Session M47: Metal-Insulator and Other Electronic Phase Transitions: Experiment III

Size effect on voltage-induced metal-insulator transition in VO$_{2}$ film grown by direct thermal oxidation method

Metal-insulator transition (MIT) in vanadium dioxide (VO$_{2}$) can be induced by diverse stimuli such as temperature, voltage, light and pressure. Voltage induced MIT is of special interest for both scientific understanding and future applications. However, it is still under the debate whether the origin of voltage induced MIT comes from electrical breakdown or Joule heating effect. To figure out this origin, the electrically triggered MIT from the strip-line VO$_{2}$ film devices with dimensions of fixed width (W) of 100 $\mu $m and varied length (L) of 10, 20, 40, and 80 $\mu $m were investigated by temperature and external bias voltage dependent electrical transport, and optical microscopy. It was found that the magnitude of critical electric filed at MIT and its temperature dependence were dependent on the length of the device. In this talk, we will present the size effect on the voltage-induced metal-insulator transition in VO$_{2}$ film grown by direct thermal oxidation method and discuss the origins of voltage driven MIT and its implications.   

Effect of long-range correlations on the Metal-Insulator Transitions in Vanadium Oxides

The role of long-range electronic correlations in the metal-insulator (MIT) and structural phase (SPT) transitions in V$_{2}$O$_{3}$ and VO$_{2}$ are still under debate. In order to investigate the effect of disorder on the long-range correlations we irradiated V$_{2}$O$_{3}$ and VO$_{2}$ thin films with O$^{+}$ ion at different doses. We studied the effects on the transport and crystallographic properties as a function of the temperature across the phase transition. Both materials are sensitive to the irradiation, but effects on the transport and crystallographic properties across the phase transitions are different. We find changes in the transition temperature, lattice constant and magnitude of the MIT in both oxides. We interpreted this result as a change of the long-range order in vanadium oxides by ion irradiation. The response of VO$_{2}$ and V$_{2}$O$_{3}$ to the irradiation shed light on the SPT and MIT mechanisms.   

Electric field-induced carrier accumulation at the vanadium dioxide-dielectric interface

Classical rigid band semiconductors respond to an electric field at a dielectric interface by accumulating or depleting carriers at the interface. We investigate electrostatic field-effects in thin film devices formed from the prototypical, strongly-correlated insulator, vanadium dioxide VO$_{2}$. This material exhibits a temperature driven insulator to metal transition near room temperature. Therefore, non-trivial electric field driven electronic effects can be anticipated. We find that excess carriers can be introduced in our devices with concentrations of up to $\sim$ 5x10$^{13}$cm$^{-2}$: these field induced carriers exhibit an activated low mobility at low temperatures that is characteristic of electron localization. Field-effect conductance modulation and depletion are highly inhibited with excess carriers confined near the interface. Signatures of defect-dominated scenarios are absent. The field-effect behavior that is exhibited by our VO$_{2}$ based devices is fundamentally different from that of a classical semiconductor.   

Thermochromic characteristics of Ti-doped VO2 thin film

Utilizing metal-to-insulator transition (MIT) properties of V-oxide film, stable VO2 phase is necessary. In sputtering deposition of VO2, simple target preparation and high deposition rate are recommendable. For this, VO2 film was deposited on quartz substrate by RF magnetron sputter system under low working pressure using V2O5 target. Due to the lower sputtering yield of oxygen compared to vanadium, oxygen ion contents is usually deficient from that of target. So, the reduction of V ions was a result of charge compensation with the oxygen ions. Under lower working pressure, deposition rate become higher so that this deficiency is getting larger to cause further reduction to destabilize VO2. Preventing this, titanium oxide co-deposition was suggested to enrich oxygen source. When TiO2 was used, Ti ion has stable +4 charge state so that extra oxygen sputtered prevents V ion reduction below +4 state. But, in case of TiO, Ti ions were oxidized from +2 to +3 and +4 state and V ions with less oxidation potential should be reduced to +3 or so. Pure VO2 film had MIT at 66$^{\circ}$C and large resistivity ratio of 4 orders of magnitude from 30$^{\circ}$C to 90$^{\circ}$C. Under low working pressure, (V2O5 + TiO2) system yield fairly good films, while films with poor or absence of MIT were produced with TiO case.   

