Understanding of the Temperature--Pressure Phase Diagram of \beta-Pyrochlore Oxides: A Role of Anhamonicity on Superconductivity

概要

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We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, T_{\text{c}}, of superconductivity mediated by phonons, via specific heat measurements on \beta-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that T_{\text{c}} demonstrates unimodal dependence on pressure, where T_{\text{c}} increases more than three times by pressure. On the other hand, an electron--phonon coupling constant, \lambda, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, \omega_{\text{log}}, is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of \omega_{\text{log}} means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pressure effects. On the basis of all the results, we indicate that the unimodal pressure variation in T_{\text{c}} is attributed to two competing effects, that is, the enhancement of \lambda and the simultaneous suppression of \omega_{\text{log}} caused by the pressure evolution of anhamonicity.
The Physical Society of Japanの論文
2013-11-15