H. Hemmes

Pressure dependence of the T/sub c/ of YBa/sub 2/Cu/sub 3/O/sub 7/ up to 170 kbar

The superconducting onset temperature T/sub co/ of single-phased YBa/sub 2/Cu/sub 3/O/sub 9-//sub delta/ (with deltaapprox. =2) measured resistively in a diamond anvil cell is found to increase at a rate dT/sub co//dp = 0.043 K kbar/sup -1/ up to 170 kbar. This is much weaker than for La-Ba-Cu-O for which dT/sub co//dp = 0.64 K kbar/sup -1/. The pressure dependence of the high-T/sub c/ superconductors measured so far cannot be explained within a standard electron-phonon Bardeen-Cooper-Schrieffer theory. Predictions of resonating-valence bonds and bipolaronic theories are discussed.

Thermal expansion, compressibility and Grüneisen parameter of YBa2Cu3Ox

Abstract The thermal expansion of YBa2Cu3Ox has been measured between 4 K and 1100 K. The bulk isothermal compressibility has been determined from direct measurement of the pressure-volume relation on compacted samples by means of a simple powder model which takes into account the porosity of the sample. From these data the average Gruneisen parameter 〈γ〉 ≡ -d ln 〈ω〉 / d ln V is found to be 3.1.

Electrical properties and Tc of Bi2CaSr2Cu2O8 up to pressures of 15 GPA

Abstract By means of four-point resistivity measurements the critical temperature of Bi 2 CaSr 2 Cu 2 O 8 was studied under high pressure. Evidence for three phases, one non-superconducting and two superconducting with T c s of 80 and 110 K is found. Both T c s are only weakly pressure dependent. The non-superconducting background increases with pressure and with measurement current indicating the presence of weak links and a “weakening” of these links with pressure.

Conventional and high T-c superconductors under high pressure

Abstract By means of a cryogenic diamond anvil cell the pressure dependence of the superconducting Tc of i) palladium hydrides and deuterides and ii) various high-Tc superconductors (YBa2Cu3O7, CaLaBaCu3O7 and Bi2CaSr2Cu2O8) has been determined. The results agree with the general trend of dln Tc/dp versus Tc observed previously. A possible explanation of this trend is given and exceptions discussed.

Superconducting Pd-Hydrides and -Deuterides Synthesized under High Pressure in a Diamond-Anvil-Cell*

A diamond anvil cell for pressures up to 1 Mbar has been developed for the synthesis of metal-hydrides in thermodynamic equilibrium with compressed molecular hydrogen gas. As example of an application of this new technique results on the critical temperature and its pressure dependence for stoichiometric palladium hydrides and deuterides are presented and discussed within the context of the standard BCS-theory of superconductivity. The behaviour of PdH and PdD under pressure is compared to that of the recently discovered high-Tc superconducting metal-oxides and fundamental differences are found.

Thermodynamic properties of hydrogen between 100 and 1000 K at pressures up to 1 Mbar

Abstract Using the available data from the literature in the fluid and solid phase we have constructed a consistent equation of state for molecular hydrogen. The relevant thermodynamic properties can be derived from the equation of state.

The formation of metal hydrides under very high hydrogen pressure

Abstract The formation of metal hydrides is described in terms of a mean field lattice gas model. The parameters of this model are derived for AgH x , which is expected to become a high T c superconductor for x ∼0.8. Two alternative complete sets of pressure-composition isotherms are given for pressures up to 1 Mbar and temperatures up to 1000 K.

Volume dependence of high-Tc superconductors

Abstract The pressure dependence of Tc is found to be large for the superconductors (La Ba Cu O, La Sr Cu O) with a relatively low Tc and small for Y Ba Cu O. The implications of this trend for the theories of high-Tc superconductivity (especially those based on a Hubbard-like model) are discussed.

Pressure dependence of the Tc of superconducting metal-hydrides, measured in a diamond anvil cell

Abstract For the preparation of high concentration metal-hydrides a very large chemical potential of molecular hydrogen is needed. This can be achieved by means of a high pressure diamond anvil cell. We developed a diamond anvil cell technique for filling the sample space at cryogenic temperatures with molecular hydrogen, loading metal samples with hydrogen at high pressures and in-situ measurements of the resistance and superconducting transition of the sample.