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Despite decades of intense researches, the enigmatic pseudo-gap (PG) phase of superconducting cuprates remains an unsolved mystery. In the last 15 years, various symmetry breakings in the PG state have been discovered, spanning an intra-unit cell (IUC) magnetism, preserving the lattice translational (LT) symmetry but breaking time-reversal symmetry and parity, and an additional incipient charge density wave breaking the LT symmetry upon cooling. However, none of these states can (alone) account for the partial gapping of the Fermi surface. Here we report a hidden LT-breaking magnetism uisng polarized neutron diffraction. Our measurements reveal magnetic correlations, in two different underdoped $\rm YBa_{2}Cu_{3}O_{6.6}$ single crystals, that settle at the PG onset temperature with i) a planar propagation wave vector $(\pi,0) \equiv (0,\pi)$, yielding a doubling or quadrupling of the magnetic unit cell and ii) magnetic moments mainly pointing perpendicular to the $CuO_{2}$ layers. The LT-breaking magnetism is at short range suggesting the formation of clusters of 5-6 unit cells. Together with the previously reported IUC magnetism, it yields a hidden magnetic texture of the $CuO_{2}$ unit cells hosting loop currents, forming large supercells which may be crucial for elucidating the PG puzzle., Comment: 5 figures, supplementary file upon request

Controlling chiral magnetic domains in a multiferroic material is promising for applications in spintronics or data storage devices. The authors study the electric-field driven relaxation of multiferroic domains in NaFeGe${}_{2}$O${}_{6}$ as a function of temperature and electric field over several decades in time. An astonishingly simple combination of an Arrhenius-like temperature dependence with a Merz law for the electric-field dependence describes the relaxation times with only two parameters, indicating that multiferroic relaxation is essentially determined by domain-wall motion.

The magnetic excitations in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ are studied by polarized and unpolarized neutron scattering experiments as a function of temperature. At the scattering vector of the Fermi-surface nesting with a half-integer out-of-plane component, there is no evidence for the appearance of a resonance excitation in the superconducting phase. The body of existing data indicates weakening of the scattered intensity in the nesting spectrum to occur at very low energies. The nesting signal persists up to 290 K but is strongly reduced. In contrast, a quasiferromagnetic contribution maintains its strength and still exhibits a finite width in momentum space.

In many quantum materials, strong electron correlations lead to the emergence of new states of matter. In particular, the study in the last decades of the complex phase diagram of high temperature superconducting cuprates highlighted intra-unit-cell electronic instabilities breaking discrete Ising-like symmetries, while preserving the lattice translation invariance. Polarized neutron diffraction experiments have provided compelling evidences supporting a new form of intra-unit-cell magnetism, emerging concomitantly with the so-called pseudogap state of these materials. This observation is currently interpreted as the magnetic hallmark of an intra-unit-cell loop current order, breaking both parity and time-reversal symmetries. More generally, this magneto-electric state is likely to exist in a wider class of quantum materials beyond superconducting cuprates. For instance, it has been already observed in hole-doped Mott insulating iridates or in the spin liquid state of hole-doped 2-leg ladder cuprates., Comment: 7 figures, 1 glossary, accepted in Comptes Rendus Physique (2021)

The structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related ${\mathrm{MnWO}}_{4}$ and $\mathrm{NaFe}{({\text{WO}}_{4})}_{2}$. Both systems exhibit a competition of incommensurate spiral and commensurate spin up-up-down-down ordering along the magnetic chains. In the latter commensurate phases, the alternatingly parallel and antiparallel arrangement of ${\mathrm{Fe}}^{3+}$ or ${\mathrm{Mn}}^{2+}$ moments leads to sizable bond-angle modulation and thus to magnetic dimerization. For $\mathrm{NaFe}{({\text{WO}}_{4})}_{2}$ this structural distortion has been determined to be strongest for the low-field up-up-down-down arrangement, and the structural refinement yields a bond-angle modulation of $\ifmmode\pm\else\textpm\fi{}1.15(16)$ degrees. In the commensurate phase of ${\mathrm{MnWO}}_{4}$, superstructure reflections signal a comparable structural dimerization and thus strong magneto-elastic coupling different to that driving the multiferroic order. Pronounced anharmonic second- and third-order reflections in the incommensurate and multiferroic phase of ${\mathrm{MnWO}}_{4}$ result from tiny commensurate fractions that can depin multiferroic domains.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr_{2}IrO_{4} is particularly in the spotlight because of the so-called j_{eff}=1/2 state consisting of a quantum superposition of the three local t_{2g} orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state. Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_{eff}=1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

