You are browsing the archives of superconductivity.

A new story in Science reports that an international team of researchers have been able to turn a non-superconducting form of copper oxide into a superconductor using a strong laser burst.

The team, working in Germany, Japan and the U.K., says it hopes its discovery might open a new path to high temperature superconductivity.

“We have used light to turn a normal insulator into a superconductor,” says Andrea Cavalleri, in a news release from Oxford University.

The group says it used a femtosecond pulses of a mid-infrared laser to transform non-superconducting (above 5 K) La_1.675Eu_0.2Sr_0.125CuO₄ (LESCO_1/8) — a strip-ordered compound — into a transient superconductor. The material was at a  base temperature under 20 K superconductor, and displayed superconductivity for a fraction of a second before returning to its normal state.

“We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons ’synch up’ and superconduct,” says Cavalleri. “This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.”

Cavalleri, a professor in the Department of Physics at Oxford and the Max Planck Department for Structural Dynamics, Hamburg, continues. “That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?”

Cavalleri’s group also included researchers from the Department of Advanced Materials Science, University of Tokyo, and the RIKEN Advanced Science Institute, Japan.

Share

Physicists at Brookhaven National Laboratory have identified a single layer responsible for one such material’s ability to become superconducting. The technique, described in the Oct. 30, 2009, issue of Science, could be used to engineer ultrathin films with “tunable” superconductivity for higher-efficiency electronic devices.

The thinner the material (and the higher its transition temperature to a superconductor), the greater its potential for applications where the superconductivity can be controlled by an external electric field. “This type of control is difficult to achieve with thicker films, because an electric field does not penetrate into metals more than a nanometer or so,” explains Brookhaven physicist and the group leader Ivan Bozovic.

To explore the limits of thinness, Bozovic’s group synthesized a series of films based on the high-temperature superconducting cuprates — materials that carry current with no energy loss when cooled below a certain transition temperature. Since zinc is known to suppress the superconductivity in these materials, the scientists systematically substituted a small amount of zinc into each of the copper-oxide layers. Any layer where the zinc’s presence had a suppressing effect would be clearly identified as essential to superconductivity in the film.

This discovery opens a path toward the fabrication of electronic devices with modulated, or tunable, superconducting properties which can be controlled by electric or magnetic fields.

Share

Superconducting signature

For a while now, some scientists have thought that conditions necessary for superconductivity at higher temperatures exist. Now, a paper published in Science adds some fuel to their argument. It concerns the work of a group of U.S. and Japanese researchers – sponsored by a set of U.S. and Japanese government agencies – doing observations of low-temperature superconducting materials who say the spectroscopic signature of the materials seems to indicate that some superconductivity properties continue as the temperature increases.

“Our measurements give the most definitive spectroscopic evidence that the material we studied is a superconductor, even above the transition temperature, but one without the quantum phase coherence required for current to flow with no resistance,” said team leader Seamus Davis, a physicist at the Brookhaven National Lab and Cornell University, who led the research team. “The spectroscopic ‘fingerprint’ confirms that, at these higher temperatures, electrons are pairing up as they must in a superconductor, but for some reason they are not co-operating coherently to carry current.”

Recently there has been interest in the high-temperature possibilities of copper-oxide superconductors containing bismuth, strontium and calcium (BSCCO). Previously, Davis’ group was able to assemble a detailed spectroscopic signature containing all the quantum mechanical details of that superconducting state. Once this was established, they made spectroscopic observations of the cuprate material as it warmed above the 37 K transition temperature.

BNL scientists are now able to grow large BSCCO crystals. Credit: BNL

“We found that the characteristic signature passes unchanged from the superconducting state into the parent state - up to temperatures of at least 55 K, or 1.5 times the transition temperature,” Davis said. “We know of no explanation for why this fingerprint should remain other than that it represents the phase-incoherent superconducting state which has been proposed to exist based on other kinds of measurements.”

The group’s next step is to try to get a handle on why the cooperation between electron pairs breaks down. Their plan is to start tinkering with the doping of the copper-oxide planes in the layered material and measure the strength of quantum phase fluctuations.

Also, another group of BNL researchers has been on a similar mission, but studying how the magnetic properties of BSCCO change as temperatures are increased above normal superconducting ranges. One hurdle for this group has been creating BSCCO crystals large enough for observation, but as the accompanying picture shows, that the size problem has been solved.

“Many theorists believe that magnetism is important for high-temperature superconductivity, although they don’t agree on how it is important,” said Brookhaven physicist John Tranquada, who led the research team.

