Search

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., Comment: 24 pages, 20 figures

When bilayer graphene is rotationally faulted to an angle $\theta\approx 1.1^\circ$, theory predicts the formation of a flat electronic band and correlated insulating, superconducting, and ferromagnetic states have all been observed at partial band filling. The proximity of superconductivity to correlated insulators has suggested a close relationship between these states, reminiscent of the cuprates where superconductivity arises by doping a Mott insulator. Here, we show that superconductivity can appear without correlated insulating states. While both superconductivity and correlated insulating behavior are strongest near the flat band condition, superconductivity survives to larger detuning of the angle. Our observations are consistent with a "competing phases" picture, in which insulators and superconductivity arise from disparate mechanisms., Comment: 15 pages, 3 figures in main text, 12 figures in supplementary materials