Yu. A. Mastrikov, E. A. Kotomin, E. Heifets, J. Maier
Combining GGA-plane wave approach as implemented into the VASP - 4.6.19 computer code with a slab model, we studied in detail the atomic and electronic structure of the LaMnO3 surfaces, in both cubic and orthorhombic phases. The results obtained are compared with similar studies for other ABO3-perovskites.
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Strain engineering with different substrate facets is promising for tuning functional properties of thin film perovskite oxides. By choice of facet, different surface symmetries and chemical bond directions for epitaxial interfaces can be tailored. Here, preparation of well-defined pseudo-cubic (111)-oriented orthorhombic substrates of DyScO3 , GdScO3 , and NdGaO3 is reported. The choice of orthorhombic facet, (011)o or (101)o , both corresponding to pseudo-cubic (111)pc , gives vicinal surfaces with single or double (111pc layer terrace step heights, respectively, impacting subsequent thin fi…
The cubic-to-orthorhombic structural transition occurring in CsH at a pressure of about 17 GPa is studied by ab initio calculations. The relative stability of the competing structures and the transition pressure are correctly predicted. We show that this pressure-induced first-order transition is intimately related to a displacive second-order transition which would occur upon application of a shear strain to the (110) planes. The resulting instability is rationalized in terms of the pressure-induced modifications of the electronic structure.
The elastostatic Green function tensor approach, which was recently used to treat electrostriction in numerical simulation of domain structure formation in cubic ferroelectrics, is reviewed and extended to the crystals of hexagonal and orthorhombic symmetry. The tensorial kernels appearing in the expressions for effective nonlocal interaction of electrostrictive origin are derived explicitly and their physical meaning is illustrated on simple examples. It is argued that the bilinear coupling between the polarization gradients and elastic strain should be systematically included in the Ginzburg…
We screened six cobalt-based 18-VEC systems CoVSi, CoNbSi, CoTaSi (Si-group) and CoVGe, CoNbGe, CoTaGe (Ge-group) by the first-principles approach, with the motivation of stabilizing these orthorhombic phases into the cubic symmetry -- favorable for thermoelectrics. Remarkably, it was found that the Ge-group is energetically more favorable in the cubic symmetry than the hitherto orthorhombic phase. We account the cubic ground state of the Si-group to the interplay of internal pressure and covalent interactions. The principle of covalent interactions will provide an insight and could be vital i…
Epitaxial perovskite oxide interfaces with different symmetry of the epitaxial layers have attracted considerable attention due to the emergence of novel behaviors and phenomena. In this paper, we show by aberration corrected transmission electron microscopy that orthorhombic $\text{LaInO}_\text{3}$ films grow in form of three different types of domains on the cubic $\text{BaSnO}_\text{3}$ pseudosubstrate. Quantitative evaluation of our TEM data shows that $c_{pc}$-oriented and $a_{pc}/b_{pc}$-oriented domains are present with similar probability. While continuum elasticity theory suggests tha…
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of…
The phenomenological theory of the flexomagnetoelectric coupling in crystals of the cubic, tetragonal and orthorhombic crystal systems has been suggested. Secondary role of the crystal structure chirality was shown. Oppositely, significant role of the crystallographic point group type (symmetric, alternating, dihedral or cyclic) in the flexomagnetoelectric coupling has been derived. It was shown, that conceptually new features of the flexomagnetoelectric effects are expected in the crystals of the cyclic groups (crystal classes 4/m, 4^- and 4). Proposed verification of the theory is investigat…
While the tetragonal antiferro-electrically distorted (AFD) phase with space group $I4/mcm$ is well known for SrTiO$_3$ to occur below 105 K, there are also some hints in literature of an orthorhombic phase, either at lower temperature at high pressure. A previously proposed orthorhombic layered structure of SrTiO$_3$, known as the post-perovskite or CaIrO$_3$ structure with space group $Cmcm$ is shown to have significantly higher energy than the cubic or tetragonal phase and to have its minimum volume at larger volume than cubic perovskite. It thus cannot correspond to a high-pressure phase o…
Though the electrical responses of the various polymorphs found in ferroelectric polycrystalline thin film HfO$_2$ are now well characterized, little is currently understood of this novel material's grain sub-structure. In particular, the formation of domain and phase boundaries requires investigation to better understand phase stabilization, switching, and interconversion. Here, we apply scanning transmission electron microscopy to investigate the atomic structure of boundaries in these materials. In particular, we find orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclin…
A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO3 undergoes a phase transition from the ferroelectric alpha phase (rhombohedral, R3c) to a paraelectric beta phase (orthorhombic, Pbnm) between 820 oC and 830 oC. Co-existence of both phases over a finite temperature interval, together with abrupt changes in key structural parameters, confirm that the transition is firstorder. The beta phase corresponds to the GdFeO3-type perovskite. The metastability of BiFeO3 relative to Bi2Fe4O9 above 810 oC precludes observation of a reported cubic gamma p…
We present a temperature dependent single crystal x-ray diffraction study of twinned orthorhombic perovskites La1-xCaxMnO3, for x=0.16 and x=0.25. These data show the evolution of the crystal structure from the ferromagnetic insulating state to the ferromagnetic metallic state. The data are modelled in space group Pnma with twin relations based on a distribution of the b axis over three perpendicular cubic axes. The twin model allows full structure determination in the presence of up to six twin fractions using the single crystal x-ray diffraction data.
