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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 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 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 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 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 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 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 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 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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See detailCauchy-like relation between elastic constants in amorphous materials.
Krüger, Jan-Kristian UL; Baller, Jörg UL; Britz, Th et al

in Physical Review. B, Condensed Matter and Materials Physics (2002), 66(1), 012206-012210

It is well known that in addition to the longitudinal modulus, viscoelastic liquids show a shear stiffness at sufficiently high probe frequencies due to structural relaxations. For probe frequencies that ... [more ▼]

It is well known that in addition to the longitudinal modulus, viscoelastic liquids show a shear stiffness at sufficiently high probe frequencies due to structural relaxations. For probe frequencies that are large compared to the structural relaxation frequency, the measured elastic longitudinal and shear moduli become so-called clamped properties (c11∞ and c44∞, respectively). During freezing or polymerization of amorphous liquids, these clamped moduli behave in a strongly nonlinear fashion as a function of temperature or polymerization time. Based on Brillouin spectroscopy data we will show that there exists a linear relation between c11∞ and c44∞ over a large temperature or polymerization time range. Surprisingly, the parameters of this linear relation between the elastic moduli vary only little for different materials. Implications for the nonlinear elastic behavior at the glass transition will be discussed on the basis of mode Grüneisen parameters. [less ▲]

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See detailAnalysis of the small-angle neutron scattering of nanocrystalline ferromagnets using a micromagnetics model
Weissmüller, J.; Michels, Andreas UL; Barker, J. G. et al

in Physical Review. B, Condensed Matter and Materials Physics (2001), 63

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure ... [more ▼]

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure gives rise to strong elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for analyzing field-dependent SANS data from such materials in terms of a model based on the theory of micromagnetics. Samples of cw Ni and of electrodeposited nc Ni and nc Co were characterized by x-ray scattering and magnetometry, and were investigated by SANS both with and without polarization of the neutron beam. The variation of the differential scattering cross section with the scattering vector and with the applied magnetic field is well described by the model. Also, experimental results for the exchange stiffness constant A and for the spin-wave stiffness constant D obtained from the analysis are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens. The model supplies an “anisotropy field scattering function” that contains information on the magnitude of the magnetic anisotropy in the material, and on the characteristic length scales on which the anisotropy changes direction. The results suggest that the anisotropy may be strongly nonuniform in each crystallite, possibly due to twinning, and that some magnetic moments in the Ni samples are strongly pinned at defects. [less ▲]

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See detailExistence of An Intrinsic Glass Transition In A Silicon Rubber: Hypersonic Versus Calorimetric Properties
Krüger, Jan-Kristian UL; Mesquida, P.; Baller, Jörg UL

in Physical Review. B, Condensed Matter and Materials Physics (1999), 60(14), 10037-10041

Time-domain temperature-modulated differential scanning calorimetry and high-performance Brillouin spectroscopy have been used as experimental techniques in order to investigate the effect of the thermal ... [more ▼]

Time-domain temperature-modulated differential scanning calorimetry and high-performance Brillouin spectroscopy have been used as experimental techniques in order to investigate the effect of the thermal glass transition on the specific heat as well as on the acoustic properties, quantities which are intimately related by thermodynamics, but measured at completely different frequencies. Using a siloxane as a model substance, we demonstrate the efficiency of hypersonic relaxations until thermal freezing of the material. Introducing the optoacoustic dispersion function as a sensitive measure for structural relaxations, we show that the high-frequency α relaxations are truncated by the thermal glass transition instead of dying out at much higher temperatures. Moreover, the extrapolated static compliance of the rubbery state goes to zero close to but below the thermal glass transition temperature Tg, indicating the existence of an acoustic instability which is reminiscent of the so-called Kauzmann paradox. [less ▲]

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See detailSurface-induced organization of linear molecules on nanostructured polytetrafluoroethylene: Crystalline state of poly[vinylidenefluoride-trifluoroethylene].
Krüger, Jan-Kristian UL; Heydt, B.; Fischer, C. et al

in Physical Review. B, Condensed Matter and Materials Physics (1997), 55(6), 3497-3506

Highly crystalline and uniaxial-oriented polytetrafluoroethylene films, nanostructured in one dimension, were used to produce a fully crystalline, thermoreversible morphology in the ferroelectric ... [more ▼]

Highly crystalline and uniaxial-oriented polytetrafluoroethylene films, nanostructured in one dimension, were used to produce a fully crystalline, thermoreversible morphology in the ferroelectric copolymer poly[vinylidenefluoride-trifluoroethylene]. The resulting state is built up of regularly stacked lamellar crystals of about 100 nm thickness and lateral dimensions of about 6 μm. [less ▲]

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See detailNonlinear optical properties of Rb2ZnCl4 in the incommensurate and ferroelectric phases
Sanctuary, Roland UL; Jundt, D.; Baumert, J. et al

in Physical Review. B, Condensed Matter and Materials Physics (1985), 32(3), 1649-1660

Nonlinear optical properties of Rb2ZnCl4 are measured for the incommensurate and ferroelectric phases by means of second-harmonic generation experiments. In the temperature range of the ferroelectric lock ... [more ▼]

Nonlinear optical properties of Rb2ZnCl4 are measured for the incommensurate and ferroelectric phases by means of second-harmonic generation experiments. In the temperature range of the ferroelectric lock-in phase, the nonlinear optical coefficients d33, d32, and d24 and the Miller coefficients are determined using the wedge technique and are shown to be proportional to the spontaneous polarization. In the incommensurate phase of Rb2ZnCl4, the order of magnitude of the second-harmonic intensity corresponding to nonlinear optical coefficients allowed by the local point-group symmetry is estimated to be at least 107 times smaller than the second-harmonic intensity generated in quartz and thus difficult to detect. Furthermore, it is shown that quasi-phase-matching of the fundamental and second-harmonic waves in the incommensurate phase is limited to a very small temperature range of TC [less ▲]

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