Undistorted State Research Articles

Hidden Local Symmetry Breaking in Silver Diamondoid Compounds is Root Cause of Ultralow Thermal Conductivity

AbstractTypically, conventional structure transitions occur from a low symmetry state to a higher symmetry state upon warming. In this work, an unexpected local symmetry breaking in the tetragonal diamondoid compound AgGaTe2 is reported, which, upon warming, evolves continuously from an undistorted ground state to a locally distorted state while retaining average crystallographic symmetry. This is a rare phenomenon previously referred to as emphanisis. This distorted state, caused by the weak sd3 orbital hybridization of tetrahedral Ag atoms, causes their displacement off the tetrahedron center and promotes a global distortion of the crystal structure resulting in strong acoustic–optical phonon scattering and an ultralow lattice thermal conductivity of 0.26 W m−1 K−1 at 850 K in AgGaTe2. The findings explain the underlying reason for the unexpectedly low thermal conductivities of silver‐based compounds compared to copper‐based analogs and provide a guideline to suppressing heat transport in diamondoid and other materials.

Thermally Activated Optical Absorption into Polaronic States in Hematite.

Polaron formation, whereby an electron or hole strongly couples to a lattice distortion, inhibits the carrier mobility of many first-row transition metal oxide semiconductors. Recently reported XUV transient absorption measurements of hematite (α-Fe2O3) demonstrate formation of electron small polarons upon photoexcitation into an undistorted charge-transfer state followed by subpicosecond lattice reorganization. Here, we show that polaronic states of hematite can be accessed directly via optical transitions from the ground state in a thermally activated lattice. Thermal difference spectra collected from 30 to 573 K combined with Stokes resonance Raman spectra indicate strong coupling between optical transitions near the band-edge (2.1-2.3 eV) and zone-center a1g and longitudinal (LO) optical phonons. Density functional theory calculations of the electronic and vibrational structures of pristine and polaron-distorted hematite lattices confirm that the geometric distortion corresponding to electron small polaron formation lies along the 28-meV a1g and 81-meV LO phonon coordinates and reproduce the features observed in the experimental thermal difference and resonance Raman spectra.

A Workflow for Determining and Sizing Oligomeric Biomolecular Structures with Cryo Single Molecule Localization Microscopy

Single molecule localization microscopy (SMLM) has enormous potential for resolving subcellular structures at the nanometer scale. In order to record localization microscopy data, sample fixation is inevitable to prevent molecular motion during long recording times of minutes up to hours. Crucially, preservation of the sample's ultrastructure during fixation becomes a limiting factor when using conventional fixation methods based on chemical fixatives. Performing SMLM at cryogenic temperatures promises to resolve this issue, thus allowing for investigation of nanoscale structures in their undistorted native state.

Double differential cross sections for ionization of H by 75 keV proton impact: Assessing the role of correlated wave functions* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11974229 and 11274215).

The effect of final-state dynamic correlation is investigated for ionization of atomic hydrogen by 75-keV proton impact by analyzing double differential cross sections. The final state is represented by a continuum correlated wave (CCW-PT) function which accounts for the interaction between the projectile and the target nucleus (PT interaction). The correlated final state is nonseparable solutions of the wave equation combining the dynamics of the electron motion relative to the target and projectile, satisfying the Redmond’s asymptotic conditions corresponding to long range interactions. The transition matrix is evaluated using the CCW-PT function and the undistorted initial state. Both the correlation effects and the PT interaction are analyzed by the present calculations. The convergence of the continuous correlated final state is examined carefully. Our results are compared with the absolute experimental data measured by Laforge et al. [Phys. Rev. Lett. 103, 053201 (2009)] and Schulz et al. [Phys. Rev. A 81, 052705 (2010)], as well as other theoretical models (especially the results of the latest non perturbation theory). We have shown that the dynamic correlation plays an important role in the ionization of atomic hydrogen by proton impact. While overall agreement between theory and the experimental data is encouraging, detailed agreement is still lacking. However, such an analysis is meaningful because it provides valuable information about the dynamical correlation and PT interaction in the CCW-PT theoretical model.

