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See detailMicromagnetic modeling and small-angle neutron scattering characterization of magnetic nanocomposites
Erokhin, Sergey; Berkov, Dmitry; Gorn, Nataliya et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(024410), 1-12

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See detailMagnetic neutron scattering on nanocomposites: Decrypting cross-section images using micromagnetic simulations
Erokhin, Sergey; Berkov, Dmitry; Gorn, Nataliya et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(134418), 1-5

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See detailNeutron scattering study of the magnetic microstructure of nanocrystalline gadolinium
Döbrich, Frank UL; Kohlbrecher, Joachim; Sharp, Melissa et al

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

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See detailObservation of cross-shaped anisotropy in spin-resolved small-angle neutron scattering
Michels, Andreas UL; Honecker, Dirk UL; Döbrich, Frank UL et al

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

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See detailCoupling of excitons and defect states in boron-nitride nanostructures
Attaccalite, C.; Bockstedte, M.; Marini, A. et al

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 ▲]

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See detailVariations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory
Ziegler, D.; Gava, P.; Guettinger, J. et al

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 ▲]

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See detailAnisotropic excitonic effects in the energy loss function of hexagonal boron nitride
Galambosi, S.; Wirtz, Ludger UL; Soininen, J. A. et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(8), 081413

The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with ... [more ▼]

The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with momentum transfer parallel to the BN layers is dominated by strongly bound, quasi-two-dimensional excitons. In contrast, excitations with momentum transfer perpendicular to the layers are influenced by weakly bound three-dimensional excitons. This striking phenomenon is revealed by a combined study using high-precision nonresonant inelastic x-ray scattering measurements supported by ab initio calculations. The results are relevant in general to layered insulating systems. [less ▲]

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See detailPhonons in single-layer and few-layer MoS2 and WS2
Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84

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See detailInfluence of crystallite size and temperature on the antiferromagnetic helices of terbium and holmium metal
Michels, Andreas UL; Bick, Jens-Peter UL; Birringer, R. et al

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

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See detailDielectric function of colloidal lead chalcogenide quantum dots obtained by a Kramers-Kronig analysis of the absorbance spectrum
Moreels, Iwan; Allan, Guy; De Geyter, Bram et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(23),

We combined the Maxwell-Garnett effective medium theory with the Kramers-Kronig relations to obtain the complex dielectric function epsilon of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The ... [more ▼]

We combined the Maxwell-Garnett effective medium theory with the Kramers-Kronig relations to obtain the complex dielectric function epsilon of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The method allows extracting both real (epsilon(R)) and imaginary (epsilon(I)) parts of the dielectric function from the experimental absorption spectrum. This enables the quantification of the size-dependent oscillator strength of the optical transitions at different critical points in the Brillouin zone, strongly improving our understanding of quantum confinement effects in these materials. In addition, the static-limit sum rule yields the electronic dielectric constant from the epsilon(I) spectrum. Interestingly, values for lead chalcogenide Qdots remain close to the bulk dielectric constant. To verify these trends, we determined the dielectric constant of thin lead chalcogenide layers by ab initio calculations, and the results agree with the experimental data. [less ▲]

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See detailCorrelation functions of the spin misalignment in magnetic small-angle neutron scattering
Michels, Andreas UL

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82(2), 0244331-02443312

We have numerically calculated the autocorrelation function C(r) of the spin misalignment by means of micromagnetic theory. C(r) depends sensitively on the details of the underlying magnetic ... [more ▼]

We have numerically calculated the autocorrelation function C(r) of the spin misalignment by means of micromagnetic theory. C(r) depends sensitively on the details of the underlying magnetic microstructure and can be determined by Fourier inversion of magnetic small-angle neutron scattering data. The model system which we consider consists of a single isolated spherical nanoparticle that is embedded in an infinitely extended matrix. The particle is uniquely characterized by its magnetic anisotropy field Hp(x), whereas the matrix is assumed to be otherwise anisotropy-field free. In the approach-to-saturation regime, we have computed the static response of the magnetization to different spatial profiles of Hp(x). Specifically, we have investigated the cases of a uniform particle anisotropy, uniform core shell, linear increase, and exponential and power-law decay. From the magnetization profiles and the associated C(r), we have extracted the correlation length lc of the spin misalignment, and we have compared the applied-field dependence of this quantity with semiquantitative theoretical predictions. We find that for practically all of the considered models for the anisotropy field (except the core-shell model) the field dependence of the spin-misalignment fluctuations is quite uniquely reproduced by lc(Hi)=L+lh(Hi), where the field-independent quantity L is on the order of the particle size and lh(Hi) represents the so-called exchange length of the applied magnetic field. [less ▲]

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See detail$LDA+U$ and tight-binding electronic structure of InN nanowires
Molina-Sanchez, Alejandro UL; Garcia-Cristobal, A.; Cantarero, A. et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82

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See detailReversed polarized emission in highly strained $a$-plane GaN/AlN multiple quantum wells
Mata, R.; Cros, A.; Budagosky, J. A. et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82

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See detailInhomogeneous free-electron distribution in InN nanowires: Photoluminescence excitation experiments
Segura-Ruiz, J.; Molina-Sanchez, Alejandro UL; Garro, N. et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82

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See detailTransport through open quantum dots: Making semiclassics quantitative
Brezinova, Iva; Wirtz, Ludger UL; Rotter, Stefan et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(12),

We investigate electron transport through clean open quantum dots (quantum billiards). We present a semiclassical theory that allows to accurately reproduce quantum-transport calculations. Quantitative ... [more ▼]

