Orthorhombic Domains Research Articles

Enhanced thermoelectric property of nanostructured CaMnO3 by sol-gel hydrothermal method

Perovskite CaMnO3 nanoparticles were synthesized by sol-gel hydrothermal method at 200 °C, and the samples were annealed at different temperatures between 800 °C and 1000 °C in steps of 50 °C under oxygen (CaMnO3) and air (CaMnO2.5) atmosphere. The systematic analysis revealed the annealing temperature and atmosphere influenced significant changes in the crystal structure, crystallite size, and oxygen stoichiometry of the material. Powder XRD analysis showed that stable orthorhombic perovskite structure acquired at 950 °C. Microstructure studies confirmed that grains were spherical, and larger grain size was observed for higher annealing temperature. Raman spectra determined orthorhombic domains and the symmetry of Raman lines. Negative values of thermopower were observed, and absolute Seebeck coefficient value for CaMnO3 and CaMnO2.5 was found to be −312 μVK−1 and -287 μVK−1 at room temperature. Stoichiometric CaMnO3 nanoparticle is a suitable material for thermoelectric applications due to low activation energy and high power factor.

Evidence of precursor orthorhombic domains well above the electronic nematic transition temperature in Sr(Fe1−xCox)2As2

Raman scattering, synchrotron x-ray diffraction, specific heat, resistivity and magnetic susceptibility measurements were performed in Sr(Fe1−xCox)2As2 [] single crystals with superconducting critical temperature K and two additional transitions at 132 and 152 K observed in both specific heat and resistivity data. A quasielastic Raman signal with B2g symmetry (tetragonal cell) associated with electronic nematic fluctuations is observed. Crucially, this signal shows maximum intensity at K, marking the nematic transition temperature. X-ray diffraction shows evidence of coexisting orthorhombic and tetragonal domains between and ∼ 152 K, implying that precursor orthorhombic domains emerge over an extended temperature range above . While the height of the quasielastic Raman peak is insensitive to , the temperature-dependence of the average nematic fluctuation rate indicates a slowing down of the nematic fluctuations inside the precursor orthorhombic domains. These results are analogous to those previously reported for the LaFeAsO parent oxypnictide (Kaneko et al 2017 Phys. Rev. B 96 014506). We propose a scenario where the precursor orthorhombic phase may be generated within the electronically disordered regime () as long as the nematic fluctuation rate is sufficiently small in comparison to the optical phonon frequency range. In this regime, the local atomic structure responds adiabatically to the electronic nematic fluctuations, creating a net of orthorhombic clusters that, albeit dynamical for , may be sufficiently dense to sustain long-range phase coherence in a diffraction process up to .

Parallel point defect identification in molecular dynamics simulations without post-processing: A compute and dump style for LAMMPS

Two classes for the molecular dynamics program LAMMPS – one compute style and one dump style – are presented that are designed to identify, count, and output point defects in cascade damage and related molecular dynamics simulations. The calculations are done in parallel across multiple MPI processes, and can be done at the same time as the original simulation. This drastically reduces storage requirements by eliminating the need to post-process every atom in the system. Non-cubic lattices, free surfaces, and large voids can be eliminated from the output by suitable choices of the reference lattice. The classes are derived from LAMMPS’s Dump and Compute classes, and pose no additional overhead to LAMMPS if they are not used. PROGRAM SUMMARYProgram Title: Compute style “frenkel” and dump style “frenkel”Program Files doi:http://dx.doi.org/10.17632/p39kb4fshj.1Licensing provisions: GPLv2Programming language: C++Nature of problem: Identify and count point defects (interstitials and vacancies) in metals created by high-energy cascade damage, in parallel, without post-processing, relative to an arbitrary defect-free crystal lattice.Solution method: Defects are identified and counted in parallel, across an arbitrary number of processors, through LAMMPS’s domain decomposition techniques [1]. Defects are counted according to the occupancy of the Wigner–Seitz cell of each lattice site, but can be visualized by either Wigner–Seitz cell occupancy or by Lindemann sphere analysis. Results are then shared across all processes by message-passing.Restrictions: This implementation is likely restricted to orthorhombic domains. It is also inadvisable to use this software in the presence of grain boundaries. It is possible (with care) to use it with free surfaces.[1] S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, J. Comput. Phys. 117 (1) (1995) 1–19. doi: https://doi.org/10.1006/jcph.1995.1039.

