Tunneling Spectra in NbSe3./I/Pb junctions

NbSe₃, goes throught two phases transition inducing SDW states for T₁ = 149 K and T₂ = 59 K. These two transitions bring a partial condensation at Fermi level, the low temperature conductivity being always of metallic type. The evaluation of the width of the “gap” cannot be done simply. We employed this word “gap” althought it is incorrect here. Tunneling effect has permitted us to obtain an estimation of this “gap” for P=l bar and P=1 Kbar. We have kept tunneling spectra only for T < T₂. For T₁ > T > T₂, the width of the “gap” and thermal broadening are such that a measure is too imprecise. We have used the lead superconductivity as a test of good tunneling devices.     

A Mechanism of High-Tc Superconductivity Relating to Perovskite Crystal Structure

A mechanism of high-T_c superconductivity related to the perovskite crystal structure is proposed by a “phonon attenuation model”, which means that in the perovskite-like crystal structure, a number of “ordering” vacancies or holes occur due to the non-stoichiometric structure features. Such a reticular “spongly-like” structure facilitates lattice thermovibration attenuation perovskite (phonon attenuation) during temperature reduction. This leads to an abrupt drop of the resistivity, owing to the conduction electrons transmitted through the media without collision with irregularities in the lattice due to the lattice thermovibration. This probably is the main reason for high-T_c superconductivity of the perovskite cuprates.     

Rf-sputtered FeO films: Vapor species control in non-crystalline solid formation

It is shown that iron oxide films prepared by rf-sputtering display a wide variation in structure, composition, and film properties, depending upon the sputtering process parameters. We demonstrate that for Fe₂O₃ composition films non-crystalline solid (NCS) formation is controlled not only by substrate temperature but also by the vapor species in the vapor → solid transformation. The evidence indicates that the vapor species from an Fe₃O₄-sputtered target are primarily “molecules”. A qualitative model of the NCS formation is presented.     

Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN crystals.     

Strukturelle Umklapp-Vorgänge in metastabilen β1-Kupfer-Zink-Mischkristallen bei tiefen und höheren Temperaturen (III). Zur Gitterkinetik der isothermen Phasen-Umwandlungen

Whereas part (I) of the present series treated chiefly lattice-dynamical aspects, being important for the initiation of the β₁/α-transformation processes, and the preceding part (II) discussed explicitely the phase transformation of β₁Cu/Zn-crystals, the submitted third part, (III), handles of a lattice kinetic detailing of the results, exclusively received by X-ray measurements (transformation isotherm 360°C). Keeping the subdivision of the transformation processes into the 3 main periods (A) PREDIFFUSION, (B) “UM-.KLAPPING”, (C) POSTDIFFUSION there are the following results: (A) The quickly running heterogenization of the initial β₁-matrix lattice, β₁ ⟹ β₁ + β₁ takes place with an activation energy Q_w for the wandering of vacancies, frozen- in by quenching: “Primary vacancies”, which will disappear in sinks already after about 2 sec. (B)The first structural actions of “Umklapping”,β₁⟹β₁⟹β₁, occurring in Zn-poorβ₁-districts, will be caused dynamically by longwavy thermo-acoustic sher-vibrations – “soft modes” of the type ({110)}, 〈110〉 with wavelengths of about 86 Ä – and succeeding dissociations of dislocations in the momentary structure β₁. The activation energy of this transformation stage, being isolated from the increase of the lattice period aβ₁, is QII= 0.8 eV> QI; therefore the energy difference QII- QI⋍ 5.10⁻² eV can be interpreted as energy barrier for the structural transitions β₁⟹ β₁⟹ β₂ – As the “mean atomic volume” of the diffusionless period is smaller than that of β₁, “secondary vacancies” will be formed during this period, and so the following diffusion processes become accelerated for some time. In this subperiod, in which the first α-crystals appear and begin to grow, the activation energy is 1.0 eV. the α-crystals are formed with 12-fold crystallographical strong defined orientations relative to the β₁-matrix. (C) During the postdiffusion period, which will begin with the disappearance of the transition lattice β₂ and which is ruled by growth of the original (coherent) fcc α-crystallets, there are only diffusive direct transitions of (Zn-rich) β₁+− and of residual (Zn-poor but not more “umklapping”) β₁-districts; β₁+ α and β₁ α respectively. The growth of the α-crystallets takes place with the activation energy QIV = 1.45 eV. This energy is the sum of the activation energies for the formation and wandering of a vacancy, Q_IV=Q_B + QW, because in the meantime the excessive “secondary vacancies” are annihilated and the “mean atomic volume” of thc reacting associations is increasing; consequently QB = 0.7 eV. Caused by the increase of the “mean atomic volume” and exhibited by X-ray diffraction photographs, near the end of the isotherm reaction the (at least partially) coherent β₁/α-association suffers a self-deformation. This deformation, only taking place in the α-members of the association, will be signified already in much former reaction stages by weak satellite reflections of some α-crystal interferences (to this s. part (II)). A texture-like diffuse prolongation of several a-reflexions shows the β₁/β₂/α-lattice coherence to be removed partially in some micro-regions (very weak X-ray intensities? ) For this there are a few reasons: (1) In the moment of “Umklapping” the concerned micro-lattice regions are superplastic so that the other strong crystallographical Pβ₁α-orientation relationship can easy get lost, at least partially. (2) It is possible that also the so called “massive transformation” contributes to this effect, since this transformation operates without lattice coherency-the rather the lower the Zn-content of thc transforming lattice regions, as it is in fact realized for the Zn-poorest β₁-districts in the moment of “Umk1apping”.     

