Pressure dependence ofT_cin the MgB₂super- conductor as probed by resistivity measurements

Abstract

High-pressure resistivity experiments were performed on the recently discovered superconductor, MgB2. Tc decreases quasi-linearly with applied pressure to 1.4 GPa at a rate of -2.0(1) K GPa-1, which is somewhat larger than that derived from ac susceptibility measurements. The reduction of Tc is consistent with the BCS picture, in a similar way to the C60-based superconductors. Taking into account the pressure dependence of the unit cell volume, V, the volume coefficient of Tc, d(ln Tc)/dV is significantly large.

Dependence ofTcon hydrostatic pressure in superconductingMgB2

“…2 to have no effect on the pressure dependence T c (P ); indeed, solid helium is the softest solid known. Our value of dT c /dP differs significantly from those of other groups [17,18,19] (see discussion above) using pressure media which are either solid at RT or freeze upon cooling down at temperatures well above T c .In view of the strong compressibility anisotropy [12] and the sizeable anharmonicity and non-linear electron-phonon coupling[8] anticipated for MgB 2 , it is likely that shear stresses of sufficient magnitude will cause appreciable changes in the pressure dependence of T c , as observed for other anisotropic substances such as the superconducting oxides [25] and organic superconductors [26]. The largest shear stresses are generated by changing the pressure on a solid pressure medium, such as steatite, or using no pressure medium at all.…”

“…2 to have no effect on the pressure dependence T c (P ); indeed, solid helium is the softest solid known. Our value of dT c /dP differs significantly from those of other groups [17,18,19] (see discussion above) using pressure media which are either solid at RT or freeze upon cooling down at temperatures well above T c .…”

“…In the first experiment utilizing a fluid pressure medium (Fluorinert), Lorenz et al [17] report that T c decreases linearly with pressure to 1.8 GPa at the rate dT c /dP ≃ −1.6 K/GPa. On the other hand, using the same pressure medium, Saito et al [18] find the more rapid decrease dT c /dP ≃ −2.0 K/GPa. Both groups cite their results to argue that MgB 2 is a BCS phonon-mediated superconductor [17,18], as is also argued by Loa and Syassen [13].…”

“…On the other hand, using the same pressure medium, Saito et al [18] find the more rapid decrease dT c /dP ≃ −2.0 K/GPa. Both groups cite their results to argue that MgB 2 is a BCS phonon-mediated superconductor [17,18], as is also argued by Loa and Syassen [13]. In further experiments to 25 GPa utilizing the solid pressure medium steatite, Monteverde et al [19] find that T c decreases under pressure at differing initial rates (-0.35 K/GPa to -0.8 K/GPa), T c (P ) showing a quadratic behavior for two of the four samples studied; each of the four samples was prepared using a different synthesis procedure.…”

“…The shear stresses generated in cooling Fluorinert or other comparable liquids through the melting curve are admittedly much smaller and depend on details of the individual experimental procedures used, such as the cooling rate, change in applied force upon cooling, etc. Only experiment 4 can determine whether or not the T c (P ) dependences measured in the Fluorinert experiments [17,18] are influenced by shear stresses. To exclude the possibility of sample-dependent effects, such experiments should be carried out on a single sample.…”