PHYSICS AND INSTRUMENTATION IN NEUTRON SCATTERING (CRG-CEA/ILL) L.P. Regnault SPSMS-MDN

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1 PHYSICS AND INSTRUMENTATION IN NEUTRON SCATTERING (CRG-CEA/ILL) L.P. Regnault SPSMS-MDN Outline: - Neutron diffraction under high magnetic fields (12T and 15T) on D23 - Spherical Neutron Polarimetry on IN22 - Polarization analysis on the ladder spin-gap mode in Sr 14 Cu 24 O 41 SPSMS Scientific Council May

2 CRG-CEA INSTRUMENTS AT ILL D15 (SP+F) 2-axis-4-cirles Multidetector Diffractometer D23 (F) 2-axis diffractometer Thermal neutrons Polarized neutrons IN22 (F) 3-axis spectrometer Thermal neutrons Polarization analysis IN12 (D) 3-axis spectrometer Cold neutrons Polarization analysis

3 Upgrade of the diffractometer D23 ( ) (E.Ressouche, B. Grenier, P. Fouilloux) Fully amagnetic (12T and 15 T cryomagnet) Shorter distances Larger angular range (θ,ν) Faster displacements Better performances (signal/noise) The new D23 with the 15-T cryomagnet D23 is presently the only diffractometer at ILL on which the 12T (CEA) and 15T (ILL) vertical-field cryomagnet can be used!

4 Field dependence of the ordered magnetic moments in CsNiCl 3 and RbNiCl 3 (W.B. Yelon and D.E. Cox, PR-B 6, 204 (1972); W.J.L. Buyers et al, PRL 56, 371 (1986); Z. Tun et al., PRB 43, (1991); M. Steiner et al., JAP 61, 3953 (1987); I. Zaliznyak et al. PRB 50, 15824(1994))) Triangular lattice of Ni 2+ (S=1) chains along the c axis Magnetic structure of CsNiCl 3 Quasi-1D antiferromagnetic system (Haldane chains) T N 4.8 K (CsNiCl 3 ) ; T N 12 K (RbNiCl 3 ) chain axis (c) b a Magnetic ordering described by k=(1/3,1/3,1) Strong reduction ( 2) of the magnetic moment: m µ B in CsNiCl 3 ; m µ B in RbNiCl 3 due to the quantum fluctuations Haldane-gap systems above T N (W.J.L. Buyers et al, PRL 56, 371 (1986); M. Steiner et al., JAP 61, 3953 (1987); I. Zaliznyak et al. PRB 50, 15824(1994)) From the spinwave dispersion at low T: J 1.4 mev (16 K) J /J 2x10-2 (CsNiCl 3 ) J 2.0 mev (23 K) J /J 3x10-2 (RbNiCl 3 ) Larger field effects in CsNiCl 3

5 Field dependence of the ordered magnetic moments in CsNiCl 3 and RbNiCl 3 B. Grenier (SPSMS-MDN), I. Zaliznyak (BNL), A. Savici (BNL) (2005) D23 H // a - b H c 0.4J/gµ B m 0 (H) increases in increasing field as a consequence of the decrease of quantum fluctuations Quasi-linear behavior at least up to H c th 0.4J/gµ B : Δm 0 /m 0 30% in CsNiCl 3 Δm 0 /m 0 10% in RbNiCl 3 (explained by the different energy scales ( J)) Departure from linearity for H H c th (For other results, by E. Ressouche et al.)

6 In many strongly correlated electronic systems: Heavy-fermion systems () High-Tc superconductors (YBCO, LSCO, ) Colossal magneto resistance systems ((La,Sr)MnO 3, ) Quantum-gap low-d systems (CuGeO 3, NaV 2 O 5, ) the interplay between the lattice, magnetic (spin and orbit) and charge degrees of freedom may conduce to the existence of sizable cross correlation functions (ex: magnetic-lattice, spin-orbit, ). In the most general case, the neutron polarization P f may undergo a rotation away from P i > Need for a device capable to measure the transverse components, in addition to the longitudinal ones > Device with which P f can be analyzed independently of P i Spherical Neutron Polarimetry with CRYOPAD

7 Spherical Neutron Polarimetry (SNP) CRYOPAD: CRYOgenic Polarization Analysis Device F. Tasset, Physica B , 627 (1989) y H P1 H N1 N 1 z k i P i P i N 1 P f x//k i k f GF1, GF2: guide fields N1, N2: nutation coils IN and OUT P1, P2: precession coils IN and OUT M1, M2: Outer and inner Meissner shields (Nb) P i and P f are selected independently, thanks to a system of two coils (H ni, H Pi ) shielded by a superconducting material.

8 SPHERICAL NEUTRON POLARIMETRY (SNP) CRYOPAD optimized for Inelastic Neutron Scattering (ENPI network; ILL-CEA-JAERI collaboration) Features: Large beam size at sample (30 mm high; 20 mm wide) Accurate control of in and out polarizations over the beam size (DP 0.002; Da 0.3 ) Decoupling of in and out neutron polarization Zero-field chamber around the sample (investigation by NP of superconducting materials: YBCO, LSCO, ) Very compact set-up (distances monochromator-sample and sample-analyser identical in the polarized ans unpolarized configurations) Very low background (<30 n/hour) Precession coil IN Nobium Meissner shields Precession coil OUT

9 CRYOPAD on TAS IN22 (ILL) Longitudinal and transverse components of the polarization: x: // Q y: Q in the scattering plane z: vertical kf ki + % I"# % I"# P"# $ + % I"# + I"# Information on the purely magnetic (M), nuclear (N) terms, and their interferences (NMI and MMI terms) (α, β=x, y, z)!