A statistical reinterpretation and assessment of criteria used for measuring map projection distortion

ABSTRACT In map projections theory, various criteria have been proposed to evaluate the mean distortion of a map projection over a given area. Reports of studies are not comparable because researchers use different methods for estimating the deviation from the undistorted state. In this paper, statistical methods are extended to be used for averaging map projection distortions over an area. It turns out that the measure known as the Airy–Kavrayskiy criterion stands out as a simple statistical quantity making it a good candidate for standardization. The theoretical arguments are strengthened by a practical map projection optimization exercise.

First-principles study of the low-temperature charge density wave phase in the quasi-one-dimensional Weyl chiral compound (TaSe4)2I

Using {\it ab initio} density functional theory, we study the lattice phase transition of quasi-one-dimensional (TaSe$_4$)$_2$I. In the undistorted state, the strongly anisotropic semimetal band structure presents two non-equivalent Weyl points. In previous efforts, two possible Ta-tetramerization patterns were proposed to be associated with the low-temperature structure. Our phonon calculations indicate that the orthorhombic $F222$ CDW-I phase is the most likely ground state for this quasi-one-dimensional system. In addition, the monoclinic $C2$ CDW-II phase may also be stable according to the phonon dispersion spectrum. Since these two phases have very similar energies in our DFT calculations, both these Ta-tetramerization distortions likely compete or coexist at low temperatures. The semimetal to insulator transition is induced by a Fermi-surface-driven instability that supports the Peierls scenario, which affects the Weyl physics developed above $T_{\rm CDW}$. Furthermore, the spin-orbit coupling generates Rashba-like band splittings in the insulating CDW phases.

Freedericksz transition in a dual-frequency nematic liquid crystal

The Freedericksz transition was investigated in a dual-frequency liquid crystal consisting of a mixture of 2-chloro-4-(4-pentylbenzoyloxy)benzoate and N-(4-methoxybenzylidene)-4-butylaniline, in which dielectric anisotropy strongly depends on the frequency of the applied ac electric field. Because the threshold voltage for the transition from an undistorted state of orientation to a distorted one is a function of the dielectric anisotropy and of the Frank elastic constants, the threshold voltage was expected to depend on the frequency. The threshold voltage increased steeply as the frequency approached the crossover frequency at which the dielectric anisotropy vanished and the threshold voltage eventually diverged. Furthermore, we examined the Freedericksz transition under ac voltages of two different frequencies.

Detachment work of prolate spheroidal particles from fluid droplets: role of viscous dissipation.

The force-displacement curve for removal of an elongated solid particle from the surface of liquid droplets or gas bubbles is calculated and compared to our previous reported results for spherical particles. The surface adsorption energy for prolate particles is known to be larger than that for spheres. We show that in fact the minimum possible work done upon removal of an elongated particle from surface can be less than that for a sphere. This result is obtained when the dissipation of interfacial energy, stored in the fluid film, attaching the particles to the surface during their displacement, is properly accounted for. This dissipation is unavoidable, even if the particles are removed infinitely slowly. Once the particle actually leaves the surface, the formed liquid bridge relaxes thus dissipating any stored interfacial energy as the surface returns to its original undistorted state. The difference between the work of removal of a particle from surface and its adsorption energy is seen to become increasingly larger with smaller particle to droplet size ratios. For example, for a size ratio of 1 : 100, the work of removal is 1.93 times greater than the adsorption energy. However, we also find that for any given size ratio, there is a value of particle aspect ratio for which the work of removal of particles (combined dissipated and adsorbed energy) attains its minimum value.

