Characterization of ZYX exfoliated graphite for studies of monolayer He3 below 1 mK

Abstract

We have characterized ZYX exfoliated graphite (ZYX) by measuring the surface area and electrical resistivity perpendicular (ρ⊥) to the basal plane as a function of density. The purpose of the characterization is to seek possibilities to use this material as an adsorption substrate for studies of two dimensional (2D) He3 at ultralow temperatures (ULT) below 1 mK. A potential advantage of ZYX is its much larger single crystalline size (100–200 nm) than that of Grafoil (≈10 nm) which is an exfoliated graphite commonly used at ULT. Present results show that ZYX has sufficiently large surface area to be used in heat capacity or nuclear magnetic resonance experiments on 2D He3 samples and good thermal conductance to cool them well below 1 mK. We have demonstrated this by measuring nuclear magnetic susceptibility of monolayer solid He3 adsorbed on ZYX with a density of 1.1 g/cm3 down to 100 μK. Measured temperature dependencies of ρ⊥ and ρ∥ (resistivity parallel to the basal plane) below 70 K are discussed within the variable range hopping model plus a metallic component of carriers.