Charge density wave-templated spin cycloid in topological semimetal $NdSb_{x}Te_{2-x-\delta}$

TL;DR

This study reveals how charge density waves influence magnetic structures in NdSb_xTe_{2-x-δ}, showing a coupling between CDW and spin cycloid formation in a topological semimetal.

Contribution

It demonstrates the first detailed magnetic structure within the CDW regime of NdSb_xTe_{2-x-δ}, highlighting the coupling mechanism via conduction electrons.

Findings

Charge density wave modulates magnetic order.

Elliptical spin cycloid observed with specific wavevector.

Coupling between CDW and magnetic structure via conduction electrons.

Abstract

Magnetic topological semimetals present open questions regarding the interplay of crystal symmetry, magnetism, band topology, and electron correlations. $LnSb_{x}Te_{2-x-\delta}$ (Ln= lanthanide) is a family of square-net-derived topological semimetals that allows compositional control of band filling, and access to different topological states via an evolving charge density wave (CDW) distortion. Previously studied Gd and Ce members containing a CDW have shown complex magnetic phase diagrams, which implied that spins localized on Ln interact with the CDW, but to this date, no magnetic structures have been solved within the CDW regime of this family of compounds. Here, we report on the interplay of the CDW with magnetism in $NdSb_{x}Te_{2-x-\delta}$, by comparing the undistorted square net member $NdSb_{0.94}Te_{0.92}$ with the CDW-distorted phase $NdSb_{0.48}Te_{1.37}$, via single-crystal x-ray diffraction, magnetometry, heat capacity, and neutron powder diffraction. $NdSb_{0.94}Te_{0.92}$ is a collinear antiferromagnet with $T_N$ $\sim$ 2.7 K, where spins align antiparallel to each other, but parallel to the square net of the nuclear structure. $NdSb_{0.48}Te_{1.37}$ exhibits a nearly five-fold modulated CDW ($q_{CDW}=0.18b^*$), isostructural to other $LnSb_{x}Te_{2-x-\delta}$ at similar x. $NdSb_{0.48}Te_{1.37}$ displays more complex magnetism with $T_N = 2.3 K$, additional metamagnetic transitions, and an elliptical cycloid magnetic structure with $q_{mag}=-0.41b^*$.The magnitudes of $q_{CDW}$ and $q_{mag}$ exhibit a integer relationship, $1+2q_{mag}=q_{CDW}$, implying a coupling between the CDW and magnetic structure. Given that the CDW is localized within the nonmagnetic distorted square net, we propose that conduction electrons "template" the spin modulation via the Ruderman-Kittel-Kasuya-Yosida interaction.