Thickness dependence of electronic structures in $\mathrm{V}{\mathrm{O}}_{2}$ ultrathin films: Suppression of the cooperative Mott-Peierls transition

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Thickness dependence of electronic structures in $V O_{2}$ ultrathin films: Suppression of the cooperative Mott-Peierls transition

D. Shiga, B. E. Yang, N. Hasegawa, T. Kanda, R. Tokunaga, K. Yoshimatsu, R. Yukawa, M. Kitamura, K. Horiba, and H. Kumigashira

Phys. Rev. B 102, 115114 – Published 9 September 2020

Abstract

Through in situ photoemission spectroscopy, we investigated the change in the electronic structures and V-V dimerization of dimensionality-controlled $V O_{2}$ films coherently grown on $Ti O_{2} (001)$ substrates. In the nanostructured films, the balance between the instabilities of a bandlike Peierls transition and a Mott transition is controlled as a function of thickness. The characteristic spectral change associated with temperature-driven metal-insulator transition in $V O_{2}$ thick films holds down to 1.5 nm (roughly corresponding to five V atoms along the [001] direction), whereas $V O_{2}$ films of less than 1.0 nm exhibit insulating nature without the V-V dimerization characteristic of $V O_{2}$ . These results suggest that the delicate balance between a Mott instability and a bandlike Peierls instability is modulated at a scale of a few nanometers by the dimensional crossover effects and confinement effects, which consequently induce the complicated electronic phase diagram of ultrathin $V O_{2}$ films.

Received 1 May 2020
Revised 28 July 2020
Accepted 18 August 2020

DOI:https://doi.org/10.1103/PhysRevB.102.115114

Physics Subject Headings (PhySH)

Electronic structure Metal-insulator transition Peierls transition

Mott insulators

Photoemission spectroscopy X-ray absorption spectroscopy X-ray photoelectron spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

D. Shiga^1,2, B. E. Yang ¹, N. Hasegawa¹, T. Kanda¹, R. Tokunaga¹, K. Yoshimatsu ¹, R. Yukawa ², M. Kitamura², K. Horiba ², and H. Kumigashira ^1,2,*

¹Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980–8577, Japan
²Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305–0801, Japan

^*kumigashira@tohoku.ac.jp

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Vol. 102, Iss. 11 — 15 September 2020

