近日，美国普林斯顿大学得Ali Yazdani及其研究团队取得一项新进展。经过不懈努力，他们直接观察到磁场诱导的维格纳晶体。相关研究成果已于2024年4月10日在国际权威学术期刊《自然》上发表。

该研究团队利用高分辨率扫描隧道显微镜直接在伯纳堆叠双层石墨烯中，成像了磁场诱导的电子维格纳晶体（WC），并深入研究了其结构特性与电子密度、磁场以及温度之间的函数关系。在高磁场和最低温度下，研究人员观察到了一个处于最低朗道能级的三角形晶格电子WC。该WC具有预期的晶格常数，在填充因子ν≈0.13和ν≈0.38之间表现出鲁棒性，仅在填充因子附近与分数量子霍尔态存在竞争。

随着电子密度的增加或温度的升高，WC逐渐熔化成各向同性的液相，但仍呈现出具有WC布拉格波矢量表征的调制结构。令人意外的是，在低磁场条件下，WC转变为了各向异性条纹相，这种相通常被认为是在较高的朗道能级中形成的。对单个点阵位的分析，揭示了一些可能与WC点阵中电子的量子零点运动相关的特征。

据悉，维格纳曾预言，当电子间的库仑相互作用远超过其动能时，电子会结晶成紧密排列的晶格。目前，多种二维体系已证实存在维格纳晶体（WCs）的迹象。但遗憾的是，自发形成的经典或量子维格纳晶体始终未被直接观测到。因此，人们既未明确WC的对称性，也未对其熔化过程进行直接研究。

附：英文原文

Title: Direct observation of a magnetic-field-induced Wigner crystal

Author: Tsui, Yen-Chen, He, Minhao, Hu, Yuwen, Lake, Ethan, Wang, Taige, Watanabe, Kenji, Taniguchi, Takashi, Zaletel, Michael P., Yazdani, Ali

Issue&Volume: 2024-04-10

Abstract: Wigner predicted that when the Coulomb interactions between electrons become much stronger than their kinetic energy, electrons crystallize into a closely packed lattice. A variety of two-dimensional systems have shown evidence for Wigner crystals(WCs). However, a spontaneously formed classical or quantum WC has never been directly visualized. Neither the identification of the WC symmetry nor direct investigation of its melting has been accomplished. Here we use high-resolution scanning tunnelling microscopy measurements to directly image a magnetic-field-induced electron WC in Bernal-stacked bilayer graphene and examine its structural properties as a function of electron density, magnetic field and temperature. At high fields and the lowest temperature, we observe a triangular lattice electron WC in the lowest Landau level. The WC possesses the expected lattice constant and is robust between filling factor ν≈0.13 and ν≈0.38 except near fillings where it competes with fractional quantum Hall states. Increasing the density or temperature results in the melting of the WC into a liquid phase that is isotropic but has a modulated structure characterized by the Bragg wavevector of the WC. At low magnetic fields, the WC unexpectedly transitions into an anisotropic stripe phase, which has been commonly anticipated to form in higher Landau levels. Analysis of individual lattice sites shows signatures that may be related to the quantum zero-point motion of electrons in the WC lattice.

DOI: 10.1038/s41586-024-07212-7

Source: https://www.nature.com/articles/s41586-024-07212-7