Fluctuating magnetism and Pomeranchuk effect in multilayer graphene

TL;DR

This study reveals that multilayer graphene exhibits fluctuating magnetic moments and a Pomeranchuk effect, with entropy signatures akin to disordered local moments, despite its itinerant electron nature, indicating a decoupling of spin and valley polarization.

Contribution

It demonstrates the presence of fluctuating magnetic moments and a Pomeranchuk effect in multilayer graphene, challenging the conventional understanding of magnetism in itinerant electron systems.

Findings

Entropy signatures similar to disordered local moments.

Observation of a Pomeranchuk effect in isospin.

Finite temperature resistance minimum linked to magnetic fluctuations.

Abstract

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wave functions in f- and d-atomic orbitals. In rhombohedral multilayer graphene (RMG), in contrast, magnetism-manifesting as spontaneous polarization into one or more spin and valley flavors[1-7]-originates from itinerant electrons near a Van Hove singularity. Here, we show experimentally that the electronic entropy in this system shows signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution $\Delta$ S $\approx$ 1k$_B$/charge carrier that onsets at the Curie temperature and survives over one order of magnitude in temperature. First order phase transitions show an isospin `Pomeranchuk effect' in which the fluctuating moment phase is entropically favored over the nearby symmetric Fermi liquid[8, 9]. Our results imply that despite the itinerant nature of the electron wave functions, the spin- and valley polarization of individual electrons are decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid 3He[10]. Transport measurements, surprisingly, show a finite temperature resistance minimum within the fluctuating moment regime, which we attribute to the interplay of fluctuating magnetic moments and electron phonon scattering. Our results highlight the universality of soft isospin modes to two-dimensional flat band systems.