# Claudio GrimaldiÉcole Polytechnique Fédérale de Lausanne | EPFL · Physics Section

Claudio Grimaldi

PhD

## About

154

Publications

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3,271

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Introduction

Additional affiliations

August 2013 - present

September 2012 - July 2013

January 2008 - December 2011

## Publications

Publications (154)

To assess the number of life-bearing worlds in astrophysical environments, it is necessary to take the intertwined processes of abiogenesis (birth), extinction (death), and transfer of life (migration) into account. We construct a mathematical model that incorporates this trio of mechanisms and accordingly derive the probability distribution functi...

For about the last 60 yr the search for extraterrestrial intelligence has been monitoring the sky for evidence of remotely detectable technological life beyond Earth, with no positive results to date. While the lack of detection can be attributed to the highly incomplete sampling of the search space, technological emissions may be actually rare eno...

The connectedness percolation threshold (\(\phi_{c}\)) for spherically symmetric, randomly distributed fractal aggregates is investigated as a function of the fractal dimension (\(d_{F}\)) of the aggregates through a mean-field approach. A pair of aggregates (each of radius R) are considered to be connected if a pair of primary particles (each of d...

New ambitious initiatives searching for technosignatures from exo‐civilizations are currently being pursued and others are planned in the future, allowing exploration of the parameter search space on an unprecedented scale. Despite this intense activity and the decades‐old history of related searches, there have been only limited efforts in analyzi...

To assess the number of life-bearing worlds in astrophysical environments, it is necessary to take the intertwined processes of abiogenesis (birth), extinction (death), and transfer of life (migration) into account. We construct a mathematical model that incorporates this trio of mechanisms and accordingly derive the probability distribution functi...

The proposition that life can spread from one planetary system to another (interstellar panspermia) has a long history, but this hypothesis is difficult to test through observations. We develop a mathematical model that takes parameters such as the microbial survival lifetime, the stellar velocity dispersion, and the dispersion of ejecta into accou...

Probabilistic arguments about the existence of technological life beyond Earth traditionally refer to the Drake equation to draw possible estimates of the number of technologically advanced civilizations releasing, either intentionally or not, electromagnetic emissions in the Milky Way. Here, we introduce other indicators than Drake's number $N_D$...

Probabilistic arguments about the existence of technological life beyond Earth traditionally refer to the Drake equation to draw possible estimates of the number of technologically advanced civilizations releasing, either intentionally or not, electromagnetic emissions in the Milky Way. Here, we introduce other indicators than Drake’s number ND to...

Significance
The search for life on extrasolar worlds by way of spectroscopic biosignature detection is among the most compelling scientific endeavors of the next decades. This article explores the implications of either discovering or ruling out the presence of detectable biosignatures on planets within a few tens of light years from Earth, a dist...

One of the major goals for astronomy in the next decades is the remote search for biosignatures (i.e.\ the spectroscopic evidence of biological activity) in exoplanets. Here, we adopt a Bayesian statistical framework to discuss the implications of such future searches, both in the case when life is detected, and when no definite evidence is found....

The search for technosignatures from the Galaxy or the nearby universe raises two main questions: What are the possible characteristics of technosignatures? and How can future searches be optimized to enhance the probability of detection? Addressing these questions requires an interdisciplinary approach, combining i) the study of Anthropocene as a...

We report electrical conductivity measurements of polymethyl-methacrylate filled by onion-like carbon particles with a primary particle size of ≈5 nm. We show that the conductivity σ is exceptionally high even at very low loadings and that its low-temperature dependence follows a Coulomb gap regime at atmospheric pressure and an activated behavior...

The design of functional structures from primary building blocks requires a thorough understanding of how size, shape, and particle–particle interactions steer the assembly process. Specifically, for electrically conductive networks build from carbon nanotubes (CNTs) combining macroscopic characterization and simulations shows that the achievable c...

