  We present a theory of the dynamic magnetic susceptibility of quantum spin
liquid. The obtained results are in good agreement with experimental facts
collected on herbertsmithite ZnCu3(OH)6Cl2 and on heavy-fermion metals, and
allow us to predict a new scaling in magnetic fields in the dynamic
susceptibility. Under the application of strong magnetic fields quantum spin
liquid becomes completely polarized. We show that this polarization can be
viewed as a manifestation of gapped excitations when investigating the
spin-lattice relaxation rate.
