Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H31: Focus Session: Vanadate Thin Films |
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Sponsoring Units: DMP GMAG Chair: Dmitri Basov, University of California San Diego Room: 335 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H31.00001: Magneto-transport studies in thin film vanadium dioxide across the metal-insulator transition D. Ruzmetov, V. Narayanamurti, S. Ramanathan, D. Heiman, B.B. Claflin Temperature dependent magneto-transport measurements in magnetic fields of up to 12 Tesla were performed on thin film vanadium dioxide (VO$_{2})$ across the metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at the MIT and accounts almost entirely for the resistance change. The Hall mobility varies little across the MIT and remains low. Electrons are unambiguously found to be the major carriers on both sides of the MIT. The Hall coefficient temperature dependence in the semiconducting phase can be well fitted within a single band model. Position of the Fermi level in the band gap is estimated from the fit and an unusually large effective electron mass is found. The positive magnetoresistance at room temperature is measured to be 0.09{\%} in a 12T field. The results will be compared with single crystal data and discussed in detail. The studies are of relevance towards understanding mechanisms governing phase transition in complex oxide thin films. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H31.00002: Mott metal-insulator transition-induced electrical oscillation in VO$_{2}$ Hyun-tak Kim, Yong Wook Lee, Bong-jun Kim, Sun Jin Yun, Sungyoul Choi, Byung-gyu Chae Since Mott predicted the abrupt first-order metal-insulator transition (MIT) in 1949, one of the most important issues in contemporary solid-state physics has been to experimentally prove Mott's MIT in a strongly correlated system with electron-electron interaction. The MIT has many practical applications and is believed to facilitate the understanding of physical phenomena, such as high-$T_{c}$ superconductivity, colossal magnetoresistance, etc. In particular, in order to reveal the mechanism of the Mott MIT, many physicists have paid attention to a representative paramagnetic insulator, VO$_{2}$(4$d^{1})$, with an abrupt resistance change near 68$^{\circ}$C. The key issue is whether VO$_{2}$ is a Mott insulator, in which the abrupt MIT is not caused by a structural phase transition (SPT), or a Peierls insulator undergoing the SPT near $T_{SPT }\approx$ 68$^{\circ}$C; this question can be answered when a monoclinic and correlated metal (MCM) phase different from a normal metal is observed. Here we show an MCM phase, high frequency electrical oscillations in the MCM phase of VO$_{2}$. The oscillation possibly is generated from a temporal capacitor, which is comprised of both temporary dielectric components, arising from inhomogeneity in a VO$_{2}$ film, and MCM phases acting like electrodes. This work concluded that the electrical oscillation is a characteristic of the Mott MIT. (Ref: Applied Physics Letters 92 (2008) 162903). [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H31.00003: De-coupling Electrical and Thermal Effects in Triggering Metal-Insulator Transition in $VO_{2}$ Thin Film Devices Gokul Gopalakrishnan, Shriram Ramanathan Vanadium dioxide ($VO_{2}$) has been shown to undergo an abrupt electronic phase transition near $70^{\circ}C$ from a semiconductor to a metal, with an increase in dc conductivity of over three orders of magnitude, making it an interesting candidate for advanced electronics as well as for fundamental research into understanding the Mott transition. Recent experiments strongly suggest that this transition can be manifested independent of a structural phase transition in the system at a similar temperature, and that it can be triggered by the application of a critical field across the $VO_{2}$ thin film. To address the important question of thermal effects due to the applied field, we report the results of electro-thermal simulations on a number of common and promising device geometries showing the extent of heating caused by the leakage current in the ``off'' state of the $VO_{2}$ device. The simulation results are compared with experimentally observed device characteristics. Valuable insights into the nature of the metal-insulator transition can be obtained from such simulations and will be discussed in the presentation. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H31.00004: Properties of Complex Oxides at the Nanoscale: First Order Phase Transitions through Avalanches. Invited Speaker: Properties of complex oxides have been studied for decades, including many systems which exhibit a phase transition, among which are high temperature superconductors, multiferroics, and metal insulator (M-I) transition materials. We have studied a member of the later group, vanadium oxide (VO$_{2})$, which in spite of its long history, keeps surprising researchers today. We find that the M-I transition of nano-scaled VO$_{2}$ devices is drastically different from the smooth transition curves generally reported. The temperature driven M-I transition occurs through a series of resistance jumps ranging over two decades in magnitude, indicating that the transition between the two phases of the system happens by resistive avalanches. The avalanche magnitude follows statistically a power law similar to that observed in many other physical systems, such as Barkhausen noise in ferromagnets or sand avalanches in sand piles, and non-physical systems such as connectivity of the internet. We discuss the effects on the distribution of avalanches due to the: device dimensions, percolative nature of the measurement and interactions between the different phases within the phase transition. We present additional evidence for the importance of interactions in macroscopic FORC measurements, and their role in the opening of the hysteresis in VO$_{2}$ M-I transitions. