Levels in <Superscript>127</Superscript>La fed by the <Superscript>127</Superscript>Ce beta-decay

The low-lying levels in ¹²⁷La have been studied through the -decay of ¹²⁷Ce ( T_1/2 = 29s) produced by bombarding a ^natMo target with a 185-MeV ³⁵Cl beam. Reaction products were on-line mass-separated, and -ray singles and - coincidence measurements were performed. Conversion electrons were also measured and multipolarities of transitions have been derived. The half-life of the 210.9-keV level was determined to be (1.9±0.3)ns by the - delayed coincidence technique. The level scheme obtained has been compared with calculations based on the Nilsson model.     

Heavy-fermion behavior and spin-glass freezing in Si-stabilized amorphous alloys based on UPt3

U-Pt-Si and U-(Pt, Pd)-Si alloys have been prepared in the amorphous state by splat-cooling. The average nearest-neighbor distance is the same as the U-Pt distance in UPt₃. The low-temperature behavior of these alloys (with composition (U_0.25Pt_0.75)_1–xSi_x, x=0.18, 0.20, 0.25; and (U_0.25(Pt_1–yPd_y)_{0.75 0.8}Si_0.2,y=0.05 and 0.1) has been studied by specific heatC, magnetization and resistivity measurements in the temperature range 1.5 K to 20 K and for magnetic fields up to 6 T. Some samples were investigated down to 0.1 K and the specific heat for one sample was measured in fields up to 22 T. With ₀ =C/T| _T0 between 0.25 and 0.35 J/moleUK², the alloys can be classified as heavy-fermion materials. As opposed to UPt₃, the alloys exhibit spin-glass behavior with a concomitant decrease of ₀. The magnetization exhibits typical spin-glass features (e.g., an irreversibility line). An inverse relationship between ₀ andT _max (temperature ofC/T maximum which is proportional to the spin-glass freezing temperature), irrespective of the Pd content, is observed. The behavior can be qualitatively described by a coexistence of heavy-fermion behavior and spin-glass freezing.     

Self-avoiding random walks on the hexagonal lattice

We use the algorithm recently introduce by A. Berretti and A. D. Sokal to compute numerically the critical exponents for the self-avoiding random walk on the hexagonal lattice. We find=1.3509±0.0057±0.0023v=0.7580±0.0049±0.0046=0.519±0.082±0.077 where the first error is the systematic one due to corrections to scaling and the second is the statistical error. For the effective coordination number we find=1.84779±0.00006±0.0017 The results support the Nienhuis conjecture=43/32 and provide a rough numerical check of the hyperscaling relationdv=2–. An additional analysis, taking the Nienhuis value of=(2+2^1/2)^1/2 for granted, gives=1.3459±0.0040±0.0008     

Critical exponents for two-dimensional tracer diffusion through a changing background at concentrationc=c p

We study the diffusion of a particle on the sites of a triangular lattice of which half the sites are occupied by a background of other particles. No two particles may occupy the same site. We carry out Monte Carlo simulations for the following model: At each Monte Carlo step the tracer attempts to move to a neighboring site, which it does if the site is unoccupied. At each step, each background particle attempts to desorb with probability. If a background particle desorbs, it is replaced at a randomly chosen site on the lattice. We define R _tr ² (t)/t=D _tr. For the case=0, we calculateD ₀t ^k and findk=0.71±0.01, wheret is the number of Monte Carlo steps. When0, we calculateD _tr ~ ^– w ₁ and findw _ad=0.24±0.02. We compare this to the model in which the background particles are constrained to move to nearest neighbor sites and findD _tr ^– w ₁ withw ₁=0.28±0.03.     

Stochastic Schrödinger operators and Jacobi matrices on the strip

We discuss stochastic Schrödinger operators and Jacobi matrices with wave functions, taking values in ^l so there are 2l Lyaponov exponents ₁..._l0 _l+1..._2l =–₁. Our results include the fact that if ₁=0 on a set positive measure, thenV is deterministic and one that says that {E|exactly 2j 's are zero} is the essential support of the a.c. spectrum of multiplicity 2j.Research partially supported by USNSF under grant DMS-8416049     

Self-Similar Decay in the Kraichnan Model of a Passive Scalar

We study the two-point correlation function of a freely decaying scalar in Kraichnan's model of advection by a Gaussian random velocity field that is stationary and white noise in time, but fractional Brownian in space with roughness exponent 0<<2, appropriate to the inertial-convective range of the scalar. We find all self-similar solutions by transforming the scaling equation to Kummer's equation. It is shown that only those scaling solutions with scalar energy decay exponent a(d/)+1 are statistically realizable, where d is space dimension and =2–. An infinite sequence of invariants J _p, p=0, 1, 2,..., is pointed out, where J ₀ is Corrsin's integral invariant but the higher invariants appear to be new. We show that at least one of the invariants J ₀ or J ₁ must be nonzero (possibly infinite) for realizable initial data. Initial datum with a finite, nonzero invariant—the first being J _p—converges at long times to a scaling solution _p with a=(d/)+p, p=0, 1. The latter belongs to an exceptional series of self-similar solutions with stretched-exponential decay in space. However, the domain of attraction includes many initial data with power-law decay. When the initial datum has all invariants zero or infinite and also it exhibits power-law decay, then the solution converges at long times to a nonexceptional scaling solution with the same power-law decay. These results support a picture of a two-scale decay with breakdown of self-similarity for a range of exponents (d+)/<a<(d+2)/, analogous to what has recently been found in the decay of Burgers turbulence.     

