200 MeV silver ion irradiation induced structural modification in YBa2Cu3O7−y thin films at 89 K: An in situ x-ray diffraction study

Abstract

We report in situ x-ray diffraction (XRD) study of 200 MeV Ag ion irradiation induced structural modification in c-axis oriented YBa2Cu3O7−y (YBCO) thin films at 89 K. The films remained c-axis oriented up to a fluence of 2×1013 ionscm−2, where complete amorphization sets in. The amorphous ion tracks, the strained region around these tracks, and irradiation induced point defects are shown to control the evolution of the structure with ion fluence. Secondary electrons emanating from the ion paths are shown to create point defects in a cylindrical region of 97 nm radius, which corresponds to their maximum range in the YBCO medium. The point defects are created exclusively in the CuO basal planes of fully oxygenated YBCO, which has not been possible, by other techniques including low energy ion irradiation and thermal quenching. The point defects led to a faster decrease in the integral intensity of XRD peaks at very low fluences of irradiation (Φ≤3×1010 ionscm−2) than what can be expected from amorphous tracks. The radius of amorphous ion tracks, estimated from the fluence dependence of integral XRD peak intensity beyond this fluence, was found to be 1.9 nm. Both point defect and the strained region around amorphous ion tracks are shown to contribute to the increase in the c-parameter at 89 K. The full width at half maximum (FWHM) of XRD peaks arising mostly due to the strained region around the ion tracks showed an incubation effect up to 1012 ionscm−2, before increasing at higher fluences. Fluence dependence of FWHM gives the cross section of the strained region as 37.9 nm2, which is more than three times the cross section of the amorphous ion tracks.

Selective disorder in the CuO basal planes of YBa2Cu3O7−y by swift heavy ion induced secondary electrons

“…Ion tracks of diameter ranging from 4 to 10 nm utilizing different ion beams with S e greater than S eth have been reported in the literature [16,17]. Our in situ X-ray diffraction study [18] has revealed the diameter of the track as 4 nm for 200 MeV Ag ion irradiation on c-axis oriented YBCO thin films as is used in the present study. Increasing irradiation fluences increases the number density of ion tracks.…”

“…The elongation of the c-axis due to the resulting tensile strain however was too small to account for the observed T c decrease [16]. Further our in situ low temperature XRD study revealed that the effect of strain is noticeable only above an ion fluence 1 Â 10 12 ions cm À2 [18], whereas we observe a large rate of T c decrease (4.3 Â 10 À10 K/ions cm À2 ) even at very low fluences (u 6 5 Â 10 8 ions cm À2 ). Also, the strain surrounding the ion tracks is expected to be permanent, since the amorphous tracks once induced do not anneal out at 297 K. On the contrary, as discussed in the next section, the T c and normal state resistivity obtained after irradiating the films at 79 K, tend to recover to their pre-irradiation values on annealing at 297 K. The annealing characteristics of the T c and the normal state resistivity in fact suggest that the defects, which are created along with ion tracks due to SHI irradiation at low temperatures, are basically point defects.…”

“…To understand the origin of the permanent defects in the phase II region, we note that the lower density of amorphous material in the ion track produces radial stress on the surrounding crystalline region. The diameter of the strained region around the amorphous track was found from in situ X-ray diffraction study on irradiated YBCO thin film at low temperature to be 7 nm as compared to the track diameter of 3.9 nm [18]. Though origin of the phase II region is not known, we ascribe it to the strained region around the latent tracks due to the following reasons: (i) The fluence (1 Â 10 12 ions cm À2 ) where phase II region emerges in the present study coincides with the incubation fluence beyond which the full width at half maximum of XRD lines tend to increase [18].…”

“…The maximum energy, E 0 of the SE produced from 200 MeV Ag ion irradiation in YBCO is thus $4.1 keV. The minimum energy required for an electron to dislodge even the most loosely bound oxygen (the oxygen in the CuO chains) by elastic collision in YBCO is about 20 keV [18,23,33]. Thus, in contrast to the case of irradiation by high energy electrons [21,34], neutrons [35] and low energy ions [21], where damage is caused by elastic scattering process, the low energy SE cannot create damage in YBCO by elastic knock-on process [33,36].…”

Effects of electron beam irradiation on the superconducting properties of YBCO thin films

“…A more precise analysis for the 2θ angle of the (006) peaks of the XRD patterns show that the (006) peak shifts to the low angle as the dose of EB irradiation increases (see Fig. 3), which indicates the change of the c parameter of YBCO [21].…”