Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P27: VO2: Experiment and Theory |
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Sponsoring Units: DCMP Chair: Mengkun Liu, State University of New York, Stonybrook Room: 326 |
Wednesday, March 16, 2016 2:30PM - 2:42PM |
P27.00001: Ultrafast Pump-Probe Studies of the Light-Induced MIT and Recovery of Niobium Dioxide Thin Films Melissa Beebe, J. Michael Klopf, Salinporn Kittiwatanakul, Jiwei Lu, Stuart A. Wolf, R. Alejandra Lukaszew Niobium dioxide (NbO$_{\mathrm{2}})$ is a highly correlated binary oxide that, like vanadium dioxide (VO$_{\mathrm{2}})$, exhibits a first-order insulator-to-metal transition (IMT) at a material-dependent critical temperature, accompanied by a structural transformation from monoclinic to rutile. The nature of the IMT in VO$_{\mathrm{2}}$ has been discussed at length, while fewer studies have been carried out on NbO$_{\mathrm{2}}$. Previous studies show that the IMT can also be optically induced in VO$_{\mathrm{2}}$ on a sub-picosecond timescale; here, we present the first ultrafast pump-probe studies showing this optically-induced transition in NbO$_{\mathrm{2}}$ thin films and compare these results to similar ones carried out on VO$_{\mathrm{2}}$ thin films. [Preview Abstract] |
Wednesday, March 16, 2016 2:42PM - 2:54PM |
P27.00002: OPTICAL SPECTROSCOPY OF THE M$_{\mathrm{2}}$ AND T PHASES OF VANADIUM DIOXIDE T.J. Huffman, M.M. Qazilbash, C. Hendriks, E.J. Walter, H. Krakauer, Joonseok Yoon, Honglyoul Ju, R. Smith, G.L. Carr The salient feature of the familiar structural transition that accompanies the metal-insulator transition in bulk VO$_{\mathrm{2}}$ is a pairing of all of the vanadium ions in the M$_{\mathrm{1}}$ insulating phase. This pairing has long been thought critical to the emergence of insulating behavior. However, there exist two less familiar insulating states, M$_{\mathrm{2}}$ and T. These phases notably exhibit distinctly different V-V pairing. In the M$_{\mathrm{2}}$ phase, only half of the vanadium ions exhibit pairing while the other half carry local spin 1/2 magnetic moments and are equally spaced in quasi-one dimensional chains. The T phase has two types of inequivalent vanadium chains, each consisting of V-V pairs but with different spacing between V ions in the pairs. The M$_{\mathrm{1}}$ phase has been studied extensively with optical spectroscopy. By studying the two less familiar insulating phases, M$_{\mathrm{2}}$ and T, one can investigate how changes in V-V pairing affect the properties of the VO$_{\mathrm{2}}$ insulating state. We performed infrared and optical spectroscopy on the M$_{\mathrm{2}}$ and T phases in the same sample. Despite a clear change in the lattice structure, the inter-band transitions are insensitive to changes in the V-V pairing. This result conclusively establishes that intra-atomic Coulomb repulsion between electrons provides the dominant contribution to the energy gap in all insulating phases of VO$_{\mathrm{2}}$. Our work highlights the necessity of considering the M$_{\mathrm{2}}$ and T phases of VO$_{\mathrm{2}}$ in future experimental and theoretical research. [Preview Abstract] |
Wednesday, March 16, 2016 2:54PM - 3:06PM |
P27.00003: Photoinduced phase transitions in narrow-gap Mott insulators: the case of VO$_2$ Zhuoran He, Andrew Millis The nonequilibrium dynamics of strongly correlated electrons in photoexcited VO$_2$ is studied using the quantum Boltzmann equation and nonequilibrium Hartree-Fock methods applied to a band structure given by extended density functional theory (DFT+$U$+$V$) and realistic dynamical interactions. The initial equilibration of electrons occurs in hundreds of femtoseconds. For physically reasonable parameters, our Hartree-Fock calculation sustains a new metastable $M_1$ metal phase that is qualitatively consistent with the recent experiment of Morrison et al [1]. The long-term stability of the $M_1$ metal phase will also be considered. [1] V. R. Morrison, R. P. Chatelain, K. L. Tiwari, A. Hendaoui, A. Bruh\'acs, M. Chaker, and