Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J49: Focus Session: Metal-Metal Bonding: Vanadates and Niobates |
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Sponsoring Units: DMP Chair: Jigang Wang, Ames Lab Room: Mile High Ballroom 1C |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J49.00001: Evolution of correlated electron behavior from the surface to the bulk in Sr$_x$Ca$_{1-x}$VO$_3$ Invited Speaker: Jude Laverock Understanding correlated electron behaviour remains one of the most important, and challenging, problems in modern condensed matter physics. In correlated electron systems, the interaction between electrons is of the order of, or larger than, the electron kinetic energy, and the concept of a well-defined quasiparticle is restricted to a narrow region of energies near the Fermi level, beyond which our strict understanding of a quasiparticle with a defined dispersion relation, easily accessible through band theory, breaks down. In the last few decades, the discovery of unusual and promising behavior in strongly correlated materials has yielded effects as diverse as high temperature superconductivity, colossal magnetoresistance and multiferroics. Indeed, the functionalisation of strongly correlated materials, either in bulk crystalline form or as artificial layered heterostructures, is fast emerging as one of the most promising avenues for future advanced technologies, and key to unlocking the potential of such designed materials is a firm grasp of how electron correlations evolve at surfaces and interfaces. Here, we investigate Sr$_x$Ca$_{1-x}$VO$_3$ as a prototypical example of a strongly correlated material, exhibiting both strong Hubbard subbands and appreciable quasiparticle peaks. Using a variety of ultraviolet, soft, and hard x-ray spectroscopies, we present a detailed depth-sensitive study of the evolution in the effects of electron correlations from the sample surface to its bulk. Our results illustrate the intrinsic enhancement of the effects of electron correlations at the surface, which has important implications for the designed properties at the interface of heterostructures. Strong incoherent subbands are found to lie $\sim$ 20\% closer in energy to the coherent features in the most surface-sensitive measurements, accompanied by a $\sim$ 10\% narrowing in the overall bandwidth. Secondly, we demonstrate that resonant soft x-ray emission spectroscopy is a sensitive probe of correlated electron behavior, capable of providing complementary information to photoemission spectroscopy from a truly bulk perspective. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J49.00002: Gate-tunable gigantic changes in lattice parameters and optical properties in VO$_{2}$ Masaki Nakano, Daisuke Okuyama, Keisuke Shibuya, Naoki Ogawa, Takafumi Hatano, Masashi Kawasaki, Taka-hisa Arima, Yoshihiro Iwasa, Yoshinori Tokura The field-effect transistor provides an electrical switching function of current flowing through a channel surface by external gate voltage (VG). We recently reported that an electric-double-layer transistor (EDLT) based on vanadium dioxide (VO2) enables electrical switching of the metal-insulator phase transition, where the low-temperature insulating state can be completely switched to the metallic state by application of VG [1]. Here we demonstrate that VO2-EDLT enables electrical switching of lattice parameters and optical properties as well as electrical current. We performed in-situ x-ray diffraction and optical transmission spectroscopy measurements, and found that the c-axis length and the infrared transmittance of VO2 can be significantly modulated by more than 1{\%} and 40{\%}, respectively, by application of VG. We emphasize that these distinguished features originate from the electric-field induced bulk phase transition available with VO2-EDLT. \\[4pt] [1] M. Nakano et al., Nature 487, 459 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J49.00003: Evidences of nonthermal optically induced insulator-to-metal switching in VO$_{2}$ Zhensheng Tao, TzongĀRu T. Han, Faran Zhou, Subhendra D. Mahanti, Phillip M. Duxbury, Chong-Yu Ruan, David Torres, Nelson Sepulveda Strongly cooperative structural and electronic phase transitions at near room temperature make VO$_{2}$ a promising material for an array of high-speed applications in electronics and photonics. The critical step that limits the ultrafast performance is the structural barrier, which is result of subtle interplay between the Mott and Peierls physics. Using femtosecond electron crystallography, we examine the sequence of events resulted from this interplay and show that the cooperative behavior induced by optically induced charge doping may provide an alternative pathway for efficient ultrafast switching bypassing the high thermodynamic barrier required in the temperature-driven phase transition. