S=1/2 kagome antiferromagnet herbertsmithite: O-NMR, specific heat in high magnetic fields and thermal transport (Quentin Barthélemy / Seminar / LNCMI). – 20/10/2022, 14H

Quentin Barthélemy
(Institut Quantique, Département de physique & RQMP, Université de Sherbrooke, Sherbrooke, Québec, Canada)

Summary
Herbertsmithite ZnCu3(OH)6Cl2 is an emblematic quantum spin liquid candidate because it is the closest materialization of the nearest neighbor S = 1/2 kagome Heisenberg antiferromagnet (J ∼ 190 K) with a dynamical ground state. As main perturbations, we can list copper ions on the zinc sites between the kagome planes and a finite out-of-plane Dzyaloshinskii-Moriya component (Dz ∼ 0.06 J). First, I will present some selected developments of our 17O NMR study of a herbertsmithitesingle crystal, where we measured the low temperature static spin susceptibility and spin dynamics [1]. It allows us to show unambiguously that the spin excitation spectrum is
gapless, restoring some convergence with a Dirac cone model now advocated for in most of numerical works. Then, I will present our high field specific heat study of protonated and deuterated herbertsmithite single crystals [2]. As a fundamentalthermodynamic quantity, the low-temperature specific heat is a powerful probe of any low-energy excitation, which is both its strength and weakness. Indeed, for herbertsmithite in zero to moderate fields, the kagome contribution is masked by a contribution from magnetic defects. The use of high magnetic fields allows us, for the first time, to get rid of this parasitic contribution and single out the kagome behavior. We show that this behavior is attributed to gapless excitationswhich are unaffected by the magnetic field, at variance with predictions for fermionic spinons. Our observations are well reproduced by state-of-the-art numerical methods but challenge all the existing models so far. The proposed spin liquid ground state remains enigmatic but our study provides a delimited path for future theoretical developments. At last, I will present our recent thermal conductivity study of protonated and deuterated herbertsmithite single crystals, demonstrating theabsence of sizeable spinon heat transport and highlighting for the first time the emergence of a characteristic scale below about 20 K [3].

[1] P. Khuntia et al., Nature Physics 16, 469 (2020)
[2] Q. Barthélemy et al., Physical Review X 12, 011014 (2022)
[3] Q. Barthélemy et al., in preparation.