Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in SuperconductingLuNi2B2C

Abstract

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...

Anomalous paramagnetic effects in the mixed state ofLuNi2B2C

“…12,13 In combination with non-negligible fluctuation effects, the competition leads to unique vortex dynamics right below the H c2 line in the borocarbides, namely a reentrant vortex lattice transition. 9 Fluctuation effects near the upper critical field line wash out the anisotropy effect, stabilizing the Abrikosov hexagonal lattice. 14, 15 Here, we report the first observation of paramagnetic effects in the dc magnetization M of the mixed state of LuNi 2 B 2 C. The kink-like feature in M and the corresponding specific heat feature for H ≥ 30 kOe signify the reentrant FLL transition, which is consistent with the low-field FLL transition line inferred from small angle neutron scattering (SANS).…”

“…6,7,8,9 The transition from square to hexagonal vortex lattice occurs due to the competition between sources of anisotropy and vortex-vortex interactions. The repulsive nature of the vortex interaction favors the hexagonal Abrikosov lattice, whose vortex spacing is larger than that of a square lattice.…”

“…Since the interaction becomes isotropic, the hexagonal Abrikosov lattice is preferable, leading to the second FLL transition from square back 28,29 The diamonds are SANS data, where the numbers next to the data indicate the degrees of the azimuthal splitting with which the transition line is determined. 9 The square and rhombic shapes are forms of vortex lattices.…”

“…14,15 Here, we report the first observation of paramagnetic effects in the dc magnetization M of the mixed state of LuNi 2 B 2 C. The kink-like feature in M and the corresponding specific heat feature for H ≥ 30 kOe signify the reentrant FLL transition, which is consistent with the low-field FLL transition line inferred from small angle neutron scattering (SANS). 9 Single crystals of LuNi 2 B 2 C were grown in a Ni 2 B flux as described elsewhere 16 and were post-growth annealed at T = 1000 C • for 100 hours under high vacuum, typically low 10 −6 Torr. 17 Samples subjected to a preparation process such as grinding, were annealed again at the same condition as the post-growth annealing.…”

On The Influence of a Non-Local Electrodynamics in the Irreversible Magnetization of Non-Magnetic Borocarbides

“…First, Gammel et al 36 showed that H 2 increases as ℓ is reduced by increasing x. Later on, Eskildsen et al 38 showed that H 2 (T ) also rises as T increases.…”