Neutron diffraction study of crystal structure and antiferromagnetic ordering in the heavy fermion compound CePd2Al3

Nuclear and magnetic structures of an annealed polycrystalline sample of the heavy-fermion compound CePd₂Al₃ prepared by arc-melting were investigated by neutron powder diffraction. The chemical structure corresponds well to the ordered hexagonal PrNi₂Al₃-type structure. The antiferromagnetic structure of CePd₂Al₃ with an ordered magnetic moment _Ce=0.47(2) _B at saturation is remarkably similar to that in the heavy fermion superconductor UPd₂Al₃. The additional incommensurate magnetic structures reported previously both for UPd₂Al₃ and CePd₂Al₃ are not observed in the present sample of CePd₂Al₃. At 1.4 K the magnetoresistivity of CePd₂Al₃ measured up to 14 T indicates only one field-induced phase transition at 3.0 T.     

Frobeniusautomorphismen endlicher Gruppen

Ohne ZusammenfassungHerrnGottfried Köthe zum 60. Geburtstag am 25. 12. 1965 gewidmet     

Dynamics of vector spin glasses with random anisotropy

The dynamics of vector spin glasses with additional random anisotropy is investigated in the mean field approximation. We find a cross-over of the upper critical line from a behaviour of Heisenberg spins with a field-temperature dependence , for large fieldsH, to Ising like behaviourT _f –TT ^2/3 , for small fields and fixed anisotropy, in agreement with results of Kotliar and Sompolinsky. Here, andT _f are characteristic spin glass temperatures. In addition, one has a second line with reversed behaviour which presumably represents a cross-over line from weak to strong non-ergodicity. The local transverse susceptibility _T () varies for large fields and 0 along the upper critical line as ^vT , with a critical exponentV _T = 1/2 – 11D/60 J, whereD andJ are the anisotropy and exchange coupling constants, respectively. On the Ising-like part of the upper critical line one has isotropic spin glass parameters,q _L =q _T , and susceptibilities, and a critical exponent, which is similar to that of Ising spins along the de Almeida-Thouless line.Dedicated to B. Mühlschlegel on the occasion of his 60th birthdaySFB Aachen-Jülich-Köln     

Constant time steady gradient NMR diffusometry using the secondary stimulated echo

A pulse sequence producing a second stimulated echo is suggested for the compensation of relaxation and residual dipolar interaction effects in steady gradient spin echo diffusometry. Steady field gradients of considerable strength exist in the fringe field of NMR magnets, for instance. While the absolute echo time of the second stimulated echo is kept constant throughout the experiment, the interval between the first two radiofrequency pulses is augmented leading to a modulation of the amplitude of that second stimulated echo by self-diffusion only. The unique feature of this technique is that it is of a single-scan/single-echo-signal nature. That is, no reference signals neither of the same pulse sequence nor of separate experiments are needed. The new method was tested with poly(ethylene oxide) melts and proved to provide reliable data for (time dependent) self-diffusion coefficients down to the physical limit (D approximately 10(-15)m(2)/s) when flip-flop spin diffusion starts to become effective.     

Singlet Fission in Carbene-Derived Diradicaloids

Herein, we present a new class of singlet fission (SF) materials based on diradicaloids of carbene scaffolds, namely cyclic (alkyl)(amino)carbenes (CAACs). Our modular approach allows the tuning of two key SF criteria: the steric factor and the diradical character. In turn, we modified the energy landscapes of excited states in a systematic manner to accommodate the needs for SF. We report the first example of intermolecular SF in solution by dimer self-assembly at cryogenic temperatures.     

