S. Murakawa

Characterization of ZYX exfoliated graphite for studies of monolayer 3He below 1 mK

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) 3He 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 3He samples and good thermal conductance to cool them well below 1 mK. We have demonstrated this by measuring nuclear magnetic susceptibility of monolayer solid 3He 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 withi...

NMR Measurements on New Quantum Phases in 2D 3He

Monolayer 3He is an ideal experimental system for studying strongly correlated two dimensional fermions. We measured magnetic properties of the second layer of 3He adsorbed on 4He‐preplated graphite by NMR in a wide temperature (70 μK. < T < 300 mK) and density (4.9 < p <9.8 nm−2) ranges. The measured magnetization isotherms can be classified into four distinct regions depending on density. The present NMR results support the scenario given by the recent heat capacity measurements that the region just below the density for the 4/7 commensurate phase is a hole‐doped Mott localized phase.

Experimental apparatus for heat capacity measurements of 2D 3He in magnetic fields

Abstract Previous heat capacity measurement in zero magnetic field for the low-density second-layer solid 3 He adsorbed on graphite showed a peculiar double peak structure. From this measurement a “quantum spin liquid (QSL)” ground state has been suggested. Increasing the magnetic field, theories predict a transition from a QSL phase to a long-range ordered phase called “uuud” at a certain threshold field, which is a measure of the spin gap of the QSL phase. Here we describe an experimental setup for heat capacity measurements of 2D 3 He to test the predictions at temperatures below 200 μK in magnetic fields up to 1.2 T .

NMR Measurements of 2D 3He Adsorbed on ZYX Exfoliated Graphite

We describe first successful usage of ZYX exfoliated graphite to cool adsorbed two dimensional (2D) 3He to ugtra low temperatures (ULT). ZYX has much better characteristics as an adsorption substrate such as larger crystallite (platelet) size and smaller mosaic angle than Grafoil, a widely used exfoliated graphite in ULT experiments. In spite of the relatively small surface area of ZYX, we could perform a nuclear magnetization measurement of paramagnetic 2D solid 3He with continuous wave NMR below 10 mK. The data follow nicely the Curie law down to 200 µK. This indicates that ZYX can be used in experiments to study detailed density dependences of low temperature properties of 2D 3He including possible BCS transitions. We also show a preliminary resugt of pulsed NMR measurements of high density 2D fluid 3He adsorbed on ZYX preplated with a 4He monolayer.

Large mass enhancement in 3He monolayer fluids adsorbed on graphite preplated with 4He

A fluid monolayer of 3He adsorbed on a graphite surface is an ideal twodimensional (2D) Fermi system. We have observed very large mass enhancement of the quasiparticles (m*/m=20±5) in preliminary heat-capacity measurements for high density 3He fluids adsorbed on graphite preplated with a 4He monolayer (3He/4He/gr). This could be divergence of m* toward the critical density for the 4/7 phase with a triangular lattice, which is a registered phase commensurate against the first layer. The observed m* is larger than those reported for other 2D 3He systems, e.g., m*/m=13 for 3He/HD/HD/gr system, presumably because of the higher critical density. The apparent divergence of m* toward the different Critical densities strongly suggests the Mott–Hubbard type transition between the 2D Fermi fluid phase and the registered phase under a strong onsite hard core repulsion between the 3He atoms.

Characterization of ZYX graphite for studies of two-dimensional 3He at ultra-low temperatures

Abstract So far, theoretically predicted two-dimensional (2D) superfluidity in monolayer 3 He has not been observed yet. This may be due to the smaller single crystalline size (≃10 nm ) of Grafoil substrate than the expected superfluid coherence length (⩾100 nm ) , or pair breaking caused by trapped atoms on substrate heterogeneity. To eliminate these problems, we have made an NMR sample cell using ZYX grade exfoliated graphite substrate. This substrate has at least 10 times larger crystalline size than Grafoil. NMR measurements are now under way to search for possible 2D and quasi 2D superfluidity in 3 He thin films.