Pseudogap

From Wikipedia, the free encyclopedia

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.

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.

[edit] Mechanism

Holes that are doped into an antiferromagnetic insulator tend to form stripes (an exhibition of "self-organised quasi-one-dimensional electronic character"). This is because of the tendency of an antiferromagnet to expel holes. The accompanying lateral confinement of the intervening Mott-insulating regions induces a spin gap or pseudogap in the environment of the stripes. The pairing mechanism is the generation of a spin gap (or pseudogap) in the antiferromagnetic environment in proximity to the metallic stripes.

A pseudogap can be seen with several different experimental methods, but probably the most popular is ARPES (Angle Resolved Photoemission Spectroscopy), which can measure the density of states of the electrons in a material. It does this by shooting photons at a material and then counting the number of electrons released, categorised by both angle and energy. With conservation of momentum and energy, one can determine the energy of the electron and thus build a picture of the density of states.