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In the first part of this paper the results of X-ray and neutron diffraction studies of the structural aspects related to oxygen insertion in the CuOx planes of YBa2Cu3O6+x compounds are presented. The second part is devoted to the inelastic neutron scattering study of the spin dynamics in the three regimes (weakly doped, heavily doped, and overdoped) of the metallic phases of YBa2Cu3O6+x.

Inelastic-neutron-scattering measurements have been performed to determine the imaginary part of the dynamical susceptibility, \ensuremath{\chi}\ensuremath{''}(Q,\ensuremath{\omega}), of a ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Zn}}_{\mathit{y}}$${)}_{3}$${\mathrm{O}}_{6.97}$ sample exhibiting a superconducting transition at ${\mathit{T}}_{\mathit{c}}$=69 K. Zinc substitution induces striking modifications of the energy dependence of \ensuremath{\chi}\ensuremath{''}(Q,\ensuremath{\omega}) but magnetic fluctuations remain peaked at the antiferromagnetic wave vector, ${\mathit{Q}}_{\mathrm{AF}}$, at all investigated energies. In the superconducting state of the zinc-free compound, \ensuremath{\chi}\ensuremath{''}(Q,\ensuremath{\omega}) is restricted to a narrow energy range, \ensuremath{\Elzxh}\ensuremath{\omega}=33--47 meV, displaying a spin gap at ${\mathit{E}}_{\mathit{G}}$=33 meV and a resonant enhancement at ${\mathit{E}}_{\mathrm{\ensuremath{\tau}}}$=39 meV, both features vanishing upon heating up above ${\mathit{T}}_{\mathit{c}}$. In the y=0.02 substituted sample in the superconducting state, there is still an energy band in the range 32--47 meV but no clear resonance, and a signal is now observed in the low energy range, though the line shape of \ensuremath{\chi}\ensuremath{''}(Q,\ensuremath{\omega}) indicates some reminiscence of the spin gap of the pure compound. \textcopyright{} 1996 The American Physical Society.

Inelastic-neutron-scattering experiments have been performed to study the spin-excitation spectrum in the overdoped regime of the high-${\mathit{T}}_{\mathit{c}}$ superconducting system ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$, x=0.97. All the observed magnetic fluctuations are peaked at the antiferromagnetic wave vector. In the superconducting state, the magnetic response is restricted over a limited energy range (\ensuremath{\Elzxh}\ensuremath{\omega}=33\char21{}46 meV). The imaginary part of the magnetic susceptibility is characterized by two contributions defined by different q widths and different temperature dependences. A resonant contribution, which displays a narrower q width, appears to be connected with superconductivity. The two contributions can be accounted for in the framework of the t-t\ensuremath{'}-J model in the presence of superconductivity. The main new feature in the overdoped regime is the partial disappearance of magnetic correlations for temperatures above the superconducting temperature. \textcopyright{} 1996 The American Physical Society.

The spin dynamics in single crystals of YBa 2 Cu 3 O 6 + x has been successfully investigated, by inelastic neutron scattering (INS) experiments, as a function of temperature in the metallic state over the whole doping range from the weakly doped ( x = 0.45, 0.51) to the heavily doped ( x = 0.69, 0.83, 0.92) and the overdoped ( x = 0.97, 1) regimes. Dynamical AF-correlations persist in all the metallic state. Actually, the imaginary part of the magnetic susceptibility χ″ consists of two contributions which have different doping and temperature dependences. At low temperature, χ″ exhibits an energy gap in any superconducting samples which becomes much weaker close to the insulating-metallic transition. To emphasize the characteristic features of the spin dynamics in YBCO, INS results obtained by other groups are compared with our experiments. Several theoretical approaches, which intend to describe the energy line shape of the dynamical magnetic susceptibility, are also discussed.

