Electronic structure of ytterbium-implanted GaN at ambient and high pressure: experimental and crystal field studies

The results of high-pressure low-temperature optical measurements in a diamond-anvil cell of bulk gallium nitride crystals implanted with ytterbium are reported in combination with crystal field calculations of the Yb(3+) energy levels. Crystal field analysis of splitting of the (2)F(7/2) and (2)F(5/2) states has been performed, with the aim of assigning all features of the experimental luminescence spectra. A thorough analysis of the pressure behavior of the Yb(3+) luminescence lines in GaN allowed the determination of the ambient-pressure positions and pressure dependence of the Yb(3+) energy levels in the trigonal crystal field as well as the pressure-induced changes of the spin-orbit coupling coefficient.     

Importance of interplane coupling on the magnetic phases of quasi-two-dimensional tantalites

We propose a three-dimensional model to describe magnetic interactions in a class of tantalite compounds of compositions A(x)A'(1-x)Ta(2)O(6), with A,A' = Fe, Co or Ni. Due to the quasi-two-dimensional nature of the magnetism in these compounds, experimental data have been previously interpreted using two-dimensional models. These are anisotropic Heisenberg models or Ising models and include competing exchange interactions from different neighbors. Taking into account all the relevant exchange terms, which include interplane interactions, we show that the latter allows us to understand the various low-temperature magnetic phases observed by neutron diffraction in this family of compounds. This is done by studying the eigenvalues of the exchange-interaction matrix in wavevector space for different sets of coupling parameters, of which those relative to in-plane interactions have been obtained from high-temperature series analysis of the magnetic susceptibility. This approach is rather general and the model presented here is directly applicable to isostructural compounds like ASb(2)O(6).     

Surface stress of Ni adlayers on W(110): the critical role of the surface atomic structure

Puzzling trends in surface stress have been reported experimentally for Ni/W(110) as a function of Ni coverage. In order to explain this behavior, we have performed a density-functional-theory study of the surface stress and atomic structure of the pseudomorphic and several different possible 1 × 7 configurations for this system. For the 1 × 7 phase, we predict a different, more regular atomic structure than previously proposed based on surface x-ray diffraction. At the same time, we reproduce the unexpected experimental change of surface stress between the pseudomorphic and 1 × 7 configuration along the crystallographic surface direction which does not undergo density changes. We show that the observed behavior in the surface stress is dominated by the effect of a change in Ni adsorption/coordination sites on the W(110) surface.     

Ab?initio characterization of graphene nanoribbons and their polymer precursors

Bottom-up fabrication of graphene nanoribbons (GNRs) from halogen-terminated aromatic precursors is a promising method for achieving atomically precise nanoribbons at competitive yields. GNR fabrication proceeds via the polymerization of the precursors and successive dehydrogenation. By first principles density functional theory calculations, we perform a systematic characterization of the polymeric precursors and the corresponding graphene nanoribbons in terms of structural and electronic properties, and we compute the Raman and infrared spectra. The band structure properties are examined by considering the bonding features and the partial charge densities of the structures. The physical origin of the infrared and Raman peaks is investigated in terms of the morphology and vibrational properties of the precursors and products. We show that light spectroscopy provides a unique fingerprint for each type of GNR, which may be used to monitor the quality of the final products and the kinetics of the synthesis process.     

Evolution from a non-Fermi liquid Kondo lattice to intermediate valence behaviour in CeRhSn(1-x)In(x)

We present investigations of the magnetic and electric transport properties, specific heat, and electronic structure of the intermetallic and strongly correlated system of CeRhSn(1-x)In(x) compounds. The main goal of this paper is to determine the hybridization energy between the f electron and conduction electron states, V(cf), and its influence on the ground state properties of the system. The complementary experimental data are discussed on the basis of the Anderson model for a periodic Kondo lattice. CeRhSn is known as a non-Fermi liquid, while CeRhIn is a valence fluctuating system. We discuss the ground state properties of CeRhSn(1-x)In(x) and compare the results with those obtained for the doped Ce-based Kondo insulators.     

Stability of an aqueous quadrupole micro-trap

The recently demonstrated functionality of an aqueous quadrupole micro- or nano-trap opens a new avenue for applications of Paul traps, like confinement of a charged biomolecule which requires a water environment for its chemical stability. Besides the strong viscosity forces, the motion of a charged particle in the aqueous trap is subject to dielectrophoretic and electrophoretic forces. In this study, we describe the general conditions for stability of a charged particle in an aqueous quadrupole trap. We find that for typical micro-trap parameters, the effects of both dielectrophoresis and electrophoresis significantly influence the trap stability. In particular, an aqueous quadrupole trap could play the role of a synthetic virtual nanopore for the third generation of DNA sequencing technology.     

Reversible switching of single tin phthalocyanine molecules on the InAs(111)A surface

Individual tin phthalocyanine (SnPc) molecules adsorbed on the InAs(111)A surface were studied by low-temperature scanning tunnelling microscopy (STM) at 5?K. Consistently with the nonplanar molecular structure, SnPc adopts two in-plane adsorption geometries with the centre Sn atom either above (SnPc(up)) or below (SnPc(down)) the molecular plane. Depending on the current and bias applied to the tunnel junction, the molecule can be reversibly switched between the two conformations, implying a controlled transfer of the Sn atom through the molecular plane. The SnPc(down) conformer is characterized by an enhanced surface bonding as compared to the SnPc(up) conformer. SnPc(up) molecules can be repositioned by the STM tip by means of lateral manipulation, whereas this is not feasible for SnPc(down) molecules. The reversible switching process thus enables one to either laterally move the molecule or anchor it to the semiconductor surface.     

High-pressure Raman scattering and an anharmonicity study of multiferroic wolframite-type Mn(0.97)Fe(0.03)WO(4)

A high-pressure Raman scattering study of wolframite-type Mn(0.97)Fe(0.03)WO(4) is presented up to 10.4?GPa. The phonon wavenumbers vary linearly with pressure. The mode Grüneisen parameters are larger for many bending and lattice modes when compared to the stretching modes due to the larger compressibility of Mn(Fe)O(6) octahedra when compared to WO(6) octahedra. Combining the pressure-dependent Raman data of this work with the temperature-dependent Raman data on this crystal previously reported by us has allowed estimation of the temperature-dependent pure lattice and intrinsic anharmonic contributions to the observed total Raman shifts as a function of temperature. It has been found that the observed unusual hardening of the 884, 698 and 674?cm(-1) stretching modes upon heating from 4 to about 150-200?K followed by the usual softening above 150-200?K is a result of a positive intrinsic anharmonic contribution and a negative pure lattice contribution; i.e.,?up to about 150-200?K the anharmonic contribution surpasses the lattice contribution and the total Raman shift is slightly positive whereas above 150-200?K the lattice contribution becomes dominant and the Raman bands exhibit the usual softening with increasing temperature.