Hanle effect in (In,Ga)As quantum dots: Role of nuclear spin fluctuations

Abstract

The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified with changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model describing cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to determine the parameters of the electron-nuclear spin system of the studied quantum dots.

Spin dynamics of quadrupole nuclei in InGaAs quantum dots

“…As it is discussed in Ref. 26, such behavior of nuclear field is responsible for the W-structure in Hanle curves. Subsequent decrease of the B N dx component completes the W-structure.…”

“…1 The W-structure, in particular, the dips in the structure, are formed due to the large nuclear field, which magnifies the effect of external magnetic field on the electron spin. 26,43 At larger magnetic fields, i.e., at the wings of Hanle curves, the electron polarization is additionally suppressed by the nuclear field added to the external magnetic field. Therefore, it would be expected that the modulation of excitation modulation suppressing the nuclear polarization should partially restore the electron polarization at the wings of Hanle curve.…”

“…7 In this case, the stability of nuclear spin system should be determined by processes of longitudinal spin relaxation of quadrupole nuclei with characteristic time T 1 >> T 2 . Although many publications are devoted to the nuclear spin polarization 6,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] (see also review articles 2, [27][28][29] ), there are very few works where relaxation dynamics is studied for quadrupole nuclei. [30][31][32] In this paper we report on experimental study of spin dynamics of quadrupole nuclei in the singly-charged (In,Ga)As/GaAs QDs.…”

Spin phenomena in quantum dots revealed by trion photoluminescence

“…We propose a model considering cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electronnuclear spin system of the studied quantum dots [11].…”

“…The number of such atoms is order of 10 5 usually and the highly coupled system of carrier spin interacting with the effective magnetic field of nuclear spins (Overhauser field) is the typical example of multi-degrees freedom system. Here the brief review of recent study results of such system [2][3][4][5][6][7][8][9][10][11][12] will be presented.…”