The nuclear spin systems in CdTe/(Cd,Zn)Te and CdTe/(Cd,Mg)Te quantum wells (QWs) are studied using a multistage technique combining optical pumping and Hanle effect-based detection. The samples demonstrate drastically different nuclear spin dynamics in zero and weak magnetic fields. In CdTe/(Cd,Zn)Te, the nuclear spin relaxation time is found to strongly increase with the magnetic field, growing from 3 s in zero field to tens of seconds in a field of 25 G. In CdTe/(Cd,Mg)Te the relaxation is an order of magnitude slower, and it is field-independent up to at least 70 G. The differences are attributed to the nuclear spin relaxation being mediated by different kinds of resident electrons in these QWs. In CdTe/(Cd,Mg)Te, a residual electron gas trapped in the QW largely determines the relaxation dynamics. In CdTe/(Cd,Zn)Te, the fast relaxation in zero field is due to interaction with localized donor-bound electrons. Nuclear spin diffusion barriers form around neutral donors when the external magnetic field exceeds the local nuclear field, which is about $B_L\approx 0.4$ G in CdTe. This inhibits nuclear spin diffusion towards the donors, slowing down relaxation. These findings are supported by theoretical modeling. In particular, we show that the formation of the diffusion barrier is made possible by several features specific to CdTe: (i) the large donor binding energy (about 10 meV), (ii) the low abundance of magnetic isotopes (only $\approx 30$% of nuclei have nonzero spin), and (iii) the absence of nuclear quadrupole interactions between nuclei. The two latter properties are also favorable to nuclear spin cooling via optical pumping followed by adiabatic demagnetization. Under nonoptimized conditions we have reached sub-microkelvin nuclear spin temperatures in both samples, lower than all previous results obtained in GaAs.

中文翻译：

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宽 CdTe 量子阱中供体束缚电子和自由电子介导的核自旋弛豫

采用光泵浦和基于汉勒效应的检测相结合的多级技术研究了 CdTe/(Cd,Zn)Te 和 CdTe/(Cd,Mg)Te 量子阱 (QW) 中的核自旋系统。这些样品在零磁场和弱磁场中表现出截然不同的核自旋动力学。在 CdTe/(Cd,Zn)Te 中，核自旋弛豫时间随着磁场的增加而强烈增加，从零磁场中的 3 秒增长到 25 G 磁场中的数十秒。 ）Te 弛豫速度要慢一个数量级，并且与场无关，高达至少 70 G。差异归因于这些 QW 中不同类型的驻留电子介导的核自旋弛豫。在 CdTe/(Cd,Mg)Te 中，量子阱中捕获的残余电子气很大程度上决定了弛豫动力学。在 CdTe/(Cd,Zn)Te 中，零场中的快速弛豫是由于与局部供体束缚电子的相互作用。当外部磁场超过局部核场时，在中性供体周围形成核自旋扩散势垒，约为${乙}_{\mathrm{大号}}\approx 0.4$CdTe 中的 G。这抑制了核自旋向供体的扩散，减缓了弛豫。这些发现得到了理论模型的支持。特别是，我们表明扩散势垒的形成是通过 CdTe 特有的几个特征实现的：（i）大的供体结合能（约 10 meV），（ii）磁性同位素的低丰度（仅$\approx 30$％的原子核具有非零自旋），以及（iii）原子核之间不存在核四极相互作用。后两个特性也有利于通过光泵浦随后绝热退磁进行核自旋冷却。在非优化条件下，我们在两个样品中都达到了亚微开尔文核自旋温度，低于之前在砷化镓中获得的所有结果。