Dynamic nuclear spin polarization in self-assembled quantum dots under zero magnetic field

Abstract

We studied the zero-field dynamic nuclear spin polarization in a single In0.75Al0.25As/Al0.3Ga0.7As quantum dot. Even without any external magnetic field, the positive trion excited by the circularlypolarized light generated an Overhauser field of up to ∼0.8 T. From the excitation power dependences of the Overhauser field and degree of circular polarization of the photoluminescence spectra, the relation between the Overhauser field and Knight field under zero external magnetic field was revealed clearly. In addition, we found that the nuclear depolarization rate decreased as the magnitude of the longitudinal magnetic field increased, which seemed to be caused by the influence of the quadrupolar interaction of nuclear spins. Further, the key parameters describing the dynamics of a coupled electron-nuclear spin system, the electron g-factor and the fluctuation of the Overhauser field, were evaluated in a typical single InAlAs quantum dot.

In-plane nuclear field formation investigated in single self-assembled quantum dots

“…This is because the relative strength of QI to B x reduces with increasing |B x | and the NSP component perpendicular to B x is easy to relax. In our InAlAs QDs, the magnitude of the quadrupolar field is estimated to be ∼280 mT from other experiments [22]. Further, we assume an anisotropic nature in the nucleus or electron g-tensor, that is, the sign of the in-plane g-factor is opposite to that of z-component.…”

“…Figure 6 ⊥ |, and it indicates that the in-plane gfactor anisotropy is negligible. In addition to the in-plane gfactors, the electron and hole g-factors in z-direction of this QD were evaluated independently to be g e z = −0.34 ± 0.02 and g h z = 2.57 ± 0.01 by the method canceling an opticallyinduced nuclear magnetic field in z-direction by a longitudinal field [22]. The obtained set of the electron g-factor, (g e…”

“…The details of the experimental apparatus is seen in Ref. 22. The energy resolution of our measurement system is ≤ 5 µeV by spectral fitting.…”

Quadrupolar effect on nuclear spin depolarization in single self-assembled quantum dots

“…The observed ΔE Z corresponds to an OHS (ΔE OHS ), and by using the relation ÁE OHS ¼ g e z B B N (g e z : the electron g factor, μ B : the Bohr magneton), B N is evaluated to be −760 mT, where the g e z ¼ þ0:34 obtained from the other measurement is considered. 24) In addition, the DCP of X + -PL is evaluated as ∼60%, which is larger than the value expected from the frozen fluctuation model where 〈S z 〉 decreases to one third of the initial value owing to the random distribution of the direction and magnitude of the fluctuating Overhauser field, ΔB N . 7,11) Note that the DCP of ∼33% is the maximum value under the condition of |B z + B N | = 0, and it is achieved when the photoinjected 〈S z 〉 equals unity after energy relaxation to the ground state.…”

Sign identification of electron and hole out-of-plane g factors by utilizing nuclear spin switch in single quantum nanostructures

“…Although we vary the excitation polarization systematically for the demonstration, the magnitude and sign of the out-of-plane g factors can be deduced by observation of the PL spectra at only three excitation polarization π y (or π x ), σ − , and σ + under a positive B z . Further, this method offers advantages to evaluate the fluctuation of B n directly by combining the electron Zeeman part of ∆E and DCP in the case of X + [34].…”