Nanoelectromechanical Single-electron Transistor Research Articles

Parametric Resonance in Nanoelectromechanical Single Electron Transistors

We show that the coupling between single-electron charging and mechanical motion in a nanoelectromechanical single-electron transistor can be utilized in a novel parametric actuation scheme. This scheme, which relies on a periodic modulation of the mechanical resonance frequency through an alternating source-drain voltage, leads to a parametric instability and emergence of mechanical vibrations in a limited range of modulation amplitudes. Remarkably, the frequency range where instability occurs and the maximum oscillation amplitude, depend weakly on the damping in the system. We also show that a weak parametric modulation increases the effective quality factor and amplifies the system's response to the conventional actuation that exploits an AC gate signal.

Self-excitation of single nanomechanical pillars

Self-excitation is a mechanism that is ubiquitous for electromechanical power devices such as electrical generators. This is conventionally achieved by making use of the magnetic field component in electrical generators (Nedic and Lipo 2000 IEEE/IAS Conf. Records (Rome, Italy) vol 1 pp 51–6), a good and widely visible example of which is the wind turbine farm (Muljadi et al 2005 J. Sol. Energy Eng.127 581–7). In other words, a static force, such as the wind acting on rotor blades, can generate a resonant excitation at a certain mechanical frequency. For nanomechanical systems (Craighead 2000 Science290 1532–5; Roukes 2001 Phys. World14 25–31; Cleland 2003 Foundations of Nanomechanics (Berlin: Springer); Ayari et al 2007 Nano Lett.7 2252–7; Koenig et al 2008 Nat. Nanotechnol.3 482–4) such a self-excitation (SE) mechanism is also highly desirable, because it can generate mechanical oscillations at radio frequencies by simply applying a dc bias voltage. This is of great importance for low-power signal communication devices and detectors, as well as for mechanical computing elements. For a particular nanomechanical system—the single electron shuttle—this effect was predicted some time ago by Gorelik et al (Phys. Rev. Lett.80 4526–9). Here, we use a nanoelectromechanical single electron transistor (NEMSET) to demonstrate self-excitation for both the soft and hard regimes, respectively. The ability to use self-excitation in nanomechanical systems may enable the detection of quantum mechanical backaction effects (Naik et al 2006 Nature443 193–6) in direct tunneling, macroscopic quantum tunneling (Savelev et al 2006 New J. Phys.8 105–15) and rectification (Pistolesi and Fazio 2005 Phys. Rev. Lett.94 036806–4). All these effects have so far been overshadowed by the large driving voltages that had to be applied.

Transport of Spin-Polarized Electrons through an NEM-SET Structure with Ferromagnetic Electrodes

We have investigated the spin-polarized electron transport in a magnetic nanoelectromechanical single-electron-transistor (NEM-SET), with an oscillating quantum dot (QD) coupled to two ferromagnetic electrodes. The interplay between the electronic and mechanical degrees of freedom is considered by using the quantum master equation method within Wigner phase-space. We present a concrete picture for the transition of the QD oscillations from the tunneling state to the shuttling one by analysis of the electron occupation, the effective potential and amplitude probability distribution. It is found that the development of dynamic shuttle instability is dependent on the relative orientation of two leads’ magnetizations, which arises a pronounced spin valve effect. For an asymmetric NEM-SET structure, besides the spin valve effect, we unexpectedly find that the shuttle instability is additionally dependent on the the bias-voltage polarities, exhibiting a sizable current rectification. The coexistence of two effects makes it possible to control the spin valve effect electrically or control the rectification magnetically. Subject Index: 304, 355, 375, 394

Diodelike effect in nanoelectromechanical shuttle devices

We have analyzed the effect of bias-voltage polarities on the shuttle instability in an asymmetrically nanoelectromechanical single-electron transistor (NEM-SET) using the Wigner phase space approach. The electric-field-induced shift in oscillation equilibrium position together with the asymmetrical tunneling rate leads to the symmetry breaking of electron occupations in the quantum dot for positively and negatively biased directions. The symmetry-broken occupations make the works done by the electric field in different biased directions unequal. As a consequence, the threshold voltage for the onset of shuttle instability is lower for a positive bias voltage than for a negative one. Between two thresholds, the NEM-SET device exhibits a pronounced rectification effect.

Shuttle instability induced by an ac gate in a nanoelectromechanical single-electron transistor

We theoretically investigate shuttle instability induced by an ac gate in a nanoelectromechanical single-electron transistor containing a moveable grain. The ac gate causes periodic variations of charge on the grain, which turn (in the bias field) to driving force of grain vibrations. The forced grain vibrations and related electron transport show drastic and hysteretic changes for variations of the bias or the gate frequency due to transitions in transport mechanisms between tunneling and shuttling.

Giant shot noise due to mechanical transportation of spin-polarized electrons

We show that single-electron ``shuttling'' of electrons in a magnetic nanoelectromechanical single-electron transistor device can be an efficient tool for studying electron spin-flip relaxation on quantum dots. The reason is traced to a spin blockade of the mechanically aided shuttle current that occurs in devices with highly polarized and collinearly magnetized leads. This results in giant peaks in the shot-noise spectral function, wherein the peak heights are only limited by the rate of electronic spin flips. Therefore, we show that nanomechanical spectroscopy of the spin-flip rate is possible, allowing spin-flip relaxation times as long as $10\text{ }\ensuremath{\mu}\text{s}$ to be detected.

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.

AC gate effects on a nano-electromechanical single electron transistor

AC gate effects on electron transport of a nano electromechanical single electron transistor having a moveable nano particle in the tunnel regime are theoretically studied. The AC gate drives the nano particle vibrations, giving rise to resonance of the vibrations at the natural frequency ω0 of the confining potential and fractional magnitudes of ω0. The current and noise show remarkable increase with respect the gate frequency, reflecting the dynamic response of the nano particle vibrations to the gate. Artificial shuttle mechanism is confirmed under certain situation.

Kondo shuttling in a nanoelectromechanical single-electron transistor

We investigate theoretically a mechanically assisted Kondo effect and electric charge shuttling in nanoelectromechanical single-electron transistor (NEM-SET). It is shown that the mechanical motion of the central island (a small metallic particle) with the spin results in the time dependent tunneling width which leads to effective increase of the Kondo temperature. The time-dependent oscillating Kondo temperature T_K(t) changes the scaling behavior of the differential conductance resulting in the suppression of transport in a strong coupling- and its enhancement in a weak coupling regimes. The conditions for fine-tuning of the Abrikosov-Suhl resonance and possible experimental realization of the Kondo shuttling are discussed.

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.

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 E<<E(q) a new quantum regime of shuttle vibrations occurs. We show that in the latter regime small quantum fluctuations result in large (i.e., finite in the limit variant Planck's over 2pi -->0) shuttle vibrations.

Quantum description of shuttle instability in a nanoelectromechanical single-electron transistor

The voltage dependence of nanoelectromechanical effects in a system where the quantized mechanical vibrations of a quantum dot are coupled to coherent tunneling of electrons through a single level in the dot is studied. It is found that there are two different regimes depending on the value of an applied voltage. If the bias voltage is below a certain threshold value, then the state of the mechanical subsystem is located near its ground state. If the bias voltage is above the threshold value, then the system becomes unstable which manifests itself in the expectation value of the displacement being an oscillating function of time with an exponentially increasing amplitude. This can be interpreted as a shuttle instability in a quantum regime.