In this letter, we describe the stabilization of indium antimonide (InSb) in the high-pressure orthorhombic phase (InSb-III) at ambient conditions. Until now, InSb-III has only been observed above 9 GPa, or at around 3 GPa as a metastable structure during the phase transition from cubic zinc blende (InSb-I) to orthorhombic InSb-IV. The crystalline phase transition from InSb-I to InSb-III was driven by an ultrashort, laser-generated, non-hydrostatic pressure pulse. The transition occurred in preferred orientations locked to the initial orientation of the InSb-I crystal, breaking the symmetry of…
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…
Nanoscale characterization of atomic positions in orthorhombic perovskite thin films
本研究利用多种显微表征与第一性原理计算，对DyScO$_3$基底上生长的50 nm LaVO$_3$薄膜进行纳米尺度结构分析，确定了其晶格参数、取向及稀土元素反极性位移，并解析了所有元素的原子位置。
Role of host/guest coupling in stabilizing the phases of the over-tolerant hybrid perovskite MHyPbX3
该研究从主客体耦合视角解释杂化钙钛矿MHyPbX₃相变序列的组分依赖性，通过第一性原理计算和分子动力学模拟验证能量竞争机制，为预测其他杂化钙钛矿稳定相提供新思路。
We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta$_2$O$_5$. Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy high-symmetry structural model where all Ta and O atoms have correct formal oxidation states of +5 and -2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the Ta$_2$O$_3$…
Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$, which crystallizes in a C15 cubic structure, can absorb up to 5 H/f.u. and its pressure-composition isotherm displays a multiplateau behavior related to the existence of several hydrides with different crystal structures. At room temperature Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$H$_{x}$ hydrides (2.9 $\le$ x $\le$ 5) crystallize in three phases with cubic structure (C1, C2 and C3), two phases with monoclinic structures (M1 and M2), and one phase with orthorhombic structure (O), with the following sequence for increasing H concentration: C1, M1, C2, M2, C3, O. Each phase exist…
The dynamics of transformation twins in SrxBa1-xSnO3 (x=0.6,0.8) perovskite has been studied by dynamical mechanical analysis in three-point bend geometry. This material undergoes phase transitions from orthorhombic to tetragonal and cubic structures on heating. The mechanical loss signatures of the transformation twins include relaxation and frequency-independent peaks in the orthorhombic and tetragonal phases, with no observed energy dissipation in the cubic phase. The macroscopic shape, orientation and relative displacements of twin walls have been calculated from bending and anisotropy ene…
Molybdenum carbide (Mo2C) nanoparticles and thin films are particularly suitable catalysts for catalytic fast pyrolysis (CFP) as they are effective for deoxygenation and can catalyze certain reactions that typically occur on noble metals. Oxygen deposited during deoxygenation reactions may alter the carbide structure leading to the formation of oxycarbides, which can determine changes in catalytic activity or selectivity. Despite emerging spectroscopic evidence of bulk oxycarbides, so far there have been no reports of their precise atomic structure or their relative stability with respect to o…
The lattice constants of Strontium titanate perovskite has been studied using the Density functional theory formalism at the PBE and the HSE levels of theory. The study has been done in all the 3 existing phases; namely cubic, orthorhombic and tetragonal, however, experimental details are available only for the cubic phase so far.
