Liquid-induced damping of mechanical feedback effects in single electron tunneling through a suspended carbon nanotube

In single electron tunneling through clean, suspended carbon nanotube devices at low temperature, distinct switching phenomena have regularly been observed. These can be explained via strong interaction of single electron tunneling and vibrational motion of the nanotube. We present measurements on a highly stable nanotube device, subsequently recorded in the vacuum chamber of a dilution refrigerator and immersed in the 3He/4He mixture of a second dilution refrigerator. The switching phenomena are absent when the sample is kept in the viscous liquid, additionally supporting the interpretation of dc-driven vibration. Transport measurements in liquid helium can thus be used for finite bias spectroscopy where otherwise the mechanical effects would dominate the current.Comment: 4 pages, 3 figure

Negative frequency tuning of a carbon nanotube nano-electromechanical resonator

A suspended, doubly clamped single wall carbon nanotube is characterized as driven nano-electromechanical resonator at cryogenic temperatures. Electronically, the carbon nanotube displays small bandgap behaviour with Coulomb blockade oscillations in electron conduction and transparent contacts in hole conduction. We observe the driven mechanical resonance in dc-transport, including multiple higher harmonic responses. The data shows a distinct negative frequency tuning at finite applied gate voltage, enabling us to electrostatically decrease the resonance frequency to 75% of its maximum value. This is consistently explained via electrostatic softening of the mechanical mode.Comment: 4 pages, 4 figures; submitted for the IWEPNM 2013 conference proceeding

Magnetic damping of a carbon nanotube NEMS resonator

A suspended, doubly clamped single wall carbon nanotube is characterized at cryogenic temperatures. We observe specific switching effects in dc-current spectroscopy of the embedded quantum dot. These have been identified previously as nano-electromechanical self-excitation of the system, where positive feedback from single electron tunneling drives mechanical motion. A magnetic field suppresses this effect, by providing an additional damping mechanism. This is modeled by eddy current damping, and confirmed by measuring the resonance quality factor of the rf-driven nano-electromechanical resonator in an increasing magnetic field.Comment: 8 pages, 3 figure

Broken SU(4) symmetry in a Kondo-correlated carbon nanotube

Understanding the interplay between many-body phenomena and non-equilibrium in systems with entangled spin and orbital degrees of freedom is a central objective in nano-electronics. We demonstrate that the combination of Coulomb interaction, spin-orbit coupling and valley mixing results in a particular selection of the inelastic virtual processes contributing to the Kondo resonance in carbon nanotubes at low temperatures. This effect is dictated by conjugation properties of the underlying carbon nanotube spectrum at zero and finite magnetic field. Our measurements on a clean carbon nanotube are complemented by calculations based on a new approach to the non-equilibrium Kondo problem which well reproduces the rich experimental observations in Kondo transport.Comment: 8 pages, 6 figures; appendix of 14 pages, 7 figure

Mutual synchronization and clustering in randomly coupled chaotic dynamical networks

We introduce and study systems of randomly coupled maps (RCM) where the relevant parameter is the degree of connectivity in the system. Global (almost-) synchronized states are found (equivalent to the synchronization observed in globally coupled maps) until a certain critical threshold for the connectivity is reached. We further show that not only the average connectivity, but also the architecture of the couplings is responsible for the cluster structure observed. We analyse the different phases of the system and use various correlation measures in order to detect ordered non-synchronized states. Finally, it is shown that the system displays a dynamical hierarchical clustering which allows the definition of emerging graphs.Comment: 13 pages, to appear in Phys. Rev.

Guidelines for the establishment of microbiological criteria for foods

O Grupo de Trabalho Ocorrência Microbiológica na Cadeia Alimentar (GTOMCA) do Programa PortFIR considerou de grande importância o desenvolvimento de um documento que compilasse uma seleção de legislação e de informações relativas a Critérios Microbiológicos (CM), visando apoiar e facilitar, aos operadores e entidades do setor alimentar, a sua aplicação na validação do processo de produção, na segurança e/ou higiene dos géneros alimentícios, na adesão a boas práticas de fabrico dos mesmos, e/ou, ainda, na manutenção da sua qualidade durante o seu tempo de vida útil. Deste modo, o GTOMCA desenvolveu o Guia para o estabelecimento de critérios microbiológicos em géneros alimentícios, que foi publicado em abril de 2017, contemplando a identificação, caraterísticas e propósito dos CM, os fatores a considerar para a sua definição, nomeadamente: a categoria do alimento, o microrganismo e/ou as suas toxinas, os metabolitos e a virulência, os valores limite, o plano de amostragem, o tipo de utilização e consumo assim como o método de análise laboratorial, o ponto da cadeia alimentar onde se aplica, as medidas a tomar no caso de resultados não satisfatórios e a necessidade de revisão e atualização dos CM.The Working Group on Microbiological Occurrence on the Food Chain (GTOMCA) of Por tFIR Program considered unanimously, as an important need, the existence of a document with a selection and compilation of existing legislation and information concerning microbiological criteria (CM) as a tool to suppor t and facilitate its application by operators and entities in the food sector to validate the acceptability of the production process or the food safety or hygiene, the obser vance to good manufacturing practices or the maintenance of the food quality during its lifetime. So, GTOMCA developed a Guide for the establishment of microbiological criteria in foodstuf fs, which was published in April 2017, regarding the identification, characteristics and purpose of microbiological criteria, the factors to consider for its definition, identification, characteristics and purpose of CM and, as impor tant factors to consider the food categor y, the micro-organism and its metabolites, toxins and virulence factors, the limit values, the sampling plan, the type of food consumption as well as the analy tical method for testing the food, the point of the food chain where it is applied, the measures to be taken in the event of unsatisfactor y results and the need to review and update of the CM.info:eu-repo/semantics/publishedVersio

Partially and Fully Frustrated Coupled Oscillators With Random Pinning Fields

We have studied two specific models of frustrated and disordered coupled Kuramoto oscillators, all driven with the same natural frequency, in the presence of random external pinning fields. Our models are structurally similar, but differ in their degree of bond frustration and in their finite size ground state properties (one has random ferro- and anti-ferromagnetic interactions; the other has random chiral interactions). We have calculated the equilibrium properties of both models in the thermodynamic limit using the replica method, with emphasis on the role played by symmetries of the pinning field distribution, leading to explicit predictions for observables, transitions, and phase diagrams. For absent pinning fields our two models are found to behave identically, but pinning fields (provided with appropriate statistical properties) break this symmetry. Simulation data lend satisfactory support to our theoretical predictions.Comment: 37 pages, 7 postscript figure