This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

/pdf/phonons-and-related-crystal-properties-from-density-3mdybdn8w1.pdf

Phonons and related crystal properties from density-functional perturbation theory

A simple formulation of a generalized gradient approximation for the exchange and correlation energy of electrons has been proposed by Perdew, Burke, and Ernzerhof (PBE) [Phys. Rev. Lett. 77, 3865 (1996)]. Subsequently Zhang and Yang [Phys. Rev. Lett. 80, 890 (1998)] have shown that a slight revision of the PBE functional systematically improves the atomization energies for a large database of small molecules. In the present work, we show that the Zhang and Yang functional (revPBE) also improves the chemisorption energetics of atoms and molecules on transition-metal surfaces. Our test systems comprise atomic and molecular adsorption of oxygen, CO, and NO on Ni(100), Ni(111), Rh(100), Pd(100), and Pd(111) surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion locally.

/pdf/improved-adsorption-energetics-within-density-functional-1uykzz0ob0.pdf

Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals

The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and ...

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

Non-volatile memory devices are so named because they retain information when power is interrupted; thus they are important computer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories that use ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarization-cycling characteristics. Early work focused on lead zirconate titanate (PZT) but, when films of this material were grown on metal electrodes, they generally suffered from a reduction of Pr (‘fatigue’) with polarity switching. Strontium bismuth tantalate (SBT) and related oxides have been proposed to overcome the fatigue problem3, but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-substituted bismuth titanate thin films provide a promising alternative for FRAM applications. The films are fatigue-free on metal electrodes, they can be deposited at temperatures of ∼650 °C and their values of Pr are larger than those of the SBT films.

Lanthanum-substituted bismuth titanate for use in non-volatile memories

Observation of orbiting resonances in the integral cross section of H–Xe

The vibronic spectra of the large organic molecules tetracene (C18H12), pentacene (C24H14), base-free porphin (C20H14N4) and base-free phthalocyanine (C32H18N8) have been investigated in large (N ≈ 103–104)
4He droplets using laser induced fluorescence. The spectra show sharp zero phonon lines (Δν ⩽ 0.2 cm−1), accompanied by weaker phonon wings on the blue side. The very small inhomogeneous broadening of the zero phonon lines supports the unique properties of liquid He as an extremely gentle ultra-cold matrix for high resolution spectroscopy of larger molecules. Pronounced structures in the phonon wings, which are characteristic for each molecule, are attributed to the excitation of vibrational modes of the helium atoms located in shells surrounding the molecule.

The phonon wings in the (S1 ← S0) spectra of tetracene, pentacene, porphin and phthalocyanine in liquid helium droplets

The photoionization of neutral liquid helium droplets (mean particle number 〈N〉=102–107) was studied using synchrotron radiation at photon energies ranging from 15 to 30 eV. Mass spectra as well as total and mass selective ion yields were measured as a function of the photon energy for different droplet sizes. The experiments indicate that ionization occurs not only by a direct process at photon energies above the atomic ionization potential but also at energies below the threshold by an autoionization process. The latter ionization mechanism proceeds via the electronically excited states of the neutral droplet, which show a strong neutral droplet size dependence. For large neutral droplets HeN(〈N〉≳104) retarding field measurements established that a predominant part of the total ion yield results from larger cluster ions He+k(k≳103). These measurements also show that a decay by fluorescence emission is much more probable than one by ionization following the photoexcitation process. In droplets with embed...

The photoionization of large pure and doped helium droplets

The microscopic diffusional and vibrational motion of sodium atoms on a Cu(001) surface have been studied with high-resolution helium atom scattering. The data are analyzed through a Langevin dynamics simulation for the adatoms. The observed values for the temperature and parallel wave-vector dependence of the width of the quasielastic peak are well described by the theory with an appropriate choice of potential energy surface and the frictional coupling. The theoretical prediction for the position and width of the lateral frustrated translational mode using the same potential and friction are in excellent agreement with the corresponding experimental data. The simulation thus provides a complete determination of the adatom dynamics including the value of the diffusion constant.

Experimental and theoretical investigation of the microscopic vibrational and diffusional dynamics of sodium atoms on a Cu(001) surface

The optical absorption of size‐selected helium cluster ions was studied via photofragmentation spectroscopy. Absorption cross sections were measured for He+n (n=3, 4, 10, 21, and 30). A broad absorption peak was found, which for He+3 is centered at ≊5.3 eV, and which with increasing cluster size shifts slightly to the red. In addition, the kinetic energy release to the ionized and neutral photofragments was measured by a time‐of‐flight technique for cluster sizes between n=3 and 10. From the energy balance the total binding energy of the He+3 trimer ion was determined to be 2.6±0.15 eV. The results further indicate that a charged linear trimer acts as a core molecule for the sizes n=4–7. For n≳7, an additional isomer with a tetramer core is identified. The results are compared with recent ab initio calculations.

http://cluster.physik.uni-freiburg.de/lit/preprints/Hepap.pdf

Absorption spectroscopy and photodissociation dynamics of small helium cluster ions

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