Papers by Louis-Pierre Regnault
Neutron News, 2017
Neutron News Volume 28 • Number 2 • 2017 17 This contribution is a collection of individual opini... more Neutron News Volume 28 • Number 2 • 2017 17 This contribution is a collection of individual opinions, coordinated by communications offi cers at various neutron facilities. It does not necessarily refl ect the exact scientifi c details; please consult the original publications for a complete and accurate coverage on the subject. It aims to showcase work conducted at neutron sources that confi rmed the predictions of the winners of the 2016 Nobel Prize in Physics or that builds upon their theories.*
Europhysics Letters (EPL), 1987
The influence of a symmetry breaking field on the triangular 3-q incommensurate phase of quartz i... more The influence of a symmetry breaking field on the triangular 3-q incommensurate phase of quartz is discussed. The appearance of a 1-q stripe phase between the β phase and the 3-q phase is expected when a uniaxial stress σ is applied in the (001) plane. In the (σ, T)-plane the 1-q phase is bound by two transition lines which meet at Ti. Elastic-neutron-scattering experiments demonstrate that such a stress-induced 1-q phase is actually observed in quartz and the experimental phase diagram is in qualitative agreement with theory.
Europhysics Letters (EPL), 1996
Neutron inelastic-scattering experiments on the itinerant ferromagnet UFe2 have shown that the st... more Neutron inelastic-scattering experiments on the itinerant ferromagnet UFe2 have shown that the strong hybridization between the U 5 f and Fe 3 d electrons results in a number of major changes compared to the physics of the isostructural RFe2(R = Tb,Ho, and Er) compounds. We have observed that the Fe-Fe exchange interaction in this compound is actually greater at low
The T-x phase diagram of YBa_2Cu_3O_6+x has two well known regions: an antiferromagnetic insulato... more The T-x phase diagram of YBa_2Cu_3O_6+x has two well known regions: an antiferromagnetic insulator (AFI) for x0.45. In AFI, the excitation is that of spin waves, with well understood dispersions. In the optimally doped SC phase (T_c=93K), there is a 41 meV magnetic resonance, localized in both momentum and energy transfer that appears below T_c. We will present our latest
We report the latest extensive unpolarized neutron scattering study on the general structure of b... more We report the latest extensive unpolarized neutron scattering study on the general structure of both even and odd magnetic excitation spectra across and well above Tc in two underdoped superconducting bilayer cuprate compounds: YBa_2Cu_3O_6.5 (Tc = 52K) and YBa_2Cu_3O_6.7 (Tc = 67K). A systematic picture on the evolution of the dynamical susceptibility between 1meV to ~ 100meV from undoped regime
Physical Review Letters, 1997
We have studied magnetic excitations in the dimerized spin-Peierls phase of CuGeO 3 , by high res... more We have studied magnetic excitations in the dimerized spin-Peierls phase of CuGeO 3 , by high resolution inelastic neutron scattering. We measured the welldefined spin triplet dispersive mode which is gapped throughout the whole Brillouin zone. We also observed that this mode is separated by an unexpected second gap of ≈ 2 meV from a continuum of magnetic excitations extending to higher energy. The first gap (or 'triplet gap') and its associated dispersive mode is due to the breaking of a singlet dimer into a delocalized triplet. We propose that the second gap (or 'solitonic gap') and the continuum correspond to dissociation of that triplet into two unbound spin-1/2 solitons that are separated by a dimerized region of arbitrary length.
Europhysics Letters (EPL), 1990
The proton relaxation rate 1/T1 is measured in NENP as a function of both temperature (2 ≲ T ≲ 40... more The proton relaxation rate 1/T1 is measured in NENP as a function of both temperature (2 ≲ T ≲ 40 K) and field (1 ≲ H ≲ 8 T). A quantitative analysis of 1/T1 is given, which confirms a recent theory by Affleck describing the effects of a field on the excitation spectrum of a Haldane spin system. It is also shown that the application of a field results in a slowing-down of the fluctuations and that an “anomaly” occurs when the nonmagnetic-magnetic transition is approached. This anomaly is well interpreted by assuming a softening of the quantum energy gap.
Europhysics Letters (EPL), 1986
ABSTRACT The critical behavior of the d-dimensional Ising model in a transverse field H** PERPEND... more ABSTRACT The critical behavior of the d-dimensional Ising model in a transverse field H** PERPEND at temperatures near T equals and for H** PERPEND yields H//c** PERPEND , are predicted to be the same as for a (d plus 1)-dimensional Ising model in zero field as a function of temperature for T yields T//c. We present evidence for such a dimensionality cross-over by studying the field-dependent critical behavior of the staggered magnetization of the d equals 3 Ising-type antiferromagnet MnCl//2 multiplied by (times) 4H//2O at low temperatures.
