Vortex structure in superconducting iron pnictide single crystals

The vortex structure in the iron pnictide single crystals Ba_1?x K _x Fe₂As₂ and Sr_1?x K _x Fe₂As₂ of the 122 type and SmFeAsO_1?x F _x of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.     

Vortex dynamics in YBaCuO single crystals with point- and line-like defects-flux creep studies

Studies of vortex dynamics and pinning were conducted in irradiated Y₁Ba₂Cu₃O₇ single crystals by observing the decay of supercurrents with time. The materials contained either point-like defects from proton irradiation or line-like defects from irradiation with energetic heavy ions. Analysis of the first case gives clear support for collective creep theory. With linear defects, the experiments give evidence for thermal smearing of the vortex pinning potential at higher temperatures, as predicted theoretically.     

Vortex states in antiferromagnetic crystals

It is shown that axially symmetric two-dimensional nonuniform states can exist in easy-axis and cubic antiferromagnets lacking inversion symmetry, in the form of two-dimensional spatially modulated structures (magnetic vortex lattices) and isolated two-dimensional structures (vortices). The structure and equilibrium dimensions of the lattices and vortices have been determined by numerical solution of differential equations. Fiz. Tverd. Tela (St. Petersburg) 40, 1486–1493 (August 1998)     

Surface structure of large germanium single crystals

The dependence between structural-defect generation and the growth kinetics of germanium single crystals pulled from melts is investigated via selective chemical etching and the optical, atomic-force and scanning electron microscopy techniques. It is ascertained that the surface microrelief of germanium crystals grown from melts by means of directional crystallization contains protrusions and cavities with spatial periodicities of 5 and 50 μm. The values of the kinetic coefficients are estimated. It is demonstrated that the main part of the crystals is formed according to the normal mechanism with the kinetic coefficients β_k = 2 × 10^?5 m s^?1 K^?1.     

Domain structure of silicon iron single crystals

The model of a new domain structure arising after the magnetization of silicon iron single crystals in planes of the (110) type at an angle of 0°<Θ<-55° to the axis of easy magnetization is considered. Using this model the angular dependence of the domain-structure characteristics is established; it agrees closely with direct observations. On magnetizing a single crystal in the angular range 55° <Θ≤ 90° to the easy axis, layers with a uniform resultant magnetization parallel to the [001] direction are formed.     

Electronic structure of CsCdCl3 single crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 53, No. 6, pp. 988–991, December, 1990.     

Defect structure in EuS single crystals grown from the melt

The defect structure in EuS single crystals grown form the melt is studied by etch pitting, scanning and high-voltage electron microscopy. Circular and square etch pits and a second phase in the shape of thin hexagonal platelets are observed by etching. Microprobe analysis indicates the platelets to consist of Eu metal. In the transmission electron microscope, smoothly curved dislocations and helical dislocations, small dislocation loops and inclusions associated with dislocations are observed. The possible origin of the detected dislocation structure is considered with reference to climb and glide processes occurring during cooling down the grown crystals. The results corroborate the glide geometry of the NaCl lattice for EuS. On leave from Institute of Physics, Academic Sinica, Peking, VR China     

Dislocation structure of copper single crystals in high-temperature creep test

It has been shown that in the high-temperature creep test at 0·85 kp mm^?2 and 0·7T _m (T _m is the melting temperature in °K) the dislocation substructure of copper single-slip oriented crystals is formed practically at the early stage of the test. The subsequent changes of substructure consist in the growth of subgrain misorientation and in the transformation of tangled subgrain boundaries into regular dislocation networks. There is a pronounced tendency to the formation of subboundaries in the {111} and {110} planes.     

Sputtering of boron nitride single crystals of different structure

Results obtained in studying the sputtering of single crystals of hexagonal, rhombohedral, and cubic boron nitride modifications by computer simulations are reported. Data on sputtering the (0001) face of BN in two graphite-like modifications and the (111) face of BN crystals with a cubic lattice are presented. The energy and angular dependences of the sputtering yields and spatial and energy distributions of sputtered particles are considered for the cases of normal and oblique ion incidence. Specific features of the anisotropy of spatial distributions of sputtered particles and mechanisms of their formations are analyzed.     

Tunneling spectroscopic study of the electronic structure in TTF-TCNQ single crystals

The electronic structure near the Fermi-level in a TTF-TCNQ single crystal was experimentally determined using a tunneling spectroscopical technique. It was found that the electronic state-density at the Fermi-level in a TTF-TCNQ crystal is non-zero at room temperature, and this fact is consistent with its metallic nature.     

Multiphysical simulation analysis of the dislocation structure in germanium single crystals

To grow high-quality germanium crystals is one of the most important problems of growth industry. The dislocation density is an important parameter of the quality of single crystals. The dislocation densities in germanium crystals 100 mm in diameter, which have various shapes of the side surface and are grown by the Czochralski technique, are experimentally measured. The crystal growth is numerically simulated using heat-transfer and hydrodynamics models and the Alexander–Haasen dislocation model in terms of the CGSim software package. A comparison of the experimental and calculated dislocation densities shows that the dislocation model can be applied to study lattice defects in germanium crystals and to improve their quality.     

Vortex liquid crystals in anisotropic type II superconductors

In an isotropic type II superconductor in a moderate magnetic field, the transition to the normal state occurs by vortex lattice melting. In certain anisotropic cases, the vortices acquire elongated cross sections and interactions. Systems of anisotropic, interacting constituents generally exhibit liquid crystalline phases. We examine the possibility of a two step melting in homogeneous type II superconductors with anisotropic superfluid stiffness from a vortex lattice into first a vortex smectic and then a vortex nematic at high temperature and magnetic field. We find that fluctuations of the ordered phase favor an instability to an intermediate smectic-A in the absence of intrinsic pinning.     

Defect structure of Bi2−xAsxTe3 single crystals

Single crystals Bi_2−xAs_xTe₃ were prepared by a modified Bridgman technique. The samples were characterized by X-ray diffraction analyses and measurement of Hall coefficient and electrical conductivity. Atomic absorption spectroscopy (AAS) was used for determination of actual content of As in the samples. The doping of Bi₂Te₃ with As leads to a decrease of the free carriers concentration while their mobility increases. The observed effects are discussed within a point defect model of Bi_2−xAs_xTe₃ crystals.     

Extended x-ray absorption fine structure studies of oriented single crystals

Linearly polarized synchrotron radiation is used to study the orientation dependence of the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of single crystal zinc. The spectrum some 5–1200 eV above the K photoabsorption threshold is found to be markedly anisotropic, as would be expected from the established formalism generalized to crystals of lower than cubic symmetry. Zinc thus provides a prototype for molecular crystals of low symmetry where one wishes to study particular bond directions.     

Vortex structure in ferromagnetic superconductors

It is shown that in ferromagnetic superconductors the polarization of the local spins induced by the persistent current in a vortex is quantized along with the magnetic field due to the current. Because of the polarization the vortex structure strongly depends on temperature. Near the ferromagnetic transition temperatures T_M all the superconductors become type I even if they are extremely of type II far above T_M.     

Vibronic structure and the anisotropy of excitons in cubic LiH single crystals

The shape of one-phonon sidebands of exciton in LiH is studied both in reflection and edge luminescence spectra. The main structure of the shape is found to reflect the phonon density of states. This fact is interpreted as a result of a high anisotropy of the exciton band.