Session D5: Vanadium Oxides; Metal-insulator Transitions

Metal-Insulator Transition in ultrathin CaVO$_{3}$ Films

Bulk CaVO$_{3}$ (CVO) with a 3d$^{1}$ electronic configuration has been found to exhibit metallic and Pauli paramagnetic behavior. We have synthesized epitaxial ultrathin films of CVO on single crystal (100) SrTiO$_{3}$ substrates by pulsed electron deposition. The CVO films were capped with 2.5nm SrTiO$_{3}$ layer. Compared with single crystal CVO, thin film CVO demonstrated very large resistance ratios (RR), e.g., R (300K)/R (2K) more than 3000. The temperature dependent Hall measurement showed mostly that the large RR determined by the change in mobility, we observed the metal-insulator transition at $\sim $ 100K in CVO ultrathin films with thickness below 4nm, which was not observed in either thick CVO films or STO films. Above 100K, ultrathin CVO exhibited the metallic behavior, and below 100 K, it became an insulator. The emergence of MIT could be attributed to a pseudogap that appeared at Fermi surface with decreasing film thickness, indicating a transition from 3D metal to 2D insulator transition in ultrathin CVO films at temperatures lower than 100K. A metal-insulator transition was further characterized by I-V measurements, the insulator phase was only observed with driven current below 100K and 2$\times $10$^{-6}$A.   

Thermoelectric response in the incoherent transport region near Mott transition: the case study of La$_{1-x}$Sr$_x$VO$_3$

We report a systematic investigation on the high-temperature thermoelectric response in a typical filling-control Mott transition system La$_{1-x}$Sr$_x$VO$_3$. In the vicinity of the Mott transition, incoherent charge transport appears with increasing temperature and the thermopower undergoes two essential crossovers, asymptotically approaching the limit values expected from the entropy consideration, as known as Heikes formula. By comparison with the results of the dynamical mean field theory, we show that the thermopower in the Mott critical state mainly measures the entropy per charge carrier that depends on electronic degrees of freedom available at the measurement temperature. Our findings verify that the Heikes formula is indeed applicable to the real correlated electron systems at practical temperatures ($T>200$ K).   

First-principles calculations of phonons in VO$_2$

Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition at 340K. This is accompanied by a structural transition from a metallic, high-temperature rutile phase to a low-temperature monoclinic insulating phase. Recently, it has become possible to produce single crystal platelets of VO$_2$ deposited on a oxidized silicon substrate. These micro-crystals are under strain which can potentially alter their properties compared to bulk samples. Infrared micro-spectroscopy on these samples permits accurate measurements of their electronic and phonon properties as the mircro-crystals are driven reversibly across the temperature-driven insulator-to-metal transition (IMT). We present {\it ab-initio} calculations of phonons in the rutile and monoclinic phases of VO$_2$. These calculations were performed using first-principles density functional theory using both LDA and LDA+U. The effect of the Hubbard parameters and strain on both phases is discussed. We compare our results to the single crystal measurements and previous experimental results.   

Infrared micro-spectroscopy of strained VO$_2$ micro-crystals

The temperature-driven insulator-to-metal transition (IMT) in vanadium dioxide (VO$_2$) is accompanied by a structural instability (SI). The IMT and SI lead to a drastic change in the electronic properties, crystal structure, and lattice dynamics. We performed infrared micro-spectroscopy on single crystal platelets of VO$_2$ deposited on oxidized silicon substrate by physical vapor deposition. The firm attachment of these micro-crystals to the substrate causes strain which can alter their properties compared to bulk samples. We report infrared data on these micro-crystals and demonstrate both their electronic and phonon properties in the monoclinic M1 phase and the rutile phase. We also compare their infrared conductivity to that of bulk single crystals and thin films. Finally, we compare infrared-active phonon features to first-principles density functional theory calculations.   

Ultrafast Pump Probe Transmission Spectroscopy of VO$_2$

We have performed nondegenerate pump-probe transmission spectroscopy, pumping with an above the band gap (1.5 eV) 50 fs pulse and probing with a 0.4 eV (below the band gap) pulse to monitor the dynamics of the formation of the metallic phase in vanadium dioxide ($\mathrm{VO_2}$). Below the percolation threshold ($<$ 330 K), we find an initial drop in transmission consistent with electron-hole generation across the band gap, while in the fully metallic phase ($<$ 365 K), we see an initial rise in transmission due to transient heating of electrons at the Fermi surface. In the transitional region, the data show complex time dependence consistent with the nucleation and growth of metallic domains in the semiconducting phase and ultrafast heating of metallic precursors in the insulating phase.   

