Signatures of long-range spin-spin interactions in an (In,Ga)As quantum dot ensemble

TL;DR

This study reveals that long-range spin-spin interactions in quantum dot ensembles influence electron spin dephasing times, with experimental and simulation data showing how laser spectral width and spin coupling affect coherence.

Contribution

It introduces a cluster theory with a Heisenberg interaction model to explain spin dephasing, highlighting the role of long-range interactions in quantum dot ensembles.

Findings

Dephasing time depends weakly on laser spectral width.

Spin-spin interactions synchronize electron spins, counteracting dephasing.

Simulation matches experimental data with mean coupling of ~0.26 μeV.

Abstract

We present an investigation of the electron spin dynamics in an ensemble of singly-charged quantum dots subject to an external magnetic field and laser pumping with circularly polarized light. The spectral laser width is tailored such that different groups of quantum dots are coherently pumped. Surprisingly, the dephasing time $T^*$ of the electron spin polarization depends only weakly on the laser spectral width. These findings can be consistently explained by a cluster theory of coupled quantum dots with a long range electronic spin-spin interaction. We present a numerical simulation of the spin dynamics based on the central spin model that includes a quantum mechanical description of the laser pulses as well as a time-independent Heisenberg interaction between each pair of electron spins. We discuss the individual dephasing contributions stemming from the Overhauser field, the distribution of the electron $g$-factors and the electronic spin-spin interaction as well as the spectral width of the laser pulse. This analysis reveals the counterbalancing effect of the total dephasing time when increasing the spectral laser width. On one hand, the deviations of the electron $g$-factors increase. On the other hand, an increasing number of coherently pumped electron spins synchronize due to the spin-spin interaction. We find an excellent agreement between the experimental data and the dephasing time in the simulation using an exponential distribution of Heisenberg couplings with a mean value $\overline{J}\approx 0.26\,\mathrm{\mu eV}$.