College Park, Maryland June 6 - 10 , 2004
M4-A3 (4:30 PM): Field-Induced Square Vortex Lattices in High-Temperature Superconductors
J. Mesot, R. Gilardi (LNS, ETHZ & PSI, Switzerland), S. P. Brown, E. M. Forgan (Univ. Birmingham, UK), A. J. Drew, S. L. Lee (Univ. St-Andrews, UK), C. D. Dewhurst, R. Cubitt (Institut Laue Langevin), T. Uefuji, K. Yamada (Tohoku University)
The interest in the vortex matter of high-Tc superconductors has been renewed by the recent observations, by means of small angle neutron scattering (SANS), of square vortex lattices (VL) at high magnetic fields in La2-xSrxCuO4  and YBa2Cu3O7 . These observations are important since a square symmetry is indicative of the coupling of the VL to a source of anisotropy in the CuO2 planes, such as those provided by a d-wave superconducting gap , momentum dependent Fermi velocities , or the presence of stripes . In order to discriminate between the above- mentioned theoretical models, SANS experiments as a function of doping and up to 10.5 Tesla have been performed. The outcome of the measurements will be presented.
We will report here as well on the first SANS measurements of an ordered VL in the electron-doped cuprate superconductor Nd2-xCexCuO4 . A square VL with the nearest neighbors oriented 45 degrees from the Cu-O bond directions is observed down to unusually low magnetic fields. Moreover, the vortex lattice intensity is found to decrease faster than expected with increasing magnetic field, possibly indicating a transition to a more disordered vortex glass phase.
Similarities and differences between electron- and hole-doped high-temperature superconductors will be discussed.
This work was supported by the Swiss National Science Foundation, the Engineering and Physical Sciences Research Council of the U.K. and the Ministry of Education and Science of Japan.
 R. Gilardi et al., Phys. Rev. Lett. 88 (2002) 217003.
 S.P. Brown et al., Phys. Rev. Lett. 92 (2004) 067004.
 M. Ichioka et al.,Phys. Rev. B, 59, 8902, (1999).
 N. Nakai et al., Phys. Rev. Lett. 89 (2002) 237004.
 Franz et al., Phys. Rev. Lett. 88 (2002) 257005.
 R. Gilardi et al., to be submitted.
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