Isospin Pomeranchuk effect and the entropy of collective excitations in twisted bilayer graphene (2008.10830v4)

Abstract: In condensed matter systems, higher temperatures typically disfavors ordered phases leading to an upper critical temperature for magnetism, superconductivity, and other phenomena. A notable exception is the Pomeranchuk effect in 3He, in which the liquid ground state freezes upon increasing the temperature due 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. Most strikingly a resistivity peak appears at high temperatures near superlattice filling factor nu = -1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted field magnetotransport and thermodynamic measurements of the inplane magnetic moment show that the resistivity peak is adiabatically connected to a finite-field magnetic phase transition 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 it relative to an isospin unpolarized Fermi liquid phase at elevated temperatures. Measurements of the entropy, S/kB indeed find it to be of order unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered physical spins. In contrast to 3He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness, with implications for the nature of finite temperature transport as well as the mechanisms underlying isospin ordering and superconductivity in twisted bilayer graphene and related systems.

Overview of Isospin Pomeranchuk Effect and Collective Excitations in Twisted Bilayer Graphene

The paper of twisted bilayer graphene (TBG) at the magic angle has unveiled a plethora of novel physical phenomena owing to its highly tunable band structure and interaction-induced phases. This paper investigates the finite temperature dynamics within TBG, specifically focusing on the isospin Pomeranchuk effect and the behavior of collective excitations. The authors present findings on a temperature-induced phase transition linked to isospin degrees of freedom, with implications for understanding and potentially leveraging high-temperature states in graphene-based systems.

Key Findings

The primary discovery reported is the appearance of a resistivity peak at high temperatures near the superlattice filling factor $\nu = -1$, where no commensurate correlated phase is evident at lower temperatures. This anomalous peak suggests an entropically driven phase transition driven by the large entropy of disordered isospin moments, akin to the Pomeranchuk effect observed in solidifying $^3$He. Measurements indicate that the isospin ferromagnetic phase, stabilized at elevated temperatures, exhibits a finite-field magnetic phase transition that coincides with increased resistivity.

Thermodynamic measurements reveal a significant contribution from disordered spins to this entropy, providing a plausible mechanism for the stabilization of the high-temperature ferromagnetic phase over the isospin unpolarized Fermi liquid phase. In-plane magnetic field experiments further validate these findings, showing the emergence of this ferromagnetic state under field-induced conditions.

Implications and Future Directions

These findings imply a small isospin stiffness in TBG, which is critical for understanding finite temperature transport behaviors and the mechanisms of isospin ordering and superconductivity. The research highlights the role of collective excitations of electronic origin that strongly couple to charge carriers, potentially impacting the phase diagram and transport properties of TBG at finite temperatures.

The entropy-driven transition suggests that soft neutral excitations play a significant role in the physics of flat band systems. The demonstrated connection between disordering isospin moments and temperature-dependent resistivity changes could illuminate pathways for engineering high-temperature electronic states and improve the understanding of basic mechanisms in TBG and related materials.

Conclusion

This paper provides valuable insights into the thermodynamic behavior of twisted bilayer graphene under varying temperature and magnetic field conditions, outlining the potential role of isospin degrees of freedom in dictating electronic properties. Future work could focus on expanding these studies to explore the interplay between these collective excitations and other emergent phases, including superconductivity, with the ultimate goal of harnessing these properties for technological applications in nanoscale devices and beyond.