We present Raman spectroscopy results backed by first-principles calculations and investigate the nature of possible spin-phonon coupling (SPC) in a Gd-doped Y$_2$CoMnO$_6$ (YGCMO) double perovskite oxide. The influence of Gd substitution, A-site ordering, and anti-site disorder is also studied. YGCMO exhibits anti-site disorder leading to both ferromagnetic (between Co and Mn) and antiferromagnetic interactions (Co-Co, Mn-Mn, Gd-Co/Mn), making the SPC quite intriguing. An analysis of the temperature-dependent phonon frequencies for the stretching modes of YGCMO indicates that SPC here possibly emerges from the simultaneous presence of competing ferromagnetic and antiferromagnetic interactions. The SPC strength comes out to be 0.29 cm$^{-1}$. Our density functional theory (DFT) calculations show that Phonon modes shifted towards lower frequency with Gd doping. Similarly, A-site ordring and anti-site disorder significantly alter the Raman spectra. Experimental findings are also corroborated by first-principles DFT calculations, which indicate that anti-site disorder and Gd doping enhances SPC in YGCMO. This implies a strong influence of A-site cationic radii, and B-site (Co/Mn) ordering on SPC in the bulk double perovskite systems. The phonon dynamics of YGCMO are, therefore, correlated with magnetic ordering, indicating potential applications in spintronics devices.
Combining experimental investigations and first-principles DFT calculations, we report physical and magnetic properties of Gd-substituted Y$_2$CoMnO$_6$ double perovskite, which are strongly influenced by antisite-disorder-driven spin configurations. On Gd doping, Co and Mn ions are present in mixed-valence (Co$^{3+}$, Co$^{2+}$, Mn$^{3+}$ and Mn$^{4+}$) states. Multiple magnetic transitions have been observed: i) paramagnetic to ferromagnetic transition is found to occur at \textit{T}$_C$=95.5 K, ii) antiferromagnetic transition at \textit{T}$_N$=47 K is driven by $3d-4f$ polarisation and ant…
First-principles electronic structure calculations have been performed for the double perovskite Bi$_2$CoMnO$_6$ in its non-centrosymmetric polar state using generalized gradient approximation plus Hubbard U approach. We find that while Co is in a high spin state, Mn is in an intermediate spin state. The calculated dynamical charge tensors are anisotropic reflecting a low-symmetry structure of the compound. Magnetic structure dependent phonon frequencies indicate the presence of spin-phonon coupling. Using Berry phase method, we obtain a spontaneous electronic polarization of 5.88 $μC/cm^2$ wh…
We have investigated low energy nuclear spin excitations in double perovskite compounds R$_2$CoMnO$_6$ (R = Y,Tb) by inelastic neutron scattering with a high-resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 $μ$eV for Y$_2$CoMnO$_6$ and also for Tb$_2$CoMnO$_6$ at T = 2 K in both energy loss and energy gain sides. We interpret these inelastic peaks to be due to the transitions between the hyperfine split nuclear levels of $^{59}$Co nucleus. The inelastic peaks move towards the central elastic peak and finally merge with it at the magnetic ordering temperature …
Electron- and hole-doped La$_2$CoMnO$_6$(LCMO) are investigated using first principles DFT calculations. Hole and electron doping are achieved respectively by introducing Sr$^{2+}$ at La$^{3+}$ sites and by inducing O-site vacancies in LCMO. Electronic structure calculations suggest that hole doping alters the charge and valence state of Co ions, whereas electron doping influences the Mn ions. Introduction of defects is found to enhance antisite disorder(ASD) at Co/Mn site, which is expected to influence the magnetodielectric properties of the system. Our calculations suggest that while ASD an…
Manganites with double perovskite structure are being attractive because of their interesting magnetoelectric and dielectric responses. Particularly, colossal dielectric constant (CDC) behavior has been observed in La$_2$CoMnO$_6$ (LCMO), at radio frequencies and room temperature. In this paper, we employed infrared-reflectivity spectroscopy onto a LCMO ceramic obtained through a modified Pechini's method to determine the phonon contribution to the intrinsic dielectric response of the system and to investigate the CDC origin. The analysis of the main polar modes and of the obtained phonon para…
