Session B19: Metal Insulator transitions in Vanadates: exp/theory

Nanoscale Thermal Mapping of VO$_2$

We present a method for nanoscale thermal imaging of insulating thin films. We image the local temperature of the metal-insulator transition in a VO$_2$ film, and investigate the role of Joule heating in two-terminal geometry. By sweeping the voltage applied to a conducting atomic force microscope tip in contact mode, we locally trigger and detect the transition to the metallic phase. By fitting the Poole-Frenkel conduction regime immediately preceding the transition, we extract the local temperature. Finally, we find grains displaying two electronic transitions, consistent with a locally stable intermediate insulating phase.\\[4pt] We acknowledge financial support from Harvard's Nanoscale Science and Engineering Center, funded by NSF grant PHY 01-17795 and the Sloan Fellowship. Adam Pivonka acknowledges the support of the New York Community Trust--George Merck Fund. Magdalena Huefner acknowledges the support of the Deutsche Forschungsgemeinschaft (HU 1960/11).   

Strain-dependent Metal-Insulator Tansition in VO$_{2}$ single-crystalline thin films

Vanadium dioxide (VO$_{2}$ has a near room temperature metal insulator transition (T$_{\mathrm{MIT}}$ $\sim$ 340 K) accompanied by a structural transition making the origin of this transition controversial. In this work, we have continuously changed T$_{\mathrm{MIT}}$ by as much as 60 K in VO$_{2}$ (001) single crystalline thin films by using RuO$_{2}$ buffer layers. We observe a decrease in the T$_{\mathrm{MIT}}$ as a function of decreasing c-axis length in the rutile phase which is unexpected from a one-dimensional Peierls model. By performing complementary bulk-sensitive spectroscopic measurements, namely, x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS), we identify changes in orbital occupation and electron-electron correlations as a function of strain in the metallic state that explain the observed T$_{\mathrm{MIT}}$ dependence on strain.   

Broadband Infrared Spectroscopy of Vanadium Dioxide Films Under the Influence of Strain

Vanadium dioxide (VO$_{\mathrm{2}})$ undergoes a phase transition between an insulating monoclinic phase and a conducting rutile phase. Even in this simple, stoichiometric material, a complete explanation of the phase transition has proved elusive. This transition, like phase transitions in other correlated electron systems, involves interacting electronic, lattice, and orbital degrees of freedom. This leads to physical properties that are particularly sensitive to small changes in external parameters such as strain. VO$_{\mathrm{2~}}$films grown on different substrates are subject to differing strain effects that often lead to a shift in the transition temperature. Broadband infrared (IR) and optical spectroscopy allows us to examine the electronic structure and dynamics as well as IR-active, zone-center phonons of strained films grown on sapphire and quartz. Comparing and contrasting the IR and optical properties of these films, and those of bulk crystals, will provide insight into the influence of strain on the electronic and lattice degrees of freedom.   

In-situ studies on the Martensitic-type transition in VO$_{2}$ thin films

We present in-situ kinetic studies across metal-insulator transition in epitaxial and polycrystalline VO$_{2}$ thin films through electrical resistance and stress measurements along with TEM investigations. Variable temperature wafer curvature experiments enable the probing of in situ stress relaxation kinetics associated with the structural component of the metal insulator transition. Primarily, no time or drive rate dependence is observed in the stress relaxations providing insight into the athermal nature of phase transition kinetics. However, proximate to the phase transition boundary, minor fraction of isothermal component that show time dependence in both stress relaxation and electrical measurement is captured. In situ electron diffraction and micro structural observations across the metal insulator transition provide evidence for martensitic type transition in polycrystalline VO$_{2}$ thin films. The studied aspects of time independent, Martensitic type, athermal transition kinetics along with negligible fraction of isothermal kinetics have significance in understanding the dynamics of structural phase transitions that accompany electronic property changes.   

T$_{c}$ anisotropy and phase separation in strained Vanadium Dioxide films

We report Infrared near field study on strain induced transition temperature (T$_{c})$ anisotropy in vanadium dioxide (VO$_{2})$ films via direct visualization of a spontaneous structural and electronic phase separation. The films are epitaxially grown on [110]$_{R}$ or [100]$_{R}$ TiO$_{2}$ substrates and exhibit large uniaxial strain. By mapping the film topography with AFM and electronic percolation with Infrared scattering scanning near-field optical microscopy, a temperature dependent electron-lattice correlation can be clearly observed. Our work sheds a new light onto the nature of the Tc anomaly in metal-insulator transition and leads to the possibility of controlling the material's properties through strain induced phase separation.   

