Alessio Filippetti

Why are there any magnetic ferroelectrics

Abstract Multiferroic magnetoelectrics are rare materials which are both ferromagnetic and ferroelectric in the same phase. In this paper we show that their scarcity results from transition metal d electrons, which are essential for magnetism, de-stabilizing the off-center ferroelectric distortion. By studying some known multiferroics, we identify the unconventional driving forces which can cause ferromagnetism and ferroelectricity to occur simultaneously.

Self-interaction errors in density-functional calculations of electronic transport.

All density-functional calculations of single-molecule transport to date have used continuous exchange-correlation approximations. The lack of derivative discontinuity in such calculations leads to the erroneous prediction of metallic transport for insulating molecules. A simple and computationally undemanding atomic self-interaction correction (SIC) opens conduction gaps in I-V characteristics that otherwise are predicted metallic, as in the case of the prototype Au/ditholated-benzene/Au junction.

Self-interaction-corrected pseudopotential scheme for magnetic and strongly-correlated systems

Local-spin-density-functional (LSDA) calculations may be affected by severe errors when applied to the study of magnetic and strongly correlated materials. Some of these faults can be traced back to the presence of the spurious self-interaction in the density functional. Since the application of a fully self-consistent self-interaction correction is highly demanding even for moderately large systems, we pursue a strategy of approximating the self-interaction-corrected potential with a nonlocal, pseudopotentiallike projector, first generated within the isolated atom and then updated during the self-consistent cycle in the crystal. This scheme, whose implementation is totally uncomplicated and particularly suited for the pseudopotental formalism, dramatically improves the LSDA results for a variety of compounds with a minimal increase of computing cost.

Magnetic stress as a driving force of structural distortions: the case of CrN

We show that the observed transition from rock salt to orthorhombic P(nma) symmetry in CrN can be understood in terms of stress anisotropy. Using local spin density functional theory, we find that the imbalance between stress stored in spin-paired and spin-unpaired Cr nearest neighbors causes the rock-salt structure to be unstable against distortions and justifies the observed antiferromagnetic ordering. This stress has a purely magnetic origin and may be important in any system where the coupling between spin ordering and structure is strong.

Competition between magnetic and structural transitions in CrN

CrN is observed to undergo a paramagnetic to antiferromagnetic transition accompanied by a shear distortion from cubic NaCl-type to orthorhombic structure. Our first-principle plane wave and ultrasoft pseudopotential calculations confirm that the distorted antiferromagnetic phase with spin configuration arranged in double ferromagnetic sheets along [110] is the most stable. Antiferromagnetic ordering leads to a large depletion of states around Fermi level, but it does not open a gap. Simultaneous occurence of structural distortion and antiferromagnetic order is analyzed.

Anomalous relaxations and chemical trends at III-V semiconductor nitride nonpolar surfaces

Relaxations at nonpolar surfaces of semiconductor III-V compounds result from a competition between dehybridization and charge transfer. First-principles calculations for the (110) and (10

First principles study of structural, electronic and magnetic interplay in ferroelectromagnetic yttrium manganite

1\ifmmode\bar\else\textasciimacron\fi{}

Methylammonium fragmentation in amines as source of localized trap levels and the healing role of Cl in hybrid lead-iodide perovskites

0) faces of zinc-blende and wurtzite AlN, GaN, and InN reveal an anomalous behavior as compared with ordinary III-V semiconductors. Additional calculations for GaAs and ZnO suggest close analogies with the latter. We interpret our results in terms of the larger ionicity (charge asymmetry) and bonding strength (cohesive energy) in the nitrides with respect to other III-V compounds, both essentially due to the strong valence potential and absence of p core states in the lighter anion. The same interpretation applies to Zn II-VI compounds.

Doping-dependent band structure of LaAlO3/SrTiO3 interfaces by soft x-ray polarization-controlled resonant angle-resolved photoemission

Abstract We present results of local spin density approximation pseudopotential calculations for the ferroelectromagnet, yttrium manganite (YMnO 3 ). The origin of the differences between ferroelectric and non-ferroelectric perovskite manganites is determined by comparing the calculated properties of yttrium manganite in its ferroelectric hexagonal and non-ferroelectric orthorhombic phases. In addition, orthorhombic YMnO 3 is compared with the prototypical non-ferroelectric manganite, lanthanum manganite. We show that, while the octahedral crystal field splitting of the cubic perovskite structure causes a centro-symmetric Jahn–Teller distortion around the Mn 3+ ion, the markedly different splitting in hexagonal perovskites creates an electronic configuration consistent with ferroelectric distortion. We explain the nature of the distortion, and show that a local magnetic moment on the Mn 3+ ion is a requirement for it to occur.

Thermally Activated Point Defect Diffusion in Methylammonium Lead Trihalide: Anisotropic and Ultrahigh Mobility of Iodine

The resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as suited photo-absorber in solar cell devices. Here we investigate by ab-initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation CH