Electron spin resonance and optical absorption spectra of Cu2+ ions in Na2SO4-ZnSO4 glasses

Electron spin resonance (ESR) and optical absorption spectra of Cu²⁺ ions in Na₂SO₄-ZnSO₄ glasses have been studied. The ESR spectra of Cu²⁺ ion-doped glasses exhibited a pronounced peak atg=2.07 and a shallow quadruplet atg=2.35, the latter arising from the hyperfine splitting of g. ESR spectra of Cu²⁺ ion-doped glasses were also studied by varying the concentration of Cu²⁺ions, temperature and composition of the glasses. The optical absorption spectra exhibited a broad absorption band in the near infrared region, which is attributed to²B_1g²B_2g transition. By correlating the ESR and optical absorption data, the bonding orbital coefficients ² and ₁ ² for Cu²⁺ ions have been evaluated.     

Low-temperature properties of the heavy-fermion compound CeRu2Si2 at the metamagnetic transition

Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu₂Si₂ are reported. The field variation of the linear term of the specific heat derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB B*. Similar experiments performed on a single-crystalline Ce_0.95La_0.05Ru₂Si₂ sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu₂Si₂ approaches a magnetic instability atB* forT 0.     

Synthesis and Luminescent Properties of ZnGa2S4:Eu,F and ZnGa2O4:Eu,F

ZnGa₂S₄:Eu,F and ZnGa₂O₄:Eu,F were synthesized and characterized by x-ray diffraction and photoluminescence (PL) measurements. ZnGa₂S₄:Eu,F has a tetragonal structure (sp. gr. ) with a= 5.272 Å and c= 10.451 Å, and ZnGa₂O₄:Eu,F has a cubic structure (sp. gr. Fd3m) with a= 8.32 Å. The PL spectrum of ZnGa₂S₄:Eu,F consists of a broad band (FWHM = 1.11 eV) at 565 nm due to the Eu²⁺ 5D ¹ 7F ² transition, and the spectrum of ZnGa₂O₄:Eu,F shows four emissions due to the Eu^{3+ 5} D ₀ ⁷ F ₄ (_max = 682 nm), ⁵ D ₀ ⁷ F ₂ (_max = 615 nm), ⁵ D ₀ ⁷ F ₁ (_max = 595 nm), and ⁵ D ₀ ⁷ F ₀ (_max = 584 nm) transitions.     

Metamagnetic-like transition in CeRu2Si2?

Extensive magnetization and magnetoresistivity experiments on CeRu₂Si₂ are described, with special emphasis on the metamagnetic-like transition that occurs atH*=80 kOe. By contrast to the low-field susceptibility, the differential susceptibility atH* almost diverges atT0. It is suggested that high magnetic fields (H>H*) restore a situation where the interactions between particles have collapsed. Comparison is made with other well-known examples: TiBe₂, UPt₃, TmSe, and³He.     

The Attractive Hubbard Model in 2-D: Is It Capable of Describing a Pseudogap and Preformed Pairs?

Deviations from Fermi liquid behavior are well documented in the normal state ofthe cuprate superconductors, and some of these differences seem to be related topretransitional features appearing at temperatures above T _c. The observationof a pseudogap, e.g., in ARPES experiments, is a familiar example of this physics. Onepotential explanation for this behavior involves preformed pairs with finite lifetimesexisting in the normal state above T _c. In this way, two characteristictemperatures can be established. A higher one T* at which pairs begin toform and the actual T _c at which a phase-coherent superconducting phaseis established. In order to test these ideas we have investigated the negative UHubbard model in two dimensions in the fully self-consistent ladder approximation atlow electron densities. In the non-self-consistent version of this theory the systemalways shows an instability toward Bose-condensation of infinite lifetime pairs. In contrastto this, pairs obtain a finite lifetime due to pair–pair interaction and thesharp two-particle bound state is strongly lifetime broadened when self-consistency isapplied. A quasiparticle scattering rate which varies linearly with temperature is alsofound. The fully self-consistent calculation we were able to perform employed ak -averaged approximation in which the self-energyloses its k -dispersion due to ak -average. This approximation is found to preservethe essential physics.     

The cubic autocatalator: The influence of the quadratic autocatalytic and the uncatalysed reactions

The proptotype chemical reaction scheme, the cubic autocatalator, A + 2B 3B; B C is taken in a closed system, with A formed from the precursor P by the simple step P A. The pooled chemical approximation is invoked whereby the concentration of P can be assumed to remain constant throughout. The effects of allowing the quadratic autocatalytic reaction A + B 2B and the uncatalysed reaction A B in the scheme are considered in detail. The full scheme is described by the non-dimensional parameters µ (measuring the reaction rate of the initiation step) ands andr (measuring the reaction rates of the quadratic autocatalytic and the uncatalysed steps respectively). It is shown, provided only thatr ors (or both) are non-zero, no matter how small, the solution remains bounded for all (positive) values of µ, whereas withr =s = 0 the solution is bounded only for µ > µ₀ (µ₀ = 0.900 32). It is shown that withr = 0 ands 0 the governing equations have a Hopf bifurcation at µ = 1 –s producing a stable limit cycle which exists for all µ in 0 < µ <1 –s. The behaviour of these limit cycles as µ 0 is also discussed.     

