arXiv subjects

P. Franchetti

Publications and source records attributed to P. Franchetti.

3 recordsLinked to original sources

arXiv2026

Cubic-Zn(1-x)Mn(x)Te optical modes across the phonon and phonon-polariton regimes

Alloying offers a convenient route to tune the physical properties of semiconductors, at the price, however, of local lattice/bond distortions needed to accommodate the lattice mismatch. Being nearly lattice-matched and moreover highly resistant to lattice and bond distortions thanks to a largely covalent bonding, cubic Zn(1-x)Mn(x)Te sets a benchmark to test the sensitivity of lattice vibrations to minimum lattice and bond distortions induced by alloying. We explore experimentally the two transverse optical regimes of cubic Zn(1-x)Mn(x)Te (x<0.7), with ab initio calculations in support: (1) the phonon-polariton regime at quasi-infinite (photon-like) wavelengths averaging over the alloy disorder, probed by forward-Raman scattering, and (2) the purely-mechanical phonon regime down to minimum wavelengths nearly matching the lattice constant zooming into the alloy disorder, achieved by inelastic neutron scattering. The short Zn-Te bond is found to exhibit a compact optical duo distinguishing homo from hetero local bond environments (1 bond: 2 mode), as explained by the percolation model. This deviates from the historical Zn(1-x)Mn(x)Te MREI (modified-random-element-isodisplacement) scheme inspired from classical Raman and infrared studies, in which the optical modes are blind to the bond environment (1-bond: 1-mode). So, the test is positive, reinforcing the status of the percolation scheme as a robust decryptor of (even small) lattice distortions induced by alloying through the lattice dynamics.

cond-mat.mtrl-sci↗

arXiv2025

Phonon-polaritons in Zn(1-x)MgxTe (x<0.09): A Raman scattering study

Phonon-polaritons (PP) are phonon-photon coupled modes. Using near-forward Raman scattering, the PP of the cubic Zn(1-x)MgxTe (x<0.09) semiconductor alloy could be measured. While the PP-coupling hardly develops in pure ZnTe, minor Mg-alloying suffices to stabilize a long-lifetime PP strongly bound to the lattice, i.e., with a pronounced phonon character, and yet a fast one originating from the highly dispersive photon-like bottleneck of the PP-dispersion. By combining the advantages of a phonon and of a photon, the long-lifetime PP generated by minor Mg-alloying of ZnTe marks an improvement over the PP of pristine ZnTe, that, from the Raman cross section calculation, can only achieve a balanced compromise between the two kinds of advantages, intensity and speed. The discussion of the PP-related lattice dynamics of Zn(1-x)MgxTe (x<0.09) is grounded in a preliminary study of the lattice macro- and microstructure using X-ray diffraction and solid-state nuclear magnetic resonance, respectively, and further relies on ab initio calculations of the native phonon modes behind the PP in the Mg-dilute limit of Zn(1-x)MgxTe (x~0), considering various Mg-isotopes.

cond-mat.mtrl-sci↗

arXiv2023

Vibrational and mechanical properties of the highly mismatched (Cd,Be)Te semiconductor alloy : Experiment and ab initio calculations

The (Cd, Be)Te semiconductor alloy that exhibits a dramatic mismatch in bond covalency and stiffness clarifying its vibrational and mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted (Zn, Be) chalcogenides. The test is done by way of experiment (x smaller than 0.11) combining Raman scattering with X ray diffraction at high pressure and ab initio calculations (x around 0, 0.5 and 1). The (macroscopic) bulk modulus B drops below the CdTe value on minor Be incorporation, at variance with a linear B versus x increase predicted ab initio, thus hinting at large anharmonic effects in the real crystal. Yet, no anomaly occurs at the microscopic (bond) scale as the regular bimodal PM Raman signal predicted ab initio for the BeTe bond in minority (x around 0 and 0.5) is (barely) detected experimentally. Although at large Be content (x around 1) the same bimodal signal relaxes down to inversion, an unprecedented case, specific pressure dependencies of the regular (x around 0 and 0.5) and inverted (x around 1) BeTe Raman doublets are in line with PM predictions. Hence, the PM applies as such to (Cd, Be)Te albeit in a relaxed form, without further refinement. This enhances the scheme validity as a generic descriptor of phonons in alloys.

cond-mat.mtrl-sci↗