[Paper Review] Evidence of Dirac Quantum Spin Liquid in YbZn2GaO5

Rabindranath Bag, Sijie Xu+8 more|arXiv (Cornell University)|May 31, 2023

Advanced Condensed Matter Physics7 citations

TL;DR

The paper presents thermodynamic and inelastic neutron scattering evidence that YbZn2GaO5 hosts a U(1) Dirac quantum spin liquid on a triangular lattice, with a spin-1/2 ground state and Dirac-like spinon excitations. The results align with a J1-J2 XXZ model and show a T^2 heat capacity and gap features at Gamma but gapless spectra at M and K.

ABSTRACT

The emergence of a quantum spin liquid (QSL), a state of matter that can result when electron spins are highly correlated but do not become ordered, has been the subject of a considerable body of research in condensed matter physics. Spin liquid states have been proposed as hosts for high-temperature superconductivity and can host topological properties with potential applications in quantum information science. The excitations of most quantum spin liquids are not conventional spin waves but rather quasiparticles known as spinons, whose existence is well established experimentally only in one-dimensional systems; the unambiguous experimental realization of QSL behavior in higher dimensions remains challenging. Here, we investigate the novel compound YbZn2GaO5, which hosts an ideal triangular lattice of effective spin-1/2 moments with no detectable inherent chemical disorder. Thermodynamic and inelastic neutron scattering measurements performed on high-quality single crystal samples of YbZn2GaO5 exclude the possibility of long-range magnetic ordering down to 0.06 K, demonstrate a quadratic power law for the specific heat and reveal a continuum of magnetic excitations in parts of the Brillouin zone. Both low-temperature thermodynamics and inelastic neutron scattering spectra suggest that YbZn2GaO5 is a U(1) Dirac QSL with spinon excitations concentrated at certain points in the Brillouin zone. We advanced these results by performing additional specific heat measurements under finite fields, further confirming the theoretical expectations for a Dirac QSL on the triangular lattice of YbZn2GaO5.

Motivation & Objective

• Motivate the search for intrinsic quantum spin liquid states in two-dimensional triangular lattices without chemical disorder.
• Characterize the ground state of YbZn2GaO5 via thermodynamics and neutron scattering.
• Differentiate between Dirac spin liquid and spinon Fermi surface scenarios in this system.
• Assess how the J1-J2 XXZ model captures the observed spectra and thermodynamics.

Proposed method

• Synthesize high-quality single crystals of YbZn2GaO5 and verify structural purity to rule out disorder.
• Measure ultra-low temperature heat capacity and extract magnetic contribution showing a T^2 dependence.
• Perform inelastic neutron scattering on powder and single crystals to map low-energy excitations and identify gaps at high-symmetry points.
• Fit crystal field levels from INS to confirm a Kramers doublet ground state with effective spin-1/2.
• Compute dynamical structure factors S(q,ω) for the J1-J2 XXZ model using large-scale matrix product state simulations (DMRG/TDVP) and compare with INS data.
• Use a J2/J1 ≈ 0.12 and Δ ≈ 1.35 to reproduce observed spectra and gap behavior at Γ vs M/K.

Experimental results

Research questions

• RQ1 Does YbZn2GaO5 lack long-range magnetic order down to the lowest measured temperatures?
• RQ2 Is the low-energy excitation spectrum consistent with a U(1) Dirac quantum spin liquid rather than a spinon Fermi surface state or disorder-induced effects?
• RQ3 Can a J1-J2 XXZ model with small J2/J1 reproduce the INS spectra and the Γ-point gap while showing gapless excitations at M and K?
• RQ4 Does the heat capacity follow a T^2 scaling indicative of Dirac spinons in a triangular lattice QSL?

Key findings

• YbZn2GaO5 shows no magnetic ordering down to 0.06 K.
• Magnetic heat capacity scales as T^2 at low temperature, supporting a U(1) Dirac QSL interpretation.
• INS reveals a gap near the Γ point while exhibiting gapless continuum at M and K, consistent with Dirac spinons at those points.
• CEF fitting confirms a Kramers doublet ground state with effective spin-1/2 for Yb3+ and a first excited state above 441 K.
• Theoretical S(q,ω) from a J1-J2 XXZ model (J2/J1=0.12, Δ=1.35) agrees with the observed spectra in relevant energy ranges.
• A disorder-free triangular lattice Yb-based compound exhibits signatures aligning with a U(1) Dirac QSL rather than spinon Fermi surface behavior.