Spin-polarization effect on electron attachment to atomic hydrogen on liquid helium surface

No Heading Reaction rates of electron attachment to atomic hydrogen are measured as a function of magnetic field. The reaction takes place in a two-dimensional mixture of hydrogen atoms and electrons on liquid helium surface. Surface electron density, measured by using vibrating capacitor electrometer technique, decreases when H atoms are introduced. Applied high magnetic field suppresses electron attachment, H + e^– H^–, as well as hydrogen recombination, H + H H₂. Since the electronic state of negative hydrogen, H^–, is spin singlet, electron attachment is suppressed by spin-polarization. Possible microscopic mechanisms to explain the measured magnetic field dependence of the reaction kinetics are discussed.PACS numbers: 67.65.+z, 68.     

Mixtures of (N2)1−x : (O2) x at High Pressures and Low Temperatures

We performed Raman measurements at 18 K and pressures up to 25 GPa in order to construct a tentative phase diagram of the (N ₂)_1–x :(O ₂)_x –system at low temperatures. We varied the composition of the mixed system over the whole concentration range. Here we focused on the systems with high nitrogen concentration and pressures above 2 GPa. It is known that at room temperature oxygen is highly solvable in the –phase of N ₂. The experimental results show that oxygen suppresses the disorder–order transition –N ₂.     

Kinetics of Reactions of Np and Pu Ions with Hydrazine Derivatives: XVII. Reaction of Pu(VI) with Hydroxyethylhydrazine

Reduction of Pu(VI) to Pu(III) with hydroxyethylhydrazine (HOC₂H₄N₂H₃) in HNO₃ solutions involves the following consecutive steps²: Pu(VI) + HOC₂H₄N₂H₄ Pu(V) + ...; Pu(V) + HOC₂H₄N₂H₄ ⁺ Pu(IV) + ...; Pu(V) + Pu(III) 2Pu(IV); and Pu(IV) + HOC_2H4N₂H₄ ⁺ Pu(III) + .... The overall kinetic equations of these steps were suggested, and their rate constants and activation energies were determined. The mechanisms of the four reaction steps, consistent with the experimental kinetic data, are discussed.     

On the analysis of tunneling data for A15 superconductors

We examine the applicability of the standard McMillan inversion of Eliashberg's equations for superconductors with a nonconstant electronic density of statesN(). We do this usign simple models forN() and a realistically shapedN() taken from recent band structure work for Nb₃Sn. It turns out that peak structure inN() near _F may lead to gross errors in the derived Eliashberg function ²F() when the energy dependence ofN() is omitted in the inversion procedure. For Nb₃Sn, this leads to a 40% overestimate of when ²F() is evaluated via the standard McMillan program.     

Transport properties and determination of the basic parameters of superconductors with overlapping bands

The thermoconductivity and ultrasonic attenuation in non-single-band super-conductors are investigated. It is proved that the ratio ₂/₁ atT T _c is equal to that atT=0. It is also shown that ultrasonic measurements and data on the phonon thermoconductivity can be used very effectively for definition of the basic parameters of the superconductors with overlapping bands.     

A new formulation of Gor'kov's theory of clean type II superconductors

From Gor'kov's theory a functional for the free energy of the flux line lattice is derived which depends explicitly on the potentials andA. It may be interpreted as an extension of the Ginzburg-Landau functional to arbitrary temperatures. For periodic || ² andH=rotA, the functional is a rapidly converging series, of which often the main term (corresponding to the Brandt-Pesch-Tewordt approximation) plus a correction term depending on the lattice structure suffices. From the cutoff series all previously known results for the special casesT T _c ,B H _c2 ,orB 0 are reproduced by minimization with respect to || ² andH.     

Strong coupling effects on the upper critical field of the heavy-fermion superconductor UBe13

The upper critical fieldH _c2 (T) of a high quality single crystal of UBe₁₃ is studied with very low noise resistivity measurements. It shows a large but finite slope of —45 T/K, an unusual temperature dependence with an inflexion point atT/T _C 0.5 and a large saturated limit forT 0 ofH _c2 (0) = 14 T. The complete temperature dependence ofH _c2 (T) can be described by a simple model of strong coupling superconductor, assuming a full Pauli limitation and the occurence of a non-uniform superconducting state (FFLO state) at low temperature.     

