Tilt-modulus enhancement of the vortex lattice in the layered superconductor 2H-NbSe2

The field dependence of the pinning force has been studied in thin single crystals of the layered superconductor 2H-${\mathrm{NbSe}}_2$ in fields directed perpendicular to the layers. At high fields a peak effect is observed which sets in at about ${\mathit{B}}_{\mathrm{co}}$≊0.8${\mathit{B}}_{\mathit{c}2}$. Below this field the pinning force agrees well with the theory of two-dimensional collective pinning. The onset of the peak is triggered by the transition to three-dimensional flux-line lattice (FLL) disorder at the field ${\mathit{B}}_{\mathrm{co}}$. Comparison of the crossover field with the criterion set by the collective-pinning theory reveals that the tilt modulus of the FLL in a layered superconductor is considerably reduced. The reduction factor corresponds very well to recent theoretical predictions. These results are of importance for the prediction of depinning and flux-line lattice melting in all kinds of anisotropic superconductors.