Mössbauer study of mixed valent silicides Eu(Ir1−x Pd x )2Si2

The solid solutions Eu(Ir_1–x Pd _x )₂Si₂, which exist for 0x0.125 and 0.75x1. cristallize with the tetragonal ThCr₂Si₂-type structure. The variation of the europium valence with composition has been thoroughly studied at temperatures 4.2T293 K by¹⁵¹Eu Mössbauer resonance. For 0x0.125 the europium valence at room temperature decreases asx increases. For 0.75x1 the valence transition temperature Eu³⁺Eu²⁺ increases asx increases.     

Mössbauer spectroscopy,X-ray diffraction and magnetic measurements of iron-nickel ultrafine particles

Iron-nickel ultrafine particles with a composition in the Invar region (38–50% Ni) were prepared by the gas-evaporation-coalescence technique. The chemical composition was checked by electronprobe microanalysis, while X-ray diffraction Rietveld refinement was used to characterize the structure as well as to estimate the particle size. The temperature and field dependence of the magnetizationM(B, T) was measured for 0B25 kOe in the temperature range 4.2 KT400 K. Transmission Mössbauer spectra were taken at room temperature and at liquid helium temperature. The results obtained show that the predominant phase is a disordered Ni-rich alloy.On leave from Physics Department, University of Khartoum, P.O. Box 321, Khartoum, Sudan.     

A Monomial Basis for the Virasoro Minimal Series M(p,p′) : The Case 1<p′/p<2

Quadratic relations are given explicitly in two cases of chiral conformal field theory, and monomial bases of the representation spaces are constructed by using the Fourier components of the intertwiners. The first case is the (2,1) primary fields for the (p,p)-minimal series M_r,s (1rp–1,1sp–1) for the Virasoro algebra where 1<p/p<2. We restrict ourselves to the case p3, for which the (2,1) primary field exists. The second case is the intertwiners corresponding to the two-dimensional representation for the level k integrable highest weight modules V() (0k) for the affine Lie algebra      

Photoelectric characteristics of materials Pb1-xSnxTe?In? subject to microwave and infrared emission

Current/voltage (CV) characteristics of solid solutions Pb_1–xSn_xTeln (with D. 22x3. 24 and 0.2N_In1.0 atomic percent) were studied at He temperatures, in presence of background at 300K. Also the photoconductivity of these materials was measured in conditions of background screening and illumination at wavelengths 100µm and 2mm.CV-characteristics are interpreted within a model of two-electron capturing by Jahn-Teller centers. Photoconductivity in millimeter and submillimeter wavebands, having relaxation times less than 10^–5 sec, is related with heating of electronic gas in process of electromagnetic power absorption by the free carriers.     

Properties of theL p -solutions of linear impulsive equations in a banach space

Conditions for the existence ofL _p-solutions (1p) of linear impulsive equations in a Banach space are found.     

Mössbauer study of the perovskite series Sr(Fe1−xTix)Oy

⁵⁷Fe Mssbauer effect in the perovskite series Sr(Fe_1–xTi_x)O_y (0.0x0.99; y3) at RT and 78K has been analyzed. The influences of the Ti composition on the coexitence of Fe⁴⁺ and Fe³⁺, the ratio of Fe⁴⁺ to the total Fe, and the Y value have been studied. When 0x0.6 the ratio of Fe³⁺ (II) doublet to the total Fe increases as the final firing temperature increases, agreeing with the result obtained from the positron lifetime measurements. When x0.7, the Fe³⁺ (II) doublet disappears. In the range of x=0.9 and x=0.99, there still exists a considerable content of Fe⁴⁺ in samples, disagreeing with the result obtained from the chemical analysis by Clevenger. The relations between the electrical resistivity and the thermistor material constant B of negative temperature characteristic varying with Ti composition have been discussed.     

Statistics of the One-Dimensional Riemann Walk

The Riemann walk is the lattice version of the Lévy flight. For the one-dimensional Riemann walk of Lévy exponent 0<<2 we study the statistics of the support, i.e., set of visited sites, after t steps. We consider a wide class of support related observables M(t), including the number S(t) of visited sites and the number I(t) of sequences of adjacent visited sites. For t we obtain the asymptotic power laws for the averages, variances, and correlations of these observables. Logarithmic correction factors appear for =2/3 and =1. Bulk and surface observables have different power laws for 1<2. Fluctuations are shown to be universal for 2/3 <2. This means that in the limit t the deviations from average M(t)M(t)–M-0304;(t-0304;) are fully described either by a single M independent stochastic process (when 2/3 <1) or by two such processes, one for the bulk and one for the surface observables (when 1<<2).     

