Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y21: Vanadium Oxides I |
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Sponsoring Units: DCMP Chair: Jiang Wei, Tulane University Room: 201 |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y21.00001: Oxygen Effect and Mechanism of Insulator-to-Metal Transition using Indirect Band in Vanadium Dioxide Tetiana Slusar, Jin Cheol Cho, Ahrum Sohn, Jeongyong Choi, Dong-Wook Kim, Hyun-Tak Kim The vanadium dioxide VO$_{2}$ is known as the Mott insulator and it undergoes the insulator-to-metal transition (IMT) near $T_{c}=$340 K. Under the external influence (doping, strain etc.) the $T_{c}$ is reduced. To explain this behavior, we have used the Mott criterion proposing the critical carrier density $n_{c}$ at MIT (metal to insulator). In this case, Mott derived $n_{c}$ for the transition from metal to insulator. Therefore, $n_{c}$ is regarded as the minimum carrier density in metal phase. However, in the reverse transition (insulator to metal), $n_{c}$ cannot clarify the above behavior. Thus, a new model has been required. Here, we have grown VO$_{2}$ thin films under the different oxygen partial pressure conditions and studied the influence of oxygen deficiency on $T_{c}$ of the IMT. For the analysis, we have measured the Hall voltage near $T_{c}$ and a change of work function. Based on the experimental data we have proposed the model explaining $T_{c}$ changes in VO$_{2}$. This model describes the excitation process of bounded charges in direct and indirect energy bands as a criterion of the IMT. Moreover, it is shown that it can serve as universal mechanism that describes the physics of the IMT in many MIT materials from the new point of view, different from that, suggested by Mott. It is the fundament for operation of new devices. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y21.00002: First principles calculations of insulator-to-metal transition in photoexcited monoclinic VO2 Lede Xian, Matteo Gatti, Pierluigi Cudazzo, Daniel Wegkamp, Marc Herzog, Christina McGahan, Robert Marvel, Richard Haglund, Martin Wolf, Julia Stahler, Angel Rubio The insulator-to-metal phase transition of VO2 has been discovered for a long time, but its origin remains elusive. In recent experiments, ultrafast band gap collapse of monoclinic VO2 upon photo excitation was observed through time resolved photoelectron spectroscopy (TRPES). In order to study this issue, we have performed first principles calculations based on many-body perturbation theory. We show that the band gap in monoclinic phase is extremely sensitive to small changes in the occupation of the localized d bands of V atoms. In particular, the photo-induced hole doping in VO2 can strongly alter the dynamical screening, which then leads to a collapse of the band gap. Our results support the experimental findings and point to the electronic origin of the insulator-to-metal phase transition of monoclinic VO2 in the TRPES experiments. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y21.00003: Extraordinary Inhibition of the Field-effect by Bound Quasiparticles at the Interface of a Dielectric and the Metal-Insulator Transition Material VO$_{2}$ Koen Martens, Jaewoo Jeong, Nagaphani Aetukuri, Charles Rettner, Nikhil Shukla, Eugene Freeman, Davoud Esfahani, Francois Peeters, Teya Topuria, Phil Rice, Alexander Volodin, Benoit Douhard, Wilfried Vandervorst, Mahesh Samant, Suman Datta, Stuart Parkin An electric field applied normal to the interface of a dielectric and the prototypical, strongly-correlated semiconductor VO$_{2}$ is anticipated to lead to non-trivial phenomena. This field-effect allows for key insight into VO$_{2}$ physics. Field-effect modulation of channel current and carrier depletion in a field-effect device are found to be extraordinarily highly inhibited and no Metal-Insulator Transition is induced by the gate field for excess carriers up to 5x10$^{13}$cm$^{-2}$. The field-induced excess charge consists of bound quasi particles, as demonstrated by their activated and low excess carrier field-effect mobility. Small polarons as excess carriers in VO$_{2}$ consistently explain the observed field-effect, mobility and absence of depletion. The physics required to describe semiconducting VO$_{2}$'s$_{\, }$field-effect is fundamentally different from classical semiconductor physics. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y21.00004: Phase inhomogeneity near the electrically driven insulator-metal transition in VO$_2$ nanobeams Sujay Singh, Gregory Horrocks, Peter Marley, Zhenzhong Shi, Sarbajit Banerjee, G. Sambandamurthy Vanadium oxide (VO${_2}$) exhibits an insulator to metal transition (IMT) at $T_C$ $\sim$ 342 K and this transition is amenable to triggering by voltage, light and strain. We present results from transport measurements (both AC and DC) on individual nanobeams of single crystalline VO${_2}$ across the electrically driven transition from the insulating phase. Recent works in correlated electron systems have debated the individual roles of mechanisms such as Joule heating, percolation and avalanche in driving the transition. In our samples, the calculated average temperature of the nanobeams due to Joule heating is less than $T_C$ near the IMT, suggesting that an inhomogeneous phase develops and filamentary conduction paths likely drive the transition. At low bias values, the conduction is dictated by Joule heating and percolation-like events. The occurrence of avalanche-type events at higher bias likely induces the formation of filamentary pathways, thereby precluding further need for percolation. [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y21.00005: An ultrafast nano-infrared study of the photo-induced insulator-to-metal transition in Vanadium Dioxide Aaron Sternbach, Mengkun Liu, Martin Wagner, Ruben Iraheta, Tetiana Slusar, Alfred Leitenstorfer, Hyun-Tak Kim, Richard Averitt, Dimitri Basov We have devised and implemented the technique of time resolved scanning near-field optical microscopy to study the inhomogeneous development of a phase transition in the time domain with 20 nanometer spatial resolution and 100 femtosecond temporal resolution. The subject of our study is Vanadium Dioxide (VO$_{2})$, which is a canonical correlated electron system that exhibits an insulator to metal transition (IMT). We observe an abrupt rise in the photoconductivity at several hundred femtoseconds followed by a slow rise, which takes place on the order of several hundred picoseconds. Our measurement resolves the rise time of the IMT in individual sites, and we further observe inhomogeneous dynamics that are dependent on local strain. Our results pave the way for studying a plethora of systems where phase transitions involve inhomogeneities and phase separation. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y21.00006: Anomaly in the Metal-to-Insulator Transition of V$_{2}$O$_{3}$ Thin Films Under Pressure Ilya Valmianski, Gabriel Ramirez, Siming Wang, Christian Urban, Xavier Battle, Ivan K. Schuller We present results of electrical transport measurements in highly textured V$_{2}$O$_{3}$ thin films of varying thickness under hydrostatic pressure from 100 kPa to 1.6 GPa. All films presented $\sim$ 4 orders of magnitude resistance change at the Metal-to-Insulator Transition (MIT). Morphological and structural characterization was performed using in- and out-of-plane X-ray diffractometry and Atomic Force Microscopy before and after pressurization. We found an anomalous pressure dependence of the MIT for pressures above 500 MPa that deviates from the bulk behavior. Furthermore, we found an irreversible change in the MIT temperature, which coincides with a morphological but not crystal structure change in the film. The obtained anomalous pressure dependence suggests a difference between bulk and thin film MIT mechanisms. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y21.00007: Neutron and x-ray scattering studies of the lattice dynamics in VO2 Olivier Delaire, John D. Budai, Jiawang Hong, Michael Manley, Chen Li, Eliot Specht, Ayman Said, Bogdan Leu, Jon Tischler, Douglas Abernathy, Lynn Boatner, Robert McQueeney Vanadium dioxide exhibits a metal-insulator transition at 340K, concomitant with a structural distortion from rutile to monoclinic on cooling. While much attention has been given to purely electronic aspects of the transition and the band-gap opening, less information has been available about the lattice dynamics (phonons), and their role in the thermodynamics of this important phase transition. We report detailed x-ray and neutron scattering measurements of the phonon dispersions and density of states in VO2, and their influence on the thermodynamics [1]. We show that the entropy of the transition is dominated by the large phonon entropy of the rutile phase, which stabilizes it at high temperature. This phonon entropy arises from soft, strongly-anharmonic phonons across much of the Brillouin zone. The origin of this softness and strong anharmonicity are discussed. [1] J. D. Budai*, J. Hong*, M. E. Manley, E. D. Specht, C. W. Li, J. Z. Tischler, D. L. Abernathy, A. H. Said, B. M. Leu, L. A. Boatner, R. J. McQueeney, and O. Delaire, Nature 2014, DOI:10.1038/nature13865 Support by DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division; APS and SNS facilities supported by DOE-BES. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y21.00008: Toward Nano-Electronics Applications of Metal-Insulator Transition Materials - Atomic Layer Deposition of VO$_{2}$ and a Selector Device Concept Antony Peter, Koen Martens, Iuliana Radu, Nuo Xu, Geert Rampelberg, Christoph Adelmann, Christophe Detavernier, Marc Heyns, Malgorzata Jurczak We discuss advances toward applications of Metal-Insulator Transition (MIT) materials in nano-electronics based on the prototypal MIT material VO$_{2}$. A fabrication friendly method to deposit VO$_{2}$ is required for VO$_{2}$ applications. VO$_{2}$ films deposited by techniques suitable for manufacturing, including Atomic Layer Deposition (ALD), have typically been non-continuous and have shown a strongly degraded MIT when film thickness was below 40-50 nm. We show how the nanoscale morphology of VO$_{2}$ films can be controlled to realize smooth ultrathin (thinner than 10 nm) crystalline films with ALD. We demonstrate that the films possess both a structural and an electronic transition. The film resistivity of ultrathin films changes by more than two orders of magnitude across the MIT. Incipient nanoelectronics based on Metal-Insulator-Transition (MIT) materials currently features promising device concepts that require further development and understanding. A candidate first nanoelectronic application for MIT materials is a selector element, which is used to prevent sneak currents in dense cross bar memory arrays. Making use of simulations we elaborate a device concept for a selector element based on MIT materials such as VO$_{2}$ and SmNiO$_{3}$. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y21.00009: Tungsten-doped vanadium dioxide thin films for THz analog optical applications Gulten Karaoglan-Bebek, Nadim Hoque, Mark Holtz, Zhaoyang Fan, Ayrton Bernussi The Mott transition of vanadium dioxide (VO2) has been widely studied, with abrupt changes in electrical and optical properties at temperature approximately 70 C. The phase transition properties of thin vanadium dioxide films can be changed by doping with tungsten making it a prospective candidate to realize tunable optical devices at terahertz (THz) frequencies. Tungsten incorporation into the vanadium dioxide film yields a wider transition window and a lower transition temperature allowing practical use in analog-like continuous applications. Our results reveal characteristic metal-insulator phase temperature and width of 40$^{\mathrm{\thinspace }}$C and 35 C, respectively, for the film with the highest W content. We show that the refractive index of W-doped vanadium dioxide can be continuously tuned and this provides precise control of the transmission properties of the vanadium dioxide films and discuss future active THz optical devices for analog applications. We demonstrate that W-doped vanadium dioxide films can be also used as anti-reflective coating at THz frequencies but at temperatures much lower than that observed for undoped films. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y21.00010: Distinct substrate effects on the gate-induced metallic states in VO$_{2}$ thin films Masaki Nakano, Daisuke Okuyama, Masaichiro Mizumaki, Hiroyuki Osumi, Masaro Yoshida, Takahisa Arima, Masaki Takata, Masashi Kawasaki, Yoshinori Tokura, Yoshihiro Iwasa The idea of utilizing electric-double layers for controlling electronic phases of condensed matters by external voltages, namely EDLT, has attracted growing attention. Of particular interest is EDLT based on VO2, enabling electrical control of ``bulk'' electronic phases over the electrostatic screening length, as proven by transport, optical, and structural measurements [1-3]. We attributed this unique feature to electrostatic effects, but there are other models proposed from electrochemical viewpoints. Here we show that the reversibility of the device operation strongly depends on the substrates, suggesting a governing mechanism can differ depending on the substrates. We found that EDLT with VO2 films grown on lattice-matched TiO2 substrates show reversible gating effects, whereas those on hexagonal Al2O3 substrates become irreversible, although in both cases metallic states can be induced electrically. X-ray absorption spectra taken before and after the gating experiments also indicated distinct substrate effects on the valence states of vanadium at the gate-induced metallic states. \\[4pt] [1] Nature 487, 459 (2012);\\[0pt] [2] Appl. Phys. Lett. 103, 153503 (2013);\\[0pt] [3] Appl. Phys. Lett.104, 023507 (2014). [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y21.00011: Platform for applying uniform strain to VO2 nanobeams and other small crystals Bosong Sun, Ana Sanchez, Richard Beanland, Tauno Palomaki, David Cobden Many properties of crystals, including symmetry, equilibrium phase, band structure, and Bloch state properties such as Berry curvature and even topology, can be modified by strain. Conversely, controlling strain properly is essential for determining the intrinsic properties of many complex materials. We are therefore exploring ways to apply uniaxial stress to small crystals, ranging from two-dimensional materials to nanowires. For example, we have developed a platform for making samples with controlled built-in stress. In the case of VO2~nanobeams, this allows us to perform systematic studies on each of the several phases (R, M1, M2 and T) involved in the metal-insulator transition in a single sample using multiple microscopies, including transmission electron microscopy. Amongst other things we can image the T phase, study the propagation of interphase boundaries, explore the origin of putative noncentrosymmetricity, and seek diffuse contrast to locate the spinodal lines of the transition. