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Fabio Ricci

Publications and source records attributed to Fabio Ricci.

8 recordsLinked to original sources
arXiv2025

Roles of Defects and Sb-doping in the Thermoelectric Properties of Full-Heusler Fe2TiSn

The potential of Fe2TiSn full-Heusler compounds for thermoelectric applications has been suggested theoretically, but not yet grounded experimentally, due to the difficulty of obtaining reproducible, homogeneous, phase pure and defect free samples. In this work, we study Fe2TiSn1-xSbx polycrystals (x from 0 to 0.6), fabricated by high-frequency melting and long-time high-temperature annealing. We obtain fairly good phase purity, homogeneous microstructure and good matrix stoichiometry. Although intrinsic p-type transport behavior is dominant, n-type charge compensation by Sb doping is demonstrated. Calculations of formation energy of defects and electronic properties carried out in the density functional theory formalism reveal that charged iron vacancies VFe2- are the dominant defects responsible for the intrinsic p-type doping of Fe2TiSn in all types of growing conditions except Fe-rich. Additionally, Sb substitutions at Sn site give rise either to SbSn, SbSn1+ which are responsible for n-type doping and magnetism (SbSn) or to magnetic SbSn1- which act as additional p-type dopants. Our experimental data highlight good thermoelectric properties close to room temperature, with Seebeck coefficients up to 56 microV/K in the x=0.2 sample and power factors up to 4.8x10^-4 W m^-1 K^-2 in the x=0.1 sample. Our calculations indicate the appearance of a pseudogap in Ti-rich conditions and large Sb doping, possibly improving further the thermoelectric properties.

cond-mat.mtrl-sci↗
arXiv2019

Magnetic instabilities in doped Fe$_2YZ$ full-Heusler thermoelectric compounds

Thermoelectricity is a promising avenue for harvesting energy but large-scale applications are still hampered by the lack of highly-efficient low-cost materials. Recently, Fe$_2YZ$ Heusler compounds were predicted theoretically to be interesting candidates with large thermoelectric power factor. Here, we show that under doping conditions compatible with thermoelectric applications, these materials are prone to an unexpected magnetic instability detrimental to their thermoelectric performance. We rationalize the physics at the origin of this instability, provide guidelines for avoiding it and discuss its impact on the thermoelectric power factor. Doing so, we also point out the shortcomings of the rigid band approximation commonly used in high-throughput theoretical searches of new thermoelectrics.

cond-mat.mtrl-sci↗
arXiv2019

Density functional perturbation theory within non-collinear magnetism

We extend the density functional perturbation theory formalism to the case of non-collinear magnetism. The main problem comes with the exchange-correlation (XC) potential derivatives, which are the only ones that are affected by the non-collinearity of the system. Most of the present XC functionals are constructed at the collinear level, such that the off-diagonal (containing magnetization densities along $x$ and $y$ directions) derivatives cannot be calculated simply in the non-collinear framework. To solve this problem, we consider here possibilities to transform the non-collinear XC derivatives to a local collinear basis, where the $z$ axis is aligned with the local magnetization at each point. The two methods we explore are i) expanding the spin rotation matrix as a Taylor series, ii) evaluating explicitly the XC for the local density approximation through an analytical expression of the expansion terms. We compare the two methods and describe their practical implementation. We show their application for atomic displacement and electric field perturbations at the second order, within the norm-conserving pseudopotential methods.

physics.comp-ph↗
arXiv2015

Unveiling the room temperature magnetoelectricity of troilite FeS

The amazing possibility of magnetoelectric crystals to cross couple electric and magnetic properties without the need of time-dependent Maxwell's equations has attracted a lot of interest in material science. This enthusiasm has re-emerged during the last decade where magnetoelectric and multiferroic crystals have captivated a tremendous number of studies, mostly driven by the quest of low-power-consumption spintronic devices. While several new candidates have been discovered, the desirable magnetoelectric coupling at room temperature is still sparse and calls for new promising candidates. Here, we show from first-principles studies that the troilite phase of the iron sulfide based compounds, one of the most common mineral of Earth, Moon, Mars or meteors, is magnetoelectric up to temperatures as high as 415 K.

cond-mat.mtrl-sci↗
arXiv2015

Strain effects in monolayer Iron-Chalcogenide superconductors

The successful fabrication of one monolayer FeSe on SrTiO$_{3}$ represented a real breakthrough in searching for high-T$_{c}$ Fe-based superconductors (Ref. 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.

cond-mat.supr-con↗
arXiv2013

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.

cond-mat.supr-con↗