Cu₃O_7−δ, YBa₂(Cu_2.98Zn_0.02)O_7−δ and GdBa₂Cu₃O_7−δ. The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T_co, which is close to the point where resistivity vanishes. The specific heat results, when analyzed as dC/dT, show a weak but reproducible cusp-like anomaly at T_co. This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.     

Symmetry classification of states in high temperature superconductors

The symmetry classification of possible singlet and also triplet states in the case of the 2-dim square lattice, and 3-dim tetragonal and orthorhombic lattices is examined. If particle-hole symmetry is present then an additional symmetry classification is possible. However in the lower symmetry crystal structures that actually occur, no distinction on symmetry grounds can be drawn between usuals-wave, extendeds-wave and some of thed-wave states.     

Are the high temperature superconductors real metals?

A comparison of the experimental band structure of oxygen on Cu(110) and the theoretical band structure of the prototype material for the high temperature superconductors La₂CuO₄ is performed. This comparison suggests, that the theoretical band structure describes the occupied electronic states of La₂CuO₄ adequately.     

Magnetic fluctuations of stripes in the high temperature cuprate superconductors

Within the time-dependent Gutzwiller approximation for the Hubbard model we compute the magnetic fluctuations of vertical metallic stripes with parameters appropriate for La(1.875)Ba(0.125)CuO(4) (LBCO). For bond- and site-centered stripes the excitation spectra are similar, consisting of a low-energy incommensurate acoustic branch which merges into a "resonance peak" at the antiferromagnetic wave vector and several high-energy optical branches. The acoustic branch is similar to the result of theories assuming localized spins whereas the optical branches are significantly different. Results are in good agreement with a recent inelastic neutron study of LBCO.     

Breakdown of one-parameter scaling in quantum critical scenarios for high-temperature copper-oxide superconductors

We show that if the excitations which become gapless at a quantum critical point also carry the electrical current, then a resistivity linear in temperature, as is observed in the copper-oxide high-temperature superconductors, obtains only if the dynamical exponent z satisfies the unphysical constraint, z < 0. At fault here is the universal scaling hypothesis that, at a continuous phase transition, the only relevant length scale is the correlation length. Consequently, either the electrical current in the normal state of the cuprates is carried by degrees of freedom which do not undergo a quantum phase transition, or quantum critical scenarios must forgo this basic scaling hypothesis and demand that more than a single-correlation length scale is necessary to model transport in the cuprates.     

Nonlocal composite spin-lattice polarons in high temperature superconductors

The nonlocal nature of the polaron formation in t - t '- t' - J model is studied in large lattices up to 64 sites by developing a new numerical method. We show that the effect of longer-range hoppings t' and t' is a large anisotropy of the electron-phonon interaction (EPI) leading to a completely different influence of EPI on the nodal and antinodal points in agreement with the experiments. Furthermore, nonlocal EPI preserves polaron's quantum motion, which destroys the antiferromagnetic order effectively, even in the strong coupling regime, although the quasiparticle weight in angle-resolved-photoemission spectroscopy is strongly suppressed.     

Observation of sharp-line lanthanide fluorescence in high temperature superconductors

Lanthanide fluorescence has been observed for the first time in the high temperature superconductors that have been recently discovered. Laser excited fluorescence and fluorescence excitation spectra of La_1.85-xRE_xSr_0.15CuO₄ for RE=Pr and Eu and x=0.02, 0.18, 0.36 and 1.85 are presented. There is substantial inhomogeneous broadening caused by the Sr²⁺ in the materials containing a single rare earth and additional broadening caused by the ionic size mismatch in the materials with two rare earths. The samples have 3 crystallographic sites for the lanthanide ion. No fluorescence was observed for the higher temperature superconductors, Y₁Ba₂Cu₃O_9-δ, doped with 10 and 100% Eu³⁺.     

Scaling analysis of high temperature superconductors under pressure

The scaling relation of single parameter scaling hypothesis is applied to the study of the scaling behavior of high temperature superconductors under pressure. The data of resistance and specific heat coefficient under various pressures are scaled onto a universal curve according to this scaling relation. The scaling parameters are pressure dependent while temperature independent. It is found that the controlling parameter Bi equals to the relative critical temperature tcP, which indicates that the superconducting energy gap at the zero temperature 2Δs0 is the controlling parameter in this scaling.