title: Spontaneous symmetry-breaking vortex lattice transitions in pure niobium.
abstract: We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals of pure niobium with the magnetic field applied parallel to a fourfold symmetry axis, so as to induce frustration between the cubic crystal symmetry and hexagonal VL coordination expected in an isotropic situation. We observe new VL structures and phase transitions; all the VL phases observed (including those with an exactly square unit cell) spontaneously break some crystal symmetry. One phase even has the lowest possible symmetry of a two-dimensional Bravais lattice. This is quite unlike the situation in high-Tc or borocarbide superconductors, where VL structures orient along particular directions of high crystal symmetry. The causes of this behavior are discussed.
authors.affiliation: Institut Laue-Langevin, BP 156, F-38042 Grenoble, France.
authors.forename: M
authors.initials: M
authors.lastname: Laver
authors.forename: E M
authors.initials: EM
authors.lastname: Forgan
authors.forename: S P
authors.initials: SP
authors.lastname: Brown
authors.forename: D
authors.initials: D
authors.lastname: Charalambous
authors.forename: D
authors.initials: D
authors.lastname: Fort
authors.forename: C
authors.initials: C
authors.lastname: Bowell
authors.forename: S
authors.initials: S
authors.lastname: Ramos
authors.forename: R J
authors.initials: RJ
authors.lastname: Lycett
authors.forename: D K
authors.initials: DK
authors.lastname: Christen
authors.forename: J
authors.initials: J
authors.lastname: Kohlbrecher
authors.forename: C D
authors.initials: CD
authors.lastname: Dewhurst
authors.forename: R
authors.initials: R
authors.lastname: Cubitt