National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Nat Nanotechnol. 2016 Feb;11(2):137-42. doi: 10.1038/nnano.2015.258. Epub 2015 Nov 23.
Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn(2+), Co(2+) and so on) couple to band carriers via strong sp-d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit. In nanoscale volumes, the statistical fluctuations of N spins are expected to generate giant effective magnetic fields Beff, which should dramatically impact carrier spin dynamics, even in the absence of any applied field. Here we directly and unambiguously reveal the large Beff that exist in Mn(2+)-doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300-600 GHz) spin precession of photoinjected electrons is observed, indicating Beff ∼ 15 -30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. These signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn(2+) moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.
在半导体中，强量子限制可以将能带电子和空穴的波函数压缩到纳米级体积，显著增强它们之间以及与单个掺杂剂的相互作用。在磁性掺杂半导体中，顺磁掺杂剂（如 Mn(2+)、Co(2+)等）通过强 sp-d 自旋交换与带载子耦合，因此可以在受限几何形状中使用少数甚至单个杂质自旋实现巨大的磁光效应。然而，重要的是，在这种少数自旋极限下，热力学自旋涨落变得越来越相关。在纳米体积中，N 个自旋的统计涨落预计会产生巨大的有效磁场 Beff，即使没有施加任何磁场，这也应该会对载流子自旋动力学产生巨大影响。在这里，我们使用超快光学光谱直接且明确地揭示了 Mn(2+)掺杂 CdSe 胶体纳米晶体中存在的大 Beff。在零外磁场下，观察到光注入电子的极快（300-600 GHz）自旋进动，表明电子的 Beff 约为 15-30 T。在外磁场中，进动频率超过 2 THz。这些信号源于电子在外加磁场中围绕随机场的进动，这是由于嵌入的 Mn(2+)磁矩统计上不完全抵消，从而揭示了磁极化子形成的初始相干动力学，并强调了磁化涨落在纳米材料中对载流子自旋动力学的重要性。
