A hole-Cr$^{+}$ nano-magnet in a semiconductor quantum dot

TL;DR

This paper investigates a novel hole-Cr$^+$ nano-magnet in a CdTe quantum dot, demonstrating stable ferromagnetic coupling, controllable spin states via optical pumping, and a long spin relaxation time at low temperature.

Contribution

It introduces a stable hole-Cr$^+$ complex in a quantum dot and explores its ferromagnetic properties and spin dynamics, a new approach in diluted magnetic semiconductors.

Findings

Stable Cr$^+$ ions in quantum dots can form ferromagnetic complexes.

Optical pumping can control the magnetic states of the nano-magnet.

Long spin relaxation times (~20 μs) are achievable at low temperature.

Abstract

We study a new diluted magnetic semiconductor system based on the spin of the ionized acceptor Cr$^+$. We show that the negatively charged Cr$^+$ ion, an excited state of the Cr in II-VI semiconductor, can be stable when inserted in a CdTe quantum dot (QD). The Cr$^+$ attracts a heavy-hole in the QD and form a stable hole-Cr$^+$ complex. Optical probing of this system reveals a ferromagnetic coupling between heavy-holes and Cr$^+$ spins. At low temperature, the thermalization on the ground state of the hole-Cr$^+$ system with parallel spins prevents the optical recombination of the excess electron on the 3$d$ shell of the atom. We study the dynamics of the nano-magnet formed by the hole-Cr$^+$ exchange interaction. The ferromagnetic ground states with M$_z$=$\pm$4 can be controlled by resonant optical pumping and a spin relaxation time in the 20 $\mu$s range is obtained at T=4.2 K. This spin memory at zero magnetic field is limited by the interaction with phonons.