Pseudogap

The term 'pseudogap' was coined by Nevill Mott in 1968 to indicate a minimum in the density of states at the Fermi energy, N(EF), resulting from Coulomb repulsion between electrons in the same atom, a bandgap in a disordered material or a combination of these.[1] In the modern context pseudogap is a term from the field of high-temperature superconductivity which refers to an energy range (normally near the Fermi energy) which has very few states associated with it. This is very similar to a 'gap', which is an energy range that contains 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 to cuprate high-temperature superconductors, existing in underdoped specimens at temperatures above the superconducting transition temperature.

A pseudogap is only observed in hole-doped Cuprate superconductors. And the isotope effect is only emerged on the boundary of phases between superconducting and pseudogap state.

Interestingly, only certain electrons 'see' this gap. The gap, which should be associated with an insulating state, only exists for electrons traveling parallel to the copper-oxygen bonds. Electrons traveling at 45 degrees to this bond can move freely throughout the crystal. The Fermi surface therefore consists of Fermi Arcs forming pockets centered 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 super-conduct. 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.

Recently a pseudogap state has also been reported in strongly disordered conventional superconductors like TiN [2] and NbN.[3]

Contents

Origin

A pseudogap can be seen with several different experimental methods. One of the first observations was in specific heat measurements of YBa2Cu3O6+x by Loram et al.[4] The pseudogap is also apparent in ARPES (Angle Resolved Photoemission Spectroscopy) and STM (Scanning tunneling microscope) 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 pairs In this scenario, electrons form pairs at a temperature T* that can be much larger than the critical temperature Tc where superconductivity appears. T* of the order of 300K have been measured in underdoped cuprates where Tc is about 80K. The superconductivity does not appear at T* because large phase fluctuations [5] of the pairing field cannot order at this temperature. The pseudogap is then produced by non coherent fluctuations of the pairing field.[6] The pseudogap is a normal state precursor of the superconducting gap due to local, dynamic pairing correlations.[7] This point of view is supported confirmed by a quantitative approach of the attractive pairing model [8] to specific heat experiments.

2. The scenario of a non superconducting related pseudogap In this class of scenarios, many different origins have been put forward: like the formation of electronic stripes, anti-ferromagnetic ordering, exotic order parameter competing with superconductivity.

References

  1. ^ N. F. Mott (1968). "Metal-Insulator Transition". Reviews of Modern Physics 40 (4): 677–683. Bibcode 1968RvMP...40..677M. doi:10.1103/RevModPhys.40.677. 
  2. ^ Benjamin Sacépé, Claude Chapelier, Tatyana I. Baturina, Valerii M. Vinokur, Mikhail R. Baklanov and Marc Sanquer (2010). "Pseudogap in a thin film of a conventional superconductor". Nature Communications 1 (9): 140. Bibcode 2010NatCo...1.....S. doi:10.1038/ncomms1140. 
  3. ^ Mintu Mondal, Anand Kamlapure, Madhavi Chand, Garima Saraswat, Sanjeev Kumar, John Jesudasan, L. Benfatto, Vikram Tripathi and Pratap Raychaudhuri (2011). "Phase fluctuations in a strongly disordered s-wave NbN superconductor close to the metal-insulator transition". Physical Review Letters 106 (4): 047001. Bibcode 2011PhRvL.106d7001M. doi:10.1103/PhysRevLett.106.047001. 
  4. ^ J. W. Loram, K. A. Mirza, J. R. Cooper, and W. Y. Liang (1993). "Electronic specific heat of YBa2Cu3O6+x'' from 1.8 to 300 K". Physical Review Letters 71 (11): 1740–1743. Bibcode 1993PhRvL..71.1740L. doi:10.1103/PhysRevLett.71.1740. 
  5. ^ V.J. Emery, S.A. Kivelson, (1995). "Importance of phase fluctuations in superconductors with small superfluid density". Nature 374 (6521): 434–437. Bibcode 1995Natur.374..434E. doi:10.1038/374434a0. 
  6. ^ Marcel Franz (2007). "Superconductivity: Importance of fluctuations". Nature Physics 3 (10): 686–687. Bibcode 2007NatPh...3..686F. doi:10.1038/nphys739. 
  7. ^ Mohit Randeria and Nandini Trivedi (1998). "Pairing Correlations abobe Tc and pseudogaps in underdoped cuprates". Journal of Physics and Chemistry of Solids 59 (10–12 pages = 1754–1758). 
  8. ^ Philippe Curty and Hans Beck, (2003). "Thermodynamics and Phase Diagram of High Temperature Superconductors". Physical Review Letters 91 (25): 257002. arXiv:cond-mat/0401124. Bibcode 2003PhRvL..91y7002C. doi:10.1103/PhysRevLett.91.257002. PMID 14754139. 

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