Mössbauer Study of La1−x Ca x Mn1−y 57Fe y O3 with x=0,0.25; y=0.01

Temperature dependent Mössbauer measurements are done on the samples of La_1–x Ca _x Mn_1–y ⁵⁷Fe _y O₃ with x=0 and 0.25, and y=0.01. With decreasing temperature, the specimen with x=0.25 shows a paramagnetic to ferromagnetic transition around 175 K. In the specimen x=0.0, the temperature dependence of both the center shift () and the recoilless fraction (f) can be fitted very well with the Debye theory with a _D=320±50 K. But for the specimens with x=0.25, f and show distinct deviations from the Debye behavior in the temperature range in which the resistivity shows a sharp decrease. Dips observed in both the f and around the transition temperature suggest that the Jahn–Teller distortion observed in these systems is dynamic in nature.     

Strain analysis and optical characterization of LPE InGaAsP laser-confining layers

The compositional dependence of lattice parameters and strains induced by lattice mismatch of liquid phase epitaxial (LPE) In_1–x Ga _x AS _y P_1–y /InP with smallx andy are studied. The measured elastic strain is proportional to the lattice mismatch within certain critical limits; as much as 78% of the mismatch strain is found to be accommodated elastically under both compression and tension. The near-band edge absorption and photoluminescence measurements yield the free electron and hole recombination probability and the compositional dependence of the energy gap at low temperature. The influence of the lattice mismatch on the optical properties is discussed.On leave from Institute of Semiconductors, Chinese Academy of Sciences, Peking, ChinaOn leave from Instituto de Fisica, Universidade Estadual de Campinas, Campinas, S.P. 13100, Brazil     

Propagation parameters of the beam extracted from a diode pumped Nd:YAG ceramic slab laser with a hybrid stable–unstable resonator

We analyze the propagation properties of the beam extracted from a diode pumped ceramic Nd:YAG slab laser adopting a hybrid stable–unstable resonator. Such a resonator produces a beam characterized by an Hermite–Gauss mixture-like distribution in one transverse direction and a hard-edge diffracted distribution on the other transverse direction. The beam propagation parameters M_x² and M_y² are measured for different values of the diodes driving current. We obtain a beam parameter product smaller than 3 mm mrad in both transverse directions and in the whole range of powers, up to an extraction of 220 W in a QCW regime.     

Invariant curves for area-preserving twist maps far from integrable

One-parameter families of area-preserving twist maps of the formF (x, y)=(x +y +f(x),y +f(x)) are considered. Various invariant curves, for the maps corresponding tof(x)=sin andf(x)=sinx+(1/50) sin(5x), are rigorously constructed forlarge values of the nonlinearity parameter . For larger values of , close to critical, some numerical experiments are briefly discussed.     

Mössbauer study of substituted barium ferrite BaFe11−x−y Co0.5Ti0.5Ni x ZnyO19−r

Substituted barium ferrite BaFe_11–x–y Co_0.5Ti_0.5Ni _x Zn_yO_19–r powders were prepared using a coprecipitation method and investigated by X-ray diffraction (XRD) and⁵⁷Fe Mössbauer spectroscopy. The results show that the as-prepared magnetic powders possess the typical hexagonal structure and demonstrate both a good dispersibility and a narrow particle size distribution. The hyperfine fields for all sites decrease slightly asx (ory) increases. The Ni²⁺ ions prefer to occupy the 2a and 12k sites, and Zn²⁺ ions occupy the 4f_IV site.     

Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R _∥/R _⊥) whereR _∥ andR _⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10⁻³ in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.     

Acoustic wave propagation through the boundary between a liquid and the Heusler ferromagnetic alloy

Incidence of an acoustic wave upon a plane boundary between a liquid and a ferromagnetic crystal is considered. The ferromagnet is the Ni_2+x+y Mn_1−x Ga_1−y Heusler alloy with a shape memory, which is in the region of the premartensite or martensite phase transition in temperature. The directions of propagation and polarization and the amplitudes of the reflected and transmitted quasilongitudinal and quasitransverse waves in the (110) plane of the crystal are determined. Starting from a certain critical angle of incidence, a longitudinal wave in the crystal becomes inhomogeneous and gliding along the boundary with an accompanying surface oscillation. In the vicinity of the phase transition point, this wave may be radiated into the crystal bulk. Proceeding from the experimental data by Trivisonno for ultrasonic velocities and absorption in a Ni₂MnGa crystal, numerical estimates are obtained for the aforementioned acoustic effects.     

