Magnetization Curve of Second Layer Anti-Ferromagnetic Solid 3He on Graphite

Two dimensional anti-ferromagnetic solid ³He on graphite in the so called 4/7 phase is a highly frustrated quantum spin system and its ground state is considered to be a gapless spin liquid. To investigate the detailed magnetic behavior, the magnetization curve has been measured below 1 mK in high magnetic fields, by use of NMR over a wide frequency range up to 84 MHz. The magnetization of the 4/7 phase seems to have a plateau at half the saturation magnetization, which is not inconsistent with results obtained with a double gradient Faraday magnetometer.      

Nuclear Magnetism of Submonolayer Solid 3He on Graphite

Submonolayer solid ³ He adsorbed on graphite has been studied with NMR measurement down to the order of 0.1 mK. The magnetization curves are well fitted to the Curie-Weiss law in the whole investigated density region (5.0 nm ^–2 - 8.5 nm ^–2 ). The obtained Weiss temperatures are found to be positive at a commensurate phase and an incommensurate one. However they are negative in the intermediate density region, which suggests the existence of an antiferromaynetic phase also in the Submonolayer. The change from an antiferro to a ferromagnetic behavior at around 7.3 nm ^–2 is similar to that of the second layer, and can be explained by a multiple spin exchange model. Therefore the ferromagnetic behavior in the second layer is not necessarily due to the liquid overlayer. While the ferromagnetic behavior at the phase is not reproduced by a simple density dependence of competing multiple exchange interactions and indicates the importance of the corrugation potential from the graphite substrate.     

Thermal Relaxation in 3He Solid Films on Graphite

The thermal conductance (κ) between ³He solid films and graphite substrates is measured in weak magnetic fields of 0, 150, 300, and 600 Oe by a relaxation method. κ is markedly reduced in a magnetic field of 150 Oe and shows complicated magnetic-field-dependent variations. Previous measurements have revealed that heat flows along ³He solid films over a long distance and then flows into the graphite substrate at some local spots. The reduction in κ in magnetic field indicates an increase in the length of the thermal flow path and a decrease in the number of local spots through which heat is transferred to the graphite substrate. These suggest the important role of magnetic impurities present in the graphite substrate. The temperature-dependent variation in the ratio of κ and heat capacity (C), κ/C, is nearly unchanged when the magnetic field increases from 150 to 600 Oe, while κ shows complicated variations. κ/C shows a clear minimum at a temperature around 1 mK, which depends on the areal density of ³He. Above and below this temperature, heat transfer is thought to take place along the solid ³He film, respectively by phonons and spin excitations.     

Static Structure Factor of Two-Dimensional Liquid 3He Adsorbed on Graphite

Liquid ³He is a model system for strongly correlated Fermi liquids. For this reason, many X-ray and neutron scattering experiments have been performed to understand the structure and dynamics of this quantum fluid. We have recently shown that two-dimensional liquid ³He sustains long-lived zero-sound excitations at large wave-vectors (Godfrin et al., Nature, 483:576, 2012). Here we show that its static structure factor can be obtained with reasonable accuracy by integrating the experimental S(Q,ω) over a suitable energy range. A good agreement is found between the static structure factor deduced from the experiment and theoretical models: Quantum Monte Carlo simulations and Dynamical Many Body Theory (DMBT). At high wave-vectors, the experimental values are underestimated because of the limited accessible phase space; nevertheless, even at atomic wave-vectors a semi-quantitative agreement is observed with the theoretical predictions.     

Thermal Relaxation of 3He Solid Films Adsorbed on Graphite

No Heading The thermal conductivity between the ³He solid films and the graphite substrate was measured by the relaxation method between 100 K and 1 mK. The areal-density depedence of the thermal conductivity shows behavior similar to that of the exchange frequency J both in the submonolayer and in the second layer. These facts indicate that heat is transferred by magnetic mechanisms in the ³He solid film itself. They also imply that the ³He solid film is thermally connected with the graphite substrate only at some local spots.PACS numbers: 67.70.+n, 67.80.Gb     

Low Temperature Magnetization of Two Dimensional 3He on Three Layers of HD Preplated Graphite

We have measured the low temperature magnetization of submonolayer ³He on three layers of HD preplated graphite. NMR measurement has been performed by progressively adding ⁴He to the system, to prevent ³He atoms from being trapped in substrate heterogeneities and to change the areal density of ³He. The exchange constant (J) is found to have a similar density dependence to that for two layers of HD preplated one. The magnetization in the antiferromagnetic region increases gradually down to 100 K and shows no evidence corresponding to a spin gap.     