Pressure Evolution of X-ray Raman Spectra in a Novel Monoclinic V2O3 Metal

V2O3 is a prototypical metal-to-insulator transition system, where the transition always coincides with a corundum-to-monoclinic structural transition in temperature-dependent studies. However, recent pressure-dependent study demonstrates that the two transitions can be decoupled, showing a novel monoclinic metallic phase above a critical pressure Pc around 33 GPa. Here we study the corresponding pressure evolution of electronic structure with X-ray Raman scattering. The spectra do not exhibit any appreciable difference at low pressures, but broaden substantially across Pc. Multiplet calculations with additional screening channels from coherent quasiparticles indicate a weakened screening effect at high pressures. This could result from a decreased coherent quasiparticle strength due to enhanced electronic correlation, suggesting that V2O3 in the high-pressure monoclinic phase is a critical correlated metal on the verge of Mott- insulating behavior.   

Ab Initio calculations of phonons in metallic rutile and insulating monoclinic M1 and M2 VO$_2$

Vanadium dioxide (VO$_2$) undergoes a first-order metal-insulator transition (MIT) at 340\,K from a metallic, high-temperature rutile phase (R) to an insulating, low-temperature monoclinic phase (M1). Under tensile strain, two other insulating phases, a second monoclinic phase (M2) and a low symmetry triclinic phase (T), are also known to exist. Recently, Park {\em et al.}\footnote{J. H. Park, Nature {\bf 500}, 431 (2013).} observed a solid-state triple point of these phases in strained VO$_2$ nanobeams. More recently, phonon frequencies for strain-stabilized M2 have been observed.\footnote{M. M. Qazilbash, private communication} Understanding the vibrational properties of these phases may help resolve questions surrounding the long-debated issue of the respective roles of electronic correlation and Peierls mechanisms in driving the MIT. We will present {\em ab initio} DFT and DFT+U calculations of phonon frequencies for the M2 phase and compare these to measured results and to previous calculations and measurements for the R and M1 phases.\footnote{T. J. Huffman et al., PRB {\bf 87},115121 (2013).}   

Anharmonicity in complex oxides: case of VO$_2$

Harmonic and quasi-harmonic models of lattice dynamics are widely successful in explaining thermodynamic properties of materials, including in complex oxides. However, in some cases, strong anharmonicity can critically affect physical properties, and a (quasi) harmonic model is not sufficient to capture these important features. In this talk, we present the results of ab initio molecular dynamics studies of anharmonicity in VO$_2$. Our simulations provide good agreement with measurements of phonon dispersions and diffuse scattering. Other implications of strong anharmonicity will also be discussed.   

Hole-lattice Coupling and Photo-induced Insulator-metal Transition in VO2

In this talk, we will present a theory [PRB \textbf{88}, 035119 (2013)] that is able to explain the photo-induced insulator-metal transition in VO2 and the related transient and multi-time-scale structural dynamics upon photo-excitation. Holes created by photo-excitation weaken the V-V bonds and eventually break V-V dimers in the M1 phase when the laser fluence reaches a critical value. The breaking of the V-V bonds in turn leads to an immediate electronic phase transition from an insulating to a metallic state while the crystal lattice remains monoclinic in shape.   

Dynamically Tracking the Strain Across the Metal-Insulator Transition in VO$_2$ Measured Using Electromechanical Resonators

We study the strain state of doubly clamped VO$_2$ nanobeam devices by dynamically probing resonant frequency of the nanoscale electromechanical device across the metal$-$insulator transition. Simultaneous resistance and resonance measurements indicate M1-M2 phase transition in the insulating state with a drop in resonant frequency concomitant with an increase in resistance. The resonant frequency increases by 7~MHz with the growth of metallic domain (M2-R transition) due to the development of tensile strain in the nanobeam. Our approach to dynamically track strain coupled with simultaneous resistance and resonance measurements using electromechanical resonators enables the study of lattice-involved interactions more precisely than static strain measurements.   