Positron annihilation studies on the electronic structure of Pd−Ag−H system

Angular Correlation of Annihilation Radiation (ACAR) is shown to be useful to examine the electronic structure of -phase Pd_1–y Ag _y H _x system. Hydrogen absorption by Pd_1–y Ag _y alloys results in the increase of both nearly free andd-localized electron numbers in compliance with the KKR-CPA calculation outcomes. The investigation of -phase systems, PdH _x hydrides as well as Pd_1–y Ag _y H _x materials, failed because of a high concentration of lattice defects.     

Inelastic light scattering in strontium borate glasses in the systemxSrO.(1−x)B2O3 and (SrCl2) y .[xSrO.(1−y−x)B2O3]1−y

Raman spectra of strontium borate binary glasses in the systemxSrO.(1-x)B₂O₃ forx=0.20, 0.25, 0.30, 0.35, 0.40, and 0.50 and ternary glasses in the system (SrCl₂) _y .[xSrO.(1−y−x)B₂O₃]_1−y fory=0.10, 0.20, 0.30 and 0.40 andx=0.20 and 0.35, are reported. Raman spectra of the glasses show experimental evidence of glass network modifying nature of SrO in borate matrix. SrO causes a change of boron atom coordination number from 3 to 4 resulting in the complex structural groupings comprising of BO₄ and BO₃ units. Strontium cations are not easily accommodated in the glass structure and tend to break up the network at high concentration (x) of SrO, causing non-bridging oxygens. The effect of temperature variation of binary glasses has also been studied. The introduction of SrCl₂ to the binary stronium borate glass causes a large change in intensity of low frequency Raman scattering whereas there is no change in the vibrational dynamics in the high frequency region. The temperature reduced Raman spectra represents true vibrational density of states. Martin-Brenig model developed for the low frequency region has been discussed to obtain the structural correlation range in the glass.     

The infrared spectra of solid solutions of some iron and chromium oxides

Oxides Fe_2–x Cr _x O₃ (corundum-type) and Fe_3–y ,Cr _y O₄ (spinel-type) were studied by transmission IR spectroscopy. The wavenumbers corresponding to characteristic absorption maxima depend linearly on the composition of both types of solid solutions, i.e. onx andy. The results show that IR spectroscopy can contribute to the analysis of solid solutions of iron and chromium oxides and of the oxidation products of Fe-Cr alloys.The authors are indebted to Ing. P. Holba for the preparation of synthetic oxide samples. The cooperation of Yu. Borodkin and I. Zuika (Riga) who made the measurements in the far IR region is gratefully acknowledged.     

Muon spin relaxation study on magnetism in high quality single crystal of a high transition temperature superconductor La2−xSrxCuO4−σ (0.11≤x≤0.14)

The positive muon spin relaxation method is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2−xSr_xCuO_4−σ (0.11≤x≤0.14). The well characterized crystal ofx=0.11 (T _c=34.5 K) with nearly complete flux exclusion exhibits spin freezing at 8K(T _f) with significant spin fluctuation up to 20 K. The onset of spin fluctuation andT _f decrease against increasingx towardsx=0.15, suggesting an existence of a magnetic phase boundary aroundx whereT _c becomes maximum.     

Magnetism and candidate muon-probe sites in RBa2Cu3O y

Key results of zero-field (ZF) and transverse-field (TF) muon-spin-relaxation (μSR) experiments on superconducting and insulating RBa₂Cu₃O _y (R123 _y , with R=Eu, Gd, Pr and Pr/Y:y=6, 7) are examined. The chemical behavior of the positive muon probe is addressed, and muon-oxygen bonding is shown to occur in all these cuprates. To explain magnetic fields at muon-probe sites in Pr _x Y_1−x Ba₂Cu₃O _y (0<=x<0.5,y=7 andx=0,y=6) samples, improvements on the reported magnetic structures from neutron diffraction are necessary. Cu magnetism in Pr123y (y=6,7) is observed belowT _N1, which is near RT. The magnetism seen belowT _N2 can be interpreted assuming an additional ordering in the Cutt-O chain layers. Alternatively, Pr ordering is also considered as the cause of the second phase transition. Considering the specific muon-probe location, a more detailed interpretation can be provided for the μSR parameters, measured in the normal and mixed states of these unconventional superconductors.     

Scattering approach to the von Klitzing effect

Earlier results of the scattering theoretical approach to the quantum Hall effect are simplified and generalized. Finite size corrections to the plateau values are found to be not of order (l/L _x ) but (l ² /L _x L _y )²,l,L _x ,L _y being the magnetic length, and the sample dimensions inx andy-direction respectively. An expression for the current parallel to the electric field in terms of a scattering matrix is derived. In the weak scattering regime this expression leads to a vanishing diagonal conductivity _xx .     

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd_2−x Ce _x CuO_4−y and a large single crystal of Nd₂CuO_4−y . We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La_2−x Sr _x CuO_4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd₂CuO_4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.     

