[Paper Review] Spectrum and low-temperature bulk properties of triangular quantum spin liquid candidate NaYbSe$_2$

A. O. Scheie, Minseong Lee+19 more|arXiv (Cornell University)|Jun 25, 2024

Advanced Condensed Matter Physics5 citations

TL;DR

The study uses neutron scattering, AC calorimetry under pressure, and AC susceptibility to show a gapped quantum spin liquid state in NaYbSe2 on a triangular lattice, with a ~2.1 μeV gap and no magnetic order down to 100 mK.

ABSTRACT

We report neutron scattering, pressure-dependent AC calorimetry, and AC magnetic susceptibility measurements of triangular lattice NaYbSe$_2$. We observe a continuum of scattering, which is reproduced by matrix product simulations, and no phase transition is detected in any bulk measurements. Comparison to heat capacity simulations suggest the material is within the Heisenberg spin liquid phase. AC Susceptibility shows a frequency-dependent peak at 40 mK, as has been observed in several triangular magnets.

Motivation & Objective

• Motivate the search for quantum spin liquids on triangular lattices, particularly in Yb-based delafossites.
• Determine whether NaYbSe2 hosts a QSL ground state by combining spectroscopic and thermodynamic probes.
• Identify the presence and size of a low-energy spin gap and relate it to theoretical QSL models.
• Assess proximity to QSL phases by comparing NaYbSe2 to KYbSe2 and other AYbSe2 compounds.

Proposed method

• Inelastic neutron scattering to map the dispersion and search for a spin continuum and gaps.
• AC calorimetry under hydrostatic pressure to look for thermodynamic signatures of ordering transitions.
• AC magnetic susceptibility to detect low-temperature gaps and potential ordering.
• Quantum many-body simulations (matrix product states and TPQ) to model J1–J2 triangular lattice Hamiltonians and interpret spectra and specific heat.
• Comparative analysis with KYbSe2 and other A YbSe2 compounds to gauge proximity to QSL phases.

Experimental results

Research questions

• RQ1 Does NaYbSe2 exhibit long-range magnetic order at low temperatures or under pressure?
• RQ2 Is the magnetic excitation spectrum gapped, and if so, what is the gap size?
• RQ3 What is the appropriate spin Hamiltonian (J1–J2 with anisotropy) that describes NaYbSe2, and is it in the QSL phase?
• RQ4 Does NaYbSe2 correspond to a gapped Z2 QSL rather than a gapless U(1) Dirac QSL or other competing phases?

Key findings

• NaYbSe2 shows a highly dispersive continuum in neutron scattering with no static magnetic order down to 100 mK.
• AC susceptibility indicates a low-energy gap of approximately 2.1 μeV.
• Heat capacity and TPQ/MPS simulations place NaYbSe2 near or inside the triangular QSL region, consistent with a larger J2/J1 (~0.07) and easy-plane anisotropy.
• The data rule out a gapless U(1) Dirac QSL, favoring a gapped Z2 QSL as the natural explanation for the spectrum.
• Under pressures up to 2 GPa, no sharp ordering transition is observed in AC calorimetry.
• The neutron spectra are qualitatively consistent with a J1–J2 model with easy-plane anisotropy, and MPS spectra support this interpretation within finite-size limitations.