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Showing results 1 to 20 of 39
   Parafermion braiding in fractional quantum Hall edge states with a finite chemical potentialGroenendijk, Solofo  ; ; et alin 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 ▲] Detailed reference viewed: 12 (1 UL) Critical size limits for collinear and spin spiral magnetism in CoCr$_2$O$_4$; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2019) The multiferroic behavior of CoCr$_2$O$_4$ 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 CoCr$_2$O$_4$ 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 CoCr$_2$O$_4$ 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 CoCr$_2$O$_4$. <P /> [less ▲] Detailed reference viewed: 15 (1 UL)Evidence for the formation of nanoprecipitates with magnetically disordered regions in bulk Ni50Mn45In5 Heusler alloys; Titov, Ivan ; Malyeyev, Artem et alin Physical Review. B, Condensed Matter and Materials Physics (2019), 99 Detailed reference viewed: 134 (17 UL)Critical size limits for collinear and spin-spiral magnetism in $CoCr_2\mathrmO_4$; ; 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 ▲] Detailed reference viewed: 32 (1 UL)Erratum: Third-order effect in magnetic small-angle neutron scattering by a spatially inhomogeneous medium [Phys. Rev. B 91, 054404 (2015)]; Michels, Andreas in Physical Review. B, Condensed Matter and Materials Physics (2019), 99 Detailed reference viewed: 67 (0 UL)Unconventional superconductivity in the extended Hubbard model: Weak-coupling renormalization group; Schmidt, Thomas ; in Physical Review. B, Condensed Matter and Materials Physics (2018) Detailed reference viewed: 105 (0 UL)Third-order effect in magnetic small-angle neutron scattering by a spatially inhomogeneous medium; Michels, Andreas in Physical Review. B, Condensed Matter and Materials Physics (2015), 91 Detailed reference viewed: 119 (10 UL)Screening of electron-phonon coupling in graphene on Ir(111); Molina-Sanchez, Alejandro ; Wirtz, Ludger et alin 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 ▲] Detailed reference viewed: 193 (12 UL)Anomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots; ; 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 ▲] Detailed reference viewed: 123 (1 UL)Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2Molina-Sanchez, Alejandro ; ; et alin 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 ▲] Detailed reference viewed: 192 (4 UL)Theory of magnetic small-angle neutron scattering of two-phase ferromagnetsHonecker, Dirk ; Michels, Andreas in Physical Review. B, Condensed Matter and Materials Physics (2013), 87(224426), 1-10 Detailed reference viewed: 120 (22 UL)Analysis of magnetic neutron-scattering data of two-phase ferromagnetsHonecker, Dirk ; ; et alin Physical Review. B, Condensed Matter and Materials Physics (2013), 88 Detailed reference viewed: 104 (18 UL)Magnetic ordering in nanocrystalline gadolinium: A neutron diffraction study; Michels, Andreas ; Döbrich, Frank et alin Physical Review. B, Condensed Matter and Materials Physics (2013), 87(064408), 1-6 Detailed reference viewed: 107 (20 UL)Dielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride; Molina-Sanchez, Alejandro ; et alin Physical Review. B, Condensed Matter and Materials Physics (2013), 88 Detailed reference viewed: 148 (7 UL)Observation of cross-shaped anisotropy in spin-resolved small-angle neutron scatteringMichels, Andreas ; Honecker, Dirk ; Döbrich, Frank et alin Physical Review. B, Condensed Matter and Materials Physics (2012), 85(184417), 1-5 Detailed reference viewed: 80 (3 UL)Micromagnetic modeling and small-angle neutron scattering characterization of magnetic nanocomposites; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(024410), 1-12 Detailed reference viewed: 83 (3 UL)Neutron scattering study of the magnetic microstructure of nanocrystalline gadoliniumDöbrich, Frank ; ; et alin Physical Review. B, Condensed Matter and Materials Physics (2012), 85(094411), 1-17 Detailed reference viewed: 86 (2 UL)Magnetic neutron scattering on nanocomposites: Decrypting cross-section images using micromagnetic simulations; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(134418), 1-5 Detailed reference viewed: 78 (2 UL)Variations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory; ; 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 ▲] Detailed reference viewed: 165 (2 UL)Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride; Wirtz, Ludger ; et alin 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 ▲] Detailed reference viewed: 104 (7 UL)
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