Bistability of a slow mechanical oscillator coupled to a laser-driven two-level system

It has been recently proposed that single molecule spectroscopy could be employed to detect the motion of nano-mechanical resonators. Estimates of the coupling constant (g) between the molecular two-level system and the oscillator indicate that it can reach values much larger than the mechanical resonating pulsation (omega_m) and the two-level system linewidth (Gamma). Other experimental realization of the same system are also approching this strong coupling regim. In this paper we investigate the behavior of the system in the limit for slow mechanical oscillator omega_m << Gamma}. We find that, for sufficiently large coupling, the system undergoes a bistability reminiscent of that observed in optical cavities coupled to mechanical resonators.Comment: 8 pages, 7 Figure

From semiconductors to superconductors: a simple model for pseudogaps

We consider a two dimensional semiconductor with a local attraction among the carriers. We study the ground state of this system as a function of the semiconductor gap. We find a direct transition from a superconducting to an insulating phase for no doping at a critical value, the single particle excitations being always gapped. For finite doping we find a smooth crossover. We calculate the critical temperature due to both the particle excitations and the Berezinkii-Kosterlitz-Thouless transition.Comment: 14 pages. Accepted for publication on Eur. Phys. Jour.

Tunable spin-polaron state in a singly clamped semiconducting carbon nanotube

We consider a semiconducting carbon nanotube (CNT) laying on a ferromagnetic insulating sub-strate with one end depassing the substrate and suspended over a metallic gate. We assume that the polarised substrate induces an exchange interaction acting as a local magnetic field for the electrons in the non-suspended CNT side. Generalizing the approach of I. Snyman and Yu.V. Nazarov [Phys. Rev. Lett. 108, 076805 (2012)] we show that one can generate electrostatically a tun-able spin-polarized polaronic state localized at the bending end of the CNT. We argue that at low temperatures manipulation and detection of the localised quantum spin state is possible

Dynamics and Current Fluctuations in AC driven Charge Shuttle

The behavior of a charge shuttle under a pure AC field has been recently considered theoretically and experimentally. If the system presents an asymmetry in the tunneling amplitudes the device acts as a nano-electromechanical rectifier, transforming a pure AC voltage field into a direct curren. In this paper we first review the model and the appearance of the rectifying effect for bias voltages below the threshold of self-oscillation. We discuss in some details the dynamics of the central island that, like the current, presents strong dependence on the forcing AC field frequency. In presence of both a constant and a small oscillating bias voltage we analyze the transition from the static to self-oscillating solution. We then consider current fluctuations (full counting statistics) for periodic motion of the grain. We explicitly evaluate the current noise numerically and we find that it shows clear signatures of correlated transport at certain locking frequencies. In the adiabatic limit we obtain a simple expression for the full-counting statistics and calculate explicitly the first four moments.Comment: 16 pages, 7 Figures. Invited paper for the special issue on NEMS on New Journal of Physic

Electro-Mechanical Transition in Quantum dots

The strong coupling between electronic transport in a single-level quantum dot and a capacitively coupled nano-mechanical oscillator may lead to a transition towards a mechanically-bistable and blocked-current state. Its observation is at reach in carbon-nanotube state-of-art experiments. In a recent publication [Phys. Rev. Lett. 115, 206802 (2015)] we have shown that this transition is characterized by pronounced signatures on the oscillator mechanical properties: the susceptibility, the displacement fluctuation spectrum and the ring-down time. These properties are extracted from transport measurements, however the relation between the mechanical quantities and the electronic signal is not always straightforward. Moreover the dependence of the same quantities on temperature, bias or gate voltage, and external dissipation has not been studied. The purpose of this paper is to fill this gap and provide a detailed description of the transition. Specifically we find: (i) The relation between the current-noise and the displacement spectrum. (ii) The peculiar behavior of the gate-voltage dependence of these spectra at the transition. (iii) The robustness of the transition towards the effect of external fluctuations and dissipation

Steps and facets at the surface of soft crystals

We consider the shape of crystals which are soft in the sense that their elastic modulus $\mu$ is small compared to their surface tension $\gamma$, more precisely $\mu a < \gamma$ where $a$ is the lattice spacing. We show that their surface steps penetrate inside the crystal as edge dislocations. As a consequence, these steps are broad with a small energy which we calculate. We also calculate the elastic interaction between steps a distance $d$ apart, which is a $1/d^2$ repulsion. We finally calculate the roughening temperatures of successive facets in order to compare with the remarkable shapes of lyotropic crystals recently observed by P. Pieranski et al. Good agreement is found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.

Self-consistent theory of molecular switching

We study the model of a molecular switch comprised of a molecule with a soft vibrational degree of freedom coupled to metallic leads. In the presence of strong electron-ion interaction, different charge states of the molecule correspond to substantially different ionic configurations, which can lead to very slow switching between energetically close configurations (Franck-Condon blockade). Application of transport voltage, however, can drive the molecule far out of thermal equilibrium and thus dramatically accelerate the switching. The tunneling electrons play the role of a heat bath with an effective temperature dependent on the applied transport voltage. Including the transport-induced "heating" selfconsistently, we determine the stationary current-voltage characteristics of the device, and the switching dynamics for symmetric and asymmetric devices. We also study the effects of an extra dissipative environment and demonstrate that it can lead to enhanced non-linearities in the transport properties of the device and dramatically suppress the switching dynamics

Detection of ultrafast oscillations in Superconducting Point-Contacts by means of Supercurrent Measurements

We present a microscopic calculation of the nondissipative current through a superconducting quantum point contact coupled to a mechanical oscillator. Using the non-equilibrium Keldysh Green function approach, we determine the current-phase relation. The latter shows that at certain phases, the current is sharply suppressed. These dips in the current-phase relation provide information about the oscillating frequency and coupling strength of the mechanical oscillator. We also present an effective two-level model from which we obtain analytical expressions describing the position and width of the dips. Our findings are of relevance for nanomechanical resonators based on superconducting materials.Comment: 8 pages, 5 figures. Published in Phys. Rev.