Thermoelectric Property in Orthorhombic-Domained SnSe Film.

Single-crystal SnSe exhibits extremely high thermoelectric properties, and fabrication of SnSe films is promising for practical application and basic research on properties. However, the high thermoelectric properties have not yet been reported in SnSe films and their thermoelectric properties and nanostructure have not yet been analyzed in detail. In the present study, a-axis-oriented epitaxial SnSe films were prepared to discuss the thermoelectric properties of the SnSe films. While the electrical conductivity of the films was orders of magnitude smaller than that in the single crystals at room temperature, surprisingly, the thermoelectric property (power factor) of the films was slightly higher than that in the single crystals at high temperatures (∼300 °C). The SnSe films contained orthorhombic domain boundaries with a spacing of several hundred nanometers. The orthorhombic domain boundaries caused carrier scattering and degraded the mobility of the films at room temperature, but their effect decreased with increasing temperature. Thus, the carrier scattering at domain boundaries results in characteristic temperature dependence of thermoelectric properties in the SnSe films. High thermoelectric properties at high temperatures were successfully achieved in the SnSe films in spite of the existence of domain boundaries, demonstrating the possibility of high-performance of SnSe thermoelectric films.

Switchable polar spirals in tricolor oxide superlattices

There are increasing evidences that ferroelectric states at the nanoscale can exhibit fascinating topological structures including polar vortices and skyrmions, akin to those observed in the ferromagnetic systems. Here we report the discovery of a new type of polar topological structure, an ordered array of nanoscale spirals, in the PbTiO3/BiFeO3/SrTiO3 tricolor ferroelectric superlattice system obtained via phase-field simulations. This polar spiral structure is composed of fine ordered semi-vortex arrays with vortex cores forming a wavy distribution. It is demonstrated that the tricolor system has an ultrahigh Curie temperature of ∼1000 K and a temperature of ∼800 K for the phase transformation from spiral structure to in-plane orthorhombic domain structure, demonstrating a great thermal stability. The periodicity phase-diagram is constructed, showing multiple phases from inplane domain, polar spiral, and polar vortex to flux-closure with increasing ferroelectric layer thickness. Moreover, the spiral structure has a net in-plane polarization that could be switched by an experimentally-feasible irrotational in-plane field. The switching process involves a metastable vortex state and is fully reversible. This discovery could open up a new routine to design novel multiferroic topological structures with enhanced stability and tunability towards potential future applications in next-generation electronics.

Polarized-light and electron microscopy study of the static domain structure of ferroic Fe3B7O13I boracite at room temperature

The static domain structure of ferroic Fe3B7O13I, (abbreviated as Fe-I) synthetic boracite crystals has been analyzed by polarized-light microscopy (PLM) and by field emission scanning electron microscopy (FE-SEM) at room temperature. As for most boracites, Fe-I shows dielectric, elastic, magnetic and optical properties of unusual interest. Upon cooling, Fe-I boracite shows a sequence of structural phase transitions from the cubic -43m to the orthorhombic mm2 (Tc = 348K), the monoclinic m (Tc = 218 K) and the trigonal 3m phases (Tc = 191 K). At room temperature, the Fe-I crystals present an orthorhombic domain structure which consists of six ferroic domains. Our results show that both techniques: PLM and FE-SEM are complementary for the visualization of ferroic domains.

In-plane anisotropy of the electric field gradient in Ba(Fe1−xCox)2As2 observed by As75 NMR

We have performed $^{75}\mathrm{As}$ NMR measurements on single crystals to investigate the nematic behavior via the in-plane anisotropy of the electronic state at the As site far from Co impurities in the representative iron arsenides $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$. From the analysis of the angular dependence of the NMR satellites in the $c$ plane using the binominal distribution, we find that there is the in-plane fourfold symmetry breaking, namely, the orthorhombic-type anisotropy in the electric field gradient (EFG) at the As site with no Co atom at the nearest neighboring Fe sites even in the tetragonal phase of both ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}(x\ensuremath{\ne}0)$. The NMR spectrum in the antiferromagnetically ordered state of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ is shown not to support a nanotwin model on the basis of the nematic order proposed from the pair-distribution analysis of neutron scattering data. Based on results of the $x$ and temperature $T$ dependences of the in-plane anisotropy in the wide $x$ and $T$ ranges, the symmetry breaking is concluded to come from the local orthorhombic domains induced by disorder such as Co impurities or lattice imperfections. Furthermore, we find that the asymmetry parameter of EFG $\ensuremath{\eta}$ obeys the Curie-Weiss law which may be governed by nematic susceptibility, and the Weiss temperature becomes zero at ${x}_{\mathrm{c}}\ensuremath{\sim}0.05$ in $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$.

