One of the greatest unsolved problems in condensed matter physics is explaining how electrons pair up in the copper-oxide materials that superconduct at temperatures above 100 K. Some theorists believe that the place to start in straightening out this mystery is to understand better how the cuprates behave at normal temperatures, long before they become superconducting.
University of Illinois physicist Philip Phillips suggests that the solution might be the existence of a previously overlooked doubly charged particle, one that mediates interactions among electrons lying in planes filled with copper and oxygen atoms. This particle would be distinct from a Cooper pair, the charge carrier in a superconductor. The new particle would be a boson that carries twice the charge of an electron, but is not made out of elementary excitations.
Nonetheless, it emerges from the strong repulsions among the electrons and persists above and below the superconducting transition temperature.
It is ironic, and revealing, that the cuprates (in their undoped state) are Mott insulators. In ordinary insulators every possible electron state is filled (with two electrons of opposite spin orientation). Under these circumstances, no electrical current is possible and the material is
insulating. In a Mott insulator things are rather counterintuitive.
Only half the electronic states are occupied but still no electrical current flows (http://www.aip.org/pnu/2003/split/645-2.html). This state of affairs comes about because strong electron repulsions prevent any electron motion.
When extra electrons or holes are introduced into a Mott insulator through the addition of dopant atoms, Mott insulators change drastically. One change is that the allowed energy bands in the material do not remain static, as in a semiconductor.
This lack of rigidity of the energy bands facilitates the appearance of the new particle, says Philips (email@example.com).
But what kind of collective excitation is this? Concentrate for the moment on the electrons in the sample. Semiconductors and most materials obey the standard counting principle that the removal of an electron leaves behind one empty state.
In a doped Mott insulator, by contrast, each hole leaves behind two empty states. This indicates that the electron that is removed ultimately did not reside in a single electronic state but must have been in a superposition of two
states. The question is, how does one describe the extra state. This question has now been answered by a new theory by Philips and his Illinois colleagues (Leigh et al., Physical Review Letters).
Some experimental results support this theory (see Graf et al., Phys Rev Lett, 9 February 2007 ). The Illinois work shows that the proposed charge-2e particle binds to the hole and produces the missing state. Philips believes that this particle is responsible for the normal state of the cuprates, including the odd “pseudogap state,” the condition in which some electrons in the material seem to be paired even at temperatures above where superconductivity sets in.