Ballistic Phonons Reveal Strange Attenuation In Lead Superconductor

"Lead is a conventional superconductor with not-so-conventional properties," said Jim Wolfe, a UI professor of physics and a researcher at the university’s Frederick Seitz Materials Research Laboratory. "Anomalies in the lattice dynamics, specific heat and acoustic attenuation in lead superconductors have puzzled researchers for several decades. Now, we are much closer to a viable explanation." Several years ago, Albert Overhauser – a physicist at Purdue University – proposed a theory to account for some of this odd behavior. Overhauser suggested that the ground state for lead possessed an unusual spin-density-wave structure not normally found in superconductors.

"If spin-density waves did, indeed, exist in lead, they would create a large anisotropy in the superconducting gap," Wolfe said. "We thought this anisotropy might be revealed by imaging the transmission of phonons through a single crystal of high-purity lead."

Wolfe’s research group invented the method of phonon imaging in order to examine the propagation and scattering of high-frequency phonons in crystals at low temperatures. The technique – which measures the spatial pattern of heat flux emanating from a point source – can probe anisotropies in the superconducting gap of conventional superconductors.

To image phonons, Wolfe and his graduate student, Jonathan Short, use a laser pulse to generate thermal energy at a point on the surface of a supercooled crystal. They record the arrival of the thermal energy after it propagates through the crystal lattice to a detector – a small, superconducting aluminum bolometer. Scanning the laser beam, they piece together many measurements to create a time-lapse movie showing phonon movement.

"In our experiment, we found certain directions in which the phonons are attenuated, even though the usual expectation for lead is that the superconducting gap is very isotropic," Wolfe said. "This suggests there are directions in the superconductor where the energy gap is much lower than usual."

For a large energy gap, phonons can propagate ballistically – that is, without scattering off electrons. A significant reduction in the gap along specific directions will result in highly anisotropic attenuation of these phonons, producing a pattern of dark lines in a phonon image.

"Our experiment provides an interesting piece of evidence that seems to support Overhauser’s theory," Wolfe said. "The present theoretical challenge is to apply the spin-density-wave theory to the specific electronic structure of lead and see if the experimental results are reproduced in detail."

Short and Wolfe presented their latest findings March 16 during a meeting of the American Physical Society, held in Seattle. The U.S. Department of Energy funded the work.