Magnetoresistance and collective Coulomb blockade in super-lattices of ferromagnetic CoFe nanoparticles

We report on transport properties of millimetric super-lattices of CoFe nanoparticles surrounded by organic ligands. R(T)s follow R(T) = R_0.exp(T/T_0)^0.5 with T_0 ranging from 13 to 256 K. At low temperature I(V)s follow I=K[(V-V_T)/V_T]^ksi with ksi ranging 3.5 to 5.2. I(V) superpose on a universal curve when shifted by a voltage proportional to the temperature. Between 1.8 and 10 K a high-field magnetoresistance with large amplitude and a strong voltage-dependence is observed. Its amplitude only depends on the magnetic field/temperature ratio. Its origin is attributed to the presence of paramagnetic states present at the surface or between the nanoparticles. Below 1.8 K, this high-field magnetoresistance abruptly disappears and inverse tunnelling magnetoresistance is observed, the amplitude of which does not exceed 1%. At this low temperature, some samples display in their I(V) characteristics abrupt and hysteretic transitions between the Coulomb blockade regime and the conductive regime. The increase of the current during these transitions can be as high as a factor 30. The electrical noise increases when the sample is near the transition. The application of a magnetic field decreases the voltage at which these transitions occur so magnetic-field induced transitions are also observed. Depending on the applied voltage, the temperature and the amplitude of the magnetic field, the magnetic-field induced transitions are either reversible or irreversible. These abrupt and hysteretic transitions are also observed in resistance-temperature measurements. They could be the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64, 041302 (R), 2001] or could also be interpreted as a true phase transition between a Coulomb glass phase to a liquid phase of electrons

Optimizing adiabaticity in quantum mechanics

A condition on the Hamiltonian of a time-dependent quantum mechanical system is derived which, if satisfied, implies optimal adiabaticity (defined below). The condition is expressed in terms of the Hamiltonian and in terms of the evolution operator related to it. Since the latter depends in a complicated way on the Hamiltonian, it is not yet clear how the condition can be used to extract useful information about the optimal Hamiltonian. The condition is tested on an exactly-soluble time-dependent problem (a spin in a magnetic field), where perfectly adiabatic evolution can be easily identified.Comment: 5 pages, 2 figure

The Dovekie, Alle alle, as a Spring Migrant in Eastern Lancaster Sound and Western Baffin Bay

The distribution and numbers of dovekies during spring migration were studied by aerial surveys of eastern Lancaster Sound (1976, 1978, 1979) and western Baffin Bay (1978, 1979). Dovekies that nest in northwest Greenland migrated north through the study area during May. Extrapolations of recorded densities indicate that a peak of ~14 million dovekies may have been present in eastern Lancaster Sound and northwest Baffin Bay in mid-May 1978: fewer were present in May 1979, although the migration was more protracted and total numbers migrating through these areas may have been similar. Dovekies preferred offshore pack ice habitats with moderate to heavy ice cover. Possible reasons for the highly variable distributions in the three years are discussed.Key words: dovekie, Alle alle, spring migration, Lancaster Sound, Baffin Bay, aerial survey

Remote Photonic THz Generation using an Optical Frequency Comb and Multicore Fiber

This paper proposes and demonstrates a photonic THz generation technique based on an optical frequency comb and multicore fiber (MCF) transmission, with the advantage of remote generation with great reconfigurability and reduced digital signal processing (DSP). The feasibility of the proposed technique is evaluated experimentally comparing the performance when transmitting a data wavelength and a local oscillator for optical heterodyning over a single core or over different cores in a 1-km MCF link. The proposed remote photonic THz generation technique is demonstrated employing a 16QAM 12.5 GBd signal. A short wireless transmission at 183 GHz center frequency with 0.25 m antenna-separation is achieved after 1 km MCF, meeting the soft-decision decision forward error correction (SD-FEC) bit error rate (BER) recommendation of 210-2. The analysis includes the received photocurrent range for which the BER meets the standard hard-decision forward error correction (HD-FEC) recommendation of 3.810-3 BER. The BER performance is analyzed considering different DSP configurations, with and without frequency offset estimation (FOE) and decision-driven least mean squares (DD LMS) equalization. The performance of the remote photonic THz generation technique is evaluated comparatively against traditional free-running laser transmission over MCF, confirming the advantage of using comb generation to reduce the frequency offset fluctuation and simplify the DSP

Multicore fiber-assisted photonic sub-THz generation for full-duplex wireless transmission

This paper evaluates experimentally a centralized radio access network (C-RAN) based on multi-core fiber (MCF) for the transmission of high-bandwidth signals in the sub-THz band. We compare the system performance when the data and carrier wavelengths to be mixed at the receiver for optical heterodyning are transmitted over the same or over different cores of a MCF link. Full-duplex transmission on MCF is evaluated using the same received wireless signal downconverted and transmitted back as uplink over the same carrier wavelength. The performance with different digital signal processing (DSP) configurations and with higher-power interference in the other cores are also analyzed in this work. Successful provision of 12.5 GBd 16QAM signals is achieved after 1-km of 7-core MCF transmission including a short wireless link at 182 GHz, obtained with optical heterodyning. C-RAN implementation with MCF simplifies the remote nodes as all the lasers can be located in the central office, while minimizing the number of lasers needed at the central office thanks to wavelength re-use. It also provides more flexibility to the system, as it enables using the same LO for different purposes (i.e. THz generation of other data or optical modulation for uplink transmission)

Local Leaders in Random Networks

We consider local leaders in random uncorrelated networks, i.e. nodes whose degree is higher or equal than the degree of all of their neighbors. An analytical expression is found for the probability of a node of degree $k$ to be a local leader. This quantity is shown to exhibit a transition from a situation where high degree nodes are local leaders to a situation where they are not when the tail of the degree distribution behaves like the power-law $\sim k^{-\gamma_c}$ with $\gamma_c=3$. Theoretical results are verified by computer simulations and the importance of finite-size effects is discussed.Comment: 4 pages, 2 figure

Dispersive fields in de Sitter space and event horizon thermodynamics

When Lorentz invariance is violated at high energy, the laws of black hole thermodynamics are apparently no longer satisfied. To shed light on this observation, we study dispersive fields in de Sitter space. We show that the Bunch-Davies vacuum state restricted to the static patch is no longer thermal, and that the Tolman law is violated. However we also show that, for free fields at least, this vacuum is the only stationary stable state, as if it were in equilibrium. We then present a precise correspondence between dispersive effects found in de Sitter and in black hole metrics. This indicates that the consequences of dispersion on thermodynamical laws could also be similar.Comment: 19 pages. Black and White version on Phys.Rev.D serve

Multimodal stimulus coding by a gustatory sensory neuron in Drosophila larvae.

Accurate perception of taste information is crucial for animal survival. In adult Drosophila, gustatory receptor neurons (GRNs) perceive chemical stimuli of one specific gustatory modality associated with a stereotyped behavioural response, such as aversion or attraction. We show that GRNs of Drosophila larvae employ a surprisingly different mode of gustatory information coding. Using a novel method for calcium imaging in the larval gustatory system, we identify a multimodal GRN that responds to chemicals of different taste modalities with opposing valence, such as sweet sucrose and bitter denatonium, reliant on different sensory receptors. This multimodal neuron is essential for bitter compound avoidance, and its artificial activation is sufficient to mediate aversion. However, the neuron is also essential for the integration of taste blends. Our findings support a model for taste coding in larvae, in which distinct receptor proteins mediate different responses within the same, multimodal GRN