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Isospin Pomeranchuk effect in twisted bilayer graphene
Nature ( IF 42.778 ) Pub Date : 2021-04-07 , DOI: 10.1038/s41586-021-03409-2
Yu Saito, Fangyuan Yang, Jingyuan Ge, Xiaoxue Liu, Takashi Taniguchi, Kenji Watanabe, J. I. A. Li, Erez Berg, Andrea F. Young

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 3He, in which the liquid ground state freezes upon increasing the temperature1, 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 graphene2. 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 transition3 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 3He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness4,5, with implications for the nature of finite-temperature electron transport6,7,8, as well as for the mechanisms underlying isospin ordering and superconductivity9,10 in twisted bilayer graphene and related systems.



中文翻译:

扭曲双层石墨烯中的同位旋Pomeranchuk效应

在冷凝物系统中,较高的温度通常不利于有序相,从而导致磁性,超导性和其他现象的临界温度较高。一个例外是3 He中的Pomeranchuk效应,其中由于顺磁性固相的大熵,液态基态在温度1升高时冻结。在这里,我们表明,相似的机制描述了魔角扭曲双层石墨烯2中自旋和谷基同位旋的有限温度动力学。。值得注意的是,尽管在低温极限中没有出现相称相关相的迹象,但在超晶格填充系数-1附近的高温下仍会出现电阻率峰值。倾斜磁场的磁输运和面内磁矩的热力学测量表明,电阻率峰值与有限磁场的相变有关3在该系统上,系统将产生有限的同位旋极化。这些数据暗示了Pomeranchuk型机制,其中在较高的温度下,铁磁相中无序的同位旋矩的熵相对于同位旋未极化的费米液相稳定了相。我们发现,以玻耳兹曼常数为单位的熵约为每单位单元面积的单位数量级,其可测量分数受平面内磁场的抑制,与无序自旋的贡献一致。然而,与3 He相反,在该转变期间没有观察到热力学量的不连续性。我们的发现表明,同位旋刚度较小,为4,5,这对有限温度电子输运的性质6,7,8有影响。,以及在双层石墨烯及其相关系统中的等位旋有序和超导性9,10的潜在机理。

更新日期:2021-04-08
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