arXiv subjects

Ravi Kashikar

Publications and source records attributed to Ravi Kashikar.

At least 19 recordsLinked to original sources

arXiv2026

Emergence of polar monoclinic phase in heterohalogen substituted CsGeX$_3$

The occurrence of ferroelectricity in inorganic germanium-based halide perovskites has provided an alternative to oxide counterparts. Using first-principles methods, we have studied CsGeX$_3$ materials with heterohalogen substitution at the X site in a 2:1 configuration. Structurally, such variation alters the octahedral environment more strongly than in pristine materials, giving rise to a polar monoclinic phase at room temperature. The occurrence of the monoclinic phase is also confirmed through the energetics of structures generated by the displacements of atoms in accordance with soft mode eigenvectors of the dynamical matrix along various directions. In the chemically tuned phase, the polarization is along [101] and increases by 10-15\% compared to pristine ones. The electronic structure analysis reveals that spin-splitting energy ranges from 25 to 250 meV in the valence band and from 9 to 80 meV in the conduction band in chemically tuned structures. In addition, these structures exhibit Rashba and persistent spin textures, which are coupled to the polarization direction. The parameters of the symmetry-dependent \textbf{k.p } Hamiltonian provide insights into the strength of spin-splitting and the nature of spin-texture. The semiconducting and spin-polarized nature of CsGeX$_3$ materials makes them strong candidates for Datta-Das spin transistors.

cond-mat.mtrl-sci↗

arXiv2025

DFT-based insight into finite-temperature properties of ferroelectric perovskites with lone-pair: the case of CsGeX$_3$ (X=Cl, Br, I)

Ferroelectrics remain in the focus of scientific attention for decades owing to their fundamental and practical appeal. Recently, ferroelectricity has been demonstrated in semiconducting halide perovskites, offering both a rare combination of ferroelectricity and semiconductivity in the same material and a possible alternative to the prevailing perovskite oxide ferroelectrics. We propose a route to simulating such materials at finite temperatures capable of reproducing key experimental and first-principle data, such as Curie temperature, phase transition sequence, spontaneous polarization, and soft mode frequencies. The key methodological finding is the superior performance of hybrid exchange correlation functionals in parametrization of effective Hamiltonians for ferroelectrics with lone pair. The parametrization for effective Hamiltonians for CsGeX$_3$, (X=Cl, Br, I) is reported. The application of methodology to study polarization reversal in CsGeX$_3$, allows for the development of a ``minimalistic" model for polarization reversal in ferroelectrics that provides an insight into the mechanisms of polarization reversal and its key features, such as the relationship between the coercive field, temperature, and AC field frequency. Importantly, the model reveals the origin of the well-known and ever-puzzling overestimation of coercive fields in computations. Furthermore, we report a variety of finite-temperature properties of CsGeX$_3$, ferroelectrics, such as dielectric susceptibility, pyroelectric coefficients, and energy storage density, which reveal that these halide perovskites possess properties comparable to their oxide counterparts. We believe that our work provides significant methodological advancements, deepens fundamental understanding of ferroelectrics, and reveals the potential of halide perovskite ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2024

Large electrically and chemically tunable Rashba-Dresselhaus effects in Ferroelectric CsGeX$_3$ (X=Cl, Br, I) perovskites

Rashba-Dresselhaus effects, which originate from spin-orbit coupling and allow for spin manipulations, are actively explored in materials following the pursuit of spintronics and quantum computing. However, materials that possess practically significant Rashba-Dresselhaus effects often contain toxic elements and offer little opportunity for tunability of the effects. We used first-principles simulations to reveal that the recently discovered halide ferroelectrics in the CsGeX$_3$ (X=Cl, Br, I) family possess large and tunable Rashba-Dresselhaus effects. In particular, they give origin to the spin splitting of up to 171meV in valence band of CsGeI$_3$. The value is chemically tunable and can decrease by 25% and 70% for CsGeBr$_3$ and CsGeCl$_3$, respectively. Such chemical tunability could result in engineering of desired values through solid solution technique. Application of electric field was found to result in structural changes that could both decrease and increase spin splitting leading to electrical tunability of the effect. In the vicinity of conduction and valence band extrema, the spin textures are mostly of Rashba type which is promising for spin-to-charge conversion applications. The spin directions are coupled with the polarization direction leading to Rashba-ferroelectricity co-functionality. Our work identifies lead-free perovskite halides as excellent candidates for spin-based applications and is likely to stimulate further research in this direction.

