G. Saemann-Ischenko

Plastic deformation in SiO2 induced by heavy-ion irradiation

Abstract Samples of vitreous silica and quartz were irradiated by different high-energy heavy ions at various temperatures. The dimensions of the samples were measured at room temperature before and after irradiation with different fluences. In the low-fluence range vitreous silica exhibits a long-known compaction which saturates at a density increase of about 3%. In the high-fluence range vitreous silica is, like other amorphous materials, subject to radiation-induced plastic deformation: the dimensions perpendicular to the ion beam grow without tendency to saturation whereas the dimension parallel to it shrinks. The deformation yield per incident ion is the higher, the lower the temperature and the larger the electronic energy loss. The samples of quartz are amorphized in the low-fluence range and subsequently undergo plastic deformation in the same way as vitreous silica. A model describing this plastic deformation is proposed and compared to the experimental results.

Low temperature neutron irradiation of the high-Tc-superconductors YBa2Cu3O7 and Y1.2Ba0.8CuO4

Abstract Single phase YBa 2 Cu 3 O 7 and multiphase Y 1.2 Ba 0.8 CuO 4 have been irradiated with fast reactor neutrons up to fluences of φt = 9.4 × 10 18 cm −2 ( E > 0.1 MeV). With increasing neutron fluence, we find an enhanced decrease of T c , considerable broadening of the transition width and an increase of ρ ( T = 100 K).

Magnetization of superconducting YBa2Cu3O7 and Ni- and Zn-substituted samples

Abstract We report dc-magnetization measurements of YBa 2 Cu 3 O 7 and effects of substituting Cu by Ni and Zn. The differential susceptibilities dM/dH exhibit characteristic structur e, which we relate to the anisotropy of the lower critical fields H c1 ,⊥ c and H c1 ,∥ c . Kinks in the H c1 -curves are observed for all samples. Parabolic approximation of H c1 (T) leads to a second, characteristic temperature below T c , for which we discuss possible explanations.

A cems and absorption Mössbauer spectroscopy study of iron arachidate Langmuir-Blodgett films

Abstract Langmuir-Blodgett multilayers of ferric arachidate on silicon wafers have been investigated by means of conversion electron Mossbauer spectroscopy (CEMS) and absorption Mossbauer spectroscopy at different temperatures between room temperature and 4.2 K without and with an external magnetic field of 5 T. The Mossbauer spectra show Fe 3+ . A change of the relative line intensities in the measured spectra dependent on the observation direction is visible. It was found that the lines in the spectra are significantly broadened, which is due to a distribution of electric field gradients and isomer shifts. The reason for this is a slightly disordered iron environment. The quadrupole splitting is linearly correlated to the isomer shift. At low temperatures antiferromagnetic ordering has been observed and the orientations of the internal magnetic field and the electric field gradient in the crystal system have been determined. Due to its chemical structure this substance is an example of a quasi two dimensional system.

Preparation, Structure, Transport Properties and Photoemission Studies of the High-Tc-Superconductors YBa2Cu3O4-δ and La1-xSrxCuO4

Single-phase bulk-YBa2Cu3O9-δ was prepared by solid-state reaction under oxygen atmosphere as well as exclusively in air, observing a transition from semi- to superconducting behaviour depending on the preparation. Low temperature powder x-ray diffraction from 300K down to 10K was carried out to investigate the structure. The upper critical field is estimated to Bc2 (T=0) = 110T (Tc-midpoint), critical current densities of up to 180A/cm2 are determined at different temperatures in magnetic fields up to 7T. Photoemission experiments show, that grains are only coupled by semiconducting interfaces and reveal a strong hybridization between O-2p and Cu-3d states.

Superconductivity and electrical resistivity of amorphous Nb75Ge25 and Nb80Si20 after heavy ion irradiation at low temperature

Abstract Low temperature irradiation of thin films of Nb75Ge25 (Tc = 3.2 K) and Nb80Si20 (Tc = 4.7 K) with 20 MeV sulfur ions leads to an increase of Tc of about 0.5 K and a decrease of ϱ of about 1.5 to 3.5%. Annealing up to room temperature partly restores the initial values. Qualitatively the results can be explained by irradiation induced smearing of the structure factor, which is partially recovered by annealing.

Preparation, superconducting and normal state transport properties of granular, n-doped Ln2-xCexCuO4-δ, Ln = Nd, Sm

We report on superconducting and normal state transport properties of the electron-doped cuprates Ln2-xCexCuO4-δ with Ln=Nd, P Sm. Irreversible behaviour of bulk samples after applying a magnetic field at low temperatures is observed. This feature as well as the two step transition in R(T) and the `semiconducting behaviour of the polycrystalline samples above Tc is explained in terms of granularity. Phase-diagrams for both Nd2-xCexCuO4-δ and Sm2-xCexCuO4-δ are presented. There is evidence for a solubility limit of Ce-ions in Sm2CuO4 within the superconducting concentration range. We show results for the Hall-coefficient, which is negative all over the investigated temperature range. The preparation process has been monitored by means of X-ray analysis and TGA/DTA.

Properties of disordered Mo3Ge thin films with A15 structure

Abstract Heavy ion irradiation of A15 Mo3Ge with the low transition temperature Tc = 1.45 K raises Tc to ⪆ 6 K. For the first time in A15 compounds a Tc degradation (≈ 3 K) after having passed through a maximum is observed until near 3.5 K the saturation value is reached. The effects on Tc are interpreted by variations of the deduced (Hc2, ϱ) density of states at the Fermi level. This picture consistently explains the Tc degradation of the high-Tc A15 compounds.

Tailored defects in HTSC — What can we learn from swift heavy ion irradiation?

Abstract Since the discovery of superconductivity above liquid nitrogen temperature in copper oxide based compounds many experiments have been performed on the one hand to study their mixed state properties and on the other hand to check their technological applicability. Due to their well defined cylindrical shape latent particle tracks (also known as columnar defects) are a powerful tool to investigate the mixed state in high temperature superconductors (HTSC) as they provide optimized pinning centers. Therefore in this paper we will present experiments performed with HTSC samples (epitaxial thin films on SrTiO 3 substrates, single crystals and technical tapes) containing columnar defects. We will outline the influence of these linear defects on the resistive phase transition and on transport I–V curves of YBa 2 Cu 3 O 7−δ (123) thin films. All HTSC known up to now exhibit a pronounced structure-related anisotropy. Hence we present magnetic and transport measurements concerning the angular dependence of the critical current in various irradiated HTSC compounds. Furthermore experiments with technical tapes of (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10 (Bi2223) will be shown giving an overview of problems concerning technological applications. Finally in irradiated and nonirradiated Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) single crystals the flux line lattice is visualized via the high resolution Bitter decoration technique.

Electronic structure of Nd2CuO4−yFy and Nd2−xCexCuO4−δ studied by photoemission spectroscopy

Abstract Resonant photoemission spectroscopy was used to identify the various features in the valence band spectra of these n-type superconducting materials. On doping no significant changes in the occupied density of states are observed, except the narrow F 2p-peak in the F-doepd system. Core-level spectroscopy revealed, that CE is tetravalent in these compounds, and that the trivalence of Nd is not affected by the presence of F. In both cases a significant, doping-dependent admixture of Cu 3+ to the Cu 3d 9 -states is observed.