Session J3: The Kavli Foundation Special Symposium: Nobelist Perspectives on 100 Years of Superconductivity

Theoretical work on superconductivity up to 1956

In this talk I survey some of the attempts to understand superconductivity which preceded the 1957 work of Bardeen, Cooper and Schrieffer, and conclude by asking if there are any lessons we can draw for our current efforts to understand apparently ``non-BCS'' superconductors such as the cuprates and ferropnictides.   

The Exceptional Properties of Superconductivity in Cuprates

Copper oxides are the only materials that have transition temperatures, $T_c$, \textit{above} the boiling point of liquid nitrogen, with a maximum $T_c^m$ of 162 K under pressure. Their structure is layered, with one to several CuO$_2$ planes, and upon hole doping, their transition temperature follows a dome-shaped curve with a maximum at $T_c^m$. In the underdoped regime, i.e., below $T_c^m$, a pseudogap $T$* is found, with $T$* always being larger than $T_c$, a property unique to the copper oxides [1]. In the superconducting state, Cooper pairs (two holes with antiparallel spins) are formed that exhibit coherence lengths on the order of a lattice distance \textit{in the} CuO$_2$ plane and one order of magnitude less perpendicular to it. Their macroscopic wave function is parallel to the CuO$_2$ plane near 100\% $d$ at their surface, but only 75\% $d$ and 25\% $s$ in the bulk, and near 100\% $s$ perpendicular to the plane in YBCO. There are two gaps with the same $T_c$ [2]. As function of doping, the oxygen isotope effect is novel and can be quantitatively accounted for by a two-band vibronic theory [3]. These cuprates are intrinsically heterogeneous in a dynamic way. In terms of quasiparticles, bipolarons are present at low doping, and aggregate upon cooling [1], so that probably ramified clusters and/or stripes are formed, leading over to a more Fermi-liquid-type behavior at large carrier concentrations above $T_c^m$. \newline \newline [1] For an overview, see: K.A.\ M\"uller, J. Phys:\ Condens.Matter \textbf{19}, 251002 (2007). \newline [2] R.\ Khasanov, A.\ Shengelaya \textit{et al.}, Phys.\ Rev.Lett.\ \textbf{98}, 0570007 (2007). \newline [3] H.\ Keller, A.\ Bussmann-Holder, and K.A.\ M\"uller, Materials Today \textbf{11}, 38 (2008).   

Discovery of Superconductive Tunneling

Some times unlikely events happens: How can a mechanical engineer from Norway end up with a Nobel Prize in Physics? I had the great fortune to receive the prize in Physics for using electron tunneling to measure the energy gap in superconductors. In this talk I will recollect some of the events that led to this discovery and hopefully be able to convey to you some of the fun and excitement of that area. My great fortune was really to be at the right place at the right time, where I had access to outstanding and helpful physicists. If you become real interested, you may look up the talk at the web site http://nobelprize.org/ .   

Superconductivity and particle physics

I will briefly review and give a modern perspective on some classic applications of the ideas of superconductivity theory to fundamental particle physics: spontaneous chiral symmetry breaking in vacuum QCD, the Higgs mechanism in electroweak theory, and color superconductivity in dense hadronic matter; and also the confinement problem. Then I will discuss some frontier topics that carry the ideas further and in new directions: supersymmetry and the superHiggs mechanism, exotic quantum statistics of superconducting vortices.   

Superfluidity in an Atomic Gas of Strongly Interacting Fermions

What is the benefit of realizing superfluidity in a gas a million times more dilute than air? Such systems consist of well-separated atoms which can be observed and manipulated with the control and precision of atomic physics, and which can be treated with first-principles calculations. By implementing scattering resonances, we have realized the strong-coupling limit of the Bardeen Schrieffer-Cooper (BCS) mechanism and observed a normalized transition temperature of 15\% of the Fermi temperature, higher than in any superconductor. By tuning the strength of the interactions, the BEC-BCS crossover is realized. When the population of the two spin states is imbalanced, pairing is frustrated; and superfluidity is quenched at the Chandrasekhar-Clogston limit. These studies illustrate a new approach to condensed-matter physics where many-body Hamiltonians are realized in dilute atomic gases.