The search for life in the universe is a major theme of astronomy and astrophysics for the next decade. Searches for technosignatures are complementary to searches for biosignatures, in that they offer an alternative path to discovery, and address the
question of whether complex (i.e. technological) life exists elsewhere in the Galaxy. This approac...

A relationship based upon analogy is explored between (i) a lattice-based model for percolation by cylinders that employs distinct site types with unequal occupation probabilities in order to capture heterogeneities in the particle dispersion, and (ii) the well-studied mean-field lattice gas model. The strength of the correlation that sites in the...

Significance
Ongoing and future initiatives in the search for extraterrestrial intelligence (SETI) will explore the Galaxy on an unprecedented scale to find evidence of communicating civilizations beyond Earth. Here, we construct a Bayesian formulation of SETI to infer the posterior probability of the mean number of radio signals crossing Earth, gi...

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

The Milky Way Galaxy contains an unknown number, $N$, of civilizations that emit electromagnetic radiation (of unknown wavelengths) over a finite lifetime, $L$. Here we are assuming that the radiation is not produced indefinitely, but within $L$ as a result of some unknown limiting event. When a civilization stops emitting, the radiation continues...

A lattice model for continuum percolation by cylindrical rods is generalized to account for inhomogeneities in the volume fraction that are indicative of particle clustering or aggregation. The percolation threshold is evaluated from a formalism that uses two different categories of occupied sites (denoting particles) with different occupation prob...

Carbon nanotubes (CTNs) with large aspect-ratios are extensively used to establish electrical connectedness in polymer melts at very low CNT loadings. However, the CNT size polydispersity and the quality of the dispersion are still not fully understood factors that can substantially alter the desired characteristics of CNT nanocomposites. Here we d...

Rashba spin-orbit coupling emerges in materials lacking of structural inversion symmetry, such as heterostructures, quantum wells, surface alloys and polar materials, just to mention few examples. It yields a coupling between the spin and momentum of electrons formally identical to that arising from the weakly-relativistic limit of the Dirac equati...

Suspensions of hard core spherical particles of diameter $D$ with inter-core connectivity range $\delta$ can be described in terms of random geometric graphs, where nodes represent the sphere centers and edges are assigned to any two particles separated by a distance smaller than $\delta$. By exploiting the property that closed loops of connected s...

The lack of evidence for the existence of extraterrestrial life, even the simplest forms of animal life, makes it is difficult to decide whether the search for extraterrestrial intelligence (SETI) is more a high-risk, high-payoff endeavor than a futile attempt. Here we insist that even if extraterrestrial civilizations do exist and communicate, the...

The recent experimental discovery of three-dimensional (3D) materials hosting a strong Rashba spin-orbit coupling calls for the theoretical investigation of their transport properties. Here we study the zero temperature dc conductivity of a 3D Rashba metal in the presence of static diluted impurities. We show that, at variance with the two-dimensio...

We discuss the transport properties of a disordered two-dimensional electron gas with strong Rashba spin-orbit coupling. We show that in the high-density regime where the Fermi energy overcomes the energy associated with spin-orbit coupling, dc transport is accurately described by a standard Drude's law, due to a non-trivial compensation between th...

Hybrid halide perovskite photovoltaic materials show a remarkable light conversion efficiency in various optoelectronic devices. In the fabrication of these solar cells, light emitting diodes, laser and photodetector prototypes the thickness of the perovskite is an important parameter since the light is absorbed within a thin layer of a few hundred...

The problem of continuum percolation in dispersions of rods is reformulated
in terms of weighted random geometric graphs. Nodes (or sites or vertices) in
the graph represent spatial locations occupied by the centers of the rods. The
probability that an edge (or link) connects any randomly selected pair of nodes
depends upon the rod volume fraction...

We investigate the transport properties of the Rashba model in the strong
spin-orbit coupling regime. We show that as soon as the energy associated with
SO interaction overcomes the Fermi energy and it becomes the largest energy
scale in the system the dc conductivity computed by Kubo formula shows
substantial deviations from usual Drude transport...

