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We develop a shell model (SM) for potassium dihydrogen phosphate (KDP) which is fitted to ab initio (AI) results that include nonlocal van der Waals corrections. The SM is comprehensively tested by comparing results of structural, vibrational and ferroelectric properties with AI and experimental data. The relaxed structural parameters are in very good agreement with the AI results and the available experimental data. The $\Gamma$-point phonons and the total phonon densities of states (DOSs) in the ferroelectric and paraelectric phases calculated with the developed SM are in good overall agreement with the corresponding AI and experimental data. We also compute the effective Debye temperature as a function of $T$ which shows good accordance with the corresponding AI and experimental results. Classical molecular dynamics (MD) simulations obtained with the developed SM show a FE-PE phase transition at $\approx 360$ K in remarkable agreement with ab initio MD calculations., Comment: 12 pages, 3 figures, 3 tables

Path integral Monte Carlo (PIMC) simulations with very simple models were used in order to unveil the physics behind the isotope effects in H-bonded ferroelectrics. First, we studied geometrical effects in the H-bonds caused by deuteration with a general three-site model based on a back-to-back double Morse potential plus a Morse potential between oxygens, fitted to explain different general features for a wide set of H-bonded compounds. Our model results show the Ubbelohde or geometrical effect (GE), i.e., the expansion of the H-bond with deuteration, in agreement to what is observed in H-bonded ferroelectrics with short H-bonds. Moreover, adjusting the potential parameters to ab initio results, we have developed a 1D model which considers the bilinear proton-proton interaction in mean-field to study nuclear quantum effects that give rise to the GE in KDP crystals. PIMC simulations reveal that protons tunnel more efficiently than deuterons along the 1D chain, giving rise to a strong attraction center that pulls the oxygens together. This mechanism, which is based on the correlation between tunneling and geometrial modifications of the H-bonds, leads to a strong GE in the ordered phase of the chain at low temperature which is in good agreement with the experimental data., Comment: 12 pages, 6 figures, 3 tables

The Raman spectrum of Ba2CuGe2O7, a tetragonal insulator which develops Dzyaloshinsky- Moriya helical magnetism below TN = 3.2 K, has been detected at temperatures varying from 300 to 80 K in a single crystal, with the radiation polarized either in the ab plane or along the c axis of its tetragonal cell. 29 phonon lines out of the 35 allowed by the Raman selection rules for the present geometry were observed, and their vibrational frequencies were found in overall good agreement with those provided by shell-model calculations. Together with the previous report on the infrared-active phonons [A. Nucara et al., Phys. Rev. B 90, 014304 (2014)] the present study provides an exhaustive description, both experimental and theoretical, of the lattice dynamics in Ba2CuGe2O7.

The lattice dynamics of Ba$_2$CuGe$_2$O$_7$, a compound which develops Dzyaloshinsky-Moriya (DM) helical magnetism below $T_N$ = 3.2 K, has been studied by measuring the infrared reflectivity of a single crystal with the radiation polarized both in the $ab$ plane and along the $c$ axis of its tetragonal cell, from 7 K to 300 K. In this compound, where the unit cell has no inversion symmetry, fourteen $E$ phonon modes of the $ab$ plane, out of the eighteen predicted, and all the ten $B_2$ modes of the $c$ axis, have been observed. They have been assigned to the atomic motions by a comparison with shell-model calculations, which provided vibrational frequencies in good agreement with the experiment, while most calculated intensities turned to be much lower than the experimental values. This discrepancy has been tentatively explained by assuming strong electron-phonon interactions, a hypothesis supported by the failure of the $f$- sum rule if restricted to the phonon region. Indeed, we observe a remarkable increase in the oscillator strengths at $T$'s low but higher than $T_N$, which suggests that the dielectric constant of Ba$_2$CuGe$_2$O$_7$ may increase at those temperatures., Comment: 17 pages, 6 figures, 4 tables

We study approximate solutions of the gravitational lens equation and corresponding lens magnification factor near the critical point. This consideration is based on the Taylor expansion of the lens potential in powers of coordinates and an introduction of a proximity parameter characterising the closeness of a point source to the caustic. Second-order corrections to known approximate solutions and magnification are found in case of a general fold point. The first-order corrections near a general cusp are found as well. Key words: gravitational lensing, methods: analytical

