Showing papers by "Francesco Mauri published in 2013"

Ab initio variational approach for evaluating lattice thermal conductivity

Abstract: We present a first-principles theoretical approach for evaluating the lattice thermal conductivity based on the exact solution of the Boltzmann transport equation. We use the variational principle and the conjugate gradient scheme, which provide us with an algorithm faster than the one previously used in literature and able to always converge to the exact solution [Omini and Sparavigna, Physica B: Condens. Matter 212, 101 (1995)]. Three-phonon normal and umklapp collisions, isotope scattering, and border effects are rigorously treated in the calculation. Good agreement with experimental data for diamond is found. Moreover we show that by growing more enriched diamond samples it is possible to achieve values of thermal conductivity up to three times larger than those commonly observed in isotopically enriched diamond samples with $99.93%$ C${}^{12}$ and $0.07$ C${}^{13}$.

Anharmonic properties from a generalized third order ab~initio approach: theory and applications to graphite and graphene

Abstract: We have implemented a generic method, based on the $2n+1$ theorem within density functional perturbation theory, to calculate the anharmonic scattering coefficients among three phonons with arbitrary wave vectors. The method is used to study the phonon broadening in graphite and graphene mono- and bilayers. The broadening of the high-energy optical branches is highly nonuniform and presents a series of sudden steps and spikes. At finite temperature, the two linearly dispersive acoustic branches TA and LA of graphene have nonzero broadening for small wave vectors. The broadening in graphite and bilayer graphene is, overall, very similar to the graphene one, the most remarkable feature being the broadening of the quasiacoustical Z-polarized branch. Finally, we study the intrinsic anharmonic contribution to the thermal conductivity of the three systems, within the single mode relaxation time approximation. We find the conductance to be in good agreement with experiments in the out-of-plane direction but underestimate by a factor 2 in-plane.

First-Principles Theory of Anharmonicity and the Inverse Isotope Effect in Superconducting Palladium-Hydride Compounds

Abstract: Palladium hydrides display the largest isotope effect anomaly known in the literature. Replacement of hydrogen with the heavier isotopes leads to higher superconducting temperatures, a behavior inconsistent with harmonic theory. Solving the self-consistent harmonic approximation by a stochastic approach, we obtain the anharmonic free energy, the thermal expansion, and the superconducting properties fully ab initio. We find that the phonon spectra are strongly renormalized by anharmonicity far beyond the perturbative regime. Superconductivity is phonon mediated, but the harmonic approximation largely overestimates the superconducting critical temperatures. We explain the inverse isotope effect, obtaining a -0.38 value for the isotope coefficient in good agreement with experiments, hydrogen anharmonicity being mainly responsible for the isotope anomaly.

Signature of the two-dimensional phonon dispersion in graphene probed by double-resonant Raman scattering

Abstract: In this article we unravel the origin of the two-phonon D + D �� peak and its asymmetric line shape by combining experimental data of single-layer graphene with a full twodimensional calculation of the double-resonant Raman process based on fourth-order perturbation theory. We show that the main peak originates from phonons along the K highsymmetry line and that the asymmetry is due to phonons from the two-dimensional Brillouin zone. The analysis of the asymmetric line shape in experiment provides a direct probe of the two-dimensional phonon dispersion. We further show how the D + D �� peak evolves with the number of graphene layers.

A biocompatible calcium bisphosphonate coordination polymer: towards a metal-linker synergistic therapeutic effect?

Abstract: A novel, biocompatible, calcium-based coordination polymer was hydrothermally synthesized using as linker the bone antiresorptive bisphosphonate alendronate. Its structure was determined using single-crystal X-ray diffraction and characterized by chemical analysis, infrared spectroscopy (IR), X-ray powder diffraction (XRPD), solid state nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). Its bioactivity was finally evaluated in vitro, revealing a very high stability towards simulated body fluid.

