Doppler Effect Reveals Gap Nodes, Physics Research Highlights, University of Illinois at Urbana-Champaign

Professor Myron Salamon and graduate student Tuson Park, with colleagues at Pohang University, report the first direct observation of a Doppler effect in the energy of excitations in a flowing superfluid. They believe this phenomenon should be common to many so-called unconventional superconductors, those whose Cooper pairs have symmetries more complex than originally envisaged in the classic BCS theory of superconductivity.

In particular, if the pairs have a symmetry similar to d-orbitals in atoms, certain directions in momentum space (nodes) would exist, along which electronic excitations, termed quasiparticles, could arise without an energy gap. In the vortex state of such superconductors, the zero-energy density of states of such nodal quasiparticles depends on the angle between the vortex line and the direction of the node.

In experiments at the University of Illinois, Salamon and Park detected this density-of-states modulation directly for the first time via low-temperature heat capacity and studied its behavior as a function of field and temperature. The figure shows three-dimensional surface plots of the magnetic field-angle (x axis) and field intensity (y axis) dependence of the in-plane specific-heat ratio at 2.5 K. (Click on the figure for a higher-resolution image.) The upper plot represents a numerical calculation of the specific heat due to nodal quasiparticles in a 3D system, and the lower plot presents experimental data taken in the vortex phase. The sample used, YNi₂B₂C, is a member of a fascinating class of superconductors that are believed to exhibit d-symmetry. The findings have been accepted for publication in Physical Review Letters.

The Illinois work was supported in part by the National Science Foundation (Grant No. 99-72087). X-ray measurements were carried out in the Center for Microanalysis of Materials, University of Illinois at Urbana-Champaign, which is partially supported by the U.S. Department of Energy (Grant DEFG02-91-ER45439). Any opinion, findings, and conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the National Science Foundation or the Department of Energy.