Recently, “application of electric field (E-field)” has received considerable attention as a new method to induce novel quantum phenomena since application of E-field can tune the electronic states directly with obvious scientific and industrial advantages over other turning methods. However, E-field-induced Mott transitions are rare and typically require high E-field and low temperature. Here we report that the multiband Mott insulator Ca2RuO4 shows unique insulator-metal switching induced by applying a dry-battery level voltage at room temperature. The threshold field Eth ~40 V/cm is much weaker than the Mott gap energy. Moreover, the switching is accompanied by a bulk structural transition. Perhaps the most peculiar of the present findings is that the induced metal can be maintained to low temperature by a weak current.

We show that the pressure-temperature phase diagram of the Mott insulator Ca2RuO4 features a metal-insulator transition at 0.5GPa: at 300K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal; at T ≤ 12K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. We compare the metallic state to that of the structurally related p-wave superconductor Sr2RuO4, and discuss the importance of structural distortions, which are expected to couple strongly to pressure. PACS numbers: 71.30+h, 75.30Kz, 74.70Pq, and 74.62Fj

We studied the crystal and magnetic structure of Ca2RuO4 by different diffraction techniques under high pressure. The observed first order phase transition at moderate pressure (0.5 GPa) between the insulating phase and the metallic high pressure phase is characterized by a broad region of phase coexistence. The following structural changes are observed as function of pressure: a) a discontinuous change of both the tilt and rotation angle of the RuO6-Octahedra at this transition, b) a gradual decrease of the tilt angle in the high pressure phase (p>0.5 GPa) and c) the disappearance of the tilt above 5.5GPa leading to a higher symmetry structure. By single crystal neutron diffraction at low temperature, the ferromagnetic component of the high pressure phase and a rearrangement of antiferromagnetic order in the low pressure phase was observed.

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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...