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H31.00005: FORC Analysis of the thermal hysteresis at the Metal Insulator Transition in VO$_2$ Juan Ramirez, Amos Sharoni, Jonathan Dubi, Maria E. Gomez, Ivan K. Schuller We use the first order reversal curve method (FORC) in order to obtain a quantitative analysis of the temperature-driven metal-insulator transition hysteresis in VO$_2$ thin films. By studying the hysteresis properties of resistance vs. temperature we were able to obtain information regarding inter-domain interactions. An unexpected tail like feature in the contour plot of the FORC distribution indicates the existence of irreversible regions outside of the hysteresis loop. This irreversibility may arise from metallic domains present at temperatures below the closing of the hysteresis, which interact with the surrounding medium and change the reversal path relative to one coming from a \emph{fully} insulating state. We develop a model where the driving force which opens hysteresis in VO$_2$ are inter-domain interactions. This model is intrinsically different from the Preisach model that is usually used to describe hysteresis, since it identifies a microscopic origin of the hysteresis, and provides physical parameters to characterize it. Work Supported by the US Department of Energy, AFOSR and the Colombian agencies COLCIENCIAS and the Excellence Center for Novel Materials, CENM. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H31.00006: Electric field effects on phase transition and electronic transport mechanisms in vanadium oxide thin films Changhyun Ko, Shriram Ramanathan Metal-insulator transitions (MIT) in complex oxide thin films are of great interest from both scientific and application perspectives. Vanadium oxide is a model system to exploit strongly correlated electronic phenomena such as Mott transition. Furthermore wide attention for application has been attracted by its transition functionality that can be tuned in terms of various external parameters: temperature, electric field, photoexcitation, and stress. The tunable conduction states allow creating novel devices whose functions are controlled by multiple parameters. We report on recent observations of electric-field assisted phase transition in vanadium oxide thin films. The conduction mechanisms were analyzed using both in-plane and out-of-plane modes and matched well. In the insulator phase, Poole-Frenkel emission was suggested to govern non-ohmic behavior at high field regime, while under low field application, ohmic conduction with activation energy of $\sim $0.24 eV was observed. Activation energy of ohmic conduction in the metallic phase was $\sim $0.08 eV. These preliminary results are encouraging towards exploring correlated oxides for computing device technologies. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H31.00007: Conductivity Dynamics in the Correlated Metallic State of V$_{2}$O$_{3}$ M. Liu, B. Pardo, M.M. Qazilbash, S.J. Yun, B.G. Chae, B.J. Kim, H.T. Kim, D.N. Basov, R.D. Averitt V$_{2}$O$_{3}$ is a strongly correlated electron material that undergoes a transition from antiferromagnetic insulator at low temperatures to a strongly correlated metal above $\sim $140K. We report on time resolved spectroscopic studies of V$_{2}$O$_{3}$ thin films where we have observed coherent oscillations in the far-infrared conductivity following excitation with a 35-fs optical pulse. The resultant $\sim $100 ps conductivity oscillations result from the optically induced generation of strain which modulates the orbital overlap and hence the conductivity thus revealing a strong coupling of carriers to the lattice in the metallic state. This contrasts with other vanadates such as VO$_{2}$ where this effect is not observed. We will discuss the potential of V$_{2}$O$_{3}$ as a candidate material for investigating photoinduced phase transitions. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H31.00008: Subdomain studies of the metal-insulator transition in VO$_{2}$ nanobeams Jiang Wei, Jae Hyung Park, Jacob Beedle, Zenghui Wang, Wei Chen, Geeta Yadav, David Cobden In many correlated materials, domain structure causes the bulk properties to differ from those on the sub-domain level. In addition, near first-order phase transitions it leads to transition broadening, hysteresis, and sample degradation. Studies of nanoscale crystals enable investigations of the domain-free homogeneous material. We demonstrate this by working with nanobeams of vanadium dioxide, thereby discovering or clarifying multiple aspects of its famous metal-insulator transition at 67$^{\circ}$C. Amongst them are that the transition to the metal occurs at a constant value of the resistivity of the insulating phase; large supercooling of the homogeneous metallic phase is possible; and the activation energy in the insulating phase is consistent with the optical gap, in contrast with earlier reports on bulk samples. The nanobeams also enable new classes of experiments, including investigating a single metal-insulator interphase wall, employing nanomechanical effects to determine the equilibrium transition temperature, and investigating the dynamics of a phase transition in quasi-one-dimensional geometry. