Triplet and in-gap magnetic states in the ground state of the quantum frustrated FCC antiferromagnet Ba2YMoO6

TL;DR

This study investigates the magnetic excitations in the frustrated FCC antiferromagnet Ba2YMoO6, revealing a singlet-triplet excitation and in-gap states indicative of complex quantum magnetic behavior.

Contribution

It provides new neutron spectroscopy data showing the presence of singlet-triplet excitations and in-gap states, advancing understanding of quantum ground states in frustrated magnets.

Findings

Identification of a 28 meV singlet-triplet excitation

Observation of in-gap magnetic states resembling spin-polaron states

Absence of static long-range magnetic order at low temperatures

Abstract

The geometrically frustrated double perovskite Ba2YMoO6 is characterized by quantum s=1/2 spins at the Mo5+ sites of an undistorted face-centered cubic (FCC) lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a non-magnetic spin-singlet ground state. We report new time-of-flight and triple-axis neutron spectroscopy of Ba2YMoO6 that shows a 28 meV spin excitation with a bandwidth of ~4 meV, which vanishes above ~125 K. We identify this as the singlet-triplet excitation out of a singlet ground state, and further identify a weaker continuum of magnetic states within the gap, reminiscent of spin-polaron states arising due to weak disorder.