Heat capacity of mixtures of ³He-⁴He confined to coupled 1 μm boxes

We have measured the heat capacity of helium mixtures confined to lithographically created cylindrical boxes whose height, 1.08 μm, equals their diameter. The specific heat at constant concentration x, C_px, is renormalized due to the ³He impurity. Thus, in order to observe critical behavior, a conversion must be made to a specific heat at constant φ = μ₃ - μ₄, C_pφ. The confined system's specific heat, near it's maximum, has values which rise above the bulk system's specific heat at the same temperature. This is unexpected and is true for both C_px and C_pφ. This enhancement, not observed with pure ⁴He, becomes more dramatic as x and the correlation length increase. The shift of the maximum with x is much larger for the boxes than for 2D confined mixtures. These observations might be related to a "band structure" effect associated with the 18.5 nm channels which connect the cylinders.