Freezing of spin dynamics and omega/T scaling in underdoped cuprates

The memory function approach to spin dynamics in doped antiferromagnetic insulator combined with the assumption of temperature independent static spin correlations and constant collective mode damping leads to omega/T scaling in a broad range. The theory involving a non universal scaling parameter is used to analyze recent inelastic neutron scattering results for underdoped cuprates. Adopting modified damping function also the emerging central peak in low-doped cuprates at low temperatures can be explained within the same framework.Comment: 4 pages, 5 figures; to appear in Journal of Physics: Conference Series (ICM2009 Conference, Karlsruhe, Germany

Spin susceptibility of underdoped cuprates: the case of Ortho-II YBa_2Cu_3O_{6.5}

Recent inelastic neutron scattering measurements found that the spin susceptibility of detwinned and highly ordered ortho-II YBa_2Cu_3O_{6.5} exhibits, in both the normal and superconducting states, one-dimensional incommensurate modulations at low energies which were interpreted as a signature of dynamic stripes. We propose an alternative model based on quasiparticle transitions between the arcs of a truncated Fermi surface. Such transitions are resonantly enhanced by scattering to the triplet spin resonance. We show that the anisotropy in the experimental spin response is consistent with this model if the gap at the saddle points is anisotropic.Comment: 5 fives, 3 postscript figure

Theory of the in-plane anisotropy of magnetic excitations in YBa_{2}Cu_{3}O_{6+y}

A pronounced xy-anisotropy was observed in recent neutron scattering experiments for magnetic excitations in untwinned YBa_{2}Cu_{3}O_{6+y} (YBCO). The small anisotropy of the bare band structure due to the orthorhombic crystal symmetry seems to be enhanced by correlation effects. A natural possibility is that the system is close to a Pomeranchuk instability associated with a d-wave Fermi surface deformation (dFSD). We investigate this possibility in the bilayer t-J model within a self-consistent slave-boson mean-field theory. We show that the dFSD correlations drive a pronounced xy-anisotropy of magnetic excitations at low doping and at relatively high temperatures, providing a scenario for the observed xy-anisotropy in optimally doped as well as underdoped YBCO, including the pseudogap phase.Comment: magnetic excitations in the even channel for YBCO are presented; the proceedings of the M2S-HTSC VIII conferenc

Magnetic order in the pseudogap phase of high-TCT_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 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.Comment: 3 figure

Crossover from weak to strong pairing in unconventional superconductors

Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap to the critical temperature, Tc. We present an extensive comparison of the gap ratios among many single- and multiband superconductors from simple metals to high-Tc cuprates and iron pnictides. Contrary to the recently suggested universality of this ratio in Fe-based superconductors, we find that the coupling in pnictides ranges from weak, near the BCS limit, to strong, as in cuprates, bridging the gap between these two extremes. Moreover, for Fe- and Cu-based materials, our analysis reveals a universal correlation between the gap ratio and Tc, which is not found in conventional superconductors and therefore supports a common unconventional pairing mechanism in both families. An important consequence of this result for ferropnictides is that the separation in energy between the excitonic spin-resonance mode and the particle-hole continuum, which determines the resonance damping, no longer appears independent of Tc.Comment: 15 pages, 3 figures, 5 tables with an exhaustive overview of the published gap and spin-resonance measurements in Fe-based superconductors. New in V3: updated references. To be published in Phys. Rev.

Physics of π\pi-Meson Condensation and High Temperature Cuprate Superconductors

The idea of condensation of the Goldstone $\pi$-meson field in nuclear matter had been put forward a long time ago. However, it was established that the normal nuclear density is too low, it is not sufficient to condensate $\pi$-mesons. This is why the $\pi$-condensation has never been observed. Recent experimental and theoretical studies of high temperature cuprate superconductors have revealed condensation of Goldstone magnons, the effect fully analogous to the $\pi$-condensation. The magnon condensation has been observed. It is clear now that quantum fluctuations play a crucial role in the condensation, in particular they drive a quantum phase transition that destroys the condensate at some density of fermions

Fe-based superconductors: unity or diversity?

Does the high temperature superconductivity observed in the newly discovered iron-pnictide materials represent another example of the same essential physics responsible for superconductivity in the cuprates, or does it embody a new mechanism?Comment: Some minor errors in the figure and in the reference in the published version are corrected. 2 pages, 2 figure

Resonant magnetic excitations at high energy in superconducting YBa2Cu3O6.85\bf YBa_2Cu_3O_{6.85}

A detailed inelastic neutron scattering study of the high temperature superconductor $\rm YBa_2Cu_3O_{6.85}$ 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 $\rm CuO_2$ 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 can be understood by considering the momentum shape of the electron-hole spin-flip continuum of d-wave superconductors. This provides new insight into the interplay between collective spin excitations and the continuum of electron-hole excitations.Comment: 5 figure