N. Momono

Strong pairing interactions in the underdoped region of Bi2Sr2CaCu2O8+σ

Abstract Doping effects on the superconducting gap of Bi2Sr2CaCu2O8+σ have been studied for 0.1 ⪅p⪅ 0.21 by STM/STS. The gap is consistent with dx2−y2 symmetry in the p-range examined. The gap magnitude 2 Δ0 increases with decreasing p even below p0 ∼ 0.17, at which Tc starts to decrease, and seems to correlate with the development of an antiferromagnetic correlation. The in-plane resistivity is reduced just like a superconducting precursor below T ∗ , at which pseudogaps open in the electronic and magnetic energy spectra. Furthermore, the reduced gap 2 Δ 0 /K B T ∗ is nearly constant for p ⪅ 0.21, 4 ∼ 5, comparable to the mean field value 2 Δ0/kBTc0 ∼ 4.3 for d-wave pairing states. These results suggest convincingly that incoherent pairs are preformed well above Tc at low doping levels where the pairing interaction is extremely strong but Tc is largely reduced.

Low-temperature electronic specific heat of La2−xSrxCuO4 and La2−xSrxCu1−yZnyO4. Evidence for a d wave superconductor

Abstract The electronic specific heat C el was studied at T ⩽ 10 K both on La 2− x Sr x CuO 4 samples and on La 2− x Sr x Cu 1− y O 4 samples below and above y c at which the superconductivity disappears. In superconducting La 2− y Sr x CuO 4 samples, the C el contains only a T 2 component at T « T c in a x range between 0.14 and 0.22, suggesting a gapless superconductor with line nodes. A T linear component of C el appears in superconducting La 2− x Sr x Cu 1− y Zn y O 4 ( y y c samples, and its coefficient, γ, increases markedly with Zn content. In samples with various Zn contents, the relation between T c / T c ( y = 0) and γ/γ( y ≥ y c ) can be understood in terms of strong impurity scattering in a d wave superconductor.

Energy gap evolution in the tunneling spectra of Bi2Sr2CaCu2O8+δ

On the basis of the tunneling spectra in Bi2212/vaccum/Bi2212 junctions fabricated using STM, we report that, in the electronic excitations, there exist two kinds of pseudogaps; one with a characteristic energy comparable to the superconducting (SC) gap and another one that is 3 to 4 times larger. The smaller energy-scale pseudogap (SPG) develops progressively below temperature T * , which nearly scales with the SC gap amplitude Δ 0 at T ≪ T c , in addition to the larger energy-scale pseudogap (LPG), which already exists above T * . The SPG smoothly develops into the SC-state gap with no tendency to close at T c .

Evidence for nodes in the superconducting gap of La2−xSrxCuO4. T2 dependence of electronic specific heat and impurity effects

Abstract The electronic heat Cel of La2−xSrxCuO4 (0.16 ≲ x ≲ 0.22) exhibits a T2 dependence at T ⪡ Tc. Substitution of as little as 0.3–0.5% Zn for Cu changes the T2 dependence of Cel into a quite different T dependence, described by the sum of T-linear and T3 terms, which is characteristic of a superconductor with lines of nodes in the gap such as a d wave superconductor. The relation between Tc/Tc0 and γ/γN for La2−xSrxCu1−yZnyO4 can be explained in terms of strong impurity scatterings in a d wave superconductor. The superconducting gap 2Δ0 of La2−xSrxCuO4, estimated from the T2 term of Cel on the basis of a nodal superconductor, decreases markedly with Sr content at x ≳ 0.16; this reduction of 2Δ0 seems to correlate with the suppression of a spin-singlet formation in the normal state. The ratio 2Δ0/T0(=7.5) for the optimum superconductor (x = 0.16) is consistent with the reported in a recent Raman scattering experiment.

Direct Evidence for an Intrinsic Square Vortex Lattice in the Overdoped High- Tc Superconductor La1.83Sr0.17CuO4+δ

We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+delta) single crystal (slightly overdoped, x = 0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d-wave order parameter or the presence of stripes.

Tuning competing orders in La2-xSrxCuO4 cuprate superconductors by the application of an external magnetic field

We report the results of a combined muon-spin rotation and neutron-scattering study on La2-xSrxCuO4 in the vicinity of the so-called 1/8 anomaly. Application of a magnetic field drives the system toward a magnetically ordered spin-density-wave state, which is fully developed at 1/8 doping. The results are discussed in terms of competition between antiferromagnetic and superconducting order parameters.

Coherent d-Wave Superconducting Gap in Underdoped La2-xSrxCuO4 by Angle-Resolved Photoemission Spectroscopy

We present angle-resolved photoemission spectroscopy (ARPES) data on moderately underdoped La

Electronic Specific Heat of La2-xSrxCuO4: Pseudogap Formation and Reduction of the Superconducting Condensation Energy

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Electronic structure near the 1/8-anomaly in La-based cuprates

Sr

Direct Observation of the Flux-Line Vortex Glass Phase in a Type II Superconductor

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