•A Cu/Ti neutron supermirror was developed and optimized for high overall and low spin-dependent neutron reflectivities.•Control of the roughness growth allowed to achieve 90% reflectivity at the reflection edge (polarized and non-polarized).•The magnetization of this coating was reduced to a level between 30 and 50 times below that of NiMo/Ti supermirrors.•The spin-flip reflectivity of < 1.8 × 10−5 makes these neutron mirrors suitable for extremely low-spin-flip applications [56].

Inelastic neutron scattering experiments on Sr_{2}RuO_{4} determine the spectral weight of the nesting induced magnetic fluctuations across the superconducting transition. There is no observable change at the superconducting transition down to an energy of ∼0.35  meV, which is well below the 2Δ values reported in several tunneling experiments. At this and higher energies magnetic fluctuations clearly persist in the superconducting state. Only at energies below ∼0.3  meV can evidence for partial suppression of spectral weight in the superconducting state be observed. This strongly suggests that the one-dimensional bands with the associated nesting fluctuations do not form the active, highly gapped bands in the superconducting pairing in Sr_{2}RuO_{4}.

The design of the first cold neutron triple-axis spectrometer at the China Advanced Research Reactor is presented. Based on the Monte Carlo simulations using neutron ray-tracing program McStas, the parameters of major neutron optics in this instrument are optimized. The neutron flux at sample position is estimated to be 5.6 ×107n/cm2/s at neutron incident energy Ei=5meV when the reactor operates normally at the designed 60MW power. The performances of several neutron supermirror polarizing devices are compared and their critical parameters are optimized for this spectrometer. The polarization analysis will be realized with a flexible switch from the unpolarized experimental mode.

We present neutron scattering studies of the inter-planar correlations in the high-temperature superconductor La1.88Sr0.12CuO4 (T_c=27 K). The correlations are studied both in a magnetic field applied perpendicular to the CuO2 planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L~0 is probed. Interestingly, the results of quench-cooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of L provides evidence that the incommensurate signal is twinned with the dominating and sub-dominant twin displaying peaks at even or odd L, respectively.

We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at T-0 = 17.5K in URu2Si2. While confirming previous results for the HO and PM phases, our data reveal a pronounced wave-vector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model containing a fit parameter 1/Gamma which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at (Q) over right arrow (1) approximate to (1.4,0,0) show an abrupt steplike suppression of 1/Gamma below T-0, whereas excitations at (Q) over right arrow (0) = (1,0,0), associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of 1/Gamma at T-0. Therefore, at the critical HO temperature T-0, LMAF fluctuations become nearly critical as well. This is the behavior expected of a "supervector" order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space.

Inelastic neutron scattering techniques have been used to study the field-induced multiferroic transition and the temperature dependence of magnetic excitations in TbMnO3. The significant changes in the spin-wave spectra across the field-induced transition perfectly agree with a rotation of the cycloidal spiral plane and with efficient pinning in the commensurate high-field phase. Further analysis of the Q dependence allows the identification of an electromagnon in the multiferroic high-field phase whose energy and polarization precisely matches previous infrared data. This and the zero-field temperature dependence of a zone-center magnon, which exactly agrees with that of an optically detected excitation, document that the inverse Dzyaloshinskii-Moriya interaction induces an electromagnon hybrid excitation in TbMnO3.

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The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu(2)(BO(3))(2) is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature, while the integrated spectral weight can be explained by an isolated dimer model up to 10 K. Analyzing this anomalous spectral line shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long lived if no thermally populated triplons are nearby but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry-Sutherland lattice.

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu$_2$(BO$_3$)$_2$ is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature while the integrated spectral weight can be explained by an isolated dimer model up to 10~K. Analyzing this anomalous spectral line-shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long-lived if no thermally populated triplons are near-by but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry--Sutherland lattice.

