Shigenobu Sakita

Superconducting energy gap of La1.85Sr0.15CuO4 single crystals from break-junction tunneling

Abstract The break-junction tunneling technique has been applied to measure the superconducting gap of La 1.85 Sr 0.15 CuO 4 single crystals with T c = 33 K. Since this technique provides the cleanest junction interface, the influence of degraded surface layers can be avoided. The observed gap structures possess BCS-like characteristics with sharp gap peaks and very low leakage below the gap voltage. A straightforward interpretation of these features can be given by the s-wave symmetry of the pairing state. The ratio of 2 Δ / k B T c as a measure of the pairing strength is estimated to be 8.9 ± 0.2.

Huge Lattice Softening in CeRu2 without Structural Transition

Symmetrized elastic moduli of high-quality single-crystalline CeRu 2 have been measured between 2 and 300 K by means of an ultrasonic technique. Both transverse moduli ( C 11 - C 12 )/2 and C 44 soften markedly from 300 to ∼20 K without structural transition. In contrast, the bulk modulus C B shows a normal temperature dependence without softening. These results indicate that there exists a marked structural fluctuation corresponding to a pure transverse strain in the normal conducting state of CeRu 2 . Our analysis, taking account of the deformation potential coupling, reveals that a narrow band with relatively high density of states at E F is responsible for the huge lattice softening.

Structural Transitions and Localization in La2-x-yNdySrxCuO4 with p - 1/8.

In order to clarify the influence of lattice deformation on the electronic states, we have measured ultrasound velocity and X-ray diffraction of La 2- x - y Nd y Sr x CuO 4 (LNSCO) with a particular carrier number of p ∼1/8. As temperature is reduced, LNSCO exhibits a first-order transition to the low-temperature tetragonal phase P 4 2 / n c m , after a second-order transition from the well-known orthorhombic phase B m a b to another orthorhombic phase P c c n . A semiconductor-like upturn is observed in resistivity, indicating tendency of localization in the normal state in these low-temperature phases of P c c n and P 4 2 / n c m . These structural changes are also reflected on the magnetic properties, which are well understood only with taking into consideration the antiferromagnetic coupling between Nd and Cu spins.

Phonon Raman Scattering Study of K 2NiF 4 Type Oxide Superconductors

La (P 1 ) and apical oxygen (P 2 ) vibrations along the c -axis, which are A 1 g -symmetry in the tetragonal phase ( I 4/ m m m ), have been systematically investigated for K 2 NiF 4 -type oxide superconductors; La 2 CuO 4+δ (δ=0, 0.014, 0.063), La 1.85 Sr 0.15 CuO 4 , and Sr 2 RuO 4 . For La 2 CuO 4+δ , with increasing of the excess oxygen concentration (δ), the intensity of P 2 decreases exponentially as a function of δ at room temperature. This decrease can be understood by the shielding effect due to the doped carriers along the c -axis. With decreasing temperature, the intensity of P 2 increases for La 2 CuO 4.063 and La 1.85 Sr 0.15 CuO 4 at low temperature, while La 2 CuO 4 and Sr 2 RuO 4 do not show such anomaly. This might be explained by the decrease of the shielding effect due to the charge confinement along the c -axis. The appearance of the excess oxygen vibration along the c -axis shows that the transition at about 250 K is due to the freezing of the itineracy of the excess oxygen for La 2 C...

Nonmetallic behavior of out-of-plane conduction in La2−xSrxCuO4

Abstract Pressure effect on the out-of-plane and in-plane resistivities has been investigated for single-crystalline La2− x Sr x CuO4 with x = 0.145 and 0.25. The out-of-plane conduction for optimally-doped La2− x Sr x CuO4 is explained in terms of a hopping process between the CuO2 planes which have anomalous coherent conduction. For highly-doped La2− x Sr x CuO4, the out-of-plane conduction is approaching a coherene metallic regime.

Transport properties of detwinned crystals of La2−xSrxCuO4

Abstract We report out-of-plane and in-plane resistivities of detwrmined La 2− x Sr x CuO 4 crystals. It is found that the conduction along the c -direction within the twin boundaries is not at all negligible compared with the intrinsic conduction along the c -axis within a domain. The anisotropy in the in-plane resistivity is interpreted as due to anisotropic scattering by phonons.

Nonmetallic conduction along thec axis in La2−x Sr x CuO4−δ

For single-crystalline La2−xSrxCuO4−δ (LSCO) we have investigated the effect of scattering at the twin boundaries and at the defects of oxygen on the resistivity along thec-axis ρc. These scattering processes, which affect the in-plane resistivity only very weakly, noticeablydecrease ρc.

INVESTIGATION OF ELECTRON-PHONON INTERACTION IN LA2-XSRXCUO4 BY RAMAN SCATTERING

Abstract Phonon Raman scattering spectra in the (c, c) geometry for La 2− x Sr x CuO 4 ( x =0, 0.1, 0.113, 0.15, 0.22) have been systematically measured. With increasing x , we have found that the line shape of the apical oxygen vibration along the c -axis becomes asymmetric and the background intensity increases. The line shape is analysed by a Fano-type interference model with the electronic excitation. The results indicate the existence of some correlation between the electron–phonon coupling constant and superconducting temperature for La 2− x Sr x CuO 4 .

Electronic and structural instabilities of single-crystalline La2−x−yNdySrxCuO4 near 1/8 doping

Abstract We have performed ultrasonic measurements, X-ray diffraction, resistivity and susceptibility measurements in order to investigate the correlation between the structural instability and the electronic states in the normal state of single-crystalline La 2- x − y Nd y Sr x CuO 4 with x =1/8 and y =0.4. There exist two structural phase transitions at low temperatures, one of which is of the 2nd order (OMT-OLT) and the other is of the 1st order (OLT-TLT). The phase transition to the TLT phase appears to couple to the electronic instability leading to the local suppression of superconductivity.

Pressure effects on out-of-plane resistivity in La2−xSrxCuO4

Abstract We have investigated pressure and temperature dependence of out-of-plane resistivity ϱc for La2−xSrxCuO4. By applying pressure, we found a drastic decrease in ϱc which reaches 50% at 8 GPa. If the out-of-plane conduction is caused by the tunneling or hopping between adjacent CuO2 layers, then the strong pressure dependence of ϱc is understood in terms of a small variation of the interlayer spacing. In comparison with a change of ϱc(T) estimated from the thermal contraction of interlayer spacing, the well-known T-linear term in ϱc does not mean the coherent band conduction. Therefore, the nonmetallic ϱc(T) is a key feature for understanding the exotic electronic states of high-Tc cuprates. We propose one of the possible functional forms of ϱc(T) which reasonably include both the high-temperature T-linear term AT + B and the semiconductor-like upturn at low temperatures.