Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H38: Phase Transitions in Vanadium Oxides |
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Sponsoring Units: DCMP Chair: Jenny Hoffman, Harvard University Room: F149 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H38.00001: THz spectroscopy of optically and thermally induced metallic states in nanogranular vanadium dioxide T.L. Cocker, L.V. Titova, S. Fourmaux, H.-C. Bandulet, D. Brassard, J.-C. Kieffer, M.A. El Khakani, F.A. Hegmann VO$_{2}$ is a model correlated electron system that undergoes an insulator-metal phase transition when heated above 341K or pumped by an ultrafast laser pulse. Potential technological applications look to utilize its drastic change in optical and electronic properties and rapid switching upon photoexcitation. Progress hinges on understanding the nature of carrier conduction in the metallic state, while the driving mechanism behind the phase transition is interesting from a fundamental research perspective. We present a characterization of the electron dynamics in nanogranular VO$_{2}$ via time-resolved terahertz (THz) spectroscopy. A hysteresis is observed in the THz complex conductivity as a function of temperature. The shape of the photo-induced transient conductivity matches this signature at early times, but evolves into a different form at later times ($>$20 ps). The Drude-Smith model is used to analyze and explain our results. [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H38.00002: Three-terminal field effect switches probing the electrically triggered Metal-Insulator Transition in Vanadium dioxide Gokul Gopalakrishnan, Changhyun Ko, Dmitry Ruzmetov, Venkatesh Narayanamurti, Shriram Ramanathan Electrostatic control of the Metal-Insulator Transition (MIT) in correlated oxides is valuable, both as a probe of the nature of the phase transition, as well as being a critical aspect of novel switching devices based on Mott insulators. Of much recent interest among this class of materials, is vanadium dioxide (VO$_2$), a correlated semiconductor which exhibits a thermally induced MIT close to room temperature, and has also been shown to undergo an ultra-fast switching of conductivity by optical and electrical means. Among many of the experiments demonstrating an electrically triggered transition, however, the attendant phenomenon of Joule heating in the current channel raises questions about the triggering mechanism. To carefully address this issue, we explore the fabrication of three terminal field-effect devices, in which the resistance of a VO$_2$ based channel may be modulated by a gate electric field in the absence of any significant current induced heating. In this talk we present details of the fabrication, the technical challenges involved in implementing them, and results of gated I-V measurements performed on these devices along with our interpretation of the observed effects. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H38.00003: Time resolved measurements for voltage induced metal-insulator-transition in VO$_{2}$ nanostructures Li Gao, Xin Jiang, See-Hun Yang, Masamitsu Hayashi, Rai Moriya, Stuart S. P. Parkin The metal-insulator-transition (MIT) induced by applying a voltage laterally across vanadium oxide nanostructures with a size of a few 100 nm are studied using time resolved measurement techniques from 295 to 330 K. We find the transition time from its high resistance phase to low resistance phase shows no clear dependence on both temperature and applied voltage pulse. However, there usually exists an incubation time before the MIT actually occurs when a large enough voltage is applied. This incubation time varies from less than 1 to more than 10 ns, and shows exponential dependence on both temperature and voltage. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 8:48AM |
H38.00004: Metal-insulator transition at 180-degree orientational domain walls in VO$_{2}$ A. Tselev, E. Strelcov, I.A. Luk'yanchuk, V. Meunier, W. Shelton Jr, K. Jones, R. Proksch, A. Kolmakov, S.V. Kalinin Appearance of unusual phenomena at interfaces of different materials due to symmetry breaking and atomic, electronic, or spin reconstructions is well established area of intensive research. However, domain walls in ferroic materials can also show unusual behavior. Here, using scanning microwave microscopy we study metal-insulator transitions in a plane-parallel structure of 180-degree orientational domains in quasi-2D nanocrystalline VO$_{2}$ nanoplatelets. The results strongly suggest that domain walls in this structure are in metastable metallic state at temperatures below $T_{MIT}$, which is supported by phenomenological considerations based on Landau-Ginzburg approach. Remarkably, \textit{ab initio} density functional calculations indicate that ferroelastic domain walls of this type possess metallic character at low temperatures, which should be ascribed to elevated structural symmetry of the domain walls. [Preview Abstract] |
