Guiti Zolfagharkhani

Controllable nanomechanical memory element

We report the realization of a completely controllable high-speed nanomechanical memory element fabricated from single-crystal silicon wafers. This element consists of a doubly-clamped suspended nanomechanical beam structure, which can be made to switch controllably between two stable and distinct states at a single frequency in the megahertz range. Because of their sub-micron size and high normal-mode frequencies, these nanomechanical memory elements offer the potential to rival the current state-of-the-art electronic data storage and processing.

Evidence for quantized displacement in macroscopic nanomechanical oscillators.

We report the observation of discrete displacement of nanomechanical oscillators with gigahertz-range resonance frequencies at millikelvin temperatures. The oscillators are nanomachined single-crystal structures of silicon, designed to provide two distinct sets of coupled elements with very low and very high frequencies. With this novel design, femtometer-level displacement of the frequency-determining element is amplified into collective motion of the entire micron-sized structure. The observed discrete response possibly results from energy quantization at the onset of the quantum regime in these macroscopic nanomechanical oscillators.

Quantum friction in nanomechanical oscillators at millikelvin temperatures

We report low-temperature measurements of dissipation in megahertz-range, suspended, single-crystal nanomechanical oscillators. At millikelvin temperatures, both dissipation (inverse quality factor) and shift in the resonance frequency display reproducible features, similar to those observed in sound attenuation experiments in disordered glasses and consistent with measurements in larger micromechanical oscillators fabricated from single-crystal silicon. Dissipation in our single-crystal nanomechanical structures is dominated by internal quantum friction due to an estimated number of roughly 50 two-level systems, which represent both dangling bonds on the surface and bulk defects.

Spectral response of a gigahertz-range nanomechanical oscillator

We report the measurement and simulation of the transverse displacement spectrum of a multi-element nanomechanical oscillator at previously inaccessible frequencies of up to 3GHz. The detected displacement signal is enhanced in the high-frequency range by the presence of high-order resonance modes generated by coherent motion of individual elements. The spectrum reveals a rich structure with groups of peaks forming quasibands. The spectral structure is qualitatively analogous to atomic emission spectra.

Spin-mechanical device for detection and control of spin current by nanomechanical torque

We propose a spin-mechanical device to control and detect spin currents by mechanical torque. Our hybrid nanoelectromechanical device, which contains a nanowire with a ferromagnetic-nonmagnetic interface, is designed to measure or induce spin-polarized currents. Since spin carries angular momentum, a spin-flip or spin-transfer process involves a change in angular momentum and hence, a torque, which enables mechanical measurement of spin flips. Conversely, an applied torque can result in spin polarization and spin current.

Temperature dependence of a nanomechanical switch

We present the effect of temperature on the switching characteristics of a bistable nonlinear nanomechanical beam. At megahertz-range frequencies, we find that it is possible to controllably change the state of the system between two stable mechanical states defined by the hysteresis brought on by nonlinear excitation. We find that the introduction of increased temperature results in a loss of switching fidelity, and that temperature acts as an effective source of external noise on the dynamics of the system.

Energy measurement in nonlinearly coupled nanomechanical modes

We report direct measurements of average vibration energy in a high frequency flexural resonance mode achieved via an-harmonic elastic coupling to a fundamental vibration mode of a nanomechanical resonator. The second order coupling effect produces a frequency shift of the read-out mode as a function of the mean square of the excitation amplitude of the high order mode. We measure frequency shifts at the lowest driving amplitudes, down to the noise floor of the experimental setup. With implementation of existing ultra-sensitive amplifiers, the reported technique will enable direct measurements of quantized energy transitions in low-thermal occupation number nanomechanical resonators.

Signal Processing and Control in Nonlinear Nanomechanical Systems

Bestriding the realms of classical and quantum mechanics, nanomechanical structures offer great promise for a huge variety of applications, from computer memory elements \cite{badzey04} and ultra-fast sensors to quantum computing. Intriguing as these possibilities are, there still remain many important hurdles to overcome before nanomechanical structures approach anything close to their full potential. With their high surface-to-volume ratios and sub-micron dimensions, nanomechanical structures are strongly affected by processing irregularities and susceptible to nonlinear effects. There are several ways of dealing with nonlinearity: exceptional fabrication process control in order to minimize the onset of nonlinear effects or taking advantage of the interesting and oftentimes counterintuitive consequences of nonlinearity. Here, we present evidence for the use of stochastic resonance as a means of coherent signal amplification for use in nanomechanical devices. Aside from being simply one more system in which the phenomenon has been demonstrated, nanoscale systems \cite{lee03} are interesting because of their proximity to the realm of quantum mechanics. The combination of stochastic resonance and quantum mechanics has been the subject of intense theoretical activities \cite{wellens00, goychuk99, grif96, lof94} for many years; nanomechanical systems present a fertile ground for the study of a broad variety of novel phenomena in quantum stochastic resonance. Additionally, the physical realization of such nonlinear nanomechanical strings offer the possibility of studying a whole class of phase transition phenomena, particularly those modeled by a Landau-Ginzburg quantum string \cite{benzi85, hu99}.

Quantized Displacement in Nanomechanical Oscillators at Millikelvin Temperatures

Recent technological advances have enabled the creation of nanomechanical structures with resonance frequencies exceeding 2 GHz. The oscillators are nanomachined single‐crystal structures of silicon, designed to provide two distinct sets of coupled elements with very low and very high frequencies. At higher temperatures — in the so‐called classical regime, the response to an applied driving force is continuous. At millikelvin temperatures these nanomechanical oscillators enter the quantum regime of mechanical motion with a manifestly discrete response to the applied drive. The observed discrete response most likely results from the onset of the quantum regime in these macroscopic nanomechanical oscillators. These new experiments offer exciting prospects for the studies of quantum measurement and quantum computation, involving macroscopic quantum mechanical oscillators.