Superconductivity of Cobalt and Quantum Computing with Superconductivity

by Prof. Bhupendra N. Dev (Centre for Quantum Engineering, Research and Education (CQuERE), TCG Centres for Research and Education in Science and Technology, Kolkata (India))

Friday 8 Sept 2023, 14:00 → 15:00 Europe/Berlin

CFEL Seminar Room I-III (Build. 99)

 

Normal metals possessing strong long-range magnetic order, e.g. the ferromagnetic transition metals Fe, Co and Ni, do not exhibit superconductivity. However, under special conditions they may show superconductivity. Fe loses its magnetism at pressures above 10 GPa and was found to exhibit superconductivity at temperatures below 2 K at pressures between 15 and 30 GPa. However, no superconductivity was found in Co and Ni under any conditions. It was predicted theoretically that, face-centred cubic (fcc) cobalt would lose its magnetic moment at a high density.

Recently, we discovered a high-density (HD) nonmagnetic (NM) phase of cobalt with fcc structure in polycrystalline cobalt thin films grown on silicon [1]. X-ray reflectivity (XRR) experiments first revealed this high-density state. Polarised neutron reflectivity (PNR) experiments confirmed this high-density state and, in addition, showed that this high-density Co is also nonmagnetic. This loss of magnetism of Co led us to look for its superconductivity, and we found it to be a superconductor below 5.4 K [2]. Interestingly, the Co film actually has a self-organised trilayer structure: HDNM-Co/normal-Co/HDNM-Co, as revealed by XRR and PNR experiments. HDNM-Co is superconducting (S) and normal-Co is ferromagnetic (F). Thus the present system resents a self-organised S/F/S hybrid structure. Such S/F/S hybrid structures have applications in spintronics and quantum information processing [3]. 

 

Superconductivity has been successfully utilised in building quantum computers. Qubits, as opposed to bits in classical computers, are the building blocks of quantum computers. I will explain how qubits are realised using superconducting materials and some basics of quantum computation. Aluminium is the standard superconductor used in building superconducting qubits. Another superconductor, CoSi₂, might provide less noisy qubits compared to Al. I will present our initial results on the superconductivity of CoSi₂, which we prepared by thermal annealing of a superconducting Co film on Si [4].

 

[1] Banu Nasrin, S. Singh, B. Satpati, A. Roy, S. Basu, P. Chakraborty, H. C. P. Movva, V. Lauter and 

B. N. Dev, Sci. Rep., 7 (2017) 41856. 

[2] Nasrin Banu, M. Aslam, Arpita Paul, S. Banik, S. Das, S. Datta, A. Roy, I. Das, G. Sheet, U. V. Waghmare, S. Ramakrishnan and B. N. Dev, EPL, 131 (2020) 47001 

[3] B. Baek, W. H. Rippard, S. P. Benz, S. E. Russek, P. D. Dresselhaus, Nat. Commun., 5 (2014) 3888. 

[4] “In search of new superconducting materials for quieter qubits”, S. Mandal, B. Biswas, S. Purkait, B. Karmakar, B. Satpati, A. Roy and B. N. Dev, To be presented in the conference on “Superconducting Qubits and Algorithms”, Munich, 29 August to 01 September, 2023.