Field-induced hexagonal to square transition of the vortex lattice in overdoped La1.8Sr0.2CuO4

Abstract

We report on a small angle neutron scattering study of the vortex lattice in overdoped La 2-x Sr x CuO 4 (x=0.2) up to high magnetic fields (9.5 Tesla) applied perpendicular to the CuO 2 planes. At low magnetic fields we observe a crossover from hexagonal to square coordination of the vortex lattice. This field-induced transition confirms the results obtained in slightly overdoped La 2-x Sr x CuO 4 (x=0.17).

Square Vortex Lattice at Anomalously Low Magnetic Fields in Electron-DopedNd1.85Ce0.15CGilardi1, Mesot2, Brown3 et al. 2004Phys. Rev. Lett.

“…These theoretical predictions are consistent with the small-angle neutron scattering (SANS) observation of a transition from an Abrikosov-like hexagonal VL to a square VL in the hole-doped HTSC YBa 2 Cu 3 O 7 (YBCO) [21]. However, similar measurements on overdoped La 2−x Sr x CuO 4 (LSCO) [22] show a square lattice with nearest neighbors oriented at 45 • to the nodes of the superconducting order parameter. The orientation of the nodal directions in the heavy-fermion superconductor CeCoIn 5 is at present uncertain [23], so it is unclear if the recent SANS observations of the VL in this material [24] confirm theoretical expectations.…”

“…However, the orientation becomes rather more disordered as the field is decreased below 50 mT and at 20 mT the intensity distribution becomes ring-like. A similar distribution of intensity at low magnetic fields has been observed in LSCO [22], and was attributed to the superposition of diffraction patterns from various domain orientations of hexagonal coordination, since the value of σ at low fields was consistent with that of an hexagonal VL. Moreover, by rotating the c-axis 10 • away from the field direction, the degeneracy of the VL system could be reduced, and the hexagonal coordination of the VL in LSCO was confirmed.…”

“…Moreover LSCO exhibits a pronounced anisotropy in the Fermi velocity. Both the shape of the Fermi surface and the Fermi velocity anisotropy would indicate (via nonlocal effects) the VL orientation actually observed in LSCO [22]. However, both d-wave and nonlocal effects should only be important at fields which are substantial fraction of B c2 , when the inter-vortex spacing is comparable to the coherence length.…”

Vortex imaging and vortex lattice transitions in superconducting Sr2RuO4single crystals

“…It is, however, well established that Fermi surface and order-parameter anisotropies can lead to other forms of vortex ordering, that is, a square vortex lattice. 6,33 Heeb and Agterberg 20 have used an extended London theory (κ 1) for a two component p-wave order parameter, to investigate the ground state vortex structure in Sr 2 RuO 4 as a function of Fermi surface anisotropy, |ν| 1, and applied field. They predict a continuous triangular → rectangular → square field-driven transition, with switching fields that are strongly dependent on the value of ν.…”

Spin density wave induced disordering of the vortex lattice in superconducting La2−xSrxCuO

“…1, the appearance of magnetism is, in essence, concomitant with the suppression of SANS intensity with field and underdoping. Drawing upon results from the literature [22][23][24] and observations reported herein, we are also able to plot the VL structure at low temperature T versus magnetic field H and doping x.…”