Giant magnetic anisotropy of InAs/GaAs quantum dots doped with a single Mn atom

In singly Mn-doped InAs/GaAs quantum dots (QDs), the Mn atom is a magnetic impurity of acceptor type which, at low temperatures, remains in a neutral state A⁰. Its total spin is well described by an effective spin J=1 featuring a strong anisotropy related both to its position and to local strains. We have observed that this effect combined to the sp-d exchange with an electron-hole pair, may lead to a spectacular phenomenon: the Zeeman splitting of the QD optical transitions into circularly polarized components by a magnetic field applied perpendicularly to the optical axis.

These unconventional optical selection rules, which drastically contrast to those of non-magnetic QDs, have been evidenced in the micro-photoluminescence of single quantum dots embedding a single Mn atom (Fig.1a). To explain these observations we have developed a theoretical model of the spin-dependent interactions including (i) the local strain anisotropy and (ii) the anisotropic exchange related to the relative Mn position. Our model which provides a very good agreement (see Fig.1b), shows that the sp-d exchange acts as a strong longitudinal magnetic field of up to ~50 T resulting in a giant magnetic anisotropy which hinders the mixing of heavy-hole ⇑ and ⇓ spins by a transverse magnetic field. The optical transitions keep therefore a circular polarization.

Besides their interest for controlling a single spin in condensed matter, such ±'magnetic' quantum dots offer the unique means to explore the magnetism at a microscopic level. For that matter, our present results already question the current theory of the sp-d exchange in a very diluted magnetic semiconductor, which predicts a much weaker interaction than directly observed here.

Figure 1: (a) Optical recombination of a positively charged exciton (X+) in a quantum dot containing a single Mn impurity A0. (b) Experimental and simulated density-plots of µ-PL spectra as a function of a longitudinal (top) and perpendicular (bottom) magnetic field, measured in σ± circularly polarizations as indicated. The clear asymmetry between the σ+ and σ- PL signal indicates a strong circular polarization of the transitions.

Contacts :

Olivier Krebs (Optics of Semiconductor nanoStructures research team)

Aristide Lemaître (Elaboration and Physics of Epitaxial Structures research team)

Further reading

O. Krebs, E. Benjamin, and A. Lemaître, Phys. Rev. B 80, 165315 (2009)