The Effect of Synthesis Conditions on the Phase Composition and Structure of EuBa2Cu3O6 + δ Samples

The composition and the structure of ceramic EuBa2Cu3O6 + δ (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu n Ba m Cum + nO y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu n Ba m Cum + nO y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature Tc of the superconducting transition of each member of the series depends on the average oxidation state of copper $$\overline {Cu} $$ . At $$\overline {Cu} $$ < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At $$\overline {Cu} $$ = 2.28, five members of the Eu n Ba m Cum + nO y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with Tc = 82–90 K.

Direct visualization of phase separation between superconducting and nematic domains in Co-doped CaFe2As2 close to a first-order phase transition

We show that biaxial strain induces alternating tetragonal superconducting and orthorhombic nematic domains in Co substituted CaFe2As2. We use Atomic Force, Magnetic Force and Scanning Tunneling Microscopy (AFM, MFM and STM) to identify the domains and characterize their properties, finding in particular that tetragonal superconducting domains are very elongated, more than several tens of micron long and about 30 nm wide, have the same Tc than unstrained samples and hold vortices in a magnetic field. Thus, biaxial strain produces a phase separated state, where each phase is equivalent to what is found at either side of the first order phase transition between antiferromagnetic orthorhombic and superconducting tetragonal phases found in unstrained samples when changing Co concentration. Having such alternating superconducting domains separated by normal conducting domains with sizes of order of the coherence length opens opportunities to build Josephson junction networks or vortex pinning arrays and suggests that first order quantum phase transitions lead to nanometric size phase separation under the influence of strain.

Electronic anisotropies revealed by detwinned angle-resolved photo-emission spectroscopy measurements of FeSe

We report high resolution angle-resolved photo-emission spectroscopy (ARPES) measurements of detwinned FeSe single crystals. The application of a mechanical strain is used to promote the volume fraction of one of the orthorhombic domains in the sample, which we estimate to be detwinned. While the full structure of the electron pockets consisting of two crossed ellipses may be observed in the tetragonal phase at temperatures above 90 K, we find that remarkably, only one peanut-shaped electron pocket oriented along the longer a axis contributes to the ARPES measurement at low temperatures in the nematic phase, with the expected pocket along b being not observed. Thus the low temperature Fermi surface of FeSe as experimentally determined by ARPES consists of one elliptical hole pocket and one orthogonally-oriented peanut-shaped electron pocket. Our measurements clarify the long-standing controversies over the interpretation of ARPES measurements of FeSe.

Intrinsic Micromechanism of Multi-step Structural Transformation in MnNi Shape Memory Alloys

Simulation of the multi-step transformation of cubic matrix → multi-variant tetragonal domain → orthorhombic domain was realized by phase-field method. The intrinsic micromechanism of the second-step transformation in MnNi alloys was studied. It was found that the orthorhombic variant originated from the tetragonal variant with similar orientation, and bar-shaped orthorhombic phase firstly occurred around the interface of twinning bands. The second-step transformation resulted in localized variation of internal stress.

Polarized Light Microscopy Study on the Reentrant Phase Transition in a (Ba1 – xKx)Fe2As2 Single Crystal with x = 0.24

A sequence of structural/magnetic transitions on cooling is reported in the literature for hole-doped iron-based superconductor (Ba1 − xKx)Fe2As2 with x = 0.24. By using polarized light microscopy, we directly observe the formation of orthorhombic domains in (Ba1 − xKx)Fe2As2 (x = 0.24) single crystal below a temperature of simultaneous structural/magnetic transition TN ~ 80 K. The structural domains vanish below ~30 K, but reappear below T = 15 K. Our results are consistent with reentrance transformation sequence from high-temperature tetragonal (HTT) to low temperature orthorhombic (LTO1) structure at TN ~ 80 K, LTO1 to low temperature tetragonal (LTT) structure at Tc ~ 25 K, and LTT to low temperature orthorhombic (LTO2) structure at T ~ 15 K.