We analyze the critical connectivity of systems of penetrable $d$-dimensional
spheres having size distributions in terms of weighed random geometrical
graphs, in which vertex coordinates correspond to random positions of the
sphere centers and edges are formed between any two overlapping spheres. Edge
weights naturally arise from the different radi...

The Critical Path Approximation ("CPA") is integrated with a lattice-based
approach to percolation to provide a model for conductivity in nanofibre-based
composites. Our treatment incorporates a recent estimate for the anisotropy in
tunnelingbased conductance as a function of the relative angle between the axes
of elongated nanoparticles. The condu...

We study the dependence of the electrical conductivity on the gold concentration of Au-implanted polymethylmethacrylate (PMMA) and alumina nanocomposite
thin films. For Au contents larger than a critical concentration, the conductivity of Au-PMMA and Au-alumina is well described by percolation in two dimensions, indicating that the critical correla...

Graphene-based SU8 nanocomposite is developed as a new conductive polymer, which benefits from remarkable electrical conductivity of graphene, along with photo-patternability and transparency of SU8. Well defined structures with minimal resolution of 10 μm have been successfully patterned using photolithography technique. The composite exhibits unp...

While the tunneling conductance between two spherical-like conducting
particles depends on the relative inter-particle distance, the wave function
overlap between states of two rod-like particles, and so the tunneling
conductance, depends also on the relative orientation of the rod axes. Modeling
slender rod-like particles as cylindrical quantum we...

Nanogranular metal composites, consisting of immiscible metallic and
insulating phases deposited on a substrate, are characterized by two distinct
electronic transport regimes depending on the relative amount of the metallic
phase. At sufficiently large metallic loadings, granular metals behave as
percolating systems with a well-defined critical co...

We study by simulation and theory how the addition of insulating spherical particles affects the conductivity of fluids of conducting rods, modeled by spherocylinders. The electrical connections are implemented as tunneling processes, leading to a more detailed and realistic description than a discontinuous percolation approach. We find that the sp...

The statistical analysis of the length distribution of catalytic chemical vapour deposition synthesized multi-walled carbon nanotubes cut by planetary ball milling is reported. The nanotube lengths follow a log-normal distribution in a broad range of grinding time and rotational speed. We show that the scale parameter of the distribution, which equ...

We have characterized the electrical conductivity of the composite which
consists of multi-walled carbon nanotubes dispersed in SU8 epoxy resin.
Depending on the processing conditions of the epoxy (ranging from
non-polymerized to cross-linked) we obtained tunneling and percolating-like
regimes of the electrical conductivity of the composites. We in...

We show that the formation of a gel by conducting colloidal particles leads to a dramatic enhancement in bulk conductivity, due to interparticle electron tunneling, combining predictions from molecular-dynamics simulations with structural measurements in an experimental colloid system. Our results show how colloidal gelation can be used as a genera...

The connectedness percolation threshold (η_{c}) and critical coordination number (Z_{c}) of systems of penetrable spherocylinders characterized by a length polydispersity are studied by way of Monte Carlo simulations for several aspect ratio distributions. We find that (i) η_{c} is a nearly universal function of the weight-averaged aspect ratio, wi...

The current theoretical understanding of the electron transport mechanism in thick-film resistors is reviewed and critically discussed in relation to the transport problem for other classes of conductor-insulator composites. The filler concentration and temperature dependencies of transport are analysed in terms of both percolation and tunnelling (...

In conductor-insulator nanocomposites in which conducting fillers are dispersed in an insulating matrix, the electrical connectedness is established by inter-particle tunneling or hopping processes. These systems are intrinsically non-percolative and a coherent description of the functional dependence of the conductivity σ on the filler properties,...

