Coupling of a high-energy excitation to superconducting quasiparticles in a cuprate from Coherent Charge Fluctuation Spectroscopy

Dynamical information on spin degrees of freedom of proteins or solids can be obtained by Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR). A technique with similar versatility for charge degrees of freedom and their ultrafast correlations could move forward the understanding of systems like unconventional superconductors. By perturbing the superconducting state in a high-Tc cuprate using a femtosecond laser pulse, we generate coherent oscillations of the Cooper pair condensate which can be described by an NMR/ESR formalism. The oscillations are detected by transient broad-band reflectivity and found to resonate at the typical scale of Mott physics (2.6 eV), suggesting the existence of a non-retarded contribution to the pairing interaction, as in unconventional (non Migdal-Eliashberg) theories.Comment: Accepted for publication in the Proceedings of the National Academy of Sciences of the U.S.A. (PNAS

Charge generation in organic solar cell materials studied by terahertz spectroscopy

We have investigated the photophysics in neat films of conjugated polymer PBDTTPD and its blend with PCBM using terahertz time-domain spectroscopy. This material has very high efficiency when used in organic solar cells. We were able to identify a THz signature for bound excitons in neat PBDTTPD films, pointing to important delocalization in those excitons. Then, we investigated the nature and local mobility (orders of magnitude higher than bulk mobility) of charges in the PBDTTPPD:PCBM blend as a function of excitation wavelength, fluence and pump-probe time delay. At low pump fluence (no bimolecular recombination phenomena), we were able to observe prompt and delayed charge generation components, the latter originating from excitons created in neat polymer domains which, thanks to delocalization, could reach the PCBM interface and dissociate to charges on a time scale of 1 ps. The nature of the photogenerated charges did not change between 0.5 ps and 800 ps after photo-excitation, which indicated that the excitons split directly into relatively free charges on an ultrafast time scale

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site. The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization

The fate of electron–hole pairs in polymer:fullerene blends for organic photovoltaics

There has been long-standing debate on how free charges are generated in donor:acceptor blends that are used in organic solar cells, and which are generally comprised of a complex phase morphology, where intermixed and neat phases of the donor and acceptor material co-exist. Here we resolve this question, basing our conclusions on Stark effect spectroscopy data obtained in the absence and presence of externally applied electric fields. Reconciling opposing views found in literature, we unambiguously demonstrate that the fate of photogenerated electron–hole pairs—whether they will dissociate to free charges or geminately recombine—is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower. Our insights are important to further develop rational approaches towards material design and processing of organic solar cells, assisting to realize their purported promise as lead-free, third-generation energy technology that can reach efficiencies over 10%

Assembly of β-Cu2V2O7/WO3 heterostructured nanocomposites and the impact of their composition on structure and photoelectrochemical properties

Multinary metal oxides and their heterostructures play a key role as light absorbers in the production of solar chemicals. Synthetic tunability is crucial to understand the impact of composition and structure on the photoelectrochemical performance. Here, we assemble \u3b2-Cu2V2O7/WO3 heterostructured nanocomposites using a novel seeded-growth approach which allows an unprecedented compositional tunability. A 10 fold increase in the net photocurrent density towards sulfite oxidation was measured for the nanocomposite with the lowest loading of WO3 (\u3b2-Cu2V2O7\u2006:\u2006WO3 = 1\u2006:\u20060.1) as compared to the bare \u3b2-Cu2V2O7 counterpart. This improvement is attributed to the formation of an intimate junction between the two metal oxides which favors charge transfer and separation. An increase in the WO3 content results in the formation of macroscopic phase segregated domains which reduce these interfacial areas, thus degrading the phototoelectrochemical performance of the nanocomposites. While highlighting the effectiveness of heterostructuring and the importance of compositional tunability, this study points at the emerging need of techniques to control and to probe the intrinsic inhomogeneity of these complex inorganic heterojunctions

A Longitudinal Approach to Appropriation of Science Ideas: A Study of Students’ Trajectories in Thermodynamics

This article provides an empirical analysis of data collected during the implementation of a teaching proposal on thermodynamics in a class of 20 students (17 year olds) of a scientifically-oriented secondary school in Italy. During the activities, each student made evident progress in gaining intellectual autonomy as they took part in the teaching/learning dynamics of the classroom. Although the research had, up to now, an empirical orientation, this paper aims to provide a contribution for advancing theory development in physics education research. The study gives an example of the application of a specific model of teaching/learning (the Model of Longitudinal Development) that acted as “framework for action” in the design of the learning environment as “properly complex territory.” The data analysis gives indications for how to develop a theoretical understanding of the concept of “personal learning trajectory” and it provides a basis for exploring the factors that can trigger intellectual autonomy