Sigfrid Strässler

Tunneling-percolation origin of nonuniversality: Theory and experiments

A vast class of disordered conducting-insulating compounds close to the percolation threshold is characterized by nonuniversal values of transport critical exponent t, in disagreement with the standard theory of percolation which predicts t

Gauge factor enhancement driven by heterogeneity in thick-film resistors

We present a simple picture of the gauge factor (GF) enhancement in highly heterogeneous materials such as thick-film resistors. We show that when the conducting phase is stiffer than the insulating one, the local strains within the latter are enhanced with respect to the averaged macroscopic strain. Within a simple model of electron tunneling processes, we show that the enhanced local strain leads to values of GF higher than those expected for a homogeneous system. Moreover, we provide formulas relating the enhancement of GF to the elastic and microstructural characteristics of thick-film resistors.

Piezoresistivity and conductance anisotropy of tunneling-percolating systems

We propose a theory of the origin of transport nonuniversality in disordered insulating-conducting compounds based on the interplay between microstructure and tunneling processes between metallic grains dispersed in the insulating host. We show that if the metallic phase is arranged in quasi- one-dimensional chains of conducting grains, then the distribution function of the chain conductivities g has a power-law divergence for g-->0, leading to nonuniversal values of the transport critical exponent t. We evaluate the critical exponent t by Monte Carlo calculations on a cubic lattice, and show that our model can describe universal as well nonuniversal behaviors of transport depending on the value of few microstructural parameters. Such a segregated tunneling-percolation model can describe the microstructure of a quite vast class of materials known as thick-film resistors, which display universal or nonuniversal values of t depending on the composition.

Superconductivity of Rb3C60: breakdown of the Migdal-Eliashberg theory

Abstract:In this paper, through an exhaustive analysis within the Migdal-Eliashberg theory, we show the incompatibility of experimental data of Rb3C60 with the basic assumptions of the standard theory of superconductivity. For different models of the electron-phonon spectral function α2F(Ω) we solve numerically the Eliashberg equations to find which values of the electron-phonon coupling λ, of the logarithmic phonon frequency and of the Coulomb pseudopotential μ* reproduce the experimental data of Rb3C60. We find that the solutions are essentially independent of the particular shape of α2F(Ω) and that, to explain the experimental data of Rb3C60, one has to resort to extremely large couplings: λ = 3.0±0.8. This results differs from the usual partial analyses reported up to now and we claim that this value exceeds the maximum allowed λ compatible with the crystal lattice stability. Moreover, we show quantitatively that the obtained values of λ and strongly violate Migdals theorem and consequently are incompatible with the Migdal-Eliashberg theory. One has therefore to consider the generalization of the theory of superconductivity in the nonadiabatic regime to account for the experimental properties of fullerides.

High Tc Superconductivity in MgB2 by Nonadiabatic Pairing

The evidence for the key role of the sigma bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the sigma bands. We show that the nonadiabatic theory leads to a coherent interpretation of T(c) = 39 K and the boron isotope coefficient alphaB = 0.30 without invoking very large couplings and it naturally explains the role of the disorder on T(c). It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.

Critical behaviour of the piezoresistive response in RuO2?glass composites

We re-analyse earlier measurements of resistance R and piezoresistance K in RuO2-based thick-film resistors (TFRs). The percolating nature of transport in these systems is well accounted by values of the transport exponent t larger than its universal value t2.0. Furthermore, we show that the RuO2 volume fraction dependence of the piezoresistance data fit well with a logarithmically divergence at the percolation threshold. We argue that the universality breakdown and diverging piezoresistive response could be understood in the framework of a tunnelling–percolating model proposed a few years ago to apply in carbon-black–polymer composites. We propose a new tunnelling–percolating theory based on the segregated microstructure common to many TFRs, and show that this model can in principle describe the observed universality breakdown and the diverging piezoresistance.

Segregated tunneling-percolation model for transport nonuniversality

We propose a theory of the origin of transport nonuniversality in disordered insulating-conducting compounds based on the interplay between microstructure and tunneling processes between metallic grains dispersed in the insulating host. We show that if the metallic phase is arranged in quasi- one-dimensional chains of conducting grains, then the distribution function of the chain conductivities g has a power-law divergence for g-->0, leading to nonuniversal values of the transport critical exponent t. We evaluate the critical exponent t by Monte Carlo calculations on a cubic lattice, and show that our model can describe universal as well nonuniversal behaviors of transport depending on the value of few microstructural parameters. Such a segregated tunneling-percolation model can describe the microstructure of a quite vast class of materials known as thick-film resistors, which display universal or nonuniversal values of t depending on the composition.

Gauge factor of thick-film resistors: Outcomes of the variable-range-hopping model

Despite a large amount of data and numerous theoretical proposals, the microscopic mechanism of transport in thick-film resistors remains unclear. However, recent low-temperature measurements point toward a possible variable-range-hopping mechanism of transport. Here, we examine how such a mechanism affects the gauge factor of thick-film resistors. We find that at sufficiently low temperatures T, for which the resistivity follows the Mott’s law R(T)∼exp(T0/T)1/4, the gauge factor (GF) is proportional to (T0/T)1/4. Moreover, the inclusion of Coulomb gap effects leads to GF∼(T0′/T)1/2 at lower temperatures. In addition, we study a simple model which generalizes the variable-range-hopping mechanism by taking into account the finite mean intergrain spacing. Our results suggest a possible experimental verification of the validity of the variable-range hopping in thick-film resistors.

Longitudinal and Transverse Piezoresistive Response of Granular Metals

Keywords: Technologie des Couches Epaisses Reference LPM-ARTICLE-2001-004View record in Web of Science Record created on 2006-06-22, modified on 2016-08-08

A random-resistor network model of voltage trimming

In industrial applications, the controlled adjustment (trimming) of resistive elements via the application of high voltage pulses is a promising technique, with several advantages with respect to more classical approaches such as the laser cutting method. The microscopic processes governing the response to high voltage pulses depend on the nature of the resistor and on the interaction with the local environment. Here we provide a theoretical statistical description of voltage discharge effects on disordered composites by considering random resistor network models with different properties and processes due to the voltage discharge. We compare standard percolation results with biased percolation effects and provide a tentative explanation of the different scenarios observed during trimming processes.