Financed scholarships:

• A common mechanism to Cooperative Effects in Many Body Quantum Systems
• Quantum protocols for super-efficient solar cells based on layered semiconductors
• Cooperative electronic dynamics in high-Tc superconductors and corundum heterostructures
• Modeling and simulations of cardiovascular system in pathological situations
• Characterization of source and sinks of O3, NO and NO2 in lowland deciduous forests. Measurements and modeling aspects
• Transition metal oxides photoelectrodes for hydrogen production: the challenge of nanogranular materials

A common mechanism to Cooperative Effects in Many Body Quantum Systems

Student: Elisa Zanardini

Background and motivation

Cooperative quantum effects emerge from the interaction between the constituents of a system and they are often connected to important functional aspects. It has been proved their robustness to noise and for this reason they might be essential to develop quantum devices able to work at room temperature. There are many examples of cooperative effects in condensed matter physics such as superconductivity, superradiance, plasmon excitation and giant resonances in nuclei. Therefore one guess a common mechanics underlying them all, as was pointed out by U. Fano (Rev. Mod. Phys. 64, 1992). For instance, the superconducting gap in ultra-small metallic nanoparticles is mathematically equivalent to the superradiant gap in an ensemble of two-level systems. One of the fundamental aspects which leads to collective phenomena is the presence of long-range interactions between the constituents of a system. Indeed, long-range forces can induce extended coherent states separated from the rest of the spectrum by an energy gap. Such a gap is the main responsible for the robustness of these coherent states. Long-range forces can be due to  Coulomb interactions, as in the case of plasmons excitation, or they can be originated by the common interaction with a single mode of an external environment. For instance, in the case of superconductivity the phonon mediates an attractive interaction between the electrons (which is long ranged in momentum space), or in the case of superradiance where the interaction is mediated by the electromagnetic field. Engineering the external environment allows us to control the nature of the long-range forces, and thus the collective effects induced by them. This is what has been recently achieved in ion traps (P.Richerme et al, Nature, 511, 198, 2014) or in superconducting qubits in an open one-dimensional transmission line (AF Van Loo et al, Science 342, 1494, 2013). This Ph.D. project will follow different directions: (i) to deepen the understanding of a common theoretical framework able to explain the emergence of cooperative phenomena in many body systems, such as many excitation superradiance and superconductivity. In particular following previous results [NC Chávez et al., Eur. Phys. Jour. B 92, 1-12 (2019)]  we would like to study the connections between superconductivity and superradiance. This will allow us to understand the general conditions for the emergence of robust cooperative quantum effects; (ii) to consider the interplay of cooperativity and noise in realistic systems relevant for  quantum transport and out-of-equilibrium dynamics in many-body systems. Superradiance has been recently discovered in perovskites superlattices [G. Rainò et al, Nature, 2018 563, 671]. Under small fluence an interpretation of the experimental results was given by our group in [F.Mattiotti et al, Nano Lett. 2020, 20 7382]. On the other hand, for large fluence, and thus for many excitations present in the system, a coherent theoretical framework has still to be developed.  In this project we would like to pursue precisely this line of research in collaboration with the theoretical (B. Janko) and experimental (M. Kuno) groups at the Univ. of Notre Dame. (iii) Another interesting application of many body cooperative effects is in the framework of cavity physics. In this context hybrid light matter states, called polaritonic states, can be created. It has been demonstrated experimentally that polaritons can undergo a BEC transition [J.Littlewood, Physics Today 63, 8, 42 (2010)], forming a condensate. It would be very interesting to investigate the possibility of having a BEC-BCS transition for polaritons. This could help to create a superconductor for excitons, which might be very relevant for ENERGY science applications and Quantum Information Processing.

Supervisors

Prof. Fausto Borgonovi, UCSC, Italy, fausto.borgonovi@unicatt.it
Prof. Boldizsar Janko, Univ. Notre Dame, US bjanko@nd.edu
Dr. Luca Celardo, Università di Firenze, Italy, giuseppeluca.celardo@unifi.it
 

Quantum protocols for super-efficient solar cells based on layered semiconductors

