Mott-Hubbard quantum criticality in paramagnetic CMR pyrochlores

We present a correlated ab initio description of the paramagnetic phase of ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$, employing a combined local density approximation with multiorbital dynamical mean-field theory treatment. We show that the insulating state observed in this colossal magnetoresistance pyrochlore is determined by strong Mn intra- and inter-orbital local electron-electron interactions. Hybridization effects are reinforced by the correlation-induced spectral weight transfer. Our result coincides with optical conductivity measurements, whose low-energy features are well accounted for by our theory. We also study the disorder-driven insulator-metal transition of doped compounds, showing the proximity of ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ to quantum phase transitions, in agreement with recent measurements.

Figure 1

Pyrochlore crystal structure: Tl and O are, respectively, represented by larger and small spheres (see Ref. 23). The Mn atoms are placed at the centers of the depicted $\mathrm{Mn}\text{−}\mathrm{O}6$ corner-sharing octahedra.

Figure 2

(Color online) LDA band structure for paramagnetic ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$. ${\mathrm{O}}_1\left({\mathrm{O}}_2\right)$ denotes oxygen ions nearest to Mn (Tl).

Figure 3

(Color online) $\mathrm{LDA}+\mathrm{DMFT}$ (paramagnetic ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$): orbital-resolved (upper panels) and total (lower panel) Mn densities of states, for $U=6.0$, 7.0, $8.0\mathrm{eV}$, $J_H=1\mathrm{eV}$.

Figure 4

(Color online) Low-energy view of $\mathrm{LDA}+\mathrm{DMFT}$ orbital-resolved (upper panels) and total (lower panel) DOS, for $J_H=1.0\mathrm{eV}$ and values of $U$ below and above the metal-insulator transition point.

Figure 5

(Color online) Paramagnetic ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$: $t_{2g}$ optical conductivity calculated with $\mathrm{LDA}+\mathrm{DMFT}$ for $U=7.0\mathrm{eV}$ and $U=7.5\mathrm{eV}$, compared with experiment at $T=295\mathrm{K}$ (see Ref. 21). Inset: Low-energy conductivity: $\mathrm{LDA}+\mathrm{DMFT}$ vs experiment at $T=295\mathrm{K}$.

Figure 6

(Color online) Effect of disorder: orbital-resolved (upper panels) and total (lower panel) DOS for ${\mathrm{Tl}}_{2-x}{\mathrm{Bi}}_x{\mathrm{Mn}}_2{\mathrm{O}}_7$. $U=7.0\mathrm{eV}$ and $v=2.5\mathrm{eV}$.

Figure 7

(Color online) Effect of $U$ in the disordered system: orbital-resolved (upper panels) and total (lower panel) DOS for $x=0.1$, $v=2.5\mathrm{eV}$, and two values of $U$.