The spontaneous exchange bias (SEB) effect is a remarkable phenomenon recently observed in some reentrant spin-glass materials. Here we investigate the SEB in La$_{1.5}$(Sr$_{0.5-x}$Ba$_{x}$)CoMnO$_{6}$ double-perovskites, a system with multifarious magnetic phases for which a notable increase in the exchange bias field is observed for intermediate Sr/Ba concentrations. The Ba to Sr substitution leads to the enhancement of the crystal lattice, which is accompanied by the raise of both the effective magnetic moment ($μ_{eff}$) and the antiferromagnetic (AFM) transition temperature that is obser…
The origin of both the Ising chain magnetism and ferroelectricity in Ca$_3$CoMnO$_6$ is studied by $ab$ $initio$ electronic structure calculations and x-ray absorption spectroscopy. We find that Ca$_3$CoMnO$_6$ has the alternate trigonal prismatic Co$^{2+}$ and octahedral Mn$^{4+}$ sites in the spin chain. Both the Co$^{2+}$ and Mn$^{4+}$ are in the high spin state. In addition, the Co$^{2+}$ has a huge orbital moment of 1.7 $μ_B$ which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange st…
We present the results of first-principles calculations on two possible terminations of the (001) surfaces of SrTiO3 (STO), BaTiO3 (BTO), and PbTiO3 (PTO) perovskite crystals. Atomic structure and the electronic configurations were calculated for different 2D slabs, both stoichiometric and non-stoichiometric, using hybrid (B3PW) exchange-correlation technique and reoptimized basis sets of atomic (Gaussian) orbitals. Results are compared with previous calculations and available experimental data. The electronic density distribution near the surface and covalency effects are discussed in details…
Double perovskite oxides have gained in importance and exhibit negative magnetoresistance, which is known as colossal magnetoresistance (CMR) effect. Using a La$_2$CoMnO$_6$ (LCM) thin film we prove that the physical consequences of the CMR effect do also influence the electron energy loss spectrometry (EELS) signal. We observe a change of the band gap at low energy losses and are able to study the magnetisation with chemical sensitivity by employing energy loss magnetic chiral dichroism (EMCD) below the Curie temperature T$_C$ where the CMR effect becomes significant.
We have determined the lattice dynamics of MgSiO$_3$ perovskite (bridgmanite) by a combination of single-crystal inelastic x-ray scattering and ab initio calculations. We observe a remarkable agreement between experiment and theory, and provide accurate results for phonon dispersion relations, phonon density of states and the full elasticity tensor. The present work constitutes an important milestone to extend this kind of combined studies to extreme conditions of pressure and temperature, directly relevant for the physics and the chemistry of Earth's lower mantle.
We present an $\textit{ab initio}$ study of the thermodynamic properties of cubic CaSiO$_3$ perovskite (CaPv) over the pressure and temperature range of the Earth's lower mantle. We compute the anharmonic phonon dispersions throughout the Brillouin zone by utilizing the phonon quasiparticle approach, which characterizes the intrinsic temperature dependence of phonon frequencies and, in principle, captures full anharmonicity. Such temperature-dependent phonon dispersions are used to calculate $\textit{ab initio}$ free energy in the thermodynamic limit ($N \rightarrow \infty$) within the framewo…
We have developed first-principles models, based on a general parametrization of the full potential-energy surface, to investigate the lattice-dynamical properties of perovskite oxides. We discuss the application of our method to prototypic ferroelectric PbTiO3, showing that its properties are drastically affected by a competition between structural instabilities. Indeed, we confirm previous indications that the destructive interaction between the ferroelectric and antiferrodistortive (involving rotations of the O6 octahedra) soft modes shifts PbTiO3's Curie temperature by hundreds of degrees.…
A first-principles derived approach is combined with the inverse Monte Carlo technique to determine the atomic orderings leading to prefixed properties in Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements between Sc and Nb atoms result in drastic changes with respect to the disordered material, including ground states of new symmetries, large enhancement of electromechanical responses, and considerable shift of the Curie temperature. We discuss the microscopic mechanisms responsible for these unusual effects.