Stable charge density wave phase in a 1T–TiSe2 monolayer

Charge density wave (CDW) phases are symmetry-reduced states of matter in which a periodic modulation of the electronic charge frequently leads to drastic changes of the electronic spectrum, including the emergence of energy gaps. We analyze the CDW state in a 1T-TiSe$_2$ monolayer within a density functional theory framework and show that, similarly to its bulk counterpart, the monolayer is unstable towards a commensurate $2{\times}2$ periodic lattice distortion (PLD) and CDW at low temperatures. Analysis of the electron and phonon spectrum establishes the PLD as the stable $T=0$ K configuration with a narrow bandgap, whereas the undistorted and semi-metalic state is stable only above a threshold temperature. The lattice distortions as well as the unfolded and reconstructed band structure in the CDW phase agree well with experimental results. We also address evidence in our results for the role of electron-electron interactions in the CDW instability of 1T-TiSe$_2$ monolayers.

Stress-Softening and Residual Strain Effects in Suture Materials

This work focuses on the experimental characterization of suture material samples of MonoPlus, Monosyn, polyglycolic acid, polydioxanone 2–0, polydioxanone 4–0, poly(glycolide-co-epsilon-caprolactone), nylon, and polypropylene when subjected to cyclic loading and unloading conditions. It is found that all tested suture materials exhibit stress-softening and residual strain effects related to the microstructural material damage upon deformation from the natural, undistorted state of the virgin suture material. To predict experimental observations, a new constitutive material model that takes into account stress-softening and residual strain effects is developed. The basis of this model is the inclusion of a phenomenological nonmonotonous softening function that depends on the strain intensity between loading and unloading cycles. The theory is illustrated by modifying the non-Gaussian average-stretch, full-network model to capture stress-softening and residual strains by using pseudoelasticity concepts. It is shown that results obtained from theoretical simulations compare well with suture material experimental data.

A Brief Review of the Current Technologies Used for the Fabrication of Metal-Molecule-Metal Junction Electrodes

Fabrication techniques for Metal-molecule-metal junction electrodes suitable to study electron tunneling through metal junctions are reviewed. The applications of current technologies such as mechanical break junction, electromigration, shadow mask lithography, focused ion beam deposition, chemical and electrochemical plating, electron-beam lithography, in fabricating vacant junction electrodes are briefly described. For biomolecular sensing applications, the size of the junction electrodes must be small enough to allow the biomolecule inserted into the junction space to connect both leads to keep the molecules in a relaxed and undistorted state. A significant advantage of using Metal-molecule-metal junction electrodes devices is that the junction can be characterized with and without the molecule in place. Any electrical artifacts introduced by the electrode fabrication process are more easily deconvoluted from the intrinsic properties of the molecule.

Multiscale simulations on the reversible plasticity of Al (0 0 1) surface under a nano-sized indenter

The incipient plasticity of Al (001) surface under a small cylindrical indenter is studied by using multiscale simulations. Results show that both reversible and irreversible plasticity occurs under the indenter. During the reversible stage, only a thin deformation twin is formed, and the plastically deformed surface will completely recover to undistorted state upon withdrawal of the indenter, otherwise, both a deformation twin and a few stacking fault ribbons are observed, and a permanent residual trace will remain on the surface, as a sign of the irreversible plasticity. Our findings suggest that the effects of some internal and external material parameters, such as indentation depth, lattice frictional force, and stacking fault energy, on the incipient surface plasticity are very important in some small contact issues. The contact force and pressure under the indenter are also analyzed. Results imply that the theoretical prediction of the critical force is suitable for estimating the beginning of reversible plastic deformation, not the onset of the irreversible plasticity.

