Electrical resistivity and Andreev reflection spectroscopy of the superconducting oxide spinelLiTi2O4

Abstract

We measured resistivity and Andreev reflection spectroscopy on $\mathrm{Li}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ polycrystalline samples $({T}_{c}\ensuremath{\approx}12.3\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ synthesized by a new method. Resistivity shows the metallic temperature dependence and is well fitted by the Bloch-Gr\"uneisen theory above $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This is consistent with the picture of electron-phonon scattering. Below $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and above ${T}_{c}$ it shows ${T}^{2}$ temperature dependence suggesting Fermi liquid behavior above ${T}_{c}$. From the ${H}_{c2}\ensuremath{-}T$ phase diagram the upper critical field ${H}_{c2}(0)$ is estimated to be about $11.6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ based on the Ginzburg-Landau theory. Point-contact Andreev reflection spectroscopy gives $2{\ensuremath{\Delta}}_{0}∕{k}_{B}{T}_{C}\ensuremath{\approx}4$ and reveals that $\mathrm{Li}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ is a typical intermediate-coupling $s$-wave BCS superconductor. We also observed the resistive hump in a normal state and the field-induced upturn near the transition region of the $\ensuremath{\rho}(T)$ curve on the aged samples. In comparison with the fresh one, it suggests that the above exotic $\ensuremath{\rho}(T)$ behavior is not an intrinsic character of the pure stoichiometric $\mathrm{Li}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ compound.