Band dispersion near the Fermi level for ${\text{VO}}_2$ thin films grown on ${\text{TiO}}_2$ (001) substrates

We have performed angle-resolved photoemission spectroscopy (ARPES) measurements of ${\text{VO}}_2$ using epitaxial thin films and observed the band dispersion near the Fermi level $\left(E_{\text{F}}\right)$ for this compound. The ${\text{VO}}_2$ thin films have been grown on ${\text{TiO}}_2$ (001) single-crystal substrates using pulsed laser deposition. The films exhibit a first-order metal-insulator transition (MIT) at 305 K. In the ARPES spectra of the metallic phase for the films, the $\text{O}2p$ band shows highly dispersive features in the binding-energy range of 3–8 eV along the $\Gamma -Z$ direction. Also, the $\text{V}3d$ state shows two dispersive bands around the $\Gamma$ point near $E_{\text{F}}$, indicative of two electron pockets centered at the $\Gamma$ point. Both electron pockets have an occupied bandwidth of about 0.4 eV. Assuming the parabolic energy bands around the $\Gamma$ point, the effective-mass ratios of the two electron pockets are estimated to be about 0.2 and 1. The present work indicates that the ARPES measurements using epitaxial thin films are promising for determining the band structure of ${\text{VO}}_2$ and thus would play a crucial role to elucidate the mechanism of the MIT in ${\text{VO}}_2$.

Figure 1

(Color online) (a) Temperature dependence of resistivity for the ${\text{VO}}_2$ thin films grown on ${\text{TiO}}_2$ (001) substrates. (b) Valence-band PES spectra of the ${\text{VO}}_2$ thin films. The upper line shows the spectrum of the insulator phase measured at 300 K. The lower line shows the spectrum of the metallic phase measured at 350 K. The spectra were taken at the photon energy of 150 eV.

Figure 2

(Color online) (a) Normal emission ARPES spectra of the ${\text{VO}}_2$ thin films in the metallic phase measured at 350 K for various photon energies. (b) Experimental band structure in the $\Gamma -Z$ direction of the ${\text{VO}}_2$ thin films obtained from the ARPES measurements. Dark parts correspond to the energy bands. $\Gamma$ point shown here is determined from the ARPES measurements of the $\text{V}3d$ state (see text).

Figure 3

(Color online) (a) Normal emission ARPES spectra of the metallic ${\text{VO}}_2$ thin films in the binding-energy range of 0–2 eV measured at 350 K. The spectra at photon energy of $h\nu =150$ and 162.5 eV show the blue line and the spectra at $h\nu =140$ and 170 eV show the green line. (b) The ARPES spectra divided by the Fermi function. Solid and open triangles represent the energy position of the band structure. The line segments indicate the upper and lower bounds for the dispersive features. (c) A momentum distribution curve at $E_{\text{F}}$ as a function of wave number $k_{\perp }$ along the $\Gamma -Z$ direction.

Figure 4

Energy position of the bands in the ARPES spectra in Fig. 3b plotted against the wave number $k_{\perp }$ along the $\Gamma -Z$ direction. The error bars represent a rough estimate of the energy where the energy band may exist, namely, the upper and lower bounds of the energy band.