Eye for Modeling Visual Distortion in Central Serous Retinopathy

To present a simple mathematical eye model capable of modeling the metamorphopsia (visual distortion) reported by patients with central serous retinopathy (CSR). A computer program was developed to perform the modeling. A simple wide-angle eye model was adapted to include a spherical edema added at the retina. The visual impression was taken as the projection of the retinal pattern back into object space, after the edema had been deflated to its undistorted state. The deflation of the edema was facilitated by assuming a simplified "spreading of photoreceptors" rule. Numerical examples have been included to demonstrate the use of the model. In particular, an Amsler grid was projected to the retina with CSR, to illustrate the associated geometrical distortion. The eye model predicted a pincushion-like distortion of an Amsler grid, that depended (for given height) on the radius of curvature of the CSR. A larger radius of curvature resulted in less pincushion distortion extending over a larger area. An eye model has been developed that simulates distortion because of CSR. The predicted visual impression of a spherical edema is a pincushion-like distortion. This model may have applications in benchmarking alternate chart designs for detecting CSR, investigating optical aberrations over the edema as well as simulating the visual effect of metamorphopsia in other retinal conditions.

Homoleptic Octahedral Methylamine Complexes of Manganese(II): Solvothermal Syntheses and Crystal Structures of [Mn(NH2CH3)6]Cl2 and the Pentaselenide [Mn(NH2CH3)6]Se5

Abstract[Mn(NH2CH3)6]Cl2 (1) and [Mn(NH2CH3)6]Se5 (2) were prepared by solvothermal reactions in liquid methylamine from MnCl2 at 150 °C for 1 and from a mixture of MnCl2, Rb2Se and selenium at 120 °C for 2. Both 1 and 2 were obtained in high yields as colorless and dark‐red crystals and represent the first homoleptic methylamine complexes with coordination number six. Compound 1 crystallizes rhombohedral (R$\bar{3}$, Z = 3) and is built of only slightly distorted octahedral [Mn(NH2CH3)6]2+ cations and Cl– anions. Compound 2 crystallizes orthorhombic (Pnna, Z = 4) and is built of octahedral [Mn(NH2CH3)6]2+ cations showing a strong angular distortion and of Se52– anions in the form of chains in transoid conformation. DFT calculations reveal an almost undistorted ground state structure for [Mn(NH2CH3)6]2+ with N–Mn–N angular distortions of 1° from orthogonality, close to the structure found for the complex in 1. The calculated energy necessary for a distortion as found in the structure of 2 is rather low and amounts to 26 kJmol–1 which is in the range typical for hydrogen bonds. The N–Mn–N angular distortions of the complex cation in 2, observed in the range of 10°, is caused by cation‐anion interactions in the crystal structure by N–H····Se hydrogen bonds.

Reformation Instability in Elastic Solids

The reformation of a body fundamentally involves the mapping of one natural reference configuration of it into another natural reference configuration. The mass and the constitutive properties of the material remain unaltered, but the overall shape of the reference configuration generally changes. If, when a natural reference configuration is distorted, there is a portion of the boundary of the body that is displacement controlled, then a reformation of the body must be such that the original displacement controlled part of the boundary and its reformation are identical. In common applications that involve reformation, the remainder of the boundary is traction-free and a reformation essentially involves a change of the morphology of this traction-free surface. For example, undulations are often a characteristic feature of the reformation of a free, plane boundary surface. Reformations are a result of a material instability and they may associate with a chemically induced diffusive processes in which particles of the body move into preferred places. Fundamentally, a reformation is generated in response to the drive to lower the total stored energy of the body. In this work we are not concerned with the physical processes that take place during reformation, but rather we are concerned with characterizing the onset of the instability. We develop a variational characterization of the reformation instability for a nonlinear elastic body and we include the effect of surface energy. As an example, we consider the axial deformation of a circular cylinder and argue that small scale nano-wires, for which the diameter-to-length ratio is sufficiently small, are expected to be stable with respect to spatial variations when extended. Moreover, we observe that if the surfacial energy function is sufficiently convex at the undistorted state such wires may also be stable with respect to spatial variations when compressed. We then show that such small scale nano-wires are unstable with respect to reformation when either extended or compressed.

