arXiv subjects

Paolo Postorino

Publications and source records attributed to Paolo Postorino.

At least 19 recordsLinked to original sources

arXiv2026

Metallic crossover through the tilt-free transition in La$_3$Ni$_2$O$_7$ at high pressure and temperature

La$_3$Ni$_2$O$_7$, a bilayer nickelate with Ruddlesden-Popper structure, undergoes a pressure-induced structural transition from a tilted Amam phase to an untilted Fmmm (or I4/mmm) phase near 10-15 GPa, concomitant with the emergence of high-T$_c$ superconductivity ($T_c \sim 80$ K). Despite intense interest, the phase boundaries and the impact of structural changes on the electronic properties remain unclear. Here, we combine high-pressure and high-temperature Raman and synchrotron-based infrared spectroscopies to map the structural and electronic evolutions. Raman measurements confirm the pressure-driven structural transition and reveal the emergence of Fano line shapes, indicating enhanced electron-phonon coupling. High-temperature data show analogous spectral signatures above 544 K, suggesting an upper temperature limit of the Amam phase within the T-P phase diagram of this system. Infrared reflectivity measurements evidence a concomitant enhanced metallicity, with a tremendous two-order-of-magnitude increase in carrier density, marking a crossover from a weakly to highly metallic state. These results establish a unified picture of the structural transition and its strong coupling to the electronic properties.

cond-mat.supr-con↗

arXiv2026

Resonantly-enhanced Raman response in graphene-capped bismuthene on SiC

Two-dimensional quantum spin Hall insulators based on atomic monolayers offer a promising route toward dissipationless electronics, yet their practical use is often limited by environmental instability. Encapsulating the system with a graphene capping layer has been shown to be a reliable method to prevent oxidation and degradation. However, the confirmation of a successful encapsulation still relies on ultra-high vacuum techniques, that considerably slow the process. Here, we present an ex situ, rapid, nondestructive and spatially resolved Raman characterization of graphene-capped bismuthene, a honeycomb monolayer of Bi on SiC. A pronounced Raman scattering peak at around 122 cm-1 is identified as the E2g phonon of bismuthene, via a comparison with density functional perturbation theory calculations. We use excitation-energy and polarization-dependent Raman measurements to enable an unambiguous assignment of the spectral features. Tuning the excitation energy close to the excitonic transition in pristine bismuthene, we observe a strong enhancement of the Raman response and the emergence of additional scattering peaks. In this regime, higher-order phonon features, as well as interfacial modes between bismuthene and the SiC substrate, become visible, suggesting the involvement of resonant scattering processes. Our results establish Raman micro-spectroscopy as a versatile tool for probing graphene-protected quantum materials, providing access to lattice dynamics and interlayer coupling.

cond-mat.mtrl-sci↗

arXiv2025

Probing enhanced electron-phonon coupling in graphene by infrared resonance Raman spectroscopy

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at $\mathbf{K}$, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D$^\prime$ peaks with respect to that measured in graphite. Comparing with fully \textit{ab initio} theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

cond-mat.mes-hall↗

arXiv2024

Fine-Tuning of the Excitonic Response in Monolayer WS2 Domes via Coupled Pressure and Strain Variation

We present a spectroscopic investigation into the vibrational and optoelectronic properties of WS2 domes in the 0-0.65 GPa range. The pressure evolution of the system morphology, deduced by the combined analysis of Raman and photoluminescence spectra, revealed a significant variation in the dome's aspect ratio. The modification of the dome shape caused major changes in the mechanical properties of the system resulting in a sizable increase of the out-of-plane compressive strain while keeping the in-plane tensile strain unchanged. The variation of the strain gradients drives a non-linear behavior in both the exciton energy and radiative recombination intensity, interpreted as the consequence of a hybridization mechanism between the electronic states of two distinct minima in the conduction band. Our results indicate that pressure and strain can be efficiently combined in low dimensional systems with unconventional morphology to obtain modulations of the electronic band structure not achievable in planar crystals.

cond-mat.mtrl-sci↗

arXiv2024

Infrared resonance Raman of bilayer graphene: signatures of massive fermions and band structure on the 2D peak

Few-layer graphene possesses low-energy carriers which behave as massive fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each sub-structure and, comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements.

cond-mat.mes-hall↗

arXiv2023

Experimental investigation of electronic interactions in collapsed and uncollapsed LaFe2As2 phases

