Defining $B_{c}$ , $B^{\ast}$ and $B_{\phi}$ for YBCO Thin Films

The accommodation field,
B*
, is generally defined to be the field at which the cross over from single vortex pinning to collective pinning occurs. It is determined from magnetization curves as the point where the
J

_c
plateau ends and it used as a convenient way of comparing the pinning properties of superconducting films. Similarly, the characteristic field,
B

_c
, can be obtained from magneto-optical (MO) images from when the flux fronts meet in the middle of the film. The matching field,
B

_phi
, at which there is one vortex line per pinning site, is sometimes thought to be the same as
B*
, but in BaZrO
₃
-doped YBa
₂
Cu
₃
O
₇
films the calculated
B

_phi
is much higher than the observed
B*
.
B

_phi
can be determined from angular dependent transport measurements. All of the field values correspond to some special case in the flux pinning in the film and relate to
J

_c
. In this work we have determined
B

_c
,
B*
and
B

_phi
for different kinds of YBCO films using MO, magnetization and transport measurements to reveal the deeper meaning of the special fields.