On the mechanism of the highly viscous flow

The asymmetry model for the highly viscous flow postulates thermally activated jumps from a practically undistorted ground state to strongly distorted, but stable structures, with a pronounced Eshelby backstress from the distorted surroundings. The viscosity is ascribed to those stable distorted structures which do not jump back, but relax by the relaxation of the surrounding viscoelastic matrix. It is shown that this mechanism implies a description in terms of the shear compliance, with a viscosity which can be calculated from the cutoff of the retardation spectrum. Consistency requires that this cutoff lies close to the Maxwell time. The improved asymmetry model compares well with experiment.

Effect of substrate-induced strain on transport and magnetic properties of epitaxial La0.66Sr0.33MnO3 thin films

Electrical transport and magnetic properties of epitaxial 500Å La0.67Sr0.33MnO3 (LSMO) thin films, grown on different substrates having different lattice strains, are found to exhibit strong correlation with Jahn-Teller (JT) strain. The authors’ study reveals that a sufficiently large JT strain gives rise to distinct insulating state in LSMO films even below the respective para-ferromagnetic Curie temperature, which is a contradiction with the established phase diagram of Sr doped manganites. The authors have presented a microscopic model for the analysis of their data instead of the usual expansion around the undistorted state. The model incorporates two relevant eg orbitals and the effect of both JT and bulk strains on the transition temperature via double exchange.

E-infinity as a fiber bundle and its thermodynamics

This paper presents a novel approach for the investigation of fractals. Using parameters of fractal organization we introduce generalized Gibbs statistic and apply it for the development of thermodynamics of fractals. Our generalization is based on a fractal generation function that produces a representation for particular states of fractal space–time. All considerations are embedded into the framework of E-infinity theory viewed as a fiber bundle. Nevertheless El Naschie’s space–time represents a particular case of undistorted fractal state which corresponds to the equilibrium state of fractal space–time in our theory. Related topics of thermodynamics of fractals are discussed.

Lattice-coupled antiferromagnet on frustrated lattices

The lattice-coupled antiferromagnetic spin model is analyzed for a number of frustrated lattices: triangular, Kagom\'e, and pyrochlore. We show, within the mean-field theory, that the classical ground state is free of lattice deformation in the triangular, Kagom\'e, and the tetrahedron-based, $\mathbf{q}=0$, ${E}_{g}$ phonon mode of the pyrochlore lattice. The $\mathbf{q}=0$, ${E}_{u}$ phonon mode of pyrochlore is shown to have an energy gain of $\ensuremath{-}2{\ensuremath{\alpha}}^{2}$ per spin over the undistorted ground state, where $\ensuremath{\alpha}$ is the spin-lattice interaction strength. Motivated by the picture of the hexagon spin cluster proposed in the recent experiment on $\mathrm{Zn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ [S. H. Lee et al., Nature (London) 418, 856 (2002)], we also analyze the hexagon-based distortion within our model. Hexagon distortions give rise to mutually orthogonal arrangement of spins for nearby hexagons, and has an energy gain of $\ensuremath{-}{\ensuremath{\alpha}}^{2}∕2$ per spin. A general criterion for the lattice instability in the spin-lattice model is discussed.

Magnetostriction in an array of spin chains under a magnetic field

We consider an array of XX spin-1/2 chains coupled to acoustic phonons and placed in a magnetic field. Treating the phonons in the mean field approximation, we show that this system presents a first order transition as a function of the magnetic field between a partially magnetized distorted state and the fully polarized undistorted state at low temperature. This behavior results from the magnetostriction of the coupled chain system. A dip in the elastic constant of the material near the saturation field along with an anomaly in the magnetic susceptibility is predicted. We also predict the contraction of the material as the magnetic field is reduced (positive magnetostriction) and the reciprocal effect i.e. a decrease of magnetization under applied pressure. At higher temperature, the first order transition is replaced by a crossover. However, the anomalies in the susceptibilities in the system near the saturation field are still present. We discuss the relevance of our analysis in relation to recent experiments on spin-1/2 chain and ladder materials in strong magnetic fields.