Crystal Ground State Research Articles

Molecular dipoles in designer honeycomb lattices

Recent advances in ultracold atoms in optical lattices and developments in surface science have allowed for the creation of artificial lattices as well as the control of many-body interactions. Such systems provide new settings to investigate interaction-driven instabilities and non-trivial topology. In this paper, we explore the interplay between molecular electric dipoles on a two-dimensional triangular lattice with fermions hopping on the dual decorated honeycomb lattice which hosts Dirac and flat band states. We show that short-range dipole-dipole interaction can lead to ordering into various stripe and vortex crystal ground states. We study these ordered states and their thermal transitions as a function of the interaction range using simulated annealing and Monte Carlo methods. For the special case of zero wave vector ferrodipolar order, incorporating dipole-electron interactions and integrating out the electrons leads to a six-state clock model for the dipole ordering. Finally, we discuss the impact of the various dipole orders on the electronic band structure and the local tunneling density of states. Our work may be relevant to studies of "molecular graphene" -- CO molecules arranged on the Cu(111) surface -- which have been explored using scanning tunneling spectroscopy, as well as ultracold molecule-fermion mixtures in optical lattices.

Stabilization of non‐native polymorphs for electrocatalysis and energy storage systems

AbstractEvolution in material centric devices like batteries and electrocatalytic reactors have predominantly been made possible via the exploitation of the thermodynamic ground state of pristine or defective bulk crystal, referred to as the “Native polymorph” (NP) here. A significant increase in the material search space is possible by utilizing “Non‐Native polymorphs (NNP),” which are materials that have different translational symmetry with respect to NP. As the NNP have a distinct coordination structure from that of the NP, critical material properties can be anticipated to be different, making NNP a potential substitute material for the aforementioned applications, which are the focus of this review. To obtain a structure–function relationship, systematic approaches to the synthesis of NNP has been demonstrated. Following certain generalities behind NNP, we classify synthesis techniques into few categories with the hope of rationalizing the underlying mechanism of these synthesis and stabilization strategies. We discuss the utility of NNPs in the context of electrochemical water electrocatalytic reactions. Typically, the NNPs have more open volume space enabling lower lithium‐ion diffusion barrier, higher lithium‐ion binding energies, thereby making NNP efficient in the context of energy storage material. However, NNP have lesser stability than the NP and methods to calibrate and improve the stability of NNP are important. Overall, the discussion of polymorphic materials by demarcating them as NP and NNP provides a systematic approach towards modulating material properties as a trade‐off between thermodynamics and kinetics of physicochemical processes. Finally, the challenges and perspectives in this emerging field are discussed.This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Energy Research & Innovation > Science and Materials Energy and Development > Science and Materials

Lack of linear magnetoelectric effect in ferrimagnetic distorted honeycomb Ni4Nb2O9

M 4 A 2O9 transition metal oxides, with M a divalent cation and A Nb or Ta, that exhibit a structure derived from corundum constitute an interesting class of materials due to their possible magnetoelectric properties. The lack of a linear magnetoelectric effect in Ni4Nb2O9, unlike Mn4Nb2O9 or Co4Nb2O9, is explained by a comprehensive investigation, combining synchrotron x-ray and neutron diffraction with magnetization, dielectric, and polarization measurements. The m′m′m magnetic point group associated with the Pb′cn′ ferrimagnetic structure induced below 76 K by the orthorhombic Pbcn structure of Ni4Nb2O9 forbids indeed such an effect. The crystal structure and magnetic ground state of Ni4Nb2O9 are discussed and compared with those of magnetoelectric P3¯c1M4A2O9 compounds whose structure derives from corundum.

Ground States of Crystalline Caps: Generalized Jellium on Curved Space.

We study the ground states of crystals on spherical surfaces. These ground states consist of positive disclination defects in structures spanning from flat and weakly curved caps to closed shells. Comparing two continuum theories and one discrete-lattice simulation, we first investigate the transition between defect-free caps to single-disclination ground states and show it to be continuous and symmetry breaking. Further, we show that ground states adopt icosahedral subgroup symmetries across the full range of curvatures, even far from the closure of complete shells. While superficially similar to other models of 2D "jellium" (e.g., superconducting disks and 2D Wigner crystals), the interplay between the free edge of caps and the non-Euclidean geometry of its embedding leads to nontrivial ground state behavior that is without counterpart in planar jellium models.

