Specific heat of the kagome antiferromagnet herbertsmithite in high magnetic fields

Herbertsmithite ZnCu3(OH)6Cl2 is an emblematic quantum spin liquid candidate because it is the closest materialization of the nearest neighbor S = ½ kagome Heisenberg antiferromagnet (J ∼ 190 K) with a dynamical ground state. As main perturbations, we can mention copper ions on the zinc sites between the kagome planes and a finite out-of-plane Dzyaloshinskii-Moriya component (Dz ∼ 0.06J).

As a fundamental thermodynamic 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 excitations which 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.