Gapless quantum spin liquid in the triangular system Sr$_{3}$CuSb$_{2}$O$_{9}$

TL;DR

This paper reports the discovery of a gapless quantum spin liquid state in the layered triangular Sr$_{3}$CuSb$_{2}$O$_{9}$ system, characterized by persistent spin dynamics and specific heat behavior indicative of Dirac spinon excitations.

Contribution

It provides experimental evidence of a gapless quantum spin liquid in a triangular lattice system with detailed thermodynamic and magnetic measurements, and links these findings to theoretical models.

Findings

No magnetic order down to 0.35 K despite large antiferromagnetic interactions.

Magnetic specific heat shows a linear plus quadratic temperature dependence.

Estimated exchange scale (~36 K) aligns with theoretical calculations.

Abstract

We report gapless quantum spin liquid behavior in the layered triangular Sr$_{3}$CuSb$_{2}$O$_{9}$ (SCSO) system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well-separated by Sb planes. Muon spin relaxation ($\mu$SR) measurements show that the $S = \frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature $\theta_{\mathrm{cw}} \simeq $ -143 K as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat ($\mathit{C}$$_{\mathrm{m}}$) below 5 K reveals a $\mathit{C}$$_{\mathrm{m}}$ $=$ $\gamma T$ + $\alpha T$$^{2}$ behavior. The significant $T$$^{2}$ contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-$T$ specific heat data, we estimate the dominant exchange scale to be $\sim $ 36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations ($\sim $ 45 K) as well as high-temperature susceptibility analysis ($\sim$ 70 K). The linear specific heat coefficient is about 18 mJ/mol-K$^2$ which is somewhat larger than for typical Fermi liquids.