The iron-based pnictide LaFe2As2 is not superconducting as-synthesized, but it becomes such below Tc around 12 K upon annealing, as a consequence of a structural transition from a phase with collapsed tetragonal crystal structure to an uncollapsed phase. In this work, we carry out specific heat, Raman spectroscopy and normal state electric and thermoelectric transport measurements in the collapsed and uncollapsed LaFe2As2 phases to gain insight into the electron interactions and their possible role in the superconducting pairing mechanism. Despite clear features of strong electron-phonon coupling observed in both phases, neither the low energy phonon spectra nor the electron-phonon coupling show significant differences between the two phases. Conversely, the Sommerfield constants are significantly different in the two phases, pointing to much higher electron correlation in the superconducting uncollapsed phase and confirming theoretical studies.

cond-mat.supr-con↗

arXiv2023

Far-Infrared Signatures for a Two-Steps Pressure-Driven Metallization in Transition Metal Dichalcogenides

We present a high-pressure investigation of the semiconductor-to-metal transition in MoS2 and WS2 carried out by synchrotron-based far-infrared spectroscopy, to reconcile the controversial estimates of the metallization pressure found in the literature and gain new insight into the mechanisms ruling this electronic transition. Two spectral descriptors are found indicative of the onset of metallicity and of the origin of the free carriers in the metallic state: the absorbance spectral weight, whose abrupt increase defines the metallization pressure threshold, and the asymmetric lineshape of the E1u peak, whose pressure evolution, interpreted within the Fano model, suggests the electrons in the metallic state originate from n-type doping levels. Combining our results with those reported in the literature, we hypothesize a two-step mechanism is at work in the metallization process, in which the pressure-induced hybridization between doping and conduction band states drives an early metallic behaviour, while the band-gap closes at higher pressures.

cond-mat.mtrl-sci↗

arXiv2022

Raman Scattering with infrared excitation resonant with MoSe$_2$ indirect band gap

Resonance Raman scattering, which probes electrons, phonons and their interplay in crystals, is extensively used in two-dimensional materials. Here we investigate Raman modes in MoSe$_2$ at different laser excitation energies from 2.33 eV down to the near infrared 1.16 eV. The Raman spectrum at 1.16 eV excitation energy shows that the intensity of high-order modes is strongly enhanced if compared to the first-order phonon modes intensity due to resonance effects with the MoSe$_2$ indirect band gap. By comparing the experimental results with the two-phonon density of states calculated with density functional theory, we show that the high-order modes originate mostly from two-phonon modes with opposite momenta. In particular, we identify the momenta of the phonon modes that couple strongly with the electrons to produce the resonance process at 1.16 eV, while we verify that at 2.33 eV the two-phonon modes lineshape compares well with the two-phonon density of state calculated over the entire Brillouin Zone. We also show that, by lowering the crystal temperature, we actively suppress the intensity of the resonant two-phonon modes and we interpret this as the result of the increase of the indirect band gap at low temperature that moves our excitation energy out of the resonance condition.

cond-mat.mtrl-sci↗

arXiv2021

Possible realization of a phononic tsunami in a wedge-shaped sample

Exploiting the theory of solitons in a nonlinear elastic medium we predict a novel phenomenon called a phononic tsunami, which is characterized by the dramatic increase of the local amplitude of phonon modes. To elucidate the possible experimental detection of this phenomenon we propose to use a wedge-shaped sample in which a sharp edge serves for the emulation of the shoaling effect and such a local enhancement can be observed. Together with eigenfrequencies of transverse and longitudinal phonon modes of a system we find the characteristic dispersion relations that can be considered as a hallmark of a phononic tsunami. We justify our predictions by means of analytical calculations and numerical simulations showing a possible realization of this nonlinear effect in such a geometry. Our results provide the framework for the implementation of new kind experiments aimed at realizing and investigating a phononic tsunami phenomenon in relevant materials.

cond-mat.mes-hall↗

arXiv2021

Broadband Infrared Study of Pressure-Tunable Fano Resonance and Metallization Transition in 2H-MoTe2