High-throughput prediction of the ground-state collinear magnetic order of inorganic materials using Density Functional Theory

We present a robust, automatic high-throughput workflow for the calculation of magnetic ground state of solid-state inorganic crystals, whether ferromagnetic, antiferromagnetic or ferrimagnetic, and their associated magnetic moments within the framework of collinear spin-polarized Density Functional Theory. This is done through a computationally efficient scheme whereby plausible magnetic orderings are first enumerated and prioritized based on symmetry, and then relaxed and their energies determined through conventional DFT + U calculations. This automated workflow is formalized using the atomate code for reliable, systematic use at a scale appropriate for thousands of materials and is fully customizable. The performance of the workflow is evaluated against a benchmark of 64 experimentally known mostly ionic magnetic materials of non-trivial magnetic order and by the calculation of over 500 distinct magnetic orderings. A non-ferromagnetic ground state is correctly predicted in 95% of the benchmark materials, with the experimentally determined ground state ordering found exactly in over 60% of cases. Knowledge of the ground state magnetic order at scale opens up the possibility of high-throughput screening studies based on magnetic properties, thereby accelerating discovery and understanding of new functional materials.

Roles of Quadrupoles in Non-Fermi Liquid Physics for Crystal Field Triplets

Crystal field triplet ground states in f2 configurations such as Pr ions have quadrupolar coupling as well as magnetic coupling to conduction electrons, leading to a nontrivial non-Fermi liquid Kon...

A charge optimized many-body potential for iron/iron-fluoride systems.

A classical interatomic potential for iron/iron-fluoride systems is developed in the framework of the charge optimized many-body (COMB) potential. This interatomic potential takes into consideration the effects of charge transfer and many-body interactions depending on the chemical environment. The potential is fitted to a training set composed of both experimental and ab initio results of the cohesive energies of several Fe and FeF2 crystal phases, the two fluorine molecules F2 and the F2-1 dissociation energy curve, the Fe and FeF2 lattice parameters of the ground state crystalline phase, and the elastic constants of the body centered cubic Fe structure. The potential is tested in an NVT ensemble for different initial structural configurations as the crystal ground state phases, F2 molecules, iron clusters, and iron nanospheres. In particular, we model the FeF2/Fe bilayer and multilayer interfaces, as well as a system of square FeF2 nanowires immersed in an iron solid. It has been shown that there exists a reordering of the atomic positions for F and Fe atoms at the interface zone; this rearrangement leads to an increase in the charge transfer among the atoms that make the interface and put forward a possible mechanism of the exchange bias origin based on asymmetric electric charge transfer in the different spin channels.

Fourth-order nematic elasticity and modulated nematic phases: a poor man’s approach

ABSTRACTWe propose an extension of Frank-Oseen’s elastic energy for bulk nematic liquid crystals which is based on the hypothesis that the fundamental deformations allowed in nematic liquid crystals are splay, twist and bend. The extended elastic energy is a fourth-order form in the fundamental deformations. The existence of bulk spontaneous modulated or deformed nematic liquid crystal ground states is investigated. The analysis is limited to bulk nematic liquid crystals in the absence of limiting surfaces and/or external fields. The non deformed ground state is stable only when Frank-Oseen’s elastic constants are positive. In case where at least one of them is negative, the ground state becomes deformed. The analysis of the stability of the deformed states in the space of the elastic parameters allows to characterise different types of deformed nematic phases. Some of them are new nematic phases, for instance a twist – splay nematic phase is predicted. Inequalities between second-order elastic constants which govern the stability of the twist–bend, splay–bend, and splay–twist states are obtained. Their stability in respect to triple splay–bend–twist deformations is investigated.