By means of extensive first-principles calculations we studied the ferroelectric phase transition and the associated isotope effect in KH2PO4 (KDP), Comment: 16 pages, 10 figures

Based on an accurate first principles description of the energetics in H-bonded KDP, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Cluster tunneling involving also heavy ions is allowed, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples selfconsistently with the proton off-centering, thus explaining both the giant isotope effect, and its close connection with geometrical effects., Comment: 4 two-column pages, 4 figures

The origin of ferroelectricity in KH_2PO_4 (KDP) is studied by first-principles electronic structure calculations. In the low-temperature phase, the collective off-center ordering of the protons is accompanied by an electronic charge delocalization from the "near" and localization at the "far" oxygen within the O-H...O bonds. Electrostatic forces, then, push the K+ ions towards off-center positions, and induce a macroscopic polarization. The analysis of the correlation between different geometrical and electronic quantities, in connection with experimental data, supports the idea that the role of tunnelling in isotopic effects is irrelevant. Instead, geometrical quantum effects appear to play a central role., Comment: 8 pages, 2 postscript figures, submitted to the X Conference on Computational Materials Science, Villasimius, Sardinia (Italy), 2000

An ab-initio Linear Augmented Plane-Wave (LAPW) calculation of the zone-centered phonon frequencies of SnO has been performed. E$_g$ symmetry has been ascribed to the mode observed at 113 cm$^{-1}$ in Raman measurements, discarding a previous B$_{1g}$ assignement. The other phonon modes measured by Raman spectroscopy are also well reproduced. A new shell-model has also been developed, that gives good agreement of the zone-centered frequencies compared to the measured data and the LAPW results. Specific heat measurements have been performed between 5 K and 110 K. Computation of the specific heat and the M\"{o}ssbauer recoilless fraction with the improved shell-model shows a good agreement with the experimental data as a function of temperature., Comment: 11 pages, 1 figure. to appear in Phys. Rev. B (November 1999)

We have compared the ground-state energy of several observed or proposed " 2 \sqrt{2} x 2 \sqrt{2} oxygen (O) ordered superstructures " (from now on HS), with those of "chain superstructures" (CS) (in which the O atoms of the basal plane are ordered in chains), for different compositions x in YBa2Cu3O6+x. The model Hamiltonian contains i) the Madelung energy, ii) a term linear in the difference between Cu and O hole occupancies which controls charge transfer, and iii) covalency effects based on known results for $t-J$ models in one and two dimensions. The optimum distribution of charge is determined minimizing the total energy, and depends on two parameters which are determined from known results for x=1 and x=0.5. We obtain that on the O lean side, only CS are stable, while for x=7/8, a HS with regularly spaced O vacancies added to the x=1 structure is more stable than the corresponding CS for the same x. We find that the detailed positions of the atoms in the structure, and long-range Coulomb interactions, are crucial for the electronic structure, the mechanism of charge transfer, the stability of the different phases, and the possibility of phase separation., Comment: 24 text pages, Latex, one fig. included as ps file, to be publisheb in Phys. Rev. B

Pair breaking mechanisms by impurities are investigated in the two-dimensional t-J model by exact diagonalization techniques. Analysis of binding energies, pairing correlations, dynamical spin and pair susceptibilities shows that non-magnetic impurities are more effective in suppressing pairing than magnetic ones in agreement with experimental studies of Zn- and Ni- substituted High-Tc superconductors., Comment: 4 pages, Revtex v3.0, 4 figures uuencoded, ask for hardcopies at jose@ifir.edu.ar A missleading statement in the introduction was corrected

We study by exact diagonalization the two-dimensional t-J-Holstein model near quarter filling by retaining only few phonon modes in momentum space. This truncation allows us to incorporate the full dynamics of the retained phonon modes. The behavior of the kinetic energy, the charge structure factor and other physical quantities, show the presence of a transition from a delocalized phase to a localized phase at a finite value of the electron-phonon coupling. We have also given some indications that the e-ph coupling leads in general to a suppression of the pairing susceptibility at quarter filling., Comment: 11 pages, Revtex v. 2.0, 4 figures available from authors