We present NMR measurements of a strong-leg spin-1/2 Heisenberg antiferromagnetic ladder compound (C7H10N)2CuBr4 under magnetic fields up to 15 T in the temperature range from 1.2 K down to 50 mK. From the splitting of NMR lines we determine the phase boundary and the order parameter of the low-temperature (3-dimensional) long-range-ordered phase. In the Tomonaga-Luttinger regime above the ordered phase, NMR relaxation reflects characteristic power-law decay of spin correlation functions as 1/T1 T^(1/2K-1), which allows us to determine the interaction parameter K as a function of field. We find that field-dependent K varies within the 1<K<2 range which signifies attractive interaction between the spinless fermions in the Tomonaga-Luttinger liquid.
We report 17O NMR measurements in the S=1/2 Cu2+ kagome antiferromagnet Herbertsmithite ZnCu3(OH)6Cl2 down to 45mK in magnetic fields ranging from 2T to 12T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments $lesssim 0.1muB$ and gapped excitations. The H-T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field mu0 Hc=1.55(25)T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagome Heisenberg antiferromagnet model.
