Doping Change in the Bi-2212 Superconductor Directly Induced by a Hard X-ray Nanobeam

Abstract

We describe the controlled use of a 17 keV X-ray synchrotron nanobeam to progressively change the oxygen doping level in Bi-2212 superconducting whisker-like single crystals. Our data combine structural and electrical information collected on the same crystals, showing a maximum change in the critical temperature Tc of 1.3 K and a maximum elongation of ∼1 Å in the c-axis length, compared to the as-grown conditions. Simulations of our experimental conditions by means of a finite element model exclude local heating induced by the X-ray nanobeam as a possible cause for the change in the doping level and suggest an important role of secondary electrons. These findings support the possible use of hard X-rays as a novel direct-writing, photoresist-free lithographic process for the fabrication of superconducting devices, with potential nanometric resolution and 3D capability.

Maskless X-Ray Writing of Electrical Devices on a Superconducting Oxide with Nanometer Resolution and Online Process Monitoring

“…4) demonstrate that the X-ray irradiation has driven the doping level of the trench regions out of the superconducting regime, thus forcing the superconducting current to flow along the c -axis and giving rise to a stack of IJJs. In agreement with our previous study 14 , this doping change should correspond to a local oxygen depletion that has also increased the normal state resistivity of the irradiated regions, as suggested by the increase of the total electrical resistance of the device (Fig. 4a).…”

“…Steady-state finite element model (FEM) simulations have previously highlighted that in similar experimental conditions the temperature increase is limited to ~23 K 14 . However, transient thermal effects induced by the intrinsically pulsed nature of synchrotron radiation, which could act on very small scales both in space (typical nano-probe size ≲ 100 nm) and in time (typical pulse duration ≲ 50 ps), could significantly modify this picture.…”

“…As already demonstrated in previous studies 26–28 , the non-stoichiometric oxygen content is also related to the critical temperature T c and the c -axis length. In particular, the progressive local oxygen depletion results in an elongation of the c -axis and in an increase of both the critical temperature and the normal state resistivity 14 . The possibility to follow on-line the variations of the sample resistance during the exposure to X-rays allows to stop the irradiation as soon as the desired local oxygen depletion is reached, facilitating a fine tuning of the final sample properties.…”

“…Our group already showed that a high-dose irradiation at 17 keV can affect both structural and electronic properties of Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) micro-crystals by modifying their oxygen content 14 . These results are particularly interesting since the existence of a layered structure along the crystal c -axis, consisting of alternating superconducting and insulating planes, induces the presence of intrinsic Josephson junctions (IJJs) in this superconducting oxide 15 .…”

“…However, since in Bi-2212 the electrical resistivity along the c -axis is about five orders of magnitude higher than in the ab -plane 21 , in order to exploit the intrinsic Josephson effect, the current has to be forced to flow along the c -axis by patterning the crystal by photolithographic processes or by Focused Ion Beam (FIB) etching 22 , thus introducing vacuum/oxide interfaces at some stage of the process to define the device geometry, which can be detrimental from the point of view of optoelectronics. However, the observation that focused hard X-rays are able to modify the crystal oxygen content 14 paved the way to the practical realization of a direct-write hard X-ray patterning method where only oxide/oxide interfaces are present, suggesting that a high enough X-ray dose could locally induce in the material an oxygen depletion large enough to drive it in a non-superconducting state. This idea has been recently demonstrated by fabricating a proof-of-concept IJJ device with this novel photoresist-free approach by means of synchrotron radiation, but, in spite of the nanometric sizes of the probe, only micrometric features have been obtained for the devices 23 .…”