Nanoelectromechanical Heat Engine Based on Electron-Electron Interaction.

We theoretically show that a nanoelectromechanical system can be mechanically actuated by a heat flow through it via an electron-electron interaction. In contrast to most known actuation mechanisms in similar systems, this new mechanism does not involve an electronic current nor external ac fields. Instead, the mechanism relies on deflection-dependent tunneling rates and a heat flow which is mediated by an electron-electron interaction while an electronic current through the device is prohibited by, for instance, a spin-valve effect. Therefore, the system resembles a nanoelectromechanical heat engine. We derive a criterion for the mechanical instability and estimate the amplitude of the resulting self-sustained oscillations. Estimations show that the suggested phenomenon can be studied using available experimental techniques.

Spin-mediated Photomechanical Coupling of a Nanoelectromechanical Shuttle.

We show that nanomechanical vibrations in a magnetic shuttle device can be strongly affected by external microwave irradiation through photo-assisted electronic spin-flip transitions. Mechanical consequences of these spin flips are due to a spin-dependent magnetic force, which may lead to a nanomechanical instability in the device. We derive a criterion for the instability to occur and analyze different regimes of nanomechanical oscillations. Possible experimental realizations of the spin-mediated photomechanical instability and detection of the device backaction are discussed.

Single-electron shuttle based on electron spin.

A nanoelectromechanical device based on magnetic exchange forces and electron spin flips induced by a weak external magnetic field is suggested. It is shown that this device can operate as a new type of single-electron "shuttle" in the Coulomb blockade regime of electron transport.

Quantum theory of magnetoelectromotive instability in nanoelectromechanical systems with positive differential conductance.

We consider dc-electronic transport through a nanowire suspended between two normal-metal leads in the presence of an external magnetic field. We show the very mechanism through which such a system, whose stationary current-voltage characteristic is essentially characterized by positive differential conductance, becomes unstable with respect to an onset of self-excited oscillations in electrical transport and mechanical vibrations. The self-excitation mechanism is based on the correlation between the occupancy of the quantized spin-split electronic energy levels inside the nanowire and the velocity of the nanowire with the crucial influence of strong enough retardation effects in magnetomotive coupling coming from mechanical vibrations.

Kondo force in shuttling devices: dynamical probe for a Kondo cloud.

We consider the electromechanical properties of a single-electronic device consisting of a movable quantum dot attached to a vibrating cantilever, forming a tunnel contact with a nonmovable source electrode. We show that the resonance Kondo tunneling of electrons amplifies exponentially the strength of nanoelectromechanical (NEM) coupling in such a device and make the latter insensitive to mesoscopic fluctuations of electronic levels in a nanodot. It is also shown that the study of a Kondo-NEM phenomenon provides additional (as compared with standard conductance measurements in a nonmechanical device) information on retardation effects in the formation of a many-particle cloud accompanying the Kondo tunneling. A possibility for superhigh tunability of mechanical dissipation as well as supersensitive detection of mechanical displacement is demonstrated.

Spin-controlled nanomechanics induced by single-electron tunneling.

We consider dc-electronic transport through a nanowire suspended between normal- and spin-polarized metal leads in the presence of an external magnetic field. We show that magnetomotive coupling between the electrical current through the nanowire and vibrations of the wire may result in self-excitation of mechanical vibrations. The self-excitation mechanism is based on correlations between the occupancy of the quantized electronic energy levels inside the nanowire and the velocity of the nanowire. We derive conditions for the occurrence of the instability and find stable regimes of mechanical oscillations.

Cooling of nanomechanical resonators by thermally activated single-electron transport.

We show that the vibrations of a nanomechanical resonator can be cooled to near its quantum ground state by tunneling injection of electrons from a scanning tunneling microscope tip. The interplay between two mechanisms for coupling the electronic and mechanical degrees of freedom results in a bias-voltage-dependent difference between the probability amplitudes for vibron emission and absorption during tunneling. For a bias voltage just below the Coulomb blockade threshold, we find that absorption dominates, which leads to cooling corresponding to an average vibron population of the fundamental bending mode of 0.2.

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.

Electronic Aharonov-Bohm effect induced by quantum vibrations.

Mechanical displacements of a nanoelectromechanical system shift the electron trajectories and hence perturb phase coherent charge transport through the device. We show theoretically that in the presence of a magnetic field such quantum-coherent displacements may give rise to an Aharonov-Bohm-type of effect. In particular, we demonstrate that quantum vibrations of a suspended carbon nanotube result in a positive nanotube magnetoresistance, which decreases slowly with the increase of temperature. This effect may enable one to detect quantum displacement fluctuations of a nanomechanical device.

Mechanically mediated electron transfer in model metallo-enzyme interfaces.

In this paper, we develop a physical analysis of charge transfer in the model 'metallo-enzyme' complex which consists of a synthetic redox-addressed assembly (a 'reaction center') hybridized with a quantum dot (a gold nanoparticle) and attached via molecular bridge (a spacer) to the electrode. This artificial system allows us to model electronic transduction in experimental redox ezyme-gold nanoparticle hybrid structure recently reported by Xiao et al. [Xiao, Y., Patolsky, F., Katz, E., Hainfeid, J.F., Willner, I., 2003. Science 299, 1877-1881]. We consider a photosensitive spacer that allows us to control the conductivity of the bridge by light. In this paper we will focus on a special type of nanoelectro-mechanical processes in this system and investigate single electronic effects in there.