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Figure 1
(a) Thickness dependence of Ti $2 p_{3 / 2}$ core-level spectra measured at $h ν = 1200 eV$ for $V O_{2} / Nb : Ti O_{2} (001)$ ultrathin films. Each spectrum is normalized to the incident photon flux; hence, the intensity reduction with increasing $V O_{2}$ overlayer thickness $t$ reflects the attenuation of the Ti $2 p$ signal from buried $Ti O_{2}$ by the overlayer. (b) Plot of relative intensities of the background-subtracted Ti $2 p_{3 / 2}$ core-level spectra, $I_{Ti}$ , as a function of $V O_{2}$ overlayer thickness. The experimental points are fitted to the photoemission attenuation function with the photoelectron inelastic mean-free path of 0.55(2) nm. The inset represents the logarithm plot of $I_{Ti}$ with respect to $t$ in comparison with the simulation curves assuming the intermixing of V and Ti ions at the interface with interdiffusion lengths $d$ of 0.3, 0.5, 0.7 [41], 1.2, and 3.1 nm [43]. From the comparison, it is clear that the interdiffusion length of the present film is less than 0.3 nm, which is much smaller than that of previous reports and comparable to the V-V dimer length of approximately 0.3 nm.
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Figure 2
(a) Temperature dependence of resistivity ( $ρ − T$ ) for $V O_{2} / Ti O_{2}$ (001) ultrathin films with thicknesses $t$ varying from 0.5 to 10 nm. Note that the saturated behavior of the resistivity value at lower temperatures for $t = 0.5$ and 1.0 nm is due to the limitation of our apparatus. (b) Electronic phase diagram of $V O_{2} (001)$ ultrathin films as functions of temperature and $t$ . The filled and open circles indicate $T_{MIT}$ determined from the $ρ − T$ curves. Here, $T_{MIT}$ was defined as the center of the hysteresis loop, namely, an average of the two inflection points in $ρ − T$ curves during cooling and heating. The thick and thin bars in the vertical axis represent the hysteresis and transition width in the $ρ − T$ curves [40], respectively. Note that the colors of each filled symbol correspond to those in the $ρ − T$ curves. Solid and dotted lines are merely guides for ease of visualization. The monoclinic insulating, rutile metallic, and new insulating phases are denoted by MI, RM, and NI, respectively, which are assigned from the spectroscopic results discussed later. The insets show the crystal structures of corresponding rutile and monoclinic $V O_{2}$ . Note that the characteristic V-V dimerization of $V O_{2}$ thick films is either absent or significantly suppressed in the NI phase.
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Figure 3
(a) Thickness dependence of valence-band spectra measured at $T = 320 K$ (HT rutile metallic phase for $t = 10 nm$ ) and 250 K (LT monoclinic insulating phase for $t = 10 nm$ ) for the $V O_{2} / Nb : Ti O_{2} (001)$ ultrathin films. (b) PES spectra near $E_{F}$ in an expanded energy scale. Note that the colors of each spectrum correspond to those in Figs. 1 and 2.
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Figure 4
(a) Temperature dependence of O $K$ -edge XAS spectra with different polarizations of $V O_{2} / Nb : Ti O_{2} (001)$ films with $t = 10 nm$ . XAS spectra acquired with the polarization vector $E$ perpendicular to the $c_{R}$ axis ( $E ⊥ c_{R}$ ) and parallel to the $c_{R}$ axis ( $E / / c_{R}$ ) are represented by gray and magenta lines, respectively. The XAS spectra with $E / / c_{R} (I_{/ /})$ are deduced from the expression $I_{/ /} = (4 / 3) (I - I_{⊥} / 4)$ , where $I_{⊥}$ (namely, that corresponding to $E ⊥ c_{R}$ ) and $I$ denote the spectra measured with normal $(θ = 0^{\circ})$ and grazing $(θ = 60^{\circ})$ incidence, respectively [49]. Following the assignments made in previous studies [58], the corresponding band diagrams are schematically illustrated in (b) together with the corresponding crystal structures; the first peak around 529.5 eV can be assigned to $π^{*}$ bands formed by V $3 d_{x z}$ and $3 d_{y z}$ orbitals, while the second peak can be assigned to $σ^{*}$ bands formed by $3 d_{3 z 2 - r 2}$ and $3 d_{x 2 - y 2}$ orbitals. The additional peak that emerges around 530.6 eV only for the spectra of insulating monoclinic phase $(T = 250 K)$ with $E / / c_{R}$ can be assigned to the $d_{/ /}^{*}$ state due to V-V dimerization (indicated by filled triangle). (c) Thickness dependence of O $K$ -edge XAS spectra acquired with $E / / c_{R}$ at $T = 320$ and 250 K for the $V O_{2} / Nb : Ti O_{2} (001)$ ultrathin films. Note that the colors of each spectrum correspond to those in Figs. 1, 2, 3.
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Physical Review B

covering condensed matter and materials physics

Thickness dependence of electronic structures in $V O_{2}$ ultrathin films: Suppression of the cooperative Mott-Peierls transition

D. Shiga, B. E. Yang, N. Hasegawa, T. Kanda, R. Tokunaga, K. Yoshimatsu, R. Yukawa, M. Kitamura, K. Horiba, and H. Kumigashira

Phys. Rev. B 102, 115114 – Published 9 September 2020

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Physical Review B

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Thickness dependence of electronic structures in VO2 ultrathin films: Suppression of the cooperative Mott-Peierls transition

D. Shiga, B. E. Yang, N. Hasegawa, T. Kanda, R. Tokunaga, K. Yoshimatsu, R. Yukawa, M. Kitamura, K. Horiba, and H. Kumigashira

Phys. Rev. B 102, 115114 – Published 9 September 2020

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Thickness dependence of electronic structures in $V O_{2}$ ultrathin films: Suppression of the cooperative Mott-Peierls transition