Quantum Theories and Numerical Simulations of Condensed Matter
The research activity of our group covers several scientific areas. A first area includes the development and application of numerical methods for ab-initio computation of structural, dynamical and electronic properties of quantum systems, such as molecules, clusters, crystalline and amorphous solids, surfaces and liquids. Ab-initio simulations are based on the fundamental natural laws (such as electrodynamics and quantum dynamics) and on the properties of the constituent atoms, without introducing any specific assumption or model. In ab-initio molecular dynamics, atoms evolve according to forces that explicitly depend on the electronic structure, usually computed by Density Functional Theory (DFT). While our research is mostly devoted to the basic comprehension of natural laws, our results have concrete implications, and can contribute to the development of novel functional materials and devices, related for instance to hydrogen storage, combustion batteries, photovoltaic cells, and electronics.
A second, and equally crucial research area is the study of those mechanism that determine the emergence of macroscopic quantum mechanical phenomena (coherence, superfluidiy, superconductivity). Their properties are analyzed for instance by statistical methods, DFT and quantum field theories.
Last but not least, we develop and apply novel numerical methods (tensor network methods) to study many-body quantum system properties to describe and support fundamental and applied quantum science experiments.
Staff
Full Professors:Simone Montangero, Luca Salasnich,
Associate Professors: Francesco Ancilotto, Luca Dell’Anna, Pierluigi Silvestrelli, Paolo Umari
Assistant Professors: Alberto Ambrosetti, Marco Di Liberto, Ilaria Siloi, Pietro Silvi, Iogann Tolbatov :
Post-doc
Sharad Kumar Upadhyay
PhD students
Andrea Bardin, Francesco Lorenzi, Edoardo Maria Tiburzi, Marco Uguccioni
External collaborators
Alexander Yakimenko (visiting Professor)
Research activities
Simulation of physisorption and chemisorption processes at surfaces
The comprehension of adsorption processes by means of ab-initio simulations is essential for designing and optimizing a broad variety of materials and devices, and for interpreting scattering and atomic force microscopy experiments. The adsorption of rare-gas atoms or saturated molecules such as H2 on metallic surfaces is paradigmatic for the "physisorption" phenomenon, which is characterized by weak bondings due to the equilibrium between long-ranged van der Waals attraction and short-ranged Pauli repulsion. Instead, when (strong) chemical bonds are formed between substrate and adsorbate, the process is called "chemisorption". For instance, the chemisorption of large non-saturated hydrocarbon molecules on silicon surfaces has a crucial relevance for the early growth phases of silicon carbide, an essential ingredient for the development of novel electronic devices.
Contacts: Pierluigi Silvestrelli
Graphene and carbon nanotubes
The recent experimental advances and the growing interest in nanotechnological applications have drawn particular interest on graphene and carbon nanotubes. Graphene exhibits peculiar physical features due to its two-dimensional crystalline structure. Our group investigates via ab-initio simulation techniques chemisorptions and physisorption of external atoms/molecules both on planar and corrugated grapheme. Relying on state of the art approaches developed in our group and based on density functional theory (DFT) and perturbative many-body theory GW, we characterize different types of adsorbates, aiming to explain -as a complement to experimental investigations- how chemical functionalization can alter the structural, vibrational and electronic properties of graphene. Water-graphene interactions are also subject of our investigations, given the relevance of this system as a model for hydrophobic substrates, and in view of recently realized applications of "energy harvesting". Carbon nanotubes are particularly interesting as potential devices for hydrogen storage in electric vehicles based on fuel cells. We study the interaction between hydrogen and small radius nanotubes via ab-initio DFT: we explore reaction paths, adsorption sites and hydrogen molecule orientations, relative to the carbon structure, and compute the corresponding adsorption potentials.
Contacts: Pierluigi Silvestrelli, Paolo Umari
Long-range interactions in nanostructures and biological systems
Van der Waals forces have quantum mechanical origin and can virtully act up to infinite range. By summing up, these forces become extremely important in large-scale systems, such as protein, DNA, nanostructures, two-dimensional materials, etc. Our activity is related here to a continuous improvement of the theoretical description of these forces, including many body effects. At the same time, we explore non-trivial effects, such as the role of electronic excitations induced by photoabsorption, the action of external fields, and mechanisms beyond the Born-Oppenheimer approximation.
Contacts: Alberto Ambrosetti, Pier Luigi Silvestrelli
Water structure (hydrogen bonds), interfacial water and aqueous solutions
Water -the most important liquid on earth-, owes its unusual properties to the hydrogen bond net connecting adjacent molecules. The description of water electronic structure needs to be improved specifically accounting for dispersion (van der Waals) interactions, a quantum mechanical effect due to non-local correlations between electrons. Currently we are studying the structural effects due to dispersion in hydrogen bonded systems. A better comprehension of the microscopic structure of water is a prerequisite for interpreting spectroscopic data, and should lead to improved macroscopic models for interfacial water and hydration processes, making them apt to describe metastable states either. Hydrophobic interactions are crucial in many biophysical and biochemical processes. Essentially, the hydrophobic effects accounts for the tendency of apolar groups to associate into aqueous solutions by minimizing the total external surface exposed to water; in contrast, polar groups can take part into hydrogen bonds with water molecules. We therefore study via ab-initio simulations structural, dynamical, bonding and electronic properties of water molecules close to various solutes, such as methane and methanol molecules at different concentrations.
