Critical size limits for collinear and spin-spiral magnetism in $CoCr_2\mathrmO_4$

in Physical Review. B, Condensed Matter and Materials Physics (2019), 100

The multiferroic behavior of CoCr2O4 results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size-dependent ... [more ▼]

The multiferroic behavior of CoCr2O4 results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size-dependent behavior, which collapses when reaching a critical particle size. Here the magnetic phase stability of CoCr2O4 in the size range of 3.6–14.0 nm is presented in detail using the combination of neutron diffraction with XYZ polarization analysis and macroscopic magnetization measurements. We establish critical coherent domain sizes for the formation of the spin spiral and ferrimagnetic structure and reveal the evolution of the incommensurate spin spiral vector with particle size. We further confirm the presence of ferroelectric polarization in the spin spiral phase for nanocrystalline CoCr2O4. [less ▲]

Evidence for the formation of nanoprecipitates with magnetically disordered regions in bulk Ni50Mn45In5 Heusler alloys

in Physical Review. B, Condensed Matter and Materials Physics (2019), 99

Third-order effect in magnetic small-angle neutron scattering by a spatially inhomogeneous medium

in Physical Review. B, Condensed Matter and Materials Physics (2015), 91

Anomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(11), 115313

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up ... [more ▼]

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement. [less ▲]

Theory of magnetic small-angle neutron scattering of two-phase ferromagnets

in Physical Review. B, Condensed Matter and Materials Physics (2013), 87(224426), 1-10

Magnetic ordering in nanocrystalline gadolinium: A neutron diffraction study

in Physical Review. B, Condensed Matter and Materials Physics (2013), 87(064408), 1-6

Observation of cross-shaped anisotropy in spin-resolved small-angle neutron scattering

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(184417), 1-5

Neutron scattering study of the magnetic microstructure of nanocrystalline gadolinium

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(094411), 1-17

Variations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(23),

We present Kelvin probe force microscopy measurements of single-and few-layer graphene resting on SiO2 substrates. We compare the layer thickness dependency of the measured surface potential with ab ... [more ▼]

We present Kelvin probe force microscopy measurements of single-and few-layer graphene resting on SiO2 substrates. We compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene. The ab initio calculations show that the work function of single-and bilayer graphene is mainly given by a variation of the Fermi energy with respect to the Dirac point energy as a function of doping, and that electrostatic interlayer screening only becomes relevant for thicker multilayer graphene. From the Raman G-line shift and the comparison of the Kelvin probe data with the ab initio calculations, we independently find an interlayer screening length in the order of four to five layers. Furthermore, we describe in-plane variations of the work function, which can be attributed to partial screening of charge impurities in the substrate, and result in a nonuniform charge density in single-layer graphene. [less ▲]

Coupling of excitons and defect states in boron-nitride nanostructures

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(14), 144115

The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN ... [more ▼]

The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the "free" exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra. [less ▲]