Evolution of the pseudogap temperature dependence in YBa_2Cu_3O_7-δ films under the influence of a magnetic field
arxiv(2024)
Abstract
The evolution of the temperature dependence of pseudogap Δ*(T) in
optimally doped (OD) YBa_2Cu_3O_7-δ (YBCO) films with T_c =
88.7 K under the influence of a magnetic field B up to 8 T has been studied
in detail. It has been established that the shape of Δ*(T) for various
B over the entire range from the pseudogap opening temperature T* to
T_01, below which superconducting fluctuations occur, has a wide maximum at
the BEC-BCS crossover temperature T_pair, which is typical for OD films and
untwinned YBCO single crystals. T* was shown to be independent on B,
whereas T_pair shifts to the low temperature region along with increase of
B, while the maximum value of Δ*(T_pair) remains practically
constant regardless of B. It was revealed that as the field increases, the
low-temperature maximum near the 3D-2D transition temperature T_0 is
blurred and disappears at B > 5 T. Moreover, above the Ginzburg temperature
T_G, which limits superconducting fluctuations from below, at B > 0.5 T,
a minimum appears on Δ*(T) at T_min, which becomes very pronounced
with a further increase in the field. As a result, the overall value of
Δ*(T) decreases noticeably most likely due to pair-breaking affect of a
magnetic field. A comparison of Δ*(T) near T_c with the Peters-Bauer
theory shows that the density of fluctuating Cooper pairs actually decreases
from <> ≈ 0.31 at B = 0 to
<> ≈ 0.28 in the field 8 T. The
observed behavior of Δ*(T) around T_min is assumed to be due to the
influence of a two-dimensional vortex lattice created by the magnetic field,
which prevents the formation of fluctuating Cooper pairs near T_c.
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