Quantum Theories and Numerical Simulations of Condensed Matter

Quantum Theories and Numerical Simulations of Condensed Matter


Francesco Ancilotto Simone Montangero, Pier Luigi Silvestrelli, Luca Salasnich, Paolo Umari, Luca Dell'Anna.


Lyudmyla Adamska, Giacomo Gori, Alberto Ambrosetti

Research Activity

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.

Simulation of physisorption and chemisorption processes at surfaces

(Pier Luigi Silvestrelli)
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.

Graphene and carbon nanotubes

(Pier Luigi Silvestrelli, Paolo Umari)
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.

Water structure (hydrogen bonds), interfacial water and aqueous solutions

(Pier Luigi Silvestrelli)
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.

Excitation properties from perturbative many-body theory

(Paolo Umari)
We developed specific algorithms for the computation of both neutral and charged electronic excitations, based on many-body perturbation theory (GW-BSE). Currently, we are applying these numerical methods for investigating the following topics: *) electrochemical solar cells *) perovskite solar cells )* electronic properties of organic and metal-organic molecules *) electronic properties of metallic nanoclusters *) simulation of Raman spectra in models of silica subject to compression *) study of electronic properties of DNA models and DNA fragments in solution.

Condensation and superfluidity in ultracold atoms

(Francesco Ancilotto, Luca Dell'Anna, Luca Salasnich)
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.

Classical and quantum fluids in confined geometries

(Francesco Ancilotto, Pier Luigi Silvestrelli)
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.

Disordered systems

(Luca Dell'Anna)
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. 

Quantum science and technologies

(S. Montangero)
A few decades ago, the second quantum revolution (www.qt.eu) started: it aims to exploit and engineer the quantum properties of matter to develop the quantum technologies. These new class of technologies (quantum simulations, computations, sensing and communication) promise to overcome current technologies under many aspects and to pave the way to novel routes to investigate directly the quantum properties of matter, and the fundamental limits of quantum system manipulation with potential impact ranging from chemistry, astrophysics, condensed matter and high-energy physics. Moreover, quantum technologies will grant access to novel investigation tools to probe and understand the quantum properties of nature.,
We apply tensor network methods and other numerical and theoretical quantum science methods to support and simulate experiments of cold atoms in optical lattices, trapped ions, NV centers in diamonds, circuit QED and Bose-Einstein condensates. Established collaborations include the groups in Firenze (L. Fallani), Vienna (J. Schmiedmayer), Garching (I. Bloch), Harvard (M. Lukin), e Ulm (F. Jelezko). The objectives of our activities ranges from the development of the quantum technologies building blocks (creation of entangled states, of macroscopic quantum superpositions, optimal control of quantum information etc.) to the development and the application of novel tool and methods to study strongly correlated quantum systems (lattice gauge theories, topological systems, many-body entanglement, critical systems, etc.). We operate in an interesting spot in between quantum science, condensed matter, quantum optics and high-energy physics.