Nuclear Magnetism in Two-Dimensional Solid Helium Three on Graphite

Abstract

Recent studies of nuclear magnetism of monolayer helium three ( 3 He) adsorbed on graphite are reviewed. This system provides us a fascinating testing ground for physics of highly frustrated quantum spins of two dimensions (2D) such as spin liquids. Particular emphasis is devoted to the gapless spin liquid state observed in the low-density commensurate phase (4/7 phase) in the second layer. Existing experiments on heat capacity in zero magnetic field and magnetization in low fields of this phase show unambiguously the gapless excitations down to three orders of magnitude lower temperatures than the relevant exchange interactions (%1 mK). This is a new class of spin liquid, and recently electronic counterparts are found in quasi-1D and -2D conductors. On the other hand, measured magnetic properties of the incommensurate phase at much higher densities can be described as a nearly ideal 2D S ¼ 1=2 ferromagnet on a triangular lattice. We propose the existence of a new ferromagnetic commensurate phase at a density in between the 4/7 and incommensurate phases based on new heat capacity data. None of these phases show signatures of finite temperature phase transitions being consistent with the MerminWagner theorem. The exchange interactions among 3 He nuclear spins originate from atom-atom tunnel exchanges due to the large zero-point motions. Because of the steric hindrance, higher order exchanges like three-, four-, five-, and six-spin ring exchanges are expected to compete each other in similar strengths (MSE model). The applicability of the MSE model to the experimental magnetic properties of this system including its density variation is discussed. It is satisfactory except the fact that the model does not reproduce the gapless nature of the 4/7 phase, which recently stimulates different theoretical approaches such as the Hubbard model.

Structural evolution of ferromagnetic 3He multilayers adsorbed on exfoliated graphite

“…5 which describes properly the thermodynamics of infinite and finite-sized spin systems at arbitrary temperatures and fields is applied in the present paper to interpret the experimentally observed exchange evolution in 3 He multilayers on graphite depending on total coverage of helium deposit. We give a consistent interpretation to a great number of experimental data on magnetization of 3 He monolayer on graphite and 3 He on 4 He-preplated graphite in the ferromagnetic regime. Fitting the present theory to the experimental data we extract the exchange constant J, saturation magnetization sat , M and average cluster size N at given h .…”

“…At present, it is generally agreed that namely nuclear spins of second layer solid are responsible for magnetic properties of 3 He multilayered system on graphite [3,4]. The thermodynamics of 2D 3 He in millikelvin region is completely determined by exchange processes in the subsystem of nuclear spins because at such ultralow temperature the lattice excitations in the helium system with direct interatomic van der Waals interaction of order a few Kelvins are practically frozen.…”

On the ‘Supersolid’ Response of the Second Layer of $$^4 \hbox {He}$$ 4 He on Graphite

“…Firstly, there is clear evidence, from NMR and heat capacity measurements of atomically layered 3 He films on graphite, of a solid phase at higher coverages in the second layer somewhat below promotion to the third layer. This 2D solid of S = 1/2 fermions has strong exchange interactions arising from cyclic ring exchange of 3 He atoms, with clear experimental signatures of the frustrated magnetism in the thermodynamic properties [6][7][8][9][10]. Secondly, torsional oscillator measurements on 4 He films on graphite revealed an anomalous response over a narrow coverage range in the second layer [11].…”

Fluid States of Helium Adsorbed in Nanopores

“…The 2D 4 He superfluid onset (transition) and the 2D 3 He Fermi fluids have been extensively studied for the fluid films formed on flat substrates such as Mylar sheet [2] and graphite [9]. Helium fluid films can be formed on the pore walls of Vycor glasses (porous glasses) of which pores several nm in diameter are three-dimensionally and randomly connected as shown in Fig.…”

“…2D properties of helium have been extensively studied for the substrate of graphite [9]. The phase diagrams (states) precisely determined for 4 He, 3 He and 3 He on 4 Hepreplated graphite strongly reflect the potential profile on the surface [34] as well as the helium coverages [5].…”