We investigate electron transport through clean open quantum dots (quantum billiards). We present a semiclassical theory that allows to accurately reproduce quantum-transport calculations. Quantitative agreement is reached for individual energy-dependent and magnetic field dependent elements of the scattering matrix. Two key ingredients are essential: (i) inclusion of pseudopaths which have the topology of linked classical paths resulting from diffraction in addition to classical paths and (ii) a high-level approximation to diffractive scattering. Within this framework of the pseudopath semiclassical approximation (PSCA), typical shortcomings of semiclassical theories such as violation of the anticorrelation between reflection and transmission and the overestimation of conductance fluctuations are overcome. Beyond its predictive capabilities the PSCA provides deeper insights into the quantum-to-classical crossover. [less ▲]

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See detailSynthesis and magnetic properties of cobalt nanocubes
Gräf, C. P.; Birringer, R.; Michels, Andreas UL

in Physical Review. B, Condensed Matter and Materials Physics (2006), 73(21), 2124011-2124014

We report on the synthesis, structure, and magnetic properties of cobalt nanoparticles with cubic shape. The nanoparticles with an average cube-edge length of about 50 nm were prepared by wet-chemical ... [more ▼]

We report on the synthesis, structure, and magnetic properties of cobalt nanoparticles with cubic shape. The nanoparticles with an average cube-edge length of about 50 nm were prepared by wet-chemical processing and characterized by electron microscopy, x-ray diffraction, and magnetometry. The x-ray data indicate a mixture of hcp Co and ϵ-Co. From the magnetization measurements we deduce on the presence of a ferromagnet/antiferromagnet Co/CoO interface, which gives rise to a pronounced exchange-bias effect. Between 350 K and 5 K the coercivity HC increases by almost a factor of 6, whereas the exchange-bias field HE takes on a value of about 380 Oe at 5 K and vanishes above 75 K. The variation of HC with temperature cannot be understood in terms of the various conventional types of magnetic anisotropy. [less ▲]

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See detailDipolar correlations in a nanocomposite: A neutron scattering study of Nanoperm Fe89Zr7B3Cu
Michels, Andreas UL; Vecchini, C.; Moze, O. et al

in Physical Review. B, Condensed Matter and Materials Physics (2006), 74(13), 1344071-13440713

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm ... [more ▼]

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm (Fe89Zr7B3Cu). An unusual “clover-leaf-shaped” intensity distribution on the detector is attributed to the dipolar stray fields around the nanosized iron particles, which are embedded in an amorphous magnetic matrix of lesser saturation magnetization. The dipole field induces spin disorder, correlating the spin misalignment of neighboring particles and matrix over several particle spacings. The clover-leaf-shaped anisotropy is observed over a wide range of applied magnetic field and momentum transfer. It persists up to several hundred degrees Kelvin above the Curie temperature of the matrix phase, indicating that some degree of magnetic coupling persists even when the matrix is paramagnetic. [less ▲]

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See detailPeriodic unmixing of a binary metallic vapor
Maul, J.; Strachnov, I.; Karpuk, S. et al

in Physical Review. B, Condensed Matter and Materials Physics (2005), 72(15),

We report on a type of surface structuring after short pulse laser ablation of a binary alloy. We observe the emergence of a concentric ring structure with changing elemental composition. The composition ... [more ▼]

We report on a type of surface structuring after short pulse laser ablation of a binary alloy. We observe the emergence of a concentric ring structure with changing elemental composition. The composition changes are interpreted by condensation of the ambient ablation vapor due to stress wave excitations in the ablation spot. [less ▲]

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See detailRaman spectra of BN nanotubes: Ab initio and bond-polarizability model calculations
Wirtz, Ludger UL; Lazzeri, M.; Mauri, F. et al

in Physical Review. B, Condensed Matter and Materials Physics (2005), 71(24),

We present ab initio calculations of the nonresonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are ... [more ▼]

We present ab initio calculations of the nonresonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are extracted from first-principles calculations of the polarizability tensor of a BN sheet. For light polarization along the tube axis, the agreement between model and ab initio spectra is almost perfect. For perpendicular polarization, depolarization effects have to be included in the model in order to reproduce the ab initio Raman intensities. [less ▲]

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See detailSpin structure of nanocrystalline terbium
Weissmüller, J.; Michels, Andreas UL; Michels, D. et al

in Physical Review. B, Condensed Matter and Materials Physics (2004), 69

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data are analyzed quantitatively in terms of ... [more ▼]

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data are analyzed quantitatively in terms of the correlation function of the spin misalignment. We find that up to applied fields of several tesla the magnetization remains “locked in” to the basal planes of the hcp crystal lattice of each individual crystallite, but that the in-plane orientation of the spins is highly nonuniform within each grain. This spin disorder at the nanoscale can be suppressed by a large applied field, but in the remanent state the disorder reduces the magnetization to values considerably below the Stoner limit. In field-dependent SANS, the intragrain spin disorder gives rise to a crossover of the scattering curves, and to the unusual finding that the scattering cross section at small scattering vector increases with increasing magnetic field. As the origin of the internal spin disorder within the grains, we propose an extra magnetic anisotropy energy at small grain size, presumably due to microstrain, a suggestion which is supported by analysis of ac-susceptibility data in the paramagnetic state. Our finding of a reduced remanence at small grain size is contrary to the remanence enhancement that is observed in other nanocrystalline hard magnets. We also report an unusual logarithmic field dependence of the magnetization over wide ranges of the applied field and temperature. [less ▲]

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