Agenda

Fifteen years ago, Kitaev proposed an exactly solvable model for S =1/2 Ising spins on a two-dimensional honeycomb lattice, with bond-directional anisotropic interactions, which result in a topological quantum-spin liquid (QSL) and emergent Majorana fermions. α-RuCl3 belonging to the class of two-dimensional tri-halides, forms a honeycomb lattice with weak interlayer coupling only and was proposed to be a prime candidate for the realization of Kitaev physics. However, α-RuCl3 is an antiferromagnet at low temperatures and exhibits three-dimensional magnetic order, preempting the exotic QSL ground state. Nevertheless, fractionalized quasiparticles, composed of itinerant and localized Majorana fermions still could exist at temperatures above magnetic order or just beyond a quantum critical point close to 7 T where the magnetic phase transition can be suppressed by in-plane magnetic fields.

After a broad introduction into spin liquids, into the physics of Majorana fermions and the specific characteristics of the title compound, I will discuss spectroscopic evidence of fractionalized spin excitations focusing on time-domain THz spectroscopy in α-RuCl3 in zero fields and for external magnetic fields up to 30 T following the temperature and field evolution of an extended magnetic continuum. In addition, I will report on detailed heat- capacity experiments as function temperature and magnetic field to identify a thermodynamic signature of these exotic excitations. It is found that excess heat capacity a possible fingerprint for fractionalized spin excitations exists well above magnetic order and in external magnetic fields beyond quantum criticality. In these experiments, the phonon reference turned out to be a highly problematic issue and specific care was taken to calculate the purely phonon-derived heat capacity.