Anisotropy and dimensional crossover of the vortex state inBi2Sr2

Abstract

We report muon-spin-rotation studies of the magnetic-field distribution n(B) in the vortex state in both underdoped and strongly overdoped Bi2Sr2CaCu208+z single crystals. With increasing magnetic field we observe sudden changes in n(B) that are indicative of a reduction in the dimensionality of the vortex system.The crossover field B* depends strongly on the doping state and the corresponding anisotropy y of the system.From magnetization measurements we find evidence that the second peak in the M-0 loops is related to the crossover in the vortex structure in B*.Due to their layered structure the cuprate high-T, superconductors exhibit very anisotropic electronic and magnetic properties. Numerous experiments indicate that only the blocks of CuOz layers are intrinsically superconducting while the layers of thickness s which separate them act mainly as insulating, or sometimes metallic, blocking layers and charge reservoirs. In the mixed state, e.g. , when the ex-

Muon Spin Rotation Studies of the Vortex Matter in the High-TcSuperconductor Bi2Sr2CaCu2O8+δ

“…The data have been obtained by field cooling in an applied field of 100 mT for the overdoped set, 27.5 mT for the nearly optimized set, and 5 mT for the underdoped set. In all cases the external field is well below the dimensional crossover field H* (H* ≈ 150 mT and ≈7.5 mT for the overdoped and underdoped crystals [10] and 60 mT for the optimized crystals) and the signature of a 3D vortex lattice, i.e. α 1, is observed at low temperature.…”

“…The resulting asymmetry or "skewness" of the " μSR-lineshape" can be characterized by calculating the second and third moment of the internal magnetic field distribution and defining the dimensionless parameter [9] due to a static 3D FLL [9][10][11]. A reduced value of 1 > α > 0 either indicates a disorder of the static vortex structure or else vortex dynamics in excess of the typical //SR time scale of T ≤ 10 -s s [9][10][11]. From the µSR experiments alone one cannot distinguish between these two possibilities.…”

“…Here the low-temperature vortex-state is a quasi-2D lattice of pancake vortices [9][10][11]. Roughly speaking, the phase coherence across the blocking layers is destroyed once the pancake vortices in adjacent CuO2 blocks are randomly displaced on a length scale comparable to their average spacing ao = (Φ the so-called second-peak effect observed in the M-H loops of magnetization experiments on Bi-2212 single crystals occurs due to this dimensional crossover in the vortex system.…”

“…In addition, we compare our TF-μSR results for the overdoped and underdoped single crystals with those from DC-magnetization measurements obtained on the same samples with a vibrating-sample magnetometer (VSM) [10].…”

“…The equilibrium vortex structure can be studied with techniques like small-angle neutron scattering (SANS) and transverse-field muon spin rotation (TF-μSR). Both techniques have provided valuable information on the vortex state, in particular, on the vortex matter transition at the irreversibility line (IL) and the so-called "dimensional crossover" from a 3D to a 2D vortex structure that occurs with increasing magnetic field [8][9][10].…”