Magnetism of ACFs and superfluidity of 4He adsorbed in ACFs

Abstract

Activated carbon fibers (ACFs) are a nanoporous form of carbon with huge specific surface areas caused by a three-dimensional random network of nanographites. Because of nano-size effects, non-bonding π-electron spins emerge on the nanographite. The nanographite random network produces many nanopores with a mean size of several nanometers and creates a host system for various guest molecules in ACFs. In order to investigate the magnetic properties of nanographite and the superfluidity of 4He restricted in nano-spaces, the magnetization of ACFs and superfluidity of 4He adsorbed in ACFs have been investigated. The magnetization shows an antiferromagnetic interaction between the non-bonding π-electron spins. Near the insulator-metal transition caused by heat treatment of ACFs, spin glass-like disordered magnetism observed. Up to an 4 He coverage of 22.6 μmol/m 2 , no superfluidity is observed due to the strong van der Waals force between 4 He and nanographite. Over 23.7 μmol/m 2 4He coverage, the superfluid transition is observed at approximately Tc ~ 500 mK. Upon increasing the 4 He coverage, the superfluid density increases, but no change in Tc is observed. These observations indicate that the thickness of superfluid films on nanographite is restricted by the slit type pore shape of ACFs. Copyright © 2018 VBRI Press.