By James Mitchell Crow
US researchers are beginning to understand how copper oxides can transmit electricity with no power loss at temperatures not far below -100˚C. Research which gives clues as to just how these superconductors work was presented at the Australian Institute of Physics Congress in Melbourne by Michael Norman from Argonne National Laboratory in Illinois.
Norman said his co-workers had found pockets of electrons hidden within the superconducting materials. The discovery could help unravel the mechanism of operation of superconductors, he said, and point the way towards materials that superconduct at room temperature and above.
Norman is convinced such materials exist, it is just a question of finding them. “The record for high temperature superconductors has already increased by a factor of eight. To achieve room temperature superconductivity, we only need to improve by another factor of two.”
Fermi Surface Reconstruction and the Origin of High Temperature Superconductivity
Michael R. Norman
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 USA
Quantum oscillation studies reveal a profound rearrangement of the Fermi surface in underdoped cuprates. The cause of the reconstruction, and its implication for high temperature superconductivity, is a subject of active debate.
High temperature superconductivity in copper oxides is obtained by doping carriers into a parent insulating state. For materials doped beyond the temperature at which superconductivity is maximal, a large Fermi surface i s observed as predicted by band theory. But over most of the phase diagram, the Fermi surface appears to break apart in the so-called pseudogap phase. This has been revealed by angle resolved photoemission (ARPES). Some ARPES studies indicate small pockets, others `arcs’. A variety of explanations have been put forward for this behavior, ranging from density wave formation to preformed pairs.
Recently, quantum oscillations have been seen in the cuprates. For overdoped materials, they indicate the same large Fermi surface as revealed by ARPES. But for underdoped materials, small pockets are observed. The number of these pockets, their character (electron versus hole), and their location in the Brillouin zone, are subjects of current debate.
The presence of these small pockets indicates that indeed some sort of density wave formation is occurring in the pseuodgap phase. The real question is what type – charge
density wave, spin density wave, etc. And as the experiments are done at high fields, a question emerges whether the density wave state is ` field revealed’ due to the suppression of superconductivity, or `field induced’ due to application of a large magnetic field.
In this talk, I will discuss these results in detail, speculating on the ground state which gives rise to them. I will also discuss the relation of these results to those obtained by
photoemission spectroscopy. Finally, I will speculate on what these results mean for the origin of high temperature superconductivity, and their implications for the famous Mott-Slater debate on the nature of the insulating parent state.
Michael Norman, firstname.lastname@example.org