Electron spin dynamics in a self-assembled semiconductor quantum dot: the limit of low magnetic fields

Direct Measurement of Hyperfine Shifts and Radio Frequency Manipulation of Nuclear Spins in Individual CdTe/ZnTe Quantum Dots

We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields B_{z}≳0.1  T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with an optical initialization time of ∼1  ms and polarization lifetime exceeding ∼1  s. Nuclear magnetic resonance spectroscopy of individual dots reveals strong electron-nuclear interactions characterized by Knight fields |B_{e}|≳50  mT, an order of magnitude stronger than in III-V semiconductor quantum dots. Our studies confirm II-VI semiconductor quantum dots as a promising platform for hybrid electron-nuclear spin qubit registers, combining the excellent optical properties comparable to III-V dots and the dilute nuclear spin environment similar to group-IV semiconductors.

State selective pumping reveals spin-relaxation pathways in CdSe quantum dots

The band-edge exciton in elongated CdSe nanocrystals is composed of an upper and lower manifold associated with heavy and light holes in which the energy separation is sensitive to the nanocrystal shape. Using resonant photoluminescence excitation, we probe the upper heavy hole exciton manifold and find rapid relaxation to the lower light hole manifold on a 5 ps time scale. State selective excitation allows the preparation of single quantum states in this system. We used this to map the hole spin relaxation pathways between the fine structure sublevels, which have energy splittings incommensurate with either optical or acoustic phonon energies. This reveals a hitherto unexpected hole spin-relaxation channel in these materials.

Room-Temperature Electron Spin Generation by Femtosecond Laser Pulses in Colloidal CdS Quantum Dots

We present an experimental investigation of optical spin orientation in colloidal CdS quantum dots (QDs) by a femtosecond laser pulse at room temperature. The spin carrier and its spin-generation process are clarified. Firstly, the observed spin signals of CdS QDs in time-resolved Faraday rotation measurements are shown to belong to electron carriers, by comparing the spin dephasing dynamics and Landé g factor between CdS QDs and bulk materials. Secondly, spin dynamics unaffected by the faster carrier recombination suggests that the spin-polarized electrons are not photoexcited but resident in the dots. Moreover, hole spins should dephase very fast compared with electron spins, otherwise the trion (two electrons with opposite spin orientations and one hole) recombination process will affect the resident electron spin signals. The electron spin is generated in a short time of which the excitation light is absorbed and the resident electron is excited to trion states, i.e., of pulse durations. Due to fast hole spin dephasing, trion recombination gives null spin signals, and the subsequent electron spin dynamics is controlled by its intrinsic mechanisms.

Dynamics of quantum dot nuclear spin polarization controlled by a single electron

We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1-2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.

Stabilization of the electron-nuclear spin orientation in quantum dots by the nuclear quadrupole interaction

The nuclear quadrupole interaction eliminates the restrictions imposed by hyperfine interaction on the spin coherence of an electron and nuclei in a quantum dot. The strain-induced nuclear quadrupole interaction suppresses the nuclear spin flip and makes possible the zero-field dynamic nuclear polarization in self-organized InP/InGaP quantum dots. The direction of the effective nuclear magnetic field is fixed in space, thus quenching the magnetic depolarization of the electron spin in the quantum dot. The quadrupole interaction suppresses the zero-field electron spin decoherence also for the case of nonpolarized nuclei. These results provide a new vision of the role of the nuclear quadrupole interaction in nanostructures: it elongates the spin memory of the electron-nuclear system.

Nonequilibrium nuclear-electron spin dynamics in semiconductor quantum dots

We study the spin dynamics in charged quantum dots in the situation where the resident electron is coupled to only about 200 nuclear spins and where the electron spin splitting induced by the Overhauser field does not exceed markedly the spectral broadening. The formation of a dynamical nuclear polarization as well as its subsequent decay by the dipole-dipole interaction is directly resolved in time. Because not limited by intrinsic nonlinearities, almost complete nuclear polarization is achieved, even at elevated temperatures. The data suggest a nonequilibrium mode of nuclear polarization, distinctly different from the spin temperature concept exploited on bulk semiconductors.

Spin blockade in the conduction of colloidal CdSe nanocrystal films

The conduction of thin films of n-type CdSe colloidal quantum dots is studied at low temperature and under magnetic field. At medium and high magnetic fields (10 T), the films exhibit positive magnetoresistance consistent with the variable range hopping model. At low magnetic field(<0.3 T) but in the strong electric field regime, there is a narrower magnetoresistance of order 10%-15%. The magnetoresistance shows a strong bias dependence, small and positive at low bias, increasing but still positive at higher bias, and turning negative at the highest bias. A similar behavior has been reported recently for thin film organics. Weak localization effects are ruled out. The explanation for the observations is based on spin blockade relaxed by the hyperfine interaction. The weak magnetoresistance at low bias is attributed to the diffusing paths taken by the hopping electrons. At higher bias, the more directed motion of electrons leads to increasingly positive magnetoresistance due to the more effective spin blockade. At the highest bias, the magnetoresistance becomes negative, which is attributed to the increased exchange interaction associated with the shorter tunneling distance.

Nuclear spin switch in semiconductor quantum dots

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a thresholdlike enhancement or reduction of the local nuclear field by up to 3 T can be generated by varying the pumping intensity. The excitation power threshold for such a nuclear spin "switch" is found to depend on both the external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of the spin transfer from electrons to the nuclei of the dot.