Jared B. Hertzberg, University of Maryland
Quantized vibrations of the lattice (phonons) are intrinsic to all of condensed- matter physics. Most experimental probes, however, treat the phonons in terms of thermal-equilibrium quantities such as heat capacity and thermal conductance, rather than directly addressing the vibrational quanta. I will discuss two types of experiments employing nanofabricated devices and low-temperature techniques to explore and exploit non-equilibrium phonon behavior. In the first experiment, we seek to prepare the 6 MHz vibrational mode of a nanomechanical (NEMS) structure in its quantum ground state, and to measure its motion at a precision limited by the Heisenberg uncertainty principle. By coupling the mechanical oscillator to a 5 GHz electromagnetic mode in a superconducting cavity, we reduce the mechanical energy to levels far below thermal equilibrium and enable the oscillator's motion to be determined at a precision approaching the quantum zero-point amplitude. [1] In the second experiment, we demonstrate a technique to produce a tunable narrow- band source of traveling acoustic phonons at frequencies from ~100 GHz to ~1 THz. [2] We accomplish the phonon emission and detection using excited electron states in superconducting tunnel junctions at a temperature of 0.3 K. We use this flux of acoustic energy to probe phonon transmission through ~100 nm-wide silicon nanosheets in the boundary-scattering regime. Our results illuminate the effects of phonon-surface interactions on heat flow through nano materials.
[1] J. B. Hertzberg et al, Nature Physics 6, 213 (2010).
[2] J. B. Hertzberg et al, Nano Lett. Article ASAP 10.1021/nl402701a (2013).