Showing papers by "Sofia University published in 1975"

Thermodynamics of thin liquid films

Abstract: The problem of the definition of the film thickness and its influence on the other thermodynamic quantities is discussed It is shown that at given physical state of the system, the film thickness can have various values depending on the way it has been defined A detailed analysis of the contact angle between the film and the meniscus is presented and is shown that the value of this angle depends on the definition of the film thickness If the thickness is assumed zero, the contact angle proves to be directly related to the film tension This yields a possibility for the film tension to be measured Formulae relating the parameters of the generatrix of the meniscus surface and the capillary pressure have been obtained for the topographic method ofScheludko et al

Measurement of surface tension by pulling a sphere from a liquid

Abstract: A new method for determination of the surface tension by measuring the maximum weight at the pull of a small vitreous sphere from the liquid is proposed The exact solution of the problem is found and is shown how the wetting of the sphere must be taken into account through the central angle formed between the sphere radius passing through the perimeter of wetting and the horizontal surface. A set for measuring the maximum weight and the central angle is described. The accuracy of determination of the surface tension and of the angle of wetting is estimated to be thousandths of dyn/cm and a few minutes, respectively. Some limitations of the method are discussed as well as the possibilities to overcome them. Various approximate solutions are treated and, in particular, it has been shown that the approximation previously used in the flotation model is satisfactorily accurate in that case.

Magnetic-order-induced band splitting in the thermomodulation spectra of the ferromagnetic semiconductor CdCr2Se4

Abstract: Diffuse reflectance, diffuse thermoreflectance (DTR), and specular thermoreflectance (TR) of the ferromagnetic semiconductor CdCr2Se4 are measured in the region of photon energies from 1.2 to 3.3 eV, between 95 and 320 °K. The TR spectra clearly show that a magnetic-order-induced band splitting appears below the Curie-Weiss temperature θc ≈200 °K. The results are consistent with the theory of spin-polarized bands and explain unambigously the anomalous behaviour of the absorption edge in the ferromagnetic semiconductors. A simple model is proposed for the optical band-to-band transitions in CdCr2Se4. [Russian Text Ignored]

Lifetimes of Excited Levels in MgIII, CaIII, KIV–V, CaV, and Ge I

Abstract: Radiative lifetimes of excited levels in Mg III, Ca III, K IV, K V, Ca V, and Ge I have been measured by means of the beam-foil technique in order to test the validity of recent theoretical methods to calculate lifetimes for heavier atoms and ions. Good agreement with theoretical lifetimes is obtained.

Biomagnetic Effect of the Constant Magnetic Field Action on Water and Physiological Activity

Abstract: The biophysical mechanism of the biological action of the magnetic field and “magnetic” water are connected with structural changes taking place in the external and internal cell water. The magnetobiological problem concerning the possible way of realizing these structural changes have not been made clear by now. No doubt the primary cause of the biomagnetic effect should be sought in the changes of physical and physical-chemical properties of “magnetic” water as a result of the magnetic field influence on water.

Best methods of interpolation for certain classes of differentiable functions

Abstract: For the class W(r)Lq (M;a, b), 1≤q≤∞, we construct the best method of approximation of the functionalf (x), x∈ [a, b], among all the methods using only information about the values off (k)(xi) (k=0, 1, ..., r−1; i=1, 2, ..., N).

Interface States in a Linear Model of Heterojunctions

Abstract: The necessary and sufficient conditions for the existence of interface states in a heterojunction between a one-atomic and a two-atomic crystal are derived on the basis of a linear model using the scattering theoretical method of Saxon and Hutner. A discussion of the general features of the above conditions and their interpretation is given and particular cases are visualized applying numerical analysis. Die Existenzbedingungen fur diskrete Interface-Niveaus, die an der Grenze zwischen einem einatomaren und einem zweiatomaren Kristall lokalisiert sind, werden mit Hilfe der Saxon-Hutner-Methode der S-und R-Matrizen abgeleitet. Ein lineares Modell mit δ-Poten- tialen ist der Diskussion solcher diskreten Energieniveaus zugrunde gelegt, und es wird deren Abhangigkeit von bestimmten Kristallparametern abgeleitet und numerisch berechnet.

