Dissipation in nanoelectromechanical systems

This article is a review of the dissipation processes in nanoelectromechanical systems (NEMS). As NEMS technology becomes more and more prevalent in research and engineering applications, it is of great importance to understand the dissipative mechanisms that in part define the dynamic response of such devices. The purpose of this work is to understand, sort, and categorize dominant dissipation sources and to determine their significance with respect to physics processes and engineering considerations.     

Scattering approach to backaction in coherent nanoelectromechanical systems

We present theoretical results for the backaction force noise and damping of a mechanical oscillator whose position is measured by a mesoscopic conductor. Our scattering approach is applicable to a wide class of systems; in particular, it may be used to describe point contact position detectors far from the weak tunneling limit. We find that the backaction depends not only on the mechanical modulation of transmission probabilities, but also on the modulation of scattering phases, even in the absence of a magnetic field. We illustrate our general approach with several simple examples, and use it to calculate the backaction for a movable, Au atomic point contact modeled by ab initio density functional theory.     

Control of the motion of nanoelectromechanical systems based on carbon nanotubes

A new method is proposed for controlling the motion of nanoelectromechanical systems based on carbon nanotubes. In this method, a single-walled nanotube acquires an electric dipole moment owing to the chemical adsorption of atoms or molecules at open ends of the nanotube and, then, the electric dipole moment thus induced can be set in motion under the effect of a nonuniform electric field. The electric dipole moments of chemically modified nanotubes are calculated for the first time. The possibility of controlling the motion of nanotube-based nanoelectromechanical systems with the proposed method is demonstrated using a gigahertz oscillator as an example. The operating characteristics of the gigahertz oscillator and the controlling electric field are calculated.     

Dynamics and dissipation induced by single-electron tunneling in carbon nanotube nanoelectromechanical systems

We demonstrate the effect of single-electron tunneling (SET) through a carbon nanotube quantum dot on its nanomechanical motion. We find that the frequency response and the dissipation of the nanoelectromechanical system to SET strongly depends on the electronic environment of the quantum dot, in particular, on the total dot capacitance and the tunnel coupling to the metal contacts. Our findings suggest that one could achieve quality factors of 10(6) or higher by choosing appropriate gate dielectrics and/or by improving the tunnel coupling to the leads.     

Control of the motion of nanoelectromechanical systems based on carbon nanotubes by electric fields

A new method is proposed for controlling the motion of nanoelectromechanical systems based on carbon nanotubes. In this method, a single-walled nanotube acquires an electric dipole moment owing to the chemical adsorption of atoms or molecules at open ends of the nanotube. The electric dipole moments of carbon nanotubes with chemically modified ends are calculated by the molecular orbital method. These nanotubes can be set in motion under the effect of a nonuniform electric field. The possibility of controlling the motion of nanoelectromechanical systems with the proposed method is demonstrated using a nanotube-based gigahertz oscillator as an example. The operating characteristics of the gigahertz oscillator are analyzed, and its operation is simulated by the molecular dynamics method. The controlling parameters and characteristics corresponding to the controlled operating conditions at a constant frequency for the system under investigation are determined.     

Spintronics of a nanoelectromechanical shuttle

We consider effects of the spin degree of freedom on the nanomechanics of a single-electron transistor (SET) containing a nanometer-sized metallic cluster suspended between two magnetic leads. It is shown that in such a nanoelectromechanical SET (NEM-SET) the onset of an electromechanical instability leading to cluster vibrations and shuttle transport of electrons between the leads can be controlled by an external magnetic field. Different stable regimes of this spintronic NEM-SET operation are analyzed. Two different scenarios for the onset of shuttle vibrations are found.     

Towards quantum entanglement in nanoelectromechanical devices

We study arrays of mechanical oscillators in the quantum domain and demonstrate how the motions of distant oscillators can be entangled without the need for control of individual oscillators and without a direct interaction between them. These oscillators are thought of as being members of an array of nanoelectromechanical resonators with a voltage being applicable between neighboring resonators. Sudden nonadiabatic switching of the interaction results in a squeezing of the states of the mechanical oscillators, leading to an entanglement transport in chains of mechanical oscillators. We discuss spatial dimensions, Q factors, temperatures and decoherence sources in some detail, and find a distinct robustness of the entanglement in the canonical coordinates in such a scheme. We also briefly discuss the challenging aspect of detection of the generated entanglement.     

