Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session Y06: VO2 and Related Metal Insulator TransitionsFocus
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Sponsoring Units: DCMP Chair: Fernando Reboredo, Oak Ridge National Laboratory Room: BCEC 109A |
Friday, March 8, 2019 11:15AM - 11:27AM |
Y06.00001: The role of Joule heating in the voltage-triggered insulator-to-metal transition in VO2. Alyson Spitzig, Alex J Frenzel, Jeehoon Kim, Changhyun Ko, Shriram Ramanathan, Jennifer Hoffman, Jason D Hoffman VO2 undergoes a conductivity increase of up to five orders of magnitude as the temperature is increased through 341 K. Recently, the insulator-to-metal transition (IMT) has been triggered with an applied voltage, but debate remains as to whether the transition can be due to electric field effects alone, or whether Joule heating is necessary. Here, we use a conductive AFM tip to locally apply a voltage bias and measure the current through a VO2 film. By fitting the IV curves immediately preceding the IMT to the temperature-dependent Poole-Frenkel conduction mechanism, we determine the local temperature of VO2 in the insulating state. We find that the tip has locally warmed the film to 335 K ± 4 K immediately preceding the transition. Therefore, Joule heating plays a significant role in the voltage-triggered IMT in VO2. |
Friday, March 8, 2019 11:27AM - 11:39AM |
Y06.00002: Self-consistent GW study of VO2 Andrey Kutepov The question of the metal-insulator transition (MIT) in VO2 remains open. Hybrid functionals [1] and non-self-consistent G0W0 [2] studies state that non-local exact exchange (EEX, included in hybrids and GW) is the most important. However, the arbitrariness of the percentage of EEX (hybrids) or of the starting point (G0W0) makes the agreement of the results with experiment not convincing. From the LDA+DMFT studies of VO2 [3,4] it follows that strong on-site correlations are important for MIT. However, LDA+DMFT operates with local parameters (U and J) and the argument in favor of strong correlations might be just a consequence of missing the non-local EEX in LDA+DMFT. The talk will be about the role of self-consistency in GW approach, which eliminates dependence on the starting point. It will be shown that indeed the GW level of theory provide all necessary basis for description of MIT in VO2 quantitatively. |
Friday, March 8, 2019 11:39AM - 11:51AM |
Y06.00003: Microstructural Engineering of the UV/Near-UV Photocurrent Production in VO2 Thin Film Based Detectors Jason Creeden, Scott Madaras, Doug Beringer, Melissa R Beebe, Irina Novikova, Rosa Alejandra Lukaszew We sought to optimize the photosensitivity of VO2 thin films in the near-UV (NUV) and UV regions after recent reports demonstrating it is possible to push the typical IR photoresponse of VO2 into the visible spectrum via thin film growth on TiO2:Nb substrates. By controlling the microstructure of the films via deposition parameters and substrate doping, we optimize VO2 growth for TiO2 and TiO2:Nb substrates and compare their photocurrent response using 405 nm (NUV) and 254 nm (UV) light. We found that the VO2 on TiO2:Nb heterostructure demonstrates greater photocurrent response. By measuring the external quantum efficiency (EQE), we found a dramatic photosensitivity improvement for the VO2 on TiO2:Nb compared to undoped TiO2 substrates. Notably, we demonstrated greater than 100% EQE for VO2 on TiO2:Nb for both wavelengths and an improvement in the EQE using UV in comparison to the NUV. Finally, we additionally propose a mechanism for this photoresponse which potentially allows for greater than 100% EQE. |
Friday, March 8, 2019 11:51AM - 12:03PM |
Y06.00004: An infrared investigation of the insulator-to-metal transition in a thin, epitaxially strained VO2 film David Lahneman, Patrick McArdle, Muhammad M Qazilbash, Tetiana Slusar, Hyun-Tak Kim The insulator-to-metal transition temperature of vanadium dioxide (VO2) can be tuned through epitaxial strain induced by lattice mismatch between a thin VO2 film and the substrate. Here we report infrared and optical measurements on a very thin (~ 10 nm) VO2 film on (001) TiO2 substrate with an insulator-to-metal transition temperature of ~ 305 K, just above room temperature. We map the transition as it evolves in temperature using near-field imaging at a wavelength of ~10μm from a mid-infrared laser. Using our tabletop home-built argon plasma light source, we obtain the broadband near-field infrared spectra on the pristine substrate and the film-substrate system. |
Friday, March 8, 2019 12:03PM - 12:15PM |
Y06.00005: Correlation-driven metal-insulator transitions in hydrogenated VO2 Se Young Park, So Yeun Kim, Steffen Backes, Byung Cheol Park, Junwoo Son, Hyojin Yoon, Tae Won Noh, Silke Biermann We investigate the electronic and optical properties of hydrogenated HxVO2. We find doping-dependent metal-insulator transitions as increasing hydrogen content from the optical conductivity measured by spectroscopic ellipsometry, showing a gap opening with a structural transition forming V-V dimer as increasing the H doping. The insulating phase is investigated using density functional and dynamical mean-field theory. We identify the orbital-ordered paramagnetic insulating phase as the ground state of HVO2 in which both the dimer-induced bonding-antibonding splitting and the orbital ordering stabilized by electron correlations play an important role in the metal-insulator transition. We compare the calculated optical conductivity of the metallic and insulating phases with experimental data and discuss the increase in the charge transfer energy with high hydrogen content. |