cond-mat.mtrl-sci↗

arXiv2024

Theory of ultrathin ferroelectrics: the case of CsGeBr$_3$

Ferroelectricity has recently been demonstrated in germanium-based inorganic halide perovskites. We use atomistic first-principles-based simulations to study ultra-thin CsGeBr$_3$ films with thicknesses of 4-18 nm and develop a theory for ferroelectric ultrathin films. The theory introduces (i) a local order parameter, the local polarization, which allows the identification of phase transitions into both monodomain and polydomain phases, and (ii) a dipole pattern classifier, which allows efficient and reliable identification of unique dipole patterns. Application of the theory to both halides CsGeBr$_3$ and CsGeI$_3$, as well as oxide BiFeO$_3$ ultrathin ferroelectrics, which undergo paraelectric cubic to ferroelectric rhombohedral phase transition in bulk, reveal two distinct scenarios for ultrathin films. In the first one, the films transition into a monodomain phase, which is allowed below a critical value of the residual depolarizing field. Above this critical value, the second scenario occurs, and the film undergoes a phase transition into a nanodomain phase. The two scenarios are associated with the opposite response of Curie temperature to thickness reduction. As the film's thickness decreases, the transition temperature into the monodomain phase increases while the transition temperature into the nanodomain phase decreases. The surface effects are responsible for the Curie temperature enhancement, while the stripe domain pattern is the origin of the transition temperature suppression. Application of dipole pattern classifier reveals a rich variety of nanodomain phases in halide films: nano-stripes, labyrinths, zig-zags, pillars, and lego-domains. Our work could lead to both a deeper understanding of nanoscale ferroelectrics and discoveries of unusual nanoscale dipole patterns.

cond-mat.mtrl-sci↗

arXiv2024

Giant Spin Splitting and its Origin in Methylhydrazinium Lead Halide Perovskites

Spin splitting, or removal of spin degeneracy in the electronic energy band/level is often a measure of spin-orbit coupling strength and a way to manipulate spin degrees of freedom. We use first-principles simulations to predict giant spin splitting in methylhydrazinium lead halide (MHyPbX$_3$, MHy = CH$_3$NH$_2$NH$_2$, X = Br and Cl) hybrid organic-inorganic perovskites. The values can reach up to 408.0~meV at zero Kelvin and 281.6~meV at room temperature. The origin of the effect is traced to the large distortion of PbX$_3$ framework, driven primarily by Pb ions in the ferroelectric $Γ^{3-}$ mode. The Pb displacements consist of combination of polar and antipolar arrangements and result in up to 39.2~meV/atom enhancement of the spin-orbit coupling energy in the polar phase of the materials. The spin-orbit coupling gives origin to highly persistent spin textures in MHyPbX$_3$, which are desirable for applications in spintronics and quantum computing. Our findings reveal an additional functionality for hybrid organic-inorganic perovskite and open a way for the design of more materials with giant spin splitting.

cond-mat.mtrl-sci↗

arXiv2024

Ferroelectric phases and phase transitions in CsGeBr$_3$ induced by mechanical load

First-principles-based atomistic simulations are used to reveal ferroelectric phases and phase transitions induced in a semiconductor ferroelectric, CsGeBr$_3$, by external loads: hydrostatic pressure, uniaxial and biaxial stresses, and misfit strain. Hydrostatic pressure was found to suppress the Curie point at the rate -0.45$T_C(0)$ K/GPa, where $T_C(0)$ is the zero pressure Curie temperature. Stresses and misfit strains were found to induce additional ferroelectric phase transitions and phases not available under normal conditions. We find that tensile load significantly enhances both the Curie temperature and spontaneous polarization, while compressive load has the opposite effect but with the difference that the Curie temperature is only slightly suppressed. The isothermal dependencies of polarization on pressure and stresses are highly nonlinear, which could result in large nonlinear piezoelectric responses. The phase diagrams reveal the diversity of the phases accessible through mechanical load, which include tetragonal, orthorhombic and monoclinic symmetries in addition to the rhombohedral and cubic ones realizable under normal conditions. We believe that this work reveals the potential of Ge-based halide perovskites for applications in energy converting devices, which is especially significant in the current pursuit of environmental friendly lead-free technologies.

cond-mat.mtrl-sci↗

arXiv2023

Coupling of structure and magnetism to spin splitting in hybrid organic-inorganic perovskites

