[en] The dependence of T_c on hydrostatic (He-gas) pressure for superconducting MgB₂ has been determined to 0.7 GPa. We find that T_c decreases linearly and reversibly under pressure at the rate dT_c/dP≅-1.11±0.02 K/GPa. These studies were carried out on the same sample used in earlier structural studies under He-gas pressure which yielded the bulk modulus B=147.2±0.7 GPa. The value of the logarithmic volume derivative of T_c is thus accurately determined, d ln T_c/d ln V=+4.16±0.08, allowing quantitative comparison with theory. The present results support the emerging picture that MgB₂ is a BCS superconductor with electron-phonon pairing interaction

[en] Studies of the effect of high pressure on superconductivity began in 1925 with the seminal work of Sizoo and Onnes on Sn to 0.03 GPa and have continued up to the present day to pressures in the 200 - 300 GPa range. Such enormous pressures cause profound changes in all condensed matter properties, including superconductivity. In high pressure experiments metallic elements, T_c values have been elevated to temperatures as high as 20 K for Y at 115 GPa and 25 K for Ca at 160 GPa. These pressures are sufficient to turn many insulators into metals and magnetics into superconductors. The changes will be particularly dramatic when the pressure is sufficient to break up one or more atomic shells. Recent results in superconductivity to Mbar pressures wll be discussed which exemplify the progress made in this field over the past 82 years

[en] Most lanthanide metals exhibit a volume collapse at a critical pressure Pc; for Dy this pressure is ∼73 GPa. The primary mechanism responsible for the volume collapse is a matter of debate and may involve the 4/ electrons themselves or be the result of simple pressure-induced s – d transfer in the conduction electrons. Possible mechanisms involving the 4/ electron system include: (i) valence increase, (ii) 4/ band formation, and (iii) increased 4/-conduction electron hybridization leading to a Kondo volume collapse. The present high pressure resistivity experiments on the dilute magnetic alloy Y(Dy) to 114 GPa give evidence for the validity of the Kondo volume collapse model for elemental Dy.

[en] Four-point electrical resistivity measurements were carried out on Nd metal and dilute magnetic alloys containing up to 1 at.% Nd in superconducting Y for temperatures 1.5–295 K under pressures to 210 GPa. The magnetic ordering temperature T_o of Nd appears to rise steeply under pressure, increasing ninefold to 180 K at 70 GPa before falling rapidly. Y(Nd) alloys display both a resistivity minimum and superconducting pair breaking ΔT_c as large as 38 K/at.% Nd. The present results give evidence that for pressures above 30–40 GPa, the exchange coupling J between Nd ions and conduction electrons becomes negative, thus activating Kondo physics in this highly correlated electron system. Furthermore, the rise and fall of T_o and ΔT_c with pressure can be accounted for in terms of an increase in the Kondo temperature.

[en] Of the 52 known elemental superconductors among the 92 naturally occurring elements in the periodic table, fully 22 only become superconducting under sufficiently high pressure. In the rare-earth metals, the strong local magnetic moments originating from the 4f shell suppress superconductivity. For Eu, however, Johansson and Rosengren have suggested that sufficiently high pressures should promote one of its 4f electrons into the conduction band, changing Eu from a strongly magnetic (J=7/2) 4f⁷-state into a weak Van Vleck paramagnetic (J=0) 4f⁶-state, thus opening the door for superconductivity, as in Am (5f⁶). We report that Eu becomes superconducting above 1.8 K for pressures exceeding 80 GPa, T_c increasing linearly with pressure to 142 GPa at the rate +15 mK/GPa. Eu thus becomes the 53rd elemental superconductor in the periodic table. Synchrotron x-ray diffraction studies to 92 GPa at ambient temperature reveal four structural phase transitions.

[en] Ac susceptibility measurements have been carried out on superconducting LaO_1-xF_xFeAs for x=0.07 and 0.14 under He-gas pressures to ∼0.8 GPa. Not only do the measured values of dT_c/dP differ substantially from those obtained in previous studies using other pressure media, but also the T_c(P) dependences observed depend on the detailed pressure/temperature history of the sample. A sizeable sensitivity of T_c(P) to shear stresses provides a possible explanation.

[en] Hydrostatic high-pressure studies to 17 GPa were carried out on superconducting YBa₂Cu₃O_x over the full range of oxygen content x. The observed bell-shaped T_c(P) dependences are found to depend markedly on the temperature at which the pressure is changed. The time-dependent relaxation processes are studied using both T_c and the electrical resistivity at 298 K as probes. The activation volume for oxygen diffusion is determined from the pressure-dependent activation energy and compared with estimates from a simple hard-sphere model. (c) 2000 The American Physical Society

[en] Nearly hydrostatic pressures are found to enhance the critical current density J_c through single grain boundaries in YBa₂Cu₃O_x rings and to induce superconductivity in Li. Whereas Li becomes superconducting above 20 GPa at temperatures as high as 15 K, no superconductivity was observed above 4 K in Na to 65 GPa nor in K above 4 K to 43.5 GPa or above 1.5 K to 35 GPa