  Using a holographic model, we study quantum field theories with a layer of
one CFT surrounded by another CFT, on either a periodic or an infinite
direction. We study the vacuum energy density in each CFT as a function of the
central charges, the thickness of the layer(s), and the properties of the
interfaces between the CFTs. The dual spacetimes in the holographic model
include two regions separated by a dynamical interface with some tension. For
two or more spatial dimensions, we find that a layer of CFT with more degrees
of freedom than the surrounding one can have an anomalously large negative
vacuum energy density for certain types of interfaces. The negative energy
density (or null-energy density in the direction perpendicular to the
interface) becomes arbitrarily large for fixed layer width when the tension of
the bulk interface approaches a lower critical value. We argue that in cases
where we have large negative energy density, we also have an anomalously high
transition temperature to the high-temperature thermal state.
