Condensed Matter Seminar: "In search of exotic fermions in spin-liquids - the case of a-RuCl3"

In 2006, Alexei Kitaev proposed that certain 2D honeycomb magnets with a strong spin-orbit coupling in the low-spin (S=1/2) ground state and a high degree of bond anisotropy will form a quantum-spin-liquid (QSL). This QSL is special since its excitations can be solved exactly to predict the presence of 2D Majorana Fermions. If realized in a solid-state material, these hold the promise for the technology of lossless topological qubits that survive to high temperatures. In this talk, I will present the search of these elusive quasi-particles in a two-dimensional (2D) honeycomb magnet a-RuCl3. Using a combination of thermodynamic, diffraction and neutron-scattering measurements in powder and single-crystal samples, we delve in detail the ground-state in the material, and show that this material has the ingredients necessary for satisfying the Heisenberg-Kitaev Hamiltonian. A spin-wave analysis yields that anisotropic Kitaev interactions are indeed the leading-order coupling in this material. Although the ground state is antiferromagnetic at lowest temperatures, this material shows a broad excitation spectrum independent of the long-range ordering, - which could not be explained with standard classical theories. Excitingly, when compared with the exact quantum calculations of the pure Kitaev QSL ground state, the data matches the broad fractionalized excitations expected from itinerant Majorana Fermions.