Pseudogap

A pseudogap is a term from the field of high-temperature superconductivity which describes an energy (normally near the Fermi energy) which has very few states associated with it. This is very similar to a 'gap', which is an energy that has no allowed states. Such gaps open up, for example, when electrons interact with the lattice. The Pseudogap is a zone of the Phase diagram generic.

Interestingly only certain electrons `see' this gap. The gap, which should be associated with an insulating state, only exists for electrons travelling parallel to the copper-oxygen bonds. Electrons travelling at 45 degrees to this bond can move freely throughout the crystal. The Fermi surface therefore consists of Fermi Arcs forming pockets centred on the corner of the Brillouin zone. In the pseudogap phase these arcs gradually disappear as the temperature is lowered until only four points on the diagonals of the Brillouin zone remain ungapped.

On one hand, this could indicate a completely new electronic phase which consumes available states, leaving only a few to pair up and superconduct. On the other hand, the similarity between this partial gap and that in the superconducting state could indicate that the pseudogap results from preformed cooper pairs.

Origin

A pseudogap can be seen with several different experimental methods. One of the first observations was in specific heat measurements of YBa₂Cu₃O_6+"x" by Loram "et al." [cite journal | author = J. W. Loram, K. A. Mirza, J. R. Cooper, and W. Y. Liang | title = Electronic specific heat of YBa₂Cu₃O_6+"x'' from 1.8 to 300 K | year = 1993 | journal = Physical Review Letters | volume = 71 | issue = 11 | pages = 1740–1743 | doi = 10.1103/PhysRevLett.71.1740] The pseudogap is also apparent in ARPES (Angle Resolved Photoemission Spectroscopy) data, which can measure the density of states of the electrons in a material.

Mechanism

The origin of the pseudogap is controversial and still subject to debate in the condensed matter community. Two main interpretations are emerging:

1. The scenario of preformed pairsIn this scenario, electrons form pairs at a temperature T* that can be much larger than the critical temperature T_c where superconductivity appears. T* of the order of 300K have been measured in underdoped cuprates where T_c is about 80K. The superconductivity does not appear at T* because large phase fluctuations [cite journal | author = V.J. Emery, S.A. Kivelson, | title = Phase Fluctuations in High Temperature Superconductors | year = 1995 | journal = Nature | volume = 374 | pages = 434–437 ] of the pairing field cannot order at this temperature. The pseudogap is then produced by non coherent fluctuations of the pairing field. The pseudogap is a normal state precursor of the superconducting gap due to local, dynamic pairing correlations [cite journal | author = Mohit Randeria and Nandini Trivedi | title = Pairing Correlations abobe Tc and pseudogaps in underdoped cuprates | year = 1998 | journal = Journal of Physics and Chemistry of Solids | volume = 59 | issues = 10-12 pages = 1754-1758 ] . This point of view is supported confirmed by a quantitative approach of the attractive pairing model [cite journal | author = Philippe Curty and Hans Beck, | title = Thermodynamics and Phase Diagram of High Temperature Superconductors | year = 2003 | journal = Physical Review Letters | volume = 91 | pages = 257002 ] to specific heat experiments. 2. The scenario of a non superconducting related pseudogapIn this class of scenarios, many different origins have been put forward: like the formation of electronic stripes, antiferromagnetic ordering, exotic order parameter competing with superconductivity.

References

* Emery et al. Physical Review B, Vol 56, Page 6120 (1997)
* Kyle McElroy, Nature Physics, Vol 2, Page 441 (2006)

External links

* [http://www.physorg.com/news10505.html pseudogap in non-superconducting materials]