Direct band gap narrowing and light-harvesting-potential in orthorhombic In-doped-AlFeO3 perovskite: A first principles study

Enhancing the structural stability of AlFeO3 is relevant since one of the primary reasons for its lack of engineering appeal lies in the fact that the system shows polytypism. This means that the system would have structural transitions in operation. Hence enhancing the stability of a relevant phase becomes central to utilizing the AlFeO3 phase for device applications. Here the crystal structures and ground state properties of AlFeO3 and indium-doped-AlFeO3, i.e Al1-xInxFeO3 (x = 0.25, 0.5, 0.75) are studied using first principles calculations based on density functional theory. Computed ground state properties like equilibrium lattice constants and bulk modulus are in good agreement with the available experimental data. Interestingly In-doped orthorhombic AlFeO3 has higher stability when compared to the parent structure, indicating a simple chemical approach to stabilizing this structure (hence providing a method to avoid the usual polytypism related challenges). Band structures and density of states of these materials phases are reported and analyzed. The experimentally relevant outcomes of this work are: (i) doped compositions when tuned appropriately can show a direct band gap of ∼1.12 eV and hence is promising as an active material for a single junction device structure; (ii) in another composition window, a direct band gap of ∼2–2.30 eV is readily achievable (which is appropriate for water splitting applications). Hence we show that orthorhombic In-doped AlFeO3 is a stable and a promising non-toxic material for energy harvesting applications.

Perovskite-Type InCoO3 with Low-Spin Co3+: Effect of In-O Covalency on Structural Stabilization in Comparison with Rare-Earth Series.

Perovskite rare-earth cobaltites ACoO3 (A = Sc, Y, La-Lu) have been of enduring interest for decades due to their unusual structural and physical properties associated with the spin-state transitions of low-spin Co3+ ions. Herein, we have synthesized a non-rare-earth perovskite cobaltite, InCoO3, at 15 GPa and 1400 °C and investigated its crystal structure and magnetic ground state. Under the same high-pressure and high-temperature conditions, we also prepared a perovskite-type ScCoO3 with an improved cation stoichiometry in comparison to that in a previous study, where synthesis at 6 GPa and 1297 °C yielded a perovskite cobaltite with cation mixing on the A-site, (Sc0.95Co0.05)CoO3. The two perovskite phases have nearly stoichiometric cation compositions, crystallizing in the orthorhombic Pnma space group. In the present investigation, comprehensive studies on newly developed and well-known Pnma ACoO3 perovskites (A = In, Sc, Y, Pr-Lu) show that InCoO3 does not fulfill the general evolution of crystal metrics with A-site cation size, indicating that InCoO3 and rare-earth counterparts have different chemistry for stabilizing the Pnma structures. Detailed structural analyses combined with first-principles calculations reveal that the origin of the anomaly for InCoO3 is ascribed to the A-site cation displacements that accompany octahedral tilts; despite the highly tilted CoO6 network, the In-O covalency makes In3+ ions reluctant to move from their ideal cubic-symmetry position, leading to less orthorhombic distortion than would be expected from electrostatic/ionic size mismatch effects. Magnetic studies demonstrate that InCoO3 and ScCoO3 are diamagnetic with a low-spin state of Co3+ below 300 K, in contrast to the case of (Sc0.95Co0.05)CoO3, where the high-spin Co3+ ions on the A-site generate a large paramagnetic moment. The present work extends the accessible composition range of the low-spin orthocobaltite series and thus should help to establish a more comprehensive understanding of the structure-property relation.

Excitonic metal–insulator phase transition of the Mott type in compressed calcium

It has been experimentally found that, under the static compression of a calcium crystal at room temperature, it undergoes a series of structural phase transitions: face-centered cubic lattice → body-centered cubic lattice → simple cubic lattice. It has been decided to investigate precisely the simple cubic lattice (because it is an alternative lattice) with the aim of elucidating the possibility of the existence of other (nonstructural) phase transitions in it by using for this purpose the Hubbard model for electrons with half-filled ns-bands and preliminarily transforming the initial electronic system into an electron–hole system by means of the known Shiba operators (applicable only to alternative lattices). This transformation leads to the fact that, in the new system of fermions, instead of the former repulsion, there is an attraction between electrons and holes. Elementary excitations of this new system are bound boson pairs—excitons. This system of fermions has been quantitatively analyzed by jointly using the equation-of-motion method and the direct algebraic method. The numerical integration of the analytically exact transcendental equations derived from the first principles for alternative (one-, two-, and three-dimensional) lattices has demonstrated that, in systems of two-species (electrons + hole) fermions, temperature-induced metal–insulator phase transitions of the Mott type are actually possible. Moreover, all these crystals are in fact excitonic insulators. This conclusion is in complete agreement with the analytically exact calculations of the ground state of a one-dimensional crystal (with half-filled bands), which were performed by Lieb and Wu with the aim to find out the Mott insulator–metal transition of another type.