We discuss the compatibility of the most accurate experimental data with the ordinary Migdal-Eliashberg theory of superconductivity in the fullerene compound Rb3C60. By using different model phonon spectra we conclude that the experimental data can be fitted only by invoking an electron-phonon coupling of order λ ≃ 3. This exceedingly high value is...

There is nowadays a general agreement on a key role of the σ bands in the superconducting properties of MgB2. We show that peculiar characteristics of the σ bands give rise to nonadiabatic and anharmonic effects which break the conventional Migdal-Eliashberg framework. Both these features are governed by the small value of the Fermi energy due to t...

We consider the problem of electron transport in segregated conductor-insulator composites in which the conducting particles are connected to all others via tunneling conductances, thus forming a global tunneling-connected resistor network. Segregation is induced by the presence of large insulating particles, which forbid the much smaller conductin...

In this paper we introduce an effective medium theory that is capable of describing site percolation in both lattice and continuum three-dimensional systems. By exploiting self-consistency with a complete graph or network of identical conductors, the resulting effective medium equation allows for a straightforward estimate of the percolation thresh...

A Comment on the Letter by A. Trionfi et al., [Phys. Rev. Lett. 102, 116601 (2009)]. The authors of the Letter offer a Reply.

We show by Monte Carlo simulations, critical path analysis, and effective medium theory that attraction between conducting particles dispersed in an insulating matrix can enhance the electrical tunneling conduction by several orders of magnitude compared to that of non-attracting hard-particle dispersions. By considering different potential profile...

In conductor-insulator composites in which the conducting particles are dispersed in an insulating continuous matrix the electrical connectedness is established by interparticle quantum tunneling. A recent formulation of the transport problem in this kind of composites treats each conducting particle as electrically connected to all others via tunn...

Here, we show that the conductivity of conductor-insulator composites in which electrons can tunnel from each conducting particle to all others may display both percolation and tunneling (i.e. hopping) regimes depending on few characteristics of the composite. Specifically, we find that the relevant parameters that give rise to one regime or the ot...

We noted that the tunneling-percolation framework is quite well understood at the extreme cases of percolation-like and hopping-like behaviors but that the intermediate regime has not been previously discussed, in spite of its relevance to the intensively studied electrical properties of nanocomposites. Following that we study here the conductivity...

Here the electron-phonon Holstein model with Rashba spin-orbit interaction is
studied for a two dimensional square lattice in the adiabatic limit. It is
demonstrated that a delocalized electron at zero spin-orbit coupling localizes
into a large polaron state as soon as the Rashba term is nonzero. This
spin-orbit induced polaron state has localizati...

We report on a model of polymer nanocomposites with fibrous fillers which explicitly considers the microscopic filler features and replicates the composites as random distributions of particles interconnected via electron tunneling. By exploiting the critical path method, we are able to obtain simple formulas, applicable to most nanotube and nanofi...

We report on our Monte Carlo calculations of the conductivity of monosized and conducting spherical particles dispersed in a homogeneous matrix, with interparticle transport mechanism given by electron tunneling. We show that our numerical results can be reproduced by a simple formula based on a critical path analysis and which gives also a practic...

We evaluate the percolation threshold values for a realistic model of continuum segregated systems, where random spherical inclusions forbid the percolating objects, modeled by hardcore spherical particles surrounded by penetrable shells, to occupy large regions inside the composite. We find that the percolation threshold is generally a nonmonotono...

We present an in-depth analysis of the geometrical percolation behavior in the continuum of random assemblies of hard oblate ellipsoids of revolution. Simulations were carried out by considering a broad range of aspect ratios, from spheres up to aspect-ratio-100 platelike objects, and with various limiting two-particle interaction distances, from 0...

Recent developments in thick-film technology have seen the widespread introduction of lead- free alternative to traditional conductive, dielectric and overglaze compositions. Resistors, however, have lagged behind this trend, as developing balanced and well-behaved resistive compositions from scratch is significantly more involved than other thick-...

We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2 meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in...

