Quantum and thermal fluctuations in a two-dimensional correlated band ferromagnet -- Goldstone-mode preserving investigation with self-energy and vertex corrections

TL;DR

This paper investigates ferromagnetism in a two-dimensional Hubbard model, incorporating correlation effects with a Goldstone-mode-preserving scheme, revealing how band structure and fluctuations influence magnetic stability and finite-temperature dynamics.

Contribution

It introduces a novel, Goldstone-mode-preserving approach to study correlation effects and fluctuations in 2D ferromagnets, enhancing understanding of ferromagnetic stability near van Hove filling.

Findings

Correlation effects significantly influence ferromagnetic stability.

Band dispersion and fluctuations behave differently at various wavelengths.

Thermal spin fluctuations critically affect magnetization decay and T_c in low dimensions.

Abstract

Ferromagnetism in the t-t' Hubbard model is investigated on a square lattice. Correlation effects in the form of self-energy and vertex corrections are systematically incorporated within a spin-rotationally-symmetric scheme which explicitly preserves the Goldstone mode and is therefore in accord with the Mermin-Wagner theorem. Interplay of band dispersion and correlation effects on ferromagnetic-state stability are highlighted with respect to both long- and short-wavelength fluctuations, which are shown to have substantially different behaviour. Our approach provides a novel understanding of the enhancement of ferromagnetism near van Hove filling for t'~0.5 in terms of strongly suppressed saddle-point contribution to the destabilizing exchange part of spin stiffness. Finite-temperature electron spin dynamics is investigated directly in terms of spectral-weight transfer across the Fermi energy due to electron-magnon coupling. Relevant in the context of recent magnetization measurements on ultrathin films, the role of strong thermal spin fluctuations in low dimensions is highlighted, in the anisotropy-stabilized ordered state, by determining the thermal decay of magnetization and T_c within a renormalized spin-fluctuation theory.