Devil's staircases in synthetic dimensions and gauge fields

We study interacting bosonic or fermionic atoms in a high synthetic magnetic field in two dimensions spanned by continuous real space and a synthetic dimension. Here, the synthetic dimension is provided by hyperfine spin states, and the synthetic field is created by laser-induced transitions between them. While the interaction is short-range in real space, it is long-range in the synthetic dimension in sharp contrast with fractional quantum Hall systems. Introducing an analog of the lowest-Landau-level approximation valid for large transition amplitudes, we derive an effective one-dimensional lattice model, in which density-density interactions turn out to play a dominant role. We show that in the limit of a large number of internal states, the system exhibits a cascade of crystal ground states, which is known as devil's staircase, in a way analogous to the thin-torus limit of quantum Hall systems.

Reversal of helicoidal twist handedness near point defects of confined chiral liquid crystals

Handedness of the director twist in cholesteric liquid crystals is commonly assumed to be the same throughout the medium, determined solely by the chirality of constituent molecules or chiral additives, albeit distortions of the ground-state helicoidal configuration often arise due to the effects of confinement and external fields. We directly probe the twist directionality of liquid crystal director structures through experimental three-dimensional imaging and numerical minimization of the elastic free energy and show that spatially localized regions of handedness opposite to that of the chiral liquid crystal ground state can arise in the proximity of twisted-soliton-bound topological point defects. In chiral nematic liquid crystal confined to a film that has a thickness less than the cholesteric pitch and perpendicular surface boundary conditions, twisted solitonic structures embedded in a uniform unwound far-field background with chirality-matched handedness locally relieve confinement-imposed frustration and tend to be accompanied by point defects and smaller geometry-required, energetically costly regions of opposite twist handedness. We also describe a spatially localized structure, dubbed a "twistion," in which a twisted solitonic three-dimensional director configuration is accompanied by four point defects. We discuss how our findings may impinge on the stability of localized particlelike director field configurations in chiral and nonchiral liquid crystals.

On the crystal ground state in the Schrödinger–Poisson model with point ions

A space-periodic ground state is shown to exist for lattices of point ions in R3 coupled to the Schrodinger and scalar fields. The coupling requires renormalization due to the singularity of the Coulomb self-action. The ground state is constructed by minimizing the renormalized energy per cell. This energy is bounded from below when the charge of each ion is positive. The elementary cell is necessarily neutral.

Ground states of stealthy hyperuniform potentials. II. Stacked-slider phases.

Stealthy potentials, a family of long-range isotropic pair potentials, produce infinitely degenerate disordered ground states at high densities and crystalline ground states at low densities in d-dimensional Euclidean space R^{d}. In the previous paper in this series, we numerically studied the entropically favored ground states in the canonical ensemble in the zero-temperature limit across the first three Euclidean space dimensions. In this paper, we investigate using both numerical and theoretical techniques metastable stacked-slider phases, which are part of the ground-state manifold of stealthy potentials at densities in which crystal ground states are favored entropically. Our numerical results enable us to devise analytical models of this phase in two, three, and higher dimensions. Utilizing this model, we estimated the size of the feasible region in configuration space of the stacked-slider phase, finding it to be smaller than that of crystal structures in the infinite-system-size limit, which is consistent with our recent previous work. In two dimensions, we also determine exact expressions for the pair correlation function and structure factor of the analytical model of stacked-slider phases and analyze the connectedness of the ground-state manifold of stealthy potentials in this density regime. We demonstrate that stacked-slider phases are distinguishable states of matter; they are nonperiodic, statistically anisotropic structures that possess long-range orientational order but have zero shear modulus. We outline some possible future avenues of research to elucidate our understanding of this unusual phase of matter.

Domain patterns and electric properties at domain walls in a surface normal to the direction of ferroelectric polarization in h-ErMnO3

The configuration of ferroelectric domains in a plane normal to the direction of ferroelectric polarization in hexagonal ErMnO3 (h-ErMnO3) was investigated using piezoresponse force microscopy and a three-dimensional Monte Carlo simulation of the clock model with a third dimensional coupling. Two kinds of patterns, long-range-ordered stripe domains and swirling vortex domains were observed alternatively depending on the crystal’s annealing temperature. From the Monte Carlo simulation, these vortex domain and stripe domain patterns could be understood as the topologies of an intermediate state and the ground state of the h-ErMnO3 crystal, respectively. The detailed electric properties of the polarization configuration were obtained by using conductive atomic force microscopy and revealed a tail-to-tail polarization configuration at the domain boundary that demonstrated a more conductive character than the head-to-head polarization configuration.