Student: Matteo Cossetto

Background and motivation

Despite the immense research and development effort of the last decades, the energy conversion efficiency of state-of-art solar cells remains intrinsically hampered to the Shockley-Queisser classical limit of 32%. This limit is due to thermalization processes, i.e. internal losses of the adsorbed photon energy in excess of the band gap of the light-harvesting material. Upon inspiration from quantum-coherence-enhanced photosynthesis in Nature, the possibility of a quantum-based operation of solar cells has been recently envisioned as a strategy to overcome such limitation of the classical physics. This PhD project intends to take this challenge by directly addressing the fundamental processes governing the coherent to incoherent crossover in the exciton dynamics of layered semiconductors with the ultimate purpose to realize super-efficient solar cell. Layered semiconductors are nowadays at the forefront of research for their peculiar and tunable optoelectronic properties which stem from a combination of the reduced dimensionality and the efficient light absorption. The photophysics of these materials is governed by formation of strongly bound excitons that are stable up to room temperature and whose decoherence and depopulation dynamics are dictated by competing processes occurring at different timescales. A universal feature is the occurrence of an initial coherent regime determined by the laser-induced macroscopic polarization, rapidly followed by an incoherent regime, where excitons relax toward an incoherent admixture of bound electron-hole pairs. The goal of this doctoral project is to unravel the microscopic mechanisms governing the exciton dynamics along the crossover between the two regimes which typically evolves on a femtosecond timescale. To achieve these goals, experimental techniques sensitive to both decoherence and depopulation dynamics of the photoexcited states are required. Here, we choose a combination of transient optical spectroscopy and interferometric time-resolved multi-photon photoemission spectroscopy (inter-tr-mPPE) to attain a complementary view on the ultrafast quasi-particle relaxation dynamics, the coherence of optical excitation and its dephasing in the time and energy domain. The chosen materials family is formed by metal iodide semiconductors which provide an ideal platform for investigating exciton dynamics. Due to their low dimensionality, these materials show a well-defined exciton line in the infrared-visible regime, corresponding to an exciton binding energy of hundreds of meV. Moreover, the absence of structural or electronic phase transitions in these materials helps the straightfowrad investigation of the bare exciton formation and the role of the environment such as lattice vibrations, defects, and the presence of excited free carriers on the exciton dynamics. Much of the initial experimental efforts of the project will focus on the implementation of the inter-tr-mPPE experimental setup available at Università Cattolica del Sacro Cuore and the grow and characterization of the samples exploiting the facility at KU Leuven.

Supervisors

Prof. Stefania Pagliara, Università Cattolica del Sacro Cuore, Italy, stefania.pagliara@unicatt.it
Prof. Jean-Pierre Locquet, KU Leuven, Belgium, jeanpierre.locquet@kuleuven.be
Prof. Jin Won Seo, KU Leuven, Belgium, maria.seo@kuleuven.be
Dr. Selene Mor, Università Cattolica del Sacro Cuore, Italy, selene.mor@unicatt.it

Cooperative electronic dynamics in high-Tc superconductors and corundum heterostructures

Student: Francesco Proietto

Background and motivation

Cooperative effects induced by light-matter interactions have been studied for decades. These studies have focused on atomic and molecular systems and have led to spectacular experimental findings in the realm of cavity quantum-electrodynamics (QED). In standard cavity-QED, direct interactions between matter constituents are often weak and can be neglected. In this case, collective effects are solely due to effective interactions, which emerge from the microscopic interactions between matter constituents and a common cavity mode. Recent experimental advances have made it possible to monolithically integrate graphene and other low-dimensional materials, such as transition metal dichalcogenides (TMDs) or 2D oxides, with optical microcavities, paving the way for fundamental studies of cavity QED at the nanometer scale with 2D materials as the active medium. On the other hand, the coherent interaction between light and matter can be enhanced by strong driving with coherent ultrashort light pulses. Tailored excitation schemes can lead to Floquet engineering of the electronic bandstruture or the coherent manipulation of solid-solid transitions. One of the most exciting research direction is the the development of protocols to coherently manipulate the superconducting phase in high-temperature superconductors, such as copper oxides, or to coherently control the electron dynamics in oxide heterostructures. The aim of this project is to address the possibility of using ultrashort resonant light-pulses to selectively enhance the coherence of specific electronic states that are responsible for the emergence of macroscopic quantum phases. The first platform that will be investigated is constituted by copper oxide superconductor, in which the interplay between high-energy charge-transfer optical excitations and the onset of unconventional superconductivity is well known. We will address how the formation of the superconducting condensate affects the decoherence time of charge-transfer excitations. We will investigate the possibility of coherently manipulating the electronic excitations to modify the condensate. As a second step we will investigate collective and coherent electron dynamics in heterostructures constituted by layers of corundum materials, such as Al2O3, Cr2O3, V2O3, Fe2O3, Ga2O3, in which the electronic, optical and magnetic properties can be engineered by proper heterostructuring. The PhD student will develop a coherent 2-dimensional electron spectroscopy (2DES) experiment to investigate the decoherence dynamics of optical and electronic excitations in nanostructured correlated materials. 2DES measures the third-order material coherent polarization by exploiting two coherent phase-locked pulses acting as a pump, and a third pulse acting as a probe, allowing for simultaneous resolution of excitation and detection frequency axes with fs temporal resolution. 2DES thus allows to investigate not only the population relaxation time, but directly the decoherence time of relevant modes. 2DES will be used to probe the decoherence dynamics of charge-transfer excitations in superconducting copper oxides. In particular, we will look for signatures of modification of the intrinsic decoherence dynamics driven by: i) the formation of the superconducting condensate; ii) the coupling with the coherent laser field. Similar experiments will be performed on corundum heterostructures. A crucial challenge is related to the sample dimensions, which command spatial resolution, mandatory to perform 2DES on micrometer-sized samples, possibly embedded in cavities. Much of the initial experimental efforts will focus on the implementation of a microscopy measurement scheme, to be coupled to state-of-the-art 2DES setups available at Univrsità Cattolica del Sacro Cuore, providing few-micron spatial resolution while retaining the intrinsic temporal resolution (10-20 fs).

