Developing a theory for high-temperature superconductivity in cuprate materials is one of the outstanding challenges in condensed-matter physics. However, there are many properties of the normal or non-superconducting state of the cuprates that are not understood either. One of these is the existence of a "pseudogap" in so-called underdoped cuprates. Now a team of researchers led by Juan Campuzano of the University of Illinois at Chicago claims to have observed the spontaneous breaking of time-reversal symmetry in the pseudogap state (A Kaminski et al. 2002 arXiv.org/abs/cond-mat/0203133). If confirmed, the result would represent a major step towards a theory of high-temperature superconductivity.
Campuzano and co-workers studied thin films of bismuth strontium calcium copper oxide (Bi2Sr2CaCu2O8+x), known as Bi-2212 for short. The cuprates are normally insulators, but when dopants are added – extra oxygen atoms in the case of Bi-2212 – they lose all resistance to electrical current. This happens at the superconducting transition temperature, Tc, which varies with the amount of doping. The maximum value of Tc occurs for optimal doping, and samples with more or less than optimal doping are said to be overdoped or underdoped.
One of the essential characteristics of a superconductor is the energy gap – this is the binding energy of the pairs of electrons or “holes” that are responsible for the superconductivity. However, in the mid-1990s physicists discovered evidence for a “pseudogap” that revealed itself in underdoped materials at temperatures well above Tc.
A key question is whether a phase transition occurs at the pseudogap temperature, T*, as this would imply the existence of a new form of quantum order, just as superconductivity itself is a form of order. Campuzano and co-workers measured the photocurrents that were produced when samples of Bi-2212 were irradiated with left- and right-handed circularly polarized radiation. No difference in the photocurrents was observed for overdoped samples. However, the photocurrents produced by left- and right-handed radiation were different below T* for underdoped samples.
This behaviour, which persists below Tc, suggests that time-reversal symmetry is broken in the pseudogap state, as had previously been predicted by one of Campuzano’s co-authors, Chandra Varma of Bell Labs. Knowledge of the symmetry and order properties of the pseudogap state could lead to a better understanding of high-temperature superconductivity.
