Orbital-Exchange and Fractional Quantum Number Excitations in an f-electron Metal Yb$_2$Pt$_2$Pb

TL;DR

This study reveals fractionalized quantum excitations called orbital-spinons in the f-electron metal Yb₂Pt₂Pb, challenging the notion that such phenomena are exclusive to 3d transition metals, and introduces a new charge-orbital separation concept.

Contribution

The paper demonstrates the existence of orbital-spinons in a 4f-electron system, showing fractionalization of electrons into charge and orbital components, a phenomenon previously thought limited to 3d systems.

Findings

Observation of a magnetic continuum indicating spinons with fractional spin-1/2

Detection of only longitudinal spin fluctuations, no transverse excitations

Identification of charge-orbital separation as a new fractionalization phenomenon

Abstract

Exotic quantum states and fractionalized magnetic excitations, such as spinons in one-dimensional chains, are generally viewed as belonging to the domain of 3d transition metal systems with spins 1/2. Our neutron scattering experiments on the 4f-electron metal Yb$_2$Pt$_2$Pb overturn this common wisdom. We observe broad magnetic continuum dispersing in only one direction, which indicates that the underlying elementary excitations are spinons carrying fractional spin-1/2. These spinons are the quantum dynamics of the anisotropic, orbital-dominated Yb moments, and thus these effective quantum spins are emergent variables that encode the electronic orbitals. The unique birthmark of their unusual origin is that only longitudinal spin fluctuations are measurable, while the transverse excitations such as spin waves are virtually invisible to magnetic neutron scattering. The proliferation of these orbital-spinons strips the electrons of their orbital identity, and we thus report here a new electron fractionalization phenomenon, charge-orbital separation.