New perspectives in the ultrafast spectroscopy of many-body excitations in correlated materials

Ultrafast spectroscopies constitute a fundamental tool to investigate the dynamics of non-equilibrium many-body states in correlated materials. Two-pulses (pump-probe) experiments have shed new light on the interplay between high-energy electronic excitations and the emerging low-energy properties, such as superconductivity and charge-order, in many interesting materials. Here we will review some recent results on copper oxides and we will propose the use of high-resolution multi-dimensional techniques to investigate the decoherence processes of optical excitations in these systems. This novel piece of information is expected to open a new route toward the understanding of the fundamental interactions that lead to the exotic electronic and magnetic properties of correlated materials.Comment: Invited by S.I.F. To appear in Nuovo Cimento C. 7 pages, 4 figure

Thermal boundary resistance from transient nanocalorimetry: a multiscale modeling approach

The Thermal Boundary Resistance at the interface between a nanosized Al film and an Al_{2}O_{3} substrate is investigated at an atomistic level. A room temperature value of 1.4 m^{2}K/GW is found. The thermal dynamics occurring in time-resolved thermo-reflectance experiments is then modelled via macro-physics equations upon insertion of the materials parameters obtained from atomistic simulations. Electrons and phonons non-equilibrium and spatio-temporal temperatures inhomo- geneities are found to persist up to the nanosecond time scale. These results question the validity of the commonly adopted lumped thermal capacitance model in interpreting transient nanocalorimetry experiments. The strategy adopted in the literature to extract the Thermal Boundary Resistance from transient reflectivity traces is revised at the light of the present findings. The results are of relevance beyond the specific system, the physical picture being general and readily extendable to other heterojunctions.Comment: 12 pages, 8 figure

Temperonic Crystal: a superlattice for temperature waves in graphene

The temperonic crystal, a periodic structure with a unit cell made of two slabs sustaining temperature wave-like oscillations on short time-scales, is introduced. The complex-valued dispersion relation for the temperature scalar field is investigated for the case of a localised temperature pulse. The dispersion discloses frequency gaps, tunable upon varying the slabs thermal properties. Results are shown for the paradigmatic case of a graphene-based temperonic crystal. The temperonic crystal extends the concept of superlattices to the realm of temperature waves, allowing for coherent control of ultrafast temperature pulses in the hydrodynamic regime at above liquid nitrogen temperatures.Comment: 5 pages, 3 figure

Analytical model of the acoustic response of nanogranular films adhering on a substrate

A 1D mechanical model for nanogranular films, based on a structural interface, is here presented. The analytical dispersion relation for the frequency and lifetimes of the acoustics breathing modes is obtained in terms of the interface layer thickness and porosity. The model is successfully benchmarked both against 3D Finite Element Method simulations and experimental photoacoustic data on a paradigmatic system available from the literature. A simpler 1D model, based on an homogenized interface, is also presented and its limitations and pitfalls discussed at the light of the more sophisticated pillar model. The pillar model captures the relevant physics responsible for acoustic dissipation at a disordered interface. Furthermore, the present findings furnish to the experimentalist an easy-to-adopt, benchmarked analytical tool to extract the interface layer physical parameters upon fitting of the acoustic data. The model is scale invariant and may be deployed, other than the case of granular materials, where a patched interface is involved

Temperature dependence of the thermal boundary resistivity of glass-embedded metal nanoparticles

The temperature dependence of the thermal boundary resistivity is investigated in glass-embedded Ag particles of radius 4.5 nm, in the temperature range from 300 to 70 K, using all-optical time-resolved nanocalorimetry. The present results provide a benchmark for theories aiming at explaining the thermal boundary resistivity at the interface between metal nanoparticles and their environment, a topic of great relevance when tailoring thermal energy delivery from nanoparticles as for applications in nanomedicine and thermal management at the nanoscaleComment: 4 pages, 3 figure

