Metal-insulator transitions (MIT) belong to a class of fascinating physical phenomena, which includes superconductivity, and colossal magnetoresistance (CMR), that are associated with drastic modifications of electrical resistance. In transition metal compounds, MIT are often related to the presence of strong electronic correlations that drive the system into a Mott insulator state. In these systems the MIT is usually tuned by electron doping or by applying an external pressure. However, it was noted recently that a Mott insulator should also be sensitive to other external perturbations such as an electric field. We report here the first experimental evidence of a non-volatile electric-pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4Se8. Our Scanning Tunneling Microscopy experiments show that this unconventional response of the system to short electric pulses arises from a nanometer scale Electronic Phase Separation (EPS) generated in the bulk material.
The rare-earth based pyrochlore molybdates involve orbitally degenerate electrons Hund's coupled to local moments. The large Hund's coupling promotes ferromagnetism, the superexchange between the local moments prefers antiferromagnetism, and Hubbard repulsion tries to open a Mott gap. The phase competition is tuned by the rare-earth ionic radius, decreasing which leads to change from a ferromagnetic metal to a spin disordered highly resistive ground state, and ultimately an 'Anderson-Mott' insulator. We attempt a quantitative theory of the molybdates by studying their minimal model on a pyroch…
Mott insulator to metal transitions under electric field are currently the subject of numerous fundamental and applied studies. This puzzling effect, which involves non-trivial out-of-equilibrium effects in correlated systems, is indeed at play in the operation of a new class of electronic memories, the Mott memories. However the combined electronic and thermal effects are difficult to disentangle in Mott insulators undergoing such transitions. We report here a comparison between the properties under electric field of a canonical Mott insulator and a model built on a realistic 2D resistor netw…
The fundamental building blocks of modern silicon-based microelectronics, such as double gate transistors in non-volatile Flash memories, are based on the control of electrical resistance by electrostatic charging. Flash memories could soon reach their miniaturization limits mostly because reliably keeping enough electrons in an always smaller cell size will become increasingly difficult . The control of electrical resistance at the nanometer scale therefore requires new concepts, and the ultimate resistance-change device is believed to exploit a purely electronic phase change such as the Mott…
Resistive switching can be achieved in a Mott insulator by applying current/voltage, which triggers an insulator-metal transition (IMT). This phenomenon is key for understanding IMT physics and developing novel memory elements and brain-inspired technology. Despite this, the roles of electric field and Joule heating in the switching process remain controversial. We resolve this issue by studying nanowires of two archetypical Mott insulators - VO2 and V2O3. Our findings show a crossover between two qualitatively different regimes. In one, the IMT is driven by Joule heating to the transition tem…
Mott insulators can be portrayed as "unsuccessful metals": systems in which a strong Coulomb repulsion prevents charge conduction notwithstanding the metal-like density of conduction electrons. The possibility to unlock such large density of frozen carriers with an electric field offers a tantalizing opportunity to realize new Mott-based microelectronic devices. Here we explicitly unveil how such unlocking happens by solving a simple, yet generic, model for correlated insulators using dynamical mean-field theory. Specifically, we show that the electric breakdown of a Mott insulator can occur v…
Current-voltage (IV) characteristics and the temperature dependence of the contact resistance [$R(T)$] of Au / YBa$_2$Cu$_3$O$_{7-δ}$ (optimally doped YBCO) interfaces have been studied at different resistance states. This states were produced by resistive switching after accumulating cyclic electrical pulses of increasing number and voltage amplitude. The IV characteristics and the $R(T)$ dependence of the different states are consistent with a Poole-Frenkel (P-F) emission mechanism with trapping-energy levels $E_t$ in the 0.06-0.11 eV range. $E_t$ remains constant up to a number-of-pulses-de…
One of today's most exciting research frontier and challenge in condensed matter physics is known as Mottronics, whose goal is to incorporate strong correlation effects into the realm of electronics. In fact, taming the Mott insulator-to-metal transition (IMT), which is driven by strong electronic correlation effects, holds the promise of a commutation speed set by a quantum transition, and with negligible power dissipation. In this context, one possible route to control the Mott transition is to electrostatically dope the systems using strong dielectrics, in FET-like devices. Another possibil…
We develop a real space theory of the voltage bias driven transition from a Mott insulator to a correlated metal. Within our Keldysh mean field approach the problem reduces to a self-consistency scheme for the charge and spin profiles in this open system. We solve this problem for a two dimensional antiferromagnetic Mott insulator at zero temperature. The charge and spin magnitude is uniform over the system at zero bias, but a bias $V$ leads to spatial modulation over a lengthscale $ξ(V)$ near the edges. $ξ(V)$ grows rapidly and becomes comparable to system size as $V$ increases towards a thre…
