Single-Photon Emission Mediated by Single-Electron Tunneling in Plasmonic Nanojunctions.

Recent scanning tunneling microscopy (STM) experiments reported single-molecule fluorescence induced by tunneling currents in the nanoplasmonic cavity formed by the STM tip and the substrate. The electric field of the cavity mode couples with the current-induced charge fluctuations of the molecule, allowing the excitation of photons. We investigate theoretically this system for the experimentally relevant limit of large damping rate κ for the cavity mode and arbitrary coupling strength to a single-electronic level. We find that for bias voltages close to the first inelastic threshold of photon emission, the emitted light displays antibunching behavior with vanishing second-order photon correlation function. At the same time, the current and the intensity of emitted light display Franck-Condon steps at multiples of the cavity frequency ω_{c} with a width controlled by κ rather than the temperature T. For large bias voltages, we predict strong photon bunching of the order of κ/Γ where Γ is the electronic tunneling rate. Our theory thus predicts that strong coupling to a single level allows current-driven nonclassical light emission.

Sensitivity of the mixing-current technique for the detection of mechanical motion in the coherent tunnelling regime.

In a recent publication we have studied theoretically the sensitivity of the mixing-current technique to detect nanomechanical motion by coupling the oscillator to a single-electron transistor in the incoherent tunnelling regime: [Formula: see text], where Γ is the tunnelling rate, T is the electronic temperature, [Formula: see text] and k B are the Planck and the Boltzmann constant, respectively. In this work we consider the same problem when the detection device is a quantum dot in the coherent tunnelling regime ([Formula: see text]). In order to reach the best sensitivity we find that one should enter the strong coupling regime, as described in the recent publication (Micchi et al 2015 Phys. Rev. Lett. 115 206802) where a mechanical bistability is described. In this regime the electronic detection device strongly modifies the effective potential of the oscillator and the non-linearities determine the form of the displacement fluctuation spectrum. We find theoretical upper bounds to the sensitivity for the detection of the oscillation amplitude of the oscillator. It turns out that it is convenient to work as close as possible to the bistability.

Scaling laws for the bifurcation escape rate in a nanomechanical resonator.

We report on experimental and theoretical studies of the fluctuation-induced escape time from a metastable state of a nanomechanical Duffing resonator in a cryogenic environment. By tuning in situ the nonlinear coefficient γ we could explore a wide range of the parameter space around the bifurcation point, where the metastable state becomes unstable. We measured in a relaxation process the distribution of the escape times. We have been able to verify its exponential distribution and extract the escape rate Γ. We investigated the scaling of Γ with respect to the distance to the bifurcation point and γ, finding an unprecedented quantitative agreement with the theoretical description of the stochastic problem. Simple power scaling laws turn out to hold in a large region of the parameter space, as anticipated by recent theoretical predictions. These unique findings, implemented in a model dynamical system, are relevant to all systems experiencing underdamped saddle-node bifurcation.

Mechanical Signatures of the Current Blockade Instability in Suspended Carbon Nanotubes.

Transport measurements allow sensitive detection of nanomechanical motion of suspended carbon nanotubes. It has been predicted that when the electromechanical coupling is sufficiently large a bistability with a current blockade appears. Unambiguous observation of this transition by current measurements may be difficult. Instead, we investigate the mechanical response of the system, namely, the displacement spectral function, the linear response to a driving, and the ring-down behavior. We find that by increasing the electromechanical coupling the peak in the spectral function broadens and shifts at low frequencies while the oscillator dephasing time shortens. These effects are maximum at the transition where nonlinearities dominate the dynamics. These strong signatures open the way to detect the blockade transition in devices currently studied by several groups.

Andreev bound-state dynamics in quantum-dot Josephson junctions: a washing out of the 0-π transition.

We consider a Josephson junction formed by a quantum dot connected to two bulk superconductors in the presence of Coulomb interaction and coupling to both an electromagnetic environment and a finite density of electronic quasiparticles. In the limit of a large superconducting gap we obtain a Born-Markov description of the relevant Andreev bound-states dynamics. We calculate the current-phase relation and we find that the experimentally unavoidable presence of quasiparticles can dramatically modify the 0-π standard transition picture. We show that photon-assisted quasiparticle absorption allows the dynamic switching from the 0 to the π state and vice versa, washing out the 0-π transition predicted by purely thermodynamic arguments. Spectroscopic signatures of Andreev bound-states broadening are investigated by considering microwave irradiation.

Crossover from adiabatic to antiadiabatic quantum pumping with dissipation.

Quantum pumping, in its different forms, is attracting attention from different fields, from fundamental quantum mechanics, to nanotechnology, to superconductivity. We investigate the crossover of quantum pumping from the adiabatic to the antiadiabatic regime in the presence of dissipation, and find general and explicit analytical expressions for the pumped current in a minimal model describing a system with the topology of a ring forced by a periodic modulation of frequency ω. The solution allows following in a transparent way the evolution of pumped dc current from much smaller to much larger ω values than the other relevant energy scale, the energy splitting introduced by the modulation. We find and characterize a temperature-dependent optimal value of the frequency for which the pumped current is maximal.

Charge shuttle as a nanomechanical rectifier.

