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.

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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One-sentence summary:The strongest diamagnetism among non-superconducting materials emerges in a Mott insulator when it is tuned to semimetal by current. When one applies magnetic field to the conduction electrons in a material, they exhibit cyclotron motion resulting in orbital magnetic moment. Such orbital motion leads to quantized (discrete) energy levels known as the Landau levels. In a simple system, the resultant increase in the total energy is proportional to the square of the applied magnetic field. This increase in the energy results in negative magnetic susceptibility, which does not depend on the field strength. This effect is known as the Landau diamagnetism or orbital diamagnetism (1). Another source of diamagnetism, present ubiquitously irrespective of metalicity of a material, is circulation of inner-core electrons. In most materials, both types of diamagnetism are rather weak and often hindered by other paramagnetic contributions. However, pyrolytic graphite and bismuth are well known to exhibit exceptionally large diamagnetism, as shown in Fig. 1, due to strong Landau diamagnetism originating from light-mass electrons as well as multi-orbital effects. Indeed, these materials host gapped Dirac cones in the electronic dispersions with strong hybridization between the electron and hole bands (2). In such systems, interband effects induced by 3 magnetic fields can lead to strong dimagnetism (3,4). Recently, relatively large diamagnetism is observed in some topological semimetals such as TaAs, ascribable to the Weyl electrons in the bulk electronic state (5), again with Dirac-cone dispersion. We also note that superconductors show much stronger diamagnetism but only up to the critical magnetic fields.In this Report, we show that the Mott insulator Ca2RuO4 under electric current exhibits the strongest diamagnetism among all known non-superconducting materials as presented in Fig. 1. Remarkably, even including superconducting materials, the diamagnetism of Ca2RuO4 at 7 T is comparable to that of YBa2Cu3O7−δ, which remains strongly diamagnetic even in such high fields owing to its unusually high superconducting transition temperature. As illustrated in Fig.1B-D, a truly novel feature is the switchability of the diamagnetism. From the point of view of device applications, our finding is attractive because we can control not only the Mott gap but also the large diamagnetism electronically using current. We note that negative magnetization due to local magnetic moments is found in certain ferrimagnets such as YVO3, which consists of two or more magnetic sublattices (6). However, such negative magnetization is not categorized as diamagnetism by definition.The Mott insulator Ca2RuO4 (7) is an end member of the system Ca2−xSrxRuO4, which exhibits a rich variety of magnetic, transport, and structural properties (8-12), including the spin-triplet superconductivity in Sr2RuO4 as the other end member (13,14). Ca2RuO4 exhibits metal to insulator transition at TMI = 357 K accompanied by the first-order structural t...

Exciton localization in In x Ga 1Ϫx N was studied. At 2 K, the time-integrated photoluminescence ͑PL͒ spectrum showed a Stokes shift from the absorption shoulder and broadening at the lower photon energy side. Site-selectively excited PL measurements determined the mobility edge. The exciton relaxation processes were studied by use of time-resolved PL spectroscopy. The PL decay time increased with the decrease of the detection-photon energy, indicating the dynamical features of exciton localization. In addition, we observed localized exciton luminescence turned into stimulated emission just below the mobility edge.

The Mott insulator Ca 2 RuO 4 is the subject of much recent attention following reports of emergent nonequilibrium steady states driven by applied electric fields or currents. In this paper, we carry out infrared nano-imaging and optical-microscopy measurements on bulk single crystal Ca 2 RuO 4 under conditions of steady current flow to obtain insight into the current-driven insulator-tometal transition. We observe macroscopic growth of the current-induced metallic phase, with nucleation regions for metal and insulator phases determined by the polarity of the current flow. A remarkable metal-insulator-metal microstripe pattern is observed at the phase front separating metal and insulator phases. The microstripes have orientations tied uniquely to the crystallographic axes, implying a strong coupling of the electronic transition to lattice degrees of freedom. Theoretical modeling further illustrates the importance of the current density and confirms a submicron-thick surface metallic layer at the phase front of the bulk metallic phase. Our work confirms that the electrically induced metallic phase is nonfilamentary and is not driven by Joule heating, revealing remarkable new characteristics of electrically induced insulator-metal transitions occurring in functional correlated oxides.