We report on the high-temperature (350 K <T< 1000 K) thermoelectric properties of the ternary compounds LuNiSb (cubic) and LuNiSn (orthorhombic) and a composite material (LuNiSb)$_{0.5}$(LuNiSn)$_{0.5}$. The electrical conductivity in LuNiSn is metallic, while it is semiconducting-like in LuNiSb. The Seebeck coefficient reaches -5.5 $μ$V/K at 700 K for the former compound and 66 $μ$V/K at 607 K for the latter one. The composite sample (LuNiSb)$_{0.5}$(LuNiSn)$_{0.5}$ constructed from orthorhombic matrix with cubic inclusions combines the electrical conductivity of LuNiSn with the thermoelectri…
Lattice structure can dictate electronic and magnetic properties of a material. Especially, reconstruction at a surface or heterointerface can create properties that are fundamentally different from those of the corresponding bulk material. We have investigated the lattice structure on the surface and in the thin films of epitaxial SrRuO3 with the film thickness up to 22 pseudo-cubic unit cells (u.c.), using the combination of surface sensitive low energy electron diffraction and bulk sensitive scanning transmission electron microscopy. Our analysis indicates that, in contrast to many perovski…
Crystals of solid solutions Bi(1-x)R(x)FeO(3),here R= La, Dy, Gd, were obtained with x <=0.7. Solid solutions of the stated rare earths, as x is increased from 0 to 0.7, have one and the same sequence of five crystal structures (rhombohedral C3v 6, triclinic C1 1,orthorhombic D2 6,orthorhombic D2 5, orthorhombic C2v 9). The ferroelectric-paraelectric transition occurs in rhombohedral and triclinic crystals at T=810-560°C.The high temperature modifications are orthorhombic and cubic. The orthorhombic structure C2v 9 holds up to 1180°C.The ferroelectric domain structure was distinguished in all …
Binary alloy phases of the noble metals with main group elements are analyzed in relation to body centered cubic structure with orthorhombic and hexagonal distortions. Stability of these distorted phases is considered on the base of the Fermi sphere - Brillouin zone interaction to understand the important part of the band structure energy contribution to the overall crystal energy. Examination of Brillouin zone - Fermi sphere configurations for several representative phases has shown significance of the electron energy contribution in the formation of distorted structures with superlattices an…
We present a self-interaction corrected local spin density (SIC-LSD) study of the electronic structure and possible charge order of magnetite, Fe$_{3}$O$_{4}$. The issue of charge order in magnetite is explored in both cubic and orthorhombic structures, the latter being an approximation to the true, low temperature, monoclinic structure. We find that the Verwey charge ordered phase is not the groundstate solution for this compound neither in cubic nor orthorhombic structure.
Vibrational Properties of the Bi2Se3 Orthorhombic Metastable Phase
该研究结合原位拉曼光谱与密度泛函理论计算，揭示电化学沉积Bi2Se3正交亚稳相的热致晶化、相变及其特征拉曼模式。
La0.67Sr0.33MnO3 (LSMO) thin films under compressive strain have an orthorhombic symmetry with (1-10)o and (001)o in-plane orientations. (The subscript o denotes the orthorhombic symmetry.) Here, we grew LSMO on cubic (LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT) substrates and observed a uniaxial contribution to the magnetic anisotropy which is related to the orthorhombic crystal structure. Since the lattice mismatch is equal in the two directions, the general understanding of anisotropy in LSMO, which relates the uniaxial anisotropy to differences in strain, cannot explain the results. These findings su…
Engineering Symmetry Breaking Interfaces by Nanoscale Structural-Energetics in Orthorhombic Perovskite Thin Films
本文研究LaVO3/DyScO3薄膜中应变与氧八面体连通性竞争驱动的界面形成，发现薄膜通过体内原子级平整切换平面实现90度取向转变，并利用原子尺度表征与第二性原理建模揭示其结构弛豫机制，为纳米尺度功能系统设计开辟新途径。
Pressure-induced structural transition, metallization, and topological superconductivity in PdSSe
研究通过理论预测揭示PdSSe在高压下经历结构转变、金属化及拓扑超导性，揭示其独特的电子和拓扑特性。
Analytical theory of large size intrinsic localized modes (ILMs) in anharmonic two-atomic chain is presented. It is shown that ILMs with frequencies close to the borders of phonon gap are govern by quartic anharmonicity while the effect of cubic anharmonicity is essentially reduced. This reduction effect is a consequence of small amplitude of vibrations of every second atom. As a result, an ILM cannot split down from the optical phonon band, in contradiction with the commonly accepted point of view. But it can spit up from the top of the acoustic phonon band making possible the existence of th…
Phase transitions in KNbO3: symmetry analysis and first principles calculations
本研究结合群论对称性分析与DFT-HSE06 LCAO从头算，探究KNbO3的三种温度相关相变，发现仅低温菱方相稳定、仅立方-四方相变对称允许，并以虚拟单斜相解释其余相变机制。
Spin-to-charge conversion in orthorhombic RhSi topological semimetal crystalline thin films
