Magnetic and Orbital Orders Coupled to Negative Thermal Expansion in Mott Insulators, Ca2Ru1-xMxO4 (M = Mn and Fe)

TL;DR

This study investigates how doping in Ca2RuO4 induces negative thermal expansion linked to electronic and magnetic phase transitions, revealing complex interactions among orbital, spin, and lattice degrees of freedom.

Contribution

It demonstrates that doping Ca2RuO4 with transition metals causes negative thermal expansion closely associated with metal-insulator and magnetic transitions, highlighting new physics in correlated electron systems.

Findings

Negative thermal expansion occurs near room temperature and up to 300 K.

Doping shifts metal-insulator transition and induces magnetism.

Negative thermal expansion correlates with electronic and magnetic phase changes.

Abstract

Ca2RuO4 is a structurally-driven Mott insulator with a metal-insulator transition at TMI = 357K, followed by a well-separated antiferromagnetic order at TN = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts TMI by weakening the orthorhombic distortion and induces either metamagnetism or magnetization reversal below TN. Moreover, M doping for Ru produces negative thermal expansion in Ca2Ru1-xMxO4 (M = Cr, Mn, Fe or Cu); the lattice volume expands on cooling with a total volume expansion ratio, {\Delta}V/V, reaching as high as 1%. The onset of the negative thermal expansion closely tracks TMI and TN, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval up to 300 K. These findings underscores new physics driven by a complex interplay between orbital, spin and lattice degrees of freedom.