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J49.00004: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J49.00005: Crystal structure change accompanying insulator-metal phase transition in VO$_{2}$ field-effect transistor Daisuke Okuyama, Masaki Nakano, Soshi Takeshita, Samuel Tardif, Hiroyuki Ohsumi, Keisuke Shibuya, Takafumi Hatano, Simpei Ono, Hirokatsu Yumoto, Takahisa Koyama, Haruhiko Ohashi, Masaki Takata, Masashi Kawasaki, Yoshihiro Iwasa, Takahisa Arima, Yoshinori Tokura The insulator-metal transition induced by the carrier accumulation in VO$_{2}$ field-effect transistor (FET) gated by electric double layers of ionic liquid has been extensively studied. To clarify the origin of this transition, we performed simultaneous measurements of in-situ synchrotron x-ray diffraction and resistivity on VO$_{2}$ FET at BL19LXU, SPring-8, Japan. By using micro-beam x-ray, the diffraction only on the carrier-accumulated channel of VO$_{2}$ FET can be measured. By applying a gate voltage, the VO$_{2}$ film becomes metallic. The $c$-lattice length estimated from the peak position of (0 0 2) diffraction on the channel of VO$_{2}$ film shows an increase of 1.4{\%} at 150 K. The $c$-lattice length in the metallic state hardly depends on the temperature, which is consistent with the temperature-independent-metallic resistivity. The changes of $c$-lattice length and resistivity by a gate voltage are reversible. This structural change is quite different with those of thermally-, x-ray-, and pressure-induced metallic phases. The crystal structure with elongated $c$-lattice length is realized only in the metallic state induced by the carrier accumulation. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J49.00006: Low loss millimeter-wave switches based on the Vanadium Dioxide Metal - Insulator - Transition Mark Field, Christopher Hillman, Philip Stupar, Zachary Griffith, Mark Rodwell A new ultra-low-loss and broad band millimeter wave switch technology based on the reversible metal / insulator phase transition of vanadium dioxide has been developed. We report having fabricated series configured, single-pole single-throw (SPST) switches having measured S-parameters from DC to 110 GHz. The on-state insertion loss is 0.2 dB and off-state isolation is 21 dB at 50 GHz. The resulting impedance contrast ratio, \textit{ZOFF / ZON}, is greater than 500:1 at 50 GHz (i.e. cut-off frequency \textit{fc} $\sim$ 40 THz). As a demonstration of the technology's utility, we also present the results of a 2-bit real time delay phase shifter incorporating a pair of VO2 SP4T switches. This switch technology's high impedance contrast ratio combined with its compactness, ease of integration, and low voltage operation make it an enabler of previously unachievable high-performance millimeter wave FPGAs. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J49.00007: Correlation between charges and phonons in the phase transition of VO$_{2}$ Bin Wang, Sokrates Pantelides Interplay among the microscopic degrees of freedom in transition-metal oxides can generate macroscopic quantum phenomena that provide functionality in electronic and photonic devices. Here, we report density functional calculations and molecular dynamics simulations of VO$_{2}$, which undergoes a semiconductor-to-metal phase transition accompanied by a monoclinic-to-rutile structural change at 68 $^{\circ}$ C. We find that the lattice vibration at the critical temperature generates a metallic state in the monoclinic structure, which may explain the observed metallic intermediate phase in experiments. Moreover, we find that the electron/hole doping strongly couples with the lattice vibration causing collapse of one particular phonon mode and stimulating the structural phase transition. Molecular dynamics simulations show a temperature-dependence of the required carrier density for the phonon collapse, that is, at higher temperature, fewer free carriers are required. We show that the abrupt change of the vibration results from the weakening of the V-V bonds induced by the hole doping. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J49.00008: Ru-Ru Dimers in honeycomb-layered Li$_{2}$RuO$_{3}$ Fei-Ting Huang, J.J. Yang, Y.Z. Wang, Y. Horibe, S.-W. Cheong Dark-field transmission electron microscopy and sub-{\AA} aberration-corrected scanning transmission electron microscopy (STEM) have been used to investigate the local structural properties of Li$_{2}$RuO$_{3}$ We found intriguing Ru-Ru dimerization in the Ru honeycomb skeletons associated with the spin-orbital coupling of the 4$d$ electrons below 540 K. Furthermore, we demonstrated that the Ru-Ru dimers can be delicately broken through various antiphase boundaries and chemical doping. Soliton-like walls in the Ru-Ru dimer lattice are unambiguously observed in real space, and are found to order in a periodic manner for particular situations. The correlation between macroscopic physical properties and local structural distortions in the Li$_{2}$RuO$_{3}$ will be discussed in detail. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J49.00009: Electrical field induced Metal-Insulator Transition in NbO$_{2}$ thin films at room temperature Yuhan Wang, Stuart Wolf, Jiwei Lu Highly correlated oxides that exhibit a metal to insulator transition (MIT) are of great interest because of their potential application to high performance switches. NbO$_{2}$ exhibits a MIT at 1081K accompanied by a structural transformation from rutile to a distorted variant, which makes it a potential candidate for the switching applications. By a reactive bias target ion beam deposition (RBTIBD) growth technique, we have obtained crystalline single phase NbO$_{2}$ thin films grown on Al$_{2}$O$_{3}$ (0001), Au/Al$_{2}$O$_{3}$(0001), and Pt/Al$_{2}$O$_{3}$(0001) substrates. AFM, XRD and Raman spectroscopy were used to characterize the morphology and microstructure of the NbO$_{2}$ films. We have observed electrically induced transitions from the insulating to the metallic state with two orders of magnitude change in the resistivity at room temperature. This transition occurred at an electric field between 30-100 kV/cm. We will discuss the possible mechanisms for this induced MIT. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J49.00010: Properties of MBE-grown NbO$_{2}$ thin films Alex Demkov, Andy O'Hara, Agham Posadas Niobium dioxide or NbO$_{2}$ a sister compound of the more celebrated VO$_{2}$, belongs to the class of transition metal oxides that undergo a temperature-driven metal-to-insulator transition. Using density functional theory, we explore the electronic properties of both the high-temperature metallic rutile and the low-temperature insulating distorted rutile phases. We investigate the nature of the transition and predict a large carrier concentration change even at the high transition temperature of 1080 K. We also grew thin NbO$_{2}$ films on LSAT(111) single crystal substrates using molecular beam epitaxy. The films show very good crystallinity with a single out-of-plane orientation by x-ray diffraction, and exhibit a smooth surface with the presence of three epitaxial domains as observed by reflection high energy electron diffraction. The NbO$_{2}$ stoichiometry is confirmed by x-ray photoemission measurements of the Nb 3d core level as well as the valence band. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J49.00011: Cryogenic Infrared Nano-Imaging of the Metal-Insulator Transition in V$_{2}$O$_{3}$ A.S. McLeod, E. Van Heumen, Brian C. Chapler, M.D. Goldflam, M.K. Liu, L. Anderegg, Siming Wang, J.G. Ramirez, S. Guenon, Ivan K. Schuller, D.N. Basov We report on temperature-dependent (18K-300K) near-field infrared imaging of the canonical Mott insulator V$_{2}$O$_{3}$ across its temperature-driven metal-insulator transition. This was accomplished using a home-built s-SNOM (scattering-type scanning near-field optical microscope) affording unprecedented spatial sensitivity ($\approx $20 nm) to surface optical properties with simultaneously acquired AFM topography at \textit{cryogenic temperatures}. Our V$_{2}$O$_{3}$ thin film is found to exhibit extreme nano-scale electronic heterogeneity near the Mott transition (170K) from paramagnetic metal to antiferromagnetic insulator. Through a sequence of near-field infrared images acquired across the transition, we resolve dynamic spatial correlations and competition between electronic phases, offering a direct probe of the metal/insulator fill fraction in strong agreement with macroscopic transport, magnetic susceptibility, and X-ray diffraction measurements of the same film. A statistical and tomographic analysis of our near-field images supports the interpretation of a complex 3-dimensional network of phases propagating across the Mott transition. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J49.00012: Tuning the charge ordering transitions in single nanobeams of Cu-doped vanadium pentoxide G. Sambandamurthy, Sujay Singh, Peter Marley, Sarbajit Banerjee Vanadium oxide bronze phases derived from the intercalation of metal ions (K, Pb, Cu, Ag) within V$_{2}$O$_{5}$ frameworks exhibit remarkable physical properties such as charge ordering, quantum spin phenomena and metal-insulator transitions. The intercalated ions typically alter the electronic and geometric structure of these strongly correlated materials and this opens up avenues to observe interesting phases and to tune transitions between them using a variety of external parameters. In this work, results from electrical transport measurements on single crystalline, individual nanobeams of Cu-doped vanadium pentoxide will be presented. The nanobeams undergo a metal to an insulator transition (MIT), possibly due to charge ordering, below room temperature (T$_{C}$ values depend on Cu doping levels). The charge ordered state can be progressively altered by applying an electric field and/or ionic liquid gating. The role of oxygen migration in the presence of ionic liquid and its effect on the transport characteristics will be discussed. The work is supported by NSF DMR 0847324, 0847169 and Intel Corporation. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J49.00013: Properties of the correlated metal phase induced by electrolyte gating of insulating vanadium dioxide nanobeams Sujay Singh, Gregory Horrocks, Peter Marley, Sarbajit Banerjee, G. Sambandamurthy Vanadium oxide (VO$_{2}$) undergoes a first order metal to insulator transition (MIT) and a structural phase transition (monoclinic insulator to rutile metal) near 340 K. Over the past few years, several attempts are made to trigger the MIT in VO$_{2}$ using ionic liquids (IL). Parkin's group has recently showed that IL gating leads to the creation of oxygen vacancies in VO$_{2}$ and stabilizes the metallic phase. Our goal is to study the electronic properties, changes in the stoichiometry and structure of this metallic phase created by oxygen vacancies. Electrical transport measurements on single crystal nanobeams show that the metallic phase has a higher resistance while IL gating is applied and results from Raman spectroscopy studies on any structural change during IL gating will be presented. The role of substitutional dopants (such as W, Mo) on the creation of oxygen vacancies and subsequent stabilization of metallic phase in IL gated experiments will also be discussed. The work is supported by NSF DMR 0847324 and 0847169. [Preview Abstract] |
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