Mesoscopic and microscopic aspects of the PSB formation mechanism in fatigued FCC monocrystals

Persistent zones of plastic strain localization (PSBs) can be observed in fatigued fee monocrystals above the “nucleation cycle number” N_n and the “nucleation stress” amplitude τ_n, respectively. They follow after temporary strain localizations existing at N < N_n and being discernible by oscillations of the relative shape of the hysteresis loops (V_H) or by strain bursts. Using a simple rheological model the frequency of internal stresses of mesoscopic wave length (“mesoscopic stresses”) was determined for Ni monocrystals fatigued at four different plastic strain amplitudes up to N_n. A “mesoscopic threshold stress” τ* has been found to be exceeded within the whole volume of the dislocation-dense regions (DRs) at N ≧ N_n. By comparison of τ* with the stress for the operation of Frank-Read sources of the segment lengths l₀ at the border of the DRs, it might be assumed that above N_n and τ_n respectively a mechanism of dislocation sources acts, compensating the high annihilation rate of movable dislocations in the zones of strain localization. In so doing the formation of persistent zones of strain localization becomes possible, in which the “ladder-like” dislocation structure of the final PSBs develops successively.     

X-Ray Diffraction Studies on Point Defects in II–VI Compounds

The point defect concentration in Te-rich CdTe and Hg_1-xCd_xTe annealed at various temperatures has been estimated from precision lattice parameters using a simple continuum inclusion model and compared with densities of electrically charged defects determined by high-temperature Hall and conductivity measurements. The nonstoichiometry is realized by cation vacancies. Dependent on the CdTe content, the ratio of total to charged defect concentrations varies between about unity for HgTe-rich composition and 75 for CdTe. Therefore, it is necessary to distinct between “electrical” and “chemical” stability regions.     

Phase Diagram Behaviors for Rod/Plate Liquid Crystal Mixtures

We present a simple theory of rod/plate liquid-crystal mixtures in which the angle-dependent pair interactions are assumed to be of second-rank form. Calculations are reported for varying relative anisotropies of the “rods” and “plates”. Temperature vs. mole-fraction phase diagrams show successive isotropic (I) → uniaxial ( U) and U → biaxial ( B) transitions which are first- and second-order, respectively. For each pair of species there exists a special composition ( x _rod*) for which cooling of the isotropic phase leads directly (and continuously) to a biaxial liquid. As X _rod approaches X _rod* from either side, the first-orderness of the I → U transition (i.e. discontinuities in volume, order parameter, etc.) becomes vanishingly small. Furthermore the transition temperature of the rod-(plate-) solvent is found to be depressed when “doped” by not-too-anisotropic plates (rods) and elevated when doped by sufficiently anisotropic plates (rods). These behaviors are explained in terms of simple excluded volume considerations and compared with recent experimental data on rod/plate mixtures.     

