Y. Sidis

Magnetic Order in the Pseudogap Phase of High- T C Superconductors

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 YB(2)Cu(3)O(6+) system. The observed magnetic order preserves translational symmetry of the lattice as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudogap phase in high-T(c) cuprates.

Electronic Liquid Crystal State in the High-Temperature Superconductor YBa2Cu3O6.45

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at millikelvin temperatures. We report the spontaneous onset of a one-dimensional, incommensurate modulation of the spin system in the high-transition-temperature superconductor YBa2Cu3O6.45 upon cooling below ∼150 kelvin, whereas static magnetic order is absent above 2 kelvin. The evolution of this modulation with temperature and doping parallels that of the in-plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route toward electronic liquid crystals and that nematic order can coexist with high-temperature superconductivity in underdoped cuprates.

Magnetic Order in the Pseudogap Phase of High-TCSuperconductors

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 YB(2)Cu(3)O(6+) system. The observed magnetic order preserves translational symmetry of the lattice as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudogap phase in high-T(c) cuprates.

Normal-state spin dynamics and temperature-dependent spin-resonance energy|[nbsp]|in|[nbsp]|optimally doped BaFe1.85Co0.15As2

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 Spin Excitation Spectrum in Superconducting YBa2Cu3O6.85

A comprehensive inelastic neutron scattering study of magnetic excitations in the near optimally doped high-temperature superconductor YBa2Cu3O6.85 is presented. The spin correlations in the normal state are commensurate with the crystal lattice, and the intensity is peaked around the wave vector characterizing the antiferromagnetic state of the insulating precursor, YBa2Cu3O6. Profound modifications of the spin excitation spectrum appear abruptly below the superconducting transition temperature T c, where a commensurate resonant mode and a set of weaker incommensurate peaks develop. The data are consistent with models that are based on an underlying two-dimensional Fermi surface, predicting a continuous, downward dispersion relation connecting the resonant mode and the incommensurate excitations. The magnetic incommensurability in the YBa2Cu3O6+ x system is thus not simply related to that of another high-temperature superconductor, La2– x Sr x CuO4, where incommensurate peaks persist well above T c. The temperature-dependent incommensurability is difficult to reconcile with interpretations based on charge stripe formation in YBa2Cu3O6+ x near optimum doping.

Neutron scattering from magnetic excitations in Bi 2 Sr 2 CaCu 2 O 8+δ

Many of the physical properties of the copper oxide high-temperature superconductors appear to defy the conventional ‘one-electron’ theory of metals. The development of alternative theories incorporating strong electron correlations is currently at the forefront of research in condensed-matter physics. In this context, inelastic neutron scattering can provide valuable insight into collective magnetic excitations in the copper oxide superconductors and so guide these theoretical efforts. Such measurements require large single crystals, and have hitherto been restricted to just two families of high-temperature superconductors—La2−xSrxCuO4 (ref. 1) and YBa2Cu3O6+x (ref. 2). Although the magnetic spectra of these two materials bear certain similarities, there are also important differences. In particular, a sharpresonant spin excitation dominates the spectrum in the superconducting state of YBa2Cu3O6+x (refs 3–10), but is not observed in La2−xSrxCuO4 (ref. 1). Here we report inelastic neutron scattering measurements of a different copper oxide system—Bi2Sr2CaCu2O8+δ—in which we observe a magnetic resonance peak in the superconducting state. This result implies that this excitation phenomenon is a general feature of the copper oxide superconductors, so extending the empirical basis for its theoretical description.

Unusual magnetic order in the pseudogap region of the superconductor HgBa2CuO4+δ

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 properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. 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 no and weak magnetic order, and the interpretation of muon spin relaxation measurements on YBCO 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 YBCO, 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 temperature rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point.

EVIDENCE FOR INCOMMENSURATE SPIN FLUCTUATIONS IN SR2RUO4

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

Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x.

{\bf q}_0=(\pm 0.6\pi/a, \pm 0.6\pi/a, 0)

Resonant Spin Excitation in an Overdoped High Temperature Superconductor

. This finding confirms recent band structure calculations that have predicted incommensurate magnetic responses related to dynamical nesting properties of its Fermi surface.