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See detailMagnetic structure factor of correlated moments in small-angle neutron scattering
Honecker, Dirk UL; Fernández Barquín, Luis; Bender, Philipp Florian UL

in Physical Review. B, Condensed Matter and Materials Physics (2020), 101(13), 134401

The interplay between structural and magnetic properties of nanostructured magnetic materials allows one to realize unconventional magnetic effects, which results in a demand for experimental techniques ... [more ▼]

The interplay between structural and magnetic properties of nanostructured magnetic materials allows one to realize unconventional magnetic effects, which results in a demand for experimental techniques to determine the magnetization profile with nanoscale resolution. Magnetic small-angle neutron scattering (SANS) probes both the chemical and magnetic nanostructure and is thus a powerful technique, e.g., for the characterization of magnetic nanoparticles. Here, we show that the conventionally used particle-matrix approach to describe SANS of magnetic particle assemblies, however, leads to a flawed interpretation. As a remedy, we provide general expressions for the field-dependent two-dimensional magnetic SANS cross section of correlated moments. It is shown that for structurally disordered ensembles the magnetic structure factor is in general, and contrary to common assumptions, (i) anisotropic also in zero field and (ii) that even in saturation the magnetic structure factor deviates from the nuclear one. These theoretical predictions explain qualitatively the intriguing experimental, polarized SANS data of an ensemble of dipolar-coupled iron oxide nanoparticles. [less ▲]

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See detailExperimental observation of third-order effect in magnetic small-angle neutron scattering
Metlov, Konstantin L.; Suzuki, Kiyonori; Honecker, Dirk UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2020), 101

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See detailParafermion braiding in fractional quantum Hall edge states with a finite chemical potential
Groenendijk, Solofo UL; Calzona, Alessio; Tschirhart, Hugo et al

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

Parafermions are non-Abelian anyons which generalize Majorana fermions and hold great promise for topological quantum computation. We study the braiding of Z2n parafermions which have been predicted to ... [more ▼]

Parafermions are non-Abelian anyons which generalize Majorana fermions and hold great promise for topological quantum computation. We study the braiding of Z2n parafermions which have been predicted to emerge as localized zero modes in fractional quantum Hall systems at filling factor ν=1/n (n odd). Using a combination of bosonization and refermionization, we calculate the energy splitting as a function of distance and chemical potential for a pair of parafermions separated by a gapped region. Braiding of parafermions in quantum Hall edge states can be implemented by repeated fusion and nucleation of parafermion pairs. We simulate the conventional braiding protocol of parafermions numerically, taking into account the finite separation and finite chemical potential. We show that a nonzero chemical potential poses challenges for the adiabaticity of the braiding process because it leads to accidental crossings in the spectrum. To remedy this, we propose an improved braiding protocol which avoids those degeneracies. [less ▲]

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See detailEvidence for the formation of nanoprecipitates with magnetically disordered regions in bulk Ni50Mn45In5 Heusler alloys
Benacchio, G.; Titov, Ivan UL; Malyeyev, Artem UL et al

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

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See detailCritical size limits for collinear and spin-spiral magnetism in $CoCr_2\mathrmO_4$
Zákutná, D.; Alemayehu, A.; Vlček et al

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 ▲]

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See detailErratum: Third-order effect in magnetic small-angle neutron scattering by a spatially inhomogeneous medium [Phys. Rev. B 91, 054404 (2015)]
Metlov, K.L.; Michels, Andreas UL

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

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See detailUnconventional superconductivity in the extended Hubbard model: Weak-coupling renormalization group
Wolf, Sebastian; Schmidt, Thomas UL; Rachel, Stephan

in Physical Review. B, Condensed Matter and Materials Physics (2018)

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See detailThird-order effect in magnetic small-angle neutron scattering by a spatially inhomogeneous medium
Metlov, K.L.; Michels, Andreas UL

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

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See detailScreening of electron-phonon coupling in graphene on Ir(111)
Endlich, M.; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(205403),

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon ... [more ▼]

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon branches are observed at the ¯ and ¯K point of the surface Brillouin zone. At ¯K the Kohn anomaly is weaker than observed for pristine graphene and graphite. This observation is rationalized in terms of a decrease of the electron-phonon coupling due to screening of graphene electron correlations by the metal substrate. [less ▲]

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See detailAnomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots
Habinshuti, J.; Kilian, O.; Cristini-Robbe, O. et al

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 ▲]

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See detailEffect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2
Molina-Sanchez, Alejandro UL; Sangalli, Davide; Hummer, Kerstin et al

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

We present converged ab initio calculations of the optical absorption spectra of single-layer, double-layer, and bulk MoS2. Both the quasiparticle-energy calculations (on the level of the GW approximation ... [more ▼]

We present converged ab initio calculations of the optical absorption spectra of single-layer, double-layer, and bulk MoS2. Both the quasiparticle-energy calculations (on the level of the GW approximation ) and the calculation of the absorption spectra (on the level of the Bethe-Salpeter equation) explicitly include spin-orbit coupling, using the full spinorial Kohn-Sham wave functions as input. Without excitonic effects, the absorption spectra would have the form of a step function, corresponding to the joint density of states of a parabolic band dispersion in two dimensions. This profile is deformed by a pronounced bound excitonic peak below the continuum onset. The peak is split by spin-orbit interaction in the case of single-layer and (mostly) by interlayer interaction in the case of double-layer and bulk MoS2. The resulting absorption spectra are thus very similar in the three cases, but the interpretation of the spectra is different. Differences in the spectra can be seen in the shape of the absorption spectra at 3 eV where the spectra of the single and double layers are dominated by a strongly bound exciton. [less ▲]

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See detailTheory of magnetic small-angle neutron scattering of two-phase ferromagnets
Honecker, Dirk UL; Michels, Andreas UL

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

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See detailMagnetic ordering in nanocrystalline gadolinium: A neutron diffraction study
Ryan, D. H.; Michels, Andreas UL; Döbrich, Frank UL et al

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

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See detailDielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride
Forster, F.; Molina-Sanchez, Alejandro UL; Engels, S. et al

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

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See detailAnalysis of magnetic neutron-scattering data of two-phase ferromagnets
Honecker, Dirk UL; Dewhurst, Charles; Suzuki, Kiyonori et al

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

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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 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 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 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 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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