Structural study of calcium phosphonates: a combined synchrotron powder diffraction, solid-state NMR and first-principle calculations approach

Abstract: The structures of four Ca-phosphonate phases are reported here: Ca(C6H5–PO3H)2 (1), Ca(C6H5–PO3)·2H2O (2), Ca(C4H9–PO3H)2 (3) and Ca(C4H9–PO3)·H2O (4). Structural models were obtained ab initio by using a combined synchrotron powder diffraction, solid-state nuclear magnetic resonance, and gauge including projector augmented wave (GIPAW) calculation approach. The 1H, 13C, 31P and 43Ca NMR parameters calculated from these structural models were found to be in good agreement with the experimental values, thereby indicating the high accuracy of the DFT-optimized structures. Correlations between the NMR parameters and structural features around the phosphonate were then analyzed, showing in particular the high sensitivity of the 31P asymmetry parameter ηCS and the 43Ca isotropic chemical shift to changes in local structure around the phosphonate groups and the Ca2+, respectively. Finally, the NMR data of a new mixed Na–Ca phosphonate phase, Ca1.5Na(C4H9–PO3)2, are reported.

Quantum Monte Carlo study of the protonated water dimer

Abstract: We report an extensive theoretical study of the protonated water dimer (Zundel ion) by means of the highly correlated variational Monte Carlo and lattice regularized Monte Carlo approaches. This system represents the simplest model for proton transfer (PT) and a correct description of its properties is essential in order to understand the PT mechanism in more complex acqueous systems. Our Jastrow correlated AGP wave function ensures an accurate treatment of electron correlations. Exploiting the advantages of contracting the primitive basis set over atomic hybrid orbitals, we are able to limit dramatically the number of variational parameters with a systematic control on the numerical precision, crucial in order to simulate larger systems. We investigate energetics and geometrical properties of the Zundel ion as a function of the oxygen-oxygen distance, taken as reaction coordinate. In both cases, our QMC results are found in excellent agreement with coupled cluster CCSD(T) technique, the quantum chemistry "gold standard". Calculations on proton transfer static barriers and dissociation energies display the same agreement. A comparison with density functional theory (DFT) results in the PBE approximation points out the crucial role of electron correlations for a correct description of the PT in the dimer. Our method is able to resolve the tiny energy differences (~ 0.1 Kcal/mol) and the corresponding structural variations at which the proton transfer takes place; it combines these features with a N^3-N^4 scaling with number of particles (favorable with respect to other post-DFT methods), as proven by a benchmark calculation on a larger protonated water cluster. The QMC approach used here represents a promising candidate to provide the first high-level ab initio description of PT in water.

Extended Czjzek model applied to NMR?parameter distributions in sodium?metaphosphate glass

Abstract: The extended Czjzek model (ECM) is applied to the distribution of NMR parameters of a simple glass model (sodium metaphosphate, NaPO3) obtained by molecular dynamics (MD) simulations. Accurate NMR tensors, electric field gradient (EFG) and chemical shift anisotropy (CSA) are calculated from density functional theory (DFT) within the well-established PAW/GIPAW framework. The theoretical results are compared to experimental high-resolution solid-state NMR data and are used to validate the considered structural model. The distributions of the calculated coupling constant CQ ∝ |Vzz| and the asymmetry parameter ηQ that characterize the quadrupolar interaction are discussed in terms of structural considerations with the help of a simple point charge model. Finally, the ECM analysis is shown to be relevant for studying the distribution of CSA tensor parameters and gives new insight into the structural characterization of disordered systems by solid-state NMR.

Extended Czjzek model applied to NMR parameter distributions in sodium metaphosphate glass

Abstract: The Extended Czjzek Model (ECM) is applied to the distribution of NMR parameters of a simple glass model (sodium metaphosphate, $\mathrm{NaPO_3}$) obtained by Molecular Dynamics (MD) simulations. Accurate NMR tensors, Electric Field Gradient (EFG) and Chemical Shift Anisotropy (CSA), are calculated from Density Functional Theory (DFT) within the well-established PAW/GIPAW framework. Theoretical results are compared to experimental high-resolution solid-state NMR data and are used to validate the considered structural model. The distributions of the calculated coupling constant $C_Q\propto |V_{zz}|$ and of the asymmetry parameter $\eta_Q$ that characterize the quadrupolar interaction are discussed in terms of structural considerations with the help of a simple point charge model. Finally, the ECM analysis is shown to be relevant for studying the distribution of CSA tensor parameters and gives new insight into the structural characterization of disordered systems by solid-state NMR.