Scientists have discovered a way to alter heat transport in thermoelectric materials, a finding that may ultimately improve energy efficiency as the materials convert heat flow into electricity. Caltech theorists simulating the thermoelectric material lead selenide saw something surprising—a thermal wave that did not propagate. They determined the trick to potentially increasing energy efficiency in this material was to stop heat-carrying vibrational waves without thwarting electricity-bearing electrons. To verify the discovery, they called on experimentalists to probe a real crystal. “Vibrational waves stop propagating in a perfect crystal because of nonlinear interactions between phonons,” said Michael Manley of Oak Ridge National Laboratory. The experiment used neutron scattering at ORNL’s Spallation Neutron Source and the National Institute of Standards and Technology’s Center for Neutron Research and X-ray scattering at Argonne National Laboratory’s Advanced Photon Source. The discovery improves understanding of thermoelectric performance and may enable unconventional heat transport in future materials.