In contrast to magnetic order formed by electrons' dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as "hidden order". Previously the hidden order in phase II was identified as primary antiferroquadrupolar (AFQ) and field-induced octupolar (AFO) order. Here we present a combined experimental and theoretical investigation of collective excitations in the phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in rotating field is calculated within a localized approach using the pseudo-spin presentation for the Gamma8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of inter-multipolar interactions that stabilize hidden-order phases., Comment: 15 pages with 13 figures + Supplemental Material

Intra unit cell (IUC) magnetic order observed by polarized neutron diffraction (PND) is one of the hallmarks of the pseudogap state of high-temperature copper oxide superconductors. This experimental observation, usually interpreted as a result of loop currents, has been recently challenged based on lower statistics data. We here address the crucial issue of polarization inhomogeneities in the neutron beams showing that the original data had a much better reproducibilty. Within these technical limitations, we here propose a self-consistent analysis that potentially solves the controversy. We show that all the reported PND experiments in superconducting cuprates are actually compatible with the existence of an IUC magnetism., A technical discussion following our arXiv:1710.08173v3 to the manuscript arXiv:1709.06128v3 and reply arXiv:1807.06250v1

Spin-resonance modes (SRM) are taken as evidence for magnetically driven pairing in Fe-based superconductors, but their character remains poorly understood. The broadness, the splitting and the spin-space anisotropies of SRMs contrast with the mostly accepted interpretation as spin excitons. We study hole-doped Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ that displays a spin reorientation transition. This reorientation has little impact on the overall appearance of the resonance excitations with a high-energy isotropic and a low-energy anisotropic mode. However, the strength of the anisotropic low-energy mode sharply peaks at the highest doping that still exhibits magnetic ordering resulting in the strongest SRM observed in any Fe-based superconductor so far. This remarkably strong SRM is accompanied by a loss of about half of the magnetic Bragg intensity upon entering the SC phase. Anisotropic SRMs thus can allow the system to compensate for the loss of exchange energy arising from the reduced antiferromagnetic correlations within the SC state., 10 pages, 5 figures, 1 table

The magnon dispersion of ferromagnetic SrRuO_{3} was studied by inelastic neutron scattering experiments on single crystals as a function of temperature. Even at low temperature the magnon modes exhibit substantial broadening pointing to strong interaction with charge carriers. We find an anomalous temperature dependence of both the magnon gap and the magnon stiffness, which soften upon cooling in the ferromagnetic phase. Both effects trace the temperature dependence of the anomalous Hall effect and can be attributed to the impact of Weyl points, which results in the same relative renormalization in the spin stiffness and magnon gap.

Triplet pairing in Sr_{2}RuO_{4} was initially suggested based on the hypothesis of strong ferromagnetic spin fluctuations. Using polarized inelastic neutron scattering, we accurately determine the full spectrum of spin fluctuations in Sr_{2}RuO_{4}. Besides the well-studied incommensurate magnetic fluctuations, we do find a sizable quasiferromagnetic signal, quantitatively consistent with all macroscopic and microscopic probes. We use this result to address the possibility of magnetically driven triplet superconductivity in Sr_{2}RuO_{4}. We conclude that, even though the quasiferromagnetic signal is stronger and sharper than previously anticipated, spin fluctuations alone are not enough to generate a triplet state strengthening the need for additional interactions or an alternative pairing scenario.

