An experimental mystery – the origin of the insulating state in a class of materials known as doped Mott insulators – 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 Physical 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’ 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.
“Mott insulators have many available states for electrons to occupy, so you would expect these materials to conduct like metals,” Phillips said. “Experiments have shown, however, that they act as insulators.”
Even more surprising, when Mott insulators are lightly doped with holes – thereby creating even more places for electrons to occupy – the material still refuses to conduct.
Strong electron interaction is the key to understanding doped Mott insulators, Phillips said. “All energy scales are inextricably coupled. If you attempt to separate them, you destroy the physics of the Mott state.”
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.
“In most materials, if you kill superconductivity by applying a large magnetic field, the resistivity falls to some finite value,” Phillips said. “In doped Mott insulators, however, the resistivity climbs to infinity. The background state uncovered as a result of destroying superconductivity is an insulating state.”
A future experiment could easily prove the researchers’ claims. While chemical doping causes disorder in the material, the technique of photodoping creates holes without causing disorder.
“If 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,” Phillips said.
Source: University of Illinois at Urbana-Champaign
Related stories:
Theories of high-temperature superconductivity violate Pauli principle
Scientists seeking to explain high-temperature superconductivity have been violating the Pauli exclusion principle, a team of researchers from the University of Illinois at Urbana-Champaign and Rutgers University report. Any theory that does not embrace the Pauli principle has a lot of explaining to do, they say.
The basic organizing precept behind the periodic table is the Pauli principle, which says that electrons with the same spin cannot occupy the same energy state. The Pauli principle leads to the shell structure of atoms, and is inviolate for electronic systems. Many researchers, however, have been breaking this important rule when proposing theories to explain the mechanism behind high-temperature superconductivity.
Room temperature superconductivity: One step closer to the Holy Grail of physics
Scientists at the University of Cambridge have for the first time identified a key component to unravelling the mystery of room temperature superconductivity, according to a paper published in today's edition of the scientific journal
Nature.
Hope Diamond's phosphorescence key to fingerprinting
Shine a white light on the Hope Diamond and it will dazzle you with the brilliance of an amazing blue diamond. Shine an ultraviolet light on the Hope Diamond and the gem will glow red-orange for about five minutes. This phosphorescent property of blue diamonds can distinguish synthetic and altered diamonds from the real thing, and it may also provide a way to fingerprint individual blue diamonds for identification purposes, according to a team of researchers from the Naval Research Laboratory, the Smithsonian Institution and Penn State.
Imaging 'Gridlock' in High-temperature Superconductors
Superconductivity -- the conduction of electricity with zero resistance -- sometimes can, it seems, become stalled by a form of electronic "gridlock."
Researchers Succeed in Inducing Ferromagnetism in High-k Dielectric Materials (Update)
A multidisciplinary team of researchers led by Ashutosh Tiwari of Nanostructured Materials Research Laboratory (NMRL) of University of Utah has succeeded in inducing high temperature ferromagnetism in highly insulting epitaxial CeO
2 films by dilute doping of cobalt. These films are transparent in the visible regime and exhibit a very high Curie temperature ~875K with a giant magnetic moment. It has been shown that the ferromagnetic property is intrinsic to the CeO
2 system and is not a result of any secondary magnetic phase or cluster formation. Observation of high temperature ferromagnetism in lightly doped high-k dielectric materials with giant magnetic moment and such a high transition temperature is remarkable and represents a groundbreaking step in Spintronics.
Ultra-cold temperature physics opens way to understanding and applications
Researchers doing ultra-cold temperature physics may not have to wear parkas, but they are producing the coldest temperatures ever and exploring model quantum systems that might lead to more accurate clocks and gyroscopes, quantum computers and communications as well as a better understanding of quantum physics phenomena.
An Unconventional Metal
The semiconductor silicon and the ferromagnet iron are the basis for much of mankind's technology, used in everything from computers to electric motors. In this week's issue of the journal
Nature (August 21st) an international group of scientists, including academic and industrial researchers from the UK, USA and Lesotho, report that they have combined these elements with a small amount of another common metal, manganese, to create a new material which is neither a magnet nor an ordinary semiconductor.
New logic: the attraction of magnetic computation
European researchers are the first to demonstrate functional components that exploit the magnetic properties of electrons to perform logic operations. Compatible with existing microtechnology, the new approach heralds the next era of faster, smaller and more efficient electronics.