Spontaneous Symmetry-Breaking Vortex Lattice Transitions in Pure Niobium

摘要: We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals pure niobium with the field applied parallel to a fourfold symmetry axis, so as induce frustration between cubic crystal and hexagonal VL coordination expected isotropic situation. observe new phase transitions; all phases observed (including those exactly square unit cell) spontaneously break some symmetry. One even has lowest possible two-dimensional Bravais lattice. This is quite unlike situation high-${T}_{c}$ or borocarbide superconductors, where orient along particular directions high The causes this behavior are discussed.