Breakdown of Fermi-liquid theory in a cuprate superconductor

The behaviour of electrons in solids is remarkably well described by Landau's Fermi-liquid theory, which says that even though electrons in a metal interact they can still be treated as well-defined fermions, called ``quasiparticles''. At low temperature, the ability of quasiparticles to transport heat is strictly given by their ability to transport charge, via a universal relation known as the Wiedemann-Franz law, which no material in nature has been known to violate. High-temperature superconductors have long been thought to fall outside the realm of Fermi-liquid theory, as suggested by several anomalous properties, but this has yet to be shown conclusively. Here we report on the first experimental test of the Wiedemann-Franz law in a cuprate superconductor, (Pr,Ce)$_2$CuO$_4$. Our study reveals a clear departure from the universal law and provides compelling evidence for the breakdown of Fermi-liquid theory in high-temperature superconductors.Comment: 7 pages, 3 figure

Efficient Olfactory Coding in the Pheromone Receptor Neuron of a Moth

The concept of coding efficiency holds that sensory neurons are adapted, through both evolutionary and developmental processes, to the statistical characteristics of their natural stimulus. Encouraged by the successful invocation of this principle to predict how neurons encode natural auditory and visual stimuli, we attempted its application to olfactory neurons. The pheromone receptor neuron of the male moth Antheraea polyphemus, for which quantitative properties of both the natural stimulus and the reception processes are available, was selected. We predicted several characteristics that the pheromone plume should possess under the hypothesis that the receptors perform optimally, i.e., transfer as much information on the stimulus per unit time as possible. Our results demonstrate that the statistical characteristics of the predicted stimulus, e.g., the probability distribution function of the stimulus concentration, the spectral density function of the stimulation course, and the intermittency, are in good agreement with those measured experimentally in the field. These results should stimulate further quantitative studies on the evolutionary adaptation of olfactory nervous systems to odorant plumes and on the plume characteristics that are most informative for the ‘sniffer’. Both aspects are relevant to the design of olfactory sensors for odour-tracking robots

Insect Antenna-Based Biosensors for In Situ Detection of Volatiles

Insect antennae are among the most sensitive and selective chemical-sensing organs in the animal kingdom. Insects can perceive picograms of specific volatile organic compounds per cubic meter of air in milliseconds, which is far below the detection thresholds of current analytical devices. These exceptional sensing abilities have many uses in the context of insect biotechnology. Living specimens or parts of them, such as isolated antennae or individual proteins, can serve as biosensors in the field. As volatiles occur in a crude mixture in the environment, knowing which trigger-volatiles are crucial for the insects’ perception of specific incidents is of great value. This knowledge promotes the development of selective sensors for applications, such as fire detection. In this chapter, we discuss the different technical procedures for the preparation and use of insect-based biosensors for the detection of organic volatiles, including those based on insect behavior, insect olfactory proteins, and biomimetic sensing units. We also consider the use of these applications in portable devices outside the laboratory under field conditions