μSR studies in an I2-doped phenylenediamine polymer

We report SR studies of the recently synthesized I₂-doped phenylenediamine polymer. Longitudinal-field spectra exhibit typical behavior of a spin-glass with a freezing temperatureT _g 100 K and a spontaneous static local field 500 G at low temperatures. Preliminary magnetization measurements hint of antiferromagnetic spin coupling and hysteretic behavior aroundT _g. These features suggest possible spin-glass and/or antiferromagnetic spin freezing with a high transition temperature due to the doped radical spins in this organic polymer.Postdoctoral fellow of the Humboldt Foundation.     

Low-load friction behavior of epitaxial C60 monolayers

We report on¹³C MAS NMR experiments in the low temperature phase of Rb₁C₆₀. Two different lines are observed with line shifts of 110 ppm and 50 ppm. The line at 50 ppm is a signature ofsp ³ carbons and can be interpreted in terms of a hypothetical polymerization along the crystallographica-axis. This supports a quasi 1 D structure of the A₁C₆₀ in the low temperature phase.     

Local magnetic moment and hyperfine field in hydrogenated iron and iron-vanadium alloy

The local magnetic moment and hyperfine fieldB _hf at Fe and V sites in hydrogenated iron and iron-vanadium were calculated using the discrete variational method. The variations in andB _hf with H occupation of the octahedral (O) site were considered. It was found that when H occupies the O site neighbouring an Fe atom, both local moment and hyperfine field at this atom decrease linearly with increasing number of H atoms. The rate of decrease is larger for Fe in iron as compared to iron in vanadium. On the other hand, when H resides at an O site next neighbouring an Fe atom, whether in iron metal or in iron-vanadium, the Fe magnetic moment increases slowly, while the hyperfine field remains almost constant. The V moment in iron, which is negative (–0.83 _B ), becomes less negative (–0.30 _B) as H occupies the neighboring O sites, whereas slight changes occur (–0.88 _B) when H is at the next neighbouring O site. The net effect of H on Fe in iron is to decrease the average magnetic moment at a rate of 1.2 _B per H/Fe for low H content. On the other hand, the average Fe moment in an iron-vanadium alloy increases if H resides at O sites which are immediate neigbours of V and next neighbours of Fe. This may explain the development of a magnetic state on hydrogenation of Fe-V alloys, which is exhibited by the specific heat and susceptibility measurements. The changes in the isomer shift were found to agree with experimental trends.     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

The critical point indices of ferroelectric and ferromagnetic phase transitions

Magnetic transitions are described by the critical indices0,1/3,4/3 while some ferroelectric transitions appear to give0,R~1/2,1. It is pointed out that these two sets of values for the critical indices are allowed by the scaling laws and stability conditions near the phase transitions.The authors thank Prof. R. S. Krishnan for his encouragement and Mr. B. Viswanathan for some discussions. The financial assistance from DAE and CSIR is also acknowledged.     

On the interface stiffness of the random field ising model

Recent results of Grinstein, Ma, Villain and Binder on interface roughening incontinuum andlattice random field Ising models are related by introducing an effective interface stiffness function {ei247-1}. Ford3 dimensions the continuum theory is shown to be valid for non-zero random field strengthh for all temperatures and on a length scaleL>l _d (h,T) _d (h,T). Ford=2 and smallT a smeared spin-glass transition occurs at ₂(h,T)h. It is argued, that for 3<d<5 interface roughening occurs only forh larger than a critical field strengthh _R (T).     

Inertial range spectrum of a fractal model for turbulence

The present study deals with a model for 3D turbulence which is based on a fractal set ink-space (massM(k)logk, i.e.d _f=0). The energy spectrumE(_k) is calculated by a method similar to the Monte Carlo renormalization technique of Ma [1]. To apply this we decouple the set into next-neighbour pairs of energy shells.E _k is calculated iteratively by simulating the pairs. Their rough scales are forced and their fine scales are damped by an eddy viscosity ^e. We start in the viscous range where ^e is neglected. An intertial range shows up with a spectrumE(k)k^–, =1.68±0.07, and an eddy viscosity ^e(k)k ^-,=1.3±. The possible benefit for the simulation of turbulent flows is pointed out.     

Hydrogen-like recombination X-ray lasers using ps pulse drivers

We propose a scheme for producing high gain recombination X-ray lasers on hydrogen-like Balmer transitions by irradiating fibre targets with a 2 ps Chirped Pulse Amplification CPA beam of a Nd-glass laser facility. Very high gain coefficients for H-like C, N, O, F, Na Balmer transitions are predicted. The optimum electron density and temperature for maximum gain operation scale approximately asN _e 4 × 10¹³ Z ⁷ cm^–3 and T_e 7 × 10^–3 Z ⁴ eV, respectively, at the time when maximum lasing gain appears. Significant improvement in gain performance of recombination X-ray lasers is predicted by using CPA ps pulse drivers.     

