Critical point revisited

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Here's a short summary of my present research activities

Femtoscopy PDF Print E-mail
Ultrafast Transformations in Physics, Chemistry and Biology

 Physical, chemical and biological processes often appear complex because we look at them on an extended timescale, during which many steps in the process are integrated. Despite the rich history of chemistry over two millennia, the real time observation of actual atomic motions involved in chemical reactions is very recent. Chemical bonds form, break, and evolve with awesome rapidity. Whether in isolation or in any other phase, this ultrafast transformation is a dynamic process involving the mechanical motion of electrons and atomic nuclei. The speed of atomic motion is of the order of 1 km/s and, hence, the average time required to record atomic-scale dynamics over a distance of 1 Å is in the range of 100 femtoseconds (fs). The very act of such atomic motions, the way reactions unfold and pass through their transition states, is the focus of the field of femtochemistry.

Funded by an  ERC-Starting grant (Femtoscopy), we build up a new laboratory for ultrafast spectroscopy from scratch. This is a highly interdisciplinary; research line, toiling over challenging problems in which the traditional distinction between biology, chemistry and physics is smeared out by the common ultra short timescale. Our realization of a non conventional pump probe setup for vibrational spectroscopy in the range 320-520 nm, is reported in Optics Express. Based on Stimulated Raman Scattering with broadband detection we reach the femtosecond timescale with energy resolutions which would pertain to the picosecond time domain in the Heisenberg sense. See our Chem Phys Chem paper on the ultimate resolution limit of this approach. Among our recent achievements, the experimental measurement of the ultrafast photo-induced dynamics of the exchange energy in a Heisenberg antiferromagnet, reported in Nature Photonics and the sub-picosecond energy flow in a biomolecule, appeared in Nature Chemistry .

Last Updated ( Tuesday, 04 October 2016 )
How slow is the glass flow? PDF Print E-mail
Slow and fast degrees of freedom in glass formers

Glasses and the glass transition stand, in the much quoted estimate of a Nobel laureate, as “perhaps the deepest and most interesting unsolved problem in condensed matter physics”. One of the most provocative aspects, concerns the slowing down of the dynamics on decreasing the temperature of the melt. When a liquid is cooled below its melting temperature, it usually crystallizes. However, if the quenching rate is fast enough, the system may remain in a disordered state, progressively losing its fluidity upon further cooling. When the time needed for the rearrangement of the local atomic structure reaches approximately 100 seconds, the system becomes “solid” for any practical purpose, and this defines the glass transition temperature Tg. Approaching this transition from the liquid side, different systems show qualitatively different temperature dependencies of the viscosity, and accordingly they have been classified by introducing the
concept of “fragility.”  In a highly cited paper appeared in SCIENCE magazine, we reported experimental observations that relate the microscopic properties of the glassy phase to the fragility. Based on that, we extend the fragility concept to the glassy state and indicate how to determine the fragility uniquely from glass properties well below Tg. More recently, by determining glass fragility for systems with different fictive temperature, we answer a question of of pivotal importance for glass formation theories: “Does the glass cease to flow at some finite temperature?” Our results, reported on PNAS, ultimately rules out the finite-temperature divergence of the molecular diffusion timescale in a glass.

Last Updated ( Thursday, 29 October 2015 )
Infra Red Photon Correlation Spectroscopy PDF Print E-mail
Slow Dynamics
Dynamic Light Scattering (DLS) is a well established, robust technique, for the investigation of dynamics in colloidal systems, polymers and glass forming materials. Traditionally, visible light from a laser source has been utilized as probe, with some limitations due to absorption in non transparent samples and multiple scattering in dense suspensions. More recently, the use of highly collimated X-ray beams become possible (XPCS), extending this technique at higher values of momentum transfer. We developed a further extension to perform dynamic light scattering with infrared radiation (λ = 1064 nm), opening the possibility to study non transparent systems. Applications range from the study of slow dynamics in polymeric systems to the structural relaxation in calchogenide glasses. With our first application of IRPCS, published in Physical Review Letters, we shed light on the lambda transition in liquid Sulfur, an unusual abrupt increase of viscosity upon heating wich we find to be connected to a relaxation on the millisecond timescale.
Last Updated ( Thursday, 27 August 2015 )
High Frequency Relaxations in Liquid Metals PDF Print E-mail
Dynamics in simple Liquids

Liquid metals are an outstanding example of systems combining great relevance in both industrial applications and basic science. On the one hand, they find broad technological application ranging from the production of industrial coatings (walls of refinery coker, drill pipe for oil search) to medical equipments (recostructive devices, surgical blades) or high performance sporting goods. We recently discussed the dynamical properties of liquid metals in the THz frequency region in a review paper appeared in The Reviews of Modern Physics .


Last Updated ( Thursday, 06 January 2011 )
Dynamics and Thermodynamics PDF Print E-mail
High Pressure - Systems of geophysical interest

According to conventional wisdom, a supercritical fluid is one that doesn't exhibit distinct liquid- or gas-like states. This may need to be revised in light of measurements that show a sharp change in the speed of sound in supercritical argon when it crosses a well-defined line on its pressure versus temperature phase diagram. Our results have been recently reported in two papers appeared on Physical Review Letters and Nature Physics , in which we investigate how the dynamics of a dense liquid evolves in the supercritical phase. Image

Last Updated ( Wednesday, 05 January 2011 )
Vibrational dynamics in amorphous materials PDF Print E-mail
Disordered induced properties in solids

In contrast to crystalline solids, where structural order governs dynamics and thermodynamics, the lack of long-range periodicity in amorphous materials is responsible for several unexpected properties. We combine experimental (inelastic scattering of light, neutrons and X-rays), computational (ab-initio and classical molecular dynamics) and theorethical (field techniques) methods to study anomalies in the dynamics such as non-linear dispersion relation of acoustic excitation, non-quadratic frequency dependence of sound attenuation, non-Debye vibrational density of states. Recent theorethical predictions have been reported in Physical Review Letters , while an experimental determination of the vibrational density of states at the surface of an amorphous material is reported in Nature Communications .

Last Updated ( Thursday, 29 October 2015 )
Broadband Picosecond Acoustics PDF Print E-mail
Visualizing coherent phonons in the time domain
Time domain techniques are a valuabe alternative to Inelastic Scattering techniques to study phonon dynamics, especially when energy resolution is an issue. Building on a 1 kHz amplified Ti:sapphire laser source, we developed a novel pump-probe setup for broadband picosecond acoustics using a white-light continuum probe coupled to an optical multichannel analyzer. The system allows one to access, in a single measurement, acoustic parameters such as sound velocity and attenuation all over the bandwidth of the acoustic wave-packet launched by the pump pulse (40-400 GHz). With this setup we extend the kinematic range of inelastic scattering techniques in the unexplored gap between Brillouin visible/UV and Inelastic X-ray Scattering. See all the details in our recent manuscript appeared in Applied Physics Letters , which also populated the cover of the issue.

Last Updated ( Thursday, 06 January 2011 )
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