887 research outputs found

    Forming individual magnetic biskyrmions by merging two skyrmions in a centrosymmetric nanodisk

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    When two magnetic skyrmions - whirl-like, topologically protected quasiparticles - form a bound pair, a biskyrmion state with a topological charge of NSk=±2N_\mathrm{Sk}=\pm 2 is constituted. Recently, especially the case of two partially overlapping skyrmions has brought about great research interest. Since for its formation the individual skyrmions need to posses opposite in-plane magnetizations, such a biskyrmion cannot be stabilized by the Dzyaloshinskii-Moriya-interaction (DMI), which is the interaction that typically stabilizes skyrmions in non-centrosymmetric materials and at interfaces. Here, we show that these biskyrmions can be stabilized by the dipole-dipole interaction in centrosymmetric materials in which the DMI is forbidden. Analytical considerations indicate that the bound state of a biskyrmion is energetically preferable over two individual skyrmions. As a result, when starting from two skyrmions in a micromagnetic simulation, a biskyrmion is formed upon relaxation. We propose a scheme that allows to control this biskyrmion formation in nanodisks and analyze the individual steps.Comment: This is a post-peer-review, pre-copyedit version of an article published in Scientific Reports. The final authenticated version is available online at [DOI

    Phonon limited thermoelectric transport in Pb

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    We present a fully ab initio based scheme to compute transport properties, i.e. the electrical conductivity {\sigma} and thermopower S, in the presence of electron-phonon interaction. Therefore, we explicitly investigate the k-dependent structure of the Eliashberg spectral function , the coupling strength, the linewidth and the relaxation time {\tau}. We obtain a state-dependent {\tau} and show its necessity to reproduce the increased thermopower at temperatures below the Debye temperature, without accounting for the phonon-drag effect. Despite the detailed investigations of various k and q dependencies, the presented scheme can be easily applied to more complicated systems.Comment: 9 pages, 10 figures, supplemental material adde

    Thermoelectric transport in strained Si and Si/Ge heterostructures

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    The anisotropic thermoelectric transport properties of bulk silicon strained in [111]-direction were studied by detailed first-principles calculations focussing on a possible enhancement of the power factor. Electron as well as hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of an [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole-doping was drastically suppressed due to quantum-well effects, while under electron-doping an enhanced power factor was found. With that, we state a figure of merit of ZT=0.2=0.2 and ZT=1.4=1.4 at T=\unit[300]{K} and T=\unit[900]{K} for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features