June 23, 2008

Impossible control of number electrons on the surface of a superconductor made real

A team of University of British Columbia researchers has developed a technique that controls the number of electrons on the surface of high-temperature superconductors, a procedure considered impossible for the past two decades.

Led by Physics Assoc. Prof. Andrea Damascelli, the team deposited potassium atoms onto the surface of a piece of superconducting copper oxide. The approach allows the scientists to continuously manipulate the number of electrons on ultra-thin layers of material. This level of control of electrons on surfaces will have applications beyond superconductors to other materials.

From the Nature Physics paper:In situ doping control of the surface of high-temperature superconductors

Central to the understanding of high-temperature superconductivity is the evolution of the electronic structure as doping alters the density of charge carriers in the CuO2 planes. Superconductivity emerges along the path from a normal metal on the overdoped side to an antiferromagnetic insulator on the underdoped side. This path also exhibits a severe disruption of the overdoped normal metal's Fermi surface. Angle-resolved photoemission spectroscopy (ARPES) on the surfaces of easily cleaved materials such as Bi2Sr2CaCu2O8+ shows that in zero magnetic field the Fermi surface breaks up into disconnected arcs. However, in high magnetic field, quantum oscillations at low temperatures in YBa2Cu3O6.5 indicate the existence of small Fermi surface pockets. Reconciling these two phenomena through ARPES studies of YBa2Cu3O7- (YBCO) has been hampered by the surface sensitivity of the technique. Here, we show that this difficulty stems from the polarity and resulting self-doping of the YBCO surface. Through in situ deposition of potassium atoms on cleaved YBCO, we can continuously control the surface doping and follow the evolution of the Fermi surface from the overdoped to the underdoped regime. The present approach opens the door to systematic studies of high-temperature superconductors, such as creating new electron-doped superconductors from insulating parent compounds.

UPDATE: The Vancouver Sun reports on the steps used

First, the copper oxide is put in a stainless-steel chamber kept in "outer space vacuum conditions" to avoid contaminating the sample, Damascelli explained. Atoms of potassium are then deposited onto the sample's surface, leaving behind electrons.

The second trick involves a technique that goes back to Albert Einstein's Nobel Prize-winning research into the photoelectric effect. Researchers shine light on the sample, which is absorbed by the electrons and ejected from the sample in a way that can be measured.

"This study that we do is the only way to really understand what is happening inside the superconductor," Damascelli said.

The experiment is groundbreaking for two reasons: Scientists are now able to control the number of electrons on the surface of a superconductor, and can also observe them.

"Extremely thin layers and surfaces of superconducting materials take on very different properties from the rest of the material. Electrons have been observed to re-arrange, making it impossible for scientists to study," says Damascelli. "It's become clear in recent years that this phenomenon is both the challenge and key to making great strides in superconductor research.

"The new technique opens the door to systematic studies not just of high-temperature superconductors, but many other materials where surfaces and interfaces control the physical properties," says Damascelli. "The control of surfaces and interfaces plays a vital role in the development of applications such as fuel cells and lossless power lines, and may lead to new materials altogether."

The superconductors Damascelli's team experimented on are the purest samples currently available and were produced at UBC by physicists Doug Bonn, Ruixing Liang and Walter Hardy.

Part of the study was carried out at the Advanced Light Source synchrotron in California. In the future, the design and study of novel complex materials for next-generation technologies will be carried out at the Quantum Materials Spectroscopy Center currently under construction at the Canadian Light Source in Saskatoon under Damascelli's leadership.

Profile of Andrea Damascelli