Several hole-doped BaFe2As2 compounds were recently shown to exhibit a second magnetic phase transition in the concentration range close to the full suppression of antiferromagnetic (AFM) order. At this additional transition ordered magnetic moments reorient from in-plane to out-of-plane alignment associated with a suppression of the orthorhombic distortion. We have studied the magnetic properties of such a representative hole-doped system, Ba1–xNaxFe2As2 with 0.25 ≤ x ≤ 0.40, by neutron diffraction on large single crystals. With increasing Na substitution (0.25 ≤ x ≤ 0.39) the AFM transition temperature sharply decreases, while the spin-reorientation transition temperature is rather constant, until both magnetic phases are completely suppressed at x = 0.40. For all studied Na concentrations the additional transition is related to the spin reorientation, which, however, is complete only in the middle of the concentration range of the out-of-plane phase. In the superconducting state, the intensities of magnetic Bragg reflections become heavily suppressed; this effect seems to increase for larger Tc and reaches ∼50% in Ba0.61Na0.39Fe2As2. In samples with coexisting in-plane and out-of-plane ordering, this superconductivity induced suppression of ordered moments is significantly stronger for the out-of-plane components indicating that this phase more strongly competes with superconductivity.

The spin dynamics of CuO2-layers in the high-Tc superconducting system YBa2Cu3O6+x has been extensively investigated by inelastic neutron scattering on single crystals. In this paper we review the most important results obtained on the evolution of magnetic excitation spectra from both sides of Tc.

There exists increasing evidence that the phase diagram of the high-transition temperature (Tc) cuprate superconductors is controlled by a quantum critical point. One distinct theoretical proposal is that, with decreasing hole-carrier concentration, a transition occurs to an ordered state with two circulating orbital currents per CuO2 square. Below the 'pseudogap' temperature T* (T* > Tc), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds that should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa2CuO4+d the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points toward a unifying picture of magnetism in the cuprates that will likely require a multi-band description., Including supplementary information

Polarized elastic neutron scattering measurements have been performed in the bilayer copper oxide system ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$, providing evidence for an intra-unit-cell magnetic order inside the pseudogap state. That shows time reversal symmetry breaking in that state as already reported in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ through dichroism in circularly polarized photoemission experiments. The magnetic order displays the same characteristic features as the one previously reported for monolayer ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ and bilayer ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+\mathit{x}}$, demonstrating that this genuine phase is ubiquitous of the pseudogap of high temperature copper oxide materials.

The incommensurate stripelike magnetic ordering in two single-layered manganites, Nd0.33Sr1.67MnO4 and Pr0.33Ca1.67MnO4, is found to exhibit an hourglasslike excitation spectrum very similar to that seen in various cuprates superconductors, but only for sufficiently short correlation lengths. Several characteristic features of an hourglass dispersion can be identified: enhancement of intensity at the merging of the incommensurate branches, rotation of the intensity maxima with higher energy transfer, and suppression of the outward-dispersing branches at low energy. The correlation length of the magnetic ordering and the large ratio of intra- to interstripe couplings are identified as the decisive parameters causing the hourglass shape of the spectrum.

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector ($0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},0$) with $\ensuremath{\delta}\ensuremath{\sim}0.07$ slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

The spin dynamics of CuO 2 -layers in the high-T c superconducting system YBa 2 Cu 3 O 6+x has been extensively investigated by inelastic neutron scattering on single crystals. In this paper we will review the most important results recently obtained on the evolution of magnetic excitation spectra in the weakly, heavily and overdoped regimes, from both sides of T c .

