  We numerically examine binary-sized granular mixtures confined between two
parallel walls subjected to vertical vibration using the discrete element
method. For a size ratio of $3$ between large and small particles, we study the
structure of large particles in regions where the combined two-dimensional
packing fractions of both particle sizes surpass $1$. When the fraction of
small particles is small, segregation of the large particles appears. In
contrast, as the fraction of small particles increases, an effective repulsion
between the large particles emerges over distances greater than the large
particle diameter, suppressing their segregation. Further, this effective
repulsion also induces a hexagonal phase of the large particles at packing
fractions lower than in mono-component systems. This work will provide a fresh
insight into granular physics, prompting further experimental and theoretical
study.
