Local electron correlation effects on the Fermiology of the weak itinerant ferromagnet ZrZn$_2$

TL;DR

This study compares DFT and DFT+DMFT calculations of ZrZn₂'s Fermi surface, revealing that many-body electron correlations significantly influence its topology and are better captured by DFT+DMFT, aligning more closely with experimental data.

Contribution

It demonstrates that electron correlation effects are crucial for accurately predicting the Fermi surface topology of ZrZn₂ using DFT+DMFT, improving agreement with experimental observations.

Findings

DFT+DMFT aligns better with experimental data than DFT.

Flat bands around high symmetry points are sensitive to electron correlations.

Positron annihilation can probe correlation-induced Fermi surface features.

Abstract

The Fermi surface topology plays an important role in the macroscopic properties of metals. It can be particularly sensitive to electron correlation, which appears to be especially significant for the weak itinerant ferromagnet ZrZn$_{2}$. Here, we look at the differences in the predicted Fermi surface sheets of this metallic compound in its paramagnetic phase for both density functional theory (DFT) and the combination of DFT with dynamical mean field theory (DFT+DMFT). The theoretical spectral functions evaluated at the Fermi level were used along with calculations of the electron-positron momentum density (also known as the two-photon momenutm density) in $k$-space to provide insights into the origin of certain features of the Fermi surface topology. We compare this two photon momentum density to that extracted from the positron annihilation experimental data (Phys. Rev. Lett. 92, 107003 (2004)). The DFT+DMFT densities are in better agreement with the experiment than the DFT, particularly with regard to the flat bands around the $L$ and $W$ high symmetry points. The experimental neck around $L$, which relates to a van Hove singularity, is present in DFT+DMFT but not in the DFT. We find that these flat bands, and as such the Fermi surface topology, are sensitive to the many body electron correlation description, and show that the positron annihilation technique is able to probe this. This description is significant for the observed behavior such as the Lifshiftz transition around the quantum critical point.