Voltage control of electron-nuclear spin correlation time in a single quantum dot

We demonstrate bias control of the efficiency of the hyperfine coupling between a single electron in an InAs quantum dot and the surrounding nuclear spins monitored through the positively charged exciton $X^{+}$ emission. In applied longitudinal magnetic fields, we vary simultaneously the correlation time of the hyperfine interaction and the nuclear spin relaxation time and thereby the amplitude of the achieved dynamic nuclear polarization under optical pumping conditions. In applied transverse magnetic fields, a change in the applied bias allows a switch from the anomalous Hanle effect to the standard electron spin depolarization curves.

Figure 1

Dot $A$. (a) Contour map of PL vs applied voltage in zero external magnetic field $B=0$. Around $-1.3$ V, the electron tunneling rate out of the dot has been sufficiently suppressed to permit the existence of neutral exciton ($X^0$) and biexciton ($X{X}^0$) complexes, and at $\sim 0$ V the negative trion ($X^{-}$) appears as electrons tunnel into the dot at a faster rate. (b) Overhauser shift OHS (closed circles) and degree of circular polarization ${\rho }_c$ (open squares) for the $X^{+}$ transition at $B=0$.

Figure 2

Dot $A$. (a) Overhauser shift OHS and fit with Eq. (1) (solid lines). (b) Degree of circular polarization ${\rho }_c$ of the $X^{+}$ transition in magnetic field ($B_z$) sweep for constant bias $-2.0$ V. Closed circles: sweeping the applied magnetic field up; open triangles: sweeping down. (c) Critical fields for magnetic field sweep up ($B_{z,1}$) and down ($B_{z,2}$) for different bias voltages. (d) Bias dependence of electron hyperfine correlation time (${\tau }_c$) and nuclear depolarisation time ($T_{d,\infty }$) determined with an accuracy of 10%. (e) OHS bistability when sweeping the applied voltage first in the positive, then in the negative direction, for constant $B_z=1.5$ T. (f) Corresponding ${\rho }_c$. Closed circles: sweeping applied voltage in the positive direction; open triangles: negative direction. Dotted lines are a guide to the eye.

Figure 3

Dot $A$. (a) Hanle measurements at $-1.7$ V using ${\sigma }^{-}$ excitation sweeping the transverse field $B_x$ up (blue full circles) and down (red open triangles). (b) Hanle measurements at bias $-1.1$ V (open squares), together with measurements carried out at $-1.7$ V with elliptically polarized excitation (full squares). A Lorentzian fit (black line) to the $-1.1$ V data allows us to extract ${\tau }_s^{*}$.