Theory of magnetic excitations in the Kondo lattice

TL;DR

This paper develops a theoretical model for magnetic excitations in the Kondo lattice, capturing spin and charge gaps with a simplified approximation that aligns well with numerical results.

Contribution

It introduces an effective Hamiltonian describing fermionic charge fluctuations and bosonic spin fluctuations, providing a quantitative understanding of spin and charge gaps in Kondo insulators.

Findings

Quantitative description of spin gap and charge gap

Effective Hamiltonian for spin and charge excitations

Simplest approximation matches numerical results

Abstract

We present a theory for the spin excitations of the Kondo lattice. We derive an effective Hamiltonian, which describes Fermionic spin 1/2 charge fluctuations interacting with Bosonic triplet spin fluctuations. We show that already the simplest approximation possible, i.e. replacing the magnon self-energy by the free polarization bubble for the charge excitation Greens function, gives a quantitative description of the `spin gap' and `charge gap' as obtained in numerical calculations for the Kondo insulator.