International audience; Under-doped La-214 cuprates show a charge-and spin-modulation known as " stripes " [1]. These stripe modulations are (quasi)-static close to 1/8 hole doping where superconductivity is suppressed. The pseudo-gap phase of other cuprate compounds recently also revealed charge modulation, but interpreted rather as a charge density wave (CDW) [2, 3, 4], that possibly competes with superconductivity. In this context, to better understand the interplay between the stripe phase and the superconductivity, we use angle-resolved photoemission spectroscopy to study the electronic band structure and gap in La-214 cuprates near 1/8 doping (La 2−x−y Nd y Sr x CuO 4 (x = 0.12; y = 0.0 & 0.4)) and compare with the previous results in the same system [5] and La 1.86 Ba 0.14 CuO 4 [6]. Our data shows a loss of spectral intensity towards the end of the Fermi arcs, that is possibly due to a strong renormalisation, as already pointed out elsewhere * matteo.dastuto@neel.cnrs.fr-Institut Néel CNRS-25, av des Martyrs-38042 Grenoble cedex 9; tel: (+33)(0)4 76 88 12 84 [5], with a noisy but still measurable gap. On the nodal direction no gap is observed within our statistics, but a sizeable decrease in intensity with temperature. Moreover, we do not see any shadow band, but our Fermi surface can be well modelled with a single electron band calculation in the tight binding approximation, even very close to the 1/8 doping La 2−x−y Nd y Sr x CuO 4 with and without Nd substitution.

The low energy magnetic excitations spectra of the pristine and doped quasi-one-dimensional spin chains cuprates $\mathrm{SrCu}{\mathrm{O}}_{2}$ have been investigated by inelastic neutron scattering. The momentum-integrated magnetic dynamical structure factor yields a constant integrated intensity with regard to energy in the pure compound, while it shows a strong decay, at low energies, in the compounds doped with nonmagnetic impurities, namely, $\mathrm{SrC}{\mathrm{u}}_{0.99}{\mathrm{M}}_{0.01}{\mathrm{O}}_{2}$ (with $M=\mathrm{Zn}$ or Mg) and $\mathrm{S}{\mathrm{r}}_{0.99}\mathrm{L}{\mathrm{a}}_{0.01}\mathrm{Cu}{\mathrm{O}}_{2}$ ($\mathrm{C}{\mathrm{u}}^{+}$ carrying $S=0$ created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of $\mathrm{SrCu}{\mathrm{O}}_{2}$ upon doping, stemming from disruptions of the spin chains by quantum impurities.

The magnon dispersion of ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ has been studied by polarized and unpolarized neutron scattering experiments on crystals containing 0, 1, and 10% of Ti. Ti is inserted in order to enable the growth of large, partially detwinned crystals. One percent of Ti has a negligible impact on structural and magnetic properties. Also for 10% Ti content magnetic properties still change very little, but the insulating phase is stabilized up to at least 700 K and structural distortions are reduced. The full dispersion of transverse magnons studied for 1% Ti substitution can be well described by a conventional spin-wave model with interaction and anisotropy parameters that agree with density functional theory calculations. Spin-orbit coupling strongly influences the magnetic excitations, as it is most visible in large energies of the magnetic zone-center modes arising from magnetic anisotropy. Additional modes appear at low energy near the antiferromagnetic zone center and can be explained by a sizable magnetic moment of 0.11 Bohr magnetons, which the density functional theory calculations find located on the apical oxygens. The energy and the signal strength of the additional branch are well described by taking into account this oxygen moment with weak ferromagnetic coupling to the Ru moments.

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}.

We use neutron scattering to study magnetic excitations near the antiferromagnetic wave vector in the underdoped single-layer cuprate HgBa_{2}CuO_{4+δ} (superconducting transition temperature T_{c}≈88 K, pseudogap temperature T^{*}≈220 K). The response is distinctly enhanced below T^{*} and exhibits a Y-shaped dispersion in the pseudogap state, whereas the superconducting state features an X-shaped (hourglass) dispersion and a further resonancelike enhancement. A large spin gap of about 40 meV is observed in both states. This phenomenology is reminiscent of that exhibited by bilayer cuprates. The resonance spectral weight, irrespective of doping and compound, scales linearly with the putative binding energy of a spin exciton described by an itinerant-spin formalism.

