  The in-phase (rectangular) fluxon lattice is required for achieving coherent
THz emission from stacked Josephson junctions. Unfortunately, it is usually
unstable due to mutual repulsion of fluxons in neighbor junctions, which favors
the out-of-phase (triangular) lattice. Here we experimentally study magnetic
field modulation of the critical current in small Bi-2212 mesa structures with
different sizes. A clear Fraunhofer-like modulation is observed when the field
is aligned strictly parallel to superconducting CuO planes. For long mesas the
periodicity of modulation is equal to half the flux quantum per intrinsic
Josephson junction, corresponding to the triangular fluxon lattice. However,
the periodicity is changed to one flux quantum, characteristic to the
rectangular fluxon lattice, both by decreasing the length of the mesas and by
increasing magnetic field. Thus, we demonstrate that the stationary in-phase
fluxon state can be effectively stabilized by geometrical confinement in small
Bi-2212 mesa structures.
