Spin Relaxation and Its Control of Change Tunable Quantum Dots

研究了单电子掺杂InP量子点中自旋失相弛豫和核自旋对电子自旋极化的影响。当一个InP量子点平均掺杂一个电子时，在Hanle曲线上出现了一个半宽4.6 mT的窄洛伦兹倾角。半宽4.6 mT归因于掺杂电子的自旋减相弛豫。在5k处对应的自旋相干时间为1.7 ns，由量子点中核自旋的冻结涨落决定。随着温度的升高，掺杂电子的自旋失相速率增大。利用电子自旋弛豫的泵浦探针光致发光测量，估计了超精细电子场中核自旋Larmor进动的时间周期TN。在外加磁场消失的情况下，在5k时，电子自旋极化在纵向磁场中的测量结果表明，动态核极化引起的奥弗豪泽场随着激发功率的增加而线性增加。发现核自旋冻结涨落的有效磁场为15mt。在中性InP量子点的线极化准共振激励下，观察到激子的共振自旋取向。在1.5 T和2.5 T的纵向和倾斜磁场下，发生了亮激子和暗激子的两次反交叉。在反交叉点，亮激子和暗激子相互混合，波函数混合引起共振自旋取向。由于在反交叉点上亮激子和暗激子的混合，时间分辨圆偏振清晰地显示出共振自旋取向。暗激子寿命长，可以作为载流子和自旋储子。我们证明了磁场可以控制量子点的自旋纠缠和共振自旋取向。

Spin dephasing relaxation and nuclear spin effects on the electron spin polarization in single electron doped InP quantum dots (QDs) were studied. When an InP QD is doped with one electron on average, a narrow Lorentzian dip with half-width of 4.6 mT appeared in the Hanle curve. The half-width 4.6 mT was ascribed to spin-dephasing relaxation of doped electrons constituting negative trions. The corresponding spin coherence time at 5 K is 1.7 ns, which is determined by the frozen fluctuation of nuclear spins in the QDs. With increase of temperature, the spin-dephasing rate of the doped electrons increases. Pump probe photoluminescence measurements of electron spin relaxation were used to estimate the time period TN of the Larmor precession of nuclear spins in the hyperfine field of electrons. We found TN to be s$ at 5 K, under the vanishing external magnetic field Measurements of electron spin polarization in the longitudinal magnetic field at 5 K indicated that the Overhauser field induced by the dynamic nuclear polarization increases linearly with the excitation power. The effective magnetic field of the frozen fluctuations of nuclear spins was found to be 15 mT.Resonant spin orientation of excitons was observed under linearly polarized quasiresonant excitation in neutral InP QDs. Under the longitudinal and tilted magnetic fields of 1.5 and 2.5 T, two anticrossings of bright and dark excitons take place. At the anticrossing points, bright and dark excitons mix with each other and the wave function mixing induces the resonant spin orientation. Time-resolved circular polarization clearly showed the resonant spin orientation due to mixing of bright and dark excitons at the anticrossing points. Dark excitons can work as the carrier and spin reservoir because of their long lifetime. We demonstrated that magnetic field can control the spin entanglement and the resonant spin orientation of QDs.