High density nonmagnetic superconducting cobalt in thin film

Metals possessing strong long-range magnetic order, as in antiferromagnetism or ferromagnetism, do not exhibit superconductivity [1]. This includes ferromagnetic transition metals Fe, Co and Ni. However, some elements, which do not show superconductivity under ambient pressure, become superconducting under higher pressure. Fe has been found to undergo a superconducting transition at pressures between 15 and 30 GPa with a critical temperature (Tc) of about 2 K [2]. Fe is nonmagnetic at this pressure. Superconductivity was not hitherto observed in Co and Ni under any condition. Recently, we have discovered a high-density nonmagnetic (HDNM) face-centered cubic (fcc) phase of Co in Co thin film with the help of many complementary techniques such as X-ray reflectivity (XRR), polarized neutron reflectometry (PNR), transmission electron microscopy (TEM) etc. [3]. As this phase of Co is nonmagnetic, it is prudent to search forsuperconductivity in this HDNM phase of Co. We have indeed discovered superconductivity in the high-density nonmagnetic Co thin film with a superconducting transition temperature (Tc) of ~9.5 K and a critical field (Hc) of ~ 35 kG [4]. The transition to the superconducting state has been detected by point-contact spectroscopy and drop in resistance in point contact measurements as well as in standard four probe resistivity measurement method. In 4-probe method the transition temperature was found to be ~5.4 K. The higher Tc obtained in point contact measurement is likely to be due to some unavoidable pressure in making the point contact. First-principles density functional theory calculations for this dense fcc phase of Co show that this phase is nonmagnetic and the estimated TC within the BCS theory is 0.3 K. A volume preserving strain in fcc Co is shown to result in anomalous softening of zone boundary phonons which couple strongly with electrons, and stabilize superconductivity at a relatively large temperature (>5 K). The value of TC may indeed be higher for other strain conditions.

Finally, I will briefly discuss some interesting results on switching behaviour of negative magneto resistance (MR) to positive MR and again negative MR upon ion irradiation in a Co/Cu multilayer sample. I will also discuss a little on the novel phenomenon of ion irradiation induced amorphization and recrystallization of Si at the same temperature and that is too at room temperature.

[1] D. R. Slocombe et al., Phil. Trans. R. Soc. A 373, 20140476 (2015).
[2] K. Shimizu et al., Nature 412, 316   (2001).
[3] Nasrin Banu, S. Singh, B. Satpati, A. Roy, S. Basu, P. Chakraborty, H. C. P. Movva, V. Lauter and B. N. Dev, Sci. Rep. 7, 41856 (2017).
[4] Nasrin Banu, B. N. Dev et al., https://arxiv.org/abs/1710.06114 (2017).