A study of the ϱ0 meson polarization in π− p reaction at 5 GeV/c

Experimental data on ⁰ meson polarization in ^– p reaction at 5 GeV/c are presented. Change in ⁰ meson polarization for mesons produced at smallP ² and atP ² 0·3 or produced in backward direction in CMS is demonstrated. Natural explanation of these phenomena is one pion exchange mechanism and mechanism of quark-antiquark annihilation as observed for ⁰ meson production in¯pp reactions.The authors are indebted to Ján Piút, Richard Lednický, V. S. Rumiancev and N. K. Koutsidi for discussions and critical comments.     

Enhancement of activated decay of metastable states by resonant pumping

We consider the effect of a high-frequency pumping cost on the escape rate of a classical underdamped Brownian particle out of a deep potential well. The energy dependence of the oscillation frequency(E) is assumed to be weak on the scale of thermal energy,_E(0)T(0)T/V₀ (0)[_E(0) is the derivative of(E) atE= 0,V ₀ is the barrier height,V ₀ T]. The quadratic-in- contribution to the decay rate is calculated in two different regimes: (1) for the case of resonance of the pumping frequency with the nth harmonic of the internal motion at an energye, when = n(e); (2) for a rollout region of the basic resonance near the bottom of the potential well, when ¦-(0)¦ and is the damping coefficient. In the latter case the absorption spectrum and the enhancement of the decay rate are calculated as functions of two reduced parameters, the anharmonicity of the potential,v _E (0)T/, and the resonance mismatch, [ —(0)]/. It is shown that the effect of the pumping increases with diminishing ¦v¦ and at small v is proportional tov ^–1. In this regime, the dependence on is stepwise: the pumping contribution is large for v > 0 and small for v < 0. In the frame of our theory, the decay rate is invariant against the simultaneous alternation of the signs of andv. The spectrum of the energy absorption has the standard Lorentzian shape in the absence of anharmonicity,v=0, and with increasing of ¦v¦ shifts and widens retaining its bell-shape form.     

The lattice covering time problem for sites visitedk-times

The problem of the covering time for sites visitedk-times is defined as the mean time taken by a random walk to visit each site of a lattice at leastk times. We performed the investigation using Monte Carlo simulations over one dimensional lattices, ofN sites, with periodic boundary conditions. Two different regions are investigated:Nk1 andkN1. In the former region, we obtain a behaviour of the typet _k/t ₁=a –Bk ^–0.35+A(k)N ^–0.75, (a <1.6). In the latter region we obtain two possible behaviours:t _k k ^0.95 andt _k k(lnk)^–0.5. Two formulas which have a very close behaviour.     

Theory of percolation in fluids of long molecules

We use the reference interaction site model (RISM) integral equation theory to study the percolation behavior of fluids composed of long molecules. We examine the roles of hard core size and of length-to-width ratio on the percolation threshold. The critical density _c is a nonmonotonic function of these parameters exhibiting competition of different effects. Comparisons with Monte Carlo calculations of others are reasonably good. For critical exponents, the theory yields =2=2 for molecules of any noninfinite lengthL. WhenL is very large, the theory yields _cL ^–2. These predictions compare favorably with observations of the conductivity for random assemblies of conductive fibers. The threshold region where asymptotic scaling holds requires the correlation length (/ _c ) ^–v to be much larger thanL. Evidently, the range of densities in this region diminishes asL increases, requiring that density deviations from _c be no larger thanL ^–2. Otherwise, crossover behavior will be observed.     

Low-temperature properties of amorphous (Mo1−x Ru x )0.8P0.2 alloys

Amorphous (Mo_1–x Ru _x )_0.8P_0.2 alloys (0.3x0.7) have been investigated with measurements of the specific heatC and thermal conductivity . Also the superconducting properties (critical temperatureT _c and upper critical field) have been determined. Well belowT _c , all alloys show the familiar behavior known for glasses, i.e.CT ⁿ and T _m withn1 andm2 which is attributed to tunneling states (TLS). The largeT _c allows an unambiguous determination of the coefficients ofC and . Compairing our data with literature data, we find no correlation between the TLS density of states and the glass temperature or crystallization temperature, as opposed to insulating glasses where such a correlation appears to exist. The unusual annealing behavior found previously in amorphous Zr–Ni and Zr–Cu, which was attributed to a change in the TLS relaxation-time spectrum, is confirmed in the present work.     

