Novel type of pairing in the first heavy fermion superconductor

Superconductors are materials which upon cooling to sufficiently low temperatures, lose their electrical resistance and become perfect conductors. This unusual phenomenon discovered over a century ago has a wide range of applications, notably in Magnetic Resonance Imaging (MRI) machines. This perfect conductivity arises from electrons in the material binding together to form Cooper pairs, and the nature of the “glue” holding these pairs together determines how the superconductor behaves. In the vast majority of superconductors, this glue comes from the coupling of electrons to the vibrations of the crystal lattice. However, in 1979 Professor Frank Steglich, now the director of Center for Correlated Matter at Zhejiang University, discovered a new superconductor CeCu₂Si₂, where unlike other known superconductors, this glue appeared instead to have a magnetic origin. Soon after, a new class of magnetic superconductors was revealed in the cuprate materials by Bednorz and Müller, where the superconductivity occurs at much higher temperatures than any other materials. This suggested that understanding how superconductivity with such a magnetic glue occurs may allow for superconductors with even higher transition temperatures to be found, possibly even up to room temperature.

An important characteristic of a superconductor is the energy gap, which corresponds to the energy required to destroy a Cooper pair, and is related to the pairing wave function. For a long time the energy gap of CeCu₂Si₂ was believed to have the same symmetry as the cuprate superconductors, where the gap disappears for electrons travelling along certain directions. However, very recently it was found that the energy gap of CeCu₂Si₂ is in fact fully open everywhere, much like regular non-magnetic superconductors, and therefore the nature of the pairing was not understood.

Recently, scientists from Zhejiang University, Rice University, Max-Planck-Institute for Chemical Physics of Solids and the University of Augsburg, led by Prof. Huiqiu Yuan from the Center for Correlated Matter and Department of Physics at Zhejiang University, found a resolution to this puzzle after measuring the penetration depth of CeCu₂Si₂ down to very low temperatures of just 0.04K, allowing the very small energy gap to be probed. In their paper published on May 8^th, 2018, in the Proceedings of the National Academy of Sciences (USA), they proposed a new explanation for the superconducting pairing, where the electrons within one electronic band have the same ‘d-wave’ pairing as the cuprate superconductors, but the ‘d-wave’ pairing between electrons on different bands has a different symmetry. This novel proposal accounts for both the recent evidence for a fully open superconducting gap, as well as older experiments which pointed to ‘d-wave’ superconductivity. In d­-wave superconductors the Cooper pairs have angular momentum,and in materials such as CeCu₂Si₂ where there are very strong interactions between electrons, there is also very strong repulsion. As a result, the angular momentum helps the electrons avoid each other, allowing the superconductivity to occur. These findings may have implications for understanding magnetic superconductivity beyond just the case of CeCu₂Si₂, opening up a new means of understanding superconductors with unconventional pairing.

The research was partially supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Science Challenge Project of China, and the Sino-German Cooperation Group on Emergent Correlated Materials.

The published article is available at www.pnas.org/cgi/doi/10.1073/pnas.1720291115

Experimental technique: When a superconductor is cooled through its superconducting transition, the magnetic fields inside the material are expelled, due to the spontaneous appearance of superconducting currents on the surface. As a result, magnetic fields can only penetrate a certain depth below the surface of the material, before decaying away, and the degree to which magnetic fields can enter the sample is the penetration depth. A larger density of Cooper pairs means that magnetic fields can be more efficiently screened by the surface currents, leading to a shorter penetration depth. As such, measuring the penetration depth at different temperatures reveals how the density of the Cooper pairs changes with temperature. The penetration depth of CeCu₂Si₂ was measured down to extremely low temperatures of just 0.04K, using the tunnel-diode-oscillator based technique. This is an extremely sensitive method for measuring small changes of the magnetic response of the superconductor, which were performed using a system set up by Professor Huiqiu Yuan at the Center for Correlated Matter. In recent years, Professor Yuan’s group have used this system to study a range of novel superconductors, publishing more than 20 papers in prestigious journals, including PNAS and Physical Review Letters. In 2017, Michael Smidman and Huiqiu Yuan were invited to write a review article in Reports on Progress in Physics, where they introduced their recent findings from measurements of noncentrosymmetric superconductors, which has been selected as an ESI hot paper and highly cited paper.

Experimental setup at the Center for Correlated Matter for measuring the penetration depth using the tunnel-diode-oscillator based technique.