Nonlinear conduction phenomena in the Mott insulator Ca$_{2}$RuO$_{4}$

The 4$d$-electron Mott insulator Ca$_2$RuO$_4$ has attracted considerable attention because of its rich electronic states dramatically varied by temperature change, pressure, and isovalent Sr substitution. Recently, Nakamura {\it et al}. reported an intriguing result of current-voltage characteristics in Ca$_2$RuO$_4$ and found an $E$-induced insulator-to-metal transition caused by relatively low electric field $E$ $\sim$ 50 V/cm [1]. In this study we investigate the nonlinear conduction phenomena in the Mott insulating phase of Ca$_2$RuO$_4$ with a proper evaluation for self-heating effects. By utilizing a non-contact infrared thermometer, sample temperature was accurately determined even in the presence of large Joule heating. The resistivity shows a typical insulating behavior featured by thermal activation with an energy gap, but clearly depends on the applied currents. The result is highlighted by a strong suppression of the energy gap by the electrical currents. A striking similarity to the current dependence of charge-order gap in organic insulators is discussed in terms of the nonequilibrium phase transition. \\[4pt] [1] F. Nakamura {\it et al.}, (submitted).