Axis Of Single Crystal Research Articles

Use of coherent synchrotron radiation in a study on cuprates exhibiting superconductivity and charge order

The reflectivity R( ω) of both the ab plane and the c axis of single crystals of La 1.875Ba 0.125− y Sr y CuO 4, with 0.05 ⩽ y < 0.125 has been measured down to 5 cm −1, using coherent synchrotron radiation below 30 cm −1. This extension to very low infrared frequencies allows one to detect far-infrared peaks (FIP) in the optical conductivity, which are related to charge ordering, and which can be hardly observed by using conventional sources. In the superconducting phase, one can also observe a Josephson plasma resonance in the c-axis reflectivity, and its shift with temperature. At higher frequencies, the effect of charge ordering is monitored through a depletion in the optical conductivity which extends up to 4000 cm −1.

Hydrothermal Growth of Manganese Dioxide into Three‐Dimensional Hierarchical Nanoarchitectures

AbstractNovel three‐dimensional (3D) hierarchical nanoarchitectures of ϵ‐MnO2 have been synthesized by a simple chemical route without the addition of any surfactants or organic templates. The self‐organized crystals consist of a major ϵ‐MnO2 dipyramidal single crystal axis and six secondary branches, which are arrays of single‐crystal ϵ‐MnO2 nanobelts. The growth directions of the nanobelts are perpendicular to the central dipyramidal axis, which shows sixfold symmetry. The shape of the ϵ‐MnO2 assembly can be controlled by the reaction temperature. The morphology of ϵ‐MnO2 changes from a six‐branched star‐like shape to a hexagonal dipyramidal morphology when the temperature is increased from 160 to 180 °C. A possible growth mechanism is proposed. The synthesized ϵ‐MnO2 shows both semiconducting and magnetic properties. These materials exhibit ferromagnetic behavior below 25 K and paramagnetic behavior above 25 K. The ϵ‐MnO2 system may have potential applications in areas such as fabrication of nanoscale spintronic materials, catalysis, and sensors.

Enzymatic Degradation Processes of Poly[(R)-3-hydroxybutyric acid] and Poly[(R)-3-hydroxybutyric acid-co-(R)-3-hydroxyvaleric acid] Single Crystals Revealed by Atomic Force Microscopy: Effects of Molecular Weight and Second-Monomer Composition on Erosion Rates

Enzymatic degradation processes of poly[(R)-3-hydroxybutyric acid] (P(3HB)) and poly[(R)-3-hydroxybutyric acid-co-(R)-3-hydroxyvaleric acid] (P(3HB-co-3HV)) single crystals in the presence of PHB depolymerase from Ralstonia pickettii T1 were studied by real-time and static atomic force microscopy (AFM) observations. Fibril-like crystals were generated along the long axis of single crystals during the enzymatic degradation, and then the dimensions of fibril-like crystals were analyzed quantitatively. The morphologies and sizes of fibril-like crystals were dependent on the molecular weight and copolymer composition of polymers. For all samples, the crystalline thickness gradually decreased toward a tip from the root of a fibril-like crystal after enzymatic degradation for 1 h. The thinning of fibril-like crystals may be attributed to the destruction of chain-packing structure toward crystallographic c axis by the adsorption of enzyme. From the real-time AFM images, it was found that at the initial stage of degradation the enzymatic erosion started from the disordered chain-packing region in single crystals to form the grooves along the a axis. The generated fibril-like crystals deformed at a constant rate along the a axis with a constant rate after the induction time. The erosion rate at the grooves along the a axis increased with a decrease of molecular weight and with an increase of copolymer composition. On the other hand, the erosion rate along the a axis, at the tip of the fibril-like crystal, was dependent on only the copolymer composition, and the value increased with an increase in the copolymer composition. The morphologies and sizes of fibril-like crystals were governed by both the erosion rates along the a axis at the grooves and tip of fibril-like crystals. In addition, we were able to estimated the overall enzymatic erosion rate of single crystals by PHB depolymerase from the volumetric analysis.

Orientational dependence of shape memory effects and superelasticity in CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals

The dependence of deforming stresses, shape memory effect (SME), and superelasticity (SE) on the orientation of the single crystal axis, test temperature, and disperse particle size is examined for CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals. The orientational dependence of SME, SE, and temperature interval of the development of martensitic transformations (MT) under loading and SE is established. The influence of disperse particles on magnitudes of SME, SE, and mechanical hysteresis is discussed.

