Non-adiabatic electron-lattice interactions in the copper-oxide superconductors

Transport measurements distinguish the superconductive phase from the parent antiferromagnetic and the overdoped metallic phases in La_2–xSr_xCuO₄. For small x, holes in the CuO₂ sheets form non-adiabatic polarons containing, experimentally, five Cu atoms. Within a polaron, a first-order contraction of the mean Cu-O bond length collapses the Hubbard U and transfers spectral weight from the Hubbard bands to the gap between them. In the superconductive phase above T_c, condensation into a polaron liquid produces an (k) dispersion curve, and dynamic pseudo Jahn-Teller deformations within the polaron give a Seebeck enhancement with T_max140 K. The polarons order pair-wise into 110 stripes alternating with parent-phase stripes; below T_c, the stripes are perfectly ordered. In the overdoped phase, the parent phase is squeezed out; the vibronic states become homogeneous.We thank the NSF, the Texas Advanced Research Program, and the Robert A. Welch Foundation, Houston, Texas, for financial support.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15–δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15?δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_n of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.

PACS: 74.72.Jt; 74.25.Bt; 75.40.Cx; 74.25.Fy     

Point contact investigation on Bi2Sr2CaCu2O8+y thin films

We have measured point contacts with a gold tip on Bi₂Sr₂CaCu₂O_8+y thin films and Bi₂Sr₂CaCu₂O_8+y/SrTiO₃ double layers. The results show tunneling or direct conductivity behaviour depending on the junction parameters and can be fitted by corresponding theoretical models. From fitting procedure of differential characteristics by modified Blonder-Tinkham-Klapwijk (BTK) theory the Fermi velocity v_F=5×10⁵ m/s, ratio 2(4.2K)/kT_c=7.9 and coherence length _ab=3.5 nm were obtained. The changing of the interface transmission by additional layers of different thickness corresponding to metallic or insulating behaviour is shown. In the frame of the inelastic scattering of quasiparticles the linear background of differential conductance is discussed.This work was supported by German BMFT under Contract No.l3N5924A and Slovak Grant Agency for Sciences (Grants Nos. 2/990125/93 and 2/999185/92).     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Instanton Sector of Correlated Electron Systems as the Origin of Populated Pseudo-gap and Flat “Band” Behavior: Analytic Solution

Finite temperature instantons between meta-stable vacua of correlated electronic system are solved analytically for quasi one-dimensional Hubbard model. The instantons produce dynamic symmetry breaking and connect metallic state with the dual vacua: superconducting (SC) and spin-density wave (SDW) states. The instantons spread along the Matsubara’s imaginary time and possess the structure similar to the coordinate-space solitonic lattices previously discovered in quasi one-dimensional Peierls model. On the microscopic level the inter-vacua excursion is described by mutual transformations between the “resonating quartets” of the couples of electron–hole and Cooper pairs. Spectral properties of the electrons in the “instantonic crystal” reveal pseudo-gap (PG) behavior, with finite fermionic density of states in the center of the PG and “flat-band” outside of it. Analytically derived inverse temperature scaling of the pseudo-gap and the densities of the SC and SDW condensates is discussed in the context of ARPES and STM data in high-T_c cuprates.     

Sr2YRu1?uCuuO6: Evidence for SrO-Layer Superconductivity

Sr₂YRuO₆ doped on its Ru site by Cu superconducts at the below 45 K, although its Ru and Cu are magnetically ordered at 23 K and 86 K, respectively. The SrO layers superconduct. Ba₂GdRuO₆, when doped with Cu, does not superconduct, because L = 0 Gd is not crystal-field split, and so induces Cooper pair-breaking.     

Is Tl2Ba2CuO6 a superconductor-metal array?

From structural analysis we have shown in a previous investigation that the series TlBa₂Ca _n Cu _n+1O_2n+5 and Tl₂Ba₂Ca _n Cu _n+1O_2n+6 are composed of superconductor-semiconductor arrays. In this paper, we demonstrate that the unit cell of the Tl₂Ba₂Ca _n Cu _n+1O_2n+6 series can further be viewed as composed of a composite block of superconductor-metal-semiconductor, where TlBa₂CuO₅ is the fundamental superconductor, TlO the metallic unit, and CaCuO₂ the semiconductor block. It is indicated that theT _c of Tl(2201) can be deduced from that of Tl(1201) based on Kresin's model of composite systems.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).