Contacts: Pierluigi Silvestrelli
Excitation properties from perturbative many-body theory
Through the years we developed several algorithms for the accurate computation from first principles of excitation properties, such as electronic band gaps and the macroscopic dielectric function, based on many-body perturbation theory. These methods are implemented in the open source software Quantum-Espresso and enable fast simulations of large model systems. Now we are extending our methods to the time evolution of quantum systems in response to external excitations.
Contacts: Paolo Umari
Computational analysis of metal-based complexes interacting with biomolecules
We study the mechanisms of action of metal-based complexes toward protein, DNA, and RNA targets, using the computational methods based on molecular quantum mechanics. The reactivity of metal scaffolds with the biomolecules constitutes a novel field of research which has attracted growing attention in the last years being now extensively recognized the importance of the so-called “metalation process†for the mode of action of approved or experimental metallodrugs. In this frame we are developping a novel theoretical strategy allowing to elucidate at molecular level the behavior of metal complexes with molecular models of relevant biomolecular targets.
This research is performed within the theme “Molecular-level study of structure and function of RNA and its modifications†in the framework of the project “National center for gene therapy and drug development with RNA technology
Contacts: Paolo Umari, Iogann Tolbatov
Condensation and superfluidity in ultracold atoms
We study the thermodynamics of Fermi and Bose weakly interacting gases (alkaline-metallic atoms, such as rubidium, sodium, lithium or atomic hydrogen) and trapped via magnetic or optical potentials. We analyze the elementary single- and many-particle excitations, solving both Bogoliubov-de Gennes and Popov equations. In addition, we investigate dynamical properties of Bose-Einstein condensates (BECs) employing the three-dimensional time-dependent Gross-Pitaevskii equation, describing the macroscopic wave function (order parameter) of the Bose condensate. We are also investigating the dynamics (collective excitations, free expansion, quantum vortex formation) of two-component Fermi gases at the BCS-BEC crossover, and the formation of solitons in Bose-Fermi mixtures. In particular, we are developing a reliable energy functional for the unitary Fermi gas (infinite scattering length) at zero and finite temperature. We are also analyzing the properties of Fermi gases in the presence of spin-orbit coupling in the BCS-BEC crossover, making use of analytical and numerical path integral techniques. Finally, we are working on the dynamics of many-body quantum tunneling effects for both Bose and Fermi systems in double and triple potential wells, aiming to study "Schroedinger's cat" states and quantum entanglement.
Contacts: Francesco Ancilotto, Luca Dell’Anna, Luca Salasnich
Classical and quantum fluids in confined geometries
We apply and develop computational methods apt to investigate structural and dynamical properties of quantum fluids in confined geometries. Examples include: liquid helium nanodroplets (highly quantum fluid), pure or doped with atomic/molecular impurities, liquid argon (typical classical fluid) adsorbed into nanopores, liquid para-hydrogen (weakly quantum fluid) adsorbed on nanostructures, helium in carbon nanotubes, etc. Helium and hydrogen are analyzed within a phenomenological theory based on density functional theory, while classical molecular dynamics and Monte Carlo simulations are adopted for describing classical fluids. An additional example regards the flow of water or helium through narrow carbon nanotubes. A full quantum mehcanical treatment predicts frictionless motion due a mechanism which presents several analogies with superfluidity.
Contacts: Francesco Ancilotto, Pier Luigi Silvestrelli, Alberto Ambrosetti
Disordered systems
Disorder is often present in nature, and the comprehension of many macroscopic physical phenomena governed by quantum mechanics can only be attained by explicitly considering impurities and inhomogeneities. A well known effect due to disorder is the so-called Anderson localization, that spatially confines electronic wave functions when overcoming a critical value of the local disorder, thus turning the system into an insulator. The properties of disordered systems in the presence of inter-particle interactions are instead yet to be explored. The phase transition between conducting and insulating phase is called in this case "many-body localization", and its characterization is currently object of intense studies in the context of many-body physics.
Contacts: Luca Dell’Anna
Computational study of solar cells: from materials to devices
In the last ten years the discovery and subsequent realization of new materials such as organic-inorganic perovskites raised large interest due to the high conversion efficiency that could be reached. From the very beginning our group carried out investigations by means of first-principle simulations, that enabled the comprehension of the electronic properties of such materials. More recently, we studied the interaction between electronic excitations and atomic vibrations, addressing the maximum achievable efficiency. We now prosecute the research extending both the variety of materials and the physical properties to be addressed.
Contacts: Paolo Umari
Glasses and amorphous materials
The physical properties of glasses and amorphous materials in general are determined by the atomistic-scale structure. However, this is not experimentally accessible due to the lack of structural periodicity. The comprehension of their physical behaviour requires then atomistic simulations. Our group has a solid expertise in the modelling of glasses, and in the simulation of their spectroscopic response properties. Presently, we are studying and simulating the mechanical dispersion in amorphous oxides.
Contacts: Paolo Umari
Computational Spectroscopy
The ability to simulate the same spectrocopic techniques exploited in experiments is pivotal to extract solid conclusions. Our group has a broad experience in first-principle calculations based on density functional theory (DFT), and on many-body perturbation theory, such as: direct and inverse photoemission, optical constants, macroscopic dielectric function, infrared and FTIR dielectric response, and Raman. Target systems include: bulk solids, 2D layered materials, amorphous systems, surfaces, and molecular systems.
Contacts: Paolo Umari
