Engineering topics
Publications and source records attributed to Romero, A. H..
BiTeCl under high-pressure conditions is characterized. The equation of state, vibrational features, and electronic band structure are discussed, as well as the non-pure van der Waals character of the interlayer bonding of the low-pressure phase.
GaGeTe is a layered van der Waals material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z 2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (α phase, space group R-3m, No. 166). Here we show that as-grown samples of GaGeTe show traces of at least another polytype with hexagonal non-centrosymmetric structure (β phase, space group P6 3 mc, No. 186). Moreover, we suggest that another bulk hexagonal polytype (γ phase, space group P-3m1, No. 164) could also be found near room conditions. Both α and β polytypes have been identified and characterized by means of X-ray diffraction and Raman scattering measurements with the support of ab initio calculations. We provide the vibrational properties of both polytypes and show that the Raman spectrum reported for GaGeTe almost forty years ago and attributed to the α phase, was, in fact, that of the secondary β phase. Additionally, we show that a Fermi resonance occurs in α-GaGeTe under non-resonant excitation conditions, but not under resonant excitation conditions. Theoretical calculations show that bulk β-GaGeTe is a non-centrosymmetric weak topological semimetal with even smaller lattice thermal conductivity than centrosymmetric bulk α-GaGeTe. In perspective, our work paves the way for the control and engineering of GaGeTe polytypes to design and implement complex van der Waals heterostructures formed by a combination of centrosymmetric and non-centrosymmetric layers of up to three different polytypes in a single material, suitable for a number of fundamental studies and technological applications.
The high pressure lattice dynamics of rhombohedral antimony have been studied by a combination of diffuse scattering and inelastic x-ray scattering. The evolution of the phonon behavior as a function of pressure was analyzed by means of two theoretical approaches: density functional perturbation theory and symmetry-based phenomenological phase transition analysis. Here, this paper focuses on the first structural phase transition, SbI-SbIV, and the role of vibrations in leading the transition. The phonon dispersion exhibits complex behavior as one approaches the structural transition, with the branches, corresponding to the two transitions happening at high pressure in the Va elements (A7-to-BCC and A7-to-PC) both showing softening.
Here, a classical interatomic potential for iron/iron-fluoride systems is developed in the framework of the charge optimized many-body (COMB) potential. This interatomic potential takes into consideration the effects of charge transfer and many-body interactions depending on the chemical environment. The potential is fitted to a training set composed of both experimental and ab initio results of the cohesive energies of several Fe and FeF 2 crystal phases, the two fluorine molecules F 2 and the F 2 –1 dissociation energy curve, the Fe and FeF 2 lattice parameters of the ground state crystalline phase, and the elastic constants of the body centered cubic Fe structure. The potential is tested in an NVT ensemble for different initial structural configurations as the crystal ground state phases, F 2 molecules, iron clusters, and iron nanospheres. In particular, we model the FeF 2 /Fe bilayer and multilayer interfaces, as well as a system of square FeF 2 nanowires immersed in an iron solid. It has been shown that there exists a reordering of the atomic positions for F and Fe atoms at the interface zone; this rearrangement leads to an increase in the charge transfer among the atoms that make the interface and put forward a possible mechanism of the exchange bias origin based on asymmetric electric charge transfer in the different spin channels.
Employing first-principles calculations, we have investigated the possible existence of the ferroelectric instability in the G-type antiferromagnetic NaNiF 3 and LiNiF 3 fluoroperovskites. The behavior of the unstable modes, at the cubic high-symmetry structure, is studied as a function of pressure. This study shows the vibrational landscape and define the conditions to drive the ferroelectricity in these materials, where the A-site dominates the polar instabilities.
