Metal-Insulator Transition and Orbital Order in PbRuO3

TL;DR

PbRuO3 exhibits a rare metal-insulator transition at 90 K linked to structural changes and orbital ordering, driven by hybridization and spin-orbit coupling, revealing a novel ground state in ruthenates.

Contribution

This study uncovers a unique metal-insulator transition coupled with orbital order and structural change in PbRuO3, highlighting unconventional symmetry reversal and electronic stabilization mechanisms.

Findings

Metal-insulator transition at 90 K with four orders of magnitude resistivity increase

Structural transition from Pnma to Imma phase at low temperature

Orbital hybridization and spin-orbit coupling stabilize a new ground state

Abstract

Anomalous low temperature electronic and structural behaviour has been discovered in PbRuO3. The structure (space group Pnma, a = 5.56314(1), b = 7.86468(1), c = 5.61430(1) A) and metallic conductivity at 290 K are similar to those of SrRuO3 and other ruthenate perovskites, but a sharp metal-insulator transition at which the resistivity increases by four orders of magnitude is discovered at 90 K. This is accompanied by a first order structural transition to an Imma phase (a = 5.56962(1), b = 7.74550(1), c = 5.66208(1) A at 25 K) that shows a coupling of Ru4+ 4d orbital order to distortions from Pb2+ 6s6p orbital hybridization. The Pnma to Imma transition is an unconventional reversal of the group-subgroup symmetry relationship. No long range magnetic order is evident down to 1.5 K. Electronic structure calculations show that hybridization of Pb 6s6p and Ru 4d orbitals and strong spin-orbit coupling stabilise this previously hidden ground state for ruthenate perovskites