Transport and Thermodynamic Evidence for a Marginal Fermi Liquid State in ZrZn$_2$

TL;DR

This study presents transport and thermodynamic measurements on ZrZn$_2$, revealing non-Fermi liquid behavior consistent with a marginal Fermi liquid state near ferromagnetic quantum criticality.

Contribution

It provides experimental evidence supporting a mean-field model of enhanced spin fluctuations leading to a marginal Fermi liquid state in ZrZn$_2$.

Findings

Resistivity follows a $T^{5/3}$ dependence in zero field.

Thermal conductivity shows a linear in $T$ difference from electrical resistivity.

Heat capacity exhibits an upturn at low temperatures.

Abstract

Measurements of low temperature transport and thermodynamic properties have been used to characterize the non-Fermi liquid state of the itinerant ferromagnet ZrZn$_2$. We observe a $T^{5/3}$ temperature dependence of the electrical resistivity at zero field, which becomes $T^2$ like in an applied field of 9 T. In zero field we also measured the thermal conductivity, and we see a novel linear in $T$ dependence of the difference between the thermal and electrical resistivities. Heat capacity measurements, also at zero field, reveal an upturn in the electronic contribution at low temperatures when the phonon term is subtracted. Taken together, we argue that these properties are consistent with a marginal Fermi liquid state which is predicted by a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions. We compare our data to quantitative predictions and establish this model as a compelling theoretical framework for understanding ZrZn$_2$.