Domains and ferroelectric switching pathways inCa3Ti2O7from first principles

Hybrid improper ferroelectricity, where an electrical polarization can be induced via a trilinear coupling to two non-polar structural distortions of different symmetry, has recently been experimentally demonstrated for the first time in the $n$=2 Ruddlesden-Popper compound Ca$_3$Ti$_2$O$_7$. In this paper we use group theoretic methods and first-principles calculations to identify possible ferroelectric switching pathways in Ca$_3$Ti$_2$O$_7$. We identify low-energy paths that reverse the polarization direction by switching via an orthorhombic twin domain, or via an antipolar structure. We also introduce a chemically intuitive set of local order parameters to give insight into how these paths are relevant to switching nucleated at domain walls. Our findings suggest that switching may proceed via more than one mechanism in this material.

Distinct doping dependence of critical temperature and critical current density in Ba1-xKxFe2As2 superconductor.

Since the high transition temperature (High-Tc) superconductivity was discovered in the series of materials containing iron (Fe), their potential for the applications has been extensively scrutinized. In particular, a lot of effort has been made in achieving the high current-carrying ability by revealing the vortex pinning behavior. Here, we report on the critical current density (Jc) for the pristine Ba1−xKxFe2As2 single crystals with various K concentrations (0.25 ≤ x ≤ 0.52) determined by the magnetization hysteresis loop measurements. The x-dependence of Jc is characterized by a spike-like peak at x ~ 0.30, which corresponds to the under-doped region. This behavior is distinct from a moderate Tc dome with a broad maximum spanning from x ~ 0.3 to 0.5. For the under-doped samples, with increasing magnetic field (H), a second magnetization peak in Jc is observed, whereas for the optimally- and over-doped samples, Jc monotonically decreases with H. This result emphasizes that fine tuning of doping composition is important to obtain strong flux pinning. The origin of the characteristic doping dependence of Jc is discussed in connection with the orthorhombic phase domain boundary, as well as the chemical inhomogeneity introduced by the dopant substitutions.

Effect of residual stress on nematic domains in BaFe2−xNixAs2 studied by angular magnetoresistance**Project supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07020300), the National Basic Research Program of China (Grant Nos. 2012CB821400 and 2011CBA00110), and the National Natural Science Foundation of China (Grant Nos. 11374011, 11374346, and 11305257).

We have studied the angular magnetoresistance of iron pnictides BaFe2−xNixAs2, which shows clear 180 degree periodicity as fitted by a cosine function. In the x = 0.065 sample, the phase of the two-fold symmetry changes 90 degrees above the tetragonal-to-orthorhombic structural transition temperature Ts. Since the phase at low temperature is associated with the rotation of orthorhombic domains by magnetic field, we show that even vacuum grease can push the presence of orthorhombic domains at temperatures much higher than Ts. Our results suggest that residual stress may have significant effects in studying the nematic orders and its fluctuations in iron pnictides.

Structural Domain Switching by Magnetic Fields in RAl3C3 (R = Rare Earth)

Hexagonal-to-orthorhombic structural transitions in RAl$_3$C$_3$ have been studied by X-ray diffraction for R=Yb, Tm, Er, Dy, and Lu. We report how orthorhombic domains are controlled by adjusting external magnetic fields. It is shown that orthorhombic domains in RAl$_3$C$_3$ can be aligned by the field when the R ion is magnetic. It is also shown that a single-domain state can be switched to another single-domain state by the field even in paramagnetic states with small induced moments. For R=Lu, however, it was not possible to change the multidomain state by a field of up to 70 kOe. We discuss that domain selection and switching are caused by magnetic anisotropies in the orthorhombic phase of RAl$_3$C$_3$. We also propose an idea to explain the field-induced antiferromagnetism revealed by Khalyavin et al. [Phys. Rev. B \textbf{87}, 220406 (2013)].

