Search

The magneto-optical properties of titanium dioxide systems are related to the presence of impurity states in the band gap due to oxygen vacancies. To understand about the interplay between localized electrons and structural distortions at the vacancy sites and the magneto-optical properties, we employ a self-interaction corrected density functional theory method to calculate bulk and small nanoparticles of rutile, anatase, and brookite titania. Our computations reveal bipolaron configurations associated to an oxygen vacancy with optical transition levels in the band gap. The ground state for these bipolarons is a spin-triplet state in bulk rutile TiO2 and also in the nanoparticles independently of the crystal phase, a result which may support the idea of oxygen vacancies as a source of magnetism in this material. The ground state for bipolarons in bulk anatase TiO2 is however a spin-singlet state, different from the spin-triplet configuration reported in a previous work based on hybrid functionals.

Focusing on the efficient probe and manipulation of single-particle spin states, we investigate the coupled spin and orbital dynamics of a spin 1/2 particle in a harmonic potential subject to ultrastrong spin-orbit interaction and external magnetic field. The advantage of these systems is the clear visualization of the strong spin-orbit coupling in the orbital dynamics. We also investigate the effect of a time-dependent coupling: Its nonadiabatic change causes an interesting interplay of spin and orbital motion which is related to the direction and magnitude of the applied magnetic field. This result suggests that orbital state manipulation can be realized through ultrastrong spin-orbit interactions, becoming a useful tool for handling entangled spin and orbital degrees of freedom to produce, for example, spin desirable polarizations in time interesting for spintronics implementations., Comment: 6 figures

We consider the relaxation of a spin qubit in a quantum dot propagating as a whole in a one-dimensional semiconductor with hyperfine coupling. We show that this motion leads to qualitatively new features in this process compared to static quantum dots. For a fast straightforward motion, the initial spin density decreases to zero with the relaxation rate independent of the spatial spread of the electron wave function and inversely proportional to the electron speed. However, for the oscillatory motion, the qubit acquires memory, and the dephasing becomes Gaussian rather than exponential. After some time, one third of the initial spin polarization is restored, as it happens for static qubits. This revival can occur either through periodic peaks or through a monotonous increase in the polarization, after a minimum, until a plateau has been reached. Our results can be useful for the understanding of the spin dynamics and decoherence in quantum wires.

For nanostructured materials, strain is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, and in consequence their magnetism when it applies. Here we describe a magnetic shape memory alloy in which nanostructure confinement strain influences the crystallographic phase and the electronic and magnetic properties of the resulting nanowires. We use first-principles calculations on shape memory MnAs nanostructures to study the influence of strain on crystal phases, which arises from surfaces. We show that MnAs nanowires down to two nanometers can be stable in a new crystal phase different from bulk hexagonal and induced by one-dimensionality. The changes between structures through use of strain require the existence of twin domains. Our analysis suggests that the strain-induced structural transition, demonstrated here in MnAs compounds, could be extended to other (magnetic) shape memory nanowire systems for applications ranging from mechanical to magneto-electronic devices.

We investigate the electronic and magnetic properties of (Ga,Mn)N nanocrystals using the density functional theory. We study both wurtzite and zinc-blende structures doped with one or two substitutional Mn impurities. For a single Mn dopant placed close to surface, the behavior of the empty Mn-induced state, hereafter referred to as "Mn hole", is different from bulk (Ga,Mn)N. The energy level corresponding to this off-center Mn hole lies within the nanocrystal gap near the conduction edge. For two Mn dopants, the most stable magnetic configuration is antiferromagnetic, and this was unexpected since (Ga,Mn)N bulk shows ferromagnetism in the ground state. The surprising antiferromagnetic alignment of two Mn spins is ascribed also to the holes linked to the Mn impurities located close to surface. Unlike Mn holes in (Ga,Mn)N bulk, these Mn holes in confined (Ga,Mn)N nanostructures do not contribute to the ferromagnetic alignment of the two Mn spins.

We investigate acceptor and donor states in GaN nanocrystals doped with a single substitutional impurity. Quantum dots (QD's) of zinc-blende structure and spherical shape are considered with the radius ranging from 4.5 to 67.7 A. The size-dependent energy spectra are calculated within the sp3d5s* tight-binding model, which yields a good agreement with the confinement-induced blue shifts observed in undoped QD's. The computed binding energy is strongly enhanced with respect to the experimental bulk value when the dopant is placed at the center of the smallest QD's. It decreases with increasing QD size following a scaling law that extrapolates to the bulk limit. In order to estimate the degree of localization of the bound carriers we analyze their wave functions and average radii. The resulting physical picture points to a highly localized acceptor hole, mostly distributed over the nearest-neighbor anion shell, and a much more extended donor electron. We also study off-center impurities in intermediate-size QD's. The acceptor binding energy is approximately independent of the dopant position unless it is placed within a surface shell of thickness of the order of the bulk Bohr radius, where the ionization energy abruptly drops. On the contrary, the donor binding energy gradually decreases as the impurity is moved away from the center toward the QD surface.

We investigate the optical properties of (Cd,Mn)Te quantum dots (QDs) by looking at the excitons as a function of the Mn impurities positions and their magnetic alignments. When doped with two Mn impurities, the Mn spins, aligned initially antiparallel in the ground state, have lower energy in the parallel configuration for the optically active spin-up exciton. Hence, the photoexcitation of the QD ground state with antiparallel Mn spins induces one of them to flip and they align parallel. This suggests that (Cd,Mn)Te QDs are suitable for spin-based operations handled by light.

We investigate the electronic and magnetic properties of Mn-doped CdTe nanocrystals (NCs) with 2 nm in diameter which can be experimentally synthesized with Mn atoms inside. Using the density-functional theory, we consider two doping cases: NCs containing one or two Mn impurities. Although the Mn d peaks carry five up electrons in the dot, the local magnetic moment on the Mn site is 4.65 mu_B. It is smaller than 5 mu_B because of the sp-d hybridization between the localized 3d electrons of the Mn atoms and the s- and p-type valence states of the host compound. The sp-d hybridization induces small magnetic moments on the Mnnearest- neighbor Te sites, antiparallel to the Mn moment affecting the p-type valence states of the undoped dot, as usual for a kinetic-mediated exchange magnetic coupling. Furthermore, we calculate the parameters standing for the sp-d exchange interactions. Conduction N0\alpha and valence N0\beta are close to the experimental bulk values when the Mn impurities occupy bulklike NCs' central positions, and they tend to zero close to the surface. This behavior is further explained by an analysis of valence-band-edge states showing that symmetry breaking splits the states and in consequence reduces the exchange. For two Mn atoms in several positions, the valence edge states show a further departure from an interpretation based in a perturbative treatment. We also calculate the d-d exchange interactions |Jdd| between Mn spins. The largest |Jdd| value is also for Mn atoms on bulklike central sites; in comparison with the experimental d-d exchange constant in bulk Cd0.95Mn0.05Te, it is four times smaller.