Stephen Carr

Accessibility of quantum effects in mesomechanical systems

We explore the quantum aspects of an elastic bar supported at both ends and subject to compression. If strain rather than stress is held fixed, the system remains stable beyond the buckling instability, supporting two potential minima. The classical equilibrium transverse displacement is analogous to a Ginsburg-Landau order parameter, with strain playing the role of temperature. We calculate the quantum fluctuations about the classical value as a function of strain. Excitation energies and quantum fluctuation amplitudes are compared for silicon beams and carbon nanotubes.

Cryogenic preamplification of a single-electron-transistor using a silicon-germanium heterojunction-bipolar-transistor

We examine a silicon-germanium heterojunction bipolar transistor (HBT) for cryogenic pre-amplification of a single electron transistor (SET). The SET current modulates the base current of the HBT directly. The HBT-SET circuit is immersed in liquid helium, and its frequency response from low frequency to several MHz is measured. The current gain and the noise spectrum with the HBT result in a signal-to-noise-ratio (SNR) that is a factor of 10–100 larger than without the HBT at lower frequencies. The transition frequency defined by SNRu2009=u20091 has been extended by as much as a factor of 10 compared to without the HBT amplification. The power dissipated by the HBT cryogenic pre-amplifier is approximately 5 nW to 5u2009μW for the investigated range of operation. The circuit is also operated in a single electron charge read-out configuration in the time-domain as a proof-of-principle demonstration of the amplification approach for single spin read-out.

Static buckling and actuation of free-standing mesoscale beams

We discuss the static buckling and actuation of free-standing mesoscopic beams. As the length-to-width aspect ratio of the beams is increased, the buckled shapes become more complex. Mechanical actuation confirms that the complex shapes are stable equilibria and that metastable equilibria also exist. This is consistent with a nonlinear elastic continuum model, which incorporates asymmetry in higher order buckling modes. Preliminary results indicate that, in addition to purely mechanical actuation, electromechanical mode-mode transitions can be induced by utilizing fabricated electrodes.

Crossover between quantum and thermal regimes of free-standing nanostructures

Mesomechanical systems offer the possibility to observe and control certain quantum effects. Here we develop a phonon description of transverse motion of a mesoscopic beam with fixed endpoints. We obtain expressions for the fundamental frequencies and quantum fluctuation amplitudes of these structures when subjected to longitudinal compression. We show how the frequencies and fluctuation amplitudes scale with physical dimensions and give estimates for the crossover temperatures which characterize the transition between quantum and thermal regimes.

Direct contact buckling of electrochemically grown gold nanowires

We report the electrochemical growth and direct probing of near-vertical gold nanowires several micrometers long that are attached at one end to a substrate. Controllable contact is made to the wires using a straightforward tip-probe arrangement inside a scanning electron microscope. The contact allowed us to perform mechanical and electrical measurements. Through elastic nanowire buckling, we have observed dynamic low-frequency parametric pumping caused by small vibrations of the probe. A numerical simulation of the recorded motion is consistent with an estimated value of Young’s modulus of about 80GPa, which is close to that of bulk gold.

Novel buckled shapes of free-standing mesoscopic beams

We discuss the observation of novel buckled shapes of free-standing mesoscopic beams fabricated using conventional electron-beam lithography and plasma etching. As the length-to-width aspect ratio of the beams is increased the buckled shapes become more complex. Based on analysis of beams in the low aspect ratio regime using a nonlinear elastic continuum model, we obtain an upper bound on the magnitude of the fundamental mode asymmetry and relate this to higher-order asymmetry. We also observe an electromechanical mode-mode transition using capacitively coupled gates.