Quadrupolar ordering in DyPd 3S 4

We performed the measurements of the specific heat and the magnetization along the principal axes of single crystals of DyPd 3S 4 and found that these crystals showed a quadrupolar ordering at T Q =2.7 K, which is a little lower than that of polycrystalline samples. The magnetization curves were very anisotropic and exhibited one or two anomalies in the principal directions. We have confirmed by comparing with the peaks of specific heat that these anomalies correspond to the phase boundaries between the paramagnetic and antiferroquadrupolar phase. A preliminary H– T phase diagram has been constructed along the three principal axes by plotting these magnetization anomalies together with those of the specific-heat data.

Structure dependence of ferroelectric properties of bismuth layer-structured ferroelectric single crystals

Ferroelectric properties along the a(b) axis and c axis in single crystals of various bismuth layer-structured ferroelectrics (BLSFs) were investigated. By measuring saturated P-E hysteresis curves of BLSFs, values of the saturated remanent polarization and the coercive electric field were found to be related to the Curie temperature and the number of BO6 octahedra (m) between bismuth layers, respectively, along the a(b) axis. The saturated remanent polarization was larger in the BLSF with a higher Curie temperature. This is attributed to a large atomic displacement accompanied by a high Curie temperature. In contrast, the saturated coercive electric field was smaller in the BLSF with a larger number of m. This phenomenon is assumed to be caused by the decrease in the strain energy of the octahedra from the bismuth layer, which leads to an easy movement of the octahedral cations in the direction of an applied external electric field. Along the c axis, the BLSFs with an odd number of m had the same relationship, that is, the saturated remanent polarization was larger with the high Curie temperature, and the saturated coercive field was smaller with the large m number. However, no remanent polarization was confirmed in the BLSFs with an even number of m.

Critical resolved shear stress anomalies of the L12-long-range ordered γ′-phase of the superalloy NIMONIC 105

The critical resolved shear stress τ0NIMγ′ of the multicomponent nickel-rich L12-long-range ordered γ′-phase γ′NIM has been measured in tension (t) and compression (c) tests. Precipitates of γ′NIM strengthen the commercial nickel-base superalloy NIMONIC 105. The nominal orientations hkl of the single crystal axes were: [001], [011], 1̄11, and 1̄23; the deformation temperatures (T) ranged from 283 to 1150 K. The results τ0NIMγ′(hkl,T,t/c) show the following anomalies: 1) positive slope ∂τ0γ′∂T, 2) anisotropy; 3) t/c-asymmetry. The experimental data τ0NIMγ′(hkl,T,t/c) are discussed in the light of current models.

Magnetic Anisotropy in Rare-Earth - Transition Metal Intermetallic Compounds

ABSTRACTIn this work the rare-earth anisotropy in the rare-earth - transition metal intermetallic compounds is investigated by studying the temperature-induced spin reorientation phenomena on aligned powder samples and by analyzing the temperature dependence of the magnetization measured along different principal axes of single crystals. From such investigations the crystalline-electric-field parameters of the rare-earth ions in these compounds have been derived. The results on different hard magnetic materials of the compositions R2Fe14B, RFe11 and their substituted compounds as well as R2Co17 are summarized.

Density functional theory for calculation of elastic properties of orthorhombic crystals: Application to TiSi2

A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young’s modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the experimental results. The shear anisotropic factors and anisotropy in the linear bulk modulus are obtained from the single crystal elastic constants. From the site and angular momentum decomposed density of states combined with a charge density analysis and the elastic anisotropies, the chemical bonding nature between the constituents in TiSi2 is analyzed. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. The calculated elastic properties are found to be in good agreement with experimental values when the generalized gradient approximation is used for the exchange and correlation potential.

High-field transition in TbCu 2

The high-field (critical field 39.5 T at 4.2 K) single-step metamagnetic transition was observed in the b axis of single crystal of TbCu 2 and its temperature evolution was studied. The transition is described by a single-ion Hamiltonian taking into account higher order crystal-field terms significantly contributing to the magnetocrystalline anisotropy.

Thermopower of CeNiSn under a magnetic field

We have measured the thermopower along the three crystallographic axes of single crystals at 1.3 K and 4.2 K under a magnetic field up to 10 T applied perpendicularly to the direction of the temperature gradient. There are six cases altogether for the direction of the magnetic field relative to the temperature gradient along the crystal axis. The value of the thermopower was found to decrease drastically as the increase of the magnetic field applied in parallel only to the a-axis, the axis having the largest magnetic susceptibility, inferring the destruction of the band gap due to the magnetic field.

Photon splitting in a strong electromagnetic field and the possibility of its observation in single crystals

The process of photon splitting in the field of the axes in single crystals is considered. It turns out that the observation of the process is possible only within the maximum probability region. This maximum is achieved at a photon energy of a few hundred GeV. The consideration is based on the amplitudes obtained in a quasiclassical approximation.

