PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25 T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Thus, our Letter reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.

The recently demonstrated chiral modes of lattice motion carry angular momentum and therefore directly couple to magnetic fields. Notably, their magnetic moments are predicted to be strongly influenced by electronic contributions. Here, we have studied the magnetic response of transverse optical phonons in a set of Pb _{1− x} Sn _x Te films, which is a topological crystalline insulator for x > 0.32 and has a ferroelectric transition at an x -dependent critical temperature. Polarization-dependent terahertz magnetospectroscopy measurements revealed Zeeman splittings and diamagnetic shifts, demonstrating a large phonon magnetic moment. Films in the topological phase exhibited phonon magnetic moment values that were larger than those in the topologically trivial samples by two orders of magnitude. Furthermore, the sign of the effective phonon g -factor was opposite in the two phases, a signature of the topological transition according to our model. These results strongly indicate the existence of interplay between the magnetic properties of chiral phonons and the topology of the electronic band structure.

We investigate the residual stress in diamond films grown on (001) silicon substrates as a function of film thickness. The diamond films were deposited at 1070 K by the conventional hot filament technique using a gas mixture of methane (1.0% vol) and hydrogen (99.0% vol). The film thickness, obtained from cross section scanning electron micrographs, varied from 3.0 to 42 μm as the growth time increased from 1 to 10 h. These images evidenced that the columnar growth is already established for films thicker than 10 μm. Top view micrographs revealed predominantly faceted pyramidal grains for the films at all growth stages. The grain size, obtained from these images, was found to vary linearly with film thickness. Using a high resolution x-ray diffractometer, the residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ values ranging from $\text{−60°}$ to $\text{+60°,}$ and applying the ${\sin }^2\mathrm{ \psi }$ method. For the micro-Raman spectroscopy, we used the summation method, which consists in recording and adding a large number of spectra in different places of a selected area of the sample. All Raman spectra were fitted with Lorentzian lines to separate the contribution of the pure diamond and the other nondiamond (graphite) phases. This spectral analysis performed in each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which increases from 70% to 90% as the thickness goes from 3 to 42 μm. The type and magnitude of the residual stress obtained from x-ray and micro-Raman measurements agreed well for films thicker than 10 μm. For films thinner than this value, an opposite behavior between both results was observed. We attributed this discrepancy to the domain size characteristic of each technique.

X-ray diffraction in thin layers containing small randomly placed defects is described by means of the kinematical diffraction theory and optical coherence formalism. The method enables us to calculate both the diffracted intensity and its angular distribution, so that it can be used for simulating double crystal and triple crystal x-ray diffractometry experiments. The theory has been applied to experimental data obtained from diffractometry measurements of an epitaxial ZnTe layer with mosaic structure after several steps of chemical thinning. A good agreement of the theory with experiments has been achieved.

The values of optical deformation potentials ($D_u$ and $D_d$) for PbSe and PbTe are analyzed using absorption, luminescence and x-ray diffraction data on high quality, deep quantum well $\mathrm{Pb}\mathrm{Se}∕\mathrm{Pb}\mathrm{Sr}\mathrm{Se}$ and $\mathrm{Pb}\mathrm{Te}∕\mathrm{Pb}\mathrm{Eu}\mathrm{Te}$ structures. Published optical data on $\mathrm{Pb}\mathrm{Te}∕\mathrm{Ba}{\mathrm{F}}_2$ structures were also used. It is important for these evaluations to use the value of isotropic deformation potential $\left(D_{\mathrm{iso}}\right)$ which is determined from the low temperature hydrostatic pressure experiments and the intervalley splitting which was determined from differential absorption spectroscopy. The fitting procedure was done using an accurate $\mathbf{k}\bullet \mathbf{p}$ model for the band structure near the fundamental gap including anisotropy, multivalley, and band nonparabolicity effects. Photoluminescence was measured in pulsed mode at 4 and $77\mathrm{K}$, which gives the lowest energy state in the rectangular well. The strain and well parameters were measured with a high resolution x-ray diffractometer. We find the new optical deformation potential values at low temperatures: $D_u=-0.2$ and $-0.5\mathrm{eV}$, and $D_d=5.3$ and $3.5\mathrm{eV}$ for PbSe and PbTe, respectively.