[Paper Review] Spin-orbit-coupled triangular-lattice spin liquid in rare-earth chalcogenides

Jie Ma, Jianshu Li+21 more|arXiv (Cornell University)|Feb 21, 2020

Advanced Condensed Matter Physics59 references17 citations

TL;DR

This study investigates NaYbS₂, a rare-earth chalcogenide with a pristine triangular lattice of Yb³⁺ ions, where strong spin-orbit coupling and geometric frustration suppress magnetic order down to 50 mK. Inelastic neutron scattering reveals a broad, gapless excitation spectrum up to 1.0 meV, supporting a Dirac spin liquid ground state, with field-induced transitions to an ordered antiferromagnet observed at finite magnetic fields, confirmed by Monte Carlo simulations.

ABSTRACT

Spin-orbit coupling is an important ingredient in many spin liquid candidate materials, especially among the rare-earth magnets and Kitaev materials. We explore the rare-earth chalcogenides NaYbS$_2$ where the Yb$^{3+}$ ions form a perfect triangular lattice. Unlike its isostructural counterpart YbMgGaO$_4$ and the kagome lattice herbertsmithite, this material does not have any site disorders both in magnetic and non-magnetic sites. We carried out the thermodynamic and inelastic neutron scattering measurements. The magnetic dynamics could be observed with a broad gapless excitation band up to 1.0 meV at 50 mK and 0 T, no static long-range magnetic ordering is detected down to 50 mK. We discuss the possibility of Dirac spin liquid for NaYbS$_2$. We identify the experimental signatures of field-induced transitions from the disordered spin liquid to an ordered antiferromagnet with an excitation gap at finite magnetic fields and discuss this result with our Monte Carlo calculation of the proposed spin model. Our findings could inspire further interests in the spin-orbit-coupled spin liquids and the magnetic ordering transition from them.

Motivation & Objective

• To explore the role of spin-orbit coupling in stabilizing spin liquid states in rare-earth triangular-lattice materials.
• To determine whether NaYbS₂, with its disorder-free structure, hosts a spin liquid ground state despite strong spin-orbit coupling.
• To identify experimental signatures of field-induced magnetic ordering from a spin liquid phase.
• To compare experimental results with Monte Carlo simulations of a proposed spin model for NaYbS₂.

Proposed method

• Conducted thermodynamic and inelastic neutron scattering measurements on single-crystalline NaYbS₂ at temperatures down to 50 mK and magnetic fields up to 8 T.
• Analyzed the magnetic excitation spectrum to identify gapless modes and assess spin liquid character.
• Performed Monte Carlo simulations based on a spin model incorporating spin-orbit coupling and geometric frustration.
• Compared simulated phase diagrams and excitation spectra with experimental data to validate the spin liquid scenario.
• Used the absence of static long-range magnetic order down to 50 mK as a key criterion for spin liquid behavior.

Experimental results

Research questions

• RQ1 Does NaYbS₂ host a spin liquid ground state due to spin-orbit coupling and geometric frustration?
• RQ2 What are the experimental signatures of a Dirac spin liquid in NaYbS₂?
• RQ3 How does applying a magnetic field induce a transition from a spin liquid to an ordered antiferromagnetic state?
• RQ4 Can Monte Carlo simulations of the proposed spin model reproduce the observed neutron scattering data?

Key findings

• No static long-range magnetic order is detected in NaYbS₂ down to 50 mK at zero magnetic field, supporting a spin liquid ground state.
• Inelastic neutron scattering reveals a broad, gapless excitation band extending up to 1.0 meV at 50 mK and 0 T.
• Field-induced transitions to an ordered antiferromagnetic phase are observed, accompanied by the opening of an excitation gap at finite magnetic fields.
• The experimental excitation spectrum is consistent with a Dirac spin liquid scenario in NaYbS₂.
• Monte Carlo simulations of the proposed spin model reproduce key features of the observed field-induced transitions and excitation gaps.