María del Carmen Heredia Moreno

Lattice distortion of MnAs nanocrystals embedded in GaAs: Effect on the magnetic properties

The x-ray coherent scattering in nanosized MnAs crystallites embedded in a GaAs matrix has been detected. The room-temperature interatomic distances along three orthogonal directions of the crystallites are determined. The MnAs nanocrystals are found to exhibit an anisotropically distorted hexagonal structure as compared to unstrained bulk MnAs. Despite the crystallite lattice distortion, the granular GaAs:MnAs material exhibits robust ferromagnetism, with enhanced transition temperature. The observed magnetic behavior is consistently explained by a localized double-exchange model of MnAs ferromagnetism, where magnetic order appears for large enough Mn–As–Mn distances, i.e., for weak enough p-d hybridization.

Enhanced electron correlations, local moments, and Curie temperature in strained MnAs nanocrystals embedded in GaAs

We have studied the electronic structure of hexagonal MnAs, as epitaxial continuous film on GaAs001 and as nanocrystals embedded in GaAs, by Mn 2p core-level photoemission spectroscopy. Configurationinteraction analyses based on a cluster model show that the ground state of the embedded MnAs nanocrystals is dominated by a d 5 configuration that maximizes the local Mn moment. Nanoscaling and strain significantly alter the properties of MnAs. Internal strain in the nanocrystals results in reduced p-d hybridization and enhanced ionic character of the Mn-As bonding interactions. The spatial confinement and reduced p-d hybridization in the nanocrystals lead to enhanced d-electron localization, triggering d-d electron correlations, and enhancing local Mn moments. These changes in the electronic structure of MnAs have an advantageous effect on the Curie temperature of the nanocrystals, which is measured to be remarkably higher than that of bulk MnAs.

Photoemission results on intralayer insertion at III-V/III-V junctions: A critical appraisal of the different interpretations

Several researchers have proposed that band discontinuities at semiconductor heterojunctions may be “tuned” by inserting very thin layers of foreign atoms at the interface which are thought to induce an “interface dipole.” Modifications of the apparent valence-band offset, as measured by photoelectron spectroscopy (PES), have been indeed observed upon Si insertion at GaAs–AlAs interfaces, and they have been generally interpreted as real band-offset changes. However, there is an alternative explanation of the photoemission results in terms of band-bending effects. Here, we present results of PES experiments designed to test the two opposing interpretations. We have examined the effect of Si insertion at polar (100) and nonpolar (110) interfaces, and we have studied the insertion of Si (n-type) and Be (p-type) intralayers. Similar results are obtained for polar and nonpolar interfaces, and effects of opposite sign are observed for Si and Be intralayers. These results can be readily interpreted in terms of a...

Anisotropic strain fields in granular GaAs:MnAs epitaxial layers: Towards self-assembly of magnetic nanoparticles embedded in GaAs

Granular GaAs:MnAs, consisting of MnAs nanoclusters embedded in a GaAs matrix, is a hybrid ferromagnet-semiconductor material with potential applications in information storage, magneto-optical, and spin electronics devices. It can be obtained, through phase separation, by high-temperature annealing of diluted (Ga,Mn)As films grown by molecular-beam epitaxy. The granular material thus obtained exhibits room-temperature ferromagnetism and excellent crystal quality, but the magnetic anisotropy is weak and control of the ordering of the clusters in lateral directions of the film has not been achieved yet. We have investigated the strain state of granular GaAs:MnAs films on GaAs(001) substrates at room temperature by x-ray diffraction. Two-dimensional reciprocal-space maps are presented, including x-ray reflections from the GaAs matrix and from the nanosized MnAs crystallites. Based on the x-ray diffraction results, we propose strategies to guide the assembly of the MnAs precipitates within the GaAs matrix, s...

Electronic states in arsenic-decapped MnAs (1(1)over-bar00) films grown on GaAs(001): A photoemission spectroscopy study

We examine the arsenic bonding in the near-surface region of initially arsenic-capped MnAs(11¯00) films grown on GaAs(001), as it evolves upon arsenic decapping. Line-shape analyses of high-resolution As 3d photoelectron emission spectra recorded at room temperature (RT) allow us to identify electronically distinct As-bonding states associated to bulk MnAs phases, bulk arsenic, and interfacial environments. Stable MnAs phases appear to be affected by the presence of a thin arsenic coating, an effect that could be advantageously used to enhance the ferromagnetic properties of MnAs films around RT.

Control of spatial instabilities with intracavity photonic crystals

Unwanted spatial instabilities often spoil the quality of the beam in broad area devices, but a periodic modulation of the refractive index, i.e. a photonic crystal, can be used to control the transverse properties of the emitted light. Due to non linearity and cavity losses this problem can be studied in the context of self-organization. Indeed, for large Fresnel numbers the emitted beams can exhibit not only temporal instability but also spontaneous pattern formation.

Control of spatial quantum fluctuations using photonic crystals

Optical parametric oscillators emit light with non-classical correlations between opposite spatial modes (twin beams). We consider these devices in presence of an intracavity photonic crystal, modeled by a spatial modulation of the refractive index. The introduction of photonic crystals allows to control not only the macroscopic transverse profile of the emitted light beam but also its quantum fluctuations. We employ the Q representation to study pump and signal spatial correlations.

Electronic signature of MnAs phases in bare and buried films grown on GaAs(001)

Photoelectron emission spectroscopy analyses of the arsenic bonding in the near-surface region of an initially arsenic-capped MnAs (11¯00) film grown on GaAs(001) have been carried out for progressive thermal decapping stages. Electronically distinct As-bonding states are identified and assigned to bulk MnAs phases, bulk arsenic, and interfacial environments. The arsenic coating imposes mechanical constraints to the MnAs film, in addition to those imposed by the GaAs substrate, which appear to alter the relative stability of the α and β MnAs phases around room temperature.