Ultrafast Spectroscopy and Optically-Induced Phase Transitions of Single Crystal VO2

We investigate the metal-insulator phase transition (MIT) of single crystal VO2 platelets using non-degenerate optical pump-probe spectroscopy. The pump pulse is at 800 nm and the probe pulse varies between 1.3 $\mu $m and 2.4 $\mu $m, covering the optical gap ($\sim $2 $\mu $m) of VO$_{2}$. We observe ultrafast carrier relaxation on the timescale of 0.5 ps or less in the insulating phase. Higher pump powers induce coherent acoustic phonon oscillations which we explore by adjusting probe wavelength and pump fluence. At temperatures just below the transition, we observe optically induced MIT at a time scale less than 300 fs (pulse width limited), and we investigate the dependence on crystal size, wavelength, and temperature. The properties of the insulator do not seem consistent with a conventional band semiconductor.   

THz induced insulator to metal transition in VO$_{2}$ metamaterial

We use metamaterial enhanced high field terahertz (THz) pulses (up to $\sim $4MV/cm) to induce the insulator-to-metal transition in vanadium dioxide (VO$_{2})$ thin films at 320K. Ultrafast THz field enhancement in the gaps of metamaterial split ring resonators releases free electrons in VO$_{2}$ by the Poole-Frenkel effect. The accelerated hot electrons transfer energy to the lattice via electron phonon coupling inducing the persistent metallic phase. A large nonlinear signature is observed in VO$_{2}$ as modulations of the metamaterial resonance on a picosecond time scale. Our results provide insight into electric field induced phase transitions in VO$_{2}$ and paves the way for studying nonlinear high THz field effects in many other strongly correlated materials.   

Optical control of structural evolution of ultrathin VO$_2$ film following a photoinduced metal-insulator phase transition

Using ultrafast x-ray diffraction technique, we have demonstrated that the structural symmetry in VO$_{2}$ thin film across the metal-insulator transition (MIT) can be controlled by the optical pump fluence. After an 800 nm, 50 fs laser excitation, the structural evolution of a 13nm-thick, MBE-grown VO$_{2}$ film on TiO$_{2}$ substrate was monitored by a synchrotron based x-ray diffraction with 100 picosecond (ps) time resolution. By adjusting the pump fluence, the VO$_{2}$ film can be prepared in selected structural state 100 ps after excitation, and transits through the known structural phases (tetragonal, monoclinic M2, monoclinic M1) in tens of nanoseconds time scale. When pumping at high fluence (~28 mJ/cm$^{2}$), a new transient state that exhibits lower symmetry than the thermal equilibrium tetragonal phase has been identified and its evolution pathway has been measured.   

In-situ measurements of Stress Relaxation across Metal Insulator Transition in Correlated Oxide Thin Films

Stress relaxation across the metal-insulator transition in correlated oxide thin films such as VO$_{2}$ and SmNiO$_{3}$ is of great importance since it could be directly related to the symmetry breaking structural component of the transition and also affects the properties and performance of the electronic devices significantly. We present in-situ stress relaxation measurements across the thermally triggered metal insulator transition and its impact on the transition characteristics and stability. Mesoscopic size effects, micro-patterning and geometrical confinement effects on the metal insulator transition and associated stress relaxation will be addressed. Correspondence between onset of the electrical transition with stress relaxation leads to several interesting observations regarding the transition dynamics and will be discussed.   

In-Plane Impedance Spectroscopy measurements in Vanadium Dioxide thin films

In plane Impedance Spectroscopy measurements have been done in Vanadium Dioxide thin films in the range of 100 Hz to 1 MHz. Our measurements allows distinguishing between the resistive and capacitive response of the Vanadium Dioxide films across the metal-insulator transition. A non ideal RC behavior was found in our thin films from room temperature up to 334 K. Around the MIT, an increase of the total capacitance is observed. A capacitor-network model is able to reproduce the capacitance changes across the MIT. Above the MIT, the system behaves like a metal as expected, and a modified equivalent circuit is necessary to describe the impedance data adequately.   