本研究通过逆自旋霍尔效应测量，在正交RhSi薄膜中实现了高效自旋-电荷转换，最大自旋霍尔电导率为126，界面自旋透明度达88%，为拓扑自旋电子学应用提供了新平台。
Through the use of perturbation theory, in this work we develop a method which allows for a substantial reduction in the size of the plane-wave basis used in density-functional calculations. This method may be used for both pseudopotentials and all-electron calculations and is particularly beneficial in the latter case. In all cases, the approach has the advantage of allowing accurate predictions of transferability errors for any environment. Finally, this method can be easily implemented into conjugate gradient techniques and it is therefore computationally efficient. In this work, we apply t…
Using density functional theory calculations, the ground state structure of BaFeO$_3$ (BFO) is investigated with local spin density approximation (LSDA). Cubic, tetragonal, orthorhombic, and rhombohedral types BFO are considered to calculate the formation enthalpy. The formation enthalpies reveal that cubic is the most stable structure of BFO. Small energy difference between the cubic and tetragonal suggests a possible tetragonal BFO. Ferromagnetic(FM) and anitiferromagnetic (AFM) coupling between the Fe atoms show that all the striochmetric BFO are FM. The energy difference between FM and AFM…
Structural and optoelectronic properties of α, \{beta}, γ phases of calcium titanate are studied with the implementation of first-principles quantum-chemical calculations in the framework of DFT. When optimizing the geometry, the GGA approximation was used. The relaxed lattice parameters obtained by us are identical with the experimental analogs. It has been established that the most stable phase of calcium titanate is the orthorhombic syngony, which corresponds to the results of experimental measurements. It is shown that as the transition from the α-phase to the γ-phase proceeds, the lattice…
We discuss relative roles played by the magnetic inversion symmetry breaking and the ferroelectric (FE) atomic displacements in the multiferroic state of YMnO3. For these purposes we derive a realistic low-energy model, using results of first-principles calculations and experimental parameters of the crystal structure. Then, we solve this model in the Hartree-Fock approximation. We argue that the multiferroic state in YMnO3 has a magnetic origin, and the centrosymmetric Pbnm structure is formally sufficient for explaining details of the noncentrosymmetric magnetic ground state. The relativisti…
Achieving high temperature ferromagnetism by doping transition metals thin films is seen as a viable approach to integrate spin-based elements in innovative spintronic devices. In this work we investigated the effect of Fe doping on structural properties of ZrO2 grown by atomic layer deposition (ALD) using Zr(TMHD)4 for Zr and Fe(TMHD)3 for Fe precursors and ozone as oxygen source. The temperature during the growth process was fixed at 350°C. The ALD process was tuned to obtain Fe doped ZrO2 films with uniform chemical composition, as seen by time of flight secondary ion mass spectrometry. The…
Pressure-induced transitions from ordered intermetallic phases to substitutional alloys to semi-ordered phases were studied in a series of bismuth tellurides. Using angle-dispersive x-ray diffraction, the compounds Bi4Te5, BiTe, and Bi2Te were observed to form alloys with the disordered body-centered cubic (bcc) crystal structure upon compression to above 14--19 GPa at room temperature. The BiTe and Bi2Te alloys and the previously discovered high-pressure alloys of Bi2Te3 and Bi4Te3 were all found to show atomic ordering after gentle annealing at very moderate temperatures of ~100°C. Upon anne…
Cluster variation method (CVM) and path probability method (PPM) have generally been employed to study replacive phase transitions in alloy systems. Recently, displacive phase transitions have been explored within the realm of replacive phase transition in the CVM theoretical framework by viewing displaced atoms as different atomic species, i.e., by converting a freedom of atomic displacement to a configurational freedom. The same methodology is applied to the PPM calculations in this work, and the kinetics of displacive phase transition from tetragonal to cubic phases in ZrO2 are investigated…
We present results of local spin density approximation pseudopotential calculations for the ferroelectromagnet, yttrium manganite (YMnO3). 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 YMnO3 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…
The gate defect of the ferroelectric HfO$_2$-based Si field-effect transistor (Si FeFET) plays a dominant role in its reliability issue. The first-principles calculations are an effective method for the atomic-scale understanding of gate defects. However, the first-principles study on the defects of FeFET gate stacks, i.e., metal/orthorhombic-Hf$_{0.5}$Zr$_{0.5}$O$_2$/SiO$_2$/Si structure, has not been reported so far. The key challenge is the construction of metal/orthorhombic-Hf$_{0.5}$Zr$_{0.5}$O$_2$/SiO$_2$/Si gate stack models. Here, we use the Hf$_{0.5}$Zr$_{0.5}$O$_2$(130) high-index cr…