The tunneling-percolation mechanism of conduction in disordered conductor-insulator composites is studied for a realistic continuum model where conducting and impenetrable spherical particles are dispersed in a three-dimensional continuum insulating material. Conduction between particles is via tunneling processes and a maximum tunneling distance d...

Electronic bound states around charged impurities in two-dimensional systems with structural inversion asymmetry can be described in terms of a two-dimensional hydrogen atom in the presence of a Rashba spin-orbit interaction. Here, the energy levels of the bound electron are evaluated numerically as a function of the spin-orbit interaction, and ana...

We have studied the electron-phonon coupling in electron doped SrTiO3 for which the carrier concentration ranges from a dilute gas of polarons to a polaron liquid. Here we report a comprehensive THz, infrared and optical study together with DC conductivity, Hall effect and specific heat measurements. Our THz spectra at 7 K show the presence of a ve...

The continuous progress in fabricating low-dimensional systems with large spin-orbit couplings has reached a point in which nowadays materials may display spin-orbit splitting energies ranging from a few to hundreds of meV. This situation calls for a better understanding of the interplay between the spin-orbit coupling and other interactions ubiqui...

Many properties of the superconducting state of high-Tc materials have been interpreted within the Eliashberg framework with a very large coupling (λ ≥ 3). Here we point out that the inclusion of non-adiabatic corrections acts as an effective coupling for various properties that can, therefore, be reproduced with a much smaller coupling (λ ≈ 1) wit...

In this Letter we show how, for small values of the Fermi energy compared to the spin-orbit splitting of Rashba type, a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states in the low charge density regime. We investigate its consequences on the onset of the superconductin...

The electronic self-energy is studied for a two dimensional electron gas coupled to a spin-orbit Rashba field and interacting with dispersionless phonons. For the case of a momentum independent electron-phonon coupling (Holstein model) we solve numerically the self-consistent non-crossing approximation for the self-energy and calculate the electron...

We formulate a generalized tunneling-percolation model that describes the electrical properties of conductor-insulator cellular disordered systems. This model predicts and explains the experimentally observed universal and nonuniversal behaviors of such properties and the crossover between them. Studying both the conductance and the corresponding p...

One of the major questions in the physics of high temperature is, to what extent electron-phonon coupling is important for the transport anomalies and for the superconductivity itself. One of the difficulties in addressing this issue for the cuprates, is the complexity of these materials, which are doped Mott-insulators, anti-ferromagnetic, striped...

We investigate the tunneling–percolation mechanism of DC transport nonuniversality for conducting particles of diameter φ placed in series within the bonds of length L of a regular lattice model. When L/φ→∞ the resulting bond conductance distribution function has a power-law divergence as g→0, leading to nonuniversal values of the transport exponen...

We study the isotope effects arising from the coupling of correlated electrons with dispersionless phonons by considering the Hubbard-Holstein model at half-filling within the dynamical mean-field theory. In particular we calculate the isotope effects on the quasi-particle spectral weight $Z$, the renormalized phonon frequency, and the static charg...

We derive the ac spin-Hall conductivity sigmasH(omega) of two-dimensional spin-orbit coupled systems interacting with dispersionless phonons of frequency omega0. For the linear Rashba model, we show that the electron-phonon contribution to the spin-vertex corrections breaks the universality of sigmasH(omega) at low frequencies and provides a nontri...

The values obtained experimentally for the conductivity critical exponent in numerous percolation systems, in which the interparticle conduction is by tunneling, were found to be in the range of t0 and about t0 + 10, where t0 is the universal conductivity exponent. These latter values are, however, considerably smaller than those predicted by the a...

In this work we seek to better understand the mechanisms governing pulse voltage trimming in disordered conductor-insulator composites. Therefore, we investigate the effect of the composition of thick-film resistors (TFRs) on sensitivity to high voltage pulses. We investigate four series of RuO2-based TFRs: two different glass compositions and two...