We report on the reversible, nonvolatile and polarity dependent resistive switching between superconductor and insulator states at the interfaces of a Au/YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO)/Au system. We show that the superconducting state of YBCO in regions near the electrodes can be reversibly removed and restored. The possible origin of the switching effect may be the migration of oxygen or metallic ions along the grain boundaries that control the intergrain superconducting coupling. Four-wire bulk resistance measurements reveal that the migration is not restricted to interfaces and produce signi…
Mott transitions induced by strong electric fields are receiving a growing interest. Recent theoretical proposals have focused on the Zener dielectric breakdown in Mott insulators, however experimental studies are still too scarce to conclude about the mechanism. Here we report a study of the dielectric breakdown in the narrow gap Mott insulators GaTa$_4$Se$_{8-x}$Te$_x$. We find that the I-V characteristics and the magnitude of the threshold electric field (E$_{th}$) do not correspond to a Zener breakdown, but rather to an avalanche breakdown. E$_{th}$ increases as a power law of the Mott Hub…
The effect of resistive switching in doped manganites being in the ferromagnetic state has been studied using resistive and magneto-optic methods. The visualization of magnetic structure of La0.75Sr0.25MnO3-x single crystals, and its transformation under electric current proposed local superheating of the material above the Curie temperature, which was supported by numerical calculation. The obtained results suggest a significant role of micrometer-scale inhomogeneity of manganites in phase separation, magnetic and transport properties of the material.
We investigate the dramatic switch of resistance in ordered correlated insulators, when driven out of equilibrium by a strong voltage bias. Microscopic calculations on a driven-dissipative lattice of interacting electrons explain the main experimental features of resistive switching (RS), such as the hysteretic $I$-$V$ curves and the formation of hot conductive filaments. The energy-resolved electron distribution at the RS reveals the underlying nonequilibrium electronic mechanism, namely Landau-Zener tunneling, and also justifies a thermal description where the hot-electron temperature, estim…
Disorder has intriguing consequences for correlated electronic materials, which include several families of high-temperature superconductors and resistive switching systems. We address the question of why strongly correlated d-wave superconductors, such as the cuprates, prove to be surprisingly robust against the introduction of non-magnetic impurities. We show that, very generally, both the pair-breaking and the normal state transport scattering rates are significantly suppressed by strong correlations effects arising in the proximity to a Mott insulating state. We also show that the correlat…
We report the direct observation of the electric pulse induced resistance-change (EPIR) effect at the nano scale on La1-xSrxMnO3 (LSMO) thin films by the current measurement AFM technique. After a switching voltage of one polarity is applied across the sample by the AFM tip, the conductivity in a local nanometer region around the AFM tip is increased, and after a switching voltage of the opposite polarity is applied, the local conductivity is reduced. This reversible resistance switching effect is observed under both continuous and short pulse voltage switching conditions. It is important for …
We study how strongly correlated electrons on a dissipative lattice evolve from equilibrium when driven by a constant electric field, focusing on the extent of the linear regime and hysteretic non-linear effects at higher fields. We access the non-equilibrium steady states, non-perturbatively in both the field and the electronic interactions, by means of a non-equilibrium dynamical mean-field theory in the Coulomb gauge. The linear response regime is limited by Joule heating effects and breaks down at fields orders of magnitude smaller than the quasi-particle energy scale. For large electronic…
We propose the concept of "hybridization-switching induced Mott transition" which is relevant to a broad class of ABO$_3$ perovskite materials including BiNiO$_3$ and PbCrO$_3$ which feature extended $6s$ orbitals on the A-site cation (Bi or Pb), and A-O covalency induced ligand holes. Using {\it ab initio} electronic structure and slave rotor theory calculations, we show that such systems exhibit a breathing phonon driven A-site to oxygen hybridization-wave instability which conspires with strong correlations on the B-site transition metal ion (Ni or Cr) to induce a Mott insulator. These Mott…
We report the study of metal-STO-metal memristors where the doping concentration in STO can be fine-tuned through electric field migration of oxygen vacancies. In this tunnel junction device, the evolution of the Density Of States (DoS) can be followed continuously across the Metal-Insulator Transition (MIT). At very low dopant concentration, the junction displays characteristic signatures of discrete dopants levels. As the dopant concentration increases, the semiconductor band gap fills in but a soft Coulomb gap remains. At even higher doping, a transition to a metallic state occurs where the…
Switching voltage of first-order metal-insulator transition (MIT) in VO_2, an inhomogeneous strongly correlated system, is changed by irradiating an infrared light with wavelength, 1.5 micrometer, and applying the electric field (photo-induced switching). This was predicted in the hole-driven MIT theory in which hole doping of a low concentration below 0.01% into conduction band (Fermi surface) induces the abrupt MIT as correlation effect. The switching is explained by the Mott transition not the Peierls transition.
