Universal behavior of the IMS domain formation in superconducting niobium

In the intermediate mixed state (IMS) of type-II/1 superconductors, vortex lattice (VL) and Meissner state domains coexist due to a partially attractive vortex interaction. Using a neutron-based multiscale approach combined with magnetization measurements, we study the continuous decomposition of a homogeneous VL into increasingly dense domains in the IMS in bulk niobium samples of varying purity. We find a universal temperature dependence of the vortex spacing, closely related to the London penetration depth and independent of the external magnetic field. The rearrangement of vortices occurs even in the presence of a flux freezing transition, i.e. pronounced pinning, indicating a breakdown of pinning at the onset of the vortex attraction

Symmetric and asymmetric excitations of a strong-leg quantum spin ladder

The zero-field excitation spectrum of the strong-leg spin ladder (C$_7$H$_10$N)$_2$CuBr$_4$ (DIMPY) is studied with a neutron time-of-flight technique. The spectrum is decomposed into its symmetric and asymmetric parts with respect to the rung momentum and compared with theoretical results obtained by the density matrix renormalization group method. Additionally, the calculated dynamical correlations are shown for a wide range of rung and leg coupling ratios in order to point out the evolution of arising excitations, as e.g. of the two-magnon bound state from the strong to the weak coupling limit

First proof of concept of remote attendance for future observation strategies between Wettzell (Germany) and Concepción (Chile)

Current VLBI observations are controlled and attended locally at the radio telescopes on the basis of pre-scheduled session files. Operations have to deal with system specific station commands and individual setup procedures. Neither the scheduler nor the correlator nor the data-analyst gets real-time feedback about system parameters during a session. Changes in schedules after the start of a session by remote are impossible or at least quite difficult. For future scientific approaches, a more flexible mechanism would optimize the usage of resources at the sites. Therefore shared-observation control between world-wide telescope s, remote attendance/control as well as completely unattended-observations could be useful, in addition to the classic way to run VLBI observations. To reach these goals, the Geodetic Observatory Wettzell in cooperation with the Max-Planck-Institute for Radio Astronomy (Bonn) have developed a software extension to the existing NASA Field System for remote control. It uses the principle of a remotely accessible, autonomous process cell as server extension to the Field System on the basis of Remote Procedure Calls (RPC). Based on this technology the first completely remote attended and controlled geodetic VLBI session between Wettzell, Germany and Concepción, Chile was successfully performed over 24 hours. This first test was extremely valuable for gathering information about the differences between VLBI systems and measuring the performance of internet connections and automatic connection re-establishments. During the 24h-session, the network load, the number of sent/received packages and the transfer speed were monitor ed and captured. It was a first reliable test for the future wishes to control several telescopes with one graphical user interface on different data transfer rates over large distances in an efficient way. In addition, future developments for an authentication and user role management will be realized within the upcoming NEXPReS project

Reciprocal space mapping of magnetic order in thick epitaxial MnSi films

We report grazing incidence small angle neutron scattering (GISANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film on Si(111) in the thick film limit. Providing a means of direct reciprocal space mapping, GISANS and OSR reveal a magnetic modulation perpendicular to the films under magnetic fields parallel and perpendicular to the film, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of field orientation, our data does not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct difference with bulk samples of MnSi of similar thickness under perpendicular field, in which a skyrmion lattice dominates the phase diagram. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell control the magnetic state, representing the main difference between the films and thin bulk samples

Long-range crystalline nature of the skyrmion lattice in MnSi

We report small angle neutron scattering of the skyrmion lattice in MnSi using an experimental set-up that minimizes the effects of demagnetizing fields and double scattering. Under these conditions the skyrmion lattice displays resolution-limited Gaussian rocking scans that correspond to a magnetic correlation length in excess of several hundred {\mu}m. This is consistent with exceptionally well-defined long-range order. We further establish the existence of higher-order scattering, discriminating parasitic double-scattering with Renninger scans. The field and temperature dependence of the higher-order scattering arises from an interference effect. It is characteristic for the long-range crystalline nature of the skyrmion lattice as shown by simple mean field calculations.Comment: 4 page

Large Scales - Long Times: Adding High Energy Resolution to SANS

The Neutron Spin Echo (NSE) variant MIEZE (Modulation of IntEnsity by Zero Effort), where all beam manipulations are performed before the sample position, offers the possibility to perform low background SANS measurements in strong magnetic fields and depolarising samples. However, MIEZE is sensitive to differences \DeltaL in the length of neutron flight paths through the instrument and the sample. In this article, we discuss the major influence of \DeltaL on contrast reduction of MIEZE measurements and its minimisation. Finally we present a design case for enhancing a small-angle neutron scattering (SANS) instrument at the planned European Spallation Source (ESS) in Lund, Sweden, using a combination of MIEZE and other TOF options, such as TISANE offering time windows from ns to minutes. The proposed instrument allows studying fluctuations in depolarizing samples, samples exposed to strong magnetic fields, and spin-incoherently scattering samples in a straightforward way up to time scales of \mus at momentum transfers up to 0.01 {\AA}-1, while keeping the instrumental effort and costs low.Comment: 5 pages, 8 figure