Isospin Pomeranchuk effect in twisted bilayer graphene
Isospin Pomeranchuk effect in twisted bilayer graphene
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DOI：
10.1038/s41586-021-03409-2
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发表时间：
2021-04-08
期刊：
影响因子：
64.8
通讯作者：
Young, Andrea F.
中科院分区：
文献类型：
--
作者：
Saito, Yu;Yang, Fangyuan;Young, Andrea F.
在凝聚态系统中，较高的温度通常不利于有序相，导致磁性、超导性和其他现象存在一个上临界温度。一个例外是氦 - 3中的波梅兰丘克效应，由于顺磁固相的大熵，其液态基态在温度升高时会冻结。在此我们表明，一种类似的机制描述了魔角扭转双层石墨烯中自旋和谷同位旋的有限温度动力学。值得注意的是，尽管在低温极限下没有出现相应关联相的迹象，但在接近 - 1的超晶格填充因子的高温处出现了一个电阻率峰值。倾斜场磁输运和平面内磁矩的热力学测量表明，电阻率峰值与一个有限场磁相变相关联，在该相变处系统产生有限的同位旋极化。这些数据暗示了一种波梅兰丘克型机制，其中铁磁相中无序同位旋矩的熵在较高温度下相对于同位旋非极化费米液相稳定了该相。我们发现，以玻尔兹曼常数为单位的熵约为每单位晶胞面积1个单位，其中可测量的部分被平面内磁场抑制，这与无序自旋的贡献相符。然而，与氦 - 3不同的是，在这个转变过程中，热力学量没有观察到不连续性。我们的研究结果意味着同位旋刚度较小，这对有限温度下电子输运的性质以及扭转双层石墨烯和相关系统中同位旋有序和超导的潜在机制都有影响。
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 He-3, 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 3He, 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.