R. Henes

Bio: R. Henes is an academic researcher from Max Planck Society. The author has contributed to research in topics: Electron beam processing & Irradiation. The author has an hindex of 1, co-authored 1 publications receiving 31 citations.

The effect of low-temperature electron irradiation on the transition temperature of YBa2Cu3O7−δ superconductors

Abstract: Measurements of the complex susceptibility χ=χ′−iχ″ of electron-irradiated YBa2Cu3O7−δ show a strong influence of the electron irradiation dose, Φ·t on the transition temperatureTc. For irradiation doses of Φ·t=2.2·1019 e−/cm2 we find a damage rate of ΔTc/Δ(Φ·t)=−1.6·10−19 K/(e−/cm2). It is assumed that the decrease ofTc is mainly a bulk effect due to the production of atomic defects like vacancies and interstitials in the Cu−O−Cu chains and in the basal planes of the unit cells.

Effect of particle-induced displacements on the critical temperature of YBa2Cu3O7−δ

Abstract: The particle‐induced depression of the superconducting critical temperature Tc of YBa2Cu3O7−δ is shown to be directly proportional, over seven orders of magnitude, to the nonionizing energy deposited in the lattice by primary knock‐on atoms displaced by incident electrons, protons, and heavy ions. It is concluded that ΔTc is proportional only to the average number of defects produced and can therefore be predicted for any particle, energy, and fluence from a calculation of the nonionizing energy loss.

Increased flux pinning upon thermal-neutron irradiation of uranium-doped YBa/sub 2/Cu/sub 3/O/sub 7/

Abstract: To assess fission-fragment damage as flux-pinning centers, sintered compacts of YBa/sub 2/Cu/sub 3/O/sub 7/ were prepared with uranium additions of 150 and 380 atomic ppm and exposed to thermal neutron fluences of 4.3/times/10/sup 17/, 1.2/times/10/sup 18/, and 4.0/times/10/sup 18//cm/sup 2/. Magnetic hysteresis measurements were made as functions of temperature at fields up to 2.5 T. The hysteresis at 1 T for the sample containing 150 ppm uranium increased upon an irradiation of 1.2/times/10/sup 18//cm/sup 2/ by 3.7 times at 4.5 K, 20 times at 63 K, and 8.3 times at 77 K. Much smaller increases in magnetic hysteresis were observed in undoped samples exposed to thermal neutrons. Critical-current densities were calculated from the hysteresis observed at 1 T using the critical-state model, assuming the currents to be restricted to the grains (/approx/5 /mu/m radius). For the 150 ppm, 1.2/times/10/sup 18//cm/sup 2/ sample, the 1-T intragranular critical-current densities obtained are 1.5/times/10/sup 7/, 1.0/times/10/sup 6/, and 1.4/times/10/sup 5/ A/cm/sup 2/ at temperatures of 4.5, 63, and 77 K, respectively. The critical temperatures by an ac susceptibility technique showed that the 4/times/10/sup 18//cm/sup 2/ irradiation lowered the onset critical temperatures only slightly, from 91 to 90 K and from 91.5 to 89 Kmore » for the 150 and 380 ppm samples, respectively.« less

Observation of flux penetration in YBa2Cu3O7−δ superconductors by means of the magneto-optical Faraday effect

Abstract: The penetration of the Shubnikov phase into both single-crystal and sintered YBaCuO specimens has been observed by means of the magneto-optical Faraday effect using the high Verdet constant in thin evaporated films of a mixture of EuS and EuF 2 . This method allows the direct observation of the flux motion. After cycling magnetic field from zero, the trapped flux structure in the single crystal consists of large domains which are related to the sample shape, whereas in sintered specimens only a part of the total number of the grains transforms into the Shubnikov phase.

A Review of the Magnetic Relaxation and Its Application to the Study of Atomic Defects in α-Iron and Its Diluted Alloys.

Abstract: This review presents a comprehensive survey on intensive studies performed during the last decades on point defect reactions on α-iron (a-Fe) and its diluted alloys. Our intention is to give an actual account of the knowledge accumulated on this subject, as it has been obtained predominantly by means of the magnetic after-effect (MAE) spectroscopy. After a concise introduction into the theoretical and experimental fundamentals of this technique, the main concern is focused on the presentation and detailed discussion of the MAE spectra arising - after low-temperature electron (e - )- or neutron(n)-irradiation and subsequent annealing - in: (i) high-purity α-Fe and α-Fe doped with (ii) substitutional solutes (like Ni, V, Al, Cu, Ti, Be, Si, Mn,...) or (iii) interstitial solutes (like O, H, C, N). During the course of systematic annealing treatments, these respective spectra undergo dramatic variations at specific temperatures thereby revealing in great detail the underlying intrinsic reactions of the radiation-induced defects, i.e., reorientation, migration, clustering, dissolution and finally annihilation. In alloyed Fe systems the corresponding reaction sequences are even multiplied due to additional interactions between defects and solute atoms. Most valuable information concerning formation-, dissociation- and binding enthalpies of small, mixed clusters (of the type C i V k , N i V k ; i, k ≥ 1) has been obtained in high-purity α-Fe base material which, after charging with C or N, had been e -irradiated. Concerning the basic recovery mechanisms in α-Fe, two complementary results are obtained from the analysis of the various systems: (i) in high-purity and substitutionally alloyed α-Fe the recovery in Stage-III (200 K) is governed by a three-dimensionally migrating (H I M = 0.56 eV) stable interstitial (dumbbell); (ii) following the formation and dissociation kinetics of small clusters (C 1 V k , N 1 V k ) in interstitially alloyed α-Fe the migration enthalpy of the monovacancy must hold the following relation H N M (0.76 eV) < H C M (0.84 eV) < H V1 M . These results are in clear agreement with the socalled two-interstitial model (2IM) in α-Fe - a conclusion being further substantiated by a systematic comparison with the results obtained from nonrelaxational techniques, like i.e. positron annihilation (PA), which by their authors are preferentially interpreted in terms of the one-interstitial model (1IM).

The influence of a point defect structure on the magnetic properties of YBa2Cu3O7-δ polycrystals

Abstract: Extensive measurements of magnetic hysteresis loops of YBa 2 Cu 3 O 7- δ polycrystals have been carried out presenting a survey of the magnetic properties of this material. The results are discussed within the framework of the critical state model in order to determine the volume pinning force F p and the critical current density j c . Two different point defect structures induced by low temperature electron irradiation and successive annealing experiments were examined. Although both treatments mainly change the oxygen arrangement, only the electron irradiation caused an enhancement of the critical current.