“The calculations based on the material’s electronic properties — which change dramatically as the material is cooled and transitions from its electrically resistive state to become a superconductor — predicted there would be a similar large change in magnetic characteristics below the transition temperature,” said Brookhaven physicist Guangyong Xu.

“But our direct measurements of the magnetic properties showed surprisingly little change. This implies that the model the theorists have been using to describe these magnetic properties is incomplete. It could be that the magnetism somehow drives the electronic structure, rather than the other way around — or that something underlying both magnetism and electronic structure influences both but in different ways,” Xu said.

Share

Inside of the diamond cell: In the middle is the coil system around the diamond anvil, which picks up the shielding signal from the superconducting sample. Credit: James Schilling, Washington Univ.

A duo from Washington University in St. Louis reports they have for the first time found a way to tap superconductivity properties of europium.

In work funded by the materials research division of NSF, WUSTL professor James Schilling and then-doctoral student Mathiewos Debessai found that europium becomes superconducting at 1.8 K (-456°F) and 80 GPa (790,000 atmospheres) of pressure, making it the 53rd known elemental superconductor and the 23rd at high pressure. Schilling and Debessai used a diamond anvil and coil system to conduct their measurements.

“It has been seven years since someone discovered a new elemental superconductor,” Schilling said. “It gets harder and harder because there are fewer elements left in the periodic table.”

Europium isn’t an obvious superconductor. As a rare earth element, its natural magnetic properties actually run counter to superconductivity. “Superconductivity and magnetism hate each other. To get superconductivity, you have to kill the magnetism,” Schilling explained.

However, armed with the insight that europium should be the easiest of the rare earths to lose magnetic properties under high compression, the researchers. “When europium atoms condense to form a solid, only two electrons per atom are released and europium remains magnetic. Applying sufficient pressure squeezes a third electron out and europium metal becomes trivalent. Trivalent europium is nonmagnetic, thus opening the possibility for it to become superconducting under the right conditions,” Schilling said.

“Theoretically, the elemental solids are relatively easy to understand because they only contain one kind of atom,” Schilling said. “By applying pressure, however, we can bring the elemental solids into new regimes, where theory has difficulty understanding things. When we understand the element’s behavior in these new regimes, we might be able to duplicate it by combining the elements into different compounds that superconduct at higher temperatures.”

Schilling and Debessai’s findings are published in a recent issue of Physical Review Letters in an article titled “Pressure-induced Superconducting State of Europium Metal at Low Temperatures.” Schilling is also presenting their research at the International Conference on High Pressure Science and Technology in July, 2009, in Tokyo, Japan.

Share

AFM image of crystallographic structure of a sheet of graphene. Credit: Jarillo-Herrero group

A recent article in MIT Tech Talk describes aspects of several exciting graphene research projects at MIT.

A successor to silicon? Graphene could become the successor to silicon in a new generation of microchips because of its unique electrical characteristics. Graphene could surmount the basic physical constraints that limit further development of smaller, faster chips.

Transparent electrodes? Pure graphene is transparent because of its single-atom thickness. Therefore, it can be used to make transparent electrodes for light-based applications, such as LEDs or solar cells.

Substitute for copper? Graphene also could substitute for copper to make the electrical connections between computer chips and other electronic devices. This would provide lower resistance and generate less heat.

AFM of graphene superconducting FET. The two gold-colored electrodes are made of superconducting titanium-aluminum alloy. Credit: Jarillo-Herrero group

Study quantum-mechanical effects? A team led by Pablo Jarillo-Herrero, an assistant professor of physics, is studying its basic physical properties and using graphene’s unique behavior as a way to study fundamental quantum-mechanical effects. For example, in graphene, electrons behave as if they were massless particles that propagate according to the laws of relativistic quantum mechanics, a behavior that is normally reserved to particles traveling near the speed of light in accelerators or in the cosmos. Such behavior is at the heart of the ultrahigh mobilities exhibited by graphene devices. Jarillo-Herrero says that because the material is so new and its fundamental properties still being discovered, “we have some applications in mind, but many totally new ones will for sure come up as we continue doing research.”

Graphene production? Another team, led by Jing Kong, the ITT Career Development Associate Professor of Electrical Engineering, is working on developing commercial methods to produce the material in greater quantities. The team has created sheets of graphene by chemical vapor deposition. Kong’s method uses equipment that is “very compatible to conventional semiconductor processing.” The method “is quite straightforward, and not too expensive,” she says. That’s good news for commercial applications. For specialized functions, such as computer chips, further research will be needed to improve the quality and uniformity of the graphene sheets, she says, but for other applications, such as solar-cell electrodes, the existing process allows the researchers to start the investigation.

The MIT article also includes an excellent review of the structure, properties and history of graphene.

Share