Phase Diagram of the Dzyaloshinskii-Moriya Helimagnet Ba2CuGe2O7 in Canted Magnetic Fields

The evolution of different magnetic structures of non-centrosymmetric Ba2CuGe2O7 is systematically studied as function of the orientation of the magnetic field H. Neutron diffraction in combination with measurements of magnetization and specific heat show a virtually identical behaviour of the phase diagram of Ba2CuGe2O7 for H confined in both the (1,0,0) and (1,1,0) plane. The existence of a recently proposed incommensurate double-k AF-cone phase is confirmed in a narrow range for H close to the tetragonal c-axis. For large angles enclosed by H and the c-axis a complexely distorted non-sinusoidal magnetic structure has recently been observed. We show that its critical field Hc systematically increases for larger canting. Measurements of magnetic susceptibility and specific heat finally indicate the existence of an incommensurate/commensurate transition for H /sim 9 T applied in the basal (a,b)-plane and agree with a non-planar, distorted cycloidal magnetic structure.Comment: 14 pages, 13 figure

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

Double-k phase of the Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O7

Neutron diffraction is used to re-investigate the magnetic phase diagram of the noncentrosymmetric tetragonal antiferromagnet Ba2CuGe2O7. A novel incommensurate double-k magnetic phase is detected near the commensurate-incommensurate phase transition. This phase is stable only for magnetic field closely aligned with the 4-fold symmetry axis. The results emphasize the inadequacy of existing theoretical models for this unique material, and points to additional terms in the Hamiltonian or lattice effects.Comment: 4 pages, 5 figure

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

ESR study of the spin ladder with uniform Dzyaloshinskii-Moria interaction

Evolution of the ESR absorption in a strong-leg spin ladder magnet (C$_7$H$_{10}$N$_2$)$_2$CuBr$_4$ (abbreviated as DIMPY) is studied from 300K to 400mK. Temperature dependence of the ESR relaxation follows a staircase of crossovers between different relaxation regimes. We ague that the main mechanism of ESR line broadening in DIMPY is uniform Dzyaloshinskii-Moria interaction ($|\vec{D}|=0.20$K) with an effective longitudinal component along an exchange bond of Cu ions within the legs resulting from the low crystal symmetry of DIMPY and nontrivial orbital ordering. The same Dzyaloshinskii-Moriya interaction results in the lifting of the triplet excitation degeneracy, revealed through the weak splitting of the ESR absorption at low temperatures.Comment: 13 pages, submitted to PRB, Fig.3 update

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