HIGHLY CITED PAPERS
By: Marsman, M.; Paier, J.; Stroppa, A.; et al.
Conference: 2nd Workshop on Theory Meets Industry Location: Erwin Schrodinger Inst, Vienna, AUSTRIA Date: JUN 12-14, 2007
JOURNAL OF PHYSICS-CONDENSED MATTER Volume: 20 Issue: 6 Article Number: 064201 Published: FEB 13 2008
We present an overview of the description of structural, thermochemical, and electronic properties of extended systems using several well known hybrid Hartree -Fock/density-functional-theory functionals (PBE0, HSE03, and B3LYP). In addition we address a few aspects of the evaluation of the Hartree -Fock exchange interactions in reciprocal space, relevant to all methods that employ a plane wave basis set and periodic boundary conditions.
Accurate surface and adsorption energies from many-body perturbation theory
By: Schimka, L.; Harl, J.; Stroppa, A.; et al.
NATURE MATERIALS Volume: 9 Issue: 9 Pages: 741-744 Published: SEP 2010
Kohn-Sham density functional theory is the workhorse computational method in materials and surface science(1). Unfortunately, most semilocal density functionals predict surfaces to be more stable than they are experimentally. Naively, we would expect that consequently adsorpion energies on surfaces are too small as well, but the contrary is often found: chemisorption energies are usually overestimated(2). Modifying the functional improves either the adsorption energy or the surface energy but always worsens the other aspect. This suggests that semilocal density functionals possess a fundamental flaw that is difficult to cure, and alternative methods are urgently needed. Here we show that a computationally fairly efficient many-electron approach, the random phase approximation(3) to the correlation energy, resolves this dilemma and yields at the same time excellent lattice constants, surface energies and adsorption energies for carbon monoxide and benzene on transition-metal surfaces.
Hybrid Improper Ferroelectricity in a Multiferroic and Magnetoelectric Metal-Organic Framework
By: Stroppa, A.; Barone, P.; Jain, P.; et al.
ADVANCED MATERIALS Volume: 25 Issue: 16 Pages: 2284-2290 Published: APR 24 2013
On the basis of first-principles calculations, we design a novel Cr-based metal-organic framework to be both multiferroic and magnetoelectric. The compound shows a “double-hybrid” nature: it is a hybrid organic-inorganic compound and it shows hybrid improper ferroelectricity. Here, the coupling of non-polar distortions, such as Jahn-Teller pseudo-rotations and tilting, pave the way to a polar behavior, with the coupling being realized through hydrogen bonds.
Tunable ferroelectric polarization and its interplay with spin-orbit coupling in tin iodide perovskites
By: Stroppa, Alessandro; Di Sante, Domenico; Barone, Paolo; et al.
NATURE COMMUNICATIONS Volume: 5 Article Number: 5900 Published: DEC 2014
Ferroelectricity is a potentially crucial issue in halide perovskites, breakthrough materials in photovoltaic research. Using density functional theory simulations and symmetry analysis, we show that the lead-free perovskite iodide (FA) SnI3, containing the planar formamidinium cation FA, (NH2CHNH2)(+), is ferroelectric. In fact, the perpendicular arrangement of FA planes, leading to a ‘weak’ polarization, is energetically more stable than parallel arrangements of FA planes, being either antiferroelectric or ‘strong’ ferroelectric. Moreover, we show that the ‘weak’ and ‘strong’ ferroelectric states with the polar axis along different crystallographic directions are energetically competing. Therefore, at least at low temperatures, an electric field could stabilize different states with the polarization rotated by pi/4, resulting in a highly tunable ferroelectricity appealing for multistate logic. Intriguingly, the relatively strong spin-orbit coupling in noncentrosymmetric (FA)SnI3 gives rise to a co-existence of Rashba and Dresselhaus effects and to a spin texture that can be induced, tuned and switched by an electric field controlling the ferroelectric state.
Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites
By: Bokdam, Menno; Sander, Tobias; Stroppa, Alessandro; et al.
SCIENTIFIC REPORTS Volume: 6 Article Number: 28618 Published: JUN 28 2016
The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA) PbI3 perovskite, a large range (6-55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPbX(3) perovskites we report on relativistic Bethe-Salpeter Equation calculations (GW-BSE). This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported ‘large’ values. For MAPbI(3), phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarization in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature and argue in favor of the formation of polarons.
Experimental and theoretical studies of structural phase transition in a novel polar perovskite-like [C2H5NH3][Na0.5Fe0.5(HCOO)(3)] formate
By: Ptak, Maciej; Maczka, Miroslaw; Gagor, Anna; et al.
DALTON TRANSACTIONS Volume: 45 Issue: 6 Pages: 2574-2583 Published: 2016
We report the synthesis, single crystal X-ray diffraction, and thermal, dielectric, Raman and infrared studies of a novel heterometallic formate [C2H5NH3][Na0.5Fe0.5(HCOO)(3)] ( EtANaFe). The thermal studies show that EtANaFe undergoes a second-order phase transition at about 360 K. X-ray diffraction data revealed that the high-temperature structure is monoclinic, space group P2(1)/n, with dynamically disordered ethyl-ammonium (EtA(+)) cations. EtANaFe possesses a polar low-temperature structure with the space group Pn and, in principle, is ferroelectric below 360 K. Dielectric data show that the reciprocal of the real part of dielectric permittivity above and below the phase transition temperature follows the Curie-Weiss, as expected for a ferroelectric phase transition. Based on theoretical calculations, we estimated the polarization as (0.2, 0, 0.8) mu C cm(-2), i.e., lying within the ac plane. The obtained data also indicate that the driving force of the phase transition is ordering of EtA(+) cations. However, this ordering is accompanied by significant distortion of the metal formate framework.