Lorenz number in the optimally doped and underdoped superconductorEuBa2Cu3Oy

Abstract

The temperature dependences of the Hall-Lorenz numbers $({L}_{xy})$ in a $\mathrm{Eu}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (Eu-123) single crystal before and after oxygen reduction are reported. The study is based on data on the normal state longitudinal and transversal transport coefficients. Namely, the temperature dependences of the electrical resistivity, Hall coefficient, longitudinal thermal conductivity, and transverse thermal conductivity are presented. The set of measurements was performed for an optimally doped sample $(y\ensuremath{\approx}7)$, then the oxygen content in the the same crystal was reduced to $y\ensuremath{\approx}6.65$, and the measurements were repeated. For both cases ${L}_{xy}$'s are about two times larger than the Sommerfeld value of the Lorenz number and depend weakly on temperature in a range 300--160 K. Below $T\ensuremath{\approx}160\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Hall-Lorenz number for the optimally doped sample slowly drops, while the value of ${L}_{xy}$ for the oxygen reduced sample begins to rise. Such results suggest that the electronic system in the investigated compound may be considered as a metallic one with pseudogaps that open at the Fermi level.