Layer Of 3He Research Articles

Study of magnetic phase transitions by helium-3 in DyF3 nanoparticles

The spin kinetics of liquid 3He in contact with a mixture of LaF3 (99.7 %) and DyF3 (0.3 %) powders at temperatures of 1.5–3 K was studied by pulsed nuclear magnetic resonance. DyF3 particles have ellipsoidal shape and average size 225 × 153 nm with the magnetic phase transition at temperature Tc = 2.45 K. The temperature dependence of the longitudinal and transverse magnetization relaxation rates of 3He nuclei with a mixture of LaF3 and DyF3 powders has a broad maximum near Tc, which is related to the magnetic phase transition. It is shown that the adsorbed layer of 3He is extremely sensitive to magnetic phase transitions. Preplating the particles surface with adsorbed nitrogen layers allowed to vary the distance from 3He nuclei to the surface of particles in a controlled way. Calculations of the nuclear magnetic relaxation rates with a change in the distance of helium-3 from the particle surface have been carried out, which is in good agreement with experimental data. It was shown that helium-3 is extremely sensitive to surface magnetism and can be used to study surface effects and surface magnetism.

Density profile of 3He in a nanoscale 3He-4He superfluid film determined by neutron scattering

For decades, superfluid helium has attracted the interest of the scientific community as an extremely pure realisation of a quantum liquid, only accessible at temperatures close to absolute zero. Previously, helium films have only been observed directly using X-rays. However, this method is limited to temperatures above 1 K due to the high levels of energy deposition, and it also suffers from an inability to distinguish between helium isotopes. Here we show that a 3He layer on top of a phase separated mixture film at 170 mK gradually dissolves into the 4He with increasing temperature. We also observe an anomaly in film behaviour near 300 mK and unexpected restoration of the layered structure at 1.5 K which is consistent with a re-entrant phase transition leading to the suppression of superfluidity in the film near 300 mK. Our successful application of neutron scattering to study helium films at ultra-low temperatures opens up new possibilities for future research.

Ion cyclotron resonance heating scenarios for DEMO

The present paper offers an overview of the potential of ion cyclotron resonance heating (ICRH) or radio frequency heating for the DEMO machine. It is found that various suitable heating schemes are available. Similar to ITER and in view of the limited bandwidth of about 10 MHz that can be achieved to ensure optimal functioning of the launcher, it is proposed to make core second harmonic tritium heating the key ion heating scheme, assisted by fundamental cyclotron heating 3He in the early phase of the discharge; for the present design of DEMO—with a static magnetic field strength of Bo = 5.855 T—that places the T and 3He layers in the core for f = 60 MHz and suggests centering the bandwidth around that main operating frequency. In line with earlier studies for hot, dense plasmas in large-size magnetic confinement machines, it is shown that good single pass absorption is achieved but that the size as well as the operating density and temperature of the machine cause the electrons to absorb a non-negligible fraction of the power away from the core when core ion heating is aimed at. Current drive and alternative heating options are briefly discussed and a dedicated computation is done for the traveling wave antenna, proposed for DEMO in view of its compatibility with substantial antenna–plasma distances. The various tasks that ICRH can fulfill are briefly listed. Finally, the impact of transport and the sensitivity of the obtained results to changes in the machine parameters is commented on.

Matsubara Green's function of 3He atoms floating on a 4He film adsorbed on graphite: A worm algorithm Monte Carlo study with a fictitious potential

A system consisting of 3He atoms floating on the surface of a 4He film adsorbed to a graphite substrate is simulated using the state-of-the-art worm algorithm path integral Monte Carlo technique. The 4He–4He interactions are described by the usual HFDHE2 potential of Aziz et al. [J. Chem. