Description and aims - 100 years ago superconductivity was discovered by Heike Kamerlingh Onnes in Leiden. Tempted by the unique possibility of an exact match in time and place, the workshop “100th Anniversary of Superconductivity: Hot Topics and Future Directions”, was held from 4–8 April at the Lorentz Center. The mission was not only to mark this centenary but also to capture the current opinion as regards unconventional superconductivity and to identify future directions. To do this, the workshop brought together a select band of experimentalists and theoreticians for a lively and fascinating exchange of views on experimental data, theories and opinions covering a wide variety of superconducting systems.
Scientific outcome - The topics covered at the workshop ranged from organic compounds, heavy fermions, iron-based oxides, cuprates, non-centrosymmetric materials and field-effect superconductivity, as well as string theory applied to superconductivity. An obvious challenge is to provide a unifying framework for the understanding of these diverse materials systems. A recurring theme throughout the workshop was that on varying pressure, magnetic field, or some material parameter, a quantum phase transition is often observed between the superconducting phase and a magnetically ordered phase. This connection suggests that quantum phase transitions could play an important role in the mechanism of superconductivity in a number of the relevant materials families. The questions as to which classes of superconductors this applies, and to what extent a revision of BCS theory is needed, were at the centre of discussions at the workshop.
One of the refrains of the workshop was the need to understand the ‘normal’ conducting state, of which the superconducting state is an instability. That practically all unconventional superconductors possess anomalous or strange normal states could be linked to the effects of a funnel of quantum critical matter extending above the critical point, as the relevant control parameter is tuned close to optimal Tc, the superconducting transition temperature. However, the nature of the quantum critical order differs from one material to another. For example, in several of the heavy fermion superconductors it is a spin-density wave. In the cuprates, there are a wide variety of suspects, but an increasing number of indications are found that, in addition to charge/spin stripes, anti-ferromagnetism, inhomogeneities and lattice distortions, a time-reversal symmetry breaking order occurs, which leaves translational symmetry intact, with a quantum phase transition to a Fermi liquid close to optimal doping. The potentially central role of quantum criticality in strongly interacting fermionic systems has also inspired novel theoretical approaches, and one session dealt with holographic superconductivity, anti-de Sitter/conformal field theory correspondence and the connection to string theory.
A News and Views article reporting the workshop appeared in Nature Physics: D. van der Marel and M.S. Golden: Heike’s Heritage, Nature Physics 7, 377-388 (2011).
Format of the workshop - The one week programme of the workshop consisted of 15 formal presentations and 8 discussion sessions, spread over the first four days. The last day was dedicated to the centenary celebrations themselves, held at the Boerhave Museum and the former Kamerlingh Onnes Laboratory. All in all, it was a highly stimulating and enjoyable week, with discussions - although sometimes high in volume and passion - always conducted in the forgiving manner of an extended family gathering to discuss, disagree and dream at their celebration of a remarkable milestone in physics. The organizers – and participants – are grateful to the Lorentz Center and its staff for the warm hospitality and smooth organization.