Entropic evidence for a Pomeranchuk effect in magic-angle graphene

Abstract

In the 1950's, Pomeranchuk 1 predicted that, counterintuitively, liquid 3 He may solidify upon heating, due to a high excess spin entropy in the solid phase. Here, using both local and global electronic entropy and compressibility measurements, we show that an analogous effect occurs in magic angle twisted bilayer graphene. Near a filling of one electron per moiré unit cell, we observe a dramatic increase in the electronic entropy to about per unit cell. This large excess entropy is quenched by an in-plane magnetic field, pointing to its magnetic origin. A sharp drop in the compressibility as a function of the electron density, associated with a reset of the Fermi level back to the vicinity of the Dirac point, marks a clear boundary between two phases. We map this jump as a function of electron density, temperature, and magnetic field. This reveals a phase diagram that is consistent with a Pomeranchuk-like temperature-and fielddriven transition from a low-entropy electronic liquid to a high-entropy correlated state with nearly-free magnetic moments. The correlated state features an unusual combination of seemingly contradictory properties, some associated with itinerant electrons, such as the absence of a thermodynamic gap, metallicity, and a Dirac-like compressibility, and others associated with localized moments, such as a large entropy and its disappearance with magnetic field. Moreover, the energy scales characterizing these two sets of properties are very different: whereas the compressibility jump onsets at ∼ , the bandwidth of magnetic excitations is ∼ or smaller. The hybrid nature of the new correlated state and the large separation of energy scales have key implications for the physics of correlated states in twisted bilayer graphene.

Interaction-driven band flattening and correlated phases in twisted bilayer graphene

“…4f-i. The relation of these features and various recently reported phases that emerge at elevated temperatures 36,37 remains a subject for future investigations. by J.A.…”

Developing Graphene‐Based Moiré Heterostructures for Twistronics

“…In addition to the preparation of graphene‐based moiré heterostructures, many recent studies focused on the observation of their topological and correlated properties, such as, the existence of a moiré superlattice and topological minibands, [ 10 , 17 , 23 , 24 , 25 ] the quantum anomalous Hall effect in twisted bilayer graphene (tBLG), [ 26 ] the Mott insulator in ABC‐TLG, [ 24 ] the ferromagnetism in tDBG, [ 16 ] the Pomeranchuk effect in MATBG, [ 27 ] the ferroelectricity in BLG/ h ‐BN, [ 28 ] and the exciton effect in graphene/TMDCs heterostructures. [ 29 ] A few reviews on the physical properties and applications have been reported; however, they are primarily focused on tBLG and TMDCs vdW heterostructures.…”

“…Moreover, the factors that influence the physical properties of graphene‐based moiré heterostructures are presented, and they more precisely tune the performance. Meanwhile, the typical performance of superconductivity, the correlated insulator and various newly observed properties are systematically introduced; these include the quantum anomalous Hall effect, [ 26 ] ferromagnetism, [ 34 ] ferroelectricity, [ 28 ] the Pomeranchuk effect, [ 27 ] and tunable excitons. [ 35 ] Finally, we summarize the development of twistronics based on graphene‐based moiré heterostructures, including the development limitations in fabrication, characterization, and a deep investigation into the mechanism of many‐body physics.…”

“…g) The chemical potential 𝜇 as a function of temperature T and 𝜈 0 . Reproduced with permission [27]. Copyright 2021, Springer Nature.…”

“…Copyright 2021, Springer Nature. i) The electronic entropy s as a function of 𝜈 and B ∥ at T = 10 K. Reproduced with permission [27]. Copyright 2021, Springer Nature.…”