Hydrogen doping and the metal-insulator transition in vanadium dioxide

Vanadium dioxide has a first-order metal-insulator transition (MIT) at 67 $^{\circ}$C. It has recently been shown [1] that hydrogen doping of VO$_{2}$ by spillover from a metal catalyst in hydrogen gas gradually reduces the gap in the insulating phase to zero, and eventually eliminates the MIT. The dependence on hydrogen concentration enables optical and electrical detection of the local hydrogen density. We exploit this to study the diffusion of hydrogen and its dependence on temperature, direction, strain, and phase in single-domain nanobeams and platelets of VO$_{2}$. For example, we find that diffusion is faster along the rutile c-axis, and can be significant even at the transition temperature. We also study the effects of hydrogen doping on the phase diagram, on the low temperature conductivity, and on the continuous-wave and ultrafast optical response.\\[4pt] [1]. Wei, J. \textit{et. al.} Nature Nano. \textbf{7}, 357 (2012)   

Comparative studies of electrically driven metal insulator transition in VO$_{2}$ single crystal and thin film

Electrically driven metal-insulator transition (MIT) characteristics of VO$_{2}$ single domain crystal and thin-film were investigated by temperature and external bias voltage dependent electrical transport, optical microscopy, and synchrotron-based polychromatic x-ray micro-diffraction measurements. Our results suggest that electrically driven metallic state of VO$_{2}$ is similar to that of temperature driven metallic state. However, after the electrically driven MIT, VO$_{2}$ single crystal exhibits metallic and insulating colors on the surface of the crystals simultaneously. In addition, the origin of electrically driven MIT of crystals seems different from that of electrically driven MIT films. In this talk, we will present comparative studies of electrically driven MIT of VO$_{2}$ single crystal and thin-film, and discuss the origins of electrically driven MIT and its implications.   

VO$_2$ and V$_2$O$_3$: different pathways for the same phase transition?

Decades of investigation have led to a better understanding of the properties of vanadates but a great deal remains to be explored in these scientifically fascinating and technologically relevant systems. VO$_2$ and V$_2$O$_3$ are canonical examples of these transition metal oxides, strongly influenced by both electronic correlations and structural effects. In both materials the MIT is known to occur following a variation in temperature, the application of a dc field, optical pumping and more recently the application of transient THz pulses. The question that naturally arises is whether and how the dynamics of the MIT depend on the nature of the stimulus that induced it. We will present time-resolved optical and THz investigations, including high THz field results, of thin films of V$_2$O$_3$ and VO$_2$.   

The Effect of Doping on the Metal-Semiconductor Transition in VO$_{2}$

Vanadium dioxide (VO$_{2})$ is a well-known correlated material that exhibits a metal-semiconductor transition at 340K, with several orders of magnitude change in the resistivity. In this study, we report the effect of Mn-doping and Al-doping, with different doping recipes; the films were deposited by Reactive Biased Target Ion Beam Deposition, and their single phase was confirmed by X-ray diffractometry. The different doping recipes had a very dramatic impact on the crystallinity of the vanadium dioxide films. It was found that using a lower frequency for the pulsed dc target bias was desirable for the improvement of the film quality. Both Al and Mn doping can enhance the transition; while the Al doped VO$_{2}$ also raises the transition temperature.   

Benchmark study of the application of density functional theory to correlated t$_{2g}^{1}$ vanadates

SrVO$_3$ and CaVO$_3$ are strongly correlated perovskite-structured metals belonging to the class of transition-metal oxides with a 3$d^1$ electronic configuration. Both cubic SrVO$_3$ and orthorhombically distorted CaVO$_3$ are classified as Pauli paramagnets, yet their magnetic states at low temperature remain controversial. Here, we present and discuss the results of systematic density functional theory (DFT) calculations on the atomic and magnetic structures of both SrVO$_3$ and CaVO$_3$ to shed light on this issue. We use standard and ``beyond-DFT'' exchange-correlation functionals to evaluate the stable magnetic states. We conclude by discussing both the accuracy of these methods for reproducing the atomic structures of the $t_{2g}^{1}$ vanadates and their implications on artificially structured oxide superlattices.   

