arXiv subjects

M. Frakulla

Publications and source records attributed to M. Frakulla.

3 recordsLinked to original sources

arXiv2026

Canonical strong coupling spin wave expansion of Kondo lattice magnets. II. Itinerant ferromagnets and topological magnon bands

In this paper we apply the canonical spin wave theory developed for itinerant Kondo lattice magnets in the strong coupling regime to Kondo ferromagnets, and address two general questions pertaining to their magnetic excitations. First, we compute corrections to the strong coupling (i.e., double-exchange) spin wave dispersion of itinerant ferromagnets. We show that the spin wave dispersion beyond the strong coupling limit can be mapped to the spin wave dispersion of a Heisenberg ferromagnet with farther neighbor exchange couplings, and discuss how this affects instabilities towards antiferromagnetism. Second, we examine the effect of including electronic spin-orbit coupling in the spin wave theory of Kondo ferromagnets. Including spin-orbit coupling is natural and straightforward in the formulation of the canonical spin wave expansion. Our key result is to demonstrate that the linear spin wave Hamiltonian of the itinerant Kondo ferromagnet can be mapped to the spin wave Hamiltonian of a Heisenberg ferromagnet with easy-axis Ising anisotropy and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We show that in the case of the Kane-Mele honeycomb lattice Kondo ferromagnet this leads to topological magnon bands, and discuss the implications of this result for itinerant ferromagnets more broadly.

cond-mat.str-el↗

arXiv2024

Canonical strong coupling spin wave expansion of Kondo lattice magnets. I. Effective Hamiltonian via canonical transformation

This paper develops a systematic strong coupling spin wave expansion of itinerant Kondo lattice magnets, magnets in which local moment spins are Kondo coupled to itinerant charge degrees of freedom. The strong coupling expansion is based on a canonical Schrieffer-Wolff transformation of the Hamiltonian, which is performed after $1/S$ expansion of the local moments and determined iteratively by requiring that spin-flip terms are removed at each order. We demonstrate that the canonical transformation can be viewed as an order-by-order diagonalization of the quantum Kondo coupling -- the dominant term in the strong coupling regime. A consequence is that the transformed electron operators correspond to electrons in a state of total spin $S\pm 1/2$ with the local moments, and the transformed boson operators describe spin wave excitations of the total local spin. We show that the electron degrees of freedom can be thought of as tightly bound spin polarons. We further show that the strong coupling spin wave expansion is readily extended to include the effects of spin-orbit coupling or electron pairing.

cond-mat.str-el↗

arXiv2024

Kondo-Heisenberg toy models: Comparison of exact results and spin wave expansion

In this paper we study a class of exactly solvable Kondo-Heisenberg toy models in one dimension, with the goal of comparing the exact low-energy excitations of the ferromagnetic ground state to the approximate solution obtained from spin wave theory. In doing so we employ a recently introduced strong coupling $1/S$ spin wave expansion, which effectively describes excitations of the total spin $S\pm 1/2$ on a given site (i.e., sum of local moment and electron spin). We further make use of the fact that the ground state of Kondo lattice models with quantum spins and a single electron is a ferromagnet, and that the magnetic excitations of the ferromagnet can be exactly determined. We demonstrate that the energies and eigenstates of the spin waves are in full agreement with the exact solution order-by-order in $1/S$ and $t/J_K$, the strong coupling expansion parameter. In the specific case of antiferromagnetic Kondo coupling, when the exact ground state wave function describes spin polaron, we show that the electron operators of the spin wave formalism precisely correspond to the spin polaron states. More broadly, the study of Kondo-Heisenberg toy models is shown to provide insight into the fundamental distinction between itinerant Kondo magnets and Heisenberg magnets.

cond-mat.str-el↗