Preparation of lanthanide orthophosphates by crystallisation from phosphoric acid solution

Highly crystalline solids of the composition of LnPO₄ · nH₂O (Ln = La, Ce, Pr, Nd. Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) were prepared by crystallisation from boiling phosphoric acid solution with the initial concentration 2 M/1 of H₃PO₄ and 0.02 M/1 of Ln. The number of molecules of water (n) was close to 1. The crystallisation process was completed within two hours. Scanning electron micrographs showed that the diameter of the LnPO₄ · H₂O crystalline particles was increasing within the group of elements from below 1 m (LaPO₄ · H₂O) to above 5 m (TmPO₄ · H₂O). Such large crystals of lanthanide phosphates cannot be obtained by usually applied precipitation methods. The light lanthanides (La Sm) formed clusters of shapeless particles with the tendency to form rod-like crystals. The heavy lanthanides (Ho Lu, and YPO₄ · H₂O) consisted of spherical globules. The TbPO₄ · H₂O and DyPO₄ · H₂O, formed a mixture of the spherical and rod-like crystals.     

Interacting Bose Gas in an Optical Lattice

A grand canonical system of hard-core bosons in an optical lattice is considered. The bosons can occupy randomly N equivalent states at each lattice site. The limit N is solved exactly in terms of a saddle-point integration, representing a weakly-interacting Bose gas. In the limit N there is only a condensate if the fugacity of the Bose gas is larger than 1. Corrections in 1/N increase the total density of bosons but suppress the condensate. This indicates a depletion of the condensate due to increasing interaction at finite values of N.     

Kinetic Characteristics of Production and Loss of Nitrogen Atoms in N2Plasma

The method of electron paramagnetic resonance (EPR) is used to measure the rates of production and concentration of nitrogen atoms, the rate constants of their heterogeneous loss, and some electrophysical characteristics of plasma at gas pressures of 60–533 Pa and discharge currents of 10–80 mA in the positive column of a d.c. discharge in N₂. Analysis of the dissociation processes revealed that, in addition to the decay of electron states excited from N₂(X ¹ _g ) by electron impact, reactions involving excited nitrogen molecules may play an important role in the formation of N atoms.     

Magnetic Resonance Studies of Impurity-Helium Solids Containing Hydrogen and Deuterium Impurities

Impurity-Helium Solids (Im-He) produced by injecting a mixed beam of helium and impurity gases into superfluid ⁴He have been investigated by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) techniques. NMR signals from deuterium molecules in a D₂-He solid have been studied. Only samples prepared from gaseous mixtures containing high concentrations of D₂ molecules gave observable signals. The ESR experiments were performed on H and/or D atomic impurities in Im-He solids containing H, D, H₂, and D₂ in various combinations. The exchange tunneling reactions D+H₂HD+H and D+HDD₂+H were used to generate high concentrations of H atoms (10¹⁷/cm³) in Im-He samples.     

Magnetic field and temperature dependence of the Knight shift in superconductors of small dimensions

The magnetic field dependence of the paramagnetic Knight shift in weak-coupling superconductors is evaluated within the framework of the BCS-Gor'kov theory for samples of small dimensions. The behavior of the monotonic and oscillatory parts of the Knight shift are examined in detail in the regimesT0 andTT _c ^– .     

A method of determining the thermal conductivity of powdered and fibrous insulation as a function of filler gas pressure

An examination is made of the theoretical basis and implementation of a nonstationary method of rapid measurement of the thermal conductivity of powdered and fibrous insulation under conditions of monotonic change of filler gas pressure.Notation t temperature - ,a thermal conductivity and diffusivity of test material - k, k_a relative temperature coefficients of anda - thickness of test layer - x variable layer coordinate reckoned from shell - =(x), _c excess temperature of material at section x and of core over shell - b_c, b_v rate of cooling of core and of variation of volume-mean temperature of layer - c_c, c total heat capacity of core and material - f_s, F_c area of working surfaces of shell and core - d diameter of particles of bulk material - p material porosity - volume density of material     

On strong Dickson pseudoprimes

It is known that the Lucas sequenceV _n(,c)=aⁿ + bⁿ,a, b being the roots ofx ² – x + c=0 equals the Dickson polynomial .^n–2i Lidl, Müller and Oswald recently defined a number b to be a strong Dickson pseudoprime to the parameterc (shortlysDpp(c)) if [itg_n(b, c)b modn for all b. These numbers seem to be very appropriate for a fast probabilistic prime number test. In generalizing results of the above mentioned authors a criterion is derived for an odd composite number to be ansDpp(c) for fixedc. Furthermore the optimal parameterc for the prime number test is determined.