Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals

TL;DR

This study measures the lower critical field and vortex entry barriers in Bi-2223 crystals, revealing insights into vortex dynamics, surface barriers, and the superconducting gap structure at low temperatures.

Contribution

It provides experimental data on H_c1 and vortex barriers, linking low-temperature behavior to nodal superconducting gaps, and confirms theoretical models of vortex creep and surface barriers.

Findings

H_p depends on sweep rate and temperature

Low-temperature upturn of H_p relates to vortex entry barriers

H_c1 data supports nodal superconducting gap structure

Abstract

The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined from magnetization curves for different field sweep rates dH/dt and temperatures. The obtained results are consistent with theoretical reports in the literature about vortex creep over surface and geometrical barriers. The frequently observed low-temperature upturn of H_p is shown to be related to metastable configurations due to barriers for vortex entry. Data of the true lower critical field H_c1 are presented. The low-temperature dependence of H_c1 is consistent with a superconducting state with nodes in the gap function. [PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs]