Incommensurate quantum magnet based on 4f-electron in a zigzag spin-1/2 chain of YbCuS$_2$

TL;DR

This study reveals an incommensurate elliptic helical magnetic structure in YbCuS2, a rare-earth zigzag chain magnet, combining neutron diffraction experiments with theoretical modeling to understand its quantum magnetic properties.

Contribution

It reports the discovery of an incommensurate magnetic order in YbCuS2 and links experimental findings with theoretical models to explain its quantum magnetic behavior.

Findings

Incommensurate elliptic helical magnetic structure observed.

Magnetic moments are smaller than expected for Yb$^{3+}$ ground state.

Field-induced up-up-down magnetic order at 7.5 T.

Abstract

We performed high-resolution powder neutron diffraction experiments and discovered an elliptic helical incommensurate magnetic structure in the semiconducting rare-earth magnet YbCuS2, featuring effective spin-1/2 Yb$^{3+}$ ions that form a zigzag chain. Upon cooling the sample to 0.2 K, we observed very weak magnetic peaks indexed with an incommensurate propagation vector k = [0, 0.305, 0] along the zigzag chain. The magnitude of the magnetic moment is at least one-third smaller than the expected value for the Yb$^{3+}$ Kramers doublet ground state. In an applied magnetic field, up-up-down magnetic order was observed at 7.5 T, characterized by diffraction peaks indexed with k = [0, 1/3, 0] and substantial uniform magnetic components. These observations agree well with theoretical calculations based on the density matrix renormalization group for a zigzag spin-1/2 model with isotropic Heisenberg interactions and off-diagonal symmetric $\Gamma$-type exchange interactions derived from material parameters. The theory elucidates the quantum mechanical nature of the incommensurate magnetism as remnant off-diagonal spin correlations in a nematic dimer-singlet state.