Figure 1

(Color online) (a) Number of unpaired spins $\left(N_S\right)$ as a function of gate voltage $\left(V_G\right)$ below threshold $\left(V_{th}=1\text{V}\right)$ as measured at 4.8 K in the dark (squares) and after illumination (circles), and at 2 K in the dark (triangles). Arrows on the curves recorded in the dark indicate a characteristic voltage $V_T^{\ast }$ at which $N_S$ becomes constant while decreasing $V_G$ from $V_{th}$. Lines are guide for the eyes. (b) Cartoon of density of states, $g\left(E\right)$, of 2D electrons; $g\left(E\right)$ is constant above the conduction band edge $\left(E_C\right)$ and decays below $E_C$. States below $E_F$ are occupied. In the experiment, $E_F$ can be adjusted by the gate voltage bias $V_G$. (c) Density of states of confined electrons as a function of energy below $E_C$, as obtained from data in Fig. 1a. The star indicates the calculated value $\left(1.6\times {10}^{11}{\text{meV}}^{-1}{\text{cm}}^{-2}\right)$ of the density of states in the conduction band for 2D electrons at a Si(100) surface (twofold valley degeneracy). The labeled arrows on the top axis indicate the depth of confined electrons after setting $V_G=0\text{V}$ in the dark at 5 K and 10 K, respectively.Reuse & Permissions

Figure 2

(Color online) $T_2$ experiment on electrons confined in dots at 5 K as measured at 350 mK. (a) Typical echo detected spectra after a two-pulse sequence for different $\tau$. (b) Peak intensity of the two-pulse signal as function of $2\tau$. The biexponential fit gives two characteristic $T_2$’s.Reuse & Permissions

Figure 3

Two-pulse echo decay for natural dots confined at 10 K, measured at 350 mK.Reuse & Permissions

Figure 4

Echo intensity at the end of the three-pulse experiment on natural dots confined at 5 K and measured at 350 mK. The exponential fit of the intensity as a function of t gives $T_1$.Reuse & Permissions

Figure 5

(Color online) $T_1$ and $T_2$ as a function of temperature for natural dots that show a long $T_2$. Squares and circles correspond to natural dots confined at 5 K while the star shows the $T_2$ for dots confined at 10 K and measured at 350 mK. Lines are guide for the eyes.Reuse & Permissions