Shape memory alloys inherit their macroscopic properties from their mesoscale microstructure originated from the martensitic phase transformation. In a cubic to orthorhombic transition, a single variant of marten- site can have a compatible (exact) interface with the austenite for some special lattice parameters in contrast to conventional austenite/twinned martensite interface with a transition layer. Experimentally, the phase compat- ibility results in a dramatic drop in thermal hysteresis and gives rise to very stable functional properties over cycling. Here, we investigate the microstructu…
From our previous models of martensitic transformation, the continuous matrices of atomic displacements and lattice deformations from face-centred-cubic (fcc) to body centred-cubic (bcc) phases are calculated in agreement with different possible final orientation relationships, such as Bain, Pitsch and Kurdjumov-Sachs (KS). The angular distortion introduced in the calculations appears a natural order parameter of transition. The distortion corresponding to KS is the only one that respects the parallelism of a dense direction and of a dense plane of both the fcc and bcc phases. This paper gives…
The linear muffin-tin-orbital method combined with density functional theory (local approximation) and the semiempirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the study of the $F$ centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO$_3$ crystals. Calculations for 39-atom supercells show that the two electrons are considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus…
We present a first-principles investigation of structural and elastic properties of experimentally observed phases of bulk SrRuO$_3$ - namely orthorhombic, tetragonal, and cubic - by applying density functional theory (DFT) approximations. At first, we focus our attention on the accuracy of calculated lattice constants in order to find out DFT approaches that best represent the crystalline structure of SrRuO$_3$, since many important physical quantities crucially depend on change in volume. Next, we evaluate single-crystal elastic constants, mechanical stability, and macroscopic elastic parame…
Using density-functional theory with ultrasoft pseudopotentials, we previously investigated the structural and electronic properties of the low-pressure (cubic, tetragonal, and monoclinic) phases of ZrO2 and HfO2, in order to elucidate phonon modes, Born effective charge tensors, and especially the lattice dielectric response in these phases. We now extend this previous work by carrying out similar calculations on the two high-pressure orthorhombic phases, and by providing density-of-states and band-gap information on all polymorphs. Our results show that the electronic structures and dielectr…
Atomistic Processes of high-temperature plastic deformation of nanoscale body-centered cubic tungsten
该研究开发基于电流诱导焦耳热的原位高温纳米力学测试方法，揭示钨纳米晶体在高温拉伸中依次发生形变孪晶、BCC-FCC-BCC相变和完整位错滑移。
Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms …
Results based on {\em ab initio} density functional calculations indicate a general graphitization tendency in ultrathin slabs of cubic diamond, boron nitride, and many other cubic structures including rocksalt. Whereas such compounds often show an energy preference for cubic rather than layered atomic arrangements in the bulk, the surface energy of layered systems is commonly lower than that of their cubic counterparts. We determine the critical slab thickness for a range of systems, below which a spontaneous conversion from a cubic to a layered graphitic structure occurs, driven by surface e…
Modular implementation of the linear and cubic-scaling orbital minimization methods in electronic structure codes using atomic orbitals
本文在 SIESTA 中模块化实现轨道最小化方法，借助 DBCSR、ScaLAPACK、MatrixSwitch 与 libOMM 支持稀疏/稠密矩阵和大规模并行，并验证线性缩放求解器在数百原子绝缘体系中的性能优势。
Magnetic charges, or magnetic monopoles, may form in the electronic structure of magnetic materials where ions are deprived of symmetry with respect to spatial inversion. Predicted in 2009, the strange magnetic, pseudoscalars have recently been found different from zero in simulations of electronic structures of some magnetically ordered, orthorhombic, lithium orthophosphates (LiMPO4). We prove that magnetic charges in lithium orthophosphates diffract x-rays tuned in energy to an atomic resonance, and to guide future experiments we calculate appropriate unit-cell structure factors for monoclin…
Effect of crystal symmetry of lead halide perovskites on the optical orientation of excitons
