美国加州大学圣巴巴拉分校Andrea F. Young团队研究了多层石墨烯中的波动磁性和波莫兰丘克效应。该项研究成果发表在2025年3月19日出版的《自然》杂志上。

磁性通常源于交换相互作用对f和d原子轨道中高度局域费米子波函数的影响。相比之下，在菱面体多层石墨烯（RMG）中，磁性——表现为自发极化为一种或多种自旋和谷味——起源于Van Hove奇点附近的巡回电子。

研究组通过实验表明，该系统中的电子熵表明了通常与无序局部磁矩相关的特征，这对于完全游离金属中的电子来说出乎意料。具体而言，他们发现每个电荷载流子的贡献ΔS≈1kB，起始于居里温度，并在温度下持续超过一个数量级。一阶相变显示出同位旋的“波莫兰丘克效应”，其中波动矩相比附近的对称费米液体更有熵。

该结果表明，尽管电子波函数具有巡回性质，但单个电子的自旋和谷极化是解耦的，这种现象通常与局域矩有关，例如在固体3He中发生的情况。令人惊讶的是，输运测量显示，在波动矩状态下存在有限的温度电阻最小值，研究组将其归因于波动磁矩和电子-声子散射的相互作用。该结果突显了软同位旋模对二维平带系统的普遍性。

附：英文原文

Title: Fluctuating magnetism and Pomeranchuk effect in multilayer graphene

Author: Holleis, Ludwig, Xie, Tian, Xu, Siyuan, Zhou, Haoxin, Patterson, Caitlin L., Panigrahi, Archisman, Taniguchi, Takashi, Watanabe, Kenji, Levitov, Leonid S., Jin, Chenhao, Berg, Erez, Young, Andrea F.

Issue&Volume: 2025-03-19

Abstract: Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wavefunctions in f- and d-atomic orbitals. By contrast, in rhombohedral multilayer graphene (RMG), magnetism—manifesting as spontaneous polarization into one or more spin and valley flavours1,2,3,4,5,6,7—originates from itinerant electrons near a Van Hove singularity. Here we show experimentally that the electronic entropy in this system indicates signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution ΔS≈1kB per charge carrier that begins at the Curie temperature and survives more than one order of magnitude in temperature. First-order phase transitions show an isospin ‘Pomeranchuk effect’ in which the fluctuating moment phase is entropically favoured over the nearby symmetric Fermi liquid8,9. Our results imply that, despite the itinerant nature of the electron wavefunctions, the spin and valley polarization of individual electrons is decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid 3He (ref.10). Transport measurements, surprisingly, show a finite-temperature resistance minimum in 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.

DOI: 10.1038/s41586-025-08725-5

Source: https://www.nature.com/articles/s41586-025-08725-5