Study of the silicon carbide preparation in the silicon–scandium–carbon system

Abstract: Results are given from the investigation on the SiSc–C system, which is of interest with respect to the growth of α-SiC single crystals. Data on the crystallization rate in the 1500 to 1900 °C temperature range are given. The optimal solvent compositions for growing a perfect SiC layer are determined. A possible mechanism for the formation and transport of SiC in the liquid phase is proposed. Es werden die Ergebnisse einer Untersuchung am SiScC-System mitgeteilt, das bezuglich des Wachstums von α-SiC-Einkristallen von Interesse ist. Werte der Kristallisationsgeschwindigkeit im Temperaturbereich von 1500 bis 1900 °C werden angegeben. Die optimalen Losungsbedingungen fur das Wachstum von perfekten SiC-Schichten werden bestimmt. Ein moglicher Mechanismus fur die Bildung und den Transport von SiC in der flussigen Phase wird vorgeschlagen.

On some experiments leading to a revision of the foundations of quantum mechanics

Abstract: When a homogeneous electron beam passes through a hole in a screen, according to quantum mechanics a well-known diffraction picture is obtained. It does not depend on the density ϱ of the electrons, because Schrodinger’s equation is linear. But the experiment contradicts this deduction, because actually the cross-section of the beam dilatates behind the screen for large ϱ. According to the new viewpoint on quantum mechanics proposed by the author, a generalized nonlinear Schrodinger equation is obtained. From this it follows both a dilatation of the electron beam and a change in de Broglie’s wavelength, depending on ϱ. The attempt to check these effects may turn into a crucial experiment.

LCAO HMO study of the binding energy of small clusters of lithium atoms

Abstract: The atom by atom growing of the energetically most favourable lithium clusters of 2 to 10 atoms was studied. The clusters had the structure of a body centered cubic lattice of lithium. The LCAO–HMO method was used for the energetic description of the clusters. It was assumed that there is an interaction of a given atom with its neighbours in the first six coordination spheres in the lithium lattice. All possible (two-dimensional and three-dimensional) positions of the n + 1st atom with respect to a cluster of n atoms were taken into account. It was shown that from 2 to 10 atoms the cluster grows twodimensionally and has the geometry of the lattice plane with the highest net density in the body centered cubic lattice of lithium, the 110 lattice plane. This was discussed in detail. It was shown that the bond energy per atom increases, whereas the energy of a bond between nearest neighbours decreases with increasing number of atoms in the cluster. [Russian Text Ignored].

Many‐phonon vibronic spectra of molecular crystals: II. Combination frequencies

Abstract: Vibronic spectra of molecular crystals with two phonons of different types (compound tones) are investigated in the weak coupling approximation. The foIIowing cases are considered: a) one of the phonons corresponds to a totally symmetrical vibration with linear vibronic coupling constant η2= 1; b) the purely electronic transition in the molecule is forbidden and one of the phonons corresponds t o a non-totally symmetrical vibration; c) a two-phonon vibronic spectrum with one totally symmetrical phonon and one non-totally symmetrical phonon. In a) and b) the problem is reduced to a two-particle problem with a known solution, while in c) the bands of the one-particle states are found and the absorption taking into account the role of anharmonicity of intramolecular vibrations. [Russian Text Ignored]

Many‐phonon vibronic spectra of molecular crystals: I. The role of anharrnonicity of intramolecular vibrations

Abstract: Many-phonon vibronic spectra are investigated in molecular crystals in the weak coupling approximation. A theory is developed (using adiabatic approximation) in which the anharmonicity of the intramolecular vibrations is treated as an exciton-phonon coupling mechanism. This theory is applied to vibronic spectra with slowly migrating phonons of nontotally symmetrical vibrations. Anharmonicity and the change of frequency have approximately the same impact on the intramolecular interaction and on the intensity of vibronic spectra, but due to its smaller constant, the anharmonirity is less essential to the appearance of one-particle states. Anharmonicity widens the bands of the many particle states by several ten per cent. [Russian Text Ignored].

Photometric extraction method for determination of oxacillin and its derivatives.