磁性纳米电机单电子晶体管的自旋输运特性

文章作者在研究磁性隧道结的自旋输运中引入量子点的机械振动自由度,将单电子隧穿和振动自由度耦合所导致的shuttle输运理论应用到自旋电子学中.研究结果表明,shuttle输运对自旋极化输运有很大的影响,其独特的输运性质可以用来设计自旋电子器件.文章在理论上提出具有巨磁效应的自旋阀、高性能的半导体自旋注入器以及电流的整流器.     

磁性纳米电机单电子晶体管的自旋输运特性

文章作者在研究磁性隧道结的自旋输运中引入量子点的机械振动自由度,将单电子隧穿和振动自由度耦合所导致的shuttle输运理论应用到自旋电子学中,研究结果表明,shuttle输运对自旋极化输运有很大的影响,其独特的输运性质可以用来设计自旋电子器件,文章在理论上提出具有巨磁效应的自旋阀、高性能的半导体自旋注入器以及电流的整流器.     

Spin valve effect in a magnetic nanoelectromechanical shuttle

We study spin-dependent shuttle phenomena in a nanoelectromechanical single electron transistor (NEM-SET) with magnetic leads by considering the coupling between the transport of spin-polarized electrons and mechanical oscillations of the nanometer quantum dot. It is shown that there are two different bias-voltage thresholds for the shuttle instability in electronic transport through the NEM-SET, respectively, corresponding to parallel (P) and antiparallel (AP) magnetization alignments. In between the two thresholds, the electronic transport is in the shuttling regime for the P alignment but in the tunneling regime for the AP one, resulting in a very large spin valve effect.     

Quantum shuttle phenomena in a nanoelectromechanical single-electron transistor

An analytical analysis of quantum shuttle phenomena in a nanoelectromechanical single-electron transistor has been performed in the realistic case, when the electron tunneling length is much greater than the amplitude of the zero point oscillations of the central island. It is shown that when the dissipation is below a certain threshold value, the vibrational ground state of the central island is unstable. The steady state into which this instability develops is studied. It is found that if the electric field E between the leads is much greater than a characteristic value E(q), the quasiclassical shuttle picture is recovered, while if E0) shuttle vibrations.     

Self-organization of irregular nanoelectromechanical vibrations in multimode shuttle structures

We investigate theoretically multimode electromechanical "shuttle" instabilities in dc voltage-biased nanoelectromechanical single-electron tunneling devices. We show that initially irregular (quasiperiodic) oscillations that occur as a result of the simultaneous self-excitation of several mechanical modes with incommensurable frequencies self-organize into periodic oscillations with a frequency corresponding to the eigenfrequency of one of the unstable modes. This effect demonstrates that a local probe can selectively excite global vibrations of extended objects.     

Molecular dynamics modeling and simulation of a graphene-based nanoelectromechanical resonator

A tunable graphene-resonator was investigated using classical molecular dynamics modeling and simulations. The fundamental resonance frequency variation of the graphene resonator was found to be very closely related to the average tension acting on both its edges. The initial stain-induced tension could be adjusted by using the mismatch between the negative thermal expansion coefficient of the graphene and the positive thermal expansion coefficient of the substrate, and the deflection-induced tension could be controlled by an electrostatic capacitive force due to the gate voltage. For very small initial axial-strains, the tunable range reached above several hundred gigahertz. As the initial axial-strain on the graphene-resonator increased, both the tunability and the tunable range decreased. The fundamental resonance frequency as a function of the calculated gate voltage was in good agreement with previous experiments. Considering the variables that affect the tension variation, this graphene-resonator is suitable for use as an ultra-sensitive accelerometer, thermo-sensor or weight scale, as well as many other types of sensor.     

Electron-spin manipulation and resonator readout in a double-quantum-dot nanoelectromechanical system

We demonstrate how magnetically coupling a nanomechanical resonator to a double quantum dot confining two electrons can enable the manipulation of a single electron spin and the readout of the resonator's natural frequency. When the Larmor frequency matches the resonator frequency, the electron spin in one of the dots can be selectively and coherently flipped by the magnetized oscillator. By simultaneously measuring the charge state of the two-electron double quantum dots, this transition can be detected thus enabling the natural frequency and displacement of the mechanical oscillator to be determined.     

