Quantum Spin Liquid in Herbertsmithite

We present a theory of herbertsmithite as a quantum spin liquid (QSL) made of chargeless fermionic spinons [1]. The essence of our theory is the notion of so-called fermion condensation quantum phase transition. Main manifestation of it is unlimited increase of the quasiparticles effective mass. We analyze the dynamic magnetic susceptibility of QSL compounds, and show that spinons form a continuum, and populate an approximately flat band crossing the Fermi level. Transport properties of QSL compounds shed light on their nature. We demonstrate that the thermal transport exhibits a scaling behavior, resembling that of heavy-fermion compounds, and reveal a strong magnetic field dependence of the effective mass. We propose the arrangement of the thermal transport measurements in magnetic field that could probe the low-lying elementary excitations, testing itinerant spinons excitations in Herbertsmithite. 1. M.Ya. Amusia, K.G. Popov, V.R. Shaginyan, and V. A. Stephanowich, \textit{Theory of Heavy-Fermion Compounds}, Springer Series in Solid-State Sciences \textbf{182,} (2014). .