A. Hofmann

Bio: A. Hofmann is an academic researcher from Max Planck Society. The author has contributed to research in topics: Irradiation & Electron beam processing. The author has an hindex of 4, co-authored 6 publications receiving 75 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.

Pair breaking effects of atomic disorder induced in YBa2Cu3O7 by electron irradiation

Abstract: Irradiation effects of electrons at very low temperatures (Tirr<40 K, 2.7 MeV) on the DC-susceptibility χDC above Tc of YBa2Cu3O7 have been studied by means of a SHE SQUID magnetometer. Contrary to nonirradiated stoichiometric samples which have a nearly temperature independent paramagnetic susceptibility, χDC, between 100°C and 300°C, specimens irradiated with doses of 4 × 1019 e-/cm2 show a distinct Curie-type behaviour due to the formation of local magnetic moments, superimposed on a temperature independent part. By annealing up to temperatures of 500 K recovering of the Curie-type contribution takes place in a similar way as that of Tc. The Pauli susceptibility remains constant during irradiation, indicating that there is, contrary to A15-superconductors, no significant reduction of the density of states at the Fermi level. The correlated annealing of Tc and of the Curie constant suggests a magnetic pair breaking mechanism due to radiation induced local magnetic moment.

The influence of the disorder induced by electron irradiation on Tc and magnetic susceptibility of YBa2Cu3O7

Abstract: The radiation dose dependence of Tc for sintered YBa2Cu3O7 irradiated with 2.7 MeV-electrons at very low temperatures (Tirr < 40 K) is recorded by AC magnetic susceptibility measurements. During irradiation, Tc shifts linearly with dose from 92 K down to 77 K at a dose of 3.8 · 10−19 e−/cm2. The shape of the transition is not significantly influenced by the irradiation. A big part of the radiation damage recovers during tempering up to RT with a distinct step at 180K. The high sensitivity to irradiation is not explainable only by the displacement of O-Atoms, but suggests that also Cu-sites of the CuOCuOchains are affected.

Reaction-rate theory: fifty years after Kramers

Abstract: The calculation of rate coefficients is a discipline of nonlinear science of importance to much of physics, chemistry, engineering, and biology. Fifty years after Kramers' seminal paper on thermally activated barrier crossing, the authors report, extend, and interpret much of our current understanding relating to theories of noise-activated escape, for which many of the notable contributions are originating from the communities both of physics and of physical chemistry. Theoretical as well as numerical approaches are discussed for single- and many-dimensional metastable systems (including fields) in gases and condensed phases. The role of many-dimensional transition-state theory is contrasted with Kramers' reaction-rate theory for moderate-to-strong friction; the authors emphasize the physical situation and the close connection between unimolecular rate theory and Kramers' work for weakly damped systems. The rate theory accounting for memory friction is presented, together with a unifying theoretical approach which covers the whole regime of weak-to-moderate-to-strong friction on the same basis (turnover theory). The peculiarities of noise-activated escape in a variety of physically different metastable potential configurations is elucidated in terms of the mean-first-passage-time technique. Moreover, the role and the complexity of escape in driven systems exhibiting possibly multiple, metastable stationary nonequilibrium states is identified. At lower temperatures, quantum tunneling effects start to dominate the rate mechanism. The early quantum approaches as well as the latest quantum versions of Kramers' theory are discussed, thereby providing a description of dissipative escape events at all temperatures. In addition, an attempt is made to discuss prominent experimental work as it relates to Kramers' reaction-rate theory and to indicate the most important areas for future research in theory and experiment.

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.