An ab initio structure optimization technique is newly developed to determine the valley line on a total-energy surface for zone-center distortions of ferroelectric perovskite oxides and is applied to barium titanate BaTiO3 and lead titanate PbTiO3. The proposed technique is an improvement over King-Smith and Vanderbilt's scheme [Phys. Rev. B 49, p.5828 (1994)] of evaluating total energy as a function of the amplitude of atomic displacements. The results of numerical calculations show that total energy can be expressed as a fourth-order function of the amplitude of atomic displacements in BaTi…
Hybrid halide perovskites are currently under intense investigation due to their potential applications in optoelectronics and solar cells. Among them, MAPbI$_3$ where MA stands for the methylammonium cation, exhibits ideal properties for solar cells. In attempt to identify new lead-free halide perovskites we have studied using \textit{ab-initio} electronic structure calculations in conjunction with hybrid functionals a series of MABX$_3$ compounds where B is a divalent cation and X a halogen atom. Our results suggest that the compounds under study exhibit a variety of lattice constants and en…
Halide perovskites are currently under intense investigation due to their potential applications in optoelectronics and solar cells. Among them several crystallize in low symmetry lattice structures like trigonal, hexagonal, orthorhombic and monoclinic. Employing \textit{ab-initio} electronic structure calculations in conjunction with generalized gradient approximation and hybrid functionals we study a series of perovskites with the formula A$_3$B$_2$X$_9$ which have been grown experimentally. A stands for a monovalent cation like Cs, Rb, K or the organic methylammonium molecule (MA), B is Sb …
Lattice thermal conductivity ($κ_{lat}$) of MgSiO$_3$ postperovskite (MgPPv) under the Earth's lower mantle high pressure-temperature conditions is studied using the phonon quasiparticle approach by combing \textit{ab initio} molecular dynamics and lattice dynamics simulations. Phonon lifetimes are extracted from the phonon quasiparticle calculations, and the phonon group velocities are computed from the anharmonic phonon dispersions, which in principle capture full anharmonicity. It is found that throughout the lowermost mantle, including the D" region, $κ_{lat}$ of MgPPv is ~25% larger than …
In the present paper, the electronic, structural, thermodynamic, and elastic properties of cubic $PbGeO_{3}$ perovskite oxide are presented and computed using the WIEN2K code. The structural properties have been evaluated and they are in good agreement with the theoretical and experimental data. A phonon dispersion is made and it reveals that the cubic $PbGeO_{3}$ perovskite is dynamically stable. In addition, the electronic properties of $PbGeO_{3}$ shows an opening gap energy, meaning a semiconductor behavior with an indirect band gap equal to $1.67 eV$. Moreover, the obtained elastic consta…
Halide perovskites consist a class of materials under intense investigation due to their potential technological applications like solar cells, optoelectronic devices and catalysis. Recently we have studied using electronic band structure calculations from first principles, the cubic MABX$_3$ compounds [A. Koliogiorgos et al., Comput. Mater. Sci. \textbf{138}, 92 (2017)], where MA stands for the methylammonium cation, B is a divalent cation and X a halogen. We expand our study in the case where Cs stands in place of the MA cation. Our results suggest that the Cs-based compounds exhibit also a …
The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to 'freely' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite temperature ab-initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long range anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical cor…
In the development of highly efficient photovoltaic cells, solid perovskite systems have demonstrated unprecedented promise, with the figure of merit exceeding nineteen percent of efficiency. In this paper, we investigate the optical and vibrational properties of organometallic cubic perovskite CH3NH3PbI3 using first-principles calculations. For accurate theoretical description, we go beyond conventional density functional theory (DFT), and calculated optical conductivity using relativist quasi-particle (GW) correction. Incorporating these many-body effects, we further solve Bethe-Salpeter equ…
