High-temperature superconductors (HTSCs) operate in mysterious ways, but scientists are starting to understand their peculiarities by using a state-of-the-art spectroscopy system at SSRL. One of the biggest mysteries is how a material that starts as an insulator-which does not conduct electricity-can become a high-temperature superconductor after being doped with electric carriers. Researchers Kyle Shen and Donghui Lu (both SSRL), working in Zhi-xun Shen's group at SSRL and Stanford, looked at the evolution from insulator to superconductor by studying an HTSC material at different doping concentrations. The team used angle-resolved photoemission spectroscopy (ARPES), a method of probing the electronic states in solids.

Their results, published in Science Feb. 11, contribute to creating a fundamental understanding of the perplexing physics in the mysterious HTSCs. Scientists hope to develop a theory explaining why the materials can be superconducting at a temperature much higher than conventional superconductors, and thereby how to improve the materials, currently too brittle for widespread use. The ARPES data revealed electronic states in the two-dimensional momentum space that are much stronger along the direction diagonal to the copper-oxygen bond (the "nodal" direction) than the direction parallel to the copper-oxygen bond (the "anti-nodal" direction), even through the anti-nodal direction is where the superconducting gap is the largest. The results show that the difference in momentum directions is important to the electronic structure, and put strong constraints on proposed HTSC models.

 Science. 11 Feb 2005; 307: 901 [DOI: 10.1126/science.1103627]