M. Ido

Pressure effects on superconductivity and structural phase transitions in La2−xMxCuO4 (M Ba, Sr)

Abstract Relations between the superconductivity and the crystal structure of La 2− x M x CuO 4 (M  Sr, Ba) are studied under high pressures up to 20 kbar. In the Sr-doped crystal, bulk superconductivity is observed both below and above the critical pressure P c where the high-temperature tetragonal (THT) phase is stabilized down to less than 20 K, the lowest temperature examined, instead of the orthorhombic phase. The superconducting transition temperature T c of the orthorhombic phase below P c is depressed in comparison with that of the THT phase above P c . The depression of T c is caused by a tilt of the CuO 6 octahedra in the orthorhombic phase. It is found for the Ba-doped crystal that the depression of T c for x⋍0.11−0.14 is removed, except for x⋍0.125 when a structure transition at T 1 ∼60 K disappears under high pressure. In the vicinity of x =0.125, there exists another origin for the depression of T c in addition to the structural transition at T c .

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.

Correlation between the Doping Dependences of Superconducting Gap Magnitude 2Δ 0 and Pseudogap Temperature T* in High-T c Cuprates

We report that in La 2- x Sr x CuO 4 (La214) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), the resistivity ρ and magnetic susceptibility χ decrease more sharply below T * , corresponding to the onset temperature of a spin gap (pseudogap) at least for Bi2212. In each system, T * scales with the superconducting gap magnitude 2Δ 0 ( T * ∼2Δ 0 /4.3 k B ), which increases as the doping level is lowered. We also report that 2Δ 0 is almost proportional to T max (> T * ), where T max is the temperature exhibiting a broad peak of χ and k B T max has been considered to give the measure of effective exchange energy J eff ; the α factors in 2Δ 0 ∼α k B T max ∼α J eff are nearly one and two for La214 and Bi2212, respectively.

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.

Non-ohmic conductivity of TaS3 in the low-temperature semiconducting regime

Abstract Electrical conductivity of the quasi one-dimensional conductor TaS3 was measured in the low-temperature semiconducting regime. Below 100K, the conductivity along c-axis (l-d axis) in the low field limit is characterized by the activated process with the activation energy of 250K, and the current-field (I-E) characteristic is nonlinear, dI dE increasing with the field strength. These results can be explained by the excitation of phase solitons associated with pinned CDW condensates.

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 .

Giant Resistivity Anomaly in ZrTe5

The group-IV transition-metal penta-chalcogenide, ZrTe 5 , shows a giant peak of the electrical resistivity around 150 K, suggestive of a structural phase transition.

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 observation of the coexistence of the pseudogap and superconducting quasiparticles in Bi2Sr2CaCu2O8 + y by time-resolved optical spectroscopy.

We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap (PG) and superconducting (SC) states of an underdoped Bi2Sr2CaCu2O8 + y (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy variant plancks over omegapr = 1.55 eV, it is found that the reflectivity change DeltaR/R changes its sign at exactly Tc, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below Tc, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.

Electronic states of doped holes and magnetic properties in La2−xMxCuO4 (M = Sr, Ba)

Abstract The magnetic susceptibility χ and resistivity ϱ were examined in La 2− x M x CuO 4 (M = Sr, Ba). The T -dependent part χ s of χ below x ⋍ x c ( x c = 0.15 and 0.18 for M = Ba and Sr) consists of a term arising from antiferromagnetically correlated Cu-spins χ s F ( T ), which is characterized by a broad peak at T max . On the other hand, χ s above x = x c consists of χ s F ( T ) and a Curie term ( C/T ) which is small but grows with increasing x . The superconducting transition temperature T c decreases rapidly with increasing x above x c . The relation between T c and C can be explained in terms of the magnetic depairing due to local moments. T ma x and the g -value of the Cu-spins are almost constant above x ⋍ 0.2 , although these are strongly x -dependent for x p ⋍ 0.2 go onto electronic states which differ largely in character from those below p ⋍ 0.2 . It is also found that a significant change in the ϱ - T curve occurs at x ⋍ 0.2 .