Effect of Sulfation Route and Subsequent Oxidation on Derivatization Degree and Biocompatibility of Cellulose Sulfates

Sulfated cellulose (CS) represents an interesting biopolymer due to bioactivity comparable to heparin. However, use of CS for making surface coatings or hydrogels requires the presence of reactive groups for covalent reactions. Here, an approach is presented to oxidize cellulose sulfates for subsequent cross‐linking reactions with amino groups to form imine bonds. Cellulose is sulfated by direct sulfation or acetosulfation, followed by a M alaprade oxidation. The CS obtained is characterized by elemental analysis and ¹³C‐NMR spectroscopy. The resulting oxidized cellulose sulfates (oxCS) have different degrees of sulfation ranging from 0.79 to 1.13 and oxidation degrees from 0.18 to 0.34, but also different mass average molecular mass (M_W). Toxicity studies are carried out with mouse 3T3 fibroblasts exposed to aqueous solutions of oxCS. The results show that all oxCS are non‐toxic at lower concentrations (0.5 mg mL^?1), but with both increasing degree of oxidation and concentrations, toxic effects are observed particularly for acetosulfated and lesser for direct sulfated oxCS, which is related to a decrease in the M_W of the products. It is concluded that oxCS obtained by direct sulfation with M_W above 70 kDa may represent a biocompatible material for the applications suggested above.     

Study of hadronic events inpp collisions at\sqrt s = 62GeV and comparison with hadronic events ine + e − collisions

We present an analysis of minimum bias events from proton-proton collisions at \(\sqrt s = 7GeV\) in the CERN ISR. We remove the effects of both the leading protons and compare theB=0 mesonic residue of the events to the hadronic events of similar energy produced ine ⁺ e ^? collisions. This comparison is presented in terms of the standard jet-type analyses involving quantities such as sphericity and aplanarity. We find significant differences between these data and the data frome ⁺ e ^? annihilations. The data of this experiment are consistent with the predictions of a longitudinal phase space model.     

Production of vector and tensor mesons in proton-proton collisions at\sqrt s = 52.5 GeV

Inclusive production of ?⁰,f, andg ⁰ mesons and ofK _s ⁰ ,K ^*0 (892), ? andK ^*0(1430)mesons has been measured at <y>～2.6 and <p _T >～1.1 GeV/c in proton-proton interactions at \(\sqrt s = 52.5\) GeV. The negative particle from the two-body decays of these resonances were identified by a threshold Cerenkov counter and used for triggering. Starting from the measured differential cross section, total inclusive cross sections for the vector and tensor mesons were determined using various parametrizations for they andp _T dependence of the differential cross section. The experimental results are discussed in the framework of production models based on the parton picture. The strangeness suppresion factor λ=(0.30±0.10) due toSU(3) symmetry breaking of the quark sea is derived.     

Der Einfluß von Spinfluktuationen auf die Sprungtemperatur eines Zweibandsupraleiters

The superconducting transition temperatureT _c of a two band superconductor in the presence of magnetic and nonmagnetic impurities as well as exchange fields is calculated using a new (8 × 8)-matrix formalism for the electron Green's function. In particular we investigate the influence of spin correlations onT _c near a magnetic phase transition. It is shown that in the strong interband phonon coupling limit the system behaves essentially as a one band superconductor. In the weak coupling limit we find typical deviations from one band theories.     

Surface properties of Si from photoelectric emission at room temperature and 80 °K

The position of the Fermi level with respect to the energy bands at a semiconductor surface as well as changes in the work function can be determined from the energy distributions of photoelectrically emitted electrons. Prior studies involved the photoelectric yield spectrum and required assumptions concerning the photoelectric threshold; the present method is free of such assumptions. Measurements at room temperature indicate that the Fermi level lies 0.23 eV and 0.41 eV above the top of the valence band for degenerate p and n-type materials, respectively. These results confirm those of Allen and Gobeli¹). Cooling to 80 °K increases the work function of p-type material by 0.025 eV while that of n-type Si remains unchanged; the results show that the electron and hole gases in the surface states are degenerate. The density of surface states lies between 7 × 10¹³ and 10¹⁵ eV⁻¹ cm⁻². On the cesiated surface, the Fermi level lies 0.16 eV below the conduction band at room temperature and coincides with its bottom at 80 °K.