Beware of the Pseudogap

This is a terrific article on the puzzling pseudogap that has been observed in cuprate superconductors. It reviews a paper that claim to observe the signature of two different types of pseudogaps in the normal state.

A pseudogap is the pairing of electrons in the normal state of the material but without the long-range coherence that one gets when these pairs condense to form superconductivity. The question has always been whether these pairs are "pre-form" pairs (i.e. the pairs that will eventually condenses to form the superconducting fluid), or are these competing pairs, where these don't actually condenses, but rather, took electrons away from forming the condensate. Knowing which is which is crucial not only to know if something is a red-herring or not, but also in deciphering the correct mechanism that does cause the pairing that leads to superconductivity.

The new work reviewed in this article showed that these two types of pairing do occur and can be separated out[1].

One difficulty in the cuprates has been the determination of the temperature where the pseudogap opens. If it is due to spontaneous symmetry breaking, it should open at a well-defined critical temperature. If, on the other hand, it is caused by a fluctuation of the superconducting order (similar to a finite fraction of uncondensed Cooper-pairs above Tc), one may expect that increasing the temperature erodes the pairing-amplitude in a rather gradual manner. Kondo et al. report the observation of both phenomena in a single experiment: A pseudogap opens upon cooling below a relatively high temperature, which could be the consequence of a spontaneous symmetry breaking. The experiments do not reveal which symmetry is broken. When the temperature is decreased further, a temperature is reached where N(EF) starts to diminish more rapidly. The authors take this as an indication that a second (pairing) gap begins to open on top of the pseudogap already present. The work of Kondo et al is unique, in that these two temperature scales are revealed in a single experiment.

This result might help in deciphering and interpreting all previous experiments, and for future experiments to pay close attention to which pseudogap/paring that is being measured.

See a previous report on this same issue reported earlier.

Zz.

[1] Kondo, T. et al., Nature Physics v.7, p.21 (2011).