Abstract

In a previous report, we presented experiments which suggested that ferromagnetic ordering of the spins of localized holes in GaAs/AlGaAs quantum wells could be observed when doped with shallow (Be) acceptors at impurity concentrations near the metal-insulator transition. The compensating impurity (Si) was introduced into a narrow region at the center of the barriers [4]. In this paper, we present results from magnetotransport experiments performed on similar structures, but without the compensating impurity (Si). In these samples, the compensation degree is expected to be controlled by the background defects located at the edges of the quantum wells and within the barriers. At low temperatures T ≤ 10 K, we observed isotropic, linear magnetoresistance, anomalous behavior of the Hall effect as a function of the magnetic field, and slow relaxation of resistance after the application of a magnetic field. We explain this anomalous magnetotransport as the manifestation of a ferromagnetic transition or spin glass, originating from indirect spin exchange between localized holes on impurities near the metal-insulator transition. However, we note that perfect disorder, including signs of interspin interactions, leads to unstable configurations. In what follows, we present a model in which we start with this perfect disorder, but apply a procedure to obtain a stable configuration. We show that the resulting spin structure, a “closely packed” structure of “droplets,” can reproduce the features observed in the experiment, particularly isotropic, linear magnetoresistance.

中文翻译：

---

金属-绝缘体转变附近掺杂非磁性杂质的 GaAs/AlGaAs 量子阱的自旋玻璃模型

摘要

在之前的报告中，我们提出的实验表明，当在金属-绝缘体转变附近的杂质浓度下掺杂浅 (Be) 受主时，可以观察到 GaAs/AlGaAs 量子阱中局域空穴自旋的铁磁有序性。补偿杂质 (Si) 被引入势垒中心的狭窄区域 [4]。在本文中，我们展示了在类似结构上进行的磁输运实验的结果，但没有补偿杂质（Si）。在这些样品中，补偿程度预计由位于量子阱边缘和势垒内的背景缺陷控制。在低温T ≤ 10 K 时，我们观察到各向同性、线性磁阻、霍尔效应作为磁场函数的异常行为，以及施加磁场后电阻的缓慢松弛。我们将这种异常磁输运解释为铁磁转变或自旋玻璃的表现，源于金属-绝缘体转变附近杂质上的局部空穴之间的间接自旋交换。然而，我们注意到完美的无序，包括自旋间相互作用的迹象，会导致不稳定的构型。接下来，我们提出一个模型，在该模型中我们从这种完美的无序开始，但应用一个过程来获得稳定的配置。我们表明，由此产生的自旋结构，即“液滴”的“紧密堆积”结构，可以重现实验中观察到的特征，特别是各向同性的线性磁阻。