Scientists Find Unexpected Electron Behavior in the Pseudogap of
summary written by Raven Hanna
Superconductivity is a hot topic in physics for good reason. With an electrical resistance of zero, superconductors transport electrical current with no loss of energy. Unfortunately, scientists have only found materials to be superconducting at very low temperatures, much too low for widespread use. In the 1980s, scientists discovered a class of "high-temperature" superconductors that can be used at the temperature of liquid nitrogen (~-200°C). This discovery has raised scientists' hopes that materials may exist that have superconductive properties at even higher temperatures. To find these, scientists are exploring the reasons why materials are superconducting, especially by researching the properties of special electron-pairs, called Cooper pairs, that form a coherent "dance" together. In high-temperature superconductors, a so-called pseudogap state occurs just above the superconducting temperature with no analogue in "low-temperature" superconductors. In this state, electron pairs were thought to form but not act in a coordinated manner. A full understanding of the pseudogap behavior is critical to understand why these materials are superconducting at such a high temperature and may lead to new materials with even higher superconducting transition temperatures, approaching the ultimate goal of finding room temperature superconductors.
A group of researchers led by Zhi-Xun Shen of Photon Science at SLAC and Applied Physics, Stanford University explored the behavior of electrons in a high-temperature superconducting material. They used SSRL Beam Line 5-4 for angle-resolved photoemission spectroscopy (ARPES) experiments at a variety of temperatures spanning the normal state, the pseudogap state, and the superconducting state. They were surprised to find that the electrons in the pseudogap state do not pair at all. Instead, they saw the tendency of electrons traveling in a density wave. The pseudogap state is not predominantly composed of electron pairs ready to take on Cooper pair properties but of an electronic state that probably competes with superconducting pairing by consuming electrons otherwise available for pairing.
Given their surprising findings, the researchers conclude that room temperature superconductivity, if it can exist, may rely on a different electronic behavior than what was previously assumed. They are currently working on understanding the nature of the density wave they observed and its effects on the underlying high-temperature superconductivity. This research was published online on April 4 by Nature Physics.
Makoto Hashimoto, Rui-Hua He, Kiyohisa Tanaka, Jean-Pierre Testaud, Worawat Meevasana, Rob G. Moore, Donghui Lu, Hong Yao, Yoshiyuki Yoshida, Hiroshi Eisaki, Thomas P. Devereaux, Zahid Hussain & Zhi-Xun Shen, Nature Physics, Published online: 04 April 2010 doi:10.1038/nphys1632