Citation for published version (Harvard): Roemer, AT, Jensen, P, Jacobsen, H, Udby, L, Andersen, BM, Bertelsen, M, Holm, SL, Christensen, NB, ToftPetersen, R, Skoulatos, M, Laver, M, Schneidewind, A, Link, P, Oda, M, Ido, M, Momono, N & Lefmann, K 2015, 'Field-induced inter-planar correlations in the high-temperature superconductor La 1.88 Sr 0.12 CuO 4 ', Physical Review B, vol. 91, no. 17, 174507. https://doi.org/10.1103/PhysRevB.91.174507

We present neutron-scattering studies of the interplanar magnetic correlations in the high-temperature superconductor La 1 . 88 Sr 0 . 12 CuO 4 ( T c = 27 K). The correlations are studied both in a magnetic field applied perpendicular to the CuO 2 planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the out-of-plane wave vector L , indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L ≈ 0 is probed. Interestingly, the results of quench-cooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of L provides evidence that the incommensurate signal is twinned with the magnetic scattering from the dominant and subdominant structural twin displaying peaks at even and odd values of L , respectively, in our crystal.

A versatile and portable magnetically shielded room with a field of (700 ± 200) pT within a central volume of 1 m × 1 m × 1 m and a field gradient less than 300 pT/m, achieved without any external field stabilization or compensation, is described. This performance represents more than a hundredfold improvement of the state of the art for a two-layer magnetic shield and provides an environment suitable for a next generation of precision experiments in fundamental physics at low energies; in particular, searches for electric dipole moments of fundamental systems and tests of Lorentz-invariance based on spin-precession experiments. Studies of the residual fields and their sources enable improved design of future ultra-low gradient environments and experimental apparatus. This has implications for developments of magnetometry beyond the femto-Tesla scale in, for example, biomagnetism, geosciences, and security applications and in general low-field nuclear magnetic resonance (NMR) measurements.

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu_2(BO_3)_2 is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature while the integrated spectral weight can be explained by an isolated dimer model up to 10 K. Analyzing this anomalous spectral line-shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long-lived if no thermally populated triplons are near-by but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry–Sutherland lattice.

A. T. Rømer,1,2 J. Chang,3,4,5 N. B. Christensen,6 B. M. Andersen,1 K. Lefmann,1 L. Mähler,3 J. Gavilano,3 R. Gilardi,3 Ch. Niedermayer,3 H. M. Rønnow,2 A. Schneidewind,7 P. Link,8 M. Oda,9 M. Ido,9 N. Momono,10 and J. Mesot3,4,5 1Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark 2Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 3Laboratory of Neutron Scattering, Paul Scherrer Institute, 5232-Villigen, Switzerland 4Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland 5Laboratory for Synchrotron and Neutron Spectroscopy, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 6Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 7Institut für Festkörperphysik (IFP), Technische Universität Dresden, D-01062 Dresden, Germany 8Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, 85747 Garching, Germany 9Department of Physics, Hokkaido University, Sapporo 060-0810, Japan 10Department of Applied Sciences, Muroran Institute of Technology, Muroran 050-8585, Japan (Received 22 February 2013; revised manuscript received 4 April 2013; published 22 April 2013)

We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductor La1.88Sr0.12CuO4 (T-c = 27 K). The temperature dependence of the low-energy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature T-N similar or equal to T-c inferred from elastic neutron scattering and the freezing temperature T-f similar or equal to 11 K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below 0.75 meV for temperatures smaller than T-f, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe-ordered ground state of La1.875Ba0.125CuO4. The difference between T-N and T-f implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly inelastic scattering-at least in the temperature range between T-f and T-c-which is not resolved in the present experiment. Combining the results obtained from our study of La1.88Sr0.12CuO4 with a critical reappraisal of published neutron scattering work on samples with chemical composition close to p = 0.12, where local probes indicate a sharp maximum in T-f (p), we arrive at the view that the low-energy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to p = 0.12 and superconducting spin gaps dominating elsewhere. DOI: 10.1103/PhysRevB.87.144513