An unusual spin excitation mode observed by neutron scattering has inspired numerous theoretical studies of the interplay between charged quasiparticles and collective spin excitations in the copper oxide high temperature superconductors. The mode has thus far only been observed in materials with crystal structures consisting of copper oxide bilayers, and it is notably absent in the single-layer compound La$_{2-x}$Sr$_{x}$CuO$_{4+\delta}$. Neutron scattering data now show that the mode is present in Tl$_2$Ba$_2$Cu$_{6+\delta}$, a single-layer compound with T$_c$ $\sim$ 90 K, thus demonstrating that it is a generic feature of the copper oxide superconductors, independent of the layer sequence. This restricts the theoretical models for the origin of the resonant mode and its role in the mechanism of high temperature superconductivity., Comment: Science, in press

Spin-polarized neutron diffraction experiments have revealed an unusual $q=0$ magnetic order in the model high-temperature superconductor HgBa${}_{2}$CuO${}_{4+\ensuremath{\delta}}$ (Hg1201) below the pseudogap temperature ${T}^{*}$ [Y. Li et al., Nature (London) 455, 372 (2008)]. Together with results for the structurally more complex compound YBa${}_{2}$Cu${}_{3}$O${}_{6+\ensuremath{\delta}}$ (YBCO) [B. Fauqu\'e et al., Phys. Rev. Lett. 96, 197001 (2006); H.A. Mook et al., Phys. Rev. B 78, 020506 (2008)], this establishes the universal existence of a genuine novel magnetic phase in underdoped cuprates with high maximal ${T}_{\mathrm{c}}$ (above 90 K at optimal doping). Here we report a systematic study of an underdoped Hg1201 sample (${T}_{\mathrm{c}}=75$ K), the result of which is consistent with the previously established doping dependence of the magnetic signal. We present an assumption-free analysis of all the data available for Hg1201. Depending on how the hole concentration is estimated, comparison with the results for YBCO leads to different scenarios for the competition between the $q=0$ magnetic order and the spin-density-wave order found in heavily underdoped YBCO.

We report a polarized neutron-scattering study of the orbital-like magnetic order in strongly underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.45}$ and ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{0.98}{\mathrm{Zn}}_{0.02}){}_{3}{\mathrm{O}}_{6.6}$. Their hole-doping levels are located on both sides of the critical doping ${p}_{\mathrm{MI}}$ of a metal-insulator transition inferred from transport measurements. Our study reveals a drop down of the orbital-like order slightly below ${p}_{\mathrm{MI}}$ with a steep decrease of both the ordering temperature ${T}_{\mathrm{mag}}$ and the ordered moment. Above ${p}_{\mathrm{MI}}$, substitution of quantum impurities does not change ${T}_{\mathrm{mag}}$, whereas it lowers significantly the bulk ordered moment. The modifications of the orbital-like magnetic order are interpreted in terms of a competition with electronic liquid crystal phases around ${p}_{\mathrm{MI}}$. This competition gives rise to a mixed magnetic state in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.45}$ and a phase separation in ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{0.98}{\mathrm{Zn}}_{0.02}){}_{3}{\mathrm{O}}_{6.6}$.

By inelastic neutron scattering, we have analyzed the magnetic correlations in the paramagnetic metallic region of the series Ca${}_{2\ensuremath{-}x}$Sr${}_{x}$RuO${}_{4}$, $0.2\ensuremath{\leqslant}x\ensuremath{\leqslant}0.62$. We find different contributions that correspond to two-dimensional ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors ${Q}_{1}^{\mathrm{IC}}=(0.11,0,0)$, ${Q}_{2}^{\mathrm{IC}}=(0.26,0,0)$, and ${Q}_{\ensuremath{\alpha}\ensuremath{\beta}}^{\mathrm{IC}}=(0.3,0.3,0)$. 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 Ca${}_{2\ensuremath{-}x}$Sr${}_{x}$RuO${}_{4}$ 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 Ca${}_{1.38}$Sr${}_{0.62}$RuO${}_{4}$ with a low characteristic energy of 0.2 meV at $T=1.5$ K, they are strongly suppressed in Ca${}_{1.8}$Sr${}_{0.2}$RuO${}_{4}$, 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.

