arXiv subjects

Juan Sebastian Reparaz

Publications and source records attributed to Juan Sebastian Reparaz.

3 recordsLinked to original sources

arXiv2026

Microscopic Origin of Temperature-Dependent Anisotropic Heat Transport in Ultrawide-Bandgap Rutile GeO2

Ultrawide-bandgap rutile GeO2 is emerging as a promising semiconductor for power electronics, where efficient heat dissipation is essential to suppress self-heating and ensure device reliability. However, the temperature dependence and microscopic origin of its anisotropic heat transport have remained experimentally unresolved. Here, temperature-dependent time-domain thermoreflectance measurements combined with first-principles phonon transport calculations are used to quantify the thermal conductivity of single-crystal rutile GeO2 from 80 to 350 K along [001] and [110]. At 295 K, the thermal conductivity reaches 47.5 W m^-1 K^-1 along [001] and 32.5 W m^-1 K^-1 along [110], corresponding to an anisotropy ratio of 1.46, in good agreement with theory. Rather than following a simple T^(-1) law, the thermal conductivity exhibits an approximate T^(-1.4) dependence, indicating additional scattering beyond purely three-phonon-limited transport. Mode-resolved analysis reveals that the room-temperature anisotropy originates from the combined effect of larger phonon group velocities along [001] and direction-dependent phonon lifetimes. Upon cooling, depopulation of high-frequency phonons progressively suppresses their contribution to heat transport and reduces the anisotropy. The temperature-dependent thermal boundary conductance of Al/rutile GeO2 interfaces is further resolved, and the scaled conductance indicates predominantly elastic interfacial transport. These findings establish the microscopic basis of bulk and interfacial heat transport in rutile GeO2 and position this material as a promising thermally robust platform for ultrawide-bandgap electronics.

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↗

arXiv2016

Two-Dimensional Phononic Crystals: Disorder Matters

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.

cond-mat.mtrl-sci↗