Quantum effects in radiation emitted by ultrahigh energy electrons in aligned crystals

The energy losses and the radiation spectra of electrons moving in the field of the single crystal's axes have been calculated in the energy range where the radiation in an external field becomes essentially quantum. Comparison with the experimental data has been made.

Cation self-diffusion and the isotope effect in Fe 2O 3

Self-diffusion of 59Fe parallel to the c axis in single crystals of Fe 2O 3 has been measured as a function of temperature (1150–1340°C) and oxygen partial pressure (2 × 10 −3 ⩽ p O 2 ⩽ 1 atm) The temperature dependence of the cation diffusivity in air is given by the expression D Fe ∗ = (1.9 −1.4 +5.2 × 10 9 exp(− 141.4 ± 4.0 kcal/mole RT ) cm 2/ s . The unusually large value of D 0 is interpreted in terms of the values of the preexponential terms in the reaction constants for the creation of defects in Fe 2O 3. The oxygen-partial-pressure dependence of the diffusivity indicates that cation self-diffusion occurs by an interstitial-type mechanism The simultaneous diffusion of 52Fe and 59Fe has been measured in Fe 2O 3. The small value of the isotope effect suggests that iron ions diffuse by an noncollinear interstitialcy mechanism, which is consistent with the crystal structure of Fe 2O 3.

Computer simulation of trajectories and radiation from high-energy particles moving in averaged potential of crystal axes

Abstract A computer simulation is carried out of the motion of high-energy electrons and positrons in the continuum string potential of single crystal axes with the given Miller indices, using the Moliere model. Characteristic trajectories of various types are obtained which correspond to properly channeled or quasichanneled particles. Angular distributions of particles at the exit from a crystal are analyzed as well as their radiation spectra.

Cation Self‐Diffusion in Cr2O3

Self‐diffusion of 51Cr was measured both parallel to and perpendicular to the c axis in single crystals of Cr2O3 as a function of oxygen partial pressure at 1490° and 1570°C. The oxygen‐partial‐pressure dependence of the diffusivity indicates that cation self‐diffusion occurs by a vacancy mechanism. The values of the self diffusion coefficients determined in this experiment are about 104 times smaller than those previously reported in this temperature range.

Theory of electrical conduction in organic molecular crystals. III. Temperature-independent mobilities in naphthalene

Temperature-independent electron mobilities (TIEM) observed in two crystal axes (b and c′) in naphthalene are explained by an extension of a model which was proposed previously [H. Sumi, J. Chem. Phys. 70, 3775 (1979); 71, 3403 (1979)] to explain TIEM observed in anthracene only in a single crystal axis (c′). The electron mobility in the a direction, which decreases with increasing temperature, is postulated to arise from electron motion in a one-dimensional Bloch band along the a axis. TIEM in the b and c′ directions arises because electrons hop from one a-axis Bloch band to another because of a transfer integral induced by thermal rotational vibrations of molecules. An important feature in the model is that sharp peaks in the density-of-state function of the one-dimensional band are rounded by the broadening due to phonon scattering.

Polarized i.r. reflection and Raman spectra of sodium formate crystal

Polarized i.r. reflection spectra from the (010) and the (001) planes and polarized Raman spectra for the incident lights propagating along the ′a, the b and the c axes of single crystals of sodium formate were measured. The TO and the LO frequencies were determined for a number of the i.r. active intraionic and lattice modes. In both the i.r. and the Raman spectra, the frequency of the CH in-plane deformation mode ν 5 was proved to be higher than that of the CO 2 symmetric stretching mode ν 2. The TO—LO splittings of the intraionic modes were found to be correlated closely with the corresponding gerade-ungerade splittings and the i.r. absorption intensities.

Megneto-Elastic Coupling in ZnCr2Se4

Magnetic susceptibility was measured along the screw axis of single crystals of ZnCr 2 Se 4 , a proper screw magnet, below the Neel temperature. A pronounced temperature dependence was observed and was discussed in connection with the tetragonal deformation of the crystal. Structure sensitiveness of the magnetic properties of this material was also reported.

Formation of radiation defects in single crystals of NaCl and kcl irradiated with channeled electrons with energies 10?50 keV

We present the results of investigations of the process of formation of F-centers in single crystals of NaCl and kCl upon exposure to 10–50 keV electrons with the incident electron beam directed at various angles to the crystallographic axes of single crystals. We showed that when the electrons are channeled, the rate of formation of F-centers increases, and the concentration of F-centers near the surface of the single crystals increases. We established a connection between the observed orientation phenomena in the formation of F-centers and the radiation dose and initial energy of the electrons. We determined characteristic angles for channeling the electrons.