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 dispersion of the low-energy magnetic excitations of the Pr sublattice in PrBa2Cu3O6.2 is determined by inelastic neutron scattering measurements on a single crystal. The dispersion, which shows the effect of interactions with the Cu spin-waves, is well described by a model of the coupled Cu-Pr magnetic system. This enables values for the principal exchange constants to be determined, which suggest that both Pr-Pr and Cu-Pr interactions are important in producing the anomalously high ordering temperature of the Pr sublattice. Measurements of the Cu optic spin wave mode show that the inter-layer Cu-Cu exchange is significantly lower than in YBa2Cu3O6.2.PACS numbers: 71.28.+d, 74.72.Jt, 75.10.Dg, 78.70.Nx The anomalous properties of non-superconducting PrBa 2 Cu 3 O 6+x (PrBCO) [1] remain an outstanding problem in the field of cuprate superconductivity. Its magnetic properties are strikingly different from those of other members of the RBCO family (where R is a rare earth, Y or La). Long range antiferromagnetic order in both the Cu and Pr sublattices along with semiconducting resistivity persist over the entire range of oxygen doping 0 < x < 1, and the magnetic transition in the Pr sublattice occurs at a temperature T Pr an order of magnitude higher than for other rare earths in RBCO [2]. It is now generally believed that these anomalous properties are caused by a hybridisation of the Pr 4f and O 2p orbitals. In particular, an influential model of the electronic structure of PrBCO [3] explains the absence of superconductivity by a localisation of the holes in hybridised Pr-O bonds. Given the likelihood that the electronic and magnetic properties of PrBCO, including the occurrence of superconductivity [4], both depend on this hybridisation, detailed information on the strengths and symmetries of the magnetic interactions is desirable.In this Letter, we describe inelastic neutron scattering measurements of the magnetic excitations of PrBCO made for the first time on a single crystal. Using this data and a spin wave model of the coupled Cu-Pr system we have independently determined the magnitudes of the Pr-Pr, Cu-Pr and Cu-Cu interactions. We find that the Pr-Pr interaction is unexpectedly strong and would alone lead to T Pr being substantially higher than the rare-earth ordering temperature in other RBCO. We have also identified the effects of a pseudo-dipolar component to the Cu-Pr coupling on the magnetic excitations. Finally, we show that the inter-layer Cu-Cu exchange is reduced by a factor of two in PrBCO relative to YBCO.The experiments were performed on a crystal of PrBa 2 Cu 3 O 6.2 of mass 2 g prepared by top seeding a flux. The crystal mosaic was ∼ 1 • and T Pr = 13 K. Measurements on the triple-axis spectrometers TAS 6 at Risø and IN8 at the ILL were made at energy transfers up to 10 meV and 70 meV respectively. A horizontallyfocussing analyser was employed to increase the measured signal at the expense of some resolution in the scattering vector Q. The crystal was mounted in a 4 He cryostat and m...
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