Crystallographic aspects of functioning of adenosine triphosphatase as a macromolecular biomachine

The structural characteristics and the properties of nanoscopic biological objects functioning as rotational molecular “machines” of the type of F-adenosine triphosphatase (F₀F₁-ATPase) are described in terms of physics. The fundamental relationships between the dynamic characteristics of such machines are established, as well as the kinetic coefficients in the equations relating the generalized forces acting in these molecular machines and the generalized currents. It is shown that the one-to-one correspondence between the directions of the current flowing through the biomembrane, the chemical reactions at the catalytic sites of the “stator,” and the revolution of the machine “rotor” is dictated by the symmetry of the protein subunits.     

What is the Shape of a Polymer Chain near the Theta Point?

We address the question of the conformation of a polymer near the θ point. To this end, we present an argument that the statistics of polymer rings at the θ point in two dimensions is exactly given by the statistics of the external perimeter (“hull”) of a percolation cluster. As a consequence, the fractal dimension d_f (θ) of a polymer chain at the θ point coincides with that of the hull of the percolating cluster, d_f (θ) - d_H . We also perform extensive simulations of the conventional θ point model—the interacting self-avoiding walk (ISAW)—and the smart kinetic walk (SKW). We demonstrate that the SKW predicts a higher order critical point, termed the θ′ point, which has the same critical behavior as the ISA W. We conclude that the SKW is a well-defined walk that gives the conformation of a polymer near the θ point in two dimensions. In particular, we report accurate calculations of all three tricritical point exponents that are needed to fully describe the θ point.     

An Introduction to the Solid State Polymerization of Transition-metal Complexes: A Comparative Study

Preliminary studies in this laboratory^' of topochemical and topotactic polyreactions of NiBr₂[P(CH₂CH₂CN)₃]₂ have shown that the reactions are in many cases characterized by a) product specificity, b) stereospecificity, c) “crystallographic specificity” and d) highly anisotropic “front motion” through individual single crystals. The polymerization reaction under investigation here is     

Lifshitz-Like Behaviour in Ferroelectric Liquid Crystals

In order to explain the anomalous behaviour of the critical magnetic field and the helical pitch in the vicinity of the Λ-line and the Lifshitz point in the ferroelectric liquid crystal DOBAMBC a model involving not only chiral but also non-chiral biquadratic coupling between the polarization and the tilt has been proposed. The model predicts an “S - like” dependence of the modulation wave vector close to the smectic C* - smectic A transition. High temperature resolution measurements of the helical pitch and the intensity of the diffraction sattelites indeed support the proposed model and seem to show that the pitch is finite at T_c. In such a case the Lifshitz field H_L is finite too.     

Microscopic aspects of Stretched Exponential Relaxation (SER) in homogeneous molecular and network glasses and polymers

The “diffusion to traps” model quantitatively explains “magic” stretching fractions β(Tg) for a wide variety of relaxation experiments (nearly 50 altogether) on microscopically homogeneous electronic and molecular glasses and deeply supercooled liquids by assuming that quasi-particle excitations indexed by Breit–Wigner channels diffuse to randomly distributed sinks. Here the theme of earlier reviews, based on the observation that in the presence of short-range forces only d* = d = 3 is the actual spatial dimensionality, while for mixed short- and long-range forces, d* = fd = d/2, is applied to four new spectacular examples, where it turns out that SER is useful not only for purposes of quality control, but also for defining what is meant by a glass in novel contexts. The examples are three relaxation experiments that used different probes on different materials: luminescence in isoelectronic crystalline Zn(Se,Te) alloys, fibrous relaxation in orthoterphenyl (OTP) and related glasses and supercooled melts up to 1.15T_g, and relaxation of binary chalcogen melts probed by spin-polarized neutrons (T as high as 1.5T_g). The model also explains quantitatively the appearance of SER in a fourth “sociological” example, distributions of 600 million 20th century natural science citations, and the remarkable appearance of the same “magic” values of β = 3/5 and 3/7 seen in glasses.     