I. Altarev(), S. Chesnevskaya(), W. Feldmeier(), P. Fierlinger(), A. Frei(), E. Gutsmiedl(), F. Kuchler(), P. Link(), T. Lins(), M. Marino(), S. Paul(), G. Petzoldt(), A. Pichlmaier(), R. Stoepler(), S. Stuiber(), B. Taubenheim() () Physikdepartment Technische Universitat Munchen and Excellence-Cluster ’Universe’ () Technische Universitat Munchen, Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II)

EuC_2 is a ferromagnet with a Curie-temperature of T_C ≃ 15K. It is semiconducting with the particularity that the resistivity drops by about 5 orders of magnitude on cooling through T_C, which is therefore called a metal-insulator transition. In this paper we study the magnetization, specific heat, thermal expansion, and the resistivity around this ferromagnetic transition on high-quality EuC_2 samples. At T_C we observe well defined anomalies in the specific heat c_p(T) and thermal expansion α(T) data. The magnetic contributions of c_p(T) and α(T) can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field the magnetic contributions of the specific heat and thermal expansion fulfill a Grüneisen-scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of T_C via Ehrenfest's relation, we expect a considerable increase of T_C under applied pressure due to a strong spin-lattice coupling. Furthermore the influence of weak off stoichiometries δ in EuC_2 ±δ was studied. It is found that δ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behavior of EuC_2 strongly resembles that of EuO.

By inelastic neutron scattering, we have analyzed the magnetic correlations in the paramagnetic metallic region of the series Ca2-xSrxRuO4, 0.2 <= x <= 0.62. We find different contributions that correspond to two-dimensional ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors Q(1)(IC) = (0.11,0,0), Q(2)(IC) = (0.26,0,0), and Q(alpha beta)(IC) = (0.3,0.3,0). These components constitute the measured response as a function of the Sr concentration x, of the magnetic field, and of the temperature. A generic model is applicable to metallic Ca2-xSrxRuO4 close to the Mott transition, in spite of their strongly varying physical properties. The amplitude, characteristic energy, and width of the incommensurate components vary only slightly as functions of x, but the ferromagnetic component depends sensitively on concentration, temperature, and magnetic field. While ferromagnetic fluctuations are very strong in Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T = 1.5 K, they are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of a magnetic field, and form a magnon mode above the metamagnetic transition. The inelastic neutron scattering results document how the competition between ferromagnetic and incommensurate antiferromagnetic instabilities governs the physics of this system.

EuC2 is a ferromagnet with a Curie temperature of T-C similar or equal to 15 K. It is semiconducting with the particularity that the resistivity drops by about five orders of magnitude on cooling through T-C, which is therefore called a metal-insulator transition. In this paper, we study the magnetization, specific heat, thermal expansion and the resistivity around this ferromagnetic transition on high-quality EuC2 samples. At T-C we observe well-defined anomalies in the specific heat c(p)(T) and thermal expansion alpha(T) data. The magnetic contributions of c(p)(T) and alpha(T) can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field, the magnetic contributions of the specific heat and thermal expansion fulfil a Gruneisen scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of T-C via Ehrenfest's relation, we expect a considerable increase of T-C under applied pressure due to a strong spin-lattice coupling. Furthermore, the influence of weak off-stoichiometries delta in EuC2 +/-delta was studied. It is found that delta strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behaviour of EuC2 strongly resembles that of EuO.

Combining macroscopic and diffraction methods we have studied the electric, magnetic and struc- tural properties of RE_(1-x)Ca_xTiO_3 (RE=Y, Er, Lu) focusing on the concentration range near the metal-insulator transition. The insulating phase, which is stabilized by a smaller rare-earth ionic ra- dius, exhibits charge order with a predominant occupation of the dxy orbital. The charge and orbital ordering explains the broad stability range of the insulating state in RE_(1-x)Ca_xTiO_3 with smaller rare-earth ions. The strong modulation of the Ti-O bond distances indicates sizeable modulation of the electric charge.