First-principles calculations have been carried out in order to analyze the structural, vibrational, and magnetic properties in the A -type antiferromagnetic (AFM) - M(OH) 2 metal transition hydroxides. Theoretical results demonstrate a quasi layer-by-layer magnetic behavior with strong ferromagnetic (FM) interactions in the intralayer plane and weak AFM behavior between the interlayer. This behavior corresponds to an A -type AFM behavior for - M (OH) 2 ( M = Mn, Fe, Co, and Ni) with energy differences around 3.0 meV with respect to the FM state. Structural analysis reveal a clear influence of Van der Waals (VdW) interactions in the stabilization of the phase giving high accuracy in lattice parameter when compared to experimental findings. Qualitative analysis of the magnetic exchange interactions reveals a higher exchange intralayer interaction for M = Ni, and higher exchange or dipolar interlayer interactions for M = Mn. Vibrational analysis, in the M = Ni case, demonstrate a correct IR and Raman modes assignments according to experimental results on the P ¯3 m 1 space group.
In this work, we investigate the hydrostatic pressure dependence of the zone center optical phonons of c -plane and a -plane wurtzite InN epilayers grown on GaN substrates. The longitudinal to transverse mode splitting for the A 1 and E 1 modes was found to increase with increasing pressure, whereas the associated transverse effective charge decreases for both modes as e T * ( A 1 ) = 2.93 – 9.9 × 10 - 3 P and e T * ( E 1 ) = 2.80 – 10.6 × 10 - 3 P (in units of elementary charge and P in GPa). These observations are well in line with results for other II–VI, III–V, and group-IV semiconductor compounds as far as the relation between the magnitude and sign of the pressure derivative of e T * and the bond ionicity is concerned. As the latter increases so does | ∂ e T * / ∂ P | with a sign change from positive to negative for bond ionicities around f i = 0.46 for compounds with anions belonging to the first row of the Periodic Table. A comparison of the results for InN and other nine tetrahedrally bonded compounds indicate that the pressure behavior of the transverse effective charge is mainly determined by the strength of the Pauli repulsion between cation valence electrons and those of the anion core. We also perform ab initio calculations in order to address the origin of the observed increase in linewidth of the E 2 high mode which is found to arise from a pressure-induced increase in the rate of two-phonon decay processes. In conclusion, this broadening is associated with tuning into resonance of a steep edge in the two-phonon density of states around 460 cm - 1 with the frequency of the E 2 high mode.
Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P$\bar{3}$ (No. 147, point group -3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P$\bar{3}$m1 (No. 164, point group -3m) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH) 2 , in particular for the PP$\bar{3}$m1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P$\bar{3}$m1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P$\bar{3}$m1 structure towards P$\bar{3}$. Here, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C2/m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.
Experimental investigations of the magnetic dead layer in 7.6 nm thick film of La 0.7 Sr 0.3 MnO 3 (LSMO) are reported. The dc magnetization ( M ) measurements for a sample cooled to T = 5 K in applied field H = 0 reveal the presence of negative remanent magnetization (NRM) in the M vs. H (magnetic field) measurements as well as in the M vs. T measurements in H = 50 Oe and 100 Oe. The M vs. T data in ZFC (zero-field-cooled) and FC (field-cooled) protocols are used to determine the blocking temperature T B in different H . Isothermal hysteresis loops at different T are used to determine the temperature dependence of saturation magnetization ( M S ), remanence (MR) and coercivity HC. The M S vs. T data are fit to the Bloch law, M S ( T ) = M 0 ( 1 – BT 3/2 ), showing a good fit for T < 100 K and yielding the nearest-neighbor exchange constant J/k B ≅ 18 K. The variations of T B vs. H and H C vs. T are well described by the model often used for randomly oriented magnetic nanoparticles with magnetic domain diameter ≈ 9 nm present in the dead-layer of thickness d =1.4 nm. Finally, the data available from literature on the thickness ( D ) variation of Curie temperature ( T C ) and M S of LSMO films grown under 200, 150, and 0.38 mTorr pressures of O 2 are analyzed in terms of the finite-size scaling, with M S vs. D data fit to M S ( D ) = M S (b)(1-d / D ) yielding the dead layer thickness d = 1.1 nm, 1.4 nm and 2.4 nm respectively. Brief discussion on the significance of these results is presented.