本研究在1.6 K下比较四种不同对称性铅卤钙钛矿的激子光学取向，发现高达85%的极化，且不受失谐影响，证明晶体保持空间反演对称性，排除了Dyakonov-Perel自旋弛豫机制。
The self-organization of a chemi-sorbed mono-layer is studied as a two dimensional ordering process in presence of surface stress. As proved previously for a single phase separation, a steady surface state is yielded from the competition between the domain boundary energy and the surface stress elastic energy. In the present letter, the resulting patterns are shown to depend on the interplay between the symmetries of both the internal layer order and the underlying crystal. For experimental relevance, our study is focussed on a (001) copper surface and we believe to enhance a route to stabiliz…
Emergent superconductivity is strongly correlated with the symmetry of local atomic configuration in the parent compounds of iron-based superconductors. While chemical doping or hydrostatic pressure can change the local geometry, these conventional approaches do not provide a clear pathway in tuning the detailed atomic arrangement predictably, due to the parent compounds complicated structural deformation in the presence of the tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic approach to manipulate the local structural configurations in BaFe2As2 epitaxial thin fil…
From first-principles calculations, we investigate the structural instabilities of CaMnO$_3$. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground-state, the cubic perovskite structure of CaMnO$_3$ also exhibit a weak ferroelectric instability. Although ferroelectricity is suppressed by antiferrodistortive oxygen motions, we show that it can be favored using strain or chemical engineering in order to make CaMnO$_3$ multiferroic. We finally highlight that the FE instability of CaMnO$_3$ is Mn-dominated. This illustrates that, contrary to …
We use extensive first-principles quantum mechanical calculations to show that, although the static lattice and harmonic vibrational energies are almost identical, the anharmonic vibrational energy of hexagonal ice is significantly lower than that of cubic ice. This difference in anharmonicity is crucial, stabilising hexagonal ice compared with cubic ice by at least 1.4 meV/H2O, in agreement with experimental estimates. The difference in anharmonicity arises predominantly from molecular O-H bond stretching vibrational modes and is related to the different stacking of atomic layers.
I search for the ground state structures of the kagome metals KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$ using first principles calculations. Group-theoretical analysis shows that there are seventeen different distortions that are possible due to the phonon instabilities at the $M$ $(\frac{1}{2},0,0)$ and $L$ $(\frac{1}{2},0,\frac{1}{2})$ points in the Brilouin zone of the parent $P6/mmm$ phase of these materials. I generated these structures for the three compounds and performed full structural relaxations that minimize the atomic forces and lattice stresses. I find that the $Fmmm$ phase …
Using ab initio calculations, we have investigated a cubic perovskite BaOsO3 and a few related compounds that have been synthesized recently and formally have a metallic d^4 configuration. In BaOsO3, which shows obvious 3-dimensional fermiology, a nonmagnetism is induced by a large spin-orbit coupling (SOC), which is precisely equal to an exchange splitting ~0.4 eV of the $t_{2g}$ manifold. However, the inclusion of on-site Coulomb repulsion as small as $U^c$~1.2 eV, only 1/3 of the $t_{2g}$ bandwidth, leads to the emergence of a spin-ordered moment, indicating that this system is on the verge…
The first-principles calculations have been performed to investigate the ground state properties of cubic scandium trifluoride (ScF$_3$) perovskite. Using modified hybrid exchange-correlation functionals within the density functional theory (DFT) we have comprehensively compared the electronic properties of ScF$_3$ obtained by means of the linear combination of atomic orbitals (LCAO) and projector augmented-waves (PAW) methods. Both methods allowed us to reproduce the lattice constant experimentally observed in cubic ScF$_3$ at low temperatures and predict its electronic structure in good agre…
In search of the ferromagnetic insulators for spintronic device applications, we have studied the electronic, optical, and magnetic properties ofLa2CrNiO6(LCNO) using the first principle density functional theory (DFT)and monte carlo simulation technique. Firstly, we have adopted the sol-gel method for preparation of LCNO and refined the X-ray diffracted value using the fullprof suite under the Rietveld mechanism. The obtained orthorhombic Pbnm and cubic Fm-3m space groups are used as the input of the DFT calculations. Both the structures have shown the half metallicity and insult-ing bandgap …
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