Friday, March 8, 2019 12:15PM - 12:27PM |
Y06.00006: Insights into the anomalous thermal properties of VO2 from synchrotron spectromicroscopy SUHAS KUMAR, Stephanie Bohaichuk, Lu Chen, Aditya Sood, David A Shapiro, Haidong Zhou, Aaron M Lindenberg, Lu Li, Stanley Williams, Eric Pop Within the last year there have been reports on two distinct anomalous behaviors of the thermal conductivity of VO2 across its insulator-metal transition, namely: (1) a violation of the Wiedemann Franz law and (2) a giant peak in thermal conductivity during the transition. These have not been captured by physical models, and an understanding of their origin requires in situ probing of both the electronic and structural transitions in VO2 with very high spatial and thermal resolution. Here we performed temperature-controlled scanning X-ray absorption spectromicroscopy (SXASM), using synchrotron radiation with <30 nm spatial resolution, on single crystal VO2 that exhibited these anomalies in thermal conductivity. SXASM revealed distinct signatures of the Mott electronic transition and the Peierls structural distortion, which occurred at slightly different temperatures, with the electronic transition occurring first. These results are combined with measurements of thermal conductivity, Seebeck coefficient, and electrical conductivity. We juxtapose these multi-dimensional data sets to shine light on possible correlations between the different transitions and the anomalous changes in VO2 thermal |
Friday, March 8, 2019 12:27PM - 12:39PM |
Y06.00007: Modulating metal-insulator transitions in VOX by tuning oxygen stoichiometry Minhan Lee, Yoav Kalcheim, Javier del Valle, Ivan Schuller Thermally driven metal-insulator transitions (MITs) in vanadium oxides (VOX) have been of special interest in fundamental physics and oxide electronics. The existence of multi-valence states of vanadium leads to a complicated vanadium-oxygen phase diagram. Moreover, for thin films, finite size or strain effects can alter the physical and thermodynamic properties of the V-O system. Therefore, precise oxygen stoichiometry control in these oxide films remains an outstanding issue and a clear phase stability diagram has not yet been found. We demonstrate a gas evolution technique to precisely modify the oxygen stoichiometry in VOX thin films grown on sapphire substrates. By carefully tuning the temperature-oxygen partial pressure-time relationship, controlled phase evolution between VO2, V2O3 and Magnéli phases (VnO2n-1) was achieved, along with the detailed characterization of their electrical transport and structural properties. Moreover, we show that high quality films, with well-defined MITs can be synthesized this way. |
Friday, March 8, 2019 12:39PM - 12:51PM |
Y06.00008: Non-Gaussian resistance noise behavior across the metal-insulator transition in VO2 thin films Ahmed Ali, Dasharath Adhikari, Colin P Kilcoyne, Sambandamurthy Ganapathy Vanadium dioxide (VO2) thin films exhibit a sharp metal-insulator transition (MIT) at a critical temperature ~ 330 K. The significance of a Peierls-type instability or a Mott-Hubbard-type transition near Tc has been a topic of discussion for many years. We present results from a resistance noise spectroscopy study of VO2 thin films that show that the power spectral density (PSD) of the fluctuations near Tc show a marked deviation from the metallic and insulating phases. The PSD increases by orders of magnitude and deviates from a typical 1/f behavior near Tc. The probability density function (PDF) of the fluctuation is non-Gaussian in nature in thin films near Tc whereas the fluctuations are Gaussian at all temperatures in single crystal nanobeams. Our results suggest that the transition likely occurs as a single domain phenomenon in nanobeams whereas the nucleation and propagation of multiple domains of opposite phase are significant near Tc in thin films. The influence of the coexistence of phases on electrical transport will be discussed based on our analysis of the 1/f behavior, PSD, PDF and the second spectrum of noise. |
Friday, March 8, 2019 12:51PM - 1:03PM |
Y06.00009: Resonant Hard X-ray Emission Spectroscopy of VO2 Thin Films in Metallic and Insulating Phases Sung Soo Ha, Seokjun Choi, Mohd Faiyaz, Suyong Lee, Do Young Noh Resonant X-ray emission spectroscopy (XES) was carried out to study the metal-insulator transition (MIT) of vanadium dioxide. The Vanadium K-beta emission spectra including kβ2,5 and kβ1,3 both in the metallic and insulating phase were monitored with an x-ray energy resolution of about 1.6 eV at the 3rd generation Pohang Accelerator Laboratory using a highly oriented pyrolytic graphite (HOPG) spectrometer in von-Hamos geometry. The K-beta emission intensities were monitored as the excitation x-ray energy was scanned through the Vandium K-edge to obtain the X-ray absorption near edge structure (XANES) profile. In both the emission spectra and the XANES profile, the difference between the insulating and metallic phase was clearly observed. We also observed distinct resonant behavior in the elastic/inelastic scattering intensity. The physical interpretation of the resonant behavior will be discussed in relation with the energy band structure of the insulating and metallic phase of VO2. |