Layered 5d transition iridium oxides, Sr2(Ir,Rh)O4, are described as unconventional Mott insulators with strong spin-orbit coupling. The undoped compound, Sr2IrO4, is a nearly ideal two-dimensional pseudospin-1/2 Heisenberg antiferromagnet, similarly to the insulating parent compound of high-temperature superconducting copper oxides. Using polarized neutron diffraction, we here report a hidden magnetic order in pure and doped Sr2(Ir,Rh)O4, distinct from the usual antiferromagnetic pseudospin ordering. We find that time-reversal symmetry is broken while the lattice translation invariance is preserved in the hidden order phase. The onset temperature matches that of the odd-parity hidden order recently highlighted using optical second-harmonic generation experiments. The novel magnetic order and broken symmetries can be explained by the loop-current model, previously predicted for the copper oxide superconductors., Exploring ordered states in similarly structured oxides aids the understanding of structure-property correlations. Here the authors discover an unconventional magnetic order in Sr2(Ir,Rh)O4, which is analogous to a loop-current phase in cuprate superconductors with the same lattice structure.

Within the complex phase diagram of the hole-doped cuprates, seizing the nature of the mysterious pseudogap phase is essential for unraveling the microscopic origin of high-temperature superconductivity. Below the pseudogap temperature T^{⋆}, evidence for intra-unit-cell orders breaking the fourfold rotation symmetry have been provided by neutron diffraction and scanning tunneling spectroscopy. Using polarized neutron diffraction on a detwinned YBa_{2}Cu_{3}O_{6.6} sample, we here report a distinct a-b anisotropy of the intra-unit-cell magnetic structure factor below T^{⋆}, highlighting that intra-unit-cell order in this material breaks the mirror symmetry of the CuO_{2} bilayers. This is likely to originate from a crisscrossed arrangement of loop currents within the CuO_{2} bilayer, resulting in a bilayer mean toroidal axis along the b direction.

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Ds), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5, Comment: 20 pages, 8 figures

Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa2CuO4+δ. Here we report neutron-scattering results for HgBa2CuO4+δ (superconducting transition temperature Tc≈71 K, pseudogap temperature T*≈305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped ‘hourglass' response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped and significantly enhanced below T*, and hence a prominent signature of the pseudogap state., In the cuprates, antiferromagnetic correlations might be the cause of the pseudogap phenomenon. Here the authors use neutron scattering on the tetragonal cuprate HgBa2CuO4+δ revealing commensurate antiferromagnetic excitations as a signature of the pseudogap state.

Alors que la supraconductivite atteint, en termes de temperature critique, des sommets largement superieurs a la temperature d’ebullition de l’azote, le mecanisme de la supraconduction des materiaux a base d’oxydes de cuivre reste un mystere, vingt-cinq ans apres leur decouverte. Leur structure bidimensionnelle et la presence de fortes correlations electroniques sont les indices d’une supraconductivite non conventionnelle.La cle pourrait se cacher dans la nature inattendue de l’etat metallique prefigurant cette supraconductivite.

In this paper, we first give a concise overview of recent experimental and theoretical work dealing with “electronic liquid-crystal states” which spontaneously break different symmetries of the CuO2 layers of high-T c cuprates, with an emphasis on evidence in the spin excitation spectrum. Then we describe the importance of using twin-free samples to look for evidence for fourfold symmetry breaking in the spectrum and explain the preparation procedure to obtain such samples. We present inelastic neutron scattering results for moderately underdoped YBa2Cu3O6.6(T c = 61 K) and nearly optimally doped YBa2Cu3O6.85(T c = 89 K). In YBa2Cu3O6.6, the dispersion topology changes when heating above T c from an hourglass shape with constricted, commensurate resonance peak to a “Y”-shape without resonance anomaly. This change, and the fact that the low-energy signal above T c can be described by an incommensurate, quasi-one-dimensional distribution, indicates a competition of superconductivity with an electronic liquid-crystal state. We then show a striking analogy between the difference signal I(5 K) − I(70 K) and the downward dispersing resonance mode in YBa2Cu3O6.85. We therefore argue that a resonance mode only emerges below T c, irrespective of the doping level. We finally discuss the implications of our results for the different scenarios invoked to explain the electronic liquid-crystal state in cuprates.