Dynamical electron transport through a nanoelectromechanical wire in a magnetic field

We investigate dynamical transport properties of interacting electrons moving in a vibrating nanoelectromechanical wire in a magnetic field. We have built an exactly solvable model in which the electron-electron interaction is considered nonperturbatively and the electric current and mechanical vibration are treated fully quantum mechanically on an equal footing. We demonstrate our theory by calculating the admittance of a finite-size wire, which is influenced by the magnetic field strength, the electron-electron interaction, and the complex interplay between the mechanical and the electrical energy scales. Nontrivial features including sharp resonance peaks appear in the admittance, which may be experimentally observable.     

Experimental demonstration of a laterally deformable optical nanoelectromechanical system grating transducer

We experimentally demonstrate operation of a laterally deformable optical nanoelectromechanical system grating transducer. The device is fabricated in amorphous diamond with standard lithographic techniques. For small changes in the spacing of the subwavelength grating elements, lossy propagating resonant modes in the plane of the grating cause a large change in the optical reflection amplitude. An in-plane motion detection sensitivity of 160 fm/square root(Hz) was measured, exceeding that of any other optical microelectromechanical system transducer to our knowledge. Calculations predict that this sensitivity could be improved to better than 40 fm/square root(Hz) in future designs. In addition to having applications in the field of inertial sensors, this device could also be used as an optical modulator.     

Highly enhanced thermoelectric figure of merit of a β-SiC nanowire with a nanoelectromechanical measurement approach

We developed a reliable and highly reproducible way of fabricating a one-stop measurement platform for characterizing the thermoelectric properties of individual nanowires (NWs) using a focused ion beam and a nanomanipulator. 3-ω and 1-ω signals obtained by the four-point-probe method were used in measuring the thermal and electrical conductivities of the NW. Subsequently, the Seebeck coefficient was measured by using additional nanoelectrodes including a nanoheater. The thermal conductivity of the single β-SiC NW was obtained at 86.5±3.5 W/mK. The Seebeck coefficient was obtained to be −1.21 mV/K by using the same measurement platform. Thus, the dimensionless figure of merit, ZT=σS ² T/k, was measured to be ∼0.12. This value is around 120 times higher than the reported maximum value of bulk β-SiC.     

Iterated function systems and dynamical systems

We study the relationship between measures invariant for a piecewise expanding transformation tau of a compact metric space endowed with a underlying measure and measures invariant for an iterated function system T(tau), generated by inverse branches of tau. The main result says that the tau-invariant absolutely continuous measure &mgr; is also T(tau) invariant if and only if tau is absolutely continuously conjugated with a piecewise linear transformation. Measures of maximal entropy and general equilibrium states are also discussed. (c) 1995 American Institute of Physics.     

Recommender systems

The ongoing rapid expansion of the Internet greatly increases the necessity of effective recommender systems for filtering the abundant information. Extensive research for recommender systems is conducted by a broad range of communities including social and computer scientists, physicists, and interdisciplinary researchers. Despite substantial theoretical and practical achievements, unification and comparison of different approaches are lacking, which impedes further advances. In this article, we review recent developments in recommender systems and discuss the major challenges. We compare and evaluate available algorithms and examine their roles in the future developments. In addition to algorithms, physical aspects are described to illustrate macroscopic behavior of recommender systems. Potential impacts and future directions are discussed. We emphasize that recommendation has great scientific depth and combines diverse research fields which makes it interesting for physicists as well as interdisciplinary researchers.     

Nanoelectromechanical systems

Nanoelectromechanical systems (NEMS) are nano-to-micrometer scale mechanical resonators coupled to electronic devices of similar dimensions. NEMS show promise for fast, ultrasensitive force microscopy and for deepening our understanding of how classical dynamics arises by approximation to quantum dynamics. This article begins with a survey of NEMS and then describes certain aspects of their classical dynamics. In particular, we show that for weak coupling the action of the electronic device on the mechanical resonator can be effectively that of a thermal bath, this despite the device being a driven, far-from-equilibrium system.