Co-existing Singlet and Ordered S=1/2 Moments in the Ground State of the Triclinic Quantum Magnet CuMoO4

TL;DR

CuMoO4 exhibits a unique ground state with coexisting non-magnetic singlet dimers and ordered S=1/2 moments, revealed through neutron scattering and heat capacity measurements, highlighting complex magnetic behavior in a triclinic quantum magnet.

Contribution

This study provides the first detailed experimental evidence of coexisting singlet and ordered moments in CuMoO4's ground state, combining neutron scattering and thermodynamic data.

Findings

Presence of a spin gap of ~2.3 meV indicating singlet dimers

Observation of antiferromagnetic order below T_N ~ 1.75 K

Coexistence of singlet and ordered S=1/2 moments in the ground state

Abstract

CuMoO4 is a triclinic quantum magnet based on S = 1/2 moments at the Cu2+ site. It has recently attracted interest due to the remarkable changes in its chromic and volumetric properties at high temperatures, and in its magnetic properties at low temperatures. This material exhibits a first order structural phase transition at T_C ~ 190 K as well as a magnetic phase transition at T_N ~ 1.75 K. We report low temperature heat capacity measurements as well as extensive elastic and inelastic neutron scattering measurements on powder samples taken above and below T_N. We observe neutron diffraction consistent with a simple (1/2, 0, 0) antiferromagnetic structure indicating a doubling of the a-axis periodicity below T_N. In addition, inelastic neutron scattering above a spin gap of ~ 2.3 meV is consistent with triplet excitations out of paired S = 1/2 moments which form singlet dimers. Low lying spin wave excitations are also observed and these originate from ordered S = 1/2 moments below T_N. Taken together these measurements show the ground state of CuMoO4 to display both non-magnetic singlets, and ferromagnetically-coupled spins coexisting within an antiferromagnetic structure below T_N ~ 1.75 K.