R. V. Cherbunin

Negative circular polarization of InP QD luminescence: Mechanism of formation and main regularities

The spectrum and kinetics of the circular polarization of InP quantum dot (QD) photoluminescence have been experimentally investigated under different conditions of optical excitation and at different bias voltages applied to the sample. It is established that, at a bias of about −0.1 V, the degree of photoluminescence polarization is negative and reaches −50% in limiting cases. It is concluded that the negative polarization is formed in QDs containing one recident electron per dot and is mainly caused by the optical orientation of the electron spin. It is shown that all experimentally observed regularities are well described in the framework of the model assuming the energy relaxation of photogenerated electron-hole pairs accompanied by the electron- hole spin flip-flop process.

Dynamics of nuclear polarization in InGaAs quantum dots in a transverse magnetic field

The time-resolved Hanle effect is examined for negatively charged InGaAs/GaAs quantum dots. Experimental data are analyzed by using an original approach to separate behavior of the longitudinal and transverse components of nuclear polarization. This made it possible to determine the rise and decay times of each component of nuclear polarization and their dependence on transverse magnetic field strength. The rise and decay times of the longitudinal component of nuclear polarization (parallel to the applied field) were found to be almost equal (approximately 5 ms). An analysis of the transverse component of nuclear polarization shows that the corresponding rise and decay times differ widely and strongly depend on magnetic field strength, increasing from a few to tens of milliseconds with an applied field between 20 and 100 mT. Current phenomenological models fail to explain the observed behavior of nuclear polarization. To find an explanation, an adequate theory of spin dynamics should be developed for the nuclear spin system of a quantum dot under conditions of strong quadrupole splitting.

Measurement of the Knight field and local nuclear dipole-dipole field in an InGaAs/GaAs quantum dot ensemble

We present a comprehensive investigation of the electron-nuclear system of negatively charged InGaAs/GaAs self-assembled quantum dots (QDs) under the influence of weak external magnetic fields (up to 3 mT). We demonstrate that, in contrast to conventional semiconductor systems, these small fields have a profound influence on the electron spin dynamics, via the hyperfine interaction. QDs, with their comparatively limited number of nuclei, present electron-nuclear behavior that is unique to low-dimensional systems. We show that the conventional Hanle effect used to measure electron-spin relaxation times, for example, cannot be used in these systems when the spin lifetimes are long. An individual nucleus in the QD is subject to milli-Tesla effective fields, arising from the interaction with its nearest neighbors and with the electronic Knight field. The alignment of each nucleus is influenced by application of external fields of the same magnitude. A polarized nuclear system, which may have an effective field strength of several Tesla, may easily be influenced by these milli-Tesla fields. This in turn has a dramatic effect on the electron-spin dynamics and we use this technique to gain a measure of both the dipole-dipole field and the maximum Knight field in our system thus allowing us to estimate the maximum Overhauser field that may be generated at zero external magnetic field. We also show that one may fine tune the angle which the Overhauser field makes with the optical axis.

Spin dynamics of quadrupole nuclei in InGaAs quantum dots

Photoluminescence polarization is experimentally studied for samples with (In,Ga)As/GaAs selfassembled quantum dots in transverse magnetic field (Hanle effect) under slow modulation of the excitation light polarization from fractions of Hz to tens of kHz. The polarization reflects the evolution of strongly coupled electron-nuclear spin system in the quantum dots. Strong modification of the Hanle curves under variation of the modulation period is attributed to the peculiarities of the spin dynamics of quadrupole nuclei, which states are split due to deformation of the crystal lattice in the quantum dots. Analysis of the Hanle curves is fulfilled in the framework of a phenomenological model considering a separate dynamics of a nuclear field BNd determined by the +/- 12 nuclear spin states and of a nuclear field BNq determined by the split-off states +/- 3/2, +/- 5/2, etc. It is found that the characteristic relaxation time for the nuclear field BNd is of order of 0.5 s, while the relaxation of the field BNq is faster by three orders of magnitude.

Very weak oscillating magnetic field disrupts the magnetic compass of songbird migrants

Previously, it has been shown that long-distance migrants, garden warblers (Sylvia borin), were disoriented in the presence of narrow-band oscillating magnetic field (1.403 MHz OMF, 190 nT) during autumn migration. This agrees with the data of previous experiments with European robins (Erithacus rubecula). In this study, we report the results of experiments with garden warblers tested under a 1.403 MHz OMF with various amplitudes (∼0.4, 1, ∼2.4, 7 and 20 nT). We found that the ability of garden warblers to orient in round arenas using the magnetic compass could be disrupted by a very weak oscillating field, such as an approximate 2.4, 7 and 20 nT OMF, but not by an OMF with an approximate 0.4 nT amplitude. The results of the present study indicate that the sensitivity threshold of the magnetic compass to the OMF lies around 2–3 nT, while in experiments with European robins the birds were disoriented in a 15 nT OMF but could choose the appropriate migratory direction when a 5 nT OMF was added to the stationary magnetic field. The radical-pair model, one of the mainstream theories of avian magnetoreception, cannot explain the sensitivity to such a low-intensity OMF, and therefore, it needs further refinement.

Multiple-frequency quantum beats of quantum confined exciton states

A. V. Trifonov,1 I. Ya. Gerlovin,1 I. V. Ignatiev,1 I. A. Yugova,1 R. V. Cherbunin,1 Yu. P. Efimov,2 S. A. Eliseev,2 V. V. Petrov,2 and A. V. Kavokin1, 3 Spin Optics Laboratory, Saint Petersburg State University, 198504 St. Petersburg, Russia SPbU Resource Center “Nanophotonics”, Saint Petersburg State University, 198504 St. Petersburg, Russia Physics and Astronomy School, University of Southampton, Highfield, Southampton, SO171BJ, UK (Dated: July 16, 2015)

Nuclear spin cooling by helicity-alternated optical pumping at weak magnetic fields in n -GaAs

The spin dynamics of localized donor-bound electrons interacting with the nuclear spin ensemble in

Time-resolved Hanle effect in (In,Ga)As/GaAs quantum dots

n

Carrier spin dynamics in GaAs/AlGaAs quantum wells with a laterally localizing electric potential

-doped GaAs epilayers is studied using nuclear spin polarization by light with modulated circular polarization. We show that the observed build-up of the nuclear spin polarization is a result of competition between nuclear spin cooling and nuclear spin warm-up in the oscillating Knight field. The developed model allows us to explain the dependence of the nuclear spin polarization on the modulation frequency, and to estimate the equilibration time of the nuclear spin system, which appears to be shorter than the transverse relaxation time

Inverse-phase Rabi oscillations in semiconductor microcavities

T_2