Tuesday, March 16, 2010 8:48AM - 9:00AM |
H38.00005: Non-equilibrium transport in VO$_2$ nanoparticles B. Wu, A. Zimmers, H. Aubin, R. Ghosh, Y. Liu, R. Lopez Over recent years, the insulator to metal transition of the vanadium dioxide (VO$_2$) Mott insulator has been revisited revealing surprising new features: (i) electronic inhomogeneities were mapped out by local infrared spectroscopy near the transition temperature T$_{IM}$; (ii) electric-field-induced resistance switching has been found in this material. To further investigate the insulator/metal transition, we have performed non-equilibrium transport measurements on 200nm VO$_2$ dots using a variable temperature/magnetic field conducting AFM setup. I(V) spectra of numerous VO$_2$ dots as a function of temperature have enabled us to determine the voltage threshold, the current jump and the the noise spectrum as voltage is swept across the electric-field-induced transition. [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H38.00006: Nano-optical study of the metal-insulator transition (MIT) phase behavior of individual VO$_2$ microcrystals Andrew C. Jones, Samuel Berweger, Jiang Wei, David Cobden, Markus B. Raschke Understanding the nature of the metal-insulator transition (MIT) in VO$_2$ has remained a challenging problem due to the associated structural lattice changes leading to strain, the prevailing use of polycrystalline film samples, and the limited spatial resolution in most studies, thus hindering access to the complex phase behavior due to inevitable inhomogeneities. Combining scattering-Scanning Near-field Optical Microscopy (s-SNOM) for ultrahigh spatial resolution imaging with Raman microscopy we identify the two insulating monoclinic M1 and M2 phases and associated nano-domain formation in the MIT to the metallic R phase. We deduce that the MIT is sensitively influenced by the competition between the M1, M2, and R phases with their different lattice constants subject to external and internal strain. The implications for the interpretation of the results from polycrystalline thin films studies will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H38.00007: Thermal Conductance of Nanoscale VO$_{x}$ Epitaxial Layers Dong-Wook Oh, Ivan Petrov, David Cahill We use time-domain thermoreflectance to measure the thermal conductance of VO$_{x}$ layers in epitaxial Pt/VO$_{x}$/Pt structures. In particular, the metal-insulator-transition of VO$_{2}$ at $\approx $70$^{\circ}$C allows us to systematically explore channels for heat transport between metals and correlated-electron systems. Pt/VO$_{x}$/Pt layers are deposited on a sapphire substrates by reactive DC sputtering with O2 partial pressure varied from 0{\%} to 13{\%}. The thermal conductance has a strong dependence on thickness, 3-50 nm, and oxygen content, pure V to V$_{2}$O$_{5}$. The thermal conductance of $\sim $10 nm thick layers of V in series with the two Pt/V interfaces is 1 GW/m$^{2}$-K, comparable to what is expected based on the diffuse-mismatch model for electron transport at interfaces. The conductance of $\sim $10 nm thick layers of VO$_{2}$ at room temperatures is remarkably high, 0.5 GW/m$^{2}$-K, for the series conductance of two metal-dielectric interfaces. At the metal-insulator-transition, the conductance of VO$_{2}$ layers increases by only 10{\%}, indicating that electrons in Pt and electrons in metallic VO$_{2}$ are not strongly coupled. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H38.00008: A study on the metal-insulator transition in V$_{1-x}$W$_{x}$O$_{2}$ by optical spectroscopy Byung Gyu Chae, Hyun Tak Kim, Mumtaz M. Qazilbash, Dimitri N. Basov A strong Coulomb repulsion between electrons in a system with half filling makes a Mott insulator, although the band theory predicts that the system should be metallic. Carrier doping of a Mott insulator leads to an anomalous change in the electronic structure such as the transfer of spectral weight. VO$_{2}$ having a 3$d^{1}$ electron configuration has attracted considerable attention for its classification into a Mott system or a Peierls band insulator because of the metal-insulator transition accompanying