10 Example: Quasi-2D helimagnet with planar spins BaCo 2 As 2 O 8 3D ordering below T N 5.4 K described by k IC =(0.27, 0, -1.33) Simple helical structure in the basal (a, b) plane (?) (LPR and J. Rossat-Mignod (1991)) ---> Expected behavior for a structural Bragg peak (purely longitudinal non-spin-flip process): P xx =P yy =P zz =1; P xy =P xz =P yx = = 0 (idem exp.) Structural Bragg peak x: // Q and y: Q in the scattering plane; z: vertical P!" x y z Q=(0 0 9) x 1.000± ± ±0.002 y ± ± ±0.002 z ± ± ± > Expected behavior for a simple (left- and right-handed) helix structure in the (a,b) plane: P xx =-1; -P yy =P zz 1; P xy =P xz =P yx = = 0. Magnetic Bragg peak (mainly // a * ) P!" x y z ( ) x ± ± ±0.013 y 0.013± ± ±0.008 z 0.025± ± ±0.005 Exp: P xx -1 ; -P yy P zz 0.981±0.003 ; P yz P zy 0.182±0.005 Out-of-plane component!! > m c /m b 0.1 (from P yz, P zy ) (m ( C. Boullier et al for the full results) c /m b )

11 Magnetic excitations in the ladders of Sr14Cu24O41 Cu2O3 ladders and CuO2 chains b a c E.M. Carron et al., Mat. Res. Bull. 23, 1355 (1988) [(Sr,Ca)2Cu2O3]7(CuO2)10 Interest: - Superconducting under pressure by substitution of Sr for Ca (Sr2Ca14Cu24O41 : Tc 6 K at p= 30 kbar) - Structure reminiscent of YBa2Cu3O6+x

12 SPIN-GAP IN THE LADDERS IN Sr 14 Cu 24 O 41 Intensité neutronique (neutrons/mn=800k) Sr 14 Cu 24 O 41 T=5 K chaines échelles Energie (mev) IN22 k f =4.1 A -1 Q=( ) Q=( ) unpolarized INS Non-magnetic singlet ground state Sharp degenerated triplet gap at D ladder 32 mev Splitting << 1 mev (DJ/J < 0.01) Spectral weight extending up to high energies ( 200 mev) Lattice (phonon) excitations? -----> Prediction for the AF S=1/2 2-leg ladder with J leg J rung : D ladder ~ 0.5J mev w max ~ 2.5J 250 mev Effect of 4-spin ring exchange interactions S. Brehmer et al, Phys. Rev. B 60, 329 (1999) M. Müller et al., Phys. Rev. B 66, (2002) J leg J rung mev ; J ring mev account for the 32-meV gap and the dispersion M. Matsuda et al., Phys. Rev.B 62, 8903 (2000); JAP 87, 6271 (2000)

13 MAGNETIC EXCITATIONS IN THE LADDERS Polarized neutron scattering (P 0 // Q): Strong temperature dependence Melting on place up to 150 K Maximum energy increasing from 32 mev (T 0 K) to 50 mev (T 300 K) Spin-gap > pseudo-gap T=300 K Unpolarized INS: At T 300 K, E max 0.5 J corresponds to the theoretical value for J leg J rung J 110 mev and J ring 0 The resonance-like behavior of the 32-meV mode might due to the growing of cyclic correlations

14 ANISOTROPY OF MAGNETIC CORRELATIONS (LPA/CRYOPAD) (1) P 0 // Q SF: M y +M z (spin excitations) NSF: N (phonons) NSF: N+M z SF: M y (2) P 0 vertical (3) SF: M y NSF: N+M z P 0 Q in plane (1) - (2) -----> M b (M ladder ) (1) - (3) -----> M c (M //ladder )

15 M b /M ac C. Boullier et al., Physica B 350, 40 (2004) (33 mev) (40 mev) Correlations quasi isotropic in the (a, c) ladder layers (M a M c ). Correlations strongly anisotropic along b : M b M ac g b 2.28, g ac 2.04, (g b /g a ) from ESR on CuO 2 chains (Kataev et al., PRB 64, (2001) No splitting of the 32-meV mode M b /M ac increases below K : Origin (?): -----> effect of J ring (J ring /k B K) -----> effect of copper (orbital/spin) form factor (orbital currents -----> effect of a charge (hole) order below K ( C. Boullier et al for details)

16 PROJECTS IN INSTRUMENTATION 3 He spin-filter option as beam polarizer on D23 (coll. ILL/CEA and NMI3 network) (polarized neutron diffraction up to 12 T) Multi-analyzer/multi-detector option on IN12 (coll. FZJ/CEA) (mapping of magnetic excitation spectra in the (Q, w) space) Zero-field spin-echo option on IN22 (coll. ILL/CEA and NMI3 network) (Lattice and magnetic excitation lifetimes in low-d or strongly correlated electron systems (high-t c superconductors, ) ) Acknowledgements C. Boullier (SPSMS-MDN) J.E. Lorenzo (CNRS-Grenoble) B. Grenier (SPSMS-MDN and UJF) E. Ressouche (SPSMS-MDN) C. Marin (SPSMS) G. Dhalenne (LPCES-Orsay) A. Revcolevschi (LPCES-Orsay) I. Zaliznyak (BNL) A. Savici (BNL) F. Tasset (ILL) E. Lelièvre-Berna (ILL)