A little addition of Cl to \ce{MAPbI3} has been reported to improve the material stability as well as light harvesting and carrier conducting properties of organometal trihalide perovskites, the key component of perovskite solar cell (PSC). However, the mechanism of performance enhancement of PSC by Cl addition is still unclear. Here, we apply the efficient virtual crystal approximation method to revealing the effects of Cl addition on the structural, electronic, optical properties and material stability of \ce{MAPb(I_{1-x}Cl_x)3}. Our {\it ab initio} calculations present that as the increase …
The phonon spectra of calcium, strontium, barium, radium, cadmium, zinc, magnesium, germanium, tin, and lead titanates with the perovskite structure are calculated from first principles within the density functional theory. By analyzing the unstable modes in the phonon spectra, the possible lattice distortions are determined and the energies of the corresponding phases are calculated. From analyzing the phonon spectra, force constants, and eigenvectors of TO phonons, a conclusion is drawn on the origin of the ferroelectricity in considered crystals. It is shown that the main factors determinin…
We present a first-principles investigation of ferroelectricity in layered perovskite oxide La2Ti2O7 (LTO), one of the compounds with highest Curie temperature known (1770 K). Our calculations reveal that LTO's ferroelectric transition results from the condensation of two soft modes that have the same symmetry and are strongly coupled anharmonically. Further, the leading instability mode essentially consists of rotations of the oxygen octahedra that are the basic building block of the perovskite structure; remarkably, because of the particular topology of the lattice, such O6 rotations give ra…
La1-xSrxCo0.25Fe0.75O3-delta is known as one of the best cathode materials for permeation membranes and solid oxide fuel cells. Optimization of its chemical composition is a challenging problem. One of the key properties is concentration of oxygen vacancies, controlled by their formation energies. Ab initio calculations were employed in order to study the formation of oxygen vacancies in La1-xSrxCo0.25Fe0.75O3-delta perovskites by varying the Sr content from x = 12.5% to 50%. The formation energies were obtained for different stoichiometries as functions of temperature and oxygen partial press…
Ion displacements are the cause of the ferroelectricity in perovskites. By properly shifting ions, ab initio computations have been extensively used to investigate properties of perovsites in various structural phases. In addition to the relatively simple ion displacements, perovskites have another type of structural distortion known as antiferrodistortion or oxygen octahedron tilting. The interplay between these two types of distortions have generated abundant structural phases that can be tedious to prepare for ab initio computation, especially for large supercells. Here, we design and imple…
In recent experiments the irradiation of insulators of perovskite type with swift heavy ions under glancing incidence has been shown to provide a unique means to generate periodically arranged nanodots at the surface. The physical origin of these patterns has been suggested to stem from a highly anisotropic electron density distribution within the bulk. In order to show the relevance of the electron density distribution of the target we present a model calculation for the system Xe$^{+23}$ $\to$ SrTiO$_{3}$ that is known to produce the aforementioned surface modifications. On the basis of the …
The isothermal compression of magnesium perovskite and postperovskite is examined through the F-f plot and the diagnostic plot of Vinet universal model theoretically from the ab initio quantum-mechanical calculations at the hybrid Hartree-Fock / Density Functional Theory level. A purely numerical approach, first time applied in this paper, shows that the discrepancies largely observed between studies on the perovskite and criticized in geophysical applications are due to the inadequate choice of the Birch-Murnaghan equation of state; meanwhile the Vinet model is found utterly appropriate for t…
