In a new preprint posted in arXiv on June 5, the IISc team has provided evidence of signatures of critical current — the current at which the superconductor is no longer stable and becomes resistive. The critical current data is a strong indicator of superconductivity.

Further evidence of possible superconductivity in gold-silver nanostructures has been presented in a preprint by a team of researchers led by Prof. Arindam Ghosh from the department of Physics from the Indian Institute of Science (IISc). The preprint has been posted on June 5 in the pre-print repository arXiv. Pre-prints are yet to be peer-reviewed and published in a journal.

In a revised preprint posted on May 21 in arXiv, the IISc team found the material exhibiting superconductivity. “Two of the most important properties of superconductivity are diamagnetism and zero resistance. These two were seen in the material we studied. They seem to suggest that the material becomes superconducting below a certain temperature (286 K or 13 degree C). And it can go up to 70 degree C,” says Prof. Ghosh. “At 286 K we have seen clear transition from a normal state to a superconducting state. This is more than anyone has reported.”

Still, the May 21 preprint did not have data on current-voltage characteristics and the evidence of critical current.

The article posted on June 5 fills that lacuna as it deals with current-voltage characteristics in gold-silver nanostructures with regard to superconductivity.

What typically happens in current-voltage characteristics is that when the current is increased the voltage remains zero and at a critical current the voltage suddenly increases and superconductivity is destroyed.

“The current-voltage characteristics was one of the important data that was not presented in the earlier paper. This study shows the material has some signatures of critical current — the current at which the superconductor is no longer stable and becomes resistive,” Prof. Ghosh says.

The IISc team observed that at a critical temperature of 160 K (-113.15 degree C) and critical current of little less than 10 milliampere the voltage suddenly shoots up and the gold-silver nanostructures no longer exhibit superconductivity as resistance increases rapidly. A superconductor is one which conducts electricity with zero resistance to the flow of electrons.

“The data look interesting but whether they confirm superconductivity is not sure. The data is not unambiguous,” says Prof. Pratap Raychaudhuri from the Superconductivity Lab at the Tata Institute of Fundamental Research (TIFR), Mumbai.

Referring to the figure on current-voltage characteristics, Prof. Raychaudhuri says that though the voltage increases sharply at around 10 milliampere critical current, the voltage is not zero before the sharp increase is seen. He compares the IISc work with another preprint posted by a team led by Prof. Subhankar Bedanta from the National Institute of Science Education and Research (NISER), Bhubaneshwar.

“The current-voltage characteristics [in Prof. Bedanta’s paper] show clear evidence of superconductivity,” says Prof. Raychaudhuri. “At the critical current, the voltage is zero before it increases sharply.”

Prof. Raychaudhuri adds: “It would take more effort by the authors to convincingly show that the nanomaterial is indeed superconducting. The paper posted last year had very little data. Now, more data are available. Whether the data are correct or not can be settled only though scientific discourse — peer-reviewing and other groups reproducing it.”

Clarifying about the zero voltage Prof. Ghosh says, “At temperatures slightly below 161.3 K the voltage is zero up to 10 milliampere current. The voltage may increase at higher current, which was not tested. Similar behaviour was observed in metal whiskers where superconductivity is observed at a lower temperature.”

“At a lower temperature the voltage becomes zero. This further supports the original claim. The critical current data is a strong indicator of superconductivity,” emphasises Prof. Ghosh. “We observed critical current behaviour which is one of the important characteristics of traditional superconductors.”

Published in The Hindu on June 7, 2019