Electron-electron interaction and magnon scattering in the d-electron dominated resistivity of Ni80−xFe x Si8B12 glasses

Our investigation of the electrical resistivity (T) betweenT=0.1 K and 295 K revealed for all Fe-concentrations, 2.4x16, a minimum of betweenT _min=7 K and 15 K. Above the minimum, scattering from structural disorder prevails in the paramagnetic alloy (x=2.4), while in the ferromagnetic glasses (x5.6) coherent magnon scattering appears to dominate. With one exception, (TT _min) increases as down to the lowest temperatures for all investigated Fe-contents (x10.4) in zero magnetic field as well as in applied 6 T. The value of the slopes, 6.3(6) ( cm)^–1 K^–1/2, agrees with the universal scaling behaviour reported recently by Cochrane and Strom-Olsen for systems with strong electron-electron interaction. A large, linear rise of (T<T _min,B=0) observed in the most diluted sample (x=2.4) is tentatively attributed to quantum localization in the non-reentrant spin glass phase. The magnitude of the temperature effects on (T) above as well as belowT _min indicates significantd-band contributions to the conductivity.     

Structural stability properties of friedman cosmology

A dynamical system with Robertson-Walker symmetries and the equation of the statep=, 01, considered both as a conservative and nonconservative system, is studied with respect to its structural stability properties. Different cases are shown and analyzed on the phase space (x=R ^D , y=x).     

Atomic volume dependence of the153Eu isomer shift in ScxY1−x

Mössbauer studies of dilute¹⁵³Eu in Sm_0.05Sc _x Y_0.95–x (0x0.95) reveal large changes in the isomer shift as a function ofx. The strong dependence of the isomer shift on alloy composition, is interpreted in terms of changes in the charge density at the Eu nucleus ((0)), which result from changes in atomic volume. The isomer shift was found to depend linearly on V/V (V/V changes between 0 and –0.3). A value of (–17.5±2.0)a ₀ ^–3 was obtained for d(0)/d lnV.This research was supported by a grant from the United States—Israel Binational Science Foundation (BSF), Jerusalem, Israel.     

A contribution to the domain structure of iron whiskers

The paper contains an interpretation of the domain patterns found on iron whiskers. An explanation of the closure structure at the end of a whisker grown in the [111] direction is given on the basis of a quantitative analysis. Structures, which form on whiskers strained by axial pressure and bending, are also dealt with. The results are in good agreement with experiment.

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, . [111]. , . .

     

Evidence of Hg-chain formation in Hg x TiS2: a199mHg-TDPAC study

We determined the^199mHg nuclear quadrupole interaction in the misfit or superstoichiometric compound Hg _x TiS₂ by time differential perturbed angular correlation. A unique Hg-site withv _Q =511(1) MHz and =0.410(4) was observed, irrespective of the Hg-uptake (2/3x4/3). We propose a model of Hg-Hg zig-zag chains which accounts for these observations as well as for the X-ray diffraction data.     

Solutions without phase-slip for the Ginsburg-Landau equation

We consider the Ginsburg-Landau equation for a complex scalar field in one dimension and consider initial data which have two different stationary solutions as their limits in space asx±. If these solutions are not very different, then we show that the initial data will evolve to a stationary solution by a phase melting process which avoids phase slips, i.e., which does not go through zero amplitude.     

From ferromagnetic to non-magnetic singlet ground state in CePd1-x Ni x

Magnetic, resistivity and specific heat measurements were performed on the CePd_1-x Ni _x system, which ranges between a ferromagnetic (CePd) and non-magnetic (CeNi) singlet ground state. The Curie temperature (T _c ) grows with Ni concentration, reaching a maximum value ofT _c =(10.5±0.1) K for 0.5x0.8. Both, the concentration dependence ofT _c and the Curie-Weiss temperature ( _P ) follow the predicted competition between the magnetic exchange interactions and the Kondo effect. In the crossover region (x0.9) the entropy gain shows that only the doublet crystal field ground state is involved in the singlet ground state formation.Partially supported by the Direction de la Coopération Scientifique et Technique du Ministère des Relations Extérieures of France     

More inequalities for critical exponents

A variety of rigorous inequalities for critical exponents is proved. Most notable is the low-temperature Josephson inequalitydv +2 2–. Others are 1 1 +v, 1 1 , 1,d 1 + 1/ (for d),dv, ₃ + (for d), ₄ , and _{2m 2m+2} (form 2). The hypotheses vary; all inequalities are true for the spin-1/2 Ising model with nearest-neighbor ferromagnetic pair interactions.NSF Predoctoral Fellow (1976–1979). Research supported in part by NSF Grant PHY 78-23952.     