What drives superconductivity in Pt-doped IrTe2?

The emergency of superconductivity at the edge of charge density wave (CDW) state by pressure or chemical doping in 1T transition metal dichalcogenides (TMDs) has attracted much attention in past decade [1, 2]. The interplay of CDW and superconductivity has triggered the hot debate: why does superconductivity arise after suppressing the CDW and what’s the driving force of the charge modulation? For the 5d transition metal ditelluride IrTe2, a layered compound with trigonal symmetry, previous studies have shown an ordered superstructure below *280 K, and a superconducting state when doped or intercalated with Pd or Pt [3, 4]. In IrTe2, the Fermi surface nesting has been excluded as the impetus of superstructure; instead, splitting of Te orbitals by the crystal field [5] or destabilization of polymeric Te–Te networks was proposed [6], evident from the visualized periodic dimerization of Te atoms by scanning tunneling microscopy (STM) [7]. Meanwhile, a charge disproportionated CDW by covalent bonding Ir dimers was also presented [8]. However, though the origin of the superstructure formation becomes more and more clear, the underlying mechanism of the appearance of superconductivity by doping Pd or Pt is still obscure. Recently, Ruan et al. [9] have studied the structural phase transition and electronic structure evolution in pure and Pt-doped IrTe2 single crystals using low-temperature STM combined with first-principles calculations. The authors presented a clear evidence for a commensurate 1/6 charge modulation as the ground state of high-quality IrTe2 single crystals, consistent with previous reports [7, 10]. By substituting Pt for Ir, the system exhibits longrange ordered 1/5 phase and then goes to a new threefold quasiperiodic phase. In addition to a full picture of phase evolution, the most important finding is that the Pt dopants actually act as chemical strain, generating local impurity states rather than injecting itinerant charge carriers [9]. The weakened intralayer Ir dimerization effectively leads to the recovery of polymeric Te bonds across layers, lifting the density of states at Fermi level and making the system more three-dimensional [9]. Therefore, it turns out that the higher dimensionality tips the balance in favor of superconductivity in this system. This work presents a coherent story and also stimulates us to think more about the role of strain in modulating new order.

Observations of Rabi oscillations in a non-polar InGaN quantum dot

Experimental observation of Rabi rotations between an exciton excited state and the crystal ground state in a single non-polar InGaN quantum dot is presented. The exciton excited state energy is determined by photoluminescence excitation spectroscopy using two-photon excitation from a pulsed laser. The population of the exciton excited state is seen to undergo power dependent damped Rabi oscillations.

Double-well elastic theory for twist-bend nematic phases.

The ground state of twist-bend nematic liquid crystals is a heliconical molecular arrangement in which the nematic director precesses uniformly about an axis, making a fixed angle with it. Both precession senses are allowed in the ground state of these phases. When one of the two helicities is prescribed, a single helical nematic phase emerges. A quadratic elastic theory is proposed here for each of these phases which features the same elastic constants as the classical theory of the nematic phase, requiring all of them to be positive. To describe the helix axis, it introduces an extra director field which becomes redundant for ordinary nematics. Putting together helical nematics with opposite helicities, we reconstruct a twist-bend nematic, for which the quadratic elastic energies of the two helical variants are combined in a nonconvex energy. States with minimal energy and opposite helicities may come in contact along an interface belonging to a special family of developable surfaces, including a peculiar cone, which can be constructed through a general method.

First-principles study of lattice stability of ReO3-type hypothetical TaO3

Tantalum trioxide is a layered material with potential application as an electrolyte, catalyst and transparent conducting oxide. Crystal structure and ground state properties of TaO3 has not been investigated. Here, lattice stability of TaO3 is studied using density functional theory calculations. The calculated free energy of formation shows that ReO3-type TaO3 is thermodynamically stable. Further, TaO3 is also found to fulfill all the elastic stability criterion, suggesting its mechanical stability. The calculated phonon spectrum of TaO3 does not show any imaginary modes, suggesting that the lattice is dynamically stable. Electronic band structure shows that TaO3 is a self-doped p-type metal.