The impact of ordered vacancies on the dynamic stability of perovskites is investigated under the $\textit{ab initio}$ framework with a focus on cubic BaFeO$_{3}$ ($Pm\bar{3}m$) and vacancy-ordered monoclinic BaFeO$_{2.67}$ ($P2_{1}/m$). The harmonic approximation shows that both structures are dynamically unstable at 0 K. For the monoclinic structure, the instability is related to rotational distortions of the Fe coordination tetrahedra near the ordered vacancies. $\textit{Ab initio}$ molecular dynamics simulations in combination with the introduced structural descriptor demonstrate that both…
We predict high thermoelectric efficiency in the layered perovskite La$_2$Ti$_2$O$_7$, based on calculations (mostly ab-initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit $ZT$ computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around 10$^{20}$ cm$^{-3}$. The Seebeck thermopower coefficient is between 200 and 300 $μ$V/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the …
Numerous physical properties of CaPd3Ti4O12 (CPTO) and CaPd3V4O12 (CPVO) double perovskites have been explored based on density functional theory (DFT). The calculated structural parameters fairly agree with the experimental data to confirm their stability. The mechanical stability of these two compounds was clearly observed by the Born stability criteria. To rationalize the mechanical behavior, we investigate elastic constants, bulk, shear and Young's modulus, Pugh's ratio, Poisson's ratio and elastic anisotropy index. The ductility index confirms that both materials are ductile in nature. Th…
Ab initio calculations show that the coupling between antiferrodistortive(AFD) distortions and magnetization in perovskite Bi2FeMnO6 is prohibited to make magnetization rotate as on-site Coulomb interaction($U$) is larger than 2.7 eV, where anomalies in antiferromagnetic(AFM) vectors and band gap varying with on-site Coulomb interaction can be observed. This coupling is attributed to the antisymmetric Dzyaloshinskii-Moriya interaction(DMI) driven by the eg-eg states AFM interaction and charge redistribution with respect to different AFD distortions.
Exploring epitaxial structures for electrically pumped perovskite lasers: a study of CsPb(Br,I)$_3$ based on the \textit{ab initio} Bethe-Salpeter equation
An ab initio LMTO approach and semi-empirical quantum chemical INDO method have been used for supercell calculations of basic point defects - F-type centers and hole polarons bound to cation vacancy - in partly covalent perovskite KNbO3. We predict the existence of both one-site and two-site (molecular) polarons with close absorption energies (~ 1 eV). The relevant experimental data are discussed and interpreted.
Using the dynamical matrix of a crystal obtained from ab initio calculations, we have evaluated for the first time the strength of the dynamic flexoelectric effect and found it comparable to that of the static bulk flexoelectric effect, in agreement with earlier order-of-magnitude estimates. We also proposed a method of evaluation of these effects directly from the simulated phonon spectra. This method can also be applied to the analysis of the experimental phonon spectra, being currently the only one enabling experimental characterization of the static bulk flexoelectric effect.
First principles electronic structure calculations have been carried out on ordered double perovskites Sr_2B'B"O_6 (for B' = Cr or Fe and B" 4d and 5d transition metal elements) with increasing number of valence electrons at the B-sites, and on Ba_2MnReO_6 as well as Ba_2FeMoO_6. The Curie temperatures are estimated ab initio from the electronic structures obtained with the local spin-density functional approximation, full-potential generalized gradient approximation and/or the LDA+U method (U - Hubbard parameter). Frozen spin-spirals are used to model the excited states needed to evaluate the…
By means of ab initio calculations the possibilities of switchable spin-polarized two-dimensional electron gas (2DEG) at the interface of antiferromagnetic and ferroelectric perovskites, i.e., LaMnO3/BaTiO3 superlattice, were investigated. We demonstrate that at the heterostructure with TiO2/LaO interfacial layers the two-dimensional conducting state arises mainly localized within the interfacial MnO layer. The density of states at the Fermi-level can be tuned by ferroelectric polarization reversal. The conducting state coexists with magnetic one mainly arose from Mn atoms.