TGFbeta-mediated immunoregulation.

T cell homeostasis is required for normal immune responses and prevention of pathological responses. Transforming growth factor beta (TGFbeta) plays an essential role in that regulation. Owing to its broad expression and inhibitory effects on multiple immune cell types, TGFbeta regulation is complex. Through recent advances in cell-specific targeting of TGFbeta signaling in vivo, the role of TGFbeta in T cell regulation is emerging. We demonstrated here a critical role for TGFbeta in regulating effector vs regulatory T cell homeostasis.

Coulomb promotion of spin-dependent tunneling.

We study transport of spin-polarized electrons through a magnetic single-electron transistor (SET) in the presence of an external magnetic field. Assuming the SET to have a nanometer size central island with a single-electron level we find that the interplay on the island between coherent spin-flip dynamics and Coulomb interactions can make the Coulomb correlations promote rather than suppress the current through the device. We find the criteria for this new phenomenon--Coulomb promotion of spin-dependent tunneling--to occur.

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<0) shuttle vibrations.

Electrical manipulation of nanomagnets.

We demonstrate that it is possible to manipulate the magnetic coupling between two nanomagnets by means of an ac electric field. In the scheme suggested, the magnetic coupling is mediated by a magnetic particle that is in contact with both nanomagnets via tunnel barriers. The time-dependent electric field is applied so that the height of first one barrier then the other is suppressed in an alternating fashion. We show that the result is a pumping of magnetization from one nanomagnet to the other through the mediating particle. The dynamics of the magnetization of the mediating particle allows the coupling to be switched between being ferromagnetic and being antiferromagnetic.

Mechanical Cooper pair transportation as a source of long-distance superconducting phase coherence.

Transportation of Cooper pairs by a movable single Cooper-pair box placed between two remote superconductors is shown to establish coherent coupling between them. This coupling is due to entanglement of the movable box with the leads and is manifested in the suppression of quantum fluctuations of the relative phase of the order parameters of the leads. It can be probed by attaching a high resistance Josephson junction between the leads and measuring the current through this junction. The current is suppressed with increasing temperature.

Immune-mediated eradication of tumors through the blockade of transforming growth factor-beta signaling in T cells.

Despite the existence of tumor-specific antigens and demonstrated presence of tumor-specific immune cells, the majority of tumors manage to avoid immune-mediated destruction. Various mechanisms have been suggested for tumor evasion from immune response. One such mechanism is thought to be mediated by transforming growth factor-beta (TGF-beta), an immunosuppressive cytokine found at the site of most tumors. We demonstrate here that T-cell-specific blockade of TGF-beta signaling allows the generation of an immune response capable of eradicating tumors in mice challenged with live tumor cells. In addition, we provide mechanisms through which abrogation of TGF-beta signaling leads to the enhancement of anti-tumor immunity. Our data indicate that T-cell-specific blockade of TGF-beta signaling has strong therapeutic potential to shift the balance of the immune response in favor of anti-tumor immunity.

Coherent transfer of Cooper pairs by a movable grain.

Superconducting circuits that incorporate Josephson junctions are of considerable experimental and theoretical interest, particularly in the context of quantum computing. A nanometre-sized superconducting grain (commonly referred to as a Cooper-pair box) connected to a reservoir by a Josephson junction is an important example of such a system. Although the grain contains a large number of electrons, it has been experimentally demonstrated that its states are given by a superposition of only two charge states (differing by 2e, where e is the electronic charge). Coupling between charge transfer and mechanical motion in nanometre-sized structures has also received considerable attention. Here we demonstrate theoretically that a movable Cooper-pair box oscillating periodically between two remote superconducting electrodes can serve as a mediator of Josephson coupling, leading to coherent transfer of Cooper pairs between the electrodes. Both the magnitude and the direction of the resulting Josephson current can be controlled by externally applied electrostatic fields.

Cutting edge: TGF-beta inhibits Th type 2 development through inhibition of GATA-3 expression.

TGF-beta is an important immunomodulatory cytokine that can inhibit differentiation of effector T cells. In this report, we address the molecular mechanisms through which TGF-beta inhibits differentiation of CD4(+) cells into Th type 2 cells. We demonstrate that TGF-beta inhibits GATA-3 expression in developing Th cells. We also show that inhibition of GATA-3 expression by TGF-beta is a major mechanism of inhibition of Th2 differentiation by TGF-beta as ectopic expression of GATA-3 in developing T cells overcomes the ability of TGF-beta to inhibit Th2 differentiation. TGF-beta likely inhibits GATA-3 expression at the transcriptional level and does so without interfering with IL-4 signaling.

Abrogation of TGFbeta signaling in T cells leads to spontaneous T cell differentiation and autoimmune disease.

Targeted mutation of TGFbeta1 in mice demonstrated that TGFbeta1 is one of the key negative regulators of immune homeostasis, as its absence leads to activation of a self-targeted immune response. Nevertheless, because of the highly pleiotropic properties of TGFbeta and the presence of TGFbeta receptors on most cell types, its biologic role in the regulation of immune homeostasis is not yet understood. To limit the consequences of TGFbeta effects to a single cell type, we developed a transgenic approach to abrogate the TGFbeta response in key immune cells. Specifically, we expressed a dominant-negative TGFbeta receptor type II under a T cell-specific promoter and created a mouse model where signaling by TGFbeta is blocked specifically in T cells. Using this transgenic model, we show that T cell homeostasis requires TGFbeta signaling in T cells.