Hybrid organic-inorganic perovskites are famous for the diversity of their chemical compositions, phases and phase transitions, and associated physical properties. We use a combination of experimental and computational techniques to reveal strong coupling between structure, magnetism, and spin splitting in a representative of the largest family of hybrid organic-inorganic perovskites: the formates. With the help of first-principles simulations, we find spin splitting in both conduction and valence bands of [NH$_2$NH$_3$]Co(HCOO)$_3$, induced by spin-orbit interactions, which can reach up to 14~meV. Our magnetic measurements reveal that this material exhibits canted antiferromagnetism below 15.5 K. The direction of the associated antiferromagnetic order parameter is strongly coupled with the spin splitting already in the centrosymmetric phase, allowing for the creation and annihilation of spin splitting through the application of a magnetic field. Furthermore, the structural phase transition into experimentally observed polar Pna2$_1$ phase completely changes the aforementioned spin splitting and its coupling to magnetic degrees of freedom. This reveals that in [NH$_2$NH$_3$]Co(HCOO)$_3$, the structure and magnetism are strongly coupled to spin splitting in a way that allows for its manipulation through both magnetic and electric fields. As an example, for a given point inside the Brillouin zone of centrosymmetric Pnma phase of [NH$_2$NH$_3$]Co(HCOO)$_3$, spin splitting can be turned on/off by aligning the antiferromagnetic vector along certain crystallographic directions or through inducing a polar phase by the application of an electric field. We believe that our findings offer an important step toward fundamental understanding and practical applications of materials with coupled properties.

cond-mat.mtrl-sci↗

arXiv2023

Coexistence of polar and antipolar phases in ferroelectric halide perovskite CsGeBr$_3$

Recently ferroelectricity has been demonstrated in the family of halide perovskites: CsGeX$_3$ (X=Cl, Br, I). We develop a first-principles-based computational approach to probe finite-temperature properties of \cgb. Our computations reveal the existence of a dynamic antipolar phase above the Curie temperature. The phase can be stabilized below the Curie temperature through quenching. Furthermore, competition between the polar and antipolar phases results in the formation of rare polar-antipolar domains. We demonstrate that such polar-antipolar domains can result in the formation of unusual polarization fields with head-to-head and tail-to-tail dipole arrangements, which not only offers an explanation for the recent experimental findings but could also lead to the discovery of novel topological polar structures.

cond-mat.mtrl-sci↗

arXiv2023

First-Principles Property Assessment of Hybrid Formate Perovskites

Hybrid organic inorganic formate perovskites, AB(HCOO)$_3$, is a large family of compounds which exhibit variety of phase transitions and diverse properties. Some examples include (anti)ferroelectricity, ferroelasticity, (anti)ferromagnetism, and multiferroism. While many properties of these materials have already been characterized, we are not aware of any study that focuses on comprehensive property assessment of a large number of formate perovskites. Comparison of the materials property within the family is challenging due to systematic errors attributed to different techniques or the lack of data. For example, complete piezoelectric, dielectric and elastic tensors are not available. In this work, we utilize first-principles density functional theory based simulations to overcome these challenges and to report structural, mechanical, dielectric, piezoelectric, and ferroelectric properties for 29 formate perovskites. We find that these materials exhibit elastic stiffness in the range 0.5 to 127.0 GPa , highly anisotropic linear compressibility, including zero and even negative values; dielectric constants in the range 0.1 to 102.1; highly anisotropic piezoelectric response with the longitudinal values in the range 1.18 to 21.12 pC/N, and spontaneous polarizations in the range 0.2 to 7.8 $μ$C/cm$^2$. Furthermore, we propose and computationally characterize a few formate perovskites, which have not been reported yet.

cond-mat.mtrl-sci↗

arXiv2023

Enhanced piezoelectric response at nanoscale vortex structures in ferroelectrics

The piezoelectric response is a measure of the sensitivity of a material's polarization to stress or its strain to an applied field. Using in-operando x-ray Bragg coherent diffraction imaging, we observe that topological vortices are the source of a five-fold enhancement of the piezoelectric response near the vortex core. The vortices form where several low symmetry ferroelectric phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions in which a large piezoelectric response is associated with coexisting phases in the proximity of the triple point, the largest responses for pure BaTiO3 at the nanoscale are in spatial regions of extremely small spontaneous polarization at vortex cores. The response decays inversely with polarization away from the vortex, analogous to the behavior in bulk ceramics as the cation compositions are varied away from the triple point. We use first-principles-based molecular dynamics to augment our observations, and our results suggest that nanoscale piezoelectric materials with large piezoelectric response can be designed within a parameter space governed by vortex cores. Our findings have implications for the development of next-generation nanoscale piezoelectric materials.

cond-mat.mtrl-sci↗

arXiv2022

Machine learning reveals memory of the parent phases in ferroelectric relaxors Ba(Ti$_{1-x}$,Zr$_x$)O$_3$

