C. Looney

Pressure-induced oxygen ordering phenomena in high-Tc superconductors

Abstract High-pressure experiments on polycrystalline and single-crystalline Tl 2 Ba 2 CuO 6+δ with varying oxygen content reveal the existence of distinct low (70–100 K) and high (180–270 K) temperature oxygen-defect ordering processes corresponding to activation energies E A ≈ 0.25 eV and 0.7 eV, respectively. Hall effect studies show that for both processes the effect of oxygen ordering on the superconducting transition temperature T c is the result of a transfer of hole carriers to or from the CuO 2 plane. We also report the first observation of oxygen-ordering effects in TlSr 2 CaCu 2 O 7−δ . Evidence is discussed for the existence of oxygen-ordering processes in YBa 2 Cu 3 O 7− y at both low and high temperatures which may be responsible for the phenomena of transient and persistent photo-induced superconductivity.

Effect of the pressure-temperature history on Tc and the Hall coefficient in superconducting Tl2Ba2CuO6+x

Abstract The Hall coefficient R H of Tl 2 Ba 2 CuO 6+ x has been investigated as a function of both temperature (50–110 K) and hydrostatic pressure (0–0.6 GPa) utilizing a He-gas high-pressure system. The value of the pressure derivative d R H /d P at a given temperature is found to depend systematically on the temperature at which the pressure is changed. A similar P - T path dependence for the superconducting transition temperature T c in this system was previously reported by Sieburger and Schilling who observed that the value of d T c /d P depends markedly on whether the pressure is changed at room temperature or low temperature (∼50 K). We find that the relation between T c and the Hall coefficient obtained by changing pressure at room temperature is quite different from that obtained by changing pressure at low temperature. The dependences of R H and T c on the P - T path are believed to be associated with the pressure-induced rearrangement of interstitial oxygen within the Tl-O double layer.

Effect of hydrostatic pressure on Tc in superconducting YNi2−xMxB2C (M = Co, Cu)

Abstract The dependence of the superconducting transition temperature T c on hydrostatic pressure to 0.7 GPa has been determined for four YNi 2− x M x B 2 C compounds where up to 15% of Ni is substituted by Co or Cu. In all cases T c is found to decrease with pressure in the range −0.09 to −0.19 K/GPa. It is shown that these results are consistent with BCS-like superconductivity in transition metal systems where changes in both electronic and phononic degrees of freedom are required to account for the variation of T c with pressure.

Dependence of Tc on hydrostatic pressure in β″-(ET)2SF5CH2CF2SO3 and κ-(ET)2Cu(NCS)2

Abstract The dependence of Tc on hydrostatic (He-gas) pressure is determined for the recently discovered organic superconductor β″-(ET)2SF5CH2CF2SO3 [ET = bis(ethylenedithio)-tetrathiafulvalene] with T c (0) ⋍ 5 K , yielding the pressure derivative d T c d P ⋍ −1.34 K kbar −1 . The present experiments also included κ-(ET)2Cu(NCS)2 where we find the extremely large value d T c d P ⋍ −3.84 K kbar −1 , in agreement with earlier studies. For both samples the pressure dependence Tc(P) does not depend on the temperature at which the pressure is changed.

Possibility for oxygen relaxation below 15 K in superconducting Tl2Ba2CuO6 + x

Abstract Previous high-pressure experiments to 0.8 GPa on a superconducting Tl 2 Ba 2 CuO 6 + x crystal have revealed for temperatures above 55 K distinct low- and high-temperature relaxation processes, presumably originating in the oxygen sublattice. In the present experiment, a minute ∼1 μg piece of the same crystal is placed in a diamond-anvil cell loaded with dense helium and subjected to ∼ 6 GPa hydrostatic pressure, depressing T c from 40 K to 9 K. Relaxation in T c is observed for temperatures as low as 13 K.

The influence of pressure-induced relaxation effects on Tc in TlSr2CaCu2O7−δ

Abstract High pressure experiments on polycrystalline TlSr 2 CaCu 2 O 7− δ (Tl-1212) reveal relaxation effects in T c that vary strongly with oxygen content. Unlike Tl 2 Ba 2 CuO 6+ δ , where distinct low- and high-temperature effects are found, relaxation in Tl-1212 occurs only for temperatures above 210 K. This relaxation is believed to originate from the ordering of oxygen vacancies in the TlO layer.