Current Research Topics
Interacting nodal semimetals
In collaboration with with collaboration with several researches from European institutions me and a PhD student A. Poli
investigate the influence of strong local electron-electron correlations on the transport properties of nodal semimetals.
Nodal semimetals exhibit a band structure reminiscent of graphene, with a pair of bands crossing at specific points in momentum space. Unlike graphene, however, nodal semimetals are three-dimensional materials, and this three-dimensionality ensures that these intersections are protected by either topology or symmetries, rendering them topological entities within momentum space. Our research primarily focuses on examining two competing phenomena: the protection against local electron-electron interactions and the system's tendency to open a band gap upon increasing the interaction strength.
A. Poli et al. "Interacting nodal semimetals with non-linear bands." Phys. Rev. B 109, 045118 (2024)
From strongly disordered to bad metals
In collaboration with S. Fratini (CNRS, Grenoble), V. Dobrosavljević (SMFL and Florida State University Tallahassee, FL, USA) and my former PhD student D. Di Sante (Università di Bologna, Italy) we developed a local theory to explain the large temperature behaviour of strongly disordered metals beyond the so-called Mott-Ioffe-Regel (MIR) limit, solving the unsettled question of universality of the correlations between the temperature coefficient of the resistivity and its residual (T = 0) value. The disorder-driven metal-insulator transition is fundamentally changed (e.g. in its critical exponents) with respect to the non-interacting (Anderson) scenario.
A behaviour which is typical of disordered systems may emerge from the presence of long-range interaction and geometrical frustration in systems which are not intrisically disordered. For example the long-range electron-electron interaction is able to produce an effective distributed disorder which give rise to Coulomb-glass behaviour often observed in Bechgaard salts.
On a more general point of view interaction with slow fluctuations coupled to the charge could give rise to the bad metallic regime where resistivity increase linearly far beyond the MIR limit.
Divergent Precursors of the Mott-Hubbard Transition
Is the normal phase close to the Metal to Insulator Transition really normal? We address this question using Dynamical Mean Field Theory as well as analytical calculations in the Hubbard model. In a joint collaboration with several researches from European institutions we where able to detect, for the first time, an hidden divergence of the local Bethe-Salpeter equation in the charge and particle-particle channel which sets a precise boundary within the normal metallic phase. We provide a comprehensive understanding of this elusive phenomenon by describing the local two-particles correlators in different channels (spin,charge,pairing) in terms of a suitably defined correlation functions. This analysis lead recently to unveil unexpected "attraction-from-repulsion" mechanism which can occur in the vicinity of a metal-insulator transition driven by short range electronic repulsion. This mechanism could be of relevance in explaining the high temperature superconductivity in oxides which remains an open problem since its discovery in the eighties.
Transport in organic systems
In a strict collaboration with Simone Fratini (CNRS Grenoble), we develop along the last decade a formalism which is able to explain and predict the intrinsic mobility of crystalline organic semiconductors. The charge mobility of molecular semiconductors is limited by the large fluctuation of intermolecular transfer integrals, often referred to as off-diagonal dynamic disorder, which causes transient localization of the carriers’ eigenstates. We develop a transient localization theory which recently evolves in dynamical localization correction (DLC) theory which interpolates from the standard Bloch-Boltzmann theory to the Anderson localization state.
Former Research activity
My research activity started focusing on non-equilibrium phenomena in disordered systems and then gradually evolved to theoretical study of models for interacting many-body quantum systems.
Non equilibrium dynamics
My first degree thesis, presents for the first time the full analytical time-dependent solution of the spherical p=2 spin model for spin glasses starting from different initial condition . This paper acquired renewed attention in connection of the dynamical properties of models for the glass transition. In subsequent research we studied the finite size corrections leading to a long time regime in which ergodicity is finally restored as well as we generalize this dynamical study to the p=2+n models which can be relevant models for the study of glassy relaxation (in collaboration with C. Crisanti Sapienza Rome). Non equilibrium dynamics of the witch-on transition in the context of lasers has been analysed using the formalisms of non-linear stochastic equations. In a series of papers we analyse the role of multiplicative noise in the decay from an unstable state. These research was later found applications spanning from quantitative economics as well as ecology.
DMFT
During my PhD I start, in collaboration my tutor prof. F. de Pasquale and co-tutor D. Feinberg (CNRS Neél Grenoble), a new line of research about polarons in solid state physics. The related results of my PhD thesis was later published in a [review paper]https://dx.doi.org/10.1142/S0217979290000656) where we approach the problem of polaron formation as a function of the coupling of the charge carrier to the lattice with analytical and numerical techniques.
By the early 90’s, searching for a non-perturbative tool to deal with the polaron problem, I was one of the first to apply the newly developed Dynamical Mean Field Theory (DMFT) to the electron-phonon problem. We then succesfully apply DMFT to the get the first fully non-pertubative solution to the single small polaron problem in collboration with my former student S. Fratini (now CNRS, Néel Grenoble). This paper presents for the first time the full semi-analytical solution to the single particle spectral properties of the small polaron. The method presentd in this paper is also at the basis of our subsequent research on the t-J Holstein model. Several modern approaches, among all the Momentum Average Approximation by M. Berciu and co-workers, are based on this non perturbative treatment.
The many-body problems of interacting polarons was also the focus of my research during the 2000s. In collaboration with M. Capone (SISSA, Trieste) we compare and disentangle the polaron formation mechanism and the metal insulator transition which can have a bi-polaronic origin in polaronic system. In this paper we clearly states that the conditions for polaron formation is not generally equivalent to those that guarantee the bi-polaronic metal to insulator transition. These two concept were often confused in the literature. From a technical point of view we devise a method based on exact diagonalization by Lanczos technique which allow to attack non perturbatively the many-polaron system.
Organic systems
The many-body problems of interacting large polarons was also a subject of my research during the early 2000s. With G. Rastelli we studied the problem of Wigner crystallization in a polarizable medium i.e. the phase diagram of many-body polaronic system with long range interactions. This and related studies turns out to be relevant in an unexpected context: the large dielectric constant (high-k) Field Effect Transistors. In a collaboration with the Leiden experimental group lead by A. Morpurgo (now at Geneva) we succesfully explain the charge mobilities in high-k using in terms of large interacting polarons.