arXiv subjects

Davide Sangalli

Publications and source records attributed to Davide Sangalli.

44 records · Page 3Linked to original sources

arXiv2013

The role of oxygen vacancies on the structure and the density of states of iron doped zirconia

In this paper we study, both with theoretical and experimental approach, the effect of iron doping in zirconia. Combining density functional theory (DFT) simulations with the experimental characterization of thin films, we show that iron is in the Fe3+ oxidation state and accordingly that the films are rich in oxygen vacancies (VO). VO favor the formation of the tetragonal phase in doped zirconia (ZrO2:Fe) and affect the density of state at the Fermi level as well as the local magnetization of Fe atoms. We also show that the Fe(2p) and Fe(3p) energy levels can be used as a marker for the presence of vacancies in the doped system. In particular the computed position of the Fe(3p) peak is strongly sensitive to the VO to Fe atoms ratio. A comparison of the theoretical and experimental Fe(3p) peak position suggests that in our films this ratio is close to 0.5. Besides the interest in the material by itself, ZrO2:Fe constitutes a test case for the application of DFT on transition metals embedded in oxides. In ZrO2:Fe the inclusion of the Hubbard U correction significantly changes the electronic properties of the system. However the inclusion of this correction, at least for the value U = 3.3 eV chosen in the present work, worsen the agreement with the measured photo-emission valence band spectra.

cond-mat.mtrl-sci↗

arXiv2012

Pseudo-potentials based first-principles approach to the magneto-optical Kerr effect: from metals to the inclusion of local fields and excitonic effects

We propose a first-principles scheme for the description of the magneto-optical kerr effect within density functional theory (DFT). Though the computation of Kerr parameters is often done within DFT, starting from the conductivity or the dielectric tensor, there is no formal justification to this choice. As a first steps, using as reference materials iron, cobalt and nickel we show that pseudo-potential based calculations give accurate predictions. Then we derive a formal expression for the full dielectric tensor in terms of the density-density correlation function. The derived equation is exact in systems with an electronic gap, with the possible exception of Chern insulators, and whenever the time reversal symmetry holds and can be used as a starting point for the inclusion of local fields and excitonic effects within time-dependent DFT for such systems. In case of metals instead we show that, starting from the density-density correlation function, the term which describes the anomalous Hall effect is neglected giving a wrong conductivity.

cond-mat.mtrl-sci↗

arXiv2012

New methods for the computation of the linear response within the density functional theory in spin polarized system (Metodi innovativi per il calcolo della risposta lineare in density functional theory per sistemi spin polarizzati)

In the present thesis we study absorption spectra of spin polarized isolated systems. Thus we introduce the density functional theory (DFT) formalism and its time dependent extension (TDDFT) together with the approximation used. In particula the Casida formulation of TDDFT is described. The equations for the description of open shell systems have been implemented in the abinit code. Then we used the BeH molecule as a test case. Studying the results for this molecule we underline the limits of the commonly used approximations. In particular we discuss why the the spin symmetry of excited states is broken and we propose a rule to find which excitation energies are affected by this problem. Finally, in the last part, we discuss how a better approximation could be obtained starting from the many body perturbation theory. ----- Il lavoro della presente tesi è focalizzato sullo studio dello spettro di eccitazione di sistemi isolati spin polarizzati. Quindi vengono esposte la Density Functional Theory (DFT), la sua estensione al caso dipendente dal tempo (TDDFT) e le approssimazioni utilizzate. In particolare vien esposta la formulazione di Casida della TDDFT. Le equazioni per lo studio di sistemi spin-polarizzati sono state implementate all'interno del software ABINIT. È stata poi scelta la molecola di BeH per testare l'implementazione fatta. I calcoli eseguiti ci hanno permesso di mettere in luce i limiti delle approssimazioni comunemente utilizzate. In particolare abbiamo discusso l'impossibilità di mantenere la corretta simmetria di spin in sistemi spin polarizzati. Viene quindi proposta una regola per individuare quali siano, negli spettri calcolati, le energie di eccitazione affette da questo problema. Nell'ultimo capitolo della tesi viene inoltre esposta una possibile soluzione per superare l'approssimazione adiabatica.