Two-dimensional metal-insulator transition in RuO$_{2}$ films

The complex chemical and structural nature of oxide materials make them highly susceptible to disorder. This disorder strongly influences the transport properties of these systems. By systematically varying the disorder and/or carrier concentration, many oxides can be driven through the metal--insulator transition (MIT). We have performed temperature dependant magneto-transport measurements (1.75K\textless T\textless 300K and 0\textless B\textless 8T) on 10-30 nm thick films of RuO$_{2}$ as they were driven through the MIT through calcination. The results reveal an unexpected 2-d insulator to metal transition as a function of decreasing disorder. The presentation will include an introduction to the concepts of localization in disordered materials, an overview of the thin-film sample preparation and characterization, a comparison with a 3-d oxide system (In$_{2}$O$_{3}$), and a discussion of the results in the context of a localization model.   

Insulator to Metal Transition in WO$_{3}$ Induced by Electrolyte Gating

We have modified the transport properties of thin WO$_{3}$ films by the electric field effect using ionic liquids and solid electrolytes. Atomically flat films were prepared on different substrates by RF sputtering. The huge electric field that is generated in the double-layer induces an extraordinarily large change of the mobile charge carrier density in the sample. The sheet resistance of the gated film drops by more than 10 orders of magnitude at the lowest temperature, and a clear insulator-to-metal transition is observed. The thickness dependence has been studied and the mechanism of doping by electrolyte gating will be discussed.   

Pressure Induced Insulator to Metal Transition in FBBO

We have measured resistivity vs. temperature and pressure on the fluoro-substituted oxobenzene-bridged bisdithiazolyl radical, FBBO. This is a layered, single component organic compound that is a Mott insulator at ambient pressure, due to the singly occupied molecular orbitals and an intrinsically high inter-molecular charge transfer energy barrier. Previous room temperature infrared absorption and conductivity measurements suggest that the charge gap of 0.1eV closes and the sample may become metallic at pressures above 3GPa[1]. We report direct transport measurements under various pressures on powder samples of FBBO down to low temperature, measured in an anvil pressure cell. [1] A. Mailman, \textit{et al}., J. Am. Chem. Soc. \textbf{134}, 9886 (2012).   

Metal-insulator transition of (CeO)MnAs by carrier doping

(LaO)MnPn ; (Pn $=$ P, As, Sb) are antiferromagnetic semiconductors with high N\'eel temperature by the strong Mn -- Mn magnetic interaction and they seem to be a robust system against carrier doping. (CeO)MnPn are suitable materials to study the electron correlation because the Ce 4$f$ electrons in the Ce$^{3+}$ state constitute a Mott insulator which is expected to control by carrier doping due to weaker magnetic interaction than that of the Mn case. In this study, we have investigated the carrier doping effects on the physical properties of (CeO)MnAs. The parent material (CeO)MnAs is also a magnetic semiconductor as same as the analogous case of (NdO)MnAs [1]. In this material, there are two magnetic components, one is the antiferromagnetic ordered Mn 3$d$ component, the other is the Ce paramagnetism. The CeO deficiencies provide enough carriers to change the electrical resistivity from insulating to metallic. The deficient samples show Fermi liquid like behaviors at low temperature. These drastic changes are thought to be controlled by Mott transitions.   

Magnetotransport in Iron Cobalt Silicide Nanowires

Iron silicide is a small gap insulator that can be made metallic and magnetic when doped with cobalt. With the incorporation of cobalt, Fe$_{\mathrm{1-x}}$Co$_{x}$Si undergoes an insulator-to-metal transition becoming a half metal for a wide range of $x$. The magnetic ground state is helimagnetic with distinct itinerant character. It has been demonstrated by others that an exotic intermediate magnetic vortex or skyrmion state exists between the helimagnetic and ferromagnetic phases in small applied fields. Electron transport in bulk Fe$_{\mathrm{1-x}}$ Co$_{x}$Si has been found to be dominated by electron-electron interaction effects similar to what has been found in prototypical semiconductors such as Si:P. Here we probe low temperature magnetotransport in CVD-grown Fe$_{\mathrm{1-x}}$ Co$_{x}$Si nanowires with x$=$0.05. The reduced size presents the opportunity to characterize the quantum contributions to the conductivity where the electrons are effectively confined to one dimension. The dimensionality is determined by the wire diameter which can be smaller than the electron's inelastic scattering length at low temperatures. Results of these measurements will be presented.