Nuclear Magnetism of Two Dimensional Solid 3He Adsorbed on Plated Graphite

Nuclear spin exchange in the low density two dimensional quantum solid formed by ³ He adsorbed on graphite plated by a bilayer of HD has been studied by heat capacity and magnetization measurements. The low densities result in a frustrated magnet with significantly larger effective exchange constants than observed in the antiferromagnetic second layer of ³ He on bare graphite. Promotion to a fluid overlayer occurs at 6.8 nm ^–2. A ferromagnetic anomaly is observed in magnetization measurements at 10 nm ^–2 with an effective exchange constant of 1.0 mK, about half the value observed at the corresponding anomaly on bare graphite.     

Heat Capacity of 3He Solid Film on Graphite in Magnetic Fields

Heat capacities of a submonolayer ³He solid film adsorbed on a graphite surface are measured in magnetic fields up to 400 Oe. The measured heat capacity shifts to higher temperatures with an increasing magnetic field. The amplitudes of the shifts are twenty times larger than the magnitude of the Zeeman energy, which is anomalously large. With regard to the origin of these large shifts, the reduction of frustrations and the weakening of the competition between multiple spin exchange interactions are discussed.     

Direct Nuclear Demagnetization of Two Dimensional Solid 3He Adsorbed on Graphite

Direct nuclear demagnetization has been made for two-dimensional solid ³He adsorbed on graphite. The lowest temperature is estimated to be about 10 µK from the observed magnetization of the paramagnetic solid ³He. The deviation from, ideal behavior is explained by a simple heat flow model.     

Preliminary Heat-Capacity Measurements of 2D Solid 3He Adsorbed on Graphite Preplated with 4He

We describe preliminary resugts of heat capacity measurements of the 4/7 phase in two dimensional ³He adsorbed on graphite preplated with a ⁴He monolayer in a temperature range between 0.17 and 20 mK. In zero magnetic field, we observed a double-peak structure similar to that reported previously for the same phase in a different system (³He/³He/gr) in which the gapless quantum spin-liquid ground state is proposed. The exchange interaction deduced from the high temperature data is by a factor of 30% smaller than the previous one, presumably because of the smaller lattice constant for the ³He/⁴He/gr system. The present resugt provides strong evidence that such a peculiar temperature dependence of heat capacity is characteristic of the 4/7 phase. Application of magnetic fields up to 0.65 T on this phase does not seem to change appreciably the higher temperature broad peak at around 1.4 mK.     

Possibility of a Chiral Phase Transition in Two-Dimensional 3He Solid

We study a Hamiltonian of S = 1/2 spins with two-, three and four-spin exchange interactions on the triangular lattice as a possible model of the nuclear magnetism of solid ³ He layers. The spin wave theory shows that the tetrahedral ground state, which was shown to be favoured by the four-spin exchange interaction in our previous paper, is stable against quantum fluctuations in some parameter region. Since this state has a scalar chiral long-range order, a phase transition occurs at a finite temperature even though the Hamiltonian has a full rotational symmetry in the spin space. Critical behavior of this phase transition was examined by classical Monte Carlo simulations. The specific heat diverges much more strongly than that of the 2D Ising model.     

Ultra-Low Temperature Susceptibility of a Highly Frustrated Two-Dimensional Solid 3He Magnet

We report the first measurement of the ultra-low temperature (200 ,K) susceptibility of a two-dimensional antiferromagnetic solid ³ He system, the 4/7 commensurate phase of ³ He/ ⁴ He/graphite. In the high temperature range the data has been analyzed using the series expansions of the Multiple-Spin-Exchange model. At ultra-low temperatures the susceptibility displays a very unusual temperature dependence, characteristic of a highly frustrated two-dimensional system.     