The neutron scattering from antiferromagnetic γ-manganese–copper alloys has a relatively sharp diffuse feature at the (0, 0, 1) reciprocal lattice point. The scattering near this point, in a crystal of Mn 90 Cu 10 , has been energy analysed and found to have a significant and temperature-dependent width. The shape of the peak around zero energy is complex but shows similarities with the peaks seen in spin glasses. We have analysed the peak as a sum of two Lorentzians each convoluted with the instrumental resolution. The narrower Lorentzian has a width indistinguishable from zero at 30 K but at 150 K it is almost comparable, at 0.15 THz, with the instrumental resolution. The other Lorentzian is almost 10 times wider and its width also increases with temperature, but from a finite value at 30 K. The widths of the peaks appear to be independent of scattering vector close to the (0, 0, 1) where the scattering is large. At the (0, 0, 1) position only moment components perpendicular to the antiferromagnetic direction scatter. We therefore conclude that we are seeing the dynamical slowing down of the transverse components of magnetic defects which we identify with the freezing of transverse components of the moment observed in low field susceptibility at about 50 K.

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the 42/m screw symmetry around the (1/2 1/2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode Er, as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship Er=4.3*kB*Tc holds for all studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates., Comment: To be published in Phys. Rev. B. 18 pages, 14 figures, including one interactive figure. References updated and supplied with hyperlinks (v3); Fig. 3 added with an interactive comparison of different Brillouin zones, introduction extended (v2)

We present diffraction data on a single-layered manganite La(0.42)Sr(1.58)MnO4 with hole doping (x>0.5). Overdoped La(0.42)Sr(1.58)MnO4 exhibits a complex ordering of charges, orbitals and spins. Single crystal neutron diffraction experiments reveal three incommensurate and one commensurate order parameters to be tightly coupled. The position and the shape of the distinct superstructure scattering points to a stripe arrangement in which ferromagnetic zigzag chains are disrupted by additional Mn4+ stripes.

We report an inelastic-neutron-scattering and muon-spin-relaxation study of the effect of 2% spinless (Zn) impurities on the magnetic order and dynamics of YBa(2)Cu(3)O(6.6), an underdoped high-temperature superconductor that exhibits a prominent spin pseudogap in its normal state. Zn substitution induces static magnetic order at low temperatures and triggers a large-scale spectral-weight redistribution from the magnetic resonant mode at 38 meV into uniaxial, incommensurate spin excitations with energies well below the spin pseudogap. These observations indicate a competition between incommensurate magnetic order and superconductivity close to a quantum critical point. Comparison to prior data on La(2-x)Sr(x)CuO(4) suggests that this behavior is universal for the layered copper oxides and analogous to impurity-induced magnetic order in one-dimensional quantum magnets.

A neutron scattering study reveals that the magnetic fluctuations in an iron arsenide superconductor behave according to the conventional theories of metals, unlike the cuprate superconductors. Moreover, the magnetic spin-excitation energies are sufficient to mediate the Cooper pairs that form the superconducting state.

The pseudogap phenomenon, a discontinuity in the energy level of a material's electronic spectrum, is a universal characteristic of the high transition temperature (Tc) copper oxides. The nature of the pseudogap has been a central question in condensed matter physics for more than a decade, but many of its properties remain unexplained. Recent studies have pointed to the universal existence of an unusual magnetic order below T*, the temperature below which the anomalous properties associated with the pseudogap become apparent. If confirmed, this would have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. The results of inelastic neutron scattering experiments on the superconductor HgBa2CuO4+δ (Hg1201) now reveal a fundamental collective magnetic mode associated with the unusual order, providing further support for this picture. Recent findings indicate that the pseudogap regime in the high-transition-temperature copper oxides constitutes a new phase of matter rather than a mere crossover phenomenon. These authors report inelastic neutron scattering results for HgBa2CuO4+δ that reveal a fundamental collective magnetic mode associated with the unusual order, further supporting this picture. The mode's intensity rises below the pseudogap characteristic temperature and its dispersion is weak. The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-Tc) copper oxides—a set of anomalous physical properties below the characteristic temperature T* and above Tc—has been a major challenge in condensed matter physics for the past two decades1. Following initial indications of broken time-reversal symmetry in photoemission experiments2, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T* (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point5. Here we report inelastic neutron scattering results for HgBa2CuO4+δ that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode’s intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter6. Its energy of 52–56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron–boson coupling features observed in spectroscopic studies7,8,9,10.

