Jaewoo Jeong

Suppression of Metal-Insulator Transition in VO2 by Electric Field–Induced Oxygen Vacancy Formation

Mind the Vacancies Varying the carrier density of solid-state systems to manipulate their electrical properties usually involves chemical doping, which can lead to disorder. Recently, ionic liquids have been used to form an electronic double layer on the surface of a material, tuning its carrier density by the application of an electric field. Jeong et al. (p. 1402) used liquid gating on VO2, which undergoes a metal-insulator transition close to room temperature. The liquid gating suppressed the transition to lower and lower temperatures; however, the material remained in the metallic state, even when the gating fluid was washed off. It appears that, instead of a simple electrostatic effect, the properties of VO2 are modulated by the introduction of oxygen vacancies, an electrochemical consequence of high electric fields. The results imply that careful interpretation of liquid gating experiments in condensed matter physics is needed. Electrochemistry plays a role in the ionic liquid gating of a strongly correlated oxide. Electrolyte gating with ionic liquids is a powerful tool for inducing novel conducting phases in correlated insulators. An archetypal correlated material is vanadium dioxide (VO2), which is insulating only at temperatures below a characteristic phase transition temperature. We show that electrolyte gating of epitaxial thin films of VO2 suppresses the metal-to-insulator transition and stabilizes the metallic phase to temperatures below 5 kelvin, even after the ionic liquid is completely removed. We found that electrolyte gating of VO2 leads not to electrostatically induced carriers but instead to the electric field–induced creation of oxygen vacancies, with consequent migration of oxygen from the oxide film into the ionic liquid. This mechanism should be taken into account in the interpretation of ionic liquid gating experiments.

Giant reversible, facet-dependent, structural changes in a correlated-electron insulator induced by ionic liquid gating

Significance We report a remarkable reversible change in structure of vanadium dioxide films when gated with an ionic liquid. We show that the film expands by more than 3% in the out-of-plane direction when gated to the metallic state. This giant structural change is not only more than 10 times larger than the one at the thermally controlled insulator-to-metal transition measured in the same films, but is in the opposite direction—an expansion rather than a contraction. These results are very important to the field of ionic liquid gating, which has largely ignored the possibility that the high electric fields created on gating at the liquid–oxide interface can result in significant structural changes rather than a purely electrostatic phenomenon. The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO2) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO2. Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO2 crystal facet, which we relate to the ease of electric-field–induced motion of oxygen ions along chains of edge-sharing VO6 octahedra that exist along the (rutile) c axis.

Suppression of ionic liquid gate-induced metallization of SrTiO3(001) by oxygen.

Ionic liquid gating of three terminal field effect transistor devices with channels formed from SrTiO3(001) single crystals induces a metallic state in the channel. We show that the metallization is strongly affected by the presence of oxygen gas introduced external to the device whereas argon and nitrogen have no effect. The suppression of the gating effect is consistent with electric field induced migration of oxygen that we model by oxygen-induced carrier annihilation.

Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy

Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

Improved metal-insulator-transition characteristics of ultrathin VO2 epitaxial films by optimized surface preparation of rutile TiO2 substrates

Key to the growth of epitaxial, atomically thin films is the preparation of the substrates on which they are deposited. Here, we report the growth of atomically smooth, ultrathin films of VO2 (001), only ∼2 nm thick, which exhibit pronounced metal-insulator transitions, with a change in resistivity of ∼500 times, at a temperature that is close to that of films five times thicker. These films were prepared by pulsed laser deposition on single crystalline TiO2(001) substrates that were treated by dipping in acetone, HCl and HF in successive order, followed by an anneal at 700–750  °C in flowing oxygen. This pretreatment removes surface contaminants, TiO2 defects, and provides a terraced, atomically smooth surface.

Facet-Independent Electric-Field-Induced Volume Metallization of Tungsten Trioxide Films.

Reversible metallization of band and Mott insulators by ionic-liquid gating is accompanied by significant structural changes. A change in conductivity of seven orders of magnitude at room temperature is found in epitaxial films of WO3 with an associated monoclinic-to-cubic structural reorganization. The migration of oxygen ions along open volume channels is the underlying mechanism.