Layered halide perovskites are solution-processed natural heterostructures where quantum and dielectric confinement effects down to the nanoscale strongly influence the optical properties, leading to stabilization of bound excitons. Achieving a detailed understanding of the exciton properties is crucial to boost the exploitation of these materials in energy conversion and light emission applications, with current on-going debate related to the energy order of the four components of the most stable exciton. To provide theoretical feedback and solve among contrasting literature reports, we perfo…
Nanocrystals based on metal-halide perovskites offer a promising material platform for highly efficient lighting. Using transient optical spectroscopy, we study excitation recombination dynamics in manganese-doped CsPb(Cl,Br)3 perovskite nanocrystals. We find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by ab initio calculations, we can attribute the enhanced emission rates to increased exciton localization through lattice periodicity breaking from Mn dopants, which increases exciton eff…
Density-functional simulations are used to calculate structural properties and high-symmetry phonons of the hypothetical cubic phase, the stable orthorhombic phase and an intermediate tetragonal phase of magnesium silicate perovskite. We show that the structure of the stable phase is well described by freezing in a small number of unstable phonons into the cubic phase. We use the frequencies of these unstable modes to estimate transition temperatures for cubic--tetragonal and tetragonal--orthorhombic phase transitions. These are investigated further to find that the coupling with the strain su…
Hybrid organic-inorganic halide perovskites for photovoltaics have attracted research interest due to their unique material properties, but suffered from poor material stability. In this work, we investigated the surface phase diagrams of PbI$_2$ and cubic CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) for a better understanding of precursor effect on perovskite synthesis via solid-gas reaction, by density functional theory calculations combined with thermodynamics. Using the devised slab models of PbI$_2$(001) and MAPbI$_3$(100), (110) and (111) surfaces with perfect and various vacant defect terminations, …
We report a systematic investigation of orthorhombic perovskite-type RCrO3 powder samples by Raman scattering for nine different rare earth R3+ cations (R = Y, La, Pr, Sm, Gd, Dy, Ho, Yb, Lu). The room-temperature Raman spectra and the associated phonon mode assignment provide reference data for structural investigation of the whole series of RCrO3 orthochromites and phonon ab-initio calculations. The assignment of the chromite spectra and comparison with Raman data on other orthorhombic perovskites allows correlating the phonon modes with the structural distortions in the RCrO3 series. In par…
We present an ab initio simulation of $90^{\circ}$ ferroelastic twins that were recently observed in methyl ammonium lead iodide. There are two inequivalent types of $90^{\circ}$ walls that we calculate to act as either electron or hole sinks which suggests a possible route to enhancing charge carrier separation in photovoltaic devices. Despite separating non-polar domains, we show these walls to have a substantial in-plane polarisation of $\sim 6 \phantom{|} μ\text{C}\phantom{|}\text{cm}^{-2}$, due in part to flexoelectricity. We suggest this in turn could allow for the photoferroic effect an…
Mixed-anion mixed-cation perovskites with (FAPbI$_3$)$_{1-x}$(MAPbBr$_3$)$_x$ composition have allowed record efficiencies in photovoltaic solar cells, but their atomic-scale behaviour is not well understood yet, in part because their theoretical modelling requires consideration of complex and interrelated dynamic and disordering effects. We present here an ab initio molecular dynamics investigation of the structural, thermodynamic, and electronic properties of the (FAPbI$_3$)$_{0.875}$(MAPbBr$_3$)$_{0.125}$ perovskite. A special quasi-random structure is proposed to mimic the disorder of both…
We elucidate the nature of the electron-phonon interaction in the archetypal hybrid perovskite CH$_3$NH$_3$PbI$_3$ using ab initio many-body calculations and an exactly solvable model. We demonstrate that electrons and holes near the band edges primarily interact with three distinct groups of longitudinal-optical vibrations, in order of importance: the stretching of the Pb-I bond, the bending of the Pb-I-Pb bonds, and the libration of the organic cations. These polar phonons induce ultrafast intraband carrier relaxation over timescales of 6-30 fs and yield polaron effective masses 28% heavier …
Perovskite oxides form an eclectic class of materials owing to their structural flexibility in accommodating cations of different sizes and valences. They host well known point and planar defects, but so far no line defect has been identified other than dislocations. Using analytical scanning transmission electron microscopy (STEM) and ab initio calculations we have detected and characterized the atomic and electronic structures of a novel line defect in NdTiO3 perovskite. It appears in STEM images as a perovskite cell rotated by 45 degrees. It consists of self-organized Ti-O vacancy lines rep…