We present a neutron-scattering study of the static and dynamic spin correlations in the underdoped high-temperature superconductor YBa2Cu3O6.45 in magnetic fields up to 15 T. The field strongly enhances static incommensurate magnetic order at low temperatures and induces a spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of the Fermi surface driven by the field-enhanced magnetic superstructure may thus be responsible for the unusual Fermi surface topology revealed by recent quantum-oscillation experiments.

Magnetic correlations in Ti-substituted ${\text{Sr}}_{3}{({\text{Ru}}_{1\ensuremath{-}x}{\text{Ti}}_{x})}_{2}{\text{O}}_{7}$ have been studied by elastic and inelastic neutron-scattering techniques. Below transition temperatures of the order of 20 K, the samples with 7.5 and 10% of Ti content exhibit an incommensurate spin-density wave ordering with a propagation vector ${\mathbf{q}}_{\mathbf{i}\mathbf{c}}=(0.24,0.24,0)$, which does not reflect the dominant magnetic instabilities in pure ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$. Strong inelastic correlations near ${\mathbf{q}}_{\mathbf{i}\mathbf{c}}$ were found to persist in ${\text{Sr}}_{3}{({\text{Ru}}_{1\ensuremath{-}x}{\text{Ti}}_{x})}_{2}{\text{O}}_{7}$ far above the magnetic ordering; they become gapped in the ordered state.

Spin correlations in La2-xSrxCoO4 (0.3 0.3 exhibits incommensurate spin ordering with the modulation closely following the amount of doping. These incommensurate phases strongly resemble the stripe phases observed in cuprates and nickelates, but incommensurate magnetic ordering appears only at larger Sr content in the cobaltates due to a reduced charge mobility.

International audience; Quasicrystals are long range ordered materials which lack translational invariance so that the study of their physical properties remains a challenging problem. In order to study the respective influence of the local order and of the long range order (periodic or quasiperiodic) on lattice dynamics, 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. 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 pseudo Brillouin zone in the quasicrystal. These results are compared to simulations on atomic models and using oscillating pair potentials which have been fitted against ab-initio data. The simulated response function reproduces both the dispersion relation but also the observed intensity distribution in the measured spectra. The partial vibrational density of states, projected on the cluster shells, is computed from this model.

We have studied incommensurate spin ordering in single-crystal underdoped ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ with $x\ensuremath{\sim}0.08$, 0.05, and 0.025 using neutron-scattering techniques. Static incommensurate magnetic order is observed in the ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ ($x=0.05$ and 0.025) compounds with ordering wave vectors which are rotated by $45\ifmmode^\circ\else\textdegree\fi{}$ about the commensurate (0.5,0.5,0) position, with respect to that in the superconducting $x=0.08$ material. These spin modulations are one dimensional in the $x=0.05$ and 0.025 samples, with ordering wave vectors lying along the orthorhombic ${b}^{\ensuremath{\ast}}$ direction. Such a rotation in the orientation of the static spin ordering as a function of increasing Ba doping, from diagonal to collinear, is roughly coincident with the transition from an insulating to a superconducting ground state and is similar to that observed in the related ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ system. This phenomenon is therefore a more generic property of underdoped La-214 cuprates.

Polarized beam neutron-scattering measurements on a highly perfect crystal of ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.6}$ show a distinct magnetic transition to a different phase with an onset at about 235 K, the temperature expected for the pseudogap transition. The moment is found to be about $0.1{\ensuremath{\mu}}_{B}$ for each sublattice and has a correlation length of at least $75\text{ }\text{\AA{}}$. We found the critical exponent for the magnetic neutron intensity to be $2\ensuremath{\beta}=0.37\ifmmode\pm\else\textpm\fi{}0.12$. This is the proper range for the class of transition that has no specific-heat divergence possibly explaining why none is found at the pseudogap transition.

