Nuclear spin dynamics influenced and detected by electron spin polarization in CdTe/(Cd,Mg)Te quantum wells

Nuclear spin coherence and relaxation dynamics of all constituent isotopes of an n-doped CdTe/(Cd,Mg)Te quantum well structure are studied employing optically detected nuclear magnetic resonance. Using time-resolved pump-probe Faraday ellipticity, we generate and detect the coherent spin dynamics of the resident electrons. The photogenerated electron spin polarization is transferred into the nuclear spin system, which becomes polarized and acts back on the electron spins as the Overhauser field. Under the influence of resonant radio frequency pulses, we trace the coherent spin dynamics of the nuclear isotopes ${}^{111}\mathrm{Cd}, {}^{113}\mathrm{Cd}$, and ${}^{125}\mathrm{Te}$. We measure nuclear Rabi oscillations, the inhomogeneous dephasing time $T_2^{*}$, the spin coherence time $T_2$, and the longitudinal relaxation time $T_1$. Furthermore, we investigate the influence of the laser excitation and the corresponding electron spin polarization on the nuclear spin relaxation time and find a weak extension of this time induced by interaction with the electron spins.

Figure 1

(a) Dynamics of the Faraday ellipticity of the probe beam polarization after passing through the sample versus time delay relative to the pump pulse measured at $B=1\mathrm{T}$. The left inset presents a zoom of the data around a delay of $-1.7\mathrm{ns}$ (i.e., $11.5\mathrm{ns}$ after the previous pump pulse). The red curve corresponds to the measurement in the presence of RF, affecting the nuclear polarization. The right inset shows the photoluminescence spectrum (PL) exhibiting trion (T) and exciton (X) emission lines. The arrow marks the position of the laser for the pump-probe experiment. (b) Phase shift as a function of applied continuous wave RF. Sharp resonances correspond to the NMR conditions of different isotopes at $B=1\mathrm{T}$ at $T=1.5\mathrm{K}$. The line is a guide to the eye.

Figure 2

Coherent control of the ${}^{111}\mathrm{Cd}$ isotope at $B=1\mathrm{T}$ and $f_{\text{r}}=9.06\mathrm{MHz}$. (a) Phase shift of the electron precession induced by an RF pulse. The open circles correspond to a 650-mV pulse with varied duration and the red curve is a sine fit with a Rabi period of $8.6\mu \mathrm{s}$. (b) Decay of the Ramsey fringes amplitude. The red line corresponds to an exponential decay with a time constant of 144 $\mu \mathrm{s}$, whereas the blue dashed curve corresponds to Gaussian fit with a decay constant of 122 $\mu \mathrm{s}$. (c) Decay of the spin-echo amplitude. The line is an exponential decay fit featuring a time constant of 2.5 ms. (d) Decay of the phase shift with recurring nuclear polarization. The line is an exponential decay fit with a time constant of 104 s. The measurements in panels (b)–(d) are given for RF application while the laser radiation was blocked. The sketches show the corresponding pulse sequences used in the experiments.