Absence of orbital currents in superconducting YBa2Cu4O8 using a zeeman-perturbed nuclear-quadrupole-resonance technique.

Zeeman perturbed nuclear quadrupole resonance was applied to evaluate weak magnetic fields in the context of orbital currents in cuprate superconductors. The magnetic environment of the barium atom in c-axis oriented powder samples of YBa(2)Cu(4)O(8) was investigated in the pseudogap phase at 90 K. No evidence for orbital currents was found: any static and dynamic field must be less than 0.07 and 0.7 mT, respectively.

Fermi surface and order parameter driven vortex lattice structure transitions in twin-free YBa2Cu3O7.

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

Evidence for a competition between the superconducting state and the pseudogap state of (BiPb)2(SrLa)2CuO6+delta from muon spin rotation experiments.

The in-plane magnetic penetration depth lambda ab in optimally doped (BiPb)2(SrLa)2CuO6+delta (OP Bi2201) was studied by means of muon-spin rotation. The measurements of lambda ab(-2)(T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett. 98, 267004 (2007)], which suggest that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. The remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate.

Lack of evidence for orbital-current effects in the high-temperature Y2Ba4Cu7O15-delta superconductor using 89Y nuclear magnetic resonance.

We have performed NMR measurements at the Y site on a c-axis-oriented powder sample of the cuprate superconductor Y2Ba4Cu7O15-delta to search for the possible orbital-current phase. The temperature dependence of the Y linewidth and relaxation behavior in the normal-conducting phase were studied down to 100 K. These measurements give upper limits for a static magnetic field and the amplitude of a fluctuating magnetic field at the Y site of < or approximately 0.15 and < or approximately 0.7 mT, respectively. These values provide significant constraints on possible static or quasistatic orbital currents.

Multiple gap symmetries for the order parameter of cuprate superconductors from penetration depth measurements.

The temperature dependence of the London penetration depth lambda was measured for an untwinned single crystal of YBa_{2}Cu_{3}O_{7-delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (lambda_{a};{-2} and lambda_{b};{-2}) show an inflection point in their temperature dependence which is absent in the component along the c direction (lambda_{c};{-2}). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of (s+d)-wave symmetry whereas it is mainly s wave along the c direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.