Dimensional-Crossover-Driven Metal-Insulator Transition in ${\mathrm{SrVO}}_3$ Ultrathin Films

We have investigated the changes occurring in the electronic structure of digitally controlled ${\mathrm{SrVO}}_3$ ultrathin films across the metal-insulator transition (MIT) by the film thickness using in situ photoemission spectroscopy. With decreasing film thickness, a pseudogap is formed at $E_F$ through spectral weight transfer from the coherent part to the incoherent part. The pseudogap finally evolves into an energy gap that is indicative of the MIT in a ${\mathrm{SrVO}}_3$ ultrathin film. The observed spectral behavior is reproduced by layer dynamical-mean-field-theory calculations, and it indicates that the observed MIT is caused by the reduction in the bandwidth due to the dimensional crossover.

Figure 1

Characterizations of ${\mathrm{SrVO}}_3$ films grown onto $\mathrm{Nb}\mathrm{\text{:}}{\mathrm{SrTiO}}_3$ substrates: (a) typical reflection high-energy electron diffraction intensity oscillations along with marks where PES spectra were taken and corresponding AFM images (scan area $0.5\times 0.5\mu {\mathrm{m}}^2$). (b) A typical AFM image of SVO(100ML), (c) cross-sectional TEM image of the interface between SVO and Nb:STO, and (d) reciprocal space mapping around the (303) reflection of SVO.

Figure 2

(a) In situ valence band spectra of ${\mathrm{SrVO}}_3$ thin films grown on $\mathrm{Nb}\mathrm{\text{:}}{\mathrm{SrTiO}}_3$ substrates by digitally controlling the film thickness. (b) PES spectra near $E_F$. The filled and open triangles represent the energy positions of the coherent parts and the incoherent parts, respectively.

Figure 3

(a) Symmetrized V $3d$ spectra for thickness-dependent ${\mathrm{SrVO}}_3$ ultrathin films. The filled and open triangles represent the energy positions of the coherent and incoherent parts, respectively. The dashed lines for 1–3 ML indicate the background level. (b) Plots of the quasiparticle peak intensity and the peak positions of the incoherent and coherent parts in V $3d$ spectra of SVO ultrathin films as a function of the film thickness. (c) Spectral functions calculated from layer DMFT. (d) Plots of the quasiparticle peak intensity and the centroid of the incoherent part in the calculated spectra as a function of the number of slabs. MI and PG denote the Mott-insulator and pseudogap region, respectively.