Temperature dependence of the spinS=1/2 fluctuation rate and the low-spin versus high-spin transition in native chloroperoxidase

We analyzed two preparations of native, low-spin ferric chloroperoxides as a function of temperature with the following results. (i) The spin lattice transition rateW(T) is relative slow, following a power lawW=0.035 (T/K)^4.98 (rad/s) for one of the samples. (ii) The quadrupole splitting is strongly temperature dependent, dropping from ¦E _Q¦ 2.94 mm/s at 100 K to 2 mm/s at 250 K. (iii) Starting at 190 K, the low-spin heme iron in frozen adqueous solution converts reversibly to a high-spin form, reaching 40% high-spin at 250 K. The two forms appear to be in thermal equilibrium, (iv) Optical data indicate that in a 70% glycerol glass, the conversion starts at lower temperature and reaches 50% highspin at 190 K.Supported in part by GM 49513 and GM 16406.     

A Low-Noise 230 GHz SIS Receiver for Radio Astronomy Employing Untuned Nb/AlOx/Nb Tunnel Junctions

A heterodyne receiver based on a 1/3 reduced height rectangular waveguide SIS mixer with two mechanical tuners has been built for astronomical observations of molecular transitions in the 230 GHz frequency band. The mixer used an untuned array (R_nC_j3, R_n70 ) of four Nb/AIOx/Nb tunnel junctions in series as a nonlinear mixing element. A reasonable balance between the input and output coupling efficiencies has been obtained by choosing the junction number N=4. The receiver exhibits DSB (Double Side Band) noise temperature around 50 K over a frequency range of more than 10 GHz centered at 230 GHz. The lowest system noise temperature of 38 K has been recorded at 232.5 GHz. Mainly by adjusting the subwaveguide backshort, the SSB (Single Side Band) operation with image rejection of 15 dB is obtained with the noise temperature as low as 50 K. In addition, the noise contribution from each receiver component has been studied further. The minimum SIS mixer noise temperature is estimated as 15 K, pretty close to the quantum limit v/k11 K at 230 GHz. It is believed that the receiver noise temperatures presented are the lowest yet reported for a 230 GHz receiver using untuned junctions.     

Large third-order nonlinear optical properties of C70 fullerene in the infrared regime

The nonlinear third-order optical susceptibility of C₇₀ in a toluene solution is measured for the first time by the method of degenerate four-wave mixing using 10 ns laser pulses at 1.06 m. The third-order susceptibility X _in ⁽³⁾ is 5.6×10^–12 esu for a C₇₀ toluene solution at a concentration of 0.476 g/l. The correspondent magnitude of the hyperpolarizability ₁₁₁₁ of the C₇₀ molecule is 1.2×10^–30 esu which is in a good agreement with the prediction given by the model of a free electron in a spherical box.     

Self interstitial trapping at Hf impurities in Niobium

After electron irradiation at 15 K of Niobium doped with¹⁸¹Hf the trapping of defects at the Hf impurities was observed by means of the perturbed angular correlation method. The results are interpreted as the formation of Hf-Nb mixed dumbbells which are formed at 5 K and are stable up to 80 K.     

Zero value of the Schwarzschild mass for an asymptotically Euclidian system of gravitational waves

A class of the asymptotically Euclidian space-times is shown to exist for which the Schwarzschild mass is equal to zero. The coordinate atlases of these space-times satisfy two additional conditions: _k (-gg ^0k )=0 and _ik ⁰ _0g ^ik - _ik ^k _0g ⁰ⁱ =0. In aT-orthogonal metricgs ²=g ₀₀ dt ² -g dx dx these conditions take a simple form: ₀(detg )=0 and (₀ g )(₀ g )=0.     

Theory of electron escape from the surface of liquid helium

We have studied the properties of an electron bubble close to the surface of liquid³ He, by using a Density Functional approach. We find that up to an electron-surface distanced ₀ 23 Åthe bubble is stable, while at smaller distances it becomes unstable and bursts. A potential energy barrier /K _B 38°K for the thermal emission of electrons is obtained from our results, in agreement with experiments. Even when the electron-surface distance is larger thand ₀, however, tunneling through the surface layer dominates the electron escape probability. Large deviations of the electron potential energy from its ideal value are found close to the surface. These deviations have a profound effect on the calculated decay rates of the tunneling curent, which are much smaller than those obtained previously and in semi-quantitative agreement with experiments.