The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.

Coupled mechanical oscillations were first observed in paired pendulum clocks in the mid-seventeenth century and were extensively studied for their novel sympathetic oscillation dynamics [1, 2]. In this era of nanotechnologies, coupled oscillations have again emerged as subjects of interest when realized in nanomechanical resonators for both practical applications and fundamental studies [3][4][5][6][7][8][9][10][11]. However, a key obstacle to the further development of this architecture is the ability to coherently manipulate the coupled oscillations. This limitation arises as a consequence of the usually weak coupling between the constituent nanomechanical elements. Here, we report parametrically coupled mechanical resonators in which the coupling strength can be dynamically adjusted by modulating (pumping) the stress in the mechanical elements via a piezoelectric transducer. The parametric control enables the coupling rate between the two resonators to be made so strong that it exceeds their intrinsic energy dissipation rate by more than a factor of four. This ultra-strong coupling can be exploited to coherently transfer phonon populations, namely phonon Rabi oscillations [12,13], between the mechanical resonators via two coupled vibration modes, realizing superposition states of the two modes and their time-domain control. More unexpectedly, the nature of the parametric coupling can also be tuned from a linear first-order interaction to a non-linear higher-order process in which more than one pump phonon mediates the coherent oscillations. This demonstration of multipump phonon mixing echoes multi-wave photon mixing [14] and suggests that concepts from nonlinear optics can also be applied to mechanical systems. Ultimately, the parametric pumping is not only useful for controlling classical oscillations [15] but can also be extended to the quantum regime [12,13,[16][17][18], opening up the prospect of entangling two distinct macroscopic mechanical objects [19,20].The dynamic parametric coupling is developed in GaAs-based paired mechanical beams shown in Fig. 1a, in which the piezoelectric effect is exploited to mediate all-electrical displacement transduction [21]. The frequency response of beam 1 measured by harmonically driving it while the parametric pump is deactivated displays two coupled vibration modes (Fig. 1b), where mode 1 (ω 1 = 2π × 293.93 kHz) is dominated by the vibration of beam 1 while mode 2 (ω 2 = 2π × 294.37 kHz) is dominated by the vibration of beam 2. The amplitude of mode 2 is much smaller than that of mode 1 reflecting the energy exchange due to the structural coupling via the overhang is inefficient because of the eigenfrequency difference between the two beams. This difference can be compensated by activating the parametric pump, which is induced by piezoelectrically modulating the spring constant of beam 1 with the pump frequency ω p at around the frequency difference between the two modes, ∆ω ≡ ω 2 − ω 1 (Fig. 1c).The dynamics of this system can then be expressed by the following e...

The Parametron was first proposed as a logic-processing system almost 50 years ago. In this approach the two stable phases of an excited harmonic oscillator provide the basis for logic operations. Computer architectures based on LC oscillators were developed for this approach, but high power consumption and difficulties with integration meant that the Parametron was rendered obsolete by the transistor. Here we propose an approach to mechanical logic based on nanoelectromechanical systems that is a variation on the Parametron architecture and, as a first step towards a possible nanomechanical computer, we demonstrate both bit storage and bit flip operations.