X-ray absorption spectroscopy was used to study the microscopic origin of conductance and resistive switching in chromium doped strontium titanate (Cr:SrTiO3). Differences in the x-ray absorption near edge spectroscopy (XANES) at the Cr K-edge indicate that the valence of Cr changes from 3+ to 4+ underneath the anode of our sample device after the application of an electric field. Spatially resolved x-ray fluorescence microscopy ($μ$-XRF) maps show that the Cr4+ region retracts from the anode-Cr:SrTiO3 interface after a conducting state has been achieved. This interface region is studied with …
Volatile resistive-switched state in a bulk organic conductor with a sharp metal-insulator transition
Tokyo University of Science（东京科学大学）；National Institute for Materials Science (NIMS)（国立材料研究所）；Japan Science and Technology Agency (JST)（日本科学技术振兴机构）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；Institute of Science Tokyo（东京科学大学）；RIKEN, Cluster for Pioneering Research (RIKEN-CPR)（理化学研究所前沿研究集群）
The layered compound BaCo1-xNixS2-y (0.05<x<0.2 and 0.05<y<0.2) exhibits an unusual first-order structural and electronic phase transition from a low-T monoclinic paramagnetic metal to a high-T tetragonal antiferromagnetic insulator around 200 K with huge hysteresis (~ 40 K) and large volume change (~0.01). Here we report on unusual voltage-controlled resistive switching followed by current-controlled resistive switching induced by self-heating in polycrystalline BaCo1-xNixS2-y (nominal x=0.1 and y=0.2). These were due to the steep metal to insulator transition upon heating followed by the act…
$VO_2$ undergoes the insulator-metal transition (IMT) and monoclinic-rutile structural phase transition (SPT) near $67^oC$. The IMT switching has many applications. However, there is an unresolved issue whether the IMT is a Mott transition or a Peierls transition. This complication is caused by metal and insulator coexistence, which is an inherent property of the IMT region. Thus, the acquired data in the IMT region are averaged over the two phases in many experiments. We overcome the issue by probing the electronic state of the monoclinic structure and by introducing a model that accounts for…
In many transition metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, Fe$_3$O$_4$, below the Verwey temperature, an electric field driven transition to a state of lower resistance was recently found, with hysteretic current-voltage response. We report the results of pulsed electrical conduction measurements in epitaxial magnetite thin films. We show that while the high- to low-resistance transition is driven by electric field, the hysteresis observed in $I-V$ curves results from Joule heating in the low resistance state. The sh…
Mott-Hubbard and Hund electron correlations have been realized thus far in separate classes of materials. Here, we show that a single moiré homobilayer encompasses both kinds of physics in a controllable manner. We develop a microscopic multiband model that we solve by dynamical mean-field theory to nonperturbatively address the local many-body correlations. We demonstrate how tuning with twist angle, dielectric screening, and hole density allows us to switch between Mott-Hubbard and Hund correlated states in a twisted WSe$_2$ bilayer. The underlying mechanism is based on controlling Coulomb-i…
The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resis…
The insulator-to-metal transition in Mott insulators is the key mechanism for a novel class of electronic devices, belonging to the Mottronics family. Intense research efforts are currently devoted to the development of specific control protocols, usually based on the application of voltage, strain, pressure and light excitation. The ultimate goal is to achieve the complete control of the electronic phase transformation, with dramatic impact on the performance, for example, of resistive switching devices. Here, we investigate the simultaneous effect of external voltage and excitation by ultras…
