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Title: Souping Up Superconductors
Date: 01/17/2008
Publication: Science Matters @ Berkeley
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Souping Up Superconductors

Alessandra Lanzara is a Berkeley professor of physics and a member of Lawrence Berkeley National Laboratory's Materials Sciences Division. Photo credit: courtesy Alessandra Lanzara

Imagine a world where electricity was virtually free and the means to store it limitless. Alessandra Lanzara, a Berkeley professor of physics, sees a way to reach this goal: by restringing the power grid with high temperature superconductors.

"There is a lot of waste getting electricity from its production site to your home. This is because materials that carry a current have resistivity; their conduction isn't perfect," Lanzara says.

Superconductors, on the other hand, can transmit a current without loss when chilled below a critical temperature. Power lines made of superconductors, Lanzara argues, could retain the energy now lost to waste, drastically increasing the amount available for use and decreasing its cost. Superconductors can also hold a current indefinitely without any loss of power, making them ideal for storing intermittent energy from sources like the sun.

There is a catch, however-the expense of keeping power lines cold largely offsets any gains in energy efficiency. The first superconductor, discovered in 1911, operated at a phenomenally cold -269 degrees Celsius. Since then, scientists have hit upon so-called high temperature superconductors. Made of ceramics mixed with other elements such as copper and oxygen, these materials must still be chilled to below -140 degrees Celsius to conduct electricity freely.

Lanzara uses a method called photoemissions spectroscopy to measure electron behavior within superconductors. Here, a photon of light hits a sample of graphene. The released electron reveals how it was traveling in the sample. Image credit: Alessandra Lanzara

"We are trying to understand, when you cool them down, what the driving force is behind superconductivity. If you identify the mechanism here, maybe you can use this information to engineer a new material that superconducts at a higher temperature," Lanzara says.

A superconductor's remarkable properties derive from the flow of electrons within it. Lanzara observes this movement in superconductors using a technique called photoemission spectroscopy. Using light, she excites electrons to emerge from her sample. By mapping the angles and velocities of exiting electrons, Lanzara can deduce how they were moving inside each material.

Under normal conditions, electrons are negatively charged and should repel one another. But when a metallic superconductor drops below a critical temperature, its electrons suddenly begin traveling in pairs. The movement of these particles is akin to two bowling balls rolling across a mattress. The first electron deforms the energetic space through which electrons travel. This makes a second electron following close behind likely to follow the same path. In traditional superconductors, the mattress effect can be identified by the atomic vibrations, or phonons, it triggers.

"The big question is whether this mattress effect is still at work in the new ceramic superconductors," Lanzara says.

To find out, Lanzara has developed a new twist on photoemissions spectroscopy. She measures not only the speed and trajectory of emerging electrons, but also their spin dynamics as well. This additional piece of data allows Lanzara to describe in unprecedented detail how electrons behave within a sample.

Lanzara has developed a way to improve graphene's superconductivity. Growing graphene on silicon carbide disrupts the symmetry of its atoms, and opens a gap in its electron bands crucial for its use in electronic devices. Image credit: Alessandra Lanzara

Lanzara also studies another material with unusual properties: graphene. Essentially made of carbon atoms interlocked in open rings, graphene's airy structure lets electrons travel long distances through it at ballistic speeds without hitting any obstructions. This simple lattice also allows graphene to be molded into shapes just one atom thick. Meanwhile, its high carbon content can withstand very high temperatures. For these reasons, graphene is a leading candidate to replace silicon in a new generation of tiny, superfast computer chips.

But before this can happen, scientists must learn to modulate graphene's electrical properties. Silicon functions as a transistor because there is a gap between its tightly bound valence band electrons and its more loosely bound conduction electrons.

Graphene normally lacks this gap. But Lanzara has been able to induce a gap by growing a thin layer of graphene on semiconducting substrates. Her approach could lead to methods of mass manufacturing graphene for electronic devices.

"I like basic research, but when I chose which type of physics to go into, for me it was really important to have a potential application," Lanzara says. "If I figure out what is going on with this material, maybe I can make a big change in the way we live and contribute to something really special."

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