Machine learning has been establishing its potential in multiple areas of condensed matter physics and materials science. Here we develop and use an unsupervised machine learning workflow within a framework of first-principles-based atomistic simulations to investigate phases, phase transitions, and their structural origins in ferroelectric relaxors, Ba(Ti$_{1-x}$,Zr$_x$)O$_3$. We first demonstrate the applicability of the workflow to identify phases and phase transitions in the parent compound, a prototypical ferroelectric BaTiO$_3$. We then apply the workflow on Ba(Ti$_{1-x}$,Zr$_x$)O$_3$, with $x\leq0.25$ to reveal (i) that some of the compounds bear a subtle memory of BaTiO$_3$, phases beyond the point of the pinched phase transition, which could contribute to their enhanced electromechanical response; (ii) the existence of peculiar phases with delocalized precursors of nanodomains -- likely candidates for the controversial polar nanoregions; and (iii) nanodomain phases for the largest concentrations of $x$

cond-mat.mtrl-sci↗

arXiv2022

Persistent spin textures in halide perovskites induced by uniaxial stress

Persistent spin textures are highly desirable for applications in spintronics as they may allow for long carrier spin lifetimes. However, they are also rare as only four point groups can host such textures, and even for these four groups, the emergence of persistent spin textures requires a delicate balance between coupling parameters, which control the strength of spin-momentum interactions. We use first-principles density functional simulations to predict the possibility of achieving these desirable spin textures through the application of uniaxial stress. Hybrid organic-inorganic perovskite MPSnBr$_3$ (MP = CH$_3$PH$_3$) is a ferroelectric semiconductor which exhibits persistent spin textures in the near to its conduction band minimum and mostly Rashba type in the vicinity of its valence band maximum. Application of uniaxial stress leads to the gradual evolution of the valence bands spin textures from mostly Rashba type to persistent ones under tensile load and to pure Rashba or persistent ones under compressive load. We also report that the material exhibits flexibility, rubber-like response, and both positive and negative piezoelectric constants. Combination of such properties may create opportunities for a flexible/rubbery spintronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Feeble Metallicity and Robust Semiconducting Regime in Structurally Sensitive Ba(Pb, Sn)O$_3$ Alloys

Density functional calculations are carried out to study the symmetry and substitution-driven electronic phase transition in BaPb$_{1-x}$Sn$_x$O$_3$. Two end members BaSnO$_3$ and BaPbO$_3$, are found to be insulating and metallic, respectively. In the latter case, the metallicity arises with the presence of an electron pocket, formed by Pb-s dominated conduction band edge, and a hole pocket formed O-p dominated valence bands. While electron carriers are found to be highly mobile, the hole carriers are localized. Our study reveals that an insulating phase can be realized in the metallic cubic BaPbO$_3$ in three ways in order to explore optoelectronic properties. Firstly, by lowering the symmetry of the lattice to monoclinic through rotation and tilting of the PbO$_6$ octahedra. Secondly, by hydrostatic pressure, and thirdly by alloying with Sn substitution. The presence of soft phonon modes implies the plausibility of symmetry lowering structural transitions. Furthermore, unlike the earlier reports, we find that Sn substituted BaPbO$_3$ cannot exhibits topological insulator phase due to absence of the band inversion.

cond-mat.mtrl-sci↗

arXiv2021

A Generic Slater-Koster Description of the Electronic Structure of Centrosymmetric Halide Perovskites

The halide perovskites have truly emerged as efficient optoelectronic materials and show the promise of exhibiting nontrivial topological phases. Since the bandgap is the deterministic factor for these quantum phases, here we present a comprehensive electronic structure study using first-principle methods by considering nine inorganic halide perovskites CsBX$_3$ (B = Ge, Sn, Pb; X = Cl, Br, I) in their three structural polymorphs (cubic, tetragonal and orthorhombic). A series of exchange-correlations (XC) functionals are examined towards accurate estimation of the bandgap. Furthermore, while thirteen orbitals are active in constructing the valence and conduction band spectrum, here we establish that a four orbital based minimal basis set is sufficient to build the Slater-Koster tight-binding model (SK-TB), which is capable of reproducing the bulk and surface electronic structure in the vicinity of the Fermi level. Therefore, like the Wannier based TB model, the presented SK-TB model can also be considered as an efficient tool to examine the bulk and surface electronic structure of halide family of compounds. As estimated by comparing the model study and DFT band structure, the dominant electron coupling strengths are found to be nearly independent of XC functionals, which further establishes the utility of the SK-TB model.

cond-mat.mtrl-sci↗

arXiv2020

Defining the Topological Influencers and Predictive Principles to Engineer Band Structure of Halide Perovskites

