Mechanism of Hot-Carrier Photoluminescence in Sn-Based Perovskites.

Metal halide perovskites have shown exceptionally slow hot-carrier cooling, which has been attributed to various physical mechanisms without reaching a consensus. Here, experiment and theory are combined to unveil the carrier cooling process in formamidinium (FA) and caesium (Cs) tin triiodide thin films. Through impulsive vibrational spectroscopy and molecular dynamics, much shorter phonon dephasing times of the hybrid perovskite, which accounts for the larger blueshift in the photoluminescence seen at high excitation density for FASnI compared to CsSnI is reported.

Many-Body MYP2 Force-Field: Toward the Crystal Growth Modeling of Hybrid Perovskites.

Hybrid perovskites are well-known for their optoelectronic and photovoltaic properties. Molecular dynamics simulations allow the study of these soft and ionic crystals by including dynamical effects (e.g.

Ultra-thin magnetic film with giant phonon-drag for heat to spin current conversion.

A tightly confined 2D electron gas with good carrier mobility and large spin-polarization is an essential ingredient for the implementation of spin-caloritronic conversion device technology. Here we give evidence that the SrTiO/EuTiO/LaAlO heterostructure is a prototype material for this purpose. The presence of Eu induces strong spin-polarization in the 2D electron gas spontaneously formed at the interface and ferromagnetic order at low temperature.

Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CHNHPbBr ferroelastic single crystals.

Hybrid lead halides CHNHPbX (X = I, Br, and Cl) have emerged as a new class of semiconductors for low-cost optoelectronic devices with superior performance. Since their perovskite crystal structure may have lattice instabilities against polar distortions, they are also being considered as potential photo-ferroelectrics. However, so far, research on their ferroelectricity has yielded inconclusive results and the subject is far from being settled.

A three-order-parameter bistable magnetoelectric multiferroic metal.

Using first-principles calculations we predict that the layered-perovskite metal BiMnO is a ferromagnet, ferroelectric, and ferrotoroid which may realize the long sought-after goal of a room-temperature ferromagnetic single-phase multiferroic with large, strongly coupled, primary-order polarization and magnetization. BiMnO has two nearly energy-degenerate ground states with mutually orthogonal vector order parameters (polarization, magnetization, ferrotoroidicity), which can be rotated globally by switching between ground states. Giant cross-coupling magnetoelectric and magnetotoroidic effects, as well as optical non-reciprocity, are thus expected.

Viewpoint: Atomic-Scale Design Protocols toward Energy, Electronic, Catalysis, and Sensing Applications.

Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields.

Theory of thermoelectricity in MgSb with an energy- and temperature-dependent relaxation time.

We study the electronic transport coefficients and the thermoelectric figure of merit ZT in [Formula: see text]-doped MgSb based on density-functional electronic structure and Bloch-Boltzmann transport theory with an energy- and temperature-dependent relaxation time. Both the lattice and electronic thermal conductivities affect the final ZT significantly, hence we include the lattice thermal conductivity calculated ab initio. Where applicable, our results are in good agreement with existing experiments, thanks to the treatment of lattice thermal conductivity and the improved description of electronic scattering.

Electronic properties of fluorides by efficient approximated quasiparticle DFT-1/2 and PSIC methods: BaF, CaF and CdF as test cases.

Dialkali halides are materials of great interest from both fundamental and technological viewpoints, due to their wide transparency range. The accurate determination of their electronic, excitation and optical properties in bulk and low dimensional systems is therefore of crucial importance. Moreover, it is a challenge from the theoretical point of view to deal with quasiparticle band structure calculations for such large energy gap materials, requiring very expensive methods for achieving a desirable accuracy.

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO Overlayers.

Interfaces between complex oxides constitute a unique playground for two-dimensional electron systems (2DESs), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO/SrTiO interface is one of the most studied in this regard, and its origin is determined by the polar field in LaAlO as well as by the presence of point defects, like oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decrease of the electronic mobility.

Collective Molecular Mechanisms in the CHNHPbI Dissolution by Liquid Water.

The origin of the dissolution of methylammonium lead trihalide (MAPI) crystals in liquid water is clarified by finite-temperature molecular dynamics by developing a MYP-based force field (MYP1) for water-MAPI systems. A thermally activated process is found with an energy barrier of 0.36 eV consisting of a layer-by-layer degradation with generation of inorganic PbI films and solvation of MA and I ions.

Conductivity and Local Structure of LaNiO Thin Films.

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures.

Modeling hybrid perovskites by molecular dynamics.

The topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body.

Tuning the thermal conductivity of methylammonium lead halide by the molecular substructure.

