Continuous magnetic phase transition in half-frustrated Ca2Os2O7

TL;DR

This study investigates the magnetic and electronic phase transition in Ca2Os2O7, revealing a second-order transition to a ferrimagnetic insulator at 327 K with detailed experimental and theoretical insights.

Contribution

It provides the first comprehensive experimental and first-principles analysis of the magnetic transition and insulating state in Ca2Os2O7, highlighting the roles of AFM correlation, Coulomb repulsion, and non-collinear interactions.

Findings

Transition at 327 K is second-order with a specific heat peak.

Low-temperature electronic specific heat indicates a full energy gap.

Theoretical calculations show AFM correlation and frustration are key to the insulating state.

Abstract

We present the specific heat, magnetization, optical spectroscopy measurements and the firstprinciple calculations on the Weberite structure Ca2Os2O7 single crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A \lambda-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, small amount of itinerant carriers with short life time \tau are observed, which is gapped at 20 K with a direct gap of 0:24 eV . Our first principle calculations indicate that the anti-ferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t2g bands and push them away from Fermi level(EF). On the other hand, a non-collinear magnetic interaction is needed to push the Os(4c) bands away from EF, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and non-collinear interaction all play important roles for the insulating ground state in Ca2Os2O7.