P. Müller

Low temperature irradiation of Nb3Sn and V3Si with high energy sulphur ions

Abstract The critical temperature Tc, the transition width To and the residual resistance R0 of Nb3Sn and V3Si have been measured as functions of the fluence of 20 MeV sulphur ions below 20 K and isochronal annealing up to 290 K. The temperature dependence of the resistivity is reported for samples with different irradiation-induced states of damage. Tφ and the thermal part of the resistivity are unambiguously correlated to the Tcversus fluence behavior. At high fluences the Tc of Nb3Sn again shows a minimum as a function of fluence, as reported earlier for oxygen irradiation. The temperature coefficient of the resistivity changes from positive to negative when the Tc of Nb3Sn passes through the minimum. A qualitatively different annealing behavior of Tc and R0 is observed for samples irradiated to Tc. values before and after the minimum. Highly damaged Nb3Sn shows an increase of R0 with increasing annealing temperature. The results are compared with A15 irradiations with different projectiles and with heavy ion irradiations of superconducting elements.

High disorder effects in superconducting Nb3Sn after heavy ion irradiation at low temperature

Abstract Significant decreases of transition temperatures, transition measured after irradiation with oxygen ions (25 MeV) at temperatures

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.

Superconductivity of Nb3Sn, Nb, V, Pb and Sn after low temperature irradiation with 25 MeV oxygen ions

The critical temperature Tc and transition width Tϑ of the superconductors were measured as functions of fluence and subsequent annealing. For the transition metals Nb, V and the nontransition metals Pb, Sn radiation induced resistivity ΔρB and damage rates were determined up to high fluences at irradiation temperatures T ⩽ 20 K and T ⩽ 7.2 K, respectively. Measurements of the critical current density jc(B, T) of 5 μm Nb3Sn samples (CVD) were made and Bc2, the volume pinning force density PV and scaling law were deduced. The Tc levelling off on Nb3Sn at high fluences was especially studied. In Nb an enhancement of Tc was observed and both Nb and V showed a significant broadening of Tϑ in heavily damaged state. Pb and Sn are discussed with respect to smoothing out of the anisotropy and to changes of the electron-phonon coupling strength α2 (w) and phonon spectrum F(w). Different behaviour of Tc vs. ΔρB between irradiation and annealing was observed in Nb, V and Pb, but not Sn.

Effects of low temperature irradiation with heavy ions on superconductor niobium

To study the effects of heavy ion irradiation at low temperature on type II superconductor Nb, the transition temperatureTc, the normal state residual resistivityρB, the transition widthΔTph using oxygen ions of 25 MeV and subsequent thermal annealing were measured. The samples were held at temperatures <20 K during irradiation in a cryostat for in situ measurements. The maximum oxygen fluence was about 2·1015 cm−2 corresponding a relatively high defect concentration. The heavy ion irradiation experiments are described. The critical temperatureTc decreases with increasing residual resistivityρB. In agreement with the theory and experiments, the gap anisotropy parameter is 〈a2〉=0.008, subsequent annealing shows a hysteresis ofTc versusρB. The resistivity saturation value ΔρBS = 2.55 μΩ cm was obtained and different recovery stages were found. Significant broadening of transition width during irradiation was observed.Tc andΔTph anneal to 60% in the temperature interval of (60–90) K. Oxygen induced effects as a simulation method of high neutron damage are compared with irradiation measurements using neutrons and deuterons.

Hydrogenated and irradiatedA15 Nb3Sn layers — Preparation, Rutherford scattering analysis, resistivity and superconductivity

Niobium films on sapphire were reacted in tin-vapour to Nb∼3Sn with resistance ratiosR(297 K)/R(18.3 K) up to 6 and resistively measured superconducting transition temperaturesTc up to 17.93 K. The composition Nb3+zSn1−zHx of electrolytically hydrogenated samples was determined depth dependent by Rutherford backscattering of 30 MeV32S and simultaneous detection of recoiled protons. Considerable concentration gradients in the thin layers (≈0.27 μm) were detected. The increase of resistivity ϱ with hydrogen content and the change in the temperature dependence of ϱ is analyzed. A correlation betweenTc and ρ0=ρ is found: An increase of ΔTc=0.2 K at ρ0≈25μΩcm andx≈0.03 is followed by a drastic decrease toTc<1.1 K at ρ0≈80μΩcm andx≈1. TheTc vs. ρ0 andTc vs. ϱ(T) characteristic correlations are different from “universal” irradiation or preparation induced correlations. The discrepancies can be interpreted by a stiffening of phonon modes and a band-shifting caused by the hydrogen.

Irradiation of copper with 25 MeV oxygen ions-defect production and annealing

Abstract Results of damage rate measurements and an isochronal annealing program from 15K up to room temperature after heavy ion irradiation of copper are reported. Irradiations were performed at temperatures of the recovery stages I (20 K) and I1 (76K, 90 K). The fluences of the low temperature irradiation are the largest reported in literature, resulting in defect concentrations up to 3.5·10−3 or to a radiation induced resistivity increase of 0.70 μΩcm. The damage rates can be explained by the generally accepted models of radiation damage derived for electron irradiation. In stage I1 extended trapping models have been used. The differential recovery spectra reveal all substages of stage I reported after electron irradiations. Substage IE shows to be strongly dose dependent, as well as stage III. The defect retentionf, at the end of stage I is between the values for electron and fast neutron irradiations, and clearly higher than after irradiation with α-particles.

Heavy ion irradiation of super conductor NbTi at low temperature and high fluences

Abstract Transition temperature, critical current densities and residual resistivity of NbTi50 have been measured as functions of 25 MeV oxygen irradiations at temperatures

TRANSPORT PROPERTIES, ELECTRONIC DENSITY OF STATES AND Tc IN DISORDERED A15 COMPOUNDS

Publisher Summary In this chapter, the effects of defects are discussed to investigate the idea of universal behavior of A15 compounds and their transition into the metallic glass phase at high degrees of disorder. The main point of interest is not the study of defect structures but the well-defined variation of sample parameters by the irradiation-induced defects as a continuous sample preparation procedure of the same sample. Because of the high Rutherford dislocation cross section, to achieve well-defined variation of sample parameters, fast heavy ions are the best tool. Compared with the large changes after irradiation, the effect of annealing up to room temperature is only a few percent. In the chapter, the most remarkable support for the dominance of the electronic-density of states in determining Tc of A15 compounds is obtained by a calculation of Tc with the Eliashberg formalism.

ELASTIC BACKSCATTERING AND RECOIL DETECTION ANALYSIS USING A 6 MV TANDEM ACCELERATOR

The stoichiometry of composite materials has been measured as a function of depth. Target materials were sandwich structures, Al5s, borides, hydrides, amorphous films, pieces of a TOKAMAK wall, or semiconductors. Furthermore the sputtering yield of Au by 3 keV Ar+ ions was calibrated by Rutherfordbackscattering in UHV.