Spin transfer and coherence in coupled quantum wells

Spin dynamics of optically excited electrons confined in asymmetric coupled quantum wells are investigated through time-resolved Faraday rotation experiments. The interwell coupling is shown to depend on applied electric field and barrier thickness. We observe three coupling regimes: independent spin precession in isolated quantum wells, incoherent spin transfer between single-well states, and coherent spin transfer in a highly coupled system. Relative values of the interwell tunneling time, the electron-spin lifetime, and the Larmor precession period appear to govern this behavior.

Figure 1

PL intensity plotted on a logarithmic grayscale as a function of $U_g$ and $E_d$. A CW $\mathrm{HeNe}$ laser emitting at $1.96\mathrm{eV}$ is used to excite carriers at $B_0=0\mathrm{T}$. (a) PL from sample 7-20-10 shows two Stark shifted peaks corresponding to the 7- and $10\text{−}\mathrm{nm}$ QWs without evidence of interwell coupling. (b) PL from sample 7-6-10 (i) reveals a strongly Stark shifted indirect exciton peak, and (ii) shows the quenching of the $7\text{−}\mathrm{nm}$ well PL peak and the corresponding greater intensity in the $10\text{−}\mathrm{nm}$ well peak. (c) Sample 7-2-10 shows a single PL peak which is strongly Stark shifted.

Figure 2

Dependence of time-resolved FR data on $U_g$ and $B_0$ in sample 7-6-10. (a) FR plotted in a grayscale as a function of $U_g$ and $\Delta t$ for $B_0=6\mathrm{T}$ and $E_L=1.57\mathrm{eV}$. Note the appearance of only two precession frequencies and the sharp transition between the two. (b) Line cuts of the time-resolved FR data shown in (a) for $U_g=-0.8$ and $-4.0\mathrm{V}$. Inset: ${\nu }_L$ plotted as a function of $B_0$ for two gate voltages $U_g$. Data taken at $U_g=0.0\mathrm{V}$ are shown as crosses and data taken at $U_g=-4.0\mathrm{V}$ are shown as filled circles. The solid lines are linear fits to the data.

Figure 3

(Color). Dependence of $g$ factor on $U_g$ and $d$. (a) Fourier transform of time-resolved FR data measured in sample 7-20-10 plotted on a logarithmic grayscale as a function of $\mid g\mid$ and $U_g$ at $B_0=6\mathrm{T}$ and $E_L=1.57\mathrm{eV}$. Note the presence of two $g$ factors with a weak dependence on $U_g$. Schematic band diagrams are shown in the middle and on the right-hand side for $U_g$ close to zero and for negative $U_g$, respectively. Electron spin is represented by blue arrows, while holes are shown without spin to illustrate the rapid hole spin relaxation (less than $5\mathrm{ps}$) in these systems. The thick red arrow indicates resonant excitation and detection of FR, while the thin dotted arrow refers to weaker, off-resonant FR. (b) Similar data are shown for sample 7-6-10, where switching between two $g$ factors is observed as a function of $U_g$. Panels on the right-hand-side illustrate the destructive effect of incoherent tunneling on the spin coherence of the lower energy conduction-electron state. (c) Continuous tuning of the $g$ factor is observed in sample 7-2-10 and the panels to the right schematically depict the electron ground states extending over both QWs.

Figure 4

(Color). Dependence of $g$ on QW width $w$. (a) Fourier transform of time-resolved FR data measured in sample 8-4-8 plotted in a logarithmic grayscale as a function of $\mid g\mid$ and $U_g$ at $B_0=6\mathrm{T}$ and $E_L=1.58\mathrm{eV}$. Note that the $g$ factor, $\mid g\mid =0.105$, has no observable dependence on $U_g$. (b) Similar data are plotted for sample 5.7-3.8-7.7 showing continuous tuning from $\mid g\mid =0.09$, via $\mid g\mid =0$ around $U_g=0\mathrm{V}$, and to $\mid g\mid =0.035$. The red dots map the peak position of the Fourier transform in order to guide the eye. (c) $g$ is shown as a function of $w$. Data drawn from work by Snelling et al. are plotted as crosses and a fit to this data is shown as a solid line to guide the eye. Values of $g$ extracted from FR data of our five samples for different $w$ are plotted as filled circles.