arXiv subjects

Grazia Raciti

Publications and source records attributed to Grazia Raciti.

3 recordsLinked to original sources

arXiv2026

Isolating the natural edges of bilayer graphene in gate-defined mesoscopic devices

We introduce a graphite-gated architecture for bilayer graphene devices in which the active device is completely isolated from the natural graphene edges. Using a single patterned graphite-gate layer, we realize a fully electrostatically defined Hall-bar. Longitudinal and Hall measurements reveal mesoscopic transport features, including Hall-effect quenching and magnetoresistance peaks associated with boundary scattering. The dependence of the mesoscopic features on the carrierdensity shows that the effective channel width increases with the Fermi level and the electrostatic confinement at the gate-defined boundaries, and indicates that the carriers scatter at the electrostatic boundary. Raman spectroscopy and Kelvin probe force microscopy suggest that this boundary is disordered due to the used fabrication methods. Comparably, the quantum mobility in a fieldeffect transistor fabricated with the same architecture is not limited by boundary scattering and the visibility of quantum oscillations down to 4 mT suggests a record value of 2.5 x 10^6 cm2/Vs.

cond-mat.mes-hall↗

arXiv2025

Unraveling energy flow mechanisms in semiconductors by ultrafast spectroscopy: Germanium as a case study

Semiconductor materials are the foundation of modern electronics, and their functionality is dictated by the interactions between fundamental excitations occurring on (sub-)picosecond timescales. Using time-resolved Raman spectroscopy and transient reflectivity measurements, we shed light on the ultrafast dynamics in germanium. We observe an increase in the optical phonon temperature in the first few picoseconds, driven by the energy transfer from photoexcited holes, and the subsequent decay into acoustic phonons through anharmonic coupling. Moreover, the temperature, Raman frequency, and linewidth of this phonon mode show strikingly different decay dynamics. This difference was ascribed to the local thermal strain generated by the ultrafast excitation. We also observe Brillouin oscillations, given by a strain pulse traveling through germanium, whose damping is correlated to the optical phonon mode. These findings, supported by density functional theory and molecular dynamics simulations, provide a better understanding of the energy dissipation mechanisms in semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

In-plane thermal diffusivity determination using beam-offset frequency-domain thermoreflectance with a one-dimensional optical heat source

We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experiments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically < 100 kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a linear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer.

cond-mat.mtrl-sci↗