Magnetic structure factor of correlated moments in small-angle neutron scattering

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 ▲]

Parafermion braiding in fractional quantum Hall edge states with a finite chemical potential

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 ▲]

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 ▲]

Unconventional superconductivity in the extended Hubbard model: Weak-coupling renormalization group

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

Screening of electron-phonon coupling in graphene on Ir(111)

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 ▲]

Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2

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 ▲]

Magnetic ordering in nanocrystalline gadolinium: A neutron diffraction study

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

Analysis of magnetic neutron-scattering data of two-phase ferromagnets

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

Magnetic neutron scattering on nanocomposites: Decrypting cross-section images using micromagnetic simulations

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

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