Gianni Profeta

van der Waals interaction in iron-chalcogenide superconductors

We demonstrate that the inclusion of van der Waals dispersive interaction sensibly improves the prediction of lattice constants by density functional theory in iron-chalcogenides (FeCh) superconductor compounds, namely FeSe and FeTe. We show how generalized gradient approximation (GGA) for the exchange correlation potential overestimates the out-of-plane lattice constants in both compounds when compared with experiments. In addition, GGA predicts a too weak bonding between the neutral FeCh layers, with a sensible underestimation of the bulk modulus. van der Waals corrected simulations completely solve both problems, reconciling theoretical results with experiments. These findings must be considered when dealing with theoretical predictions in FeCh compounds.

Theoretical investigation of optical conductivity in Ba(Fe1−xCox)2As2

We report on theoretical calculations of the optical conductivity of Ba [Fe(1-x)Co(x)]2 As2, as obtained from density functional theory within the full potential LAPW method. A thorough comparison with experiment shows that we are able to reproduce most of the observed experimental features, in particular a magnetic peak located at about 0.2 eV which we ascribe to antiferromagnetic ordered magnetic stripes. We also predict a large in-plane anisotropy of this feature, which agrees very well with measurements on detwinned crystals. The effect of Co doping as well as the dependence of plasma frequency on the magnetic order is also investigated.

Strain effects in monolayer iron-chalcogenide superconductors

Successful fabrication of one monolayer FeSe on SrTiO3 represented a real breakthrough in searching for high-Tc Fe-based superconductors ([1]). Motivated by this important discovery, we studied the effects of tensile strain on one monolayer and bulk iron-chalcogenide superconductors (FeSe and FeTe), showing that it produces important magnetic and electronic changes in the systems. We found that the magnetic ground state of bulk and monolayer FeSe is the block-checkerboard phase, which turns into the collinear stripe phase under in-plane tensile strain. FeTe, in both bulk and monolayer phases, shows two magnetic transitions upon increasing the tensile strain: from bicollinear in the ground state to block-checkerboard ending up to the collinear antiferromagnetic phase which could bring it in the superconducting state. Finally, the study of the mechanical properties of both FeSe and FeTe monolayers reveals their enormous tensile strain limits and opens the possibility to grow them on different substrates.

TESTING THE CHARGED ADATOM MODEL ONTO THE

Synchrotron radiation high resolution core level spectroscopy has been used to investigate the 1/3 monolayer (ML) solid solution at well-defined values of the mutual surface concentration of the Sn and Si adatoms (x = 0.24 and x = 0.40) as a function of temperature (100–300 K). For 24% Si adatoms at the surface there is low energy electron diffraction (LEED) evidence of a nascent 3 × 3 lowering of the surface symmetry at 100 K. Despite this evidence core level spectroscopy data do not show any signature of the nascent phase transition. Our evidence is in agreement with the analogous observation of no significant spectral changes at reduced temperature of the core level spectra of the isoelectronic and isostructural 1/3 ML Sn/Ge(111) surface.

{\rm Sn}_{1-x}{\rm Si}_x/{\rm Si}(111) (\sqrt{3}\times \sqrt{3}){\rm R}30^\circ

We investigate with scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations the surface structures and the electronic properties of Fe1+y Te thin films grown by pulsed laser deposition. Contrary to the regular arrangement of antiferromagnetic nanostripes previously reported on cleaved single-crystal samples, the surface of Fe1+y Te thin films displays a peculiar distribution of spatially inhomogeneous nanostripes. Both STM and DFT calculations show the bias-dependent nature of such features and support the interpretation of spin-polarized tunneling between the FeTe surface and an unintentionally magnetized tip. In addition, the spatial inhomogeneity is interpreted as a purely electronic effect related to changes in hybridization and Fe-Fe bond length driven by local variations in the concentration of excess interstitial Fe cations. Unexpectedly, the surface density of states measured by STS strongly evolves with temperature in close proximity to the antiferromagnetic-paramagnetic first-order transition, and reveals a large pseudogap of 180-250 meV at about 50-65 K. We believe that in this temperature range a phase transition takes place, and the system orders and locks into particular combinations of orbitals and spins because of the interplay between excess interstitial magnetic Fe and strongly correlated d-electrons.

ALLOY WITH HIGH RESOLUTION CORE LEVEL SPECTROSCOPY

We present an extensive theoretical and experimental study of the Mn-Ge dilute magnetic semiconductor, a material which - due to its high integrability with mainstream Si technology - may hold good promises for spintronic applications. Ab-initio calculations on several different systems containing isolated Mn impurities as well as small clusters (up to three Mn impurities) show that Mn has a tendency to segregate into the Ge matrix and to stabilize occupation of interstitial sites if these are coordinated with other Mn occupying substitutional sites nearby. Several different experimental characterizations (HRTEM, XRD, UPS, MOKE) performed on Mn ion-implanted systems are analyzed and discussed: a close comparison betweeen experimental evidences and density functional calculations allows a full understanding of the sample properties and to disentagle the contributions coming from the diluted and segregated phases. The complexity of this system shows that much has to be done still to understand the physics of these materials and to undisclose all their possible applications.