Low-temperature and long-time anomalies of a damped quantum particle

The time evolution of a damped quantum particle is discussed. Dissipation is modeled by the bilinear coupling to a set of harmonic oscillators. Using a functional integral technique that accounts for initial correlations between the particle and the reservoir, one can express the dynamics of the damped particle entirely in terms of equilibrium correlation functions. The long-time behavior of these correlations is determined for memory damping arising from the coupling to a reservoir with spectral densityI() at low frequencies, where > 0. The time evolution of nonequilibrium initial states of the damped particle is discussed. At finite temperatures an initially localized state is found to spread subdiffusively or superdiffusively, depending on . For > 2 the damping becomes ineffective for long times, and the width of a state grows kinematically. At zero temperature and for < 1, an initially localized state remains localized for all times. For 1 the state spreads, but with a slower rate than at finite temperatures. Study of arbitrary initial states indicates that the process is ergodic at finite temperatures only for 2 and at zero temperature for 1 2.     

Mössbauer studies on the series La1-xSrxFeO3

Mössbauer spectra have been recorded at 4.2 and 300 K on the series La_1–x Sr _x FeO₃, wherex varies from 0 to 1.0 in steps of 0.1. Neutron diffraction experiments have shown that the crystal structure is orthorhombic for 0x<0.3, rhombohedral for 0.4x0.7, and cubic for 0.8<x1.0. Mössbauer spectra at 4.2 K are composed of magnetic sextet components arising from different charge states of iron ions. In the orthorhombic and rhombohedral phases, the charge states Fe³⁺ and Fe⁵⁺ coexist. In the cubic phase, iron is present as Fe³⁺ and Fe⁴⁺ states. At 300 K, the samples are magnetically ordered in the range 0 x0.3 and the coexistence of Fe³⁺ and Fe⁵⁺ remains. For samples 0.4x1.0, the samples are paramagnetic. Fits to these spectra require two components, one corresponding to an Fe⁴⁺ state, the other being best described as an Fe³⁺ ion forx0.7 but forx>0.7 having a mean charge state which increases to 3.5 forx=1.0.     

Primes in Short Intervals

Contrary to what would be predicted on the basis of Cramérs model concerning the distribution of prime numbers, we develop evidence that the distribution of (x+H)–(x), for 0xN, is approximately normal with mean H and variance HlogN/H, when NHN^1–.Dedicated to Freeman Dyson, with best wishes on the occasion of his eightieth birthdayResearch supported by NSF grants DMS–0070720 and DMS–0244660.Research supported by the American Institute of Mathematics and NSF grants.     

Electrical switching behavior of bulk As–Te–Si glasses: composition dependence and topological effects

The current–voltage characteristics and electrical switching behavior of bulk As₃₀Te_70-xSi_x (2x22) and As₄₀Te_60-xSi_x (2x17) glasses have been investigated over a wide range of compositions. The glasses studied have been found to exhibit a current-controlled negative-resistance behavior and memory switching. Further, the switching voltage (V_t) is found to increase linearly with sample thickness in the range of 0.15 mm to 0.45 mm, and it decreases linearly with temperature (300–353 K). It is also observed that the variation of the switching voltage V_t of As–Te–Si glasses exhibits a sharp slope change at an average coordination number r=2.46 (for both tie lines), which is associated with the rigidity percolation in the system. Further, a minimum is seen in the switching voltage V_t at an average coordination number r=2.66, which is likely to be the chemical threshold of the system. PACS 71.55.Jv; 72.20.Ht; 72.80.Ng; 73.61.Jc; 77.80.Fm; 78.66.Jg     

Subsolidus phase relation and crystal structure in the Pr<Subscript>1+x-y</Subscript>Ba<Subscript>2-x-z</Subscript>Ca<Subscript>y+z</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7±δ</Subscript> system

The Pr_1+xBa_2-xCu₃O_7± solid solution was investigated by means of X-ray powder diffraction combined with Rietveld analysis. A Pr123 single phase could be synthesized under Pr-rich conditions by sintering at 950 °C in air. The solubility range of Pr_1+xBa_2-xCu₃O_7± solid solution is 0.08x0.80. The structure of Pr_1+xBa_2-xCu₃O_7± for 0.08x<0.30 is orthorhombic. The structure transforms into tetragonal for 0.30x0.80. To form the Pr123 single phase, the Ba sites in the Pr123 structure must have partial Pr ions, and the least amount is x=0.08. Ba ions cannot occupy the sites of Pr ions. In the Pr123 structure, Ca ions can replace Pr ions; the highest value is x=0.4 in the PrBa_2-xCa_xCu₃O_7± system under our experimental conditions. However, Ca ions cannot replace Ba ions. The ionic radius plays a more important role than the chemical properties in the substitution between Pr, Ba and Ca ions in the Pr123 structure. PACS 61.50.Ah; 64.70.Kb; 61.72.Ff