Supervisors

Prof. Claudio Giannetti, Università Cattolica del Sacro Cuore, Italy, claudio.giannetti@unicatt.it
Prof. J-P Locquet, KU Leuven, Belgium, jeanpierre.locquet@kuleuven.be

Modeling and simulations of cardiovascular system in pathological situations

Student: Muzammal Aziz

Background and motivation

Fluid dynamics modeling of blood flow and electrical propagation in myocardial tissue are fields of research widely studied nowadays.

Blood has interesting properties that vary depending on the type of vessel in which it moves. Moreover, its behavior can change deeply in pathological situations, which will be the focus of this project since they are of the utmost interest in clinical practice. Examples of a pathological condition are chronic venous disease (CVD) and coronary artery disease (CAD). CVD occurs very often during aging and affects almost 40% of the total population. CAD is the leading cause of death worldwide. In either of those pathological situations it is often necessary to use a specific drug therapy or in some cases even surgical treatments.

The mechanical contraction of the heart is governed by an electrical stimulus. Describing the trend of this stimulus is very important in pathological situations, such as cardiac arrhythmias that are an important cause of death, e.g. via cardiac arrest or via blood clot formation. Nowadays there is still a lack of comprehension of the dynamics of heart rhythm disorders, so that a mathematical model of these problems can be very useful to better understand the underlying physiology [1].

The aim of this doctoral project is to define a continuum mechanics model for such pathological conditions. Indeed, the availability of a model can then lead to the study of blood flow patterns and cardiac electrical signals through numerical simulations. Interpretation of the results obtained from the simulations can, for example, be used by clinicians to suggest ways to prevent the degeneration of the disease, drug therapies or surgery.

The main goal of this doctoral project is therefore to build up a mathematical and numerical model of pathological situations for the mutual effect of cardiac arrhythmias and the resulting flow in veins and arteries.

Remaining in the framework of continuum mechanics, the blood can be modeled as a Newtonian or a Non-Newtonian fluid. Great attention will be paid to considering blood viscosity as dependent on hematocrit or other physiological parameters that can be regulated by drug therapy [2]. In order to consider relevant pathological situations (e.g. CVD or CAD), the proposed model will be studied in different geometries and in various scenarios [3]. From a mathematical point of view, cardiac arrhythmias will be described by reaction-diffusion systems where several parameters will take into account pathological conditions. Several numerical simulations will be done and compared, employing techniques like the finite element method, and integrating when possible relevant clinical data, either patient-specific or available from literature.

The initial part of the project will focus on the fluid dynamics modeling at Università Cattolica del Sacro Cuore in Italy, under the supervision of G. Giantesio and F. Ballarin. Then, the PhD student will also perform research visit(s) with a combined length of minimum 1 year at KU Leuven in Belgium in the arrhythmia modeling group of H. Dierckx. Both disciplines will be integrated in the second half of the project.

