Nanoscale, phonon-coupled calorimetry with sub-attojoule/Kelvin resolution

We have developed an ultrasensitive nanoscale calorimeter that enables heat capacity measurements upon minute, externally affixed (phonon-coupled) samples at low temperatures. For a 5 s measurement at 2 K, we demonstrate an unprecedented resolution of Delta C similar to 0.5 aJ/K (similar to 36 000 k(B)). This sensitivity is sufficient to enable heat capacity measurements upon zeptomole-scale samples or upon adsorbates with sub-monolayer coverage across the minute cross sections of these devices. We describe the fabrication and operation of these devices and demonstrate their sensitivity by measuring an adsorbed He-4 film with optimum resolution of similar to 3 x 10(-5) monolayers upon an active surface area of only similar to 1.2 x 10(-9) m(2).