Oblique Nuclear Quadrupole Interaction in Self-Assembled Structures Based on Semiconductor Quantum Dots

Abstract

Dynamic nuclear polarization (DNP) is well recognized as being important in spintronics and quantuminformation processing. DNP gives rise to high nuclear spin polarization that not only can prolong electron-spin lifetime by generating an Overhauser field (OHF), but also has fertilized the idea to explore nuclear spin qubits. In strained quantum-dot structures (QDSs), a nuclear spin is coupled to a strain field via its quadrupole moment. It has been shown that such nuclear quadrupole interaction (NQI) can be used to achieve appreciable DNP and hence electron-spin polarization. Here, we uncover magneto-optical anomalies from a series of laterally arranged (In,Ga)As QDSs that arise from the NQI and DNP in these nanostructures. We find that the principal axis of NQI in symmetry-lowered QDSs significantly deviates from the growth direction, resulting in tilting of OHF with an angle exceeding 37°. The resulting transverse component of OHF is probed with respect to the crystallographic orientations and its influence on the DNP and ensemble spin dephasing is analyzed. We show that a high-symmetry electronic confinement potential for excitons does not guarantee a high-symmetry NQI for nuclei within the same nano-object, thereby calling for correlated optimization in the symmetry of the electronic confinement potential and that of the nuclear spin bath. Our results underline the role of oblique NQI in electron-spin decoherence and depolarization, which has so far largely been overlooked. This work thus sheds light on design rules for engineering the electronic and spin landscape of QDSs for better performance of DNP desirable for applications in spintronics and quantum computation.

Anomalous Hanle effect considered in time-resolved measurements and numerical simulations

“…The effect of the nonzero biaxiality of NQI is expressed by adding a term proportional to Î2 X, j − Î2 Y, j where ÎX, j and ÎY, j are the spin operators of jth nucleus which observe spin expectation value orthogonal to each other and the q axis. Similar procedure has already been also applied in the symmetry-lowered InGaAs QD structures [31]. Thus the nonzero biaxiality of NQI could explain the anomalous Hanle effect, in which the q axis can be close to the z axis.…”

“…2(b)]. It has been reported theoretically [30] and experimentally [31] that the major principal axis of NQI is generally tilted from the z axis in actual QDs. Because the q axis contains both the in-plane and z components, the anomalous Hanle effect and gradual decay of B n,z , which arise from the different origins independently, are observed simultaneously.…”

“…The model successfully explains the formation of B n,x and gradual decay of B n,z , but does not consider the tilt of NQI major principal axis [30]. In particular, Huang et al [31] studied magneto-optical anomalies associated with NQI in various symmetry-lowered InGaAs QD structures and convincingly demonstrated that the major principal axis of NQI is generally not along the z axis resulting in the permanent in-plane nuclear field. They further noted that they did not detect a large anisotropy in the hyperfine constant.…”