Positron mobility in polyethylene in the 60–400 K temperature range

We have determined the positron mobility (₊) in polyethylene samples (67.2% crystalline, glass transition temperatureT _g=151 K) in the 64–400 K temperature range by Doppler shift measurements. A method based on the simulataneous observation of two lines from¹³³Ba and¹³⁷Cs radioactive sources together with the positron annihilation line, was employed to measure the Doppler shift of the 511 keV line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and theS parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation. ₊ at 64 K is 31.7±0.8 cm² V^–1 s^–1 then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (₊=26.9±0.8 cm² V^–s^–). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K. From 250 K to 377 K, ₊ increases and reaches the value of 38.4±1.0 cm² V^–1s^–1. The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).     

Effect of hydrostatic pressure on the static susceptibility of the spin-glass systems Ag:Mn,Au:Mn,Cu:Mn and Au:Fe

Previous studies of the magnetic susceptibility of a Ag-3 at.% Mn spin-glass under high pressure are extended to include the systems Au-3at.% Mn, Cu-3at.% Mn and Au-3at.% Fe. In all four alloys the spin-glass freezing temperatureT ₀ increases with pressure, yielding the volume dependences lnT ₀/ lnV=–3.6±0.4, –4.1±0.6, –2.1±0.4 and –1.64±0.25, respectively. These results give information about the nature of the interaction mechanisms responsible for the spin-glass freezing phenomena. The pressure dependence of the Curie temperature of ferromagnetic Au-20at.% Fe was also studied.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Mössbauer study of oxygen-related lattice instabilities in oriented 1-2-3 HTSC

⁵⁷Fe Mössbauer effect measurements as a function of temperature between 78 and 293 K in grain-aligned YBaCuO (T _c=89 K, T _c<2 K) with bothc andc orientations revealed a sequence of anomalies in the recoilless fraction, center shift and linewidth of all⁵⁷Fe quadrupole doublets in the host lattice. These anomalies are believed to arise due to antiferroelectric phase transitions driven by a nonlinear anharmonic vibrational motion of oxygen atoms in a breathing mode configuration. The results obtained are indicative of a possible coexistence of high-T _c superconductivity and antiferroelectricity in 1-2-3 ceramics.     

Prompt gamma-ray emission from fission of239Pu by resonance neutrons I

Gamma-ray spectra from²³⁹Pu(n,f) reaction were measured with the gamma spectrometer on the pulsed reactor IBR-30. The fast 19-section ionization chamber with 1.6 g of²³⁹Pu targets was used. The fission gamma-rays within the 32 ns time window were measured using a Ge(Li) detector. Based on the measured gamma-ray spectrum in the (0.1–1.8) MeV interval, the total number of emitted-quanta-multiplicity N ^tot and total energy of -quanta per fission event, E ^tot , were calculated. The results were 7.0±0.6/fission and 6.8±0.5 MeV/fission respectively.This work is supported in part by the Grant Agency of the Slovak Academy of Science under grant GA-SAV 517/1992.     

A comparison of high-T c superconductors: Specific heat in magnetic fields at very low temperatures

We discuss specific-heat,C(T), results on YBa₂Cu₃O_7–(=0.13, 0.99) and Bi₂Sr₂CaCu₂O₈. For completion, we compare these with literature data on La_1.85Sr_0.15CuO₄ and La₂CuO₄ by emphasizing the data at lowest temperatures,T0.45K. The strong upturn inC(T)/T found atB=0 is predominantly associated with a splitting of nuclear states by quadrupolar interactions. The shift of this upturn towards higher temperatures, when magnetic fields up to 8 T are applied, indicates additional hyperfine splitting. For YBa₂Cu₃O_6.87 and Bi₂Sr₂CaCu₂O₈, subtraction of the nuclear specific heat from the raw data reveals a spin-glass derived contribution ^*(T,B), with ^* being strongly depressed upon both warming and applying externalB-fields. The probable source for ^* T are impurity phases (in YBa₂Cu₃O_7–) and/or frustrated Cu²⁺ moments created by lattice defects. Any intrinsic linear specific heat, ⁱ , due to, e.g., spinon excitations or normal electronic carriers, would have to imply ⁱ <1 mJ/K² mole_f.u. for each of these two compounds.Dedicated to Prof. Dr. G. von Minnigerode on the occasion of his 60th birthday     

A “hydrodynamic” theory of surface reaction rates

The Brownian motion of adsorbed particles is described in terms of the first four velocity moments of the distribution function (number density, momentum density, energy density and energy current density). The resulting hydrodynamic equations turn out to be sufficient for a simple derivation and extension of Kramers' results for chemical reaction rates in terms of the friction constant of an underlying Fokker-Planck equation. An interpolation formula is obtained for() containing Kramers' results for small and large as limiting cases. For temperaturesT small compared to the well depthV ₀ one finds a large regionT/V ₀/v ₀V ₀/T in which Eyring's absolute rate theory is approximately valid.On leave of absence from Physikdepartment der TUM, München-Garching     

Phase transition in an Ising-spinglass-model including next-nearest-neighbor-interaction?

