A Theory for the High-T_c Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing

A theory of highly correlated layered superconducting materials isapplied for the cuprates. Differently from an independent-electron approximation, their low-energy excitations are approached in terms of auxiliary particles representing combinations of atomic-like electron configurations, where the introduction of a Lagrange Bose field enables treating them as bosons or fermions. The energy spectrum of this field accounts for the tendency of hole-doped cuprates to form stripe-like inhomogeneities. Consequently, it induces a different analytical behavior for auxiliary particles corresponding to "antinodal" and "nodal" electrons, enabling the existence of different pairing temperatures at T^* and T_c. This theory correctly describes the observed phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover in the normal state, the existence of Fermi arcs below T^* and of a "marginal-FL" critical behavior above it. The qualitative anomalous behavior of numerous physical quantities is accounted for, including kink- and waterfall-like spectral features, the drop in the scattering rates below T^* and more radically below T_c, and an effective increase in the density of carriers with T and \omega, reflected in transport, optical and other properties. Also is explained the correspondence between T_c, the resonance-mode energy, and the "nodal gap".Comment: 28 pages, 7 figure

Lorentzian noise in the two-dimensional electron gas of AlxGa1–xAs/GaAs quantum wells

Current noise spectra SI() are reported on samples grown by the molecular beam epitaxy technique, with current-carrying contacts, acting as source and drain, and two probes extending into the two-dimensional electron gas (2DEG) of the AlGaAs/GaAs quantum well, in the range 77–295 K for frequencies of 10 Hz to 1 MHz. The time constants are almost independent of temperature and the current dependence is close to linear. The noise is interpreted as Lorentzian-modulated shot noise of the 2DEG current

1/f noise and its unusual high-frequency deactivation at high biasing currents in carbon black polymers with residual 1/fγ (γ=2.2) noise and a preliminary estimation of the average trap energy

We have performed noise measurements on 5 different carbon black polymer composite resistive gas sensors, both in an inert chemical atmosphere (dry nitrogen) and in an active chemical atmosphere (with toluene or ethanol vapour). All the sensors exhibited the presence of significant 1/f noise for biasing currents in the μA range; moreover, we show that the level of 1/f noise is strongly dependent upon the chemical environment and, in particular, the concentration of the vapour. These results, obtained for the first time with this chemically sensitive nanocomposite material, should help in the creation of circuit models and also in the design of low noise chemical sensors using carbon-black composite materials. Additionally, in the thinnest sensor, at sufficiently high biasing currents we found the deactivation of 1/f noise above a certain frequency, with an unexpected residual 1/f γ excess noise (γ around 2.2) which, to our knowledge, has not been observed before. Interestingly, this unusual excess noise was almost insensitive to the presence of either toluene or ethanol vapour; this observation may offer insight on the origins of both 1/f and the measured 1/f γ excess noise in composite polymer resistors. Finally, we have estimated the available noise energy per trap for a given adsorption process which may be used to characterize the noise fluctuations in a chemical environment. We believe that our work will also enable the construction of better SPICE models to help in the design of advanced CMOS transduction circuitry. © 2012 Elsevier B.V