Complex quantum coupling phenomena of halide perovskites are examined through ab-initio calculations and exact diagonalization of model Hamiltonians to formulate a set of fundamental guiding rules to engineer the bandgap through strain. The bandgap tuning in halides is crucial for photovoltaic applications and for establishing non-trivial electronic states. Using CsSnI$_3$ as the prototype material, we show that in the cubic phase, the bandgap reduces irrespective of the nature of strain. However, for the tetragonal phase, it reduces with tensile strain and increases with compressive strain, while the reverse is the case for the orthorhombic phase. The reduction can give rise to negative bandgap in the cubic and tetragonal phases leading to normal to topological insulator phase transition. Also, these halides tend to form a stability plateau in a space spanned by strain and octahedral rotation. In this plateau, with negligible cost to the total energy, the bandgap can be varied in a range of 1eV. Furthermore, we present a descriptor model for the perovskite to simulate their bandgap with strain and rotation. Analysis of band topology through model Hamiltonians led to the conceptualization of topological influencers that provide a quantitative measure of the contribution of each chemical bonding towards establishing a normal or topological insulator phase. On the technical aspect, we show that a four orbital based basis set (Sn-$\{s,p\}$ for CsSnI$_3$) is sufficient to construct the model Hamiltonian which can explain the electronic structure of each polymorph of halide perovskites.

cond-mat.mtrl-sci↗

arXiv2020

Pressure and Inversion Symmetry Breaking Field Driven First Order Phase Transition and Formation of Dirac Circle in Perovskites

Through model Hamiltonian studies and first-principle electronic structure calculations, we have examined the effect of inversion symmetry breaking (ISB) field and hydrostatic pressure on the band topology of halide perovskites by taking MAPbI$_3$ as a prototype. Our study shows that while hydrostatic pressure induces normal to topological insulator continuous phase transition, the ISB field makes it first order. The pressure smoothly reduces the normal bandgap, and without ISB, the system achieves a gapless state before it produces a non-trivial bandgap with inverted characters. The ISB field does not stabilize the gapless state, and therefore, the discontinuity in the bandgap with pressure gives rise to the first-order transition. Furthermore, in the non-trivial phase, the ISB field forms an invariant surface Dirac circle in the neighbourhood of TRIM, which is first of its kind. The circle is formed due to interpenetration of Dirac cones resembling the band topology of AA-stacked bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2018

Second Neighbor Electron Hopping and Pressure Induced Topological Quantum Phase Transition in Insulating Cubic Perovskites

Perovskite structure is one of the five symmetry families suitable for exhibiting topological insulator phase. However, none of the halides and oxides stabilizing in this structure exhibit the same. Through density functional calculations on cubic perovskites (CsSnX$_3$ ; X = Cl, Br, and I), we predict a band insulator -- Dirac semimetal -- topological insulator phase transition with uniform compression. With the aid of a Slater-Koster tight binding Hamiltonian, we show that, apart from the valence electron count, the band topology of these perovksites is determined by five parameters involving electron hopping among the Sn-{s, p} orbitals. These parameters monotonically increase with pressure to gradually transform the positive band gap to a negative one and thereby enable the quantum phase transition. The universality of the mechanism of phase transition is established by examining the band topology of Bi based oxide perovskites. Dynamical stability of the halides against pressure strengthens the experimental relevance.

cond-mat.mtrl-sci↗

arXiv2018

Topologically Invariant Double Dirac States in Bismuth based Perovskites: Consequence of Ambivalent Charge States and Covalent Bonding

Bulk and surface electronic structures, calculated using density functional theory and a tight-binding model Hamiltonian, reveal the existence of two topologically invariant (TI) surface states in the family of cubic Bi perovskites (ABiO$_3$; A = Na, K, Rb, Cs, Mg, Ca, Sr and Ba). The two TI states, one lying in the valence band (TI-V) and other lying in the conduction band (TI-C) are formed out of bonding and antibonding states of the Bi-$\{$s,p$\}$ - O-$\{$p$\}$ coordinated covalent interaction. Below a certain critical thickness of the film, which varies with A, TI states of top and bottom surfaces couple to destroy the Dirac type linear dispersion and consequently to open surface energy gaps. The origin of s-p band inversion, necessary to form a TI state, classifies the family of ABiO$_3$ into two. For class-I (A = Na, K, Rb, Cs and Mg) the band inversion, leading to TI-C state, is induced by spin-orbit coupling of the Bi-p states and for class-II (A = Ca, Sr and Ba) the band inversion is induced through weak but sensitive second neighbor Bi-Bi interactions.

cond-mat.mtrl-sci↗