structural change. We investigated the metal-insulator transition in thin films of tungsten-doped vanadium oxide: V$_{1-x}$W$_{x}$O$_{2}$, focusing on the lightly doped regime. Optical gap excitation at 2.5 eV for a film of $x$ = 0.012 is substantially suppressed, which coincides with the appearance of electronic spectral weight in the mid-infrared region. In films with x=0.04 we observed a strong Drude spectrum indicative of the metallic state. Our results are difficult to reconcile with the dominant role of the Peierls picture in the metal to insulator transition. [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H38.00009: Electronic and structural transformations near the insulator-to-metal transition in vanadium dioxide M. M. Qazilbash, A. Tripathi, A. Frenzel, O. G. Shpyrko, D. N. Basov, M. V. Holt, J. M. Maser, Byung-Gyu Chae, Bong-Jun Kim, Hyun-Tak Kim Vanadium dioxide (VO$_2$) undergoes an insulator-to-metal transition (IMT) at $T$ $\approx$ 340 K accompanied by a change in the lattice structure. Numerous studies of this phase transition in VO$_2$ have focused either on the electronic change or on the structural change. The interplay between the electronic and lattice degrees of freedom has been relatively unexplored. In previous work using scanning near-field infrared microscopy (SNIM), we showed that the electronic IMT in VO$_2$ films proceeds via nucleation and percolation of nanoscale metallic domains [1,2,3]. Here we present nanoscale X-ray diffraction measurements that image the structural changes in a VO$_2$ film with 40 nm spatial resolution. In addition, local resistivity and SNIM measurements of the electronic IMT in the VO$_2$ film allow us to present a coherent picture of this complex phase transition. 1. M. M. Qazilbash et al., Science 318, 1750 (2007). 2. M. M. Qazilbash et al., Phys. Rev. B 79, 075107 (2009). 3. A. Frenzel et al., Phys. Rev. B 80, 115115 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H38.00010: Electron correlations in the k-dependent electronic structure of metallic V$_{2}$O$_{3}$ O. Krupin, J.D. Denlinger, B.J. Kim, J.W. Allen, P. Metcalf Both the insulator and metal phases of vanadium sesquioxide serve as paradigms of strongly correlated electron physics. The metallic state displays an incoherent lower Hubbard band and a coherent quasiparticle (QP) peak near the Fermi level. Employing angular-resolved photoemission spectroscopy we are making the first studies of the behavior of the QP band in energy-momentum space. Here we report on electron mass renormalization near the Fermi level, and higher energy kink and ``waterfall'' features such as have been reported for different families of superconducting cuprates. Owing to a variety of coexisting interactions the precise origin of these features remains controversial and is presently actively discussed in the literature. Observation of these features in a paradigm system broadens the basis for discussing and assessing various suggested scenarios. [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H38.00011: Investigation of M2 phase in the Metal-insulator transition in vanadium dioxide nanobeams Jiang Wei, Jae Park, David Cobden, Andy Jones, Samuel Berweger, Markus Raschke The role of M2 insulating phase in the metal-insulator transition of vanadium dioxide is still unknown largely due to the reason that the M2 phase can only be stabilized by doping or pressure. We fabricated a special nanostructure based on vanadium dioxide single-crystal nanobeams, where we can tune the system to stay at single M2 phase or move along the phase boundary among different phases (M1, M2 and rutile). We used Raman spectroscopy to observe the interconversion among the three phases. Electrical transport measurement shows that M1 and M2 insulating phases have the same thermal electronic gap, but the resistivity of M2 phase is about twice as much as M1 phase'. At the phase boundary of M2 and rutile metallic phase, the resistivity of M2 insulating phase remains constant, which strongly indicates the Mott like transition (strongly correlated electrons) nature of vanadium dioxide. Supported by Army Research Office and National Science Foundation [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H38.00012: Mapping and Exploration of Extensive Stress-Temperature Phase Diagram of Vanadium Dioxide J. Cao, W. Fan, D.F. Ogletree, K. Chen, N. Tamura, M. Kunz, C. Barrett, J. Seidel, E. Saiz, J. Wu Vanadium dioxide is a prototypical strongly correlated electron material exhibiting a metal-insulator phase transition. This electronic transition