Low Temperature Properties for Correlation Functions in Classical N-Vector Spin Models

We obtain convergent multi-scale expansions for the one-and two-point correlation functions of the low temperature lattice classical N - vector spin model in d S 3 dimensions, N S 2. The Gibbs factor is taken as exp[-b(1/2 ||?f||² +l/8 || |f|² - 1 ||² + v/2||f- h||²)], \exp [-\beta (1/2 ||\partial \phi||^2 +\lambda/8 ||\, |\phi|^2 - 1 ||^2 + v/2||\phi - h||^2)], where f(x), h ? R^N\phi(x), h \in R^N, x ? Z^dx \in Z^d, |h|=1, b < ￥|h|=1, \beta < \infty, l 3 ￥\lambda \geq \infty are large and 0 < v h 1. In the thermodynamic and v ˉ 0v \downarrow 0 limits, with h = e₁, and j L ‘^½ ‘, the expansion gives áf₁(x)? = 1+0(1/b^1/2)\langle \phi_1(x)\rangle = 1+0(1/\beta^{1/2}) (spontaneous magnetization), áf₁(x)f_i(y)? = 0\langle \phi_1(x)\phi_i(y)\rangle=0, áf_i (x)f_i (y)? = c₀ D^-1(x,y)+R(x,y)\langle \phi_i (x)\phi_i (y)\rangle = c_0 \Delta^{-1}(x,y)+R(x,y) (Goldstone Bosons), i = 2, 3, ?, Ni= 2, 3,\,\ldots, N, and áf₁(x)f₁(y)?^T=R￠(x,y)\langle \phi_1(x)\phi_1(y)\rangle^T=R'(x,y), where |R(x,y)||R(x,y)|, |R￠(x,y)| < 0(1)(1+|x-y|)^d-2+r|R'(x,y)|< 0(1)(1+|x-y|)^{d-2+\rho} for some „ > 0, and c₀ is aprecisely determined constant.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

One-dimensional DLR invariant measures are regular

A system of infinite spins in one dimension is considered. The interaction is given by a pair potential –J _xyS_xS_y, whereS _x,S _y are the spins at the sitesx,y andJ _xy=J(|x–y|) whereJ(|x–y|) decreases asymptotically in an integrable way. The self-interaction makes the system superstable. It is proven that any invariant DLR measure for this system satisfies Ruelle's superstable estimates (regularity condition).     

Depinning of a Discrete Elastic String from a Random Array of Weak Pinning Points with Finite Dimensions

In possible connection with dislocation pinning by foreign atoms in alloys and vortex pinning in type II superconductors, we compute the external force required to drag an elastic string along a discrete two-dimensional random array with finite dimensions. The obstacles, with a maximum pinning force f _m are distributed randomly on a rectangular lattice with square symmetry. The system dimensions are fixed by the total course of the elastic string L _x and the string length L _y . Our study shows that Larkin’s length is larger than L _y when f _m is less than a certain bound depending on the system size as well as on the obstacle density c _s . Below such a bound an analytical theory is developed to compute the depinning threshold. Some numerical simulations allow us to demonstrate the accuracy of the theory for an obstacle density ranging from 1 to 50% and for different geometries.     

Propagation properties and M2 factors of a vortex Airy beam

We investigate the propagation properties and M² factor of a vortex Airy beam. The expression of M² factor as a function of the modulation parameters a_x and a_y is analytically obtained. The results indicate that the M² factor can achieve minimum M²=2.33 when a_x=a_y=1.026 for the m=1st order vortex Airy beam. For m=2, the M² factor can achieve minimum M²=7.53 when a_x=1.601 and a_y=0.529 (or a_y=1.601 and a_x=0.529). These results indicate that the far field divergence of the beam width in the sense of root-mean-square beam is related to modulation parameters a_x and a_y, and increases with increasing vortex order. Numerical calculations provide an intuitive physical picture for the effect of the modulation parameters on the propagation of the vortex Airy beam.     

Effects of nitrogen and vanadium on the57Fe hyperfine field of RFe12−x V x N y (R=Y,Nd)

Magnetization and⁵⁷Fe Mössbauer measurements were carried out on RFe_12–x V _x N _y compounds (R=Y and Nd,x=1.7 and 2.2) and the effects of nitrogen and vanadium atoms on the⁵⁷Fe hyperfine fields at the different iron crystallographic sites were investigated. The hyperfine field decreases with increasing number of vanadium neighbour atoms at all the iron sites. The hyperfine field is strongly enhanced in the nitrogen composition withy>1 where the compound tends to transform into an amorphous-like solid. The iron moment deduced from the hyperfine field increases more upon nitrogenation for the 8i-site than for the other sites, and exceeds the moment of bcc iron.