Researchers solve one mystery of high-temperature superconductors
An experimental mystery 鈥� the origin of the insulating state in a class of materials known as doped Mott insulators 鈥� has been solved by researchers at the University of Illinois at Urbana-Champaign. The solution helps explain the bizarre behavior of doped Mott insulators, such as high-temperature copper-oxide superconductors.
In a paper published in the Nov. 2 issue of the journal 糖心视频ical Review Letters, physics professor Philip Phillips and graduate student Ting-Pong Choy show that lightly doped Mott insulators are, in fact, still insulators. The scientists鈥� theoretical results confirm previous experimental findings obtained by other researchers.
Unlike low-temperature superconductors, which are metals, high-temperature superconductors are insulators in their normal state. This has puzzled scientists, because half of the electron states are empty.
鈥淢ott insulators have many available states for electrons to occupy, so you would expect these materials to conduct like metals,鈥� Phillips said. 鈥淓xperiments have shown, however, that they act as insulators.鈥�
Even more surprising, when Mott insulators are lightly doped with holes 鈥� thereby creating even more places for electrons to occupy 鈥� the material still refuses to conduct.
Strong electron interaction is the key to understanding doped Mott insulators, Phillips said. 鈥淎ll energy scales are inextricably coupled. If you attempt to separate them, you destroy the physics of the Mott state.鈥�
The fact that lightly doped Mott insulators are still insulators is an intrinsic property of Mott physics (that is, Mottness), the researchers claim. The insulating state is not caused by disorder, exotic excitations or something external to the system.
鈥淚n most materials, if you kill superconductivity by applying a large magnetic field, the resistivity falls to some finite value,鈥� Phillips said. 鈥淚n doped Mott insulators, however, the resistivity climbs to infinity. The background state uncovered as a result of destroying superconductivity is an insulating state.鈥�
A future experiment could easily prove the researchers鈥� claims. While chemical doping causes disorder in the material, the technique of photodoping creates holes without causing disorder.
鈥淚f experimenters create such holes and still see this insulating state, then we will know for a fact that insulating doped Mott insulators is due to Mottness,鈥� Phillips said.
Source: University of Illinois at Urbana-Champaign