Anomalous Hanle Effect in Quantum Dots : Evidence for Strong Dynamical Nuclear Polarization in Transverse Magnetic Field

TL;DR

This paper reports an anomalous Hanle effect in quantum dots, showing strong dynamical nuclear polarization that sustains electron spin polarization in high transverse magnetic fields, revealing new spin interaction mechanisms.

Contribution

It provides experimental evidence of a novel nuclear spin polarization mechanism in quantum dots under transverse magnetic fields, with implications for spintronics.

Findings

Hanle curve broadening by a factor of ~20

Bistability in Hanle curve upon magnetic sweep reversal

Abrupt increase in electron Zeeman splitting at collapse

Abstract

Hanle effect is ubiquitous in the study of spin-related phenomena and has been used to determine spin lifetime, precession and transport in semiconductors. Here, we report an experimental observation of anomalous Hanle effect in individual self-assembled InAs/GaAs quantum dots where we find that a sizeable photo-created electron spin polarization can be maintained in transverse fields as high as 1T until it abruptly collapses. The striking broadening of the Hanle curve by a factor of ~20 and its bistability upon reversal of the magnetic sweep direction points to a novel dynamical nuclear spin polarization mechanism where the effective nuclear magnetic field compensates the transverse applied field. This interpretation is further supported by the measurement of actual electron Zeeman splitting which exhibits an abrupt increase at the Hanle curve collapse. Strong inhomogeneous quadrupolar interactions typical for strained quantum dots are likely to play a key role in polarizing nuclear spins perpendicular to the optically injected spin orientation.