Friday, March 8, 2019 1:03PM - 1:15PM |
Y06.00010: Resistance fluctuation of V2O3 films near the metal-insulator transition Liyang Chen, Panpan Zhou, Yoav Kalcheim, Ivan Schuller, Douglas Natelson The metal-insulator transition in the correlated material V2O3 is a highly studied phenomenon. Recent research has shown the nanotextured phase coexistence around the transition temperature (Tc) in V2O3 films, but the dynamic electronic properties are still not well resolved. Here, we study resistance fluctuations as a function of time in both low frequency (< 300 kHz) and higher frequency (225MHz-580MHz) ranges, comparing them to show the dynamic properties at both low and high frequency. We performed measurements at temperatures above Tc, close to Tc, and below Tc. We found 1/f type noise close to and below (yet near) Tc as expected at low frequencies, and similar resistance fluctuation noise (mean square voltage fluctuations proportional to the square of the bias current) at higher frequency. However, the integrated noise intensity at high frequency exceeds our expectation based on an extrapolation of the observed low frequency data. More detailed measurements and experiments are ongoing to reveal the mechanism that causes the resistance fluctuations in V2O3 films near the metal-insulator transition. |
Friday, March 8, 2019 1:15PM - 1:27PM |
Y06.00011: The role of defects on the metal-insulator transition in V2O3 and VO2 Darshana Wickramaratne, Noam Bernstein, Igor Mazin V2O3 and VO2, are prototypical strongly-correlated materials that undergo a metal-insulator transition (MIT). The MIT in both materials leads to a concomitant structural phase transition and a magnetic phase transition in V2O3. Recent experiments [PRB 91, 205123, 2015] explored the sensitivity of the MIT to defects and demonstrated the MIT phenomenon in V2O3 is sensitive to the presence of defects while it is robust in VO2. To explore the underlying mechanisms for this behavior we performed first-principles calculations to assess the role of defects on the structural, magnetic and electronic properties of V2O3 and VO2. In V2O3 we find defects disrupt the antiferromagnetic (AFM) order in the insulating phase. We also find small polarons form through self-trapping or due to defects. We assess the impact of this phenomenon on the spin-flip energy between the metallic paramagnetic and the insulating AFM state with the goal of understanding how this impacts the MIT temperature. In VO2, we explore the impact of defects on the vanadium dimerization in the insulating phase and the impact on the MIT. |
Friday, March 8, 2019 1:27PM - 1:39PM |
Y06.00012: NMR and bulk studies on S=1/2 spin chain Bi6V3O16 Tanmoy Chakrabarty, Ivo Heinmaa, Valeriy Yu. Verchenko, P L Paulose, Raivo Stern We report the NMR and bulk properties of a vanadium based S=1/2 spin chain compound Bi6V3O16 in its low temperature phase where the magnetic V4+ ions are arranged in a one-dimensional chain mediated by V5+ and oxygen ions. We have carried out magnetic and heat capacity measurements, both static and magic angle spinning (MAS) 51V NMR measurements and analysis for unaligned powder sample. The magnetic susceptibility χ shows a broad maximum around 50 K signifying a short-range magnetic order which matches well with the S=1/2 Heisenberg chain model. In the NMR measurements, we only detected the non-magnetic 51V. The spin susceptibility calculated from the shift of the static 51V NMR spectra reproduces well the behavior observed in χ. The MAS NMR experiments reproduce the trend observed in χ and static NMR with unprecedented accuracy, and confirm that there is only one site the V4+ ions are occupying. We have also performed and analyzed spin-lattice relaxation measurements of 51V. |
Friday, March 8, 2019 1:39PM - 1:51PM |
Y06.00013: Ultrafast response of thin film vanadium dioxide grown on titanium dioxide doped with niobium Scott Madaras, Jason Creeden, Doug Beringer, Irina Novikova, Rosa Alejandra Lukaszew We have studied vanadium dioxide (VO2) films grown on titanium dioxide (TiO2) substrates to investigate the properties of the heterojunction that forms at the interface between substrate and film with the purpose of applying it as an UV photodetector. The use of niobium as dopant on TiO2 substrates has been shown to favorably modify the energy levels at the heterojunction thus promoting photocurrent generation when illuminated with UV light. To further investigate this electronic structure modifications we study the ultrafast dynamics of the insulator-metal-transition (IMT) in such samples by using a pump probe configuration. The samples are pumped with ~150 fs pulses of 400nm wavelength light, and the changes in electronic structure of the heterojunction region are detected via change in relative optical reflectance (ΔR/R) of a 800nm probe light. The VO2 on TiO2:Nb doped samples generate distinctive ΔR/R effects compared with the VO2 films deposited on plain TiO2 substrates samples that are undoped. |
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