Strain-induced coherent dynamics of coupled carriers and Mn spins in a quantum dot

Abstract

We report on the coherent dynamics of the spin of an individual magnetic atom coupled to carriers in a semiconductor quantum dot which has been investigated by resonant photoluminescence of the positively charged exciton (X + ). We demonstrate that a positively charged CdTe/ZnTe quantum dot doped with a single Mn atom forms an ensemble of optical Λ systems which can be addressed independently. We show that the spin dynamics of the X + -Mn complex is dominated by the electronMn exchange interaction and report on the coherent dynamics of the electron-Mn spin system that is directly observed in the time domain. Quantum beats reflecting the coherent transfer of population between electron-Mn spin states, which are mixed by an anisotropic strain in the plane of the quantum dot, are clearly observed. We finally highlight that this strain induced coherent coupling is tunable with an external magnetic field.Semiconductor quantum dots (QDs) are solid-state systems which permit efficient manipulation of single charge and spin. The optical properties of a QD can be used to control the spin state of carriers, nuclei as well as individual 1-4 or pairs 5,6 of magnetic atoms. The spin of one magnetic atom in a QD can be prepared by the injection of spin polarized carriers and its state can be read through the energy and polarization of the photons emitted by the QD. The insertion of a magnetic atom in a QD where the charge and strain states can be controlled offers additional degrees of freedom to tune the properties of the localized spin. Development of information processing using the spin of magnetic atoms in semiconductors will require tuning the coherent coupling between two or more spins. Controlling the exchange coupling of magnetic atoms with the spin of a carrier is an attractive approach to coherently transfer information between the atomic localized spins.Here we report on the coherent dynamics of the spin of a magnetic atom coupled to the spin of individual carriers. This dynamics has been investigated by resonant photoluminescence (PL) of the positively charged exciton (X + ) coupled to a single Mn in a p-doped CdTe/ZnTe QD 7 . We have analysed the spin dynamics of the X + -Mn complex under resonant excitation and show that it is controlled by the electron-Mn (e-Mn) interaction. We demonstrate that a positively charged Mn-doped QD forms optical Λ systems that can be addressed with resonant lasers. We show that the coherent dynamics of the coupled electron and Mn spins is extremely sensitive to the local strain at the Mn location. Oscillations in the circular polarization rate of the resonant PL reveal quantum beats between e-Mn spin states mixed by an anisotropic strain in the plane of the QD. This coherent coupling, characterized by a change in the angular momentum ∆S z = ±2, can be tuned by a magnetic field.The sample consist of Mn-doped CdTe QDs grown by molecular beam epitaxy on a p-doped ZnTe(001) substrate 8 . A positive bias voltage is applied between a 6 nm thick gold semi-transparent Schottcky gate deposi...

Resonant photoluminescence and dynamics of a hybrid Mn hole spin in a positively charged magnetic quantum dot

“…When scanning a resonant laser through the high energy transitions of the X + -Mn, three absorption resonances are observed 25 . These, labelled (1), (2) and (3) in Fig.…”

“…2(b). They correspond, for a σ+ laser, to the successive resonant excitation of the electron-Mn levels |3, +1 , |3, +2 and |2, +2 25 . These states can be expressed as linear combinations of the Mn and electron spins |S z , σ z coupled by a flip-flop:…”

“…The population transfers from level j to level i in an irreversible process associated with a coupling to a reservoir is described by a Lindblad term of the form L inc,j→i ̺ = Γ j→i 2 (2|i j|̺|j i| − ̺|j j| − |j j|̺) (25) where Γ j→i is the incoherent relaxation rate from level j to level i. Such term can describe the radiative decay of the exciton (irreversible coupling to the photon modes) or the relaxation of the carriers or Mn spin (irreversible coupling to the phonon modes).…”

“…It corresponds to the optically allowed recombination on the second branch of the Λ system. This recombination occurs with a flip-flop of the electron and Mn spins 24,25 . The energy splitting between the resonant absorption and the emission corresponds to the splitting between the two ground states of the Λ system.…”

“…It has been shown in charge tunable Mn-doped CdTe/ZnTe QDs that the hybrid Mn-hole spin can be prepared by optical orientation under excitation with circularly polarized light 24 . It was also demonstrated that a positively charged Mn-doped QD forms an ensemble of optical Λ systems that can be independently addressed 25 . This level structure suggests that the hybrid Mn-hole spin states could be coherently manipulated using two resonant optical fields 26 .…”