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 rs, 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 rs surpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, when rs exceeds ≃38. However, our sample makes a transition to an insulating state at rs≃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.

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，长期以来人们预计，随着电子密度的降低，通过排列电子自旋获得的交换能应超过动能（费米）的增强，从而导致（布洛赫）铁磁转变。在甚至更低的密度下，应该会发生另一种向（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，对这些状态的实验访问受到限制。在这里，我们探索了一个极其干净的二维电子系统中相互作用电子的基态，该系统仅限于调制掺杂的 AlAs 量子阱中。该系统中的大电子有效质量使我们能够达到非常大的相互作用参数值rs，定义为库仑能量与费米能量之比。当我们通过栅极偏置降低电子密度时，我们发现了一系列相，在质量上与上述情况一致：大密度下的顺磁相，当rs超过 35，然后是具有强烈非线性电流-电压特性的相，暗示着固定维格纳固体，当rs超过≃38。然而，我们的样品在以下温度下转变为绝缘状态：rs≃27，在自发铁磁性开始之前，这意味着除了相互作用之外，在理解现实稀电子系统的不同阶段时还必须考虑无序的作用。