We consider the charge shuttle proposed by Gorelik et al. driven by a time-dependent voltage bias. In the case of asymmetric setup, the system behaves as a rectifier. For pure ac drive, the rectified current shows a rather rich frequency dependent response characterized by frequency locking at fractional values of the external frequency. Because of the nonlinear dynamics of the shuttle, rectification is present also for very low frequencies. These effects could be useful to unveil the internal dynamics of nanomechanical devices.

Energy dependence of current noise in superconducting/normal metal junctions.

Interference of electronic waves undergoing Andreev reflection in diffusive conductors determines the energy profile of the conductance on the scale of the Thouless energy. A similar dependence exists in the current noise, but its behavior is known only in a few limiting cases. We consider a metallic diffusive wire connected to a superconducting reservoir through an interface characterized by an arbitrary distribution of channel transparencies. Within the quasiclassical theory for current fluctuations we provide a general expression for the energy dependence of the current noise.

Alterations of Fas (APO-1/CD 95) gene and its relationship with p53 in non small cell lung cancer.

The Fas (APO-1/CD95) system regulates a number of physiological and pathological processes of cell death. The ligand for Fas induces apoptosis by interacting with a transmembrane cell surface Fas receptor. The key role of the Fas system has been studied mostly in the immune system, but Fas mutations, one of the possible mechanisms for resistance to apoptosis signaling, may be involved in the pathogenesis of non-lymphoid malignancies as well. To better understand the potential involvement of Fas system in non-small cell lung cancer (NSCLC) we evaluated Fas and Fas-ligand mRNA expression by polymerase chain reaction in 102 tumor samples and in 44 normal surrounding tissues. Although over 60% of the human NSCLC analysed expressed both genes, they seem to be unable to induce apoptosis in vivo by autocrine suicide. In this regard, we investigated in 79 cases, the promoter and the entire coding region of the Fas gene by polymerase chain reaction, single strand conformation polymorphism and DNA sequencing for detecting putative alterations. Sixteen tumors (20.25 %) were found to have Fas alterations, in promoter and/or exon region. In all cases samples carried heterozygous alterations and mostly showed simultaneous mutations of p53 gene. Moreover, the quantitative analysis of Fas mRNA expression showed high levels of Fas messenger associated with p53 wild-type status alone. Taken together, these findings point to an involvement of Fas/Fas-ligand system in the development of NSCLC, suggesting that the loss of its apoptotic function might be linked to p53 alterations which contribute to the self-maintenance of cancer cells.

Off-diagonal geometric phases.

We investigate the adiabatic evolution of a set of nondegenerate eigenstates of a parametrized Hamiltonian. Their relative phase change can be related to geometric measurable quantities that extend the familiar concept of Berry phase to the evolution of more than one state. We present several physical systems where these concepts can be applied, including an experiment on microwave cavities for which off-diagonal phases can be determined from published data.

Geometric phases and multiple degeneracies in harmonic resonators

In a recent experiment Lauber et al. have deformed cyclically a microwave resonator and have measured the adiabatic normal-mode wave functions for each shape along the path of deformation. The nontrivial observed cyclic phases around a threefold degeneracy were accounted for by Manolopoulos and Child within an approximate theory. However, open-path geometrical phases disagree with experiment. By solving exactly the problem, we find unsuspected extra degeneracies around the multiple one that account for the measured phase changes throughout the path. It turns out that proliferation of additional degeneracies around a multiple one is a common feature of quantum mechanics.

Tumour necrosis factor-alpha and transforming growth factor-beta are significantly associated with better prognosis in non-small cell lung carcinoma: putative relation with BCL-2-mediated neovascularization.

Recent in vivo and in vitro studies have demonstrated a wide spectrum of biologic activities of cytokines in the pathogenesis and progression of malignancy. Tumour necrosis factor alpha (TNF-alpha) and transforming growth factor beta (TGF-beta) have emerged as two of the many host-derived mediators that seem to interfere with both antiproliferative and tumorigenic effects in malignant tumours including lung cancer. However, their association with tumour prognosis or prognostic factors has not yet been completely clarified. In this study, we assessed TNF-alpha and TGF-beta mRNA expression by RT-PCR technique in 61 NSCLC samples, demonstrating the presence of TNF-alpha and TGF-beta mRNA in 55.74% and 45.9% of cases, respectively. We also evaluated the expression of the two distinct transmembrane TNF receptors. TNFR-I and TNFR-II, with a PCR-positive signal in 70.49% and 65.57% of cases, respectively. In 49 of the 61 cases, we evaluated the prognostic impact of the two growth-inhibiting factors using the Kaplan-Meier analysis. In the univariate analysis patients without nodal metastatic involvement (P = 0.02), less advanced tumour stage (P = 0.02) or TNF-alpha and TGF-beta positive cancers (P = 0.01 and P = 0.03) showed a favourable prognosis in terms of overall survival. Since our previous studies demonstrated a significant association between NSCLC behaviour, neoangiogenesis and bcl-2 expression, we investigated the putative relation between TNF-alpha and TGF-beta on the one hand, and vascular count (as a measure of tumour angiogenesis) and bcl-2 protein expression, on the other hand. Our results showed a significant direct association between TNF-alpha and bcl-2 (P = 0.05) and an inverse association between TNF-alpha and microvessel count (P = 0.03). Moreover, as previously demonstrated, we observed a significant inverse correlation between bcl-2 protein expression and vascular count (P = 0.05), suggesting that the favourable effect of TNF-alpha on clinical outcome may be related to a bcl-2-mediated low neovascular development.