• GaAs quantum well
• manganese
• spin

Summary
III-V or II-VI semiconductors doped with magnetic atoms such as (Ga,Mn)As have gathered much interest in recent years because of their potential applications in spin-based computation and quantum information technology. While most electronic devices rely only on the charge of the electron, spin-based electronic (spintronic) devices based on these semiconductor alloys exploit the spin degrees of freedom. In these materials, in particular, the interaction of the spin of the Mn atoms mediated by free carriers leads to ferromagnetic behavior below a critical temperature that depends on the Mn concentration and can reach values close to 200 K in (Ga,Mn)As.
Despite no commercial applications of such dilute magnetic semiconductors (DMS) exist to date, the combination of electronic and magnetic properties in these systems opens potential avenues of fundamental and applied research. In recent years, however, the focus has progressively moved from the regime outlined above, dominated by macroscopic magnetic phenomena and linked to a significant doping of the host crystal with Mn atoms, to the challenging research frontier represented by the manipulation of the spin of single magnetic atoms inserted in the semiconductor crystal. The underlying idea is to acquire the knowledge to develop a new class of spintronic devices that exploit the properties of single magnetic atoms. Such atom-based technology would offer a new paradigm in future electronics.
This emerging scenario motivates the present thesis work. The goal is to probe optically the spin states of few manganese atoms in a single gallium arsenide quantum well (QW). Thanks to the remarkable progresses made in the growth of semiconductor samples, is now possible to reach ultra-dilute concentrations of Mn impurities preserving the crystal quality of the host semiconductor material. Such magnetic atoms inserted in the semiconductor heterostructure have uniform properties and relatively long spin lifetimes allowing for the optical probing of Mn spins and their manipulation.
In the diluted regime, Mn in GaAs acts as an acceptor with the hole that remains weakly bound to it. The neutral complex ground state lies ≈ 110 meV above the top of the valence band of the QW, it has a total angular momentum J=1 and a g-factor g=2.77.
We will show in this thesis that it is possible to directly probe the spin orientation of few neutral Mn acceptors in GaAs/AlGaAs quantum-well heterostructures grown at the University of Santa Barbara by the group of Prof. David Awschalom. To this end we used the resonant inelastic light (Raman) scattering (ILS) technique to detect spin-flip excitations of the neutral Mn acceptor spins between the spin-resolved levels . In a magnetic field B, the spin states of the neutral Mn acceptor split by the Zeeman effect. While by varying the magnitude of the magnetic field above one Tesla the spin-flip excitation energies follow the expected Zeeman dependence gμB, surprisingly a finite splitting of the spin levels is found at zero magnetic field. This unexpected result shines new light on the structure of spin states of Mn atoms in GaAs and calls for the development of novel theoretical models. Consistent with previous results, we also demonstrate the possibility to control the magnetic orientation of the Mn atoms by optically injecting mobile electron spins through circularly polarized laser light at zero magnetic field.
This thesis is organized as follows:
• Chapter 1 starts with a review of the basic theory of electronic states in quantum wells in the envelope function approximation. We describe the selection rules and oscillator strength of interband absorption and the possibility of optically creating a population of polarized electron spins into the conduction band (optical spin injection). We also discuss the electronic configuration of the neutral Mn acceptor in GaAs analyzing the different interactions that determine the dynamics and the orientation of its spin. Finally a section is dedicated to the growth technique, with a brief introduction to Molecular Beam Epitaxy (MBE) followed by the descriptionof the samples studied in this work.
• Chapter 2 contains a brief review of the inelastic light scattering technique used in this thesis, with a particular focus on spin-flip excitations, followed by a technical description of the low-temperature experimental equipment used in this work.
• Chapter 3 illustrates the photoluminescence of the studied (Ga,Mn)As quantum wells. After an initial investigation of the luminescence profile aimed to the identification of the resonances needed for the ILS measurements, we focus on the optical emission linked to the neutral Mn acceptor and we demonstrate the ability to link such emission to the spin polarization of the Mn atoms.
• Chapter 4 reports the main experimental results of this thesis work: the observation of spin-flip excitations between the different spin-resolved levels of the neutral Mn acceptor and their evolution as a function of the magnetic field. We measure the g-factor and offer evidence for an anomalous splitting of the spin states at zero magnetic field.