B. J. Siwick, Science 346, 445 (2014). [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P27.00004: Random Field Driven Spatial Complexity at the Mott Transition in VO$_{\mathrm{2}}$ Erica Carlson, Shuo Liu, Benjamin Phillabaum, Karin Dahmen, Narsimhamurthy Vidhyadhiraja, Mumtaz Qazilbash, Dimitri Basov We report the first application of critical cluster techniques to the Mott metal-insulator transition in vanadium dioxide. We show that the geometric properties of the metallic and insulating puddles observed by scanning near-field infrared microscopy are consistent with the system passing near criticality of the random field Ising model as temperature is varied. The resulting large barriers to equilibrium may be the source of the unusually robust hysteresis phenomena associated with the metal-insulator transition in this system. [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P27.00005: Field-Effect Modulation of Ambipolar Doping and Domain Wall Band Alignment in P-type Vanadium Dioxide Nanowires Yasen Hou, Xingyue Peng, Yiming Yang, Dong Yu The sub-picosecond metal-insulator phase transition in vanadium dioxide (VO$_{2})$ has attracted extensive attention with potential applications in ultrafast Mott transistors. However, the development of VO$_{2}$-based transistors lags behind, owing to the lack of an efficient and hysteresis-free electrostatic doping control. Here we report the first synthesis of p-type single crystalline VO$_{2\, }$nanowires via catalyst-free chemical vapor deposition. The p-type doping was unambiguously confirmed by both solid and electrochemical gating methods, and further evidenced by the scanning photocurrent microscopic measurements. Interestingly, we observed that the photocurrent spot polarity at the metal-insulator domain walls was reversibly switched by electrochemical gating, which indicates a band bending flipping. Furthermore, we eliminated the common hysteresis in gate sweep and greatly shortened the transistor response time via a hybrid gating method, which combines the merits of liquid ionic and solid gating. The capability of efficient field effect modulation of ambipolar conduction and band alignment offers new opportunities on understanding the phase transition mechanism and enables novel electronic applications based on VO$_{2}$. [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P27.00006: Electric Field-induced Resistance Switching in VO$_{2}$ Channels using Hybrid Gate Dielectric of High-$k$ Ta$_{2}$O$_{5}$/Organic material Parylene-C. Tingting Wei, Teruo Kanki, Kohei Fujiwara, Masashi Chikanari, Hidekazu Tanaka Electrostatic approach utilizing field-effect transistor (FET) with correlated electron materials provides an avenue to realize the novel devices (Mott-transistor) and to clarify condensed matter physics. In this study, we have prepared Mott-transistors using vanadium dioxide (VO$_{2})$ channels and employed hybrid gate dielectric consisted of high-$k$ material Ta$_{2}$O$_{5\, }$and organic polymer parylene-C to trigger carrier transport modulation in VO$_{2}$. Obvious resistance modulations were observed in insulating regime through time-dependent resistance measurement at varied square-shaped gate bias ($V_{G})$. Contrasting to the hysteretic response in electric double layer transistor (EDLT), an abrupt resistance switching in less than of 2-second-interval enables us to attribute such immediate modulation to pure electrostatic effect. Moreover, the maximum of resistance change was identified to appear around phase transition temperature (T$_{MI})$, which confirmed the disordered heterogeneous regime at T$_{MI}$. Taking advantage of systematic modulation using VO$_{2}$-based devices, we demonstrated the pronounced shifts of T$_{MI}$ by gate bias. Another fascinating behavior on asymmetric drop in T$_{MI}$ by hole-electron carrier doping was observed. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P27.00007: Resistance modulation in VO$_{\mathrm{2}}$ nanowires induced by an electric field \textit{via} air-gap gates Teruo Kanki, Masashi Chikanari, Tingting Wei, Hidekazu Tanaka Vanadium dioxide (VO$_{\mathrm{2}})$ shows huge resistance change with metal-insulator transition (MIT) at around room temperature. Controlling of the MIT by applying an electric field is a topical ongoing research toward the realization of Mott transistor. In this study, we have successfully switched channel resistance of VO$_{\mathrm{2}}$ nano-wire channels by a pure electrostatic field effect using a side-gate-type field-effect transistor (SG-FET) \textit{via }air gap and found that single crystalline VO$_{\mathrm{2}}$ nanowires and the channels with narrower width enhance transport modulation rate. The rate of change in resistance ( (R$_{\mathrm{0}}$-R)/R, where R$_{\mathrm{0}}$ and R is the resistance of VO$_{\mathrm{2}}$ channel with off state and on state gate voltage (V$_{\mathrm{G}})$, respectively) was 0.42 {\%} at V$_{\mathrm{G}}=$30 V in in-plane poly-crystalline VO$_{\mathrm{2}}$ channels on Al$_{\mathrm{2}}$O$_{\mathrm{3}}$(0001) substrates, while the rate in single crystalline channels on TiO$_{\mathrm{2}}$ (001) substrates was 3.84 {\%}, which was 9 times higher than that using the poly-crystalline channels. With reducing wire width from 3000 nm to 400 nm of VO$_{\mathrm{2}}$ on TiO$_{\mathrm{2}}$ (001) substrate, furthermore, resistance modulation ratio enhanced from 0.67 {\%} to 3.84 {\%}. This change can not be explained by a simple free-electron model. In this presentation, we will compare the electronic properties between in-plane polycrystalline VO$_{\mathrm{2}}$ on Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ (0001) and single crystalline VO$_{\mathrm{2}}$ on TiO$_{\mathrm{2}}$ (001) substrates, and show experimental data in detail.. [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P27.00008: Resistance noise spectroscopy across the thermally and electrically driven metal-insulator transitions in VO$_2$ nanobeams Ali Alsaqqa, Colin Kilcoyne, Sujay Singh, Gregory Horrocks, Peter Marley, Sarbajit Banerjee, G. Sambandamurthy Vanadium dioxide (VO$_2$) is a strongly correlated material that exhibits a sharp thermally driven metal-insulator transition at $T_c\sim$ 340 K. The transition can also be triggered by a DC voltage in the insulating phase with a threshold ($V_{th}$) behavior. The mechanisms behind these transitions are hotly discussed and resistance noise spectroscopy is a suitable tool to delineate different transport mechanisms in correlated systems. We present results from a systematic study of the low frequency (1 mHz $<$ f $<$ 10 Hz) noise behavior in VO$_2$ nanobeams across the thermally and electrically driven transitions. In the thermal transition, the power spectral density (PSD) of the resistance noise is unchanged as we approach $T_c$ from 300 K and an abrupt drop in the magnitude is seen above $T_c$ and it remains unchanged till 400 K. However, the noise behavior in the electrically driven case is distinctly different: as the voltage is ramped from zero, the PSD gradually increases by an order of magnitude before reaching $V_{th}$ and an abrupt increase is seen at $V_{th}$. The noise magnitude decreases above $V_{th}$, approaching the $V=0$ value. The individual roles of percolation, Joule heating and signatures of correlated behavior will be discussed. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P27.00009: Strain-dependent ultrafast dynamics of insulator-to-metal phase transition in VO2 Sergiy Lysenko, Armando Rua, Jose Figueroa, Felix Fernandez Much attention has been devoted recently to visualize and understand the strain effects in phase transition dynamics of vanadium oxide materials. In this study, using femtosecond angle-resolved light scattering technique we show strong influence of internal misfit strain in epitaxial VO2(M1) films on insulator-to-metal phase transition within less than 1 ps. Anisotropic strain in twinned domains and in domains of different size results mostly in antiphase oscillatory dynamics of coherent phonons. Depending on domain pattern and type of the substrate, this dynamics was found to be dependent on azimuthal angle and/or on spatial frequency of surface roughness. The origin of observed photoinduced antiphase oscillations is associated with compressive and tensile strain in VO2 domains which alters the initial phase of the oscillations. In contrast to pure VO2(M1), the Cr-doped VO2(M2) shows strong phonon scattering signatures with noticeable random component in the phase of coherent phonons. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P27.00010: Straining to observe the M2 phase in epitaxial VO$_2$ films Nicholas Quackenbush, Matthew Wahila, Louis Piper, Hanjong Paik, Megan Holtz, Xin Huang, Joel Brock, David Muller, Darrell Schlom, Joseph Woicik, Dario Arena It has been more than a decade since it was shown that the transition temperature T$_{MIT}$ of VO$_2$ in epitaxial thin films can be tuned by compressive and tensile strain along the rutile c-axis. Since this discovery, uniaxial strain studies of VO$_2$ nanobeams have demonstrated that compressive strain indeed lowers T$_{MIT}$, thus stabilizing the metallic rutile phase. However, even minor tensile strain induces an intermediate insulating monoclinic M2 phase. Whether this phase can be stabilized in thin films remains contentious owing to the constraints of sample and/or interface quality. Here, we present hard x-ray photoelectron spectroscopy and temperature-dependent soft x-ray absorption spectroscopy of high quality ultrathin epitaxial VO$_2$ films on TiO$_2$ (001) and (100) substrates. The VO$_2$/TiO$_2$(001) are absent of intermediate phases and maintain a MIT similar to unstrained VO$_2$, while the VO$_2$/TiO$_2$(100) films display a stable M2 phase between the M1 and rutile endpoint phases. We discuss our findings in terms of differences between uniaxial and biaxial strain. [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P27.00011: Metallic bubbles nucleation and growth in VO2 nanofilms: insights from TDDFT+DMFT Volodymyr Turkowski, Jose Mario Galicia-Hernandez, Gregorio Hernandez-Cocoletzi, Talat S. Rahman We apply a time-dependent density-functional theory + dynamical mean-field theory (TDDFT+DMFT) approach to model the response of insulating nanofilms of VO2 to perturbations by ultrafast laser pulses. We focus on the spatially-resolved metallization of the systems, and especially on the process of nucleation and time-dependence of the size of the "surface" and "bulk" metallic domains (bubbles) as a function of film width. In particular, we find that the initial universal (parameter-independent) growth of the domains (radius $R\sim t^{1/2}$), changes by the bubbles shrinking ($R\sim t^{-a}$, $a\sim 1$) as a result of Coulomb scattering effects, and eventually by post-femto-second phonon-involved relaxation of the systems to the equilibrium accompanied by infrared photoemission. The time-dependent conductivity obtained from the above results is in a good agreement with available experimental data.[1,2] [1] D.J. Hilton et al., PRL 99, 226401 (2007); [2] T.L. Cocker et al., PRB 85, 155120 (2012). [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P27.00012: Observation of impact ionization in vanadium dioxide Joshua Holleman, Michael Bishop, Carlos Garcia, Christianne Beekman, Shinbuhm Lee, Ho Nyung Lee, Efstratios Manousakis, Stephen McGill Pump-probe optical spectroscopy was used to investigate the possibility of charge carrier multiplication by impact ionization in a 100~nm film of VO$_{\mathrm{2}}$ in the M$_{\mathrm{1}}$ insulating phase. The film was excited by pump pulses with energies above and below twice the band gap energy and observed with two different probe wavelengths. The transient reflectivities of the film were then compared. We observed an enhancement of the reflectivity for the higher energy pump pulses near zero delay compared to the reflectivity for the lower energy pump pulses for both probe wavelengths. Additionally, we identified and described multiple timescales within the charge dynamics. This experiment demonstrated that impact ionization acts as a carrier multiplication process in this prototypical strongly-correlated system. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P27.00013: Visualization of local phase transition behaviors in ultrathin VO$_{2}$/TiO$_{2}$ thin films Ahrum Sohn, Terou Kanki, Hidekazu Tanaka, Dong-Wook Kim VO$_{2}$ undergoes the first order phase transition and two electronic phases can coexist near the critical temperature. We investigated evolution of the surface work function maps of epitaxial VO$_{2}$/TiO$_{2}$ thin films (thickness: 15, 30, and 45 nm) using Kelvin probe force microscopy (KPFM) measurements in the temperature range of 285-330 K. Fully strained thin films were almost free of grain boundaries and thicker films had dislocations caused by strain relaxation. The sample's work function decreases, while spanning the metal-insulator transition (MIT). The work function maps clearly revealed coexistence of the two distinct phase domains. The surface area fraction of the insulating phase near the dislocations was higher than that in other regions. Thicker films have complicated domain patterns; hence, the three-dimensional percolation model properly described the MIT behaviors. In contrast, the two-dimensional percolation model well explained the transition behaviors of uniformly strained thinner films. [Preview Abstract] |
Wednesday, March 16, 2016 5:06PM - 5:18PM |
P27.00014: A Microscopic Model for the Strongly Coupled Electron-Ion System in VO$_2$ Timothy Lovorn, Sanjoy Sarker The metal-insulator transition (MIT) in vanadium dioxide (VO$_2$) near 340 K is accompanied by a structural transition, suggesting strong coupling between electronic and lattice degrees of freedom [1]. To help elucidate this relationship, we construct and analyze a microscopic model in which electrons, described by a tight-binding Hamiltonian, are dynamically coupled to Ising-like ionic degrees of freedom. A mean-field decoupling leads to an interacting two-component (pseudo) spin-1 Ising model describing the ions. An analysis of the minimal ionic model reproduces the observed M1 and M2 dimerized phases and rutile metal phase, occurring in the observed order with increasing temperature. All three transitions are first order, as observed. We further find that both dimerization and correlations play crucial roles in describing the insulating M1 phase. We discuss why dynamical coupling of electrons and ions is key to obtain a full understanding of the phenomenology of VO$_2$, particularly in the context of the phase coexistence [2] observed near the MIT.\\[4pt] [1] D. Paquet and P. Leroux-Hugon, Phys. Rev. B {\bf 22}, 5284 (1980).\\[0pt] [2] M. M. Qazilbash et al., Science {\bf 318}, 1750 (2007). [Preview Abstract] |
Wednesday, March 16, 2016 5:18PM - 5:30PM |
P27.00015: Controlling Oxygen Vacancy Creation In Ionic Liquid Gated Vanadate Nanostructures Colin Kilcoyne, Sujay Singh, Gregory Horrocks, Peter Marley, Sarbajit Banerjee, G. Sambandamurthy Vanadium dioxide (VO$_{2}$) is a correlated material with a transition from a monoclinic insulator to a rutile metal at $\sim$ 340 K. Through ionic liquid gating, oxygen vacancies can be electrochemically induced in VO$_{2}$ and it is found that the vacancies formation is greatly facilitated in the rutile phase, leading to the suppression of the metal-insulator transition. The reversibility, the rate and kinetics of the electrochemical reaction can be readily controlled with the gate voltage sweeps suggesting a potential defect engineering route to tune the electrical and structural properties of VO$_{2}$. Vanadium pentoxide (V$_{2}$O$_{5}$) is a related system with diverse structural and electronic phases that can be obtained by intercalation of various cations. The electrochemical role of ionic liquid gating in creating new phases and modulating conductance in exfoliated thin flakes of V$_{2}$O$_{5}$ will also be presented. This work is supported by NSF DMR 0847324. [Preview Abstract] |
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