Abstract: A new extraction photometric method is developed for determination of oxacillin and its chlorinated derivates, based on the extraction of ion-pairs of the oxacillin anion and the cationic dye Crystal Violet. The method is very simple in experimental technique and permits rapid determination of the antibiotic with satisfactory precision and accuracy —coefficient of variation 2.7–4.5 %, relative error of the determination 2.0–3.1% (95% confidence limits).

Photoelectrical resonant gate transistor /PRGT/

Abstract: PRGT is a photosensitive device with resonant properties compatible with the elements of microelectronics and integrated optics. The photosensitive element is a single crystal platelet of CdS, the mechanical resonance of which determines the resonant frequency of the device. The flexural mechanical vibrations of the platelet excited from an a.c. light flux are detected and converted into an electrical signal by means of a MOS-transistor. Without optimization of the parameters of the device a mean value of the sensitivity as high as 3 A lm−1 was obtained reaching 10 A lm−1 for some devices for frequencies up to 3 kHz. A low frequency analyser for a laser Doppler anemometer is also described.

A resonant gate transistor with an active vibrator (RGTAV)

Abstract: A resonant gate transistor with an active vibrator is described in which the mechanical resonance of electrostatically excited flexural waves in thin single-crystal piezo semiconducting platelets of CdS is used for obtaining a small-sized frequency selective device. The flexural mechanical vibrations of the platelet are transformed into electrical signals according to the principle of the vibrating capacitor, making use also of the high input impedance of a MOS transistor. The amplitude of vibrations of the platelet and consequently the amplification factor of the RGTAV for the input voltages is raised as a result of the acousto-electrical interaction of the flexural wave with the drifting electrons produced by an external d.c. voltage. A number of devices were made having resonant frequencies from 1 to 10 kHz, with a Q-factor as high as 100 and an amplification factor at resonant frequency of about 0 dB.

On the electron states in a finite chain of delta-function potentials

Abstract: The fol lowing models a re wide ly discussed in t h e t h e o r y of d isordered sys tems: Anderson ' s mode l and its modif icat ions (1), one-d imens ional l i q u i d a series of shortr ange po ten t ia l s w i t h r andom spacing be tween t h e m (2), quasi-classical r andom field (3). I n (~) is cons idered a la t t ice conta in ing N equa l ly highbu t f in i te-potent ia l bar r ie rs w i th p la teaus of v a ry ing he igh t r andomly d is t r ibu ted in be tween. The barr iers and the p la teaus are enclosed be tween inf ini te ly h igh walls. The local iza t ion of t he e lec t ron s tates was sub jec ted to numer ica l ca lcula t ion as a func t ion of t h e pa rame te r charact e r i z ing the difference in he igh t of t he var ious pla teaus . The dens i ty of s ta tes for a oned imens iona l la t t ice of 5-functions wi th Cauchy d is t r ibut ion of the i r in tensi t ies was ob ta ined in (5). The condi t ion of cons tan t spacing be tween the sites, e.g. t he presence o f a la t t ice for Anderson ' s model , was app l ied in (6). This l e t t e r is concerned wi th a po ten t ia l field which is a finite one-dimensional chain or la t t ice of var ious (Lfunctions, t he field be ing zero in all po in ts except t he sites. W e should not assume t h a t t he pa ramete r s of po ten t ia l are r andom var iab les ; e.g., a nonper iod iea l bu t non random field is avai lable . The designat ions for a la t t ice were used for t h e sake of b rev i ty . On t h e l ine x a t t he po in t s n a ( a > 0; n = l , 2, ..., 5 r) N dif ferent &funct ions are located. Then t h e Schr5dinger equa t ion for a par t ic le w i th mass m is

Raman Spectra Concentrational Dependence of Naphthalene - Cyclohexane and Naphthalene-Methanol Solutions Frozen at 77° K

Abstract: The concentrational dependence of Raman spectra of frozen at 77°K naphthalene solutions in cyclohexane and in methanol is studied. An increase in the naphthalene Raman line intensity is observed for the naphthalene-cyclohexane system in a concentrational range in which a quasilinear fluorescence spectrum is recorded.

Modification of a theorem of A. A. Gonchar on the rate of rational approximation

Abstract: In this paper results of A. A. Gonchar [1] on uniform approximation by rational functions are extended to certain classes of unbounded functions.