  Wave-particle duality in quantum mechanics allows for a halo bound state
whose spatial extension far exceeds a range of the interaction potential. What
is even more striking is that such quantum halos can be arbitrarily large on
special occasions. The two examples known so far are the Efimov effect and the
super Efimov effect, which predict that spatial extensions of higher excited
states grow exponentially and double exponentially, respectively. Here, we
establish yet another new class of arbitrarily large quantum halos formed by
spinless bosons with short-range interactions in two dimensions. When the
two-body interaction is absent but the three-body interaction is resonant, four
bosons exhibit an infinite tower of bound states whose spatial extensions scale
as $R_n\sim e^{(\pi n)^2/27}$ for a large $n$. The emergent scaling law is
universal and is termed a semisuper Efimov effect, which together with the
Efimov and super Efimov effects constitutes a trio of few-body universality
classes allowing for arbitrarily large quantum halos.
