Browsing by Author "Winzer, K."

We compare full potential LDA band calculations of the Fermi surfaces areas and band masses of MgB2 and ZrB2 previously reported (1,3-5) and new dHvA data. Discrepancies in areas in MgB2 can be removed by a small shift of sigma bands relative to pi bands. Comparison of effective masses lead to orbit averaged el-ph coupling constants lambda(sigma)=1.3 and lambda(pi)=0.5, whereas for ZrB2 only weak el-ph coupling with lambda < 0.3 is found. The ARPES data(6) can be also well described by the LDA showing the presence of surface states.

Practical implementation of thermoelectric (QVD) detectors requires cryogenic thermoelectric sensors with a high figure of merit. For this requirement there can be different solutions: thin films, bulk materials and whiskers. Initial prototype QVD devices used Au-Fe thin film sensors. Another class of materials, lanthanum (cerium) hexaborides La(Ce)B-6, has higher Seebeck coefficient at cryogenic temperatures, and therefore one can expect higher figures of merit for devices based on these materials. To utilize La(Ce)B6 crystals with different La-Ce content as sensor elements we investigated their kinetic properties. Then we explored prototype devices based on bulk single-crystalline sensors. Subsequently, we manufactured and successfully tested a candidate single-photon detector device with a sharp-end hexaboride sensor and small bismuth absorber. (C) 2003 Elsevier B.V. All rights reserved.

The Fermi surface of PrNi5 has been studied by the measurements of the de Haas-van Alphen (dHvA) effect at temperatures between 0.3 and 1.8 K in magnetic fields up to 12 T. Two dHvA frequencies have been obtained. The electronic structure of PrNi5 was calculated using the full potential linearized augmented plans wave method. Five sheets of the Fermi surface and the multiple extremely cross sections were determined. First and second sheet have a hole-like structure. The agreement between theory and experiment is obtained by. a small downward shift (approximate to 0.1 eV) of the Fermi energy which is probably due to an underestimation of thr rule of 4f electrons.

We report on measurements of resistivity and magnetoresistivity, Hall effect and upper critical field on normal and superconducting heavily boron-doped diamond layer produced by microwave plasma assisted chemical vapour deposition. The magnetoresistivity shows a characteristic change as a function of the boron concentration with strong evidence for a closed Fermi surface for superconducting samples. Although experimental and theoretical results favour a conventional, weak coupling electron-phonon interpretation of the superconducting mechanism, the observed dependence of the transition temperature T-C on the hole concentration cannot be consistently described by the conventional theory. (c) 2005 Elsevier B.V. All rights reserved.

Temperature-dependent measurements of the resistivity of RuIn3 single crystals show a semiconducting behaviour, in contrast to previously published results ( Roof et al 1986 Powder Diffr. 1 20; Pottgen 1995 J. Alloys Compounds 226 59). In the high- temperature range the semiconducting gap was measured to be 0.4 - 0.5 eV. We observe an anisotropy of the resistivity along [ 110] and [ 001] orientations of the tetragonal single crystals. At low temperatures two activation energies of impurities were estimated to 1 and 10 meV. The temperature dependence of the specific heat and the band structure calculations also show a semiconducting behaviour of RuIn3.

High quality samples and absence of superconductivity down to 40 mK make of ZrB2 the best normal state reference system for the superconducting isostructural MgB2. Actually, the question of pairing has to be focused on the electron-phonon interaction in the normal state. After presenting the resistivity measurements of ZrB2, we explain the details of the Bloch-Gruneisen and Einstein models used to deduce the first results. We then compare experimental de Haas-van Alphen effect data with theoretical Fermi surfaces to present additional results on electron quasi-particle renormalization. The estimations reveal an isotropic and negligible coupling constant of in average [lambda(tr)] = 0.145. The contribution of the coupling to the optical phonon modes is 0.082, in contrast to the known larger coupling of 0.283 [ 3] to the E-2g phonon mode in MgB2.