[1] R.H. Clayton et al. Models of cardiac tissue electrophysiology: Progress, challenges and open questions, Prog. In Biophysics and Molecular Biology 104 (2011).
[2] G.P. Galdi, R. Rannacher, A.M. Robertson, S. Turek. Hemodynamical Flows, Birkauser (2008).
[3] F. Ballarin et al. Fast simulations of patient-specific haemodynamics of coronary artery bypass grafts based on a POD-Galerkin method and a vascular shape parametrization, J. Comput. Phys 315 (2016).

Supervisors

Dr. Giulia Giantesio, Università Cattolica del Sacro Cuore, Italy, giulia.giantesio@unicatt.it
Dr. Francesco Ballarin, Università Cattolica del Sacro Cuore, Italy, francesco.ballarin@unicatt.it
Dr. Hans Dierckx, KU Leuven, Belgium h.dierckx@kuleuven.be

Characterization of source and sinks of O3, NO and NO2 in lowland deciduous forests. Measurements and modeling aspects

Student: Davide Plebani

Background and motivation

Ozone (O3) is a secondary air pollutant that forms in the troposphere because of chemical reactions between nitrogen oxides and volatile organic compounds, favored by solar radiation and high temperatures. It has a strong phytotoxic action and negative consequences on the carbon sequestration capacity of natural ecosystems. Ozone is deposited on vegetation and transported down in the intra-canopy space to the soil. In tall canopies like forests, turbulence is damped, and the time scale of turbulent O3 transportation becomes comparable to that of chemical transformations, whose role cannot be neglected. The emission of NO from the forest floor (not yet completely characterized) and the emission of isoprene from the canopy contribute decisively to those chemical transformations. Moreover, in the intra-canopy space, chemistry is strongly influenced by light and temperature gradients that drive photodissociation and reaction rates.

These processes influence a substantial part of what is defined as non-stomatal deposition which, ultimately, contributes to determining the O3 concentration at the leaf level and then the dose of O3 absorbed by the vegetation through the stomata. The phytotoxic O3 dose is usually calculated using big-leaf monolayer dual sink models, at most distinguished between sunlit and shaded leaves (e.g. DO3SE model, Emberson et al., 2000). However, doubts were raised about the correct formulation of the intra-canopy transport, which is currently based on parameterizations derived from a single experiment carried out on corn fields (van Pul & Jacob, 1994). Multilayer models try to resolve the intra-canopy turbulent transport from a theoretical point of view through first order closure techniques, but forest measurements or fluid dynamic simulations to assess their reliability are still lacking. The opportunity to move toward multilayer models has been highlighted by some recent publications (Bonan, 2021), pushed also by the fact that the computational effort that these models require is no longer a major limitation.

Whichever type of model is used, the phytotoxic dose is calculated on the amount of O3 absorbed by the stomata. The latter is determined by the stomatal conductance which, in simplest models, is predicted by a Jarvisian scheme that does not include photosynthesis (Jarvis, 1976). However, there is a close relationship between stomatal conductance and photosynthetic assimilation and some empirical models of conductance, such as the Ball-Berry model (Ball et al. 1987; Collatz et al. 1991), take this dependence into account. This mechanistic representation of the stomatal behavior requires CO2 demand, CO2 supply, and water loss to be matched. The CO2 demand is modeled with the Farquhar scheme for gross photosynthesis (Farquhar et al., 1980); the CO2 supply is modeled with the Fick’s law of diffusion through the stomata, and the water loss is modeled with the equations for evapotranspiration starting from the leaf temperature, which in turn results from the resolution of the energy balance at leaf level.

Finally, in the background, there is the unresolved question of the difficulty in identifying direct signals of O3 damage to photosynthesis detectable through Net Ecosystem Exchange measurements at the ecosystem level. It would be interesting to investigate and test methodologies to identify the reduction of net CO2 assimilation directly from gas exchange measurements.

The PhD project aims at characterizing the sources and sinks of O3, NO and NO2 (and isoprene, if possible), and the possible influence of their dynamics on the CO2 fluxes in one or more lowland deciduous forests of the ICOS infrastructure network (Integrated Carbon Observation System, https://www.icos-cp.eu), by means of multi-level concentration measurements above and within the canopy, eddy covariance measurements of turbulent fluxes above and within the canopy, and soil chamber measurements of gas exchange from the forest floor. Measurements will be used to develop single layer and multilayer deposition models that include a description of stomatal and non-stomatal absorption processes and chemical transformation processes as more mechanistic as possible. Application of the models to other forests of the ICOS network and comparisons of the results in latitudinal/climatic gradients are desirable. The measurements will be carried out primarily at the ICOS IT-BFt ecosystem station which is specifically equipped for long-term flux measurements and concentration profiles. The candidate may address and deepen one or more of the open aspects highlighted above.