Detailed Study of the Phase Diagram of Fe-based Superconductor Ba(Fe1-xCox)2As2 by Super High-Resolution Neutron Diffraction Measurements

Temperature (T) dependence of Bragg reflections of Ba(Fe1-xCox)2As2 (x = 0, 0.02) have been measured in detail on assembled mm-size crystallites to avoid effects of surface and/or externally induced strains with the high-resolution neutron powder diffractometer, where we have found that the profile width of the (400)O/(040)O reflections with the orthorhombic indexing begin to increase, as T decreases, at ~270 K, much higher than the tetragonal (Tet)-orthorhombic (Ort) second order transition temperature TS (-147.5 K) without showing any indication of an phase change above TS. The coexistence of two Ort phases with different orthorhombicity exists in the region of ~140 K < T < 143 K). The profile widths of (hhl)O reflections are nearly T-independent in the entire temperature region studied here (130 K ≤ T ≤ 350 K). An additional broadening due to the Co-doping is clearly found. We discuss these results in relation to the breakdown of the 4-fold symmetry of static physical quantities found in the electrical resistivity and band splitting of the 3dyz and 3dzx and orbitals and conclude that the disappearance of the 4-fold symmetry even in the macroscopically tetragonal phase can be understood by the existence of orthorhombic domains induced by crystal defects and/or impurities.

Synthesis of CuInS2nanocrystals from a molecular complex – characterization of the orthorhombic domain structure

CuInS2 nanocrystals were synthesized by thermal decomposition of the molecular precursor [(Me3P)3Cu(SC2H4S)In(i)Pr2] in the presence of oleylamine in dioctyl phthalate. According to X-ray diffraction patterns, the as-synthesized CuInS2 nanocrystals crystallize in the wurtzite type structure. High-resolution transmission electron images and selected area electron diffraction patterns reveal a nanodomain structure. The individual domains are approximately 5-10 nm in size and characterized by short-range cation ordering, which assuming hypothetical long-range order, corresponds to an orthorhombic superstructure (space group Pmc21, a = 4.09 ± 0.01 Å, b = 7.16 ± 0.02 Å and c = 6.56 ± 0.03 Å). The domains are separated by twin and antiphase boundaries.

Crystal Structure of an Indigo@Silicalite Hybrid Related to the Ancient Maya Blue Pigment

The structure of the indigo@silicalite pigment, an analog of ancient Maya Blue, has been determined by combining X-ray Laue microdiffraction and powder diffraction techniques. After the adsorption of indigo into the calcined (monoclinic) silicalite sample, the powder diffraction pattern contained peaks from both orthorhombic (major phase) and monoclinic (minor phase) silicalite. Assuming that the orthorhombic phase was induced by the adsorption of indigo, Laue microdiffraction was used to map the unit cell changes (and thereby the indigo distribution) within a single crystal. It was found to be highly heterogeneous with empty monoclinic and indigo-induced orthorhombic domains. The Laue diffraction data indicated that the space group of the orthorhombic domains was Pnma rather than P212121. With this information, the indigo@silicalite structure could be solved and refined from the powder diffraction data. The starting positions for two independent indigo molecules, described as rigid bodies, were obtained by simulated annealing, with a first molecule positioned in the straight channel and the second one in the sinusoidal channel. The positions and occupancies of these molecules and the positions of the framework atoms were then refined using the Rietveld method. Approximately four indigo molecules per unit cell were found, two per independent site, and possible local arrangements are suggested. The size of the indigo molecule prevents the structure from being fully ordered.

TNF-α and Th2 Cytokines Induce Atopic Dermatitis–Like Features on Epidermal Differentiation Proteins and Stratum Corneum Lipids in Human Skin Equivalents

Atopic dermatitis (AD) is a chronic inflammatory skin disease in which the skin barrier function is disrupted. In this inflammatory AD environment, cytokines are upregulated, but the cytokine effect on the AD skin barrier is not fully understood. We aimed to investigate the influence of Th2 (IL-4, IL-13, IL-31) and pro-inflammatory (tumor necrosis factor alpha (TNF-α)) cytokines on epidermal morphogenesis, proliferation, differentiation, and stratum corneum lipid properties. For this purpose, we used the Leiden epidermal model (LEM) in which the medium was supplemented with these cytokines. Our results show that IL-4, IL-13, IL-31, and TNF-α induce spongiosis, augment TSLP secretion by keratinocytes, and alter early and terminal differentiation-protein expression in LEMs. TNF-α alone or in combination with Th2 cytokines decreases the level of long chain free fatty acids (FFAs) and ester linked ω-hydroxy (EO) ceramides, consequently affecting the lipid organization. IL-31 increases long chain FFAs in LEMs but decreases relative abundance of EO ceramides. These findings clearly show that supplementation with TNF-α and Th2 cytokines influence epidermal morphogenesis and barrier function. As a result, these LEMs show similar characteristics as found in AD skin and can be used as an excellent tool for screening formulations and drugs for the treatment of AD.