O. Gorochov

Superconductivity and possible carrier localization in YBa2Cu3−xZinxO7−y

Resistivity and Hall data on single phase YBa2Cu3−xZnxO7−y ceramic samples are reported. Though Tc decreases at a rate of 13 K/at.% as x increases, the carrier density seems to be little affected at 300 K. However, the Hall mobility decreases and varies hardly with temperature as x increases. The normal state resistivity behaviour changes from metallic to semiconducting like which is compared with other cuprate superconductors. The resulting decrease in Tc is discussed in terms of the possible carrier localization. Some appropriate models are briefly reviewed.

Two-dimensional defects in InSe

Two‐dimensional precipitates associated with stacking faults in layer semiconductors have previously been put forward to explain transport properties of these crystals, especially their remarkable electrical anisotropy. High‐field cyclotron resonance behavior, among others, can be accounted for by two‐dimensional accumulation layers in the vicinity of these defects. Direct evidence for the existence of these defects has been obtained by electron microscopy and x‐ray microprobe analysis in indium selenide. Planar faults act like sinks for impurity atoms. This accounts for the unique behavior of layer compounds which exhibit intrinsic behavior (low apparent carrier concentration—high mobilities) even with high (100 ppm) initial doping levels. Optical, transport, and magnetotransport properties at low temperature can be explained along this model. As regards the applications of indium selenide to the photovoltaic conversion of solar energy, the existence of these defects explains most features of this semiconductor in this respect: (i) Its comparatively low effective diffusion length parallel to the c axis. (ii) p‐ to n‐type switching under thermal annealing which allows fabrication of p‐n junctions. (iii) Low apparent carrier density which precludes abrupt profiles for p‐n structures.

Critical carrier density and revival of superconductivity in Ca substituted YSrBaCu2.6M0.4O6+z(M= Al, Fe)

Abstract We report on the preparation, X-ray diffraction, resistivity, hole density (p) and ac susceptibility measurements of Y 1−x Ca x SrBaCu 2.6 Al 0.4 O 6+z . For x=0, superconductivity (SC) is destroyed and p reaches a value of 1.5 10 21 /cm 3 . SC is dramatically restored when Ca is substituted. T c values are 29 and 47 K for x = 0.1 and 0.2 respectively. A relation between p, T c and x is observed. Similar data are obtained in the case of Fe(0.4) substituted samples though the maximum value of T c is around 30 K when 0.15 Ca is co-substituted.

Fermi liquid reflectivity of TiS2

Abstract Reflectivity measurements on single crystal TiS2 reveal an unusual temperature variation in the frequency range 500–2500cm-1. Drude model fits to the data yield a scattering rate τ-1 ≈ α[(pT)2 + ω2] that is compatible with electron-electron scattering in an anisotropic Fermi liquid. The estimated plasma frequency ωpl = 1200cm-1 agrees with Hall effect data. The temperature variation of τ(T) is in accord with the d.c. resistivity. Similarities with the heavy Fermion UPt3 and contrasts to high Tc superconductors are discussed.

Hall effect studies of YBa2Cu3−xFexO7+y

Abstract Hall data of YBa 2 Cu 3−x Fe x O 7+y indicate hole conductivity for 0≤x≤0.15. The linear dependence of 1/R H e vs T disappears for x ⋍ 0.04. 1/R H e follows a regular curve as a function of T c , identical to that observed for undoped samples with varying oxygen concentration. The data indicate that the relation between T c and the carrier density is nearly independent of the way in which the latter is varied.

Diamagnetic shielding and spin relaxation of Fe3+ ions in YBa2Cu3O7-δ

In YBa2Cu2.985Fe0.015O7, the iron substitution and (consequently) the effect on superconductivity (SC) are low. Magnetically split Mossbauer spectra are obtained in the presence of an external field of 5.5 T at low temperatures, and they have been fitted to spin relaxation spectra appropriate for Fe3+ ions in the high-spin state. The analyses show near-perfect diamagnetic shielding of a component spectrum at low temperatures, characterised by a low spin relaxation frequency and consequently a larger SC band gap Delta . It appears that planes containing Cu2 ions are mainly superconducting, though SC extends to a smaller degree to the Cu1 sites. The authors argue that the results do not imply coexistence of magnetism and SC but of isolated magnetic ions and SC.

Valence state of Tm in Yb1−xTmxSe

Abstract The lattice constant (a0), electrical resistivity (ϱ), Hall constant (RH), magnetic susceptibility (χ), and LIII-edge X-ray absorption spectra (LIII-XAS) of Yb1−xTmxSe (0⩽x⩽0.6) single crystals were measured in order to determine the mean valence ( v ) of Tm in a diamagnetic insulating host. In the semiconducting phase (x⩽0.5), χ indicates v = 2 whereas a0 leads to v = 2.10 . ϱ decreases by 5 orders of magnitude in the range 0.11⩽x⩽0.56. For the sample with x = 0.43, LIII -XAS reveals a mixed valent state of Tm.

Cu-O chains contribution to the electronic Raman scattering of a YBa2Cu3O7 single crystal

Abstract Micro Raman spectroscopy measurements have been performed inside an untwinned domain of a YBa2Cu3O7 single crystal. Polarization analysis and symmetry considerations on the band structure suggest that the low frequency electronic continuum coupled to the Ba normal mode at 117 cm-1, can be attributed to the electron-hole excitations coming from the Cu-O chains bands lying near the Fermi energy.

Preparation, electron microscopy and superconductivity of a new class of high Tc superconductors: Pb2Sr2Y1-xCaxCu3O8+δ (x = 0.5; 0.25)

Abstract Preparation, electron microscopy, resistivity (ζ) and field cooled susceptibility (χ) of Pb 2 Sr 2 Y 1- x Ca x Cu 3 O 8+ δ ( x =0.5; 0.25) are reported. They crystallise in an orthorhombic structure. ζ shows a two step transition with distinct onsets at 86 and 45 K with zero resistivity at 15 K. χ data show a diamagnetic onset at 74 K. These data while confirming the results of Cava et al (Nature 336, 211 1988) point out the influence of the preparation conditions on the superconducting properties calling for a systematic study.

Anomalous valence of Tm in Yb1-xTmxSe

The lattice parameter (a0), susceptibility ( chi ), resistivity ( rho ) and Hall effect of Yb1-xTmxSe are reported. For x<0.5, data show a valence vm=2 for Tm that is consistent with semiconducting behaviour. For x=0.84, there is a sudden decrease in a0 accompanied by an increase in vm to 2.58 Tm and rho decreases by a factor of 103. These data reflect an evolution of f-d hybridisation as x increases.