High pressure is a proven effective tool for modulating inter-layer interactions in semiconducting transition metal dichalcogenides, which leads to significant band structure changes. Here, we present an extended infrared study of the pressure-induced semiconductor-to-metal transition in 2H-MoTe2, which reveals that the metallization process at 13-15 GPa is not associated with the indirect band gap closure, occuring at 24 GPa. A coherent picture is drawn where n-type doping levels just below the conduction band minimum play a crucial role in the early metallization transition. Doping levels are also responsible for the asymmetric Fano line-shape of the E1u infrared-active mode, which has been here detected and analyzed for the first time in a Transition Metal Dichalcogenide compound. The pressure evolution of the phonon profile under pressure shows a symmetrization in the 13-15 GPa pressure range, which occurs simultaneously with the metallization and confirms the scenario proposed for the high pressure behaviour of 2H-MoTe2.

cond-mat.mtrl-sci↗

arXiv2021

Advanced Raman spectroscopy detection of oxidative damage in nucleic acid bases: probing chemical changes and intermolecular interactions in guanosine at ultralow concentration

DNA/RNA synthesis precursors are especially vulnerable to damage induced by reactive oxygen species occurring following oxidative stress. Guanosine triphosphates are the prevalent oxidized nucleosites, which can be misincorporated during replication, leading to mutations and cell death. Here, we present a novel method based on micro-Raman spectroscopy, combined with ab initio calculations, for the identification, detection, and quantification of low concentrations of oxidized nucleotides. We also show that the Raman signature in the terahertz spectral range (< 100 cm$^{-1}$) contains information on the intermolecular assembly of guanine in tetrads, which allows to further boost the oxidative damage detection limit. Eventually, we provide evidence that similar analyses can be carried out on samples in very small volumes at very low concentrations by exploiting the high sensitivity of Surface Enhanced Raman Scattering combined with properly designed superhydrophobic substrates. These results pave the way for employing such advanced spectroscopic methods for quantitatively sensing the oxidative damage of nucleotides in the cell.

physics.chem-ph↗

arXiv2021

Infrared study of the pressure-induced isostructural metallic transition in Mo$_{0:5}$W$_{0:5}$S$_{2}$

Ternary compounds of Transition Metal Dichalcogenides are emerging as an interesting class of crystals with tunable electronic properties, which make them attractive for nano-electronic and optoelectronic applications. Among them, Mo$_x$W$_{1-x}$S$_2$ is one of the most studied alloys, due to the well-known, remarkable features of its binary constituents, MoS$_2$ and WS$_2$. The band-gap of this compound can be modelled varying Mo and W percentages in the sample, and its vibrational modes result from a combination of MoS$_2$ and WS$_2$ phonons. In this work, we report transmission measurements on a Mo$_{0:5}$W$_{0:5}$S$_2$ single crystal in the far-infrared range. Absorbance spectra collected at ambient conditions enabled, for the first time, a classification of the infrared-active phonons, complementary to Raman studies. High-pressure measurements allowed to study the evolution of both the lattice dynamics and the free carrier density up to 31 GPa, indicating the occurrence of an isostructural semiconductor-to-metal transition above 18 GPa.

cond-mat.mtrl-sci↗

arXiv2019

Biophysical characterization of membrane phase transition profiles for the discrimination of Outer Membrane Vesicles (OMVs) from Escherichia coli grown at different temperatures

Dynamic Light Scattering (DLS), Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM) are physical techniques widely employed to characterize the morphology and the structure of vesicles such as liposomes or human extracellular vesicles (exosomes). Bacterial extracellular vesicles are similar in size to human exosomes, although their function and membrane properties have not been elucidated in such detail as in the case of exosomes. Here, we applied the above cited techniques, in synergy with the thermodynamic characterization of the vesicles lipid membrane using a turbidimetric technique to the study of vesicles produced by Gram-negative bacteria (Outer Membrane Vesicles, OMVs) grown at different temperatures. This study demonstrated that our combined approach is useful to discriminate vesicles of different origin or coming from bacteria cultured under different experimental conditions. We envisage that in a near future the techniques employed in our work will be further implemented to discriminate complex mixtures of bacterial vesicles, thus showing great promises for biomedical or diagnostic applications.

physics.bio-ph↗

arXiv2019

Molecular mechanisms driving the microgels behaviour: a Raman spectroscopy and Dynamic Light Scattering study

Responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) exhibit peculiar behaviours due to the competition between the hydrophilic and hydrophobic interactions of the constituent networks. The interpenetration of poly-acrilic acid (PAAc), a pH-sensitive polymer, within the PNIPAM network, to form Interpenetrated Polymer Network (IPN) microgels, affects this delicate balance and the typical Volume-Phase Transition (VPT) leading to complex behaviours whose molecular nature is still completely unexplored. Here we investigate the molecular mechanism driving the VPT and its influence on particle aggregation for PNIPAM/PAAc IPN microgels by the joint use of Dynamic Light Scattering and Raman Spectroscopy. Our results highlight that PNIPAM hydrophobicity is enhanced by the interpenetration of PAAc promoting interparticle interactions, a crossover concentration is found above which aggregation phenomena become relevant. Moreover we find that, at variance with PNIPAM, for IPN microgels a double-step molecular mechanisms occurs upon crossing the VPT, the first involving the coil-to-globule transition typical of PNIPAM and the latter associated to PAAc steric hindrance.

cond-mat.soft↗

arXiv2016

High-pressure behavior of methylammonium lead iodide (MAPbI3) hybrid perovskite

In this paper we provide an accurate high-pressure structural and optical study of MAPbI3 hybrid perovskite. Structural data show the presence of a phase transition towards an orthorhombic structure around 0.3 GPa followed by full amorphization of the system above 3 GPa. After releasing pressure the systems keeps the high-pressure orthorhombic phase. The occurrence of these structural transitions is further confirmed by pressure induced variations of the photoluminescence signal at high pressure. These variations clearly indicate that the bandgap value and the electronic structure of MAPI change across the phase transition.

cond-mat.mtrl-sci↗

arXiv2014

Interaction of a Long Alkyl Chain Protic Ionic Liquid and Water

A combined experimental/theoretical approach has been used to investigate the role of water in modifying the microscopic interactions characterizing the optical response of butyl-ammonium nitrate (BAN) water solutions. Raman spectra, dominated by the signal from the protic ionic liquid, were collected as a function of the water content, and the corresponding spatial organization of the ionic couples, as well as their local arrangement with water molecules, was studied exploiting classical molecular dynamics calculations. High quality spectroscopic data, combined with a careful analysis, revealed that water affects the vibrational spectrum BAN in solution: as the water concentration is increased, peaks assigned to stretching modes show a frequency hardening together with a shape narrowing, whereas the opposite behavior is observed for peaks assigned to bending modes. Calculation results clearly show a nanometric spatial organization of the ionic couples that is not destroyed on increasing the water content at least within an intermediate range. Our combined results show indeed that small water concentrations even increase the local order. Water molecules are located among ionic couples and are closer to the anion than the cation, as confirmed by the computation of the number of H-bonds which is greater for water-anion than for water-cation. The whole results set thus clarifies the microscopic scenario of the BAN-water interaction and underlines the main role of the extended hydrogen bond network among water molecules and nitrate anions.

physics.chem-ph↗

arXiv2012

Anisotropic compression in the high pressure regime of pure and Cr-doped vanadium dioxide

We present structural studies of V$_{1-x}$Cr$_x$O$_2$ (pure, 0.7% and 2.5% Cr doped) compounds at room temperature in a diamond anvil cell for pressures up to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied show a persistence of the monoclinic $M_1$ symmetry between 4 and 12 GPa. Above 12 GPa, the monoclinic $M_1$ symmetry changes to isostructural $M_x$ phase (space group $P2_1/c$) with a significant anisotropy in lattice compression of the $b$-$c$ plane of the $M_{1}$ phase. This behavior can be reconciled invoking the pressure induced charge-delocalization.

cond-mat.str-el↗

arXiv2012

Role of ionic liquids in protein refolding: native/fibrillar versus treated lysozyme

Several ionic liquids (ILs) are known to revert aggregation processes and improve the in vitro refolding of denatured proteins. In this paper the capacity of a particular class of ammonium based ILs to act as refolding enhancers was tested using lysozyme as a model protein. Raman spectra of ILs treated fibrillar lysozyme as well as lysozyme in its native and fibrillar conformations were collected and carefully analyzed to characterize the refolding extent under the effect of the IL interaction. Results obtained confirm and largely extend the earlier knowledge on this class of protic ILs and indicate Ethyl Ammonium Nitrate (EAN) as the most promising additive for protein refolding. The experiment provides also the demonstration of the high potentiality of Raman spectroscopy as a comprehensive diagnostic tool in this field.

cond-mat.soft↗