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y21.00012: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y21.00013: Phase Transitions in Electron Beam Deposited Cr-doped VO$_{2}$ Thin Films Dominic Rota, Kent Hallman, Davon Ferrara, Richard Haglund Three phases of the semiconducting state of VO$_{2}$ are known, denoted M$_{1}$, M$_{2}$ and T; the M$_{2}$ phase in particular has alternating vanadium chains arranged in antiferromagnetic pairs. This suggests potentially interesting magnetic and optical properties, motivating our interest in developing a robust protocol for preparing thin films of the M$_{2}$ and T phases for studies of the optically induced semiconductor-to-metal transition (SMT). A protocol for electron beam deposition of Cr-doped VO$_{2}$ (Cr$_{\mathrm{x}}$V$_{\mathrm{1-x}}$O$_{2})$ thin films was developed, allowing for low-cost and efficient fabrication of homogenous films, beginning with powder precursors for vanadia and chromia in appropriate proportions. The films were characterized by resistivity and reflectivity measurements of the SMT with concentrations of Cr dopant ranging from x$=$0 to x$=$0.04. Raman spectroscopy was used to identify the structural phase transitions and revealed that the structural phases M$_{1}$, T, and M$_{2}$ can be determined by comparing differences in the spectrum. Comparing the resistivity hysteresis curves to the Raman spectroscopic measurements, we find that single-phase structures occur for x$=$0 (M$_{1})$ and x$=$0.04 (M$_{2})$ while concentrations between the x$=$0 and x$=$0.04 exhibit a mixture of at least two structural phases, including the T phase. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y21.00014: Ab Initio phonon calculations in metallic and insulating phases of VO$_2$ Christopher Hendriks, Eric Walter, Henry Krakauer Vanadium dioxide (VO$_2$) undergoes a first-order metal-insulator transition (MIT) from the high-temperature rutile phase (R) to an insulating, low-temperature monoclinic phase (M1). Several competing insulating phases exist, with phase boundaries in a narrow temperature and strain range close to the MIT\footnote{J. H. Park, Nature {\bf 500}, 431 (2013).\label{3point}}\footnote{J. M. Atkin et al, PRB {\bf 85}, 020101 (2012).}. Recently, novel IR and Raman measurements of micro- and nano-structured VO$_2$ samples have become increasingly available; this allows the phases to be studied while avoiding many difficulties with bulk samples, such as twinning and cracking on cycling through the MIT. Theoretical calculations of vibrational properties can assist in the interpretation of such experiments\footnote{\label{huffman}T. J. Huffman et al., PRB {\bf 87}, 115121 (2013).}. We will present {\em ab initio} DFT+U calculations of phonon frequencies for the various phases and compare these to reported measurements for the R, M1\footnote{P. Schilbe et al., Mat. Sci. and Eng. A {\bf 370}, 449-452 (2004).}, M2 and T phases\footnote{C. Marini et al., PRB {\bf 77}, 235111 (2008).}. [Preview Abstract] |
Friday, March 6, 2015 10:48AM - 11:00AM |
Y21.00015: Heterogeneous Nucleation and Growth Dynamics in the Light-induced Phase Transition in Vanadium Dioxide Nathaniel Brady, Kannatassen Appavoo, Joyeeta Nag, Min-ah Seo, Rohit Prasankumar, Richard Haglund, David Hilton Vanadium dioxide is a well-known transition metal oxide that undergoes an insulator-to-metal phase transition at $T _c$ = 340 K that is accompanied by a structural distortion from monoclinic [P2$_{1}$/c for $T < T _c$] to rutile [P42/$mnm$ for $T> T _c$]. The heterogeneous nature of this phase transition is evident from the significantly smaller the thermal energy at $T _c$ when compared to the energy barrier for homogenous nucleation [see: Phys. Rev. B 65, 224113 (2002)]. The identity of the relevant defect that locally lowers this barrier enabling this phase transition, however, is currently unclear. In our talk, we will report on ultrafast optical investigations of the light-induced insulator-to-metal phase transition in samples with controlled disorder generated by substrate mismatch. Our results reveal several common features of this optical phase transition that are independent of this disorder and a small variation in threshold fluence needed to drive this phase transition that depends on the sample morphology. [Preview Abstract] |
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