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Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity 1 . The high transition temperatures (high- T c ) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials 2 . The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T c . The most obvious feature in the magnetic excitations of high- T c superconductors such as YBa 2 Cu 3 O 6+ x is the so-called ‘resonance’ 3 , 4 , 5 , 6 . Although the resonance may be strongly coupled to the superconductivity 3 , 4 , 5 , 6 , 7 , 8 , it is unlikely to be the main cause, because it has not been found in the La 2- x (Ba,Sr) x CuO 4 family and is not universally present in Bi 2 Sr 2 CaCu 2 O 8+ δ (ref. 9 ). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa 2 Cu 3 O 6.6 . We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T c , and have spectral weight far exceeding that of the ‘resonance’. The discovery of similar excitations in La 2– x Ba x CuO 4 (ref. 10 ) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

There is increasing evidence that inhomogeneous distributions of charge and spin--so-called 'striped phases'--play an important role in determining the properties of the high-temperature superconductors. For example, recent neutron-scattering measurements on the YBa2Cu3O(7-x) family of materials show both spin and charge fluctuations that are consistent with the striped-phase picture. But the fluctuations associated with a striped phase are expected to be one-dimensional, whereas the magnetic fluctuations observed to date appear to display two-dimensional symmetry. We show here that this apparent two-dimensionality results from measurements on twinned crystals, and that similar measurements on substantially detwinned crystals of YBa2Cu3O6.6 reveal the one-dimensional character of the magnetic fluctuations, thus greatly strengthening the striped-phase interpretation. Moreover, our results also suggest that superconductivity originates in charge stripes that extend along the b crystal axis, where the superfluid density is found to be substantially larger than for the a direction.

An important feature of the high-transition-temperature (high- T c ) copper oxide superconductors is the magnetism that results from the spins associated with the incomplete outer electronic shells (3d 9 ) of the copper ions. Fluctuations of these spins give rise to magnetic excitations of the material, and might mediate the electron pairing that leads to superconductivity. If the mechanism for high- T c superconductivity is the same for all copper oxide systems, their spin fluctuations should be universal. But so far, theopposite has seemed to be the case: neutron scattering data reveal clear differences between the spin fluctuations for two major classes of high- T c materials, La 2− x Sr x CuO 4 ( 1 - 3 ) and YBa 2 Cu 3 O 7− x ( 4 - 6 ), whose respective building blocks are CuO 2 layers and bilayers. Here we report two-dimensional neutron-scattering imaging of YBa 2 Cu 3 O 6.6 , which reveals that the low-frequency magnetic excitations are virtually identical to those of similarly doped La 2− x Sr x CuO 4 . Thus, the high-temperature ( T c ≲ 92 K) superconductivity of the former materials may be related to spatially coherent low-frequency spin excitations that were previously thought to be unique to the lower- T c (<40 K) single-layer La 2− x Sr x CuO 4 family.

One of the most striking properties of the high-transition-temperature (high- T c ) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset 1 , 2 , 3 , 4 , because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high- T c superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy 5 . For YBa 2 Cu 3 O 6+ x , where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6 , 7 , 8 , 9 , 10 , 11 ). Here we show that for underdoped YBa 2 Cu 3 O 6.6 , where x and T c are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO 2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high- T c superconductivity. The persistence of a field effect above T c favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors 12 , 13 , 14 , awaiting transition to the superconducting state.

Understanding the behaviour of the electrons in the high-temperature copper oxide superconductors remains a challenging problem. An important class of models 1 , 2 , 3 , 4 , 5 argues that the distribution of electronic charge and spin is not homogeneous: rather, spin and charge adopt a dynamic arrangement in which the spins on the copper form antiferromagnetic stripes, separated by domain walls containing the charge 1 , 2 , 3 , 4 , 5 . The dynamic behaviour of the spins has been extensively studied by neutron scattering, and recent results 6 have shown that the low-frequency fluctuations for different classes of materials display a universal spatial behaviour that is consistent with the stripe picture. But arguments for the existence of the stripe phases are difficult to sustain without a demonstration that charge is distributed in the domain walls. Here we report phonon measurements for the YBa 2 Cu 3 O 7- x high-temperature superconductors, which reveal the presence of charge fluctuations. The inferred periodicity is that expected if the charge is located in the domain walls separating the spin stripes. Our results therefore provide strong support for the existence of a dynamic stripe phase in the high-temperature superconductors.

Inelastic neutron scattering was used to study the wave vector- and frequency-dependent magnetic fluctuations in single crystals of superconducting YBa2Cu3O6+x. The spectra contain several important features, including a gap in the superconducting state, a pseudogap in the normal state, and the much-discussed resonance peak. The appearance of the pseudogap determined from transport and nuclear resonance coincides with formation of the resonance in the magnetic excitations. The exchange energy associated with the resonance has the temperature and doping dependences as well as the magnitude to describe approximately the electronic specific heat near the superconducting transition temperature (Tc).

A neutron scattering study shows that the spin excitations in both pnictide- and copper-oxide-based superconductors have the same four-fold symmetry. If these excitations do indeed mediate the superconductivity, the two families of materials may be more similar than previously thought. The origin of the superconducting state in the recently discovered Fe-based materials 1 , 2 , 3 is the subject of intense scrutiny. Neutron scattering 4 , 5 , 6 , 7 and NMR (ref.  8 ) measurements have already demonstrated a strong correlation between magnetism and superconductivity. A central unanswered question concerns the nature of the normal-state spin fluctuations that may be responsible for the pairing. Here we present inelastic neutron scattering measurements from large single crystals of superconducting and non-superconducting Fe 1+ y Te 1− x Se x . These measurements indicate a spin fluctuation spectrum dominated by two-dimensional incommensurate excitations extending to energies greater than 250 meV. Most importantly, the spin excitations in Fe 1+ y Te 1− x Se x have four-fold symmetry about the (1, 0) wavevector (square-lattice (π,π) point). Moreover, the excitations are described by the identical wavevector and can be characterized by the same model as the normal-state spin excitations in the high- T C cuprates 9 , 10 , 11 . These results demonstrate commonality between the magnetism in these classes of materials, which perhaps extends to a common origin for superconductivity.

We report phonon measurements for YBa2Cu3O(7-x) high-temperature superconductors, which reveal the presence of charge fluctuations. The inferred periodicity is that expected if the charge is located in the domain walls separating the spin stripes. Our results therefore provide strong support for the existence of a dynamic stripe phase in the high-temperature superconductors. (Author)

Understanding the magnetic excitations in high-temperature (high- T c ) copper-oxide superconductors is important because they may mediate the electron pairing for superconductivity 1 , 2 . By determining the wavevector ( Q ) and energy (ħ ω ) dependence of the magnetic excitations, it is possible to calculate the change in the exchange energy available to the superconducting condensation energy 3 , 4 , 5 . For the high- T c superconductor YBa 2 Cu 3 O 6+ x , the most prominent feature in the magnetic excitations is the resonance 6 , 7 , 8 , 9 , 10 , 11 , 12 . Suggestions that the resonance contributes a major part of the superconducting condensation 4 , 13 have not gained acceptance because the resonance is only a small portion of the total magnetic scattering 12 , 13 , 14 . Here, we report an extensive mapping of magnetic excitations for YBa 2 Cu 3 O 6.95 ( T c ∼93 K). Absolute intensity measurements of the full spectra allow us to estimate the change in the magnetic exchange energy between the normal and superconducting states, which is about 15 times larger than the superconducting condensation energy 15 , 16 —more than enough to provide the driving force for high- T c superconductivity in YBa 2 Cu 3 O 6.95 .