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Register a new userUltrafast triggering of insulator-metal transition in two-dimensional VSe$_2$
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Assembling transition metal dichalcogenides (TMDCs) at the two-dimensional (2D) limit is a promising approach for tailoring emerging states of matter such as superconductivity or charge density waves (CDWs). Single-layer (SL) VSe$_2$ stands out in this regard because it exhibits a strongly enhanced CDW transition with a higher transition temperature compared to the bulk in addition to an insulating phase with an anisotropic gap at the Fermi level, causing a suppression of anticipated 2D ferromagnetism in the material. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these electronic phases in SL VSe$_2$ using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap on a timescale of 480 fs, which we disentangle from the ensuing hot carrier dynamics. Our work thereby reveals that the phase transition in SL VSe$_2$ is driven by electron-lattice coupling and demonstrates the potential for controlling electronic phases in 2D materials with light.
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Rutile ($R$) phase VO$_2$ is a quintessential example of a strongly correlated bad-metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V dimerized monoclinic phase below T$_{MIT} sim 340K$. It has been experimentally shown that one can control this transition by doping VO$_2$. In particular, doping with oxygen vacancies ($V_O$) has been shown to completely suppress this MIT {em without} any structural transition. We explain this suppression by elucidating the influence of oxygen-vacancies on the electronic-structure of the metallic $R$ phase VO$_2$, explicitly treating strong electron-electron correlations using dynamical mean-field theory (DMFT) as well as diffusion Monte Carlo (DMC) flavor of quantum Monte Carlo (QMC) techniques. We show that $V_O$s tend to change the V-3$d$ filling away from its nominal half-filled value, with the $e_{g}^{pi}$ orbitals competing with the otherwise dominant $a_{1g}$ orbital. Loss of this near orbital polarization of the $a_{1g}$ orbital is associated with a weakening of electron correlations, especially along the V-V dimerization direction. This removes a charge-density wave (CDW) instability along this direction above a critical doping concentration, which further suppresses the metal-insulator transition. Our study also suggests that the MIT is predominantly driven by a correlation-induced CDW instability along the V-V dimerization direction.
The vanadates VO$_2$ and V$_2$O$_3$ are prototypical examples of strongly correlated materials that exhibit a metal-insulator transition. While the phase transitions in these materials have been studied extensively, there is a limited understanding of how the properties of these materials are affected by the presence of defects and doping. In this study we investigate the impact of native point defects in the form of Frenkel defects on the structural, magnetic and electronic properties of VO$_2$ and V$_2$O$_3$, using first-principles calculations. In VO$_2$ the vanadium Frenkel pairs lead to a non-trivial insulating state. The unpaired vanadium interstitial bonds to a single dimer, which leads to a trimer that has one singlet state and one localized single-electron $S=1/2$ state. The unpaired broken dimer created by the vanadium vacancy also has a localized $S=1/2$ state. Thus, the insulating state is created by the singlet dimers, the trimer and the two localized $S=1/2$ states. Oxygen Frenkel pairs, on the other hand, lead to a metallic state in VO$_2$, but are expected to be present in much lower concentrations. In contrast, the Frenkel defects in V$_2$O$_3$ do not directly suppress the insulating character of the material. However, the disorder created by defects in V$_2$O$_3$ alters the local magnetic moments and in turn reduces the energy cost of a transition between the insulating and conducting phases of the material. We also find self-trapped small polarons in V$_2$O$_3$, which has implications for transport properties in the insulating phase.
Vanadium dioxide (VO2) has been widely studied for its rich physics and potential applications, undergoing a prominent insulator-metal transition (IMT) near room temperature. The transition mechanism remains highly debated, and little is known about the IMT at nanoscale dimensions. To shed light on this problem, here we use ~1 nm wide carbon nanotube (CNT) heaters to trigger the IMT in VO2. Single metallic CNTs switch the adjacent VO2 at less than half the voltage and power required by control devices without a CNT, with switching power as low as ~85 ${mu}W$ at 300 nm device lengths. We also obtain potential and temperature maps of devices during operation using Kelvin Probe Microscopy (KPM) and Scanning Thermal Microscopy (SThM). Comparing these with three-dimensional electrothermal simulations, we find that the local heating of the VO2 by the CNT play a key role in the IMT. These results demonstrate the ability to trigger IMT in VO2 using nanoscale heaters, and highlight the significance of thermal engineering to improve device behaviour.
Unusual metallic states involving breakdown of the standard Fermi-liquid picture of long-lived quasiparticles in well-defined band states emerge at low temperatures near correlation-driven Mott transitions. Prominent examples are ill-understood metallic states in $d$- and $f$-band compounds near Mott-like transitions. Finding of superconductivity in solid O$_{2}$ on the border of an insulator-metal transition at high pressures close to 96~GPa is thus truly remarkable. Neither the insulator-metal transition nor superconductivity are understood satisfactorily. Here, we undertake a first step in this direction by focussing on the pressure-driven insulator-metal transition using a combination of first-principles density-functional and many-body calculations. We report a striking result: the finding of an orbital-selective Mott transition in a pure $p$-band elemental system. We apply our theory to understand extant structural and transport data across the transition, and make a specific two-fluid prediction that is open to future test. Based thereupon, we propose a novel scenario where soft multiband modes built from microscopically coexisting itinerant and localized electronic states are natural candidates for the pairing glue in pressurized O$_{2}$.
Metal-ion doping can effectively regulate the metal-insulator transition temperature in $mathrm{VO}_2$. Experiments found that the pentavalent and hexavalent ion doping dramatically reduces the transition temperature while the trivalent ion doping increases the transition temperature and induces intermediate phases. Based on the phase-field model of the metal-insulator transition in $mathrm{VO}_2$ we developed previously, we formulate a Landau potential of the metal-ion-doped $mathrm{VO}_2$ taking account of the effects of doping on the electron correlation and lattice structure. The effect of metal-ion doping on the lattice structure is accounted for in a phenomenological way. Using the Landau potential, we calculate the temperature-dopant-concentration phase diagrams of $mathrm{VO}_2$ doped with various metal ions consistent with the experiments and provide explanation to the different behaviors of different metal-ion doping. The phenomenological theory can provide estimations of phase diagrams of $mathrm{VO}_2$ doped with other metal ions.
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