Temperature dependence of secondary electron emission from SiO2-layers

By means of a “random walk model” the escape depths of secondary electrons (SE) can be obtained as a consequence of interaction with LO-phonons. The measured temperature dependence of the SE-yield leads to a high energetic LO-mode with ħω ⋍ 140 meV in amorphous SiO₂ influencing dominantly the electronic slowing down process.     

Structural relaxation of an “unusual” Pd40Cu40P20 metallic glass studied by measurements of the electrical resistance

Precise measurements of the electrical resistance upon thermo-cycling of an “unusual” Pd₄₀Cu₄₀P₂₀ glass in the initial and cold-rolled states have been performed. The uniqueness of this glass is defined by its higher density compared to the reference tetragonal Pd₂Cu₂P crystal. It has been found that this glass displays the relaxation behavior, which is quite similar to that of “usual” Pd₄₀Cu₃₀Ni₁₀P₂₀ glass with the usual relationship between the densities of glassy and crystalline states.     

Dislocation density in heteroepitaxial indium arsenide layers

In As layers have been grown by CVD on (111)B-oriented GaAs substrates. The dislocation density (N_D) distribution through the layer thickness has been studied. N_D is dependent on the mole fraction of AsCl₃ in the gaseous phase and, consequently, on the current carriers concentration. This results in the supposition that the decreasing of N_D in the layers after a “critical” thickness is due to the “pinning” of dislocations at impurity atoms which form stronger bonds with the host In or As atoms than the bond In–As, an effect which is known for bulk GaAs and InP crystals.     

Dielectric relaxation in ferroelectric TlInS2 layered crystals within metastable chaotic state

The results of investigations low frequency dielectric relaxation in layered ferroelectric TlInS₂ crystals are presented. The measurements were performed in the temperature range of 180‐230 K and in the frequency range of 5 kHz–1 MHz. Two different relaxation processes were observed in mentioned temperature interval. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The presence of two different relaxation mechanisms in TlInS₂ is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stoichiometric defects and volume diffusion in epitaxial PbTe-SnTe multilayers

The effect of annealing on multilayer structures based on epitaxial layers of PbTe-SnTe was investigated by X-ray diffraction profile changes. Decrease of satellite reflections intensity and conversion of pattern asymmetry were found. Profiles of interplanar distance and electron density deviations were determined by solving both the direct problem (numerical computation of Takagi-Taupin equations) and the inverse problem (optimisation task) according to the average lattise model. Computation showed the presence of interlayer area of PbSnTe composition containing up to 8% of stoichiometric vacancy type defects between PbTe and SnTe layers. Two diffusion mechanisms are proposed: “fast” diffusion connected with occupation of stoichiometry defects and “slow” substitutional diffusion.     

Zur Lumineszenz von Zinksiliziumphosphid

Results of cathodoluminescene measurements of ZnSiP₂-single crystals grown by different techniques are reported. The luminescence excited in the various samples showed considerable differences in intensity and spectral position. These differences are apparently caused by the irregular incorporation of recombination centres in the present stage of crystal growth. Samples grown by “vapour phase technique with ZnCl₂” and by “spontaneous crystallisation from Sn or zn melt” showed line spectra typical for the special method of crystal growing. This relation between a typical line spectrum and the method of crystal growing chosen is attributed to the participation of intrinsic defects in the recombination processes. The detection of doping elements by means of luminescence measurements is discussed for ZnSiP₂ crystals doped by adding Te, Se, Ga. or In to the Zn melt.