arXiv subjects

T. Tomita

Publications and source records attributed to T. Tomita.

23 records · Page 2Linked to original sources

arXiv2005

Negative thermal expansion of MgB$_{2}$ in the superconducting state and anomalous behavior of the bulk Grüneisen function

The thermal expansion coefficient $α$ of MgB$_2$ is revealed to change from positive to negative on cooling through the superconducting transition temperature $T_c$. The Grüneisen function also becomes negative at $T_c$ followed by a dramatic increase to large positive values at low temperature. The results suggest anomalous coupling between superconducting electrons and low-energy phonons.

cond-mat.supr-con↗

arXiv2004

Enhancement of Critical Current across Single Grain Boundaries in YBa2Cu3Ox and Search for Superconductivity in Alkali Metals

The ability of single grain boundaries in YBa2Cu3Ox bicrystalline rings to carry electrical current is found to be significantly enhanced under hydrostatic pressure. Nearly hydrostatic pressures are applied to the alkali metals Li, Na, and K. in a search for superconductivity. 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.

cond-mat.supr-con↗

arXiv2002

Dependence of the superconducting transition temperature of single- and polycrystalline MgB2 on hydrostatic pressure

The dependence of Tc for MgB2 on purely hydrostatic or nearly hydrostatic pressure has been determined to 23 GPa for single-crystalline and to 32 GPa for polycrystalline samples, and found to be in good agreement. Tc decreases from 39 K at ambient pressure to 15 K at 32 GPa with an initial slope dTc/dP = -1.11(2) K/GPa. Evidence is presented that the differing values of dTc/dP reported in the literature result primarily from shear-stress effects in nonhydrostatic pressure media and not differences in the samples. Although comparison of these results with theory supports phonon-mediated superconductivity, a critical test of theory must await volume-dependent calculations based on the solution of the anisotropic Eliashberg equations.

cond-mat.supr-con↗

arXiv2001

Hydrostatic Pressure Dependence of the Superconducting and Structural Properties of MgB2

In this paper we report parallel in situ neutron powder diffraction and Tc(P) measurements versus pressure on the same MgB2 sample in a He-gas apparatus to 0.6 GPa; in addition, we present Tc(P) measurements in a helium-loaded diamond-anvil-cell to 20 GPa on the same sample. Our results are compared with those of other groups. The high precision achieved in these measurements allows a quantitative evaluation of the change in Tc as a function of the changes in structure. Our results are consistent with electron-phonon pairing in MgB2.

cond-mat.supr-con↗

arXiv2001

The Dependence of Tc on Hydrostatic Pressure in Superconducting MgB2

The dependence of Tc on hydrostatic (He-gas) pressure has been determined to 0.7 GPa. We find that Tc decreases linearly and reversibly under pressure at the rate dTc/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 Tc is thus accurately determined, dlnTc/dlnV = +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.

cond-mat.supr-con↗