Emergence of Unconventional Superconductors

by Prof. Ram Gopal SHARMA (Inter-University Accelerator Centre, New Delhi)

Friday 28 May 2021, 09:30 → 11:00 Europe/Rome

Superconductivity has fascinated physicists, material scientists and engineers for over a century and continues to do so. New superconductors with ever-increasing transition temperatures and in very unconventional systems continue to be discovered even though theorists keep struggling hard to find an acceptable theory of superconductivity in cuprates and many new superconductors. Berndt Matthias at Bell Labs. discovered more than 1000 superconductors including Nb-Ti and Nb₃Sn. He made 6 guiding principles for successful discoveries, namely,

1. High symmetry is good; Cubic symmetry is best, 2. A high density of electronic states is good, 3. Stay away from Oxygen, 4. Stay away from Magnetism, 5. Stay away from insulators and 6. Stay away from Theorists.

Discovery of cuprate superconductor in 1986 by Bednorz and Muller in insulators (when doped) and having oxygen as a constituent violated the above guiding principle. Cuprates heralded the descent of a new era of superconductivity. Till then highest T_c was 23 K observed in Nb₃Ge sputtered films. A variety of curate superconductors  with layered structure and with ever increasing T_c, namely, YBCO (92 K), Bi-2201 (20 K), Bi-2212 (80 K), Bi-2223 (110 K), Tl-2201 (80 K), Tl-2212 (108 K), Tl- 2223  (125 K), Hg-1201 (94 K), Hg-1212 (128 K) and Tl-1223 (135 K) were discovered in quick succession.

Superconductivity was next discovered in MgB₂ at 39 K by Nagamatsu in 2001. MgB₂ behaves like a BCS superconductor but is characterized by two widely differing energy gaps, namely, 6.8 meV and 1.8 meV, two coherence lengths (13 and 51 nm) and two penetration depths (47.8 and 33.6 nm) respectively. This hinted at the presence of two species of electrons (σ bonding and π bonding) forming Cooper pairs. The material has low anisotropy of upper critical field ( Bc2abBc2c  = 1.1-1.7), high critical field, B_c2 of up to 60 T and irreversible field B_irr up to 40 T.

Another surprise came from nature in 2008 when Hosano reported superconductivity in iron based oxy-pnictides of the type LaFeAsO (1111) at 26 K. Here the contradiction is between the static electron spin ordering in Fe and dynamic formation of electron pairs of opposite spins in superconductors. T_c in excess of 50 K were reported in Sm and Nd based pnictides. The discovery opened the floodgate to the emergence of a large variety of iron based superconductors (IBSC) popularly known through the acronyms 1111, 111, 122, 11 and more. The common feature of all the 1111 type compounds is that superconductivity appears only when antiferromagnetic ordering (SDW) is fully suppressed at a certain level of doping. Coexistence of SDW and superconductivity has however been observed in a number of 122 superconductors of the type Sr/BaFe₂As₂. The strategy adopted to enhance T_c has been to dope the insulating La₂O₂ layer suitably whereby a charge, electron/hole is transferred to the Fe₂As₂ conduction layer. Thus a T_c = 38 K was reported in a K-doped (Ba_0.6K_0.4)Fe₂As₂  122 compound. Record high T_c = 203 K has been reported in Sulphur hydride which has been found to be a BCS superconductor.