In order to study superconducting physical properties by neutron scattering technique, the necessity to obtain large and high quality optimally doped Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) and underdoped Bi-2212 single crystals is prominent. Three routes have been investigated to reach this goal: synthesis under low oxygen pressure by travelling solvent floating zone technique (TSFZ), TSFZ synthesis of yttrium doped Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8+ δ and TSFZ synthesis under air followed by post-annealing treatments. With decreasing p( O 2 ) from ambient pressure to 0.1 atm during the TSFZ growth, the superconducting transition temperature, T c , transits from 87 K (slightly overdoped) to 93 K. On yttrium doped growth, a systematic study of Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8+δ , where x =0–0.5, was undertaken to correlate the nominal yttrium composition with the yttrium crystal composition and to control the T c as a function of yttrium composition. 85 K underdoped Bi-2212 crystals with a size up to 220 mm 3 and a 5 K transition width, Δ T c , were obtained by post-annealing under p( O 2 ) =0.1 atm at 450 °C.

Quasicrystals are long-range-ordered materials that lack translational invariance, so the study of their physical properties remains a challenging problem. Here, we have carried out inelastic-X-ray- and neutron-scattering experiments on single-grain samples of the Zn-Mg-Sc icosahedral quasicrystal and of the Zn-Sc periodic cubic 1/1 approximant, with the aim of studying the respective influence of the local order and of the long-range order (periodic or quasiperiodic) on lattice dynamics. Besides the overall similarities and the existence of a pseudo-gap in the transverse dispersion relation, marked differences are observed, the pseudo-gap being larger and better defined in the approximant than in the quasicrystal. This can be qualitatively explained using the concept of a pseudo-Brillouin-zone in the quasicrystal. These results are compared with simulations on atomic models and using oscillating pair potentials, and the simulations reproduce in detail the experimental results. This paves the way for a detailed understanding of the physics of quasicrystals.

A way to address the problem of phonons in quasicrystals (QCs) is to compare their dynamical response with that of approximant crystalline structures. Although the Zn2Mg Laves phase is not a periodic approximant of a QC phase, its structure is a periodic packing of Friauf polyhedra which are basic units involved in the construction of larger icosahedral atomic clusters found in Frank–Kasper type quasicrystals. We report on the experimental study of the lattice dynamics of the Zn2Mg hexagonal phase using inelastic neutron scattering with a particular attention devoted to the behavior of transverse acoustic (TA) modes. For TA modes propagating along the (T) direction, polarized along the c axis, there is a strong bending of the dispersion curve. Whereas the broadening rate is rather slow, going like q2, a strong coupling of the acoustic mode with an higher energy optical mode takes place. For TA modes propagating along the (Δ) direction, polarized in the hexagonal plane, the dispersion relation reaches much higher energy: the broadening rate is however steeper, going like q4, most likely due to a mixing of several excitations in the spectral response. In any case, the width of acoustic and optical excitations is found much smaller than in quasicrystals.

Inelastic neutron scattering is a powerful technique that can measure magnetic correlations in a large momentum and energy range. In strongly correlated electronic systems, where spin, orbital, lattice and charge degrees of freedom are entangled, it is currently used to study the magnetic properties and shed light on their role in the appearance of the exotic electronic properties, such as unconventional superconductivity. In this article, we focus on the observation by inelastic neutron scattering technique of unconventional spin triplet collective modes in the superconducting state of high temperature superconducting cuprates and its interplay with anomalies in the charge excitation spectrum. The triplet spin mode is interpreted as a spin exciton , within a spin band model. Alternative scenarii based on localized or dual (itinerant localized) models are also mentioned. To cite this article: Y. Sidis et al., C. R. Physique 8 (2007).

Understanding high-temperature superconductivity requires a prior knowledge of the nature of the enigmatic pseudogap metallic state, out of which the superconducting state condenses. In addition to the electronic orders involving charge degrees of freedoms recently reported inside the pseudogap state, a magnetic intra-unit-cell (IUC) order was discovered in various cuprates to set in just at the pseudogap temperature, T*. In nearly optimally doped YBa$_2$Cu$_3$O$_{6.85}$, polarized neutron scattering measurements, carried out on two different spectrometers, reveal new features. The order is made of finite size planar domains, hardly correlated along the c-axis. At high temperature, only the out-of-plane magnetic components correlate, revealing a strong Ising anistropy, as originally predicted in the loop current model. Below T*, a correlated in-plane response develops, giving rise the apparent tilt of the magnetic moment at low temperature. The discovery of these two regimes put stringent constraints on the intrinsict nature of IUC order, tightly bound to the pseudogap physics., Comment: 3 figures + supplementary information