is complicated by accompanied structural changes that involve one tetragonal and two monoclinic structures, each favored at different strain states. Full understanding of the driving mechanism of these coupled transitions necessitates concurrent structural and electrical measurements over all phases. These measurements are obstructed in VO$_2$ bulk and thin films, because domain structures developed at the sub-micron scale relax local strain and limit the accessible phase space. Enabled by superior mechanical property of crystalline microbeams, we demonstrate mapping and exploration of the stress-temperature phase diagram of VO$_2$ over a phase space that is more than one order of magnitude broader than previously attained. Electrical and structural properties of all three phases and transitions between them were evaluated. New structural and electronic aspects were observed along the phase transitions in high strain states. This work was supported in part by NSF under Grant No. EEC-0425914, and in part by the Laboratory Directed Research and Development Program of LBNL under the DoE Contract No. DE-AC02-05CH11231. Portions of this work were performed at the Molecular Foundry and at the Advanced Light Source, LBNL. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H38.00013: Search for a structural intermediate phase accompanying the metal-insulator transition of vanadium dioxide thin films Joyeeta Nag, Richard Haglund, Andrew Payzant, Karren More, Stephan Pauli, Phil Willmott Recent studies have shown evidence for the formation of a strongly correlated metal (SCM) phase in the metal-insulator transition (MIT) in vanadium dioxide; the spectral signatures of the SCM are distinct from the rutile metallic phase observed once the MIT is complete around 67C. We describe a search for a corresponding intermediate in the structural phase transition (SPT) from monoclinic to tetragonal, using both high-temperature X-ray diffraction (HTXRD) and synchrotron radiation experiments. The HTXRD experiments, with both epitaxial and highly ordered VO$_{2}$ films on c-cut sapphire, showed that the hysteresis in the VO$_{2}$ dielectric function derived from a Bruggeman effective-medium treatment was not congruent in temperature with that known from optical studies of the MIT. However, there was no evidence for anything other than a mixed tetragonal-monoclinic phase as the temperature was varied from below to above the critical temperature. The synchrotron experiments, carried out with samples prepared by in situ pulsed laser deposition on a-, c- and r-cut titania, on the other hand, do not rule out the existence of an intermediate structural phase, and may provide some support for such a strongly correlated structural phase. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H38.00014: Photo-induced Mott metal-insulator transition and oscillation in VO$_{2}$ Hyun-Tak Kim, Gi-Wan Seo, Bong-Jun Kim, Yong Wook Lee 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. 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 metal (MM) phase is observed. Here we show high frequency electrical oscillation a photo-induced MIT and oscillation controlled by adjusting the illumination of infrared light exposed to the VO$_{2}$ film in a two-terminal device. The photo-induced MIT is controlled by the intensity of the light and an applying voltage. The oscillations occur in the MM phase of VO$_{2}$. The oscillations possibly are generated from a temporal capacitor, which is comprised of both temporary dielectric components, arising from inhomogeneity in a VO$_{2}$ film, and MM phases acting like electrodes. This work concludes that the electrical and the optical oscillations are a characteristic of the Mott MIT. [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H38.00015: Voltage Triggered Hysteretic Switching of VO$_{2}$ at Room Temperature Jeehoon Kim, Changhyun Ko, Alex Frenzel, Shriram Ramanathan, Jennifer E. Hoffman Vanadium oxide (VO$_{2})$ is known to undergo an insulator-to-metal transition near 340K; because of this proximity to room temperature, VO$_{2}$ is a promising candidate material for technological applications such as sensors and memory devices. We use conducting atomic force microscopy to investigate the voltage triggered insulator-to-metal transition in VO$_{2}$ at the nanoscale. We observe hysteretic resistance switching as a function of locally applied electric field, at room temperature. We correlate the hysteresis loop shape with surface morphology. [Preview Abstract] |
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