Thermodynamic Magnetization of Liquid 3He

We have measured the absolute thermodynamic magnetization of ³ He in all of its liquid phases while using a new calibration technique which relies on the nuclear paramagnetism over the entire temperature range below 1 K as measured by Ramm, et al. We have directly measured the temperature dependence separately of the nuclear paramagnetism via NMR and of the total magnetism via our free-surface modulation technique. We report substantial agreement with Ramm, et al. In consequence, we find that the diamagnetism of liquid ³ He is significantly smaller than expected theoretically, a condensed matter effect not seen previously. We have also remeasured the magnitude of the magnetization discontinuity at the superfluid A-B transition and find close agreement at saturated vapor pressure with NMR-deduced values from Scholz.     

Heat Transfer Between 3He Solid Films and Graphite in Magnetic Fields

The thermal conductance between second-layer ³He solid films and a graphite substrate was measured in magnetic fields up to 300 Oe. The thermal conductance is strongly influenced by the magnetic fields. Previous measurements in zero magnetic fields have revealed that heat is transferred only by some local spots between the ³He films and graphite substrates. This suggests that the magnetic Kapitza conductance functions as the heat transfer mechanism. The results of the measurements confirm the magnetic Kapitza mechanism.     

Heat Capacities of Submonolayer Solid 3He Films on Graphite at Highly Frustrated Density Regime

No Heading The heat capacities (C) of submonolayer solid films adsorbed on a graphite surface are measured down to 100 K at areal densities between 6.6 and 7.5 nm^–2, where there has been no previous measurement below 2 mK. The temperature (T) variation of C shows almost single power low behavior, C T, in a wide temperature range. And the exponent is much smaller than –2, an expected exponent for localized spins in the high temperature limit. These behaviors should imply that the competition of multiple spin exchange interactions is strong at this density range.PACS numbers: 67.70.+n, 67.80.Gb     

Surface Magnetism of Liquid 3He on 4He Pre-plated Graphite

No Heading The nuclear susceptibility of liquid ³He in Grafoil pre-plated by 2.5 and 3.5 layers of ⁴He has been studied with a cw NMR method at temperatures between 0.7 and 100 mK under various liquid pressures. The 3.5 layers of ⁴He pre-plating suppresses a formation of the first and second solid ³He layer, eliminating most of surface magnetization at saturated vapor pressure. However, with increasing liquid pressure, a magnetization obeying a Curie Weiss law gradually grows in the same way as in the previous experiment for pure liquid ³He. This magnetization, induced by pressurization, is attributable to the formation of solid ³He layer above the pre-plated ⁴He.PACS numbers: 67.80.Jd, 75.70 Cn, 67.70.+n.     

Thermodynamic Magnetization of Normal and Superfluid 3He

We have measured the thermodynamic magnetization in the various phases of normal and superfluid ³ He. The A-B discontinuity differs from that inferred from nuclear-specific (NMR) experiments. We offer reasons why this apparent disagreement can be consistent with our current picture of super-fluid ³ He. One consequence of this measurement is an improved set of values for the phenomenological Landau-Ginzburg free energy -parameters.     

Heat Capacity of Dilute 3He–4He Films on Graphite

The heat capacity of dilute ³He–⁴He films is measured to clarify whether the second adsorbed layer of ⁴He films on graphite solidify into the so-called “4/7 phase.” The ³He areal density is fixed at 0.2 nm^?2, whereas the ⁴He areal density is gradually increased. The measured heat capacities suddenly decrease with an increasing areal density approaching that of the 4/7 phase. Above the areal density of the 4/7 phase, the heat capacities do not reduce completely to zero and have finite values. The behavior of the heat capacity does not change over a rather wide areal density regime, although it suddenly increases or recovers at around the areal density of the third-layer promotion. These behaviors can be interpreted as the separation of ³He–⁴He mixture films into a ³He-rich phase and a ⁴He-rich phase, with the ³He-rich phase solidifying into the 4/7 phase and the ⁴He-rich phase remaining fluid below the areal density of the third-layer promotion. These observations strongly suggest that a ⁴He film adsorbed on a graphite surface does not solidify into the 4/7 phase.     

Magnetization transfer between substrates mediated by3He

The system under study is a mixture of polystyrene microspheres, liquid³He, and Teflon microspheres. Using NMR, we observe the direct coupling of¹H nuclei to³He nuclei. T₁ data is used to predict the amount of coupling. The predicted coupling is then compared with the experimental observations. Experiments are conducted between 35 mK and 200 mK and in magnetic fields of 0.15 T, 0.32 T, and 0.54 T. Finally, we observed magnetization transport from¹H through³He to¹⁹F nuclei.