Quantum paramagnet near spin-state transition

TL;DR

This paper investigates the quantum criticality near a spin-state transition in LaCoO3 with Sc substitution, revealing a superposition of spin states that could inform the design of novel materials.

Contribution

It experimentally demonstrates a nearly degenerate spin-state boundary in LaCoO3 induced by Sc substitution, highlighting quantum superposition effects.

Findings

Destabilization of low-spin state with Sc substitution

Observation of anomalous paramagnetic state

Evidence of quantum superposition of spin states

Abstract

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality.