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

Abstract

We report gapless quantum spin liquid behavior in the layered triangular Sr3CuSb2O9 (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 (µSR) measurements show that the S = 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 θcw −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 (C m) below 5 K reveals a C m = γT + αT 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 ∼ 36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations (∼ 45 K) as well as high-temperature susceptibility analysis (∼ 70 K). The linear specific heat coefficient is about 18 mJ/mol-K 2 which is somewhat larger than for typical Fermi liquids.