Correlation-Driven Insulator-Metal Transition in Near-Ideal Vanadium Dioxide Films

A. X. Gray, J. Jeong, N. P. Aetukuri, P. Granitzka, Z. Chen, R. Kukreja, D. Higley, T. Chase, A. H. Reid, H. Ohldag, M. A. Marcus, A. Scholl, A. T. Young, A. Doran, C. A. Jenkins, P. Shafer, E. Arenholz, M. G. Samant, S. S. P. Parkin, and H. A. Dürr

Phys. Rev. Lett. 116, 116403 – Published 18 March 2016

Abstract

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in ${VO}_{2}$ . We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline ${VO}_{2}$ films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K ( $\pm 0.3 K$ ) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

Received 6 February 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.116403

Physics Subject Headings (PhySH)

Metal-insulator transition Transport phenomena

Devices

X-ray absorption spectroscopy X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. X. Gray^1,2,*, J. Jeong³, N. P. Aetukuri³, P. Granitzka^1,4, Z. Chen^1,5, R. Kukreja^1,6, D. Higley^1,7, T. Chase^1,7, A. H. Reid¹, H. Ohldag⁸, M. A. Marcus⁹, A. Scholl⁹, A. T. Young⁹, A. Doran⁹, C. A. Jenkins⁹, P. Shafer⁹, E. Arenholz⁹, M. G. Samant³, S. S. P. Parkin³, and H. A. Dürr^1,†

¹Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
²Department of Physics, Temple University, 1925 North 12th Street, Philadelphia, Pennsylvania 19130, USA
³IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA
⁴Van der Waals–Zeeman Institute, University of Amsterdam, 1018XE Amsterdam, The Netherlands
⁵Department of Physics, Stanford University, Stanford, California 94305, USA
⁶Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
⁷Department of Applied Physics, Stanford University, Stanford, California 94305, USA
⁸Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
⁹Advanced Light Source, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, USA

^*axgray@temple.edu
^†hdurr@slac.stanford.edu

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Vol. 116, Iss. 11 — 18 March 2016

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Figure 1
(a) In the high-temperature ( $T > T_{IMT}$ ) metallic phase, ${VO}_{2}$ forms rutile lattice structure with $P 4_{2} / m n m$ space symmetry where V atoms (yellow) occupy centers of sixfold oxygen-coordinated sites (O atoms not shown). (b) The near-Fermi-level $t_{2 g}$ states are separated in energy by the orthorhombic component of the crystal field into the twofold-degenerate $e_{g}^{π}$ ( $π^{*}$ ) states and a single $a_{1 g}$ orbital, which is aligned parallel to the rutile $c$ axis ( $c_{R}$ ) and is thus commonly denoted $d_{∥}$ . The two bands overlap in energy, and the resulting nonzero density of states at the Fermi level accounts for the metallic behavior of the rutile phase [13, 18]. (c) In the low-temperature ( $T < T_{IMT}$ ) insulating phase the lattice undergoes a structural transition to a lower-symmetry ( $P 2_{1} / c$ ) monoclinic crystal system via dimerization of the neighboring V atoms along the $c_{R}$ direction and tilting of the resultant V-V dimers along the rutile [110] and $[1 \bar{1} 0]$ directions. Each dimerized V-V atomic pair shares a singlet electronic state composed of two strongly correlated V $3 d^{1}$ electrons (shown in blue) [18]. (d) V-V dimerization splits the highly directional $d_{∥}$ orbitals into the bonding and antibonding bands, and the tilting of the dimers shifts the $π^{*}$ band to higher energies due to the increase of the $p − d$ orbital overlap [13, 18], together producing an insulating gap of 0.6 eV—the key aspect of the monoclinic phase observed via a wide variety of experimental techniques [23, 24]. An additional $d_{∥}$ feature in DOS, which arises at the onset of the conduction band (shown in blue), is a unique fingerprint of the strong electron-electron correlations within the dimers [18], accessible experimentally via polarization-dependent XAS at the O $K$ edge (see Ref. [25] and this work). (e) Temperature-dependent x-ray diffraction $θ − 2 θ$ scans across the ${TiO}_{2}$ substrate (002) and the ${VO}_{2}$ film peaks show a clear structural transition in ${VO}_{2}$ manifested by an abrupt change in the interplanar atomic spacing along the direction normal to the film surface at 295 K (inset). Concomitant XRD and electronic transport measurements were collected for the heating cycle, with 1 h temperature equilibration time between scans. Inset: Temperature-dependent resistance (purple circles) and $c_{R}$ lattice parameter (green circles) obtained concomitantly via electronic transport and x-ray-diffraction measurement, respectively. See full data set in the Supplemental Material [26].
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Figure 2
(a) Schematic diagram of the polarization-dependent XAS measurement geometry used in this study. Linearly polarized x rays are incident at 15° to the surface of the coherently strained epitaxial 10 nm thick ${VO}_{2} / {TiO}_{2} (001)$ sample, with the photon polarization set to either parallel ( $E ∥ c_{R}$ ) or perpendicular ( $E ⊥ c_{R}$ ) to the $c_{R}$ axis (along the sample normal) of the film enabling preferential probing of the strongly directional $d_{∥}$ ( $d_{x^{2} - y^{2}}$ ) and $π^{*}$ ( $d_{x z}$ and $d_{y x}$ ) orbitals depicted in (b), respectively. (c) Polarization-dependent O $K$ edge XAS measurements of ${VO}_{2}$ in the high-temperature ( $T = T_{IMT} + 15 K$ ) metallic state (shown as open and solid red symbols) and in the low-temperature ( $T = T_{IMT} - 15 K$ ) insulating state (shown as open and solid blue symbols). Shift of the leading slope of the edge towards lower energy between the insulating and the metallic phases of ${VO}_{2}$ corresponds to the collapse of the insulating band gap on the unoccupied side of the energy band diagram (upper gap collapse). Distinct dichroic signal at the onset of the absorption edge in the insulating state presents direct evidence of the additional $d_{∥}$ orbital character at the bottom of the unoccupied conduction band, which is predicted by theory [18] and is shown schematically in Fig. 1. (d) $d_{∥}$ V-V singlet peak at the onset of the conduction band (528.7 eV) and the $d_{∥}$ Peierls peak at 530.2 eV, both obtained by calculating the $I_{∥} - I_{⊥}$ XAS intensity taken directly from (c).
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Figure 3
(a) Temperature-dependent evolution of the $d_{∥}$ V-V singlet peak intensity (blue and white symbols) showing distinctly different transition temperature ( $T_{corr} = 290 K$ ) as compared to the $T_{IMT}$ (295 K), the critical temperature at which the collapse of the upper band gap (red solid symbols) as well as the structural transition (yellow solid symbols) is observed in ${VO}_{2}$ . Upper gap size is estimated by fitting the edge threshold with a polynomial function and extracting the energy (position along the $x$ axis) of the half-maximum point from the fitted curve. Error bar analysis is presented in the Supplemental Material [26]. (b)–(d) Schematic representation of the three phases of ${VO}_{2}$ implied by the experimental data in the plots on the left. (b) The low-temperature phase ( $T ≪ T_{IMT}$ ) is a monoclinic insulator with a strongly correlated singlet electronic state on each V-V dimer. At $T = T_{corr}$ the strong Coulomb correlations within the dimers soften, giving rise to a new monoclinic insulating phase shown in (c) via a purely electronic phase transition. Finally, only after the $e − e$ correlations are sufficiently diminished ( $T = T_{corr} + 3 K$ ), the system undergoes a transition to a rutile metallic phase shown in (d).
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