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H31.00009: Infrared nano-imaging of metallic and insulating domains in single crystalline vanadium dioxide nanowires Andrew Jones, Jiang Wei, David Cobden, Markus Raschke Correlated electron systems are often associated with heterogeneous electronic and structural phase transitions with ordering and domain formation on nanometer length scales. Vanadium Dioxide (VO$_{2}$) has long been a material of research focus due to behavior associated with its temperature induced metal-insulator transition (MIT) occurring around 340K. The underlying mechanism of this transition is thought to be the result of a complex interplay between the lattice and electronic degrees of freedom as the material passes through the MIT, whose origin is as of yet poorly understood. We study the nanometer scale formation of insulating and metallic domains of single crystal VO$_{2}$ nanowires bonded to silica substrate using infrared scattering-scanning near field optical microscopy (s-SNOM). Imaging contrast is obtained due to the distinct optical dielectric properties of the respective metallic and insulating phases. A hierarchy of domain sizes is observed, suggesting two distinct insulating phases in addition to the metallic phase as the material moves through the MIT. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H31.00010: Mapping the spatial scale of domain switching in heteroepitaxial vanadium dioxide thin films and nanoparticles Joyeeta Nag, Richard Haglund Vanadium dioxide is a strongly correlated electron system exhibiting a hysteretic semiconductor-to-metal transition around 67C, accompanied by a structural change from monoclinic to tetragonal and huge changes in its electrical conductivity and near-infrared transmission. As interest grows in very thin films and nanoparticles of vanadium dioxide, the spatial scale and domain structure of the metal-insulator transition are critical issues. To elucidate these questions, thin films and nanoparticles of vanadium dioxide were grown on R-, C- and A-cut sapphire substrates, and the substrate-dependent epitaxial growth habits and in-and-out-of-plane orientations were characterized by near-infrared transmission, X-ray diffraction, scanning and transmission electron microscopy. Temperature variable XRD scans at intervals of one degree were performed from the onset to the completion of the transition hysteresis to map out the percentage of coexisting domains of monoclinic and tetragonal phases in one such 100nm thick epitaxial film. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H31.00011: Novel devices in VO$_{2}$ nanobeams Jiang Wei, Jae Hyung Park, Jacob Beedle, Geeta Yadav, Zenghui Wang, David Cobden, Marco Rolandi Working with nanobeams and sheets of vanadium dioxide attached to a rigid substrate, one can avoid the difficulties associated with multiple domains and sample degradation that occur in the bulk material due to the first-order metal insulator transition. Taking advantage of this, we illustrate the possible uses of single-crystal nanobeams and sheets for making new kinds of nanoelectronic devices and switches. For switching, one can exploit the instabilities due to coupling between the metal-insulator transition and mechanical buckling or the supercooling of the metallic phase. For nanoelectronic devices we explore the patterning of a conducting layer on the surface by controlled reduction and oxidation using conducting atomic force microscopy. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H31.00012: Self-Limiting Growth of Magnetic Nanoparticles in a Glassy Matrix Sergio Picozzi, Mark Laurenzi III, Ian Pegg Nanoparticles of magnetite can be grown by heat treatment of suitable glass compositions slightly above the glass transition temperature. We have investigated the transformation kinetics and magnetic properties, including the size dependence of the Verwey transition, in such systems. The initially rapid growth is quickly arrested leading to tight size distributions that become essentially independent of time. The mean size (a few nm) is dependent on the glass composition and temperature. In this paper, we investigate a simple model in which the self-limiting nature of this process is ascribed to the experimentally observed strong dependence of the glass transition temperature of the matrix on the concentration of one of the diffusing species, which in turn gives rise to a concentration-dependent diffusivity. In addition, the relationship between the equilibrium concentration of the diffusing species and the curvature of the particle-matrix interface (the Gibbs-Thomson effect) is shown to play a prominent role. The model reproduces the essential features of the transformation kinetics, predicting an initial power law growth that becomes nearly logarithmic at long times, and identifies the key physical parameters that determine the self-limited particle size. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H31.00013: Unusual electronic states in TiO$_2$/VO$_2$ (001) multilayers. Victor Pardo, Warren Pickett Abrupt interfaces between oxides display a wealth of unexpected behavior, and the interface between a band insulator and a Mott insulator is expected to display extra richness. Several multilayered TiO$_2$/VO$_2$ structures have been studied by ab initio density functional theory techniques, including the thin VO$_2$ regime corresponding to the quantum confined Mott insulator. VO$_2$ undergoes a metal-insulator transition near room temperature, but when deposited in thin films of thickness smaller than 5 nm, the metal-insulator transition disappears. Our calculations (using the correlated LDA+U method with modest values of U and J) show that the electronic character (metallic versus insulating) changes with the number of VO$_2$ layers embedded within insulating TiO$_2$ layers: metallic for five VO$_2$ layers, semimetallic and half-metallic for three layers, and insulating for a single VO$_2$ layer. These trends, and the peculiar nature of the three VO$_2$ layer case, will be discussed in some detail. [Preview Abstract] |
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