Quantum Criticality of Spin Liquids in Novel Insulators and Magnets

Abstract

Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics, whose realization in some compounds is still under consideration. Quantum spin liquids are a promising new phases, where exotic quantum states of matter could be realized. Exotic quantum spin liquid (QSL) made of such hypothetic particles as fermionic spinons which carry spin 1/2 and no charge are considered in this chapter. Magnetic insulators with geometrical frustration produce important experimental facts shedding light on the nature of quantum spin liquid composed of spinons. We present a theory of the thermodynamic properties of quantum spin liquids, elucidating how their properties are affected by magnetic fields and describe as an example the experimental data for the herbertsmithite and HF metals. We show that the above insulators can be viewed as HF compounds, whose low temperature thermodynamics in magnetic fields is determined by a Fermi quantum spin liquid. These properties allow us to reveal their scaling behavior, which strongly resembles that observed in HF metals and two-dimensional 3 He. We also describe the dynamic magnetic susceptibility which allows us to reveal that at low temperatures quasiparticles excitations, or spinons, form a continuum, and populate an approximately flat band crossing the Fermi level. The obtained results show that the properties of compounds with quantum spin liquid are similar to those of HF metals. Thus, the compounds can be viewed as a new type of strongly correlated HF electrical insulator that possesses properties of HF metals with one exception: it resists a flow of electric charge. Transport properties of the compounds shed light on the nature of quantum spin liquid. Analysis of the heat conductivity detects its scaling behavior resembling those of both the spin-lattice relaxation rate and the magnetoresistivity. It reveals a strong magnetic field dependence of the spinons effective mass. As a result, the strongly correlated electrical insulator gains also a new magnetic feature of the matter, for the spins represented by the deconfined QSL get mobility. We show that the crystal keeps all properties of solids, but in the magnetic relation shows fluidity.