cond-mat.str-el↗

arXiv2012

Stabilization of tetragonal/cubic phase in Fe doped Zirconia grown by atomic layer deposition

Achieving high temperature ferromagnetism by doping transition metals thin films is seen as a viable approach to integrate spin-based elements in innovative spintronic devices. In this work we investigated the effect of Fe doping on structural properties of ZrO2 grown by atomic layer deposition (ALD) using Zr(TMHD)4 for Zr and Fe(TMHD)3 for Fe precursors and ozone as oxygen source. The temperature during the growth process was fixed at 350°C. The ALD process was tuned to obtain Fe doped ZrO2 films with uniform chemical composition, as seen by time of flight secondary ion mass spectrometry. The control of Fe content was effectively reached, by controlling the ALD precursor pulse ratio, as checked by X-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry. From XPS, Fe was found in Fe3+ chemical state, which maximizes the magnetization per atom. We also found, by grazing incidence X-ray diffraction, that the inclusion of Fe impurities in ZrO2 induces amorphization in thin ZrO2 films, while stabilizes the high temperature crystalline tetragonal/cubic phase after rapid thermal annealing at 600°C.

cond-mat.mtrl-sci↗

arXiv2011

Exchange-correlation effects in the monoclinic to tetragonal phase stabilization of Yttrium-doped ZrO2: a first-principles approach

We describe, within an ab-initio approach, the stabilization of the tetragonal phase vs. the monoclinic one in Yttrium-doped Zirconia. The process is believed to be influenced from different mechanisms. Indeed we show that there is a delicate balance between the change in electrostatic and kinetic energy and exchange-correlation effects. In the tetragonal phase the perturbation induced by doping is better screened at the price of sacrificing correlation energy. Our work opens the opportunity to use the same approach to predict the tetragonal phase stabilization of materials like Zirconia or Hafnia, with different and less characterized dopants.

cond-mat.mtrl-sci↗

arXiv2011

Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes

The gap oscillations caused by a magnetic flux penetrating a carbon nanotube represent one of the most spectacular observation of the Aharonov-Bohm effect at the nano--scale. Our understanding of this effect is, however, based on the assumption that the electrons are strictly confined on the tube surface, on trajectories that are not modified by curvature effects. Using an ab-initio approach based on Density Functional Theory we show that this assumption fails at the nano-scale inducing important corrections to the physics of the Aharonov-Bohm effect. Curvature effects and electronic density spilled out of the nanotube surface are shown to break the periodicity of the gap oscillations. We predict the key phenomenological features of this anomalous Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of a large metallic phase in the low flux regime of Multi-walled nanotubes, also suggesting possible experiments to validate our results.

cond-mat.mes-hall↗

arXiv2010

Vibrational properties of sp carbon atomic wires in cluster-assembled carbon films

Linear chains made by a single row of sp-hybridized carbon are predicted to display fascinating mechano-electronic properties connected with their termination and stabilization inside realistic carbon structures. The present work describes how the computed vibrational properties of cumulenic and polyynic carbon chains allow one to interpret the carbynic features observed in Raman spectra of cluster-assembled sp-sp2 films. The overall picture is consistent with the measured decay of the sp components induced by air or oxygen exposure.

cond-mat.mes-hall↗

arXiv2009

Effects of axial torsion on sp carbon atomic nanowires

Ab-initio calculations within Density Functional Theory combined with experimental Raman spectra on cluster-beam deposited pure carbon films provide a consistent picture of sp-carbon chains stabilized by sp^3 or sp^2 terminations, the latter being sensitive to torsional strain. This unexplored effect promises many exciting applications since it allows one to modify the conductive states near the Fermi level and to switch on and off the on-chain pi-electron magnetism.

cond-mat.mtrl-sci↗