We report the observation of the heavy-fermion behavior in the cerium-based bulk metallic glasses with an inherent strong structural disorder. The heavy-fermion behavior in the glasses with 4f electrons shows unique features not found in crystalline materials. The distribution of the f levels due to structural disorder produces internal variation from the Kondo regime to the valence fluctuation regime, with the heavy-fermion behavior at the crossover. The glasses might provide a model system to investigate some longstanding issues with electron strong correlation in complex solids.

Abstract On sandy ocean beaches, amongst many irregularly shaped pebbles, a surprisingly large number of pebbles that are nearly perfect ellipsoids may also be found. These elliptical stones are characterized by the density $\rho$ ρ of the mineral and by the length of their symmetry axes ( a , b , c ) with ( a  >  b  >  c ). The grinding process which forms the ab-plane of the ellipsoid relates to the rotation around the axis of the greatest moment of inertia ( c -axis) in the surf. In a recent paper by fairly simple physical arguments, it was shown that by this rotation around the c -axis the ellipticity $\varepsilon$ ε always increases. This is in agreement with the empirical observation that on sandy beaches of the oceans no spherical or disc-like pebbles can be found. If the pebble velocity during the rotation becomes too large, then hopping of the pebble sets in. This is caused by the fact that the centre-of-mass acceleration during fast rotation exceeds the gravity acceleration g . Since greater forces occur for jumping than for rolling, the hopping can have a strong influence on the b / a -ratio of the elliptical pebble. In this work, it will be shown that the b -axis is worn away more than the a -axis by erosion with jumping. In a laboratory study with a model of an elliptical cylinder, the theoretical results were checked and partially confirmed.

A thermoelectric generator, operating in a cryostat at liquid helium temperatures, is described. Energy to the generator is supplied via an external laser beam. For this prototype device the associated heat load at permanent operation is comparable with the heat load associated with power delivery via metallic wires. Estimates indicate that still better performance can be enabled with existing thermoelectric materials, thereby far exceeding efficiency of traditional cryostat wiring. We used a prototype generator to produce electric power for measuring critical currents in Nb3Sn-films at 4 K. (C) 2005 American Institute of Physics.

The complex magnetic ordering of TbNi2B2C and Tb0.4Y0.6Ni2B2C single crystals has been investigated by AC-susceptibility and magnetization measurements. Metamagnetic transitions and an anisotropic behavior within (a, b)-plane were revealed for TbNi2B2C. Alloying with Y reduces the critical field of antiferromagnetic to paramagnetic transition, eliminates the metamagnetic transitions and induces superconductivity below 4.1 K. (C) 2000 Elsevier Science B.V. All rights reserved.

In order to investigate the interaction between superconductivity and magnetic ordering resistivity, magnetisation measurements were performed on DyNi2B2C single crystals for T = 0.05-15 K and for B = 0-5 T in the crystallographic ab-plane. Measurements of the magnetisation with a torque cantilever magnetometer give strong evidence for metamagnetic states built of different ordered ferromagnetic domains. For B parallel to [100]. an anomalous large hysteresis effect of the metamagnetic transitions and also of the superconducting transitions were observed in the low-temperature range (T < 2 K). In some extreme cases, the metamagnetic state can be frozen after a field cycle sweeping up to 2 T and down to zero at 50 mK. In this case, superconductivity is totally suppressed. The superconducting state can be restored in two ways: Either with increasing temperature, or by applying a small field with the opposite field direction. (C) 2001 Elsevier Science B.V. All rights reserved.

'QVD' detectors are based on thermoelectric heat-to-voltage (Q-->V) conversion and digital (V-->D) readout. In theory, they are competitive with superconducting tunnel junction detectors and transition edge sensor, devices. We analyze the performance of the QVD detectors with different design architectures. It is concluded that the detectors with lanthanum-cerium hexaboride sensors can be very fast: up to 100 MHz counting rates for UV photons. In addition to traditional astrophysical applications, these detectors can be applied to the tasks of quantum computing and communication. (C) 2003 Elsevier B.V. All rights reserved.