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Entropic evidence for a Pomeranchuk effect in magic-angle graphene
Nature ( IF 42.778 ) Pub Date : 2021-04-07 , DOI: 10.1038/s41586-021-03319-3
Asaf Rozen, Jeong Min Park, Uri Zondiner, Yuan Cao, Daniel Rodan-Legrain, Takashi Taniguchi, Kenji Watanabe, Yuval Oreg, Ady Stern, Erez Berg, Pablo Jarillo-Herrero, Shahal Ilani

In the 1950s, Pomeranchuk1 predicted that, counterintuitively, liquid 3He may solidify on heating. This effect arises owing to high excess nuclear spin entropy in the solid phase, where the atoms are spatially localized. Here we find that an analogous effect occurs in magic-angle twisted bilayer graphene2,3,4,5,6. Using both local and global electronic entropy measurements, we show that near a filling of one electron per moiré unit cell, there is a marked increase in the electronic entropy to about 1kB per unit cell (kB is the Boltzmann constant). 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 field-driven 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 under a magnetic field. Moreover, the energy scales characterizing these two sets of properties are very different: whereas the compressibility jump has an onset at a temperature of about 30 kelvin, the bandwidth of magnetic excitations is about 3 kelvin or smaller. The hybrid nature of the present correlated state and the large separation of energy scales have implications for the thermodynamic and transport properties of the correlated states in twisted bilayer graphene.



中文翻译:

魔角石墨烯中Pomeranchuk效应的熵证据

在1950年代,Pomeranchuk 1预测,与直觉相反,液态3 He可能在加热时凝固。由于原子在空间上局部存在的固相中过量的核自旋熵很高,因此产生了这种效应。在这里,我们发现在魔角扭曲双层石墨烯2,3,4,5,6中发生了类似的作用。使用局部和全局电子熵测量,我们显示每个摩尔单元晶胞充满一个电子附近,电子熵显着增加到每个晶胞约1 k Bk B是玻耳兹曼常数)。巨大的过量熵被指向磁场的平面内磁场淬灭。可压缩性随电子密度的急剧下降,与费米能级重新回到狄拉克点附近有关,标志着两相之间的界限清晰。我们将此跃变映射为电子密度,温度和磁场的函数。这揭示了一个相图,该相图与类似Pomeranchuk的温度和场驱动的转换,即从具有几乎自由的磁矩的低熵电子液体到高熵相关态的转换一致。相关状态具有看似矛盾的特性的不寻常组合,其中一些特性与流动电子有关,例如不存在热力学间隙,金属性和类似于狄拉克的可压缩性,以及其他与局部矩相关的特性,例如大熵及其在磁场下的消失。而且,表征这两组特性的能级是非常不同的:虽然可压缩性跃迁在大约30开尔文的温度下开始出现,但磁激发的带宽大约为3开尔文或更小。当前相关态的混合性质和能级的大分离对双分子层石墨烯中相关态的热力学和输运性质有影响。尽管可压缩性跃迁在约30开尔文的温度下开始,但磁激发的带宽约为3开尔文或更小。当前相关态的混合性质和能级的大分离对双分子层石墨烯中相关态的热力学和输运性质有影响。尽管可压缩性跃迁在约30开尔文的温度下开始,但磁激发的带宽约为3开尔文或更小。当前相关态的混合性质和能级的大分离对双分子层石墨烯中相关态的热力学和输运性质有影响。

更新日期:2021-04-08
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