Experiments look set to resolve superconductivity debate

An intriguing controversy in condensed-matter physics concerns the symmetry of electron pairs in high-temperature superconductors. A collaboration between the University of California at Berkeley (UCB) and at San Diego (UCSD), and the University of Illinois at Urbana-Champaign has shown that previously conflicting tunnelling results can be reconciled in terms of the structure of the superconducting crystals. It is well known that layered cuprates such as yttrium barium copper oxide (YBCO) are superconducting at temperatures up to 150 K which is well above the theoretical limit for conventional superconductors. Although a adequate theory of high-temperature superconductivity may still be some way off, agreement on the symmetry of the electron pairing state is an important step forward in explaining the materials behaviour. In the last few years, strong experimental evidence has been presented that the superconductivity in the cuprates is unconventional, with so-called d-wave symmetry in which the electrons pair with non-zero orbital angular momentum. However, a few experiments have consistently disagreed with this picture, favouring instead the isotropic s-wave pairing that describes conventional superconductors. Resolution of this controversy is important to achieve a full understanding of the superconductivity in these materials. It was widely speculated that these supercurrents could arise from a distortion of the order parameter due to the orthorhombic structure of the YBCO crystal lattice. Such a distortion could cause an imbalance in the size of the positive and negative lobes, but this conjecture remained untested until the present work (K Kouznetsov et al 1997 Phys. Rev. Lett 79 3050). (UK)