We present nonlinear conduction phenomena in the Mott insulator Ca 2 RuO 4 investigated with a proper evaluation of self-heating effects. By utilizing a non-contact infrared thermometer, the sample temperature was accurately determined even in the presence of large Joule heating. We find that the resistivity continuously decreases with currents under an isothermal environment. The nonlinearity and the resulting negative differential resistance occurs at relatively low current range, incompatible with conventional mechanisms such as hot electron or impact ionization. We propose a possible current-induced gap suppression scenario, which is also discussed in non-equilibrium superconducting state or charge-ordered insulator.Nonlinear transport nature of strongly correlated electrons is one of the most fundamental but remaining issues in condensed matter physics. In a vicinity of correlated insulating phase, mobile electrons sense strong interactions among them and consequently anomalous metallic states are often realized, which are usually induced by temperature change, physical pressure or chemical substitutions. 1 This naturally invokes an idea that the correlated electrons in a highly non-equilibrium condition, such as in strong electric field, show exotic behaviors as well. 2 In correlated transition-metal oxides or organic salts, such a nonlinear conduction phenomenon has been extensively explored. [3][4][5][6][7] In an oxide Mott insulator, temperature variation of the threshold field for dielectric breakdown is found to be similar to that in the charge-density-wave (CDW) materials, implying a possible collective motion triggered by strong fields. 6 Indeed, a spontaneous electrical oscillation associated with notable nonlinear conduction has been reported in an organic charge-order salt, 8 which is reminiscent of the sliding motion of CDW. As a different origin for the breakdown phenomena, an unconventional avalanche process with anomalously long delay time has been suggested in the narrow-gap chalcogenide Mott insulators. 9The 4d-electron Mott insulator Ca 2 RuO 4 10,11 is a particularly suitable example for the study of nonlinear transport nature in correlated electron systems because the insulating phase of this material is highly susceptible to external perturbations such as heating, application of pressure, or chemical substitution. [12][13][14][15] At T MI ≃ 360 K, this compound exhibits a first-order metal-insulator transition, whose nature has been intensively studied as orbital order formation. [16][17][18][19][20][21][22][23] Systematic isovalent Sr substitution study has revealed that the ground state of Ca 2−x Sr x RuO 4 varies from the Mott insulator (x < 0.2) to the spin-triplet superconductor (x = 2) 24 through a spin-glass state in the broad composition range. 25 The parent compound becomes to be metallic with applying pressure as well, 15 and the higher pressure makes the system superconducting. 26 Recently, Nakamura et al. reported an electric-fieldinduced insulator-to-metal transition in Ca 2...

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We report the thermoelectric transport properties in the orbital-ordered Mott insulating phase of Ca 2 RuO 4 close to and far from equilibrium. Near equilibrium conditions where the temperature gradient is only applied to the sample, an insulating but non-monotonic temperature variation of the Seebeck coefficient is observed, which is accounted for in terms of a temperature-induced suppression of the orbital order. In non-equilibrium conditions where we have applied high electrical currents, we find that the Seebeck coefficient is anomalously increased in magnitude with increasing external current. The present result clearly demonstrates a non-thermal effect since the heating simply causes a decrease of the Seebeck coefficient, implying a non-trivial non-equilibrium effect such as a modification of the spin and orbital state in currents.