Europhysics Letters (EPL), 1997
Inelastic neutron scattering has been used to determine the dynamic spin fluctuations in an under... more Inelastic neutron scattering has been used to determine the dynamic spin fluctuations in an underdoped high temperature superconductor YBCO 6.8 single crystal. The magnetic resonance, that occurs around 40 meV in overdoped samples, is shifted to a lower energy, E r = 34 meV. A constant ratio, E r /k B T C = 4.9 ± 0.2, almost independent of the doping level, is found. According to numerous theoretical approaches, this finding supports the idea that the resonance energy is proportional (approximately twice) to the superconducting gap. 74.20.Mn Nonconventional mechanisms (spin fluctuations, polarons, and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc.) 25.40.Fq Inelastic neutron scattering 74.72.Bk Y-based cuprates
Europhysics Letters (EPL), 1987
... The crystal of NENP is orthorhombic and can be described with the space group Prima at room t... more ... The crystal of NENP is orthorhombic and can be described with the space group Prima at room temperature [9]. The nickel ions are covalently linked by nitrito groups along the b crystal axis. The Ni magnetic chains are well isolated from each other by C104 anions. ...
Europhysics Letters (EPL), 1995
ABSTRACT
Europhysics Letters (EPL), 1999
ABSTRACT
Europhysics Letters (EPL), 1999
We present the results of a high-resolution X-ray-scattering study of the order parameter critica... more We present the results of a high-resolution X-ray-scattering study of the order parameter critical exponent, beta, at the spin-Peierls phase transition in CuGeO3. As expected from previous X-ray data, there is a crossover from a 3D to a 2D regime, and the critical exponents in both regimes agree well with the values predicted for an n = 2, XY order parameter. The consequence of the presence of new topological magnetic excitations onto the spin excitation spectrum is discussed.
Europhysics Letters (EPL), 1998
ABSTRACT
Europhysics Letters (EPL), 1995
We have performed magnetic-susceptibility measurements and Cu nuclear-magnetic-resonance studies ... more We have performed magnetic-susceptibility measurements and Cu nuclear-magnetic-resonance studies on single crystals of the quasi-one-dimensional antiferromagnet CuGe1-xSixO3, with 0 ⩽ x ⩽ 0.1. The Si doping has a drastic effect on the spin-Peierls temperature TSP which decreases at TSP (x)/TSP (0) ≅ 1-50x at low Si concentrations. For x ⩾ 0.005, three-dimensional antiferromagnetic order with an easy axis along the
Magnetic Properties of Layered Transition Metal Compounds
Physical review letters, Jan 30, 2018
We use inelastic neutron scattering to study the low-energy spin excitations of the 112-type iron... more We use inelastic neutron scattering to study the low-energy spin excitations of the 112-type iron pnictide Ca_{0.82}La_{0.18}Fe_{0.96}Ni_{0.04}As_{2} with bulk superconductivity below T_{c}=22 K. A two-dimensional spin resonance mode is found around E=11 meV, where the resonance energy is almost temperature independent and linearly scales with T_{c} along with other iron-based superconductors. Polarized neutron analysis reveals the resonance is nearly isotropic in spin space without any L modulations. Because of the unique monoclinic structure with additional zigzag arsenic chains, the As 4p orbitals contribute to a three-dimensional hole pocket around the Γ point and an extra electron pocket at the X point. Our results suggest that the energy and momentum distribution of the spin resonance does not directly respond to the k_{z} dependence of the fermiology, and the spin resonance intrinsically is a spin-1 mode from singlet-triplet excitations of the Cooper pairs in the case of we...
Physica B: Condensed Matter
Neutron scattering experiments on single crystals have allowed an accurate determination of the m... more Neutron scattering experiments on single crystals have allowed an accurate determination of the magnetic phase diagram of YBa2Cu3O6+ x. For increasing oxygen content the antiferromagnetic ordering of YBa2Cu3O6 is unchanged up to x = 0.2. For x > 0.2 both TN and the ordered moment of Cu2+ ions in the Cu2 site decrease and vanish abruptly around x ⋍ 0.40.
Le Journal de Physique Colloques
It has been predicted by Haldane that the one-dimensional (ID) Heisenberg antiferromagnet (AF) wi... more It has been predicted by Haldane that the one-dimensional (ID) Heisenberg antiferromagnet (AF) with integer spin has a singlet ground state separated from the excited states by an energy gap. The striking susceptibility decrease as T-+ 0 and the absence of long range order in quasi-ID-AF compounds with S = 1, Ni (C~H S N~)~ N02C104 and two other related Ni chains, and in AgVP2Ss support the existence of Haldane gap. More detailed experiments including inelastic neutron scatering in NENP reveal a splitting of the Haldane gap by planar anisotropy, in agreement with numerical calculations.
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Papers by Louis-Pierre Regnault