Ball, J. T., Woodrow, I. E., & Berry, J. A. (1987). A model predicting stomatal conductance and its contribution to the control of photosynthesis under different environmental conditions. In Progress in photosynthesis research: volume 4 proceedings of the VIIth international congress on photosynthesis providence, Rhode Island, USA, august 10–15, 1986 (pp. 221-224). Springer Netherlands
Bonan, G. B., Patton, E. G., Finnigan, J. J., Baldocchi, D. D., & Harman, I. N. (2021). Moving beyond the incorrect but useful paradigm: reevaluating big-leaf and multilayer plant canopies to model biosphere-atmosphere fluxes–a review. Agricultural and Forest Meteorology, 306, 108435
Collatz, G. J., Ball, J. T., Grivet, C., & Berry, J. A. (1991). Physiological and environmental regulation of stomatal conductance, photosynthesis and transpiration: a model that includes a laminar boundary layer. Agricultural and Forest meteorology, 54(2-4), 107-136
Emberson, L. D., Ashmore, M. R., Cambridge, H. M., Simpson, D., & Tuovinen, J. P. (2000). Modelling stomatal ozone flux across Europe. Environmental Pollution, 109(3), 403-413 Farquhar, G. D., von Caemmerer, S. V., & Berry, J. A. (1980). A biochemical model of photosynthetic CO 2 assimilation in leaves of C 3 species. planta, 149, 78-90
Jarvis, P. G. (1976). The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field. Philosophical Transactions of the Royal Society of London. B, Biological Sciences, 273(927), 593-610
Van Pul, W. A. J., & Jacobs, A. F. G. (1994). The conductance of a maize crop and the underlying soil to ozone under various environmental conditions. Boundary-Layer Meteorology, 69, 83-99

Supervisors

Dr. Riccardo Marzuoli, UCSC, Italy, riccardo.marzuoli@unicatt.it
Prof. Bart Muys, KU Leuven, Belgium bart.muys@kuleuven.be
Dr. Angelo Finco, UCSC, Italy, angelo.finco@unicatt.it

Transition metal oxides photoelectrodes for hydrogen production: the challenge of nanogranular materials

Student: Michele Vergari

Background and motivation

Devices converting renewable energies to fuels such as H₂ may be based on photoelectrochemical water splitting: electron/hole pairs generated at two photoelectrodes (PE) drive the half-reactions producing H₂ and O₂. However, scant efficiency, photocorrosion and instability affects the state of the art PE made of ternary metal oxides (TMOs) like CuFe₂O₄. Such limitations are ascribed to the low charge transfer induced by the small polarons due to the TMO hybrid valence band orbitals, and to the high recombination rate of charge carriers at the TMO surface and bulk states. Moreover, current PE lacks a comprehensive investigation of different TMO phases, stoichiometries and transport properties for sizes below 50 nm. The project strategy is to overcome the current limits by: 1) reducing the TMO sizes by producing PE of ZnFe₂O₄ and BiFeO₃ with a nanogranular morphology (NG-TMO) at scales below 50 nm by supersonic cluster beam deposition (SCBD); 2) determining the PE morphological, optical and electrochemical behavior for two different NG-TMO compounds; 3) determining the PE transport behavior from the reaction kinetic constants (k_t for the hopping process and k_r for recombination process), as a function of TMO selected stoichiometries, phases and sizes.

The expected project breakthroughs are: 1) a new class of nanostructured PE for electrochemistry, NG-TMOs; 2) morphological, optical and stoichiometric properties correlation with PE thickness and annealing temperature; 3) Electrochemical properties correlation with the PE thickness and annealing temperature; 4) charge transport correlation with morphology, optical response, stoichiometry; 5) reveal the role of small polarons and surface recombination in NG-TMOs at scales below 50 nm.

The student will be tutored by three experienced tutors at the Università Cattolica (UCSC) for the PE synthesis and physical properties characterization, at the university of Padova (UPD) and university of Notre Dame (ND) for the PE electrochemical characterizations.

Supervisors

Prof. Luca Gavioli, UCSC, Italy, luca.gavioli@unicatt.it
Prof. Prashant Kamat, Notre Dame, US pkamat@nd.edu
Prof. Gian Andrea Rizzi, Università di Padova, Italy, gianandrea.rizzi@unipd.it