Pressure dependence of thermopower in YBa2Cu3O6.96 and YBa2Cu4O8.

Abstract

Measurements of the temperature dependencies of the thermopower {alpha}({ital T}) and the resistivity {rho}({ital T}) of polycrystalline YBa{sub 2}Cu{sub 3}O{sub 6.96} and YBa{sub 2}Cu{sub 4}O{sub 8} under different hydrostatic pressures are reported. They are interpreted to reveal an increase with pressure in the superconductive-condensate density {ital n}{sub {ital s}} of the chains. A literature report of {ital dT}{sub {ital c}}/{ital dP}=0.25 K/kbar for optimally doped Y{sub 0.9}Ca{sub 0.1}Ba{sub 2}Cu{sub 4}O{sub 8} is argued to be due to an increase in the elastic coupling between the CuO{sub 2} sheets of the double Cu-O chains as a result of the increase in chain {ital n}{sub {ital s}}. A {ital c}-axis in-phase {ital A}{sub {ital g}} vibration of the oxygen atoms of the CuO{sub 2} sheets is identified as a participant in the underlying physical mechanism responsible not only for high-{ital T}{sub {ital c}} superconductivity, but also for the pressure dependencies of {ital T}{sub {ital c}} and {alpha}({ital T}). {copyright} {ital 1996 The American Physical Society.}