By comprehensive neutron diffraction measurements we have studied the magnetic structure of aegirine $({\mathrm{NaFeSi}}_{2}{\mathrm{O}}_{6})$ in and above its multiferroic phase. Natural aegirine exhibits two magnetic transitions into incommensurate magnetic order with a propagation vector of ${\stackrel{P\vec}{k}}_{\mathrm{inc}}=(0,\ensuremath{\sim}0.78,0)$. Between 9 and 6 K, we find a transverse spin-density wave with moments pointing near the $c$ direction. Below 6 K, magnetic order becomes helical and spins rotate in the $ac$ plane. The same irreducible representation is involved in the two successive transitions. In addition, the ferroelectric polarization $\stackrel{P\vec}{P}$ appearing along the $b$ direction cannot be described by the most common multiferroic mechanism but follows $\stackrel{P\vec}{P}\ensuremath{\propto}{\stackrel{P\vec}{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\stackrel{P\vec}{S}}_{j}$. Synthetic ${\mathrm{NaFeSi}}_{2}{\mathrm{O}}_{6}$ does not exhibit the pure incommensurate helical order but shows coexistence of this order with a commensurate magnetic structure. By applying moderate pressure to natural aegirine, we find that the incommensurate magnetic ordering partially transforms to the commensurate one, underlining the nearly degenerate character of the two types of order in ${\mathrm{NaFeSi}}_{2}{\mathrm{O}}_{6}$.

We present a comprehensive study on ${\mathrm{PrCaFeO}}_{4}$ using macroscopic methods, neutron and x-ray diffraction, as well as inelastic neutron scattering. One polycrystalline and two single-crystal samples were investigated exhibiting structural phase transitions from a high-temperature tetragonal phase to an intermediate orthorhombic phase (space group $Bmeb$) at $510{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ (783 K). At approximately 240 K a second structural phase transition takes place into the space group $Pccn$ where the tilt axis of the ${\mathrm{FeO}}_{6}$ octahedra changes from the [100] to the $\ensuremath{\langle}110\ensuremath{\rangle}$ directions. Due to strong diffuse scattering at high temperatures neutron powder diffraction can only safely state that ${T}_{N}$ is above 330 K. ${\mathrm{PrCaFeO}}_{4}$ exhibits a magnetic spin-flop phase transition where the magnetic moments turn from the $b$ axis to the $c$ axis upon cooling. However, the transition temperatures and the width of this magnetic transition are strikingly different between the investigated samples, suggesting a strong influence from the real structure. Indeed, a significant difference in the oxygen content was deduced by single-crystal x-ray diffraction. The magnon dispersion was studied by inelastic neutron scattering revealing a nearest-neighbor interaction comparable to that in ${\mathrm{LaSrFeO}}_{4}$ but with smaller anisotropy gaps. A clear interlayer dispersion was observed resulting from the structural distortions and the relief of geometrical frustration due to the orthorhombic splitting.

Inelastic neutron scattering investigations on optimally doped YBCO revealed a very pronounced temperature dependence of the Cu–O in-plane bond-stretching vibrations along the (0 1 0)-direction: a downward shift of spectral weight with decreasing temperature by at least 10 meV in a narrow range of wave vectors halfway to the zone boundary. The temperature evolution starts around 200 K, well above the superconducting transition temperature. This phonon anomaly provides strong evidence for large electron–phonon coupling. It also indicates an incipient charge density wave instability within the CuO2 planes reminiscent of dynamical charge stripes. The magnetic fluctuations have been investigated in great detail on the same sample. Incommensurate spin fluctuations have been observed for energies both below and above the energy of the resonance peak at E=41 meV. However, the dispersive nature of these fluctuations as well as their apparent isotropy in the basal plane speak against an interpretation of the spin fluctuation spectrum in the framework of the classical stripe phase picture.

Inelastic neutron scattering techniques have been used to study the field-induced multiferroic transition and the temperature dependence of magnetic excitations in ${\text{TbMnO}}_{3}$. 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 $\mathbf{Q}$ 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 ${\text{TbMnO}}_{3}$.

