Pressure-Driven Metal-Insulator Transition in Hematite from Dynamical Mean-Field Theory

The local density approximation combined with dynamical mean-field theory is applied to study the paramagnetic and magnetically ordered phases of hematite ${\mathrm{Fe}}_2{\mathrm{O}}_3$ as a function of volume. As the volume is decreased, a simultaneous first-order insulator-metal and high-spin to low-spin transition occurs close to the experimental value of the critical volume. The high-spin insulating phase is destroyed by a progressive reduction of the spectral gap with increasing pressure, upon closing of which the high-spin phase becomes unstable. We conclude that the transition in ${\mathrm{Fe}}_2{\mathrm{O}}_3$ at $\approx 50\mathrm{GPa}$ can be described as an electronically driven volume collapse.

Figure 1

$\mathrm{Fe}\mathrm{\text{−}}d$ occupancies (left-hand panel) at various specific volumes in the paramagnetic (PM) state at $T=580\mathrm{K}$. The thick line marks the average $d$ occupancy per orbital. Corresponding values of the screened (diamonds) and instantaneous (circle) local moment are shown in the right-hand panel.

Figure 2

Evolution of the PM single-particle spectra with pressure ($T=580\mathrm{K}$). The HS solutions are shown in the left-hand panel and LS solutions in the right-hand panel. All curves are normalized to one. Left-hand inset: Comparison of experimental PES [7] and inverse PES [25] data (points) to the $d$ spectra (solid line) with 0.6 eV Gaussian broadening to mimic the experimental resolution. Right-hand inset: Comparison of the $v=0.8$ DMFT (smooth curves) and LDA spectra (jagged curves): the total $d$ spectra (black lines), the total $\mathrm{O}\mathrm{\text{−}}p$ spectra [gray (blue) lines].

Figure 3

Spin-polarized $\mathrm{Fe}\mathrm{\text{−}}d$ spectra at the ambient pressure for 580, 1160, and 1450 K (top to bottom). Left-hand inset: The same spectra averaged over spin. Right-hand inset: The staggered magnetization versus temperature curves for $v=1.0$ (circles, black), $v=0.9$ (squares, red), $v=0.85$ (diamonds, geen), and $v=0.8$ (triangles, blue). At empty symbols only the LS solution exists. The lines represent mean-field fits.

Figure 4

Calculated $V\mathrm{\text{−}}T$ phase diagram of hematite. The symbols mark the actual computational results: HS state (circle), LS state (diamond). Empty diamonds mark LS solutions stable only if PM symmetry is constrained; dotted circles mark HS solutions stable only if AFM LRO is allowed. The black line shows the Néel temperature estimated from fits to $v=1.0$, 0.9, and 0.85 data. The shaded area marks the estimated HS-LS coexistence region with PM constraint; the dashed lines mark the coexistence regime when AFM LRO is allowed.