The Transfermatrix-Method [1, 2] is applied to an Ising-spinglass-model on the square lattice with nearest neighbor interactionsJ ₁ positive and next nearest neighbor interactionsJ ₂ negative, when a fraction 1-x of the spins is removed at random. An example for this model is Eu _x Sr_1-x S [3, 4]. The question concerning the existence of a springlass-phase is not yet clarified. Investigation of the correlation function s ₀s_R² (correlation between two spins with distanceR), which is related to the Edwards-Anderson-orderparameter shows in the present work evidence against a spinglass-phase at finiteT. Analogous to the more common ±J-model (competing nearest neighbor interactions at random) a phase transition should only occur atT=0.     

Kondo-lattice: Kondo-effect?

The periodics-d-model is investigated by Green function methods. We find that the lnT-divergence appearing in third order perturbation theory for the electron selfenergy is smoothed out but still shows up in a characteristic decrease (exchange coupling<0) or increase (>0) of lifetime of electrons at the fermi level asT0 and a corresponding structure in the specific heat. However, in contrast to the single impurity case, the corresponding characteristic temperature does not vary like exp[–1/||] but always is of order ². From a mean field theory of the model, one finds a critical temperature for a transition into a magnetically ordered stateT _c0 which varies precisely like ². Thus an interesting competition between Kondo-effect and long-range magnetic order can be expected in this model.Work performed within the research program of the SFB 125 Aachen/Jülich/Köln     

Decay of the Two-Point Function in One-Dimensional O(N) Spin Models with Long-Range Interactions

Using Griffiths and Lieb–Simon type inequalities, it is shown that the two-point function of ferromagnetic spin models with N components in one dimension decays like the interaction J(n)n ^– provided that 1N4 and T>T _c.     

On the 3+ level in Os186 populated in Re186 Β-decay

The coincidences of the 400–1100 keV gammas with the conversion electrons of the 137 keV transition were measured with the intermediate image spectrometer modified for - coincidences. The e_L 137- 770 keV coincidences were observed. Consequently, in addition to the known 768 keV transition, there is another one with closed energy. For this transition the relative intensity ratioI ⁶³² /I ⁷⁷³ =8±2 was determined. The corrected decay scheme of Re¹⁸⁶ is presented.     

High-temperature series for scalar-field Lattice models: Generation and analysis

An implementation of the free-embedding scheme for high-temperature series generation on the body-centered cubic family of lattices in arbitrary dimensiond is, described. Series to order 21 in inverse temperature are tabulated for several scalar field models, both for the magnetic susceptibility and for the second moment of the spin correlation function. The critical behavior of a family of 3-dimensional double Gaussian models, which interpolate continuously between the spin-1/2 Ising model and the Gaussian model, is analyzed in detail away from the Gaussian model limit using confluent inhomogeneous secondorder differential approximants. With our best estimate of the correction-to-scaling exponent,=0.52±0.03, the leading exponents for the susceptibility and correlation length for this family are consistent with universality and are given by=1.237±0.002 and =0.630±0.0015, respectively, and=2–/=0.0359±0.0007.     

Analytic extrapolation inL ∞-norm: An alternative approach to “QCD Sum Rules”

In view of physical applications (especially to QCD Sum Rules), the following problem, pertaining to analytic extrapolation techniques, is studied. We are considering amplitudes, which are (real) analytic functions in the complex plane cut along=[s ₀, ). A modelF ₀(s) of the amplitude is given through the values ofF ₀(s) on some interval=[s ₂,s ₁] (withs ₁<s ₀) and the values of its discontinuity on. These values are approximate, and are supplemented by prescribed error channels, measured inL -norm (both on and). Investigating the compatibility between these data leads to an extremum problem which is solved up to a point where numerical methods can be implemented.Unité Associée au CNRS n^o040768     

Structural phase transition and internal fields in rubidium thiocyanate probed through vibrational spectroscopy

A comprehensive vibrational study of Rubidium thiocyanate (RbNCS), in the 298–448°K range reveals that the orthorhombic to tetragonal transition atT _c440°K is of order-disorder type exhibiting purely second order characteristics. A consistent order-parameter exponent 0.45±0.04 has been obtained from both Raman and IR measurements. In the Raman spectra, the appearance of sidebands in both the C–N&C–S stretching modes and their temperature dependence indicate the existence and a gradual diminution of internal electric field asT approachesT _c. These results have been discussed in the context of an anharmonic oscillator model. An activation energy (U) for the reorientational motion of NCS^– ions,U0.29±0.02 eV, has been calculated from large changes in the bandwidth (_C-N) nearT _c.