We present inelastic neutron scattering data on optimally doped Tl2Ba2CuO6+δ (T c =92 K ), a single-layer copper oxide with undistorted copper oxide planes. The data indicate a magnetic resonant mode below Tc, as previously observed in YBa2Cu3O6+δ and Bi2Sr2CaCu2O8+δ. The mode is thus a generic feature of the copper oxide superconductors.

We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at $\mathbf{Q}=(0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},{q}_{l})$. Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with a longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor of 8 weaker than the resonance signal in nearly optimally Co-doped ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, although a factor of 2 is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin-space anisotropy with a slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.

As in ${{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}\mathrm{O}}_{6+x}$ and ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{8+\ensuremath{\delta}}$, the pseudogap state in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ is characterized by the existence of an intra-unit-cell magnetic order revealed by polarized neutron scattering technique. We report here a supplementary set of polarized neutron scattering measurements for which the direction of the magnetic moment is determined and the magnetic intensity is calibrated in absolute units. These data allow a close comparison between bilayer systems ${{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}\mathrm{O}}_{6+x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and raise important questions concerning the range of the magnetic correlations and the role of disorder around optimal doping.

Commensurate antiferromagnetic ordering has been observed in the superconducting high-$Tc$ cuprate ${\rm YBa_{2}Cu_{3}O_{6.5}}$ (${\rm T_{c}}$=55 K) by polarized and unpolarized elastic neutron scattering. The magnetic peak intensity exhibits a marked enhancement at $T_{c}$. Zero-field $\mu $SR experiments demonstrate that the staggered magnetization is not truly static but fluctuates on a nanosecond time scale. These results point towards an unusual spin density wave state coexisting with superconductivity.

We review recent results of neutron scattering studies of the YBa 2 Cu 3 O 6+ x and Bi 2 Sr 2 CaCu 2 O 8+δ high temperature superconductors, with a focus on the magnetic resonance peak that dominates the spin excitation spectrum in these two compounds.

The influence of magnetic ( $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$) and nonmagnetic ( $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{0.97}{\mathrm{Ni}}_{0.03}{)}_{3}{\mathrm{O}}_{7}$ ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}80\mathrm{K}$) and ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{0.99}{\mathrm{Zn}}_{0.01}{)}_{3}{\mathrm{O}}_{7}$ ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}78\mathrm{K}$). In the Ni-substituted system, the magnetic resonance peak (which is observed at ${E}_{r}\ensuremath{\simeq}40\mathrm{meV}$ in the pure system) shifts to lower energy with a preserved ${E}_{r}/{T}_{c}$ ratio while the shift is much smaller upon Zn substitution. By contrast Zn, but not Ni, restores significant spin fluctuations around 40 meV in the normal state. These observations are discussed in the light of models proposed for the magnetic resonance peak.

We report a comprehensive polarized and unpolarized neutron scattering study of the evolution of the dynamical spin susceptibility with temperature and doping in three underdoped single crystals of the \YBCO{6+x} high temperature superconductor: \YBCO{6.5} (Tc = 52 K), \YBCO{6.7} (Tc = 67 K), and \YBCO{6.85} (T_c = 87 K). Theoretical implications of these data are discussed, and a critique of recent attempts to relate the spin excitations to the thermodynamics of high temperature superconductors is given.

A critical examination of the spin dynamics in high-T c cuprates is made in the light of recent inelastic neutron scattering results obtained by different groups. The neutron data show that incommensurate magnetic peaks in YBCO belong to the same excitation as the resonance peak observed at (π/a, π/a). Being observed only in the superconducting state, the incommensurability is rather difficult to reconcile with a stripe picture. We also discuss the link between the resonance peak spectral weight and the superconducting condensation energy.

We report first inelastic neutron scattering measurements in the normal state of Sr_2RuO_4 that reveal the existence of incommensurate magnetic spin fluctuations located at ${\bf q}_0=(\pm 0.6\pi/a, \pm 0.6\pi/a, 0)$. This finding confirms recent band structure calculations that have predicted incommensurate magnetic responses related to dynamical nesting properties of its Fermi surface.