By using state-of-the-art atomistic methods we provide an accurate estimate of thermal conductivity of methylammonium lead halide as a function of sample size and temperature, in agreement with experimental works. We show that the thermal conductivity of methylammonium lead halide is intrinsically low, due to the low sound velocity of the PbI lattice. Furthermore, by selectively analyzing the effect of different molecular degrees of freedom, we clarify the role of the molecular substructure by showing that the internal modes above 150 cm(-1) (in addition to rotations) are effective in reducing the thermal conductivity of hybrid perovskites.

Prediction of a native ferroelectric metal.

Over 50 years ago, Anderson and Blount discussed symmetry-allowed polar distortions in metals, spawning the idea that a material might be simultaneously metallic and ferroelectric. While many studies have ever since considered such or similar situations, actual ferroelectricity--that is, the existence of a switchable intrinsic electric polarization--has not yet been attained in a metal, and is in fact generally deemed incompatible with the screening by mobile conduction charges. Here we refute this common wisdom and show, by means of first-principles simulations, that native metallicity and ferroelectricity coexist in the layered perovskite Bi5Ti5O17.

Entropy-Suppressed Ferroelectricity in Hybrid Lead-Iodide Perovskites.

The actual nature of the electric polarization in hybrid lead-iodide perovskites is unveiled on the basis of ab initio and model results. A finite, albeit small electric polarization of few μC/cm(2) is found to be pervasive in this system, due to the polar-uncompensated alignment of methylammonium dimers, at least for temperature lower than the activation energy of dimer rotations; however, the presence of a large number of structural local minima corresponding to differently oriented polarization directions counteracts the stabilization of an ordered ferroelectric phase at the macroscale. According to our estimate, only for temperatures lower than 40-50 K a clear ferroelectric behavior is displayed.

Giant oscillating thermopower at oxide interfaces.

Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is one of the major open issues in the full comprehension of the charge confinement phenomenon in oxide heterostructures. Here, we investigate thermopower to study the electronic structure in LaAlO3/SrTiO3 at low temperature as a function of gate field. In particular, under large negative gate voltage, corresponding to the strongly depleted charge density regime, thermopower displays high negative values of the order of 10(4)-10(5) μVK(-1), oscillating at regular intervals as a function of the gate voltage.

Ferromagnetism and orbital order in a topological ferroelectric.

We explore via density functional calculations the magnetic doping of a topological ferroelectric as an unconventional route to multiferroicity. Vanadium doping of the layered perovskite La(2)Ti(2)O(7) largely preserves electric polarization and produces robust ferromagnetic order and, hence, proper multiferroicity. The marked tendency of dopants to cluster into chains results in an insulating character at generic doping.

Spontaneous 2-dimensional carrier confinement at the n-type SrTiO3/LaAlO3 interface.

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3/LaAlO3 interface as a function of the sheet carrier density n(s) via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d(xy) levels within a thin (≲2  nm) interface-adjacent SrTiO3 region for n(s) lower than a threshold value n(c)∼10(14)  cm(-2). For n(s)>n(c) a portion of charge flows into Ti 3d(xz)-d(yz) levels extending farther from the interface.

Magnetic ordering under strain and spin-Peierls dimerization in GeCuO3.

Studying from first principles the competition between ferromagnetic (FM) and antiferromagnetic (AF) interactions in the charge-transfer-insulator GeCuO3, we predict that a small external pressure should switch the uniform AF ground state to FM, and estimate (using exchange parameters computed as a function of strain) the competing AF couplings and the transition temperature to the dimerized spin-Peierls state. Although idealized as a one-dimensional Heisenberg antiferromagnet, GeCuO3 is found to be influenced by nonideal geometry and side groups.

Magnetic ordering in CuO from first principles: a cuprate antiferromagnet with fully three-dimensional exchange interactions.

We investigate the interplay of bonding and magnetism in CuO by a first-principles self-interaction-free density-functional approach. Our analysis reveals that, at variance with typical low-dimensional cuprates, a fully three-dimensional view of the exchange interactions is needed to describe accurately the magnetic ground state and low-energy excitations in CuO. The apparent one-dimensional behavior of antiferromagnetic order is due to the presence of a single spin-polarized hole of d(z)2 character.

The origin of ferroelectricity in magnetoelectric YMnO3.

Understanding the ferroelectrocity in magnetic ferroelectric oxides is of both fundamental and technological importance. Here, we identify the nature of the ferroelectric phase transition in the hexagonal manganite, YMnO(3), using a combination of single-crystal X-ray diffraction, thorough structure analysis and first-principles density-functional calculations. The ferroelectric phase is characterized by a buckling of the layered MnO(5) polyhedra, accompanied by displacements of the Y ions, which lead to a net electric polarization.