SAINTPETERSBURG STATE UNIVERSITY EXPERIMENTAL STUDY OF SPIN MEMORY

SAINT-PETERSBURG STATE UNIVERSITY EXPERIMENTAL STUDY OF SPIN MEMORY IN NANOSTRUCTURES ROMAN V. CHERBUNIN

1. About a possibility of the “spin devices” creation. CONTENTS 2. Optical orientation - method of creation of the nonequilibrium spins in semiconductors and semiconductor structures. 3. Detection of a non-equilibrium spin in semiconductors by the study a) of luminescence. b) of reflection (Faraday effect). 4. Wavelength dependence of Kerr effect. 5. Spin lifetime. Possible mechanisms of spin relaxation. 6. Our experiments. Time-resolved observation of a spin dynamics. Modification of a pump-probe method. 7. Future trends of a “spin devices”. Adaptation of quantum dots for this applications.

OUR DREEMS “LONG-TIME” SPIN MEMORY “FAST” SPIN MEMORY (DURIND ONE CLOCK) GAP QW QW SUBSTRATE We have a two dimensional array of electrons in a quantum We have a two dimensional array of excitons in a quantum well. This electrons content our information in it’s polarization. To save information electrons have do not move and do not change polarization. excitons also can content information in it’s polarization. But their lifetime is about 10 -11 s, and it is shorter than any other time. If we study spin polarization using light, it isn't ease to separate electron and exciton spin polarization.

OPTICAL ORIENTATION IN Ga. As QUANTUM WELLS Conduction band: Conduction band I = 0; S =1/2 J =1/2 ; m = ± 1/2 Al. Ga. As Valence band: Ga. As Al. Ga. As I = 1; S =1/2 J =1/2 ; m = ± 1/2 Valence band J =3/2 ; m =± 1/2 (light hole) m=± 3/2 (heavy hole) In quantum wells, to take a spin polarization in a case of photo inducted carriers is quite simple. Irradiation by circular polarized light produce 100% spin polarization. Absorption of circular polarized light in a bulk Ga. As result in an appearance of two kinds of excitons: (-3/2, +1/2) and (+1/2, -1/2). In a narrow quantum wells they energy levels are different, and we can excite them separately. Laser Glan prism Retarder λ/4 Sample in cryostat

CRATION OF ELECTRON SPIN POLARIZATION Electrons from donors Photo inducted electron – hole pairs Circularly polarized light creates polarized electrons and holes. Hole lost spin orientation in a few picoseconds (10 -12). If there are electrons from donors, they could recombine with holes as well as photo inducted ones. The remained after recombination electrons would have a preferred spin orientation. This orientation would not 100%, but for our study even 10% is still enough. Note 1: If electron – hole pairs are being produced in a bound state (exciton), they are also polarized free electrons, but this process is more complicated and is not enough studied.

SPIN POLARIZATION MEASUREMENT Electrons from donors Где Photo inducted electron – hole pairs Elliptically polarized light (luminescence) With light cooperate electron - hole pairs, whose total angular moment does not exceed unit. Electron with spin 1/2 and hole with spin 3/2 can have the total moment 1 or 2 depending on their mutual orientation. As it was mentioned above, a hole in time about ten picoseconds loses orientation. But the electron - hole pair can be highlighted only at that moment when hole orientation will return to an initial state. Thus, the luminescence will be polarized the same as an absorbed light.

USE FARADAY EFFECT FOR SPIN POLARIZATION MEASUREMENT Nonlinear Faraday effect Simple Faraday effect Medium in a magnetic field m =+1 α σ- σ+ α ~ Δn ~ B Refractive index σ+ Photon energy α m =+1 m =-1 Δ E = μB g B m =-1 Refractive index Pumped medium Photon energy σ- Different level population makes refractive indexes for σ + and σ – polarized light different. It results in light rotation.

SPIN RELAXAOTION q The main mechanism for the hole spin relaxation is interaction with phonons. q The reason of high efficiency of this mechanism is that a hole has orbit angular momentum. q There are many reasons for spin relaxation in semiconductor. All of them are based on particle motion. current magnetic field q Magnetic momentum rotate slowly around the magnetic field

MODIFICATION OF PUMP-PROBE SETUP Ti: S PULSED Laser Pulse duration: 100 fs. Repetition rate: 80 MHz (12, 5 ns). Delay length: 30 cm (600 ps) Half mirror Static delay Variable delay Probe shaper Pump shaper (Monochromator & Glan) Lens Sample in cryostat Retarder λ/4 Lens Balance photo detector T = 10 K. Light power: 800 W cm-2 Sensitivity: 10 -6 rad (10 -8 grad).