Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C45: Metal Insulator Transitions: VO2, Vanadates, and Nickelates |
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Sponsoring Units: DCMP Chair: Ivan Schuller, Univ of California - San Diego Room: LACC 505 |
Monday, March 5, 2018 2:30PM - 2:42PM |
C45.00001: Strain Effects on the Structural and Metal-Insulator Transition in Vanadium Oxides Yoav Kalcheim, Nikita Butakov, Minhan Lee, Javier Del Valle Granda, Juan Trastoy, Jon Schuller, Ivan Schuller The concomitance of the structural phase transition (SPT) and metal-insulator transition (MIT) in vanadium oxides (VO2 and V2O3) is currently under debate following suggestions that these transitions may occur at different temperatures. This has important implications for the driving mechanism of the transition. To shed light on this, we correlated the MIT and SPT of vanadium oxide thin films by means of temperature dependent X-ray diffractometry and infrared reflectivity, while using the film resistance as an internal indicator of the state of the system. We measured films of varying thicknesses, grown on different crystallographic orientations of the substrate to study how strain affects the structural and electronic transitions. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C45.00002: Electrochemically Induced Insulator-Metal-Insulator Transformations of Vanadium Dioxide Nanocrystal Films Clayton Dahlman, Gabriel LeBlanc, Amy Bergerud, Lauren Gilbert, Delia Milliron Vanadium dioxide (VO2) undergoes a metal-insulator transformation (MIT) around 68C. Alternative stimuli besides temperature have been explored to trigger the MIT, including electrochemical gating. This effect depends on film geometry, orientation and strain, and recent efforts have studied the gating transformation in epitaxial VO2 thin films. A heterostructure of VO2 has been prepared through solution deposition of colloidal V2O3 nanocrystals into a mesoporous thin film, followed by oxidative annealing. The electrochromic behavior of nanocrystalline VO2 films is explored with in situ optical transmission measurements in a temperature-controlled electrochemical cell. As expected from prior gating experiments, electrochemical reduction causes a reversible transition from the insulating monoclinic state to a reduced IR darkened state with higher conductivity and minor structural distortions. However, an unexpected transition from this darkened state to a separate insulating phase occurs upon further reduction, and the structure of this phase is explored. This sequential insulator–metal–insulator transition has not been reported in previous studies of gated epitaxial VO2 films, and demonstrates a unique functionality attributable to the film’s nanocrystalline mesostructure. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C45.00003: The metal-to-insulator transition in La1-xSrxVO3 thin films Joseph Roth, Matthew Brahlek, Lei Zhang, Roman Engel-Herbert Changing the composition x of the quaternary compound La1-xSrxVO3 allows continuous tuning of the electronic properties between the Mott insulator LaVO3, exhibiting a coupled spin and orbital ordered state below 140 K, and the paramagnetic correlated metal SrVO3. The ability to grow La1-xSrxVO3 in a self-regulated manner by hybrid molecular beam epitaxy irrespective of composition x enables an excellent control over the A-to-B site cation ratio, therefore minimizing the amount of disorder in these films[1]. We discuss the temperature dependent magnetotransport phenomena as a function of composition x with focus on the effects near the quantum critical point obtained at x~0.25. These trends are then discussed in contrast to the results obtained from films with same composition grown under nonstoichiometric conditions to elucidate the interplay between disorder effects and electron correlation in the vicinity of a quantum critical point. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C45.00004: Predicting the temperature-strain phase diagram in VO2 Chanul Kim, Chris Marianetti Predicting the phase diagram of VO$_2$, including the various structural allotopes, from first principles is a grand challenge of materials physics. The coexistence of Peierls and Mott physics suggests that a theory which can capture strong electronic correlations will be necessary to compute the total energies. Here we perform a detailed analysis of the structural energetics, and the electronic structure, of rutile and M$_1$ VO$_2$ using density functional theory (DFT) and DFT+U calculations. We demonstrate that there are qualitative failures in the structural energetics in both methods. In order to understand the nature of the failure, we build a minimal model of the structural energetics using the Peierls-Hubbard model and exactly solve it using DMRG (Density Matrix Renormalization Group); demonstrating that the on-site U has a minimal effect on the structural energetics for physical parameters. These results explain the partial success of the unorthodoxed non-spin-polarized (NSP) DFT+U results, and guide the creation of empirical corrections to the DFT+U functional. Our modified DFT+U functional is then used to predict the temperature-strain phase diagram for the rutile and M$_1$ phases. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C45.00005: Momentum distribution and Compton profile across the metal-insulator transition in vanadium dioxide Ilkka Kylanpaa, Olle Heinonen, Paul Kent, Jaron Krogel Unstrained vanadium dioxide exhibits a temperature driven metal to insulator transition (MIT) at about 340 Kelvin at ambient pressure. This strongly correlated material with a structural phase transition is studied by means of diffusion Monte Carlo (DMC) in order to accurately capture the correlation effects. We calculate experimentally observable features across the MIT such as the momentum distribution and Compton profile. The main focus is on the differences in the momentum distributions and Compton profiles between the low temperature (monoclinic M1) and high temperature (rutile) phases. We compare DMC results with those from density functional theory to assess the role of electronic correlations. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C45.00006: Time-Resolved Near-Field investigation of the Insulator to Metal transition in Vanadium Dioxide Aaron Sternbach, Peter Kissin, Tetiana Slusar, Jacob Schalch, Guangwu Duan, Kyle Lewis, Fritz Keilmann, Xin Zhang, Hyun-Tak Kim, Richard Averitt, Dimitri Basov We have preformed femtosecond time-resolved and nanometer spatially resolved measurements of the insulator-to-metal transition in Vanadium Dioxide (VO2). In order to make this work possible, we have devised and implemented a method for artifact-free nano-imaging with pulsed laser sources [1]. We observe that the transient metallic state is highly inhomogeneous. Following an ultrafast pumping event an increase in near-field signal occurs, where no significant inhomogeneity is observed for approximately fifteen picoseconds. This is followed by a second stage where significant growth of the photo-induced insulator-to-metal transition is observed to evolve inhomogeniously in real space over hundreds of picoseconds. Finally, the growth saturates after several hundred picoseconds when the photo-induced metallic phase occupies the bulk of the material. Our advances pave a pathway to study a wide range of systems with nanoscopic spatial, and ultrafast temporal resolution. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C45.00007: Magnetoresistive evidence for the origin of the metal-insulator transition in V2O3 Juan Trastoy, Alberto Camjayi, Yoav Kalcheim, Javier Del Valle Granda, Jean-Paul Crocombette, Javier Villegas, Marcelo Rozenberg, Dafiné Ravelosona, Ivan Schuller V2O3 is a strongly correlated oxide that displays simultaneously an electronic, a structural and a magnetic phase transition. The origin of these transitions has been one of the outstanding problems in condensed-matter physics. We investigated the metal-insulator transition (MIT) using magneto-transport measurements of He-irradiated V2O3 thin films. The irradiation provides the means to shift the metal-insulator transition with dose. We found an anomalous evolution of the magnetoresistance with temperature as it approaches the MIT. This behavior shifts in temperature following the MIT in irradiated samples. Dynamical mean-field theory calculations of the Hubbard model closely capture this anomalous behavior, providing strong evidence that an antiferromagnetic instability drives the opening of a gap in V2O3. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C45.00008: Resistive asymmetry in the metal-insulator transition of VO2 and V2O3 nanowires Ivan Schuller, Javier Del Valle Granda, Nareg Ghazikhanian, Yoav Kalcheim, Juan Trastoy, Minhan Lee, Marcelo Rozenberg VO2 and V2O3 are two examples of correlated oxides featuring a metal-insulator transition (MIT) at 340 K and 160 K, respectively. These transitions have attracted much attention, but few studies have addressed the effect of dimensional confinement. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C45.00009: Band Theory Description of SrVO3 and CaVO3 Sheng Xu, Yanni Gu, Xiao Shen Strontium vanadate SrVO3 and calcium vanadate CaVO3 have been considered as porotypes of correlated metals. It is usually assumed that band theory is inadequate for describing their electronic structures and methods based on strong correlations must be invoked. Here we present results from calculations based on hybrid density functional theory, a version of band theory. We show that the structural, electronic, and magnetic properties of SrVO3 and CaVO3 are correctly reproduced. The results suggest that the effects of the exchange and correlation can be correctly accounted in the hybrid density functional theory, and it may be appropriate to describe alkaline-earth vanadates without invoking strong correlation. Furthermore, the metal-insulator transitions in SrVO3 and CaVO3 are discussed using the band theory treatment. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C45.00010: Raman Scattering Study of the Low Temperature, High Pressure Structural Transition in CoV2O4 John Slimak, Taylor Byrum, Astha Sethi, Haidong Zhou, S Cooper At ambient pressures, CoV2O4 exists very near a critical value of the vanadium-vanadium separation, rV-V = 2.94 Å, for the AV2O4 spinels, below which the electronic character transitions from insulating or semiconducting to metallic. CoV2O4 has not been shown to undergo a symmetry lowering phase change at low temperatures, unlike most other AV2O4 spinels. The cubic structure has been predicted to be stable at high pressures, despite well-known strong coupling between spin, lattice, and electronic degrees of freedom in spinels. We studied the triply degenerate 192 cm-1 phonon mode with Raman spectroscopy to determine if the cubic phase remains stable at low temperatures and if there is a structural phase change associated with the pressure-induced semiconductor-metal transition. We find no evidence for a structural transition at low temperatures at ambient pressure. However, at low temperatures and above P~4 GPa, we observe a splitting of the phonon mode, indicative of a lower symmetry phase near the semiconductor-metal phase boundary. These results provide evidence that strong spin-lattice coupling plays an essential role in the pressure-tuned semiconductor-metal transition of CoV2O4. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C45.00011: Metal-Insulator Transition in a Deformable Lattice Gayan Hettiarachchi, Takehito Nakano, Halimaton Hamdan, Mohd Nazlan Mohd Muhid Metal-insulator transition (MIT) in disorder systems with critical effects of electron-lattice coupling near the mobility edge remains under debate. A discontinuous transition between small (bi)polarons and extended states was proposed within the adiabatic approximation [1]. On the other hand, theoretical treatment of the MIT in a deformable lattice proposed a continuous transition [2]. Despite many zealous theoretical efforts, this remains unresolved. In addressing this, we experimentally investigate the MIT in a deformable lattice, controlling the random potential through electron-density modulation and electron-lattice coupling strength (λ) by varying the guest element (Na, K, Rb, or Cs) encapsulated within a quasi-two-dimensional host. Physical properties were investigated using optical, magnetic, transport, and electron spin resonance measurements. We bring to light the importance of the magnitude of λ on the conducting transition (or freezing) and its continuity (or discontinuity). We conjecture that both theories [1,2] apply, perhaps in distinct coupling regimes, and propose a phase diagram for the disorder- and polaron-driven MIT in a deformable lattice. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C45.00012: Abstract Withdrawn
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Monday, March 5, 2018 4:54PM - 5:06PM |
C45.00013: Metal insulator transition to a partial disordered state in the 1/3 filled Hubbard model on a triangular lattice Matthew Enjalran, Richard Scalettar The interplay of electron filling, electron-electron interactions, and geometric frustration |
Monday, March 5, 2018 5:06PM - 5:18PM |
C45.00014: The doping phase diagram of V$_2$O$_3$ revisited I.I. Mazin, N. Berstein, F. Lechermann, R. Valenti V$_2$O$_3$ is a prototypical Mott-Hubbard insulator, with hundreds theoretical papers published in course of the last 30+ years. Intriguingly, doping with Ti rapidly supresses the low-$T$ antiferromagnetic insulator (AFI) phase so that the material becomes a paramagnetic metal (PM) at all $T$s, while doping with Cr slightly stabilizes the AFI phase, and rapidly transforms the high-$T$ PM phase into a paramagnetic insulator (PI). It's been often interpreted as a chemical pressure effect, with Ti allegedly exerting a positive, and Cr a negative pressure. However, the documented ionic radius of Ti$^{3+}$ ($R_{Ti}=0.81$ \AA) is slightly $larger$ than that $R_V=0.78$ \AA, and $R_{Cr}=0.755$ \AA is slightly $smaller$. Both experiments and our $ab$ $initio$ calculations suggest that either substitution expands the lattice, and anyway the effect is too small. In order to address this controversy, we have performed structural optimization of a 16 formula unit supercell with a Cr or Ti impurity, and found that the former $enhances$, and the latter $reduces$ the tendency to insulating behavior. We have observed this effect in both zero-$T$ LDA+U calculations and finite-$T$ DFT+DMFT calculations. Thus, we conclude that the pecularities of the V$_2$O$_3$+(Cr,Ti) phase diagram are not a simple chemical pressure effect, but indicate high sensitivity to the local geometry. This conclusion is consistent with the (theoretical) observation that the local crystal field plays a critical role in the Mott physics in V$_2$O$_3$, and with (experimental) fact that the insulating state in V$_2$O$_3$ is easily destroyed by ionic irradiation (J.G. Ramirez et al, PRB 91, 205123, 2015). |
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