Distinct Electronic Structure of the Electrolyte Gate-Induced Conducting Phase in Vanadium Dioxide Revealed by High-Energy Photoelectron Spectroscopy

The development of new phases of matter at oxide interfaces and surfaces by extrinsic electric fields is of considerable significance both scientifically and technologically. Vanadium dioxide (VO2), a strongly correlated material, exhibits a temperature-driven metal-to-insulator transition, which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO2 thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO2 thin films, we show that electronic features in the gate-induced conducting phase are distinct from those of the temperature-induced rutile metallic phase. Moreover, polarization-dependent measurements reveal that the V 3d orbital ordering, which is characteristic of the monoclinic insulating phase, is partially preserved in the gate-induced metallic phase, whereas the thermally induced metallic phase displays no such orbital ordering. Angle-dependent measurements show that the electronic structure of the gate-induced metallic phase persists to a depth of at least ∼40 Å, the escape depth of the high-energy photoexcited electrons used here. The distinct electronic structures of the gate-induced and thermally induced metallic phases in VO2 thin films reflect the distinct mechanisms by which these states originate. The electronic characteristics of the gate-induced metallic state are consistent with the formation of oxygen vacancies from electrolyte gating.

Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO(2).

The intrinsic field effect, the change in surface conductance with an applied transverse electric field, of prototypal strongly correlated VO(2) has remained elusive. Here we report its measurement enabled by epitaxial VO(2) and atomic layer deposited high-κ dielectrics. Oxygen migration, joule heating, and the linked field-induced phase transition are precluded. The field effect can be understood in terms of field-induced carriers with densities up to ∼5×10(13)  cm(-2) which are trongly localized, as shown by their low, thermally activated mobility (∼1×10(-3)  cm(2)/V s at 300 K). These carriers show behavior consistent with that of Holstein polarons and strongly impact the (opto)electronics of VO(2).

Metallization of Epitaxial VO2 Films by Ionic Liquid Gating through Initially Insulating TiO2 Layers.

Ionic liquid gating has been shown to metallize initially insulating layers formed from several different oxide materials. Of these vanadium dioxide (VO2) is of especial interest because it itself is metallic at temperatures above its metal-insulator transition. Recent studies have shown that the mechanism of ionic liquid gated induced metallization is entirely distinct from that of the thermally driven metal-insulator transition and is derived from oxygen migration through volume channels along the (001) direction of the rutile structure of VO2. Here we show that it is possible to metallize the entire volume of 10 nm thick layers of VO2 buried under layers of rutile titanium dioxide (TiO2) up to 10 nm thick. Key to this process is the alignment of volume channels in the respective oxide layers, which have the same rutile structure with clamped in-plane lattice constants. The metallization of the VO2 layers is accompanied by large structural expansions of up to ∼6.5% in the out-of-plane direction, but the structure of the TiO2 layer is hardly affected by gating. The TiO2 layers become weakly conducting during the gating process, but in contrast to the VO2 layers, the conductivity disappears on exposure to air. Indeed, even after air exposure, X-ray photoelectron spectroscopy studies show that the VO2 films have a reduced oxygen content after metallization. Ionic liquid gating of the VO2 films through initially insulating TiO2 layers is not consistent with conventional models that have assumed the gate induced carriers are of electrostatic origin.

Generation mechanism of terahertz coherent acoustic phonons in Fe

T Henighan1,2,∗ M Trigo, S Bonetti, P Granitzka, D Higley, Z Chen, M P Jiang, R Kukreja, A Gray, A H Reid, E Jal, M C Hoffmann, M Kozina, S Song, M Chollet, D Zhu, P F Xu, J Jeong, K Carva, P Maldonado, P M Oppeneer, M G Samant, S S P. Parkin, D A Reis, and H A Dürr3† PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California, USA Physics Department, Stanford University, Stanford, California, USA Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025. Van der Waals-Zeeman Institute, University of Amsterdam, 1018XE Amsterdam, The Netherlands Department of Photon Science and Applied Physics, Stanford University, Stanford, California, USA Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California, USA IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA Max-Planck Institute for Microstructure Physics, 06120 Halle (Saale), Germany Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic and Department of Physics and Astronomy, Uppsala University, P. O. Box 516, S-75120 Uppsala, Sweden (Dated: September 14, 2015)