Photoemission Study of VO2 Above and Below the Transition Temperature

Angle-resolved photoemission (ARPES) experiments on VO$_2$ have traditionally been hindered by the quality of cleaved single crystals. The lack of a clear metal-insulator transition (MIT) in low photon energy measurements has even lead to the assumption of a surface region with a different electronic structure. WIth the \textit{in situ} pulsed-laser-deposition (PLD) system available on beamline 7.0.1 at the Advanced Light Source we have grown VO$_2$(001) films on a TiO$_2$ substrate and measured the band structure above and below the transition temperature. We discuss our results in comparison with the available calculations, and we show that the MIT is clearly visible for photon energies within the UV range.   

Photoemission measurements of strained VO2

The metal-insulator transition of VO$_2$ has been a textbook example for many years, despite a clear understanding of its microscopic origins proving elusive. Recently, the promise towards novel applications of high-quality thin films, in which the properties of the transition can be tailored by applied strain, has thrust VO$_2$ back into focus. Here, we report photoemission measurements of strained VO$_2$ thin films epitaxially grown on TiO$_2$(110) and TiO$_2$(100) substrates. The applied strain for these two films lead to moderate and large compressive rutile $c$-axis strains, respectively. By making use of the incident photon polarization, we observe the changes in polarization anisotropy both across the transition and as a function of applied strain, and demonstrate how we can use this to learn more about the origin of the MIT in VO$_2$.   

Correlations of local electronic properties in vanadium dioxide thin films

We probe the local electronic properties of a vanadium dioxide thin film using scanning force microscopy. We scan a conductive cantilever in contact mode across the surface of the sample. At each point, we sweep the voltage applied to the sample, obtaining current versus voltage curves with nanonscale resolution while inducing a transition from the insulating to metallic state. We identify individual grains of $\sim $50-100 nm, and extract the electronic properties of each grain, such as transition voltage, hysteresis, dielectric constant, and metallic state resistance. We discuss the correlations between these properties.   

Correlation of Valence State and Transport Properties in VO$_{2}$ Films

Vanadium dioxide (VO$_{2})$ exhibits a metal semiconductor transition (MST) that is accompanied by the abrupt change in the electrical conductivity, optical transmittance and reflectance in infrared region, which can be used in electronic devices such as temperature sensors and electric switches. VO$_{2}$ thin films were grown on c-plane Al$_{2}$O$_{3}$ substrates with different O$_{2}$ flow rates. The XRD scans have been performed to confirm the single phase and highly textured VO$_{2}$ films despite the change in the growth parameter, i.e. the oxygen partial pressure. The valence state of vanadium on different films was investigated by soft x-ray spectroscopy. The Hall bars were fabricated for electrical transport and Hall measurements. As the O$_{2}$ flow rate increases, the XRD results show decreasing lattice parameter, hence increasing compressive strain along b-axis of monoclinic VO$_{2}$; the transport measurements also show the increasing transition temperature (T$_{MST})$ and the increasing change in resistivity associated with the strain and valence state of vanadium. Hall measurements reveal a sudden increase in carrier concentration from 10$^{20}$ to 10$^{24}$ cm$^{-3}$ around T$_{MST}$, while the mobility remains constant before and after MST. The correlation among the valence state, the strain and MST in VO$_{2}$ will be discussed.   

Metal-insulator transition in VO2 macro single domain crystals due to phase boundary motion

Metal insulator transition (MIT) characteristics of macro-size VO$_{2}$ single domain crystals, exhibiting a high resistance ratio of $\sim $ 10$^{5}$ within as small as-10$^{-3} \quad ^{\circ}$C in the vicinity of MIT temperature, were investigated by temperature-dependent electrical transport, optical microscopy, and synchrotron-based polychromatic x-ray micro-diffraction measurements. Our results clearly show that MIT initiated via inhomogeneous nucleation, proceeds with the propagation of sharp phase boundary between the metallic (R) and insulating (M1) phases, along the rutile c axis. In this talk, we will present evidences of MIT of single domain VO$_{2}$ crystals with sharp phase boundary motion and discuss the implications of our findings on to the origin of MIT and related phenomena.