Novel $P\text{−}T$ Phase Diagram of the Multiorbital Mott Insulator ${\mathrm{Sr}}_2{\mathrm{VO}}_4$

The electrical and optical properties of the Mott insulator ${\mathrm{Sr}}_2{\mathrm{VO}}_4$ are investigated under high pressure on a phase pure polycrystalline sample. The system undergoes a pressure-driven insulator to metal transition (IMT) with a crossover between 20 and 24 GPa. The effect of pressure on the thermally driven electronic changes resulting from spin-orbital ordering transitions is studied. A multiorbital analysis of the low frequency optical conductivity spectra suggests a bandwidth-controlled and orbital selective nature of the Mott IMT transition. Dramatic enhancement of the low energy spectral weight in the high pressure correlated metallic phase is explained in terms of the formation of a quasiparticle peak in the spectral function of the narrow and degenerate $d_{yz,zx}$ orbitals. Our results overall establish a novel electronic phase diagram of tetragonal ${\mathrm{Sr}}_2{\mathrm{VO}}_4$.

Figure 1

(a) Resistance as a function of temperature up to 28 GPa. (b) Temperature dependence of resistance, normalized by the value at 290 K, at various pressures. (c) Enlarged view around characteristic temperatures $T_{\max }$ and $T^{*}$ at two different pressures. (d) Sample resistance at 10.5 GPa is shown decomposed of the $ab$-plane and $c$-axis resistances, $R_{ab}$ and $R_c$, respectively.

Figure 2

(a) The phase diagram for tetragonal ${\mathrm{Sr}}_2{\mathrm{VO}}_4$, constructed from the resistance data. Low pressure orbital structures [$\mathrm{orbital}\mathrm{disordered}(\mathrm{OD})\Rightarrow \mathrm{orbital}\mathrm{liquid}(\mathrm{OL})\Rightarrow \mathrm{orbital}\mathrm{ordered}(\mathrm{OO})$, see text] are separated by $T_{\max }$ and $T^{*}$. Pressure dependence of orbital ordering transition (${\mathrm{T}}_{\text{OO}}$), as reported in Ref. [6], is also plotted for comparison. The hashed area, bounded by $T_{\min }$, indicates an unconventional metal region with $dR/dT<0$.

Figure 3

(a) Reflectance $R_{s-d}$ at the sample-diamond interface at various pressures up to 41.9 GPa at 300 K. Inset shows a comparison of the spectra recorded on a free sample in vacuum with the spectra at 0.1 GPa (in DAC), corrected for vacuum. (b) Pressure dependent optical conductivity $\sigma (\omega )$ at various pressures. (c) Integrated spectral weight $\mathrm{SW}={\int }_{{\mathrm{\Omega }}_0}^{\mathrm{\Omega }}\sigma (\omega )d\omega$ for various pressures with ${\mathrm{\Omega }}_0=1000{\mathrm{cm}}^{-1}$. (d) Pressure dependent spectral weight variation, normalized to ambient pressure value at 2500 and $8000{\mathrm{cm}}^{-1}$. Solid lines are guides for the eye.

Figure 4

Drude-Lorentz fit (thick gray) of the optical conductivity (dark blue) at (a) 0.1, (b) 15.1, and (c) 20.5 GPa. Insets are schematic views of the evolution of the spectral function (see text) and corresponding optical transitions in the Mott insulating state (from Ref. [7]) and in the correlated metallic state at high pressures. The bands originating due to transitions between LHB and UHB of $xz/yz$ orbitals are shown by red and dark red, and those from $xy$ orbitals are marked in blue.