Tracking local magnetic dynamics via high-energy charge excitations in a relativistic Mott insulator

We use time- and energy-resolved optical spectroscopy to investigate the coupling of electron-hole excitations to the magnetic environment in the relativistic Mott insulator Na$_2$IrO$_3$. We show that, on the picosecond timescale, the photoinjected electron-hole pairs delocalize on the hexagons of the Ir lattice via the formation of quasi-molecular orbital (QMO) excitations and the exchange of energy with the short-range-ordered zig-zag magnetic background. The possibility of mapping the magnetic dynamics, which is characterized by typical frequencies in the THz range, onto high-energy (1-2 eV) charge excitations provides a new platform to investigate, and possibly control, the dynamics of magnetic interactions in correlated materials with strong spin-orbit coupling, even in the presence of complex magnetic phases.Comment: 5 pages, 4 figures, supplementary informatio

Strong enhancement of d-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon

We study the hole binding energy and pairing correlations in the three-band Hubbard model coupled to an apical oxygen phonon, by exact diagonalization and constrained-path Monte Carlo simulations. In the physically relevant charge-transfer regime, we find that the hole binding energy is strongly enhanced by the electron-phonon interaction, which is due to a novel potential-energy-driven pairing mechanism involving reduction of both electronic potential energy and phonon related energy. The enhancement of hole binding energy, in combination with a phonon-induced increase of quasiparticle weight, leads to a dramatic enhancement of the long-range part of d-wave pairing correlations. Our results indicate that the apical oxygen phonon plays a significant role in the superconductivity of high-$T_c$ cuprates.Comment: 5 pages, 5 figure

Photoacoustic Sensing of Trapped Fluids in Nanoporous Thin Films: Device Engineering and Sensing Scheme

Accessing fluid infiltration in nanogranular coatings is an outstanding challenge, of relevance for applications ranging from nanomedicine to catalysis. A sensing platform, allowing to quantify the amount of fluid infiltrated in a nanogranular ultrathin coating, with thickness in the 10 to 40 nm range, is here proposed and theoretically investigated by multiscale modelling. The scheme relies on impulsive photoacoustic excitation of hypersonic mechanical breathing modes in engineered gas-phase synthesised nanogranular metallic ultathin films and time-resolved acousto-optical read-out of the breathing modes frequency shift upon liquid infiltration. A superior sensitivity, exceeding 26x103 cm^2/g, is predicted upon equivalent areal mass loading of a few ng/mm^2. The capability of the present scheme to discriminate among different infiltration patterns is discussed. The platform is an ideal tool to investigate nano fluidics in granular materials and naturally serves as a distributed nanogetter coating, integrating fluid sensing capabilities. The proposed scheme is readily extendable to other nanoscale and mesoscale porous materials.Comment: 14 pages, 4 figure

Dataset of tree inventory and canopy structure in poplar plantations in Northern Italy

The dataset reports data collected in 38 square (50 x 50m) 0.25 ha plots representative of poplar plantations in Lombardy Region (Northern Italy), which were used to calibrate optical information derived from unmanned aerial vehicle (UAV) and satellite (Sentinel-2) sensors.In each plot, the diameter at breast height was measured using a caliper; height, stem and crown volume of each tree were then derived from diameter using allometric equations developed in an independent study. Additional canopy attributes (foliage and crown cover, crown porosity, leaf area index) were derived in each plot from 12-20 optical images collected using digital cover photography (DCP).The collected data allows characterizing the assessment of structure of these plantations, along with their variation over the rotation time. Canopy and crown data also enable the evaluation of optimal rotation and tree spacing, as well as the relationship between stand and canopy structure.The raw datasets consist of 2,591 records (trees) associated with inventory measurements and 616 records (images) associated with optical canopy measurements. An R code was also provided to calculate plot-level attributes from raw data.Dataset and associated metadata are freely available at http://dx.doi.org/10.17632/ycr7w5pvkt.1