Inhomogeneous superconductivity in high-density nonmagnetic cobalt in a polycrystalline Co film

¹ Department of Materials Science, Indian Association for the Cultivation of Science 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, India
² Department of Physics, Vivekananda College, 269 - Diamond Harbour Road, Thakurpukur, Kolkata 700063, India
³ Department of Physical Sciences, Indian Institute of Science Education and Research Mohali Sector 81, S. A. S. Nagar, Manauli, PO 140306, India
⁴ Theoretical Sciences Unit, School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research Jakkur, Bangalore 560 064, India
⁵ Condensed Matter Physics Division, Saha Institute of Nuclear Physics - 1/AF Bidhannagar, Kolkata 700064, India
⁶ Microelectronic Research Center, The University of Texas at Austin - 10100 Burnet Road, Bldg 160, MER 1.606J, Austin, TX 78758, USA
⁷ Department of Condensed Matter Physics, Tata Institute of Fundamental Research - Homi Bhabha Road, Navy Nagar, Colaba, Mumbai 400005, India
⁸ Department of Physics and School of Nano Science and Technology, Indian Institute of Technology Kharagpur Kharagpur 721302, India

^(a) bhupen.dev@phy.iitkgp.ac.in

Received: 7 March 2020
Accepted: 6 August 2020

Abstract

We report the observation of inhomogeneous superconductivity (ISC) in the recently discovered high-density nonmagnetic (NM) phase of Co in thin films below an onset temperature () of 5.4 K in the absence of external magnetic field, via four-probe measurements of resistivity. Further, the point-contact spectroscopy studies also confirm superconductivity in this system. We attribute the observed ISC to the presence of nanoscale grains of high-density non-magnetic Co (FCC structure) in a thin film of conductive, normal Co (HCP structure) which is magnetic. Incomplete superconducting transition found in the bulk measurements suggests that the observed phenomenon is due to ISC of nanoscale grains of NM phase of Co. In addition, using first-principles density functional and BCS theoretical analysis of Co under hydrostatic and volume-preserving-biaxial strains, we demonstrate that superconducting of its NM phase increases anomalously with strain near its transformation to ferromagnetic phase, as a result of softening of N phonon due to strong electron-phonon coupling that is further enhanced with the biaxial strain.