The momentum-dependent Rashba and Dresselhaus spin-splitting has gained much attention for its highly promising applications in spintronics. In the present work, ab initio density functional theory calculations are performed to study the spin-splitting effect in ferroelectric oxide perovskite KIO3. Our calculations are additionally supported by symmetry adapted two-band k.p Hamiltonian. Non-negligible spin-splitting effect is observed at conduction band minimum (CBm) and valence band maximum (VBM) for rhombohedral R3m and R3c phases. Linear Rashba terms successfully explain the splitting at VB…
Halide double perovskites with alternating silver and pnictogen cations are an emerging family of photoabsorber materials with robust stability and band gaps in the visible range. However, the nature of optical excitations in these systems is not yet well understood, limiting their utility. Here, we use ab initio many-body perturbation theory within the $GW$ approximation and the Bethe-Salpeter equation approach to calculate the electronic structure and optical excitations of the double perovskite series Cs$_2$AgBX$_6$, with B=Bi$^{3+}$, Sb$^{3+}$, X = Br$^-$, Cl$^-$. We find that these materi…
We use ab initio electronic structure calculations in combination with Monte Carlo simulations to investigate the magnetic and ferroelectric properties of bulk orthorhombic HoMnO$_3$ and ErMnO$_3$. Our goals are to explain the inconsistencies in the measured magnetic properties of the orthorhombic perovskite manganites (o-$R$MnO$_3$) with small rare-earth ($R$) cations or Y, as well as the contradictions between the directions and amplitudes of the electric polarizations reported by different experimental groups. Our computations stabilize several exotic magnetic orders (so-called w-spiral, H-…
A shell model for ferroelectric perovskites fitted to results of first-principles density functional theory (DFT) calculations is strongly affected by approximations made in the exchange-correlation functional within DFT, and in general not as accurate as a shell model derived from experimental data. We have developed an isotropic shell model for BaTiO3 based on the PBEsol exchange-correlation functional, which was specifically designed for crystal properties of solids. Our shell model for BaTiO3 agrees with groundstate DFT properties and the experimental lattice constants at finite temperatur…
Metal halide perovskites exhibit a materials physics that is distinct from traditional inorganic and organic semiconductors. While materials such as CH3NH3PbI3 are non-magnetic, the presence of heavy elements (Pb and I) in a non-centrosymmetric crystal environment result in a significant spin-splitting of the frontier electronic bands through the Rashba-Dresselhaus effect. We show, from a combination of \textit{ab initio} molecular dynamics, density-functional theory, and relativistic quasi-particle \textit{GW} theory, that the nature (magnitude and orientation) of the band splitting depends o…
We report the electronic properties of the quadruple perovskite CaCu$_{3}$Ti$_{4}$O$_{12}$ as obtained via several density-functional based methods, and propose a new interpretation of optical experiments to the effect that four distinct transitions (centered around 0.7, 1.5, 2.5, and 3.5 eV) contribute to the spectrum. The comparison with experiment is satisfactory, especially after we account for the effects of spin disorder, which does not close the fundamental gap but suppresses the transition intensity. We find that some of the methods we employ tend to overestimate considerably the gaps …
In a recent publication (S. Dong et al., Phys. Rev. Lett.103, 127201 (2009)), two (related) mechanisms were proposed to understand the intrinsic exchange bias present in oxides heterostructures involving G-type antiferromagnetic perovskites. The first mechanism is driven by the Dzyaloshinskii-Moriya interaction, which is a spin-orbit coupling effect. The second is induced by the ferroelectric polarization, and it is only active in heterostructures involving multiferroics. Using the SrRuO$_3$/SrMnO$_3$ superlattice as a model system, density-functional calculations are here performed to verify …
We report an investigation of DyScO3 and GdScO3 single crystals by Raman scattering in various scattering configurations and at various wavelengths. The Raman spectra are well-defined and the reported spectral signature together with the mode assignment sets the basis for the use of Raman scattering for the investigation of RE-scandates. The observed positions of Raman modes for DyScO3 are for most bands in reasonable agreement with recent theoretical ab initio predictions of the vibrational spectrum for the same material. Further to the phonon signature, a luminescence signal is observed for …
