Observation of spontaneous ferromagnetism in a two-dimensional electron system
Observation of spontaneous ferromagnetism in a two-dimensional electron system
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DOI：
10.1073/pnas.2018248117
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发表时间：
2020-12-22
影响因子：
11.1
通讯作者：
Shayegan, M.
中科院分区：
文献类型：
--
作者：
Hossain, M. S.;Ma, M. K.;Shayegan, M.
相互作用的低密度电子系统的基态是什么？在没有无序的情况下，人们一直期望随着电子密度的降低，通过排列电子自旋获得的交换能应该超过动力学（费米）能的增强，从而导致（布洛赫）铁磁跃迁。在更低的密度下，会发生另一次向（维格纳）固体的转变，即有序的电子阵列。然而，由于缺乏同时支持高质量（低无序）和低密度电子系统的材料平台，因此对这些机制的实验访问受到限制。在这里，我们探索了一个非常干净的二维电子系统中相互作用电子的基态，该系统被限制在一个调制掺杂的AlAs量子阱中。该系统中较大的电子有效质量使我们能够达到非常大的相互作用参数r(s)值，r(s)定义为库仑能量与费米能量的比值。当我们通过栅极偏置降低电子密度时，我们发现一系列相，在质量上与上述情况一致：大密度下的顺磁相，当r(s)超过35时自发过渡到铁磁状态，然后当r(s)超过类似或等于38时，具有强烈非线性电流-电压特征的相，暗示着钉住的维格纳固体。然而，在自发铁磁性发生之前，我们的样品在r(s)近似或等于27时过渡到绝缘状态，这意味着除了相互作用外，在理解实际稀电子系统的不同相时还必须考虑无序的作用。
What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter r(s), defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when r(s) surpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, when r(s) exceeds similar or equal to 38. However, our sample makes a transition to an insulating state at r(s) similar or equal to 27, preceding the onset of the spontaneous ferromagnetism, implying that besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.