arXiv subjects

M. Peressi

Publications and source records attributed to M. Peressi.

10 recordsLinked to original sources

arXiv2007

Computational and experimental imaging of Mn defects on GaAs (110) cross-sectional surface

We present a combined experimental and computational study of the (110) cross-sectional surface of Mn $δ$-doped GaAs samples. We focus our study on three different selected Mn defect configurations not previously studied in details, namely surface interstitial Mn, isolated and in pairs, and substitutional Mn atoms on cationic sites (Mn$_{\rm Ga}$) in the first subsurface layer. The sensitivity of the STM images to the specific local environment allows to distinguish between Mn interstitials with nearest neighbor As atoms (Int$_{\rm As}$) rather than Ga atoms (Int$_{\rm Ga}$), and to identify the fingerprint of peculiar satellite features around subsurface substitutional Mn. The simulated STM maps for Int$_{\rm As}$, both isolated and in pairs, and Mn$_{\rm Ga}$ in the first subsurface layer are consistent with some experimental images hitherto not fully characterized.

cond-mat.mtrl-sci↗

arXiv2006

Non-collinear Magnetic states of Mn5Ge3 compound

Mn5Ge3 thin films epitaxially grown on Ge(111) exhibit metallic conductivity and strong ferromagnetism up to about 300 K. Recent experiments suggest a non-collinear spin structure. In order to gain deep insights into the magnetic structure of this compound, we have performed fully unconstrained ab-initio pseudopotential calculations within density functional theory, investigating the different magnetic states corresponding to Collinear (C) and Non-Collinear (NC) spin configurations. We focus on their relative stability under pressure and strain field. Under pressure, the C and NC configurations are degenerate, suggesting the possible occurrence of accidental magnetic degeneracy also in Mn5Ge3 real samples. We found a continuous transition from a ferromagnetic C low-spin state at small volumes to a NC high-spin state at higher volumes. Remarkably, the degeneracy is definitely removed under the effect of uniaxial strain: in particular, NC spin configurations is favoured under tensile uniaxial strain.

cond-mat.mtrl-sci↗

arXiv2005

ZnSe/GaAs(001) heterostructures with defected interfaces: structural, thermodynamic and electronic properties

We have performed accurate \emph{ab--initio} pseudopotential calculations for the structural and electronic properties of ZnSe/GaAs(001) heterostructures with interface configurations accounting for charge neutrality prescriptions. Beside the simplest configurations with atomic interdiffusion we consider also some configurations characterized by As depletion and cation vacancies, motivated by the recent successfull growth of ZnSe/GaAs pseudomorphic structures with minimum stacking fault density characterized by the presence of a defected (Zn,Ga)Se alloy in the interface region. We find that--under particular thermodynamic conditions--some defected configurations are favoured with respect to undefected ones with simple anion or cation mixing, and that the calculated band offsets for some defected structures are compatible with those measured. Although it is not possible to extract indications about the precise interface composition and vacancy concentration, our results support the experimental indication of (Zn,Ga)Se defected compounds in high-quality ZnSe/GaAs(001) heterojunctions with low native stacking fault density. The range of measured band offset suggests that different atoms at interfaces rearrange, with possible presence of vacancies, in such a way that not only local charges but also ionic dipoles are vanishing.

cond-mat.mtrl-sci↗

arXiv2005

Structural and magnetic properties of Mn-doped GaAs(110) surface

We have investigated STM images of the (110) cross-sectional surface of Mn-doped GaAs using first principles total-energy pseudopotential calculations. We focus on configurations with Mn interstitial in the uppermost surface layers. In particular, we have found that Mn impurities, surrounded by Ga or As atoms, introduce in both cases strong local distortions in the GaAs(110) surface, with bond length variations up to 8 % on surface and non-negligible relaxations effects propagating up to the third sub-surface layer. In both cases interstitial Mn induces a spin-polarization on its nearest neighbors, giving rise to a ferromagnetic Mn-As and to antiferromagnetic Mn-Ga configuration.

cond-mat.mtrl-sci↗

arXiv2005

Composition and strain dependence of band offsets at metamorphic In$_{x}$Ga$_{1-x}$As/In$_{y}$Al$_{1-y}$As heterostructures

We have studied the In$_{x}$Ga$_{1-x}$As/In$_{y}$Al$_{1-y}$As (001) interface using first-principles ab-initio pseudopotential calculations, focusing on the effects of alloy composition and strain state on the electronic properties. In particular we estimate a valence band offset (VBO) of 0.11 eV (InGaAs higher), including spin-orbit and self-energy corrections, for a strain-compensated configuration with homogenous composition $x=y=0.75$ on a lattice-matched substrate. Unintentional composition fluctuations which are typically limited to a few percent and different short-range order effects give rise only to small variations on the VBO, of the order of 0.1 eV or less, whereas intentional substantial changes in the alloys composition allow to achieve a high tunability of band offsets. We predict a VBO varying in a range of about 1.1 eV for interfaces between the pure arsenides in different strain states as extreme cases of composition variation at In$_{x}$Ga$_{1-x}$As/In$_{y}$Al$_{1-y}$As heterostructures.

cond-mat.mtrl-sci↗

arXiv2004

Structural properties and stability of defected ZnSe/GaAs(001) interfaces

Accurate ab-initio pseudopotential calculations within density functional theory in the LDA approximation have been performed for structural properties and stability of ZnSe/GaAs(001) defected heterostructures. There is a strong experimental evidence that ZnSe/GaAs heterostructures with minimum stacking fault density are related to the presence of a substantial concentration of Ga vacancies at interface. In order to gain insights into the still unknown microscopic maechanism governing their formation and stability, we compared the relative stability of some simple selected interface configurations, chosen taking into account charge neutrality prescription and allowing the presence of Ga vacancy next to the interface. Remarkably, our results show that, under particular thermodynamic conditions, some interfaces with vacancies are favoured over undefected ones.

cond-mat.mtrl-sci↗

arXiv2002

Electronic Properties of Mn-Compounds Under Strain

We study the physical properties of MnAs under strain by using accurate first-principles pseudopotential calculations. Our results provide new insight on the physics of strained multilayer that are grown epitaxially on different lattice mismatched substrates and which are presently of interest for spintronic applications. We compute the strain dependence of the structural parameters, electronic bands, density of states and magnetization. In the region of strain/stress that is easily directly accessible to measurements, the effects on these physical quantities are linear. We also address the case of uniaxial stress inducing sizeable and strongly non linear effects on electronic and magnetic properties.

cond-mat.mtrl-sci↗

arXiv1999

Wannier-functions characterization of floating bonds in a-Si

We investigate the electronic structure of over-coordinated defects in amorphous silicon via density-functional total-energy calculations, with the aim of understanding the relationship between topological and electronic properties on a microscopic scale. Maximally-localized Wannier functions are computed in order to characterize the bonding and the electronic properties of these defects. The five-fold coordination defects give rise to delocalized states extending over several nearest neighbors, and therefore to very polarizable bonds and anomalously high Born effective charges for the defective atoms.

cond-mat.mtrl-sci↗

arXiv1999

Floating bonds and gap states in a-Si and a-Si:H from first principles calculations

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T_5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T_5 defects.

cond-mat.mtrl-sci↗

arXiv1999

Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations

We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.

cond-mat.mtrl-sci↗