Isospin Pomeranchuk effect in twisted bilayer graphene

• Published in: Nature (London) Vol. 592; no. 7853; pp. 220 - 224
• Main Authors: Saito, Yu, Yang, Fangyuan, Ge, Jingyuan, Liu, Xiaoxue, Taniguchi, Takashi, Watanabe, Kenji, Li, J. I. A., Berg, Erez, Young, Andrea F.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 08-04-2021; Nature Publishing Group
• Subjects: 639/301/119/2795; 639/301/119/544; Bilayers; Condensed matter physics; Electrical resistivity; Entropy; Fermi liquids; Ferromagnetic phases; Ferromagnetism; Graphene; High temperature; Humanities and Social Sciences; Liquid phases; Low temperature; Magnetic fields; Magnetic moments; Magnetism; multidisciplinary; Phase transitions; Phenomenology; Science; Science (multidisciplinary); Solid phases; Spin dynamics; Superconductivity; Superlattices; Symmetry; Unit cell
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-03409-2

In condensed-matter systems, higher temperatures typically disfavour ordered phases, leading to an upper critical temperature for magnetism, superconductivity and other phenomena. An exception is the Pomeranchuk effect in
3
He, in which the liquid ground state freezes upon increasing the temperature
1
, owing to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene
2
. Notably, a resistivity peak appears at high temperatures near a superlattice filling factor of −1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted-field magnetotransport and thermodynamic measurements of the in-plane magnetic moment show that the resistivity peak is connected to a finite-field magnetic phase transition
3
at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes the phase relative to an isospin-unpolarized Fermi liquid phase at higher temperatures. We find the entropy, in units of Boltzmann’s constant, to be of the order of unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered spins. In contrast to
3
He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness
4
,
5
, with implications for the nature of finite-temperature electron transport
6
–
8
, as well as for the mechanisms underlying isospin ordering and superconductivity
9
,
10
in twisted bilayer graphene and related systems.
An electronic analogue of the Pomeranchuk effect is present in twisted bilayer graphene, shown by the stability of entropy in a ferromagnetic phase compared to an unpolarized Fermi liquid phase at certain high temperatures.