Ab initio study of metal-insulator transition in VO2

The structure distortion accompanied metal-insulator transition (MIT) of vanadium dioxide (VO$_2$) at 340K has been a matter of ongoing controversy for near four decades. It is still unclear whether the nature of this transition is due to a Peierls instability, a Mott-Hubbard transition, or other physics. Most density functional theory based methods fail to describe the nature of the electronic state in this system, further complicating theoretical description of VO$_2$. We will report on progress in applying the first principles diffusion quantum Monte Carlo method to the electronic structure of VO$_2$ in the metallic and insulator phases. By examining the energetic properties, one particle reduced density matrix, as well as other static correlations in the two phases of the system, we will comment on which of the two common descriptions is a closer representation of the physical reality of VO$_2$.   

Phonon Softenings and the Mott-spin-Peierls Transition in VO$_{2}$

To explore the driving mechanisms of the metal-insulator transition (MIT) and the structural transition in VO$_{2}$, we have investigated phonon dispersions of rutile VO$_{2}$ (\textit{R}-VO$_{2}$) in the DFT and the DFT+$U$ ($U$: Coulomb correlation) band calculations. We have found that the phonon softening instabilities occur in both cases, but the softened phonon mode only in the DFT+$U$ describes properly both the MIT and the structural transition from \textit{R}-VO$_{2}$ to monoclinic VO$_{2}$ (\textit{M$_{1}$}-VO$_{2}$). The present {\it ab-initio} phonon dispersion calculations clearly demonstrate that the Coulomb correlation effect plays an essential role of assisting the Peierls transition in \textit{R}-VO$_{2}$ and producing the spin-Peierls ground state in \textit{M$_{1}$}-VO$_{2}$.   

Examining the density functional theory description of VO$_2$ above and below the metal-insulator transition

Vanadium oxide (VO$_2$) exhibits a metal-insulator transition at 341 K, which is accompanied by a change from a tetragonal to a mononoclinic structure. We examine the electronic and magnetic properties of VO$_2$ below and above the transition point, as calculated from density functional theory (DFT) and some extensions, including hybrid DFT / Hartree-Fock functionals and Hubbard-corrected functionals. We show that the groundstate solutions obtained with either the GGA approximation or the screened hybrid functional HSE (25\% of Hartree-Fock exchange) are at odds with experimental observations for both phases. We then discuss the effect of varying amounts of Hartree-Fock exchange and values of the Hubbard parameter U on the solutions. Although the agreement of some of the calculated properties with experiment can be tuned in this way, we conclude that no single setting can describe the properties of both VO$_2$ phases simultaneously.   

Spatial complexity due to strong correlations in vanadium dioxide

Near-field scanning infrared microscopy on the Mott metal-insulator system vanadium dioxide (VO$_2$) has revealed complex nanoscale pattern formation in the form of insulating and metallic puddles near the insulator-to-metal transition [1]. We use and extend recently developed cluster techniques [2] in order to understand the fundamental physics driving this multiscale pattern formation. We map the observed metallic and insulating clusters to Ising variables by a rigorous choice of threshold amplitude, and quantify the statistics of the sizes and shapes of the geometric clusters. These in turn yield critical exponents including the cluster size distribution exponent $\tau$, and the fractal dimensions associated with the cluster formation. These quantitative measures show power-law behavior over multiple decades, revealing a delicate interplay between interactions and disorder in the material. The cluster techniques employed here can be readily applied to 2D image data in the context of other materials and measurement techniques.\newline \par \noindent [1] M. M. Qazilbash, et al., {\it Science} {\bf 318}, 1750 (2007).\newline [2] B. Phillabaum, E. W. Carlson, and K. A. Dahmen, {\it Nat. Commun.} {\bf 3}, 915 (2012).   

Structural and vibrational properties of VO2 from DFT and DFT+U calculations

Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition (MIT) at 340\,K from a metallic, high-temperature rutile phase to a insulating, low-temperature monoclinic phase. In thin films, the extremely fast switching times ($\simeq 100$~femtoseconds) of the MIT have led to many suggested device applications. Understanding the MIT driving mechanism and the long-debated importance of electronic correlation is important to these developments. We have computed the relaxed geometry and phonon frequencies using DFT and DFT+U for both phases of VO$_2$. The dependence of vibrational mode frequencies and oscillator strengths on the Hubbard $U$ parameter and their sensitivity to the Born effective charges in the insulating monoclinic phase will be reported. The calculated frequencies for $U=5$ eV are in good agreement with recent experimental infrared micro-spectroscopy measurements on single crystal platelets of VO$_2$ \footnote{T. J. Huffman et al., PRB, submitted.}. Our results indicate that strong electron-electron correlation must be included to describe the vibrational properties.