We have investigated the temperature dependence of the H c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C (Tc = 16.6 K). At temperatures below 10 K the transition onset field, H2(T ), is only weakly temperature dependent. Above 10 K, H2(T ) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T ) merging with the upper critical field, and suggests that just below the Hc2(T )-curve the flux line lattice might be hexagonal.Studies of the topology of the magnetic flux line lattice (FLL) in type-II superconductors have a long history. Early neutron scattering experiments on low-κ superconductors such as niobium revealed a multitude of different FLL symmetries and orientations, mainly determined by the symmetry of the atomic crystal structure in the plane perpendicular to the applied field [1]. This is not surprising since deviations from the hexagonal FLL characteristic of an isotropic superconductor, and the locking to the crystalline lattice are driven by the symmetry of the screening current plane and by nonlocal flux line interactions within a range determined by the coherence length ξ 0 , and the electronic mean free path ℓ. Later, similar effects were also observed in the strong type-II superconductor V 3 Si with κ ≈ 17 [2], demonstrating that nonlocal effects can be equally important in high-κ materials.Over the last couple of years, effects of nonlocality have been clearly observed in the borocarbide superconductors. The borocarbides are quaternary intermetallics with stochiometry RNi 2 B 2 C (R = Y, Gd-Lu) and a tetragonal unit cell (I4/mmm) [3]. These materials have attracted attention due to the coexistence of superconductivity (R = Y, Dy-Tm, Lu) and antiferromagnetic ordering (R = Gd-Tm). The borocarbides are strong type-II superconductors with Ginzburg-Landau (GL) parameter, κ = 6-15. The discovery of a square FLL in most of the H c phase diagram [4], which undergoes a smooth transformation into hexagonal symmetry at fields below 1 kOe [5], was the first observation of a purely field driven FLL symmetry transition.Using nonlocal corrections to the London model and incorporating the symmetry of the screening current plane obtained from band structure calculations, one is able to calculate the FLL free energy, and thereby to determine the stable FLL configuration in different fields in the borocarbides [6]. The model succesfully describes the nature of the FLL square to hexagonal symmetry evolution in the borocarbides with H c as the applied field is reduced. Qualitatively, this can be understood as driven by the four fold basal plane anisotropy which makes the vortex current paths "squarish" close to the core. At high densities this leads to a square FLL, whereas at low fields the system appears isotropic resulting in a hexagonal FLL [7]. The onset of the transition occurs as the field decreases, commensing at a critical field H 2 , determined by the range of the nonlocal inter...

The elastic moduli of the two-dimensional flux-line lattice in a uniaxial superconductor are evaluated for an arbitrary orientation of vortices (i.e., of the magnetic induction) within the crystal in intermediate fields. For the flux lines in the basal a-b plane the ratio of the simple shear moduli, corresponding to displacements normal and parallel to the basal plane, is shown to equal (ms/mi)^ where ms and m\ are the masses along the c axis and the basal plane, respectively. For YBa2Cu307 this amounts to a factor exceeding 600. The uniaxial crystal symmetry defines a preferred orientation to the flux lattice and doubles the number of independent elastic constants.

We have used magnetic decoration to study the pattern of vortices formed near an isolated sawtooth twin boundary in single-crystal YBa2Cu3-O7-5. When the pitch of the sawtooth is comparable to both the intervortex spacing and the penetration depth, we find an unusual vortex structure which has a reduced symmetry relative to the sawtooth itself. This vortex pattern arises from the magnetic interaction between the vortices and the twin boundary which occurs for an asymmetric twin. This is the first observation of a vortex structure caused by the magnetic interaction with a crystalline defect.PACS numbers: 74.60.GeMeasurements of the interactions which pin magnetic vortices to defects in the oxide superconductors are important in the quest to engineer materials with the highest possible critical currents. To date, most such studies have relied on transport and magnetization measurements [1]. In YBa2Cu307-5, the most heavily studied of the high-r^. materials, extended defects appear to be particularly promising as pinning sites. These can either be columnar pins [2] introduced through ion bombardment, screw dislocations [3], or the ubiquitous twin boundaries [4] found in most as-grown single crystals.Pinning of vortices normally arises as a result of the interaction between the normal core, of the size of the coherence length (^, and defects in the superconducting crystal [5]. On the other hand, vortex structures such as the hexagonal lattice [6] and vortex chains [7] are formed as a result of the magnetic interaction between vortices which exists on a length scale given by the penetration depth X. Magnetic interactions with defects are not generally thought to induce significant pinning. However, the oxide superconductors are strongly type II with >.>(^. Therefore the major part of the vortex energy is carried by the persistent currents which circulate around the vortex at an average distance X from the vortex axis. This suggests that purely magnetic interactions with energy of order 0oA^ per unit length of vortex line may play a role in pinning vortices to extended defects in addition to the familiar core pinning which has an energy of the same order,//.^^ 2-0^ AI Although the pinning/orc^ due to the core is still greater by a factor of X/^, the magnetic part of the interaction energy can be important in determining the equilibrium flux-lattice structure in the vicinity of an extended defect and thereby can indirectly affect the pinning of vortices to that defect.In this Letter we report on the vortex patterns seen near a peculiar type of sawtooth twin occasionally found in single-crystal YBa2Cu307-5. We find a pattern in which the vortices populate only one side of the teeth of the sawtooth. By contrast, previous observations of straight twins only showed a uniform line of vortices at the boundary. We argue that the surprisingly regular distortions of the flux-line lattice caused by this boundary can be understood by assuming purely magnetic interactions between the vortices and an asymmetric sawtooth twin. This i...