Mott-Hubbard gaps and their doping-induced collapse in strongly correlated pyrochlore ruthenates

Rapid Communication

Mott-Hubbard gaps and their doping-induced collapse in strongly correlated pyrochlore ruthenates

R. Kaneko, K. Ueda, C. Terakura, and Y. Tokura

Phys. Rev. B 102, 041114(R) – Published 17 July 2020

Abstract

We investigate the variation of charge dynamics in the course of the filling-control metal-insulator transitions (MITs) for pyrochlore $R_{2} {Ru}_{2} O_{7}$ ( $R$ being rare-earth ions). With widely changing of $R$ ions from Lu to Pr, the antiferromagnetic transition temperature systematically increases while the magnitude of the charge gap decreases toward zero at the hypothetical bandwidth-control Mott transition. It is attributable to the reduction of effective electron correlation in the Mott-Hubbard insulator regime. Doping holes give rise to the filling-control MITs from antiferromagnetic insulators to paramagnetic metals accompanied by the collapse of the Mott-Hubbard gaps. The doping-induced spectral weight transfer of the optical conductivity from the above-gap region to the in-gap region is nearly proportional to the doping level, whose rate is enhanced with the reduction of the electron correlation, in accord with the standard Mott criticality.

Received 1 May 2020
Revised 2 July 2020
Accepted 2 July 2020

DOI:https://doi.org/10.1103/PhysRevB.102.041114

Physics Subject Headings (PhySH)

Electrical conductivity Optical conductivity Phase transitions Spin-orbit coupling

Mott insulators Pyrochlores Strongly correlated systems

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

R. Kaneko ^1,2, K. Ueda ¹, C. Terakura², and Y. Tokura^1,2,3

¹Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
²RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
³Tokyo College, University of Tokyo, Tokyo 113-8656, Japan

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 102, Iss. 4 — 15 July 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Crystal structure of $R_{2} {Ru}_{2} O_{7}$ . (b) $t_{2 g}$ electronic levels of ${Ru}^{4 +} 4 d$ state. (c) Examples for possible Ru- $4 d$ spin configurations, in which the sum of spin moments within a tetrahedron is zero ( $Σ S_{i} = 0$ ). The right figure shows the all-in all-out state, as one such example. (d) Temperature dependence of magnetization for $R = \Pr, Nd, Eu, Dy, Lu$ compounds. Solid (dashed) lines represent the results measured after the zero-field-cooling (ZFC) process [field-cooling (FC)]. The black arrows denote the antiferromagnetic transition $T_{N}$ . (e) Temperature ( $T$ ) dependence of resistivity $ρ$ for $R = \Pr$ under the pressure $P = 0 GPa$ (red bold curve) and $20 GPa$ (blue bold curve). Thin lines show the second derivative of $\log ρ$ . The black arrows indicate the kink position ( $T^{*}$ ) of $ρ$ .
Reuse & Permissions
Figure 2
The Néel temperature $T_{N}$ (blue triangles) and the magnitude of the charge gap $Δ$ (red circles) measured at 10 K as a function of $R$ ionic radius $r$ . The green square represents $T^{*}$ estimated in Fig. 1. The upper scale for the hydrostatic pressure $P$ on $\Pr_{2} {Ru}_{2} O_{7}$ is calculated by an empirical relation between $P$ and $r$ observed in $R_{2} {Ir}_{2} O_{7}$ .
Reuse & Permissions
Figure 3
(a) Optical conductivity spectra of $\Pr_{2} {Ru}_{2} O_{7}$ ( $R = \Pr$ ) below the energy $ℏ ω = 5 eV$ . MH (CT) stand for Mott-Hubbard gap excitation (charge-transfer excitation). The red (green) dashed line (Lorentzian curves) is a guide to the eyes for the MH (CT) gap excitation. (b) Schematic electronic structure for $R_{2} {Ru}_{2} O_{7}$ . UH (LH) stands for upper (lower) Hubbard band of the $Ru - t_{2 g}$ state. (c) Magnified view of (a) below 1 eV. The dashed line represents an extrapolating line to estimate the magnitude of the MH gap. (d) Optical conductivity spectra for $R = \Pr, Sm, Gd, Lu$ below 2 eV. The spiky structures of the optical conductivity spectra below 0.1 eV are all due to the optical phonon modes.
Reuse & Permissions
Figure 4
(a)–(c) Temperature dependence of resistivity and (d)–(f) optical conductivity spectra of ${(\Pr_{1 - x} {Ca}_{x})}_{2} {Ru}_{2} O_{7}, {({Gd}_{1 - x} {Cd}_{x})}_{2} {Ru}_{2} O_{7}, {({Ho}_{1 - x} {Cd}_{x})}_{2} {Ru}_{2} O_{7}$ for $x = 0, 0.1, 0.2, 0.3$ . The downward vertical arrows in (d) and (e) indicate the isosbestic-point energies. (g) Spectral weight transfer $N_{D}$ (yellow hatched region) across the isosbestic point ( $ℏ ω_{c}$ ) pointed as a black arrow. Optical conductivity spectra for $x = 0$ and 0.3 in ${(\Pr_{1 - x} {Ca}_{x})}_{2} {Ru}_{2} O_{7}$ are displayed as an example. (h) Doping level ( $x$ ) dependence of $N_{D}$ of ${(\Pr_{1 - x} {Ca}_{x})}_{2} {Ru}_{2} O_{7}$ (red circles) and ${({Gd}_{1 - x} {Cd}_{x})}_{2} {Ru}_{2} O_{7}$ (green circles). Dashed lines presume the relation that $N_{D} = C x$ in the low-doped region ( $x \leq 0.2$ ). (i) $C^{- 1}$ versus $R$ ionic radius $r$ . The dashed lines and white arrows are guides to the eye. For the definitions of $N_{D}$ and $C$ , see text.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics