Spin Fractionalization in a Kondo Lattice Heterostructure

Kondo lattices are materials in which an array of localized magnetic moments (usually f- electrons) interact with a delocalized band of conduction electrons. At low temperatures, a strong quantum entanglement, the so-called Kondo coherence, develops in these systems where each local moment forms a Kondo singlet with its neighboring conduction electrons. It has been suggested that spin-charge separation and spin-fractionalization are at play in Kondo lattices.

To shed more light on the phenomena of spin fractionalization, we study the dynamical spin susceptibility in a Kondo lattice using strong Kondo coupling expansion which reduces the problem to the solution of a multi-particle Schrodinger equation. The spin excitations can be described as a simultaneous formation of a pair of spin-1/2 doublon and a spin-1/2 holon, with some residual attractive interaction between the two that naturally leads to the formation of a bound-state.

The computed susceptibility exhibits a sharp spin-1 para-magnon excitation at low energies and a continuum of excitations into fractionalized spinons at higher energies. We show that spin fractionalization can be achieved by placing the Kondo lattice in proximity to a superconductor, where the condensate of Cooper pairs within the superconductor can tunnel into/out of the Kondo lattice, transforming holons and doublons into one another, and lowering the energy of the fractionalized regime compared to the magnon bound states. We present a long-wavelength continuum model description that enables us to generalize our result to two-dimensional systems, applicable to epitaxially grown Kondo heterostructure. Such fractionalization can in principle be detected using neutron scattering.