The magnetic correlations in the charge and orbital ordered manganite ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined charge exchange (CE)-type magnetic structure with the corresponding magnons, a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ exhibits a dispersion of ferromagnetic correlations, which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high transition-temperature superconductivity. Li et al. report polarized neutron diffraction data that demonstrate that the characteristic temperature, T*, marks the onset of an unusual magnetic order, and hence a novel state of matter with broken time-reversal symmetry, for the model superconductor HgBa2CuO4+δ (Hg1201). The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with the notion that many of the unusual properties arise from the presence of a quantum-critical point. The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-Tc) superconductivity, characterized by anomalous physical properties1,2. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome3,4,5. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa2Cu3O6+δ (YBCO) resulted in contradictory claims of no6,7 and weak8,9 magnetic order, and the interpretation of muon spin relaxation measurements on YBCO10,11 and of circularly polarized photoemission experiments on Bi2Sr2CaCu2O8+δ(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa2CuO4+δ (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO14,15, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature16,17,18 rather than a phase transition temperature19,20,21. Instead, they are consistent with a variant of previously proposed charge-current-loop order19,20 that involves apical oxygen orbitals22, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point3,4,5,19.

The pseudogap is one of the most pervasive phenomena of high temperature superconductors. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature Tc, or to a hidden order parameter competing with superconductivity. Here we use inelastic neutron scattering from underdoped YBa(2)Cu(3)O(6.6) to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied "hour glass" shape of the magnetic dispersions in the superconducting state is no longer discernible in the pseudogap state and we observe an unusual "vertical" dispersion with pronounced in-plane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy excitations in the pseudogap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work., Comment: 6 pages, 8 figures, contains supplementary information. Some data from V. Hinkov et al., arXiv:cond-mat/0601048, is included

We report a neutron scattering study of the spin excitation spectrum in the superconducting state of the slightly overdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ system $({T}_{c}=87\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. We focus on the dispersion of the resonance peak in the superconducting state that is due to a $S=1$ collective mode. The measured spin excitation spectrum bears a strong similarity to the spectrum of the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ system for a similar doping level (i.e., $x\ensuremath{\sim}0.95\ensuremath{-}1$), which can be described as intersecting upward- and downward-dispersing branches. A close comparison of the threshold of the electron-hole spin flip continuum, known from angle resolved photoemission measurements in the same system, indicates that the magnetic response in the superconducting state is confined, in both energy and momentum, below the gapped Stoner continuum. In contrast to ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$, the spin excitation spectrum is broader than the experimental resolution. In the framework of an itinerant-electron model, we quantitatively relate this intrinsic energy width to the superconducting gap distribution observed in scanning tunneling microscopy experiments. Our study further suggests a significant in-plane anisotropy of the magnetic response.

The magnetic excitations in Ca 1.8 Sr 0.2 RuO 4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic transition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca 1.8 Sr 0.2 RuO 4 , therefore, has to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field.

The alpha-beta magneto-structural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 K to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an alpha-beta phase coexistence and, more important, for the stabilization of the ferromagnetic alpha-phase at higher temperature than in bulk. We explain the premature appearance of the beta-phase at 275 K and the persistence of the ferromagnetic alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature., 4 pages, 3 figures, accepted for publication in Physical Review Letters

The lattice dynamics in ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ has been studied by inelastic neutron scattering combined with shell-model calculations. The in-plane bond-stretching modes in ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ exhibit a normal dispersion in contrast to all electronically doped perovskites studied so far. Evidence for strong electron phonon coupling is found for $c$-polarized phonons suggesting a close connection with the anomalous $c$-axis charge transport in ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$. The role of this electron-phonon coupling in the superconducting pairing remains, however, a challenging open issue.

The magnetic excitations in multiferroic TbMnO3 have been studied by inelastic neutron scattering in the spiral and sinusoidally ordered phases. At the incommensurate magnetic zone center of the spiral phase, we find three low-lying magnons whose character has been fully determined using neutron-polarization analysis. The excitation at the lowest energy is the sliding mode of the spiral, and two modes at 1.1 and 2.5 meV correspond to rotations of the spiral rotation plane. These latter modes are expected to couple to the electric polarization. The 2.5 meV mode is in perfect agreement with recent infrared-spectroscopy data giving strong support to its interpretation as a hybridized phonon-magnon excitation.

Resonant magnetic modes with odd and even symmetries were studied by inelastic neutron scattering experiments in the bilayer high-${T}_{c}$ superconductor ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ over a wide doping range. The threshold of the spin excitation continuum in the superconducting state, deduced from the energies and spectral weights of both modes, is compared with the superconducting $d$-wave gap, deduced from electronic Raman scattering in the ${B}_{1g}$ symmetry on the same samples. Above a critical doping level of $\ensuremath{\delta}\ensuremath{\simeq}0.19$, both mode energies and the continuum threshold coincide. We find a simple scaling relationship between the characteristic energies and spectral weights of both modes, which indicates that the resonant modes are bound states in the superconducting energy gap, as predicted by the spin-exciton model of the resonant mode.

The magnon dispersion in the charge, orbital, and spin ordered phase in La1/2Sr3/2MnO4 has been studied by means of inelastic neutron scattering. We find excellent agreement with a magnetic interaction model based on the CE-type superstructure. The magnetic excitations are dominated by ferromagnetic exchange parameters revealing a nearly one-dimensional character at high energies. The strong ferromagnetic interaction in the charge or orbital ordered phase appears to be essential for the capability of manganites to switch between metallic and insulating phases.

One of the leading issues in high-$T_C$ superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YBa$_2$Cu$_3$O$_{6+x}$ system. The observed magnetic order preserves translational symmetry as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of an hidden order parameter characterizing the pseudogap phase in high-$T_C$ cuprates., 3 figures

We review experimental attempts using polarized neutron scattering technique to reveal the existence in high temperature superconductors of a long-range ordered state characterized by the spontaneous appearance of current loops. We draw a particular attention to our recent results (B. Fauqu\'e {\it et al.}, {\it Phys. Rev. Lett.} {\bf 96}, 197001 (2006)) that, up to now, can be explained only by the theory of circulating current proposed by C.M. Varma., Comment: presented at a conference: http://www.hmi.de/bensc/pncmi2006, to be published in Physica B

The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important., 4 pages, 4 figures, to appear in Physical Review Letters

Na$_x$CoO$_2$, the parent compound of the recently synthesized superconductor Na$_x$CoO$_2$:$y$H$_2$O, exhibits bulk antiferromagnetic order below $\sim20$ K for 0.75 $\leq x \leq$ 0.9. We have performed neutron scattering experiments in which we observed Bragg reflections corresponding to A-type antiferromagnetic order in a Na$_{0.82}$CoO$_2$ single crystal and characterized the corresponding spin-wave dispersions. The spin waves exhibit a strongly energy-dependent linewidth. The in-plane and out-of-plane exchange constants resulting from a fit to a nearest-neighbor Heisenberg model are similar in magnitude, which is unexpected in view of the layered crystal structure of Na$_{x}$CoO$_2$. Possible implications of these observations are discussed., final version, to appear in PRL

An inelastic neutron scattering experiment has been performed in the high-temperature superconductor $\rm YBa_2Cu_3O_{6.89}$ to search for an oxygen-isotope shift of the well-known magnetic resonance mode at 41 meV. Contrary to a recent prediction (I. Eremin, {\it et al.}, Phys. Rev. B {\bf 69}, 094517 (2004)), a negligible shift (at best $\leq$ +0.2 meV) of the resonance energy is observed upon oxygen isotope substitution ($^{16}$O$\to^{18}$O). This suggests a negligible spin-phonon interaction in the high-$T_c$ cuprates at optimal doping., Comment: 3 figures

Detailed neutron scattering measurements of ${\mathrm{Y}\mathrm{B}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}}_{3}{\mathrm{O}}_{6.95}$ found that the resonance peak and incommensurate magnetic scattering induced by superconductivity represent the same physical phenomenon: two dispersive branches that converge near 41 meV and the in-plane wave vector ${\mathbf{q}}_{\mathrm{A}\mathrm{F}}=(\ensuremath{\pi}/a,\ensuremath{\pi}/a)$ to form the resonance peak. One branch has a circular symmetry around ${\mathbf{q}}_{\mathrm{A}\mathrm{F}}$ and quadratic downward dispersion from $\ensuremath{\approx}41\text{ }\text{ }\mathrm{m}\mathrm{e}\mathrm{V}$ to the spin gap of $33\ifmmode\pm\else\textpm\fi{}1\text{ }\text{ }\mathrm{m}\mathrm{e}\mathrm{V}$. The other, of lower intensity, disperses from $\ensuremath{\approx}41\text{ }\text{ }\mathrm{m}\mathrm{e}\mathrm{V}$ to at least 55 meV. Our results exclude a quartet of vertical incommensurate rods in $\mathbf{q}$-$\ensuremath{\omega}$ space expected from spin waves produced by dynamical charge stripes as an origin of the observed incommensurate scattering in optimally doped YBCO.

A detailed inelastic neutron scattering study of the high temperature superconductor $\mathrm{Y}\mathrm{B}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6.85}$ provides evidence of new resonant magnetic features, in addition to the well-known resonant mode at 41 meV: (i) a commensurate magnetic resonance peak at 53 meV with an even symmetry under exchange of two adjacent $\mathrm{C}\mathrm{u}{\mathrm{O}}_{2}$ layers, and (ii) high-energy incommensurate resonant spin excitations whose spectral weight is around 54 meV. The locus and the spectral weight of these modes provides unrevealed insight about the momentum shape of the electron-hole spin-flip continuum of $d$-wave superconductors.

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-Tc) superconductivity is nucleated in ‘stripes’ aligned along one of the axes1,2,3. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-Tc material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on ‘untwinned’ YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim4, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order5. The debate about stripes has therefore been reopened.

Magnetic semiconductors are attracting high interest because of their potential use for spintronics, a new technology which merges electronics and manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently emerged as the most popular materials for this new technology. While Curie temperatures are rising towards room temperature, these materials can only be fabricated in thin film form, are heavily defective, and are not obviously compatible with Si. We show here that it is productive to consider transition metal monosilicides as potential alternatives. In particular, we report the discovery that the bulk metallic magnets derived from doping the narrow gap insulator FeSi with Co share the very high anomalous Hall conductance of (GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens up a new arena for spintronics, involving a bulk material based only on transition metals and Si, and which we have proven to display a variety of large magnetic field effects on easily measured electrical properties., 19 pages with 5 figures

The anisotropy of the magnetic incommensurate fluctuations in Sr2RuO4 has been studied by inelastic neutron scattering with polarized neutrons. We find a sizable enhancement of the out-of-plane component by a factor of 2 for intermediate energy transfer, which appears to decrease for higher energies. Our results qualitatively confirm calculations of the spin-orbit coupling, but the experimental anisotropy and its energy dependence are weaker than predicted.

Inelastic neutron scattering experiments have been carried out on a YBa 2 (Cu 0.98 Zn 0.02 ) 3 O 6.39 single crystal to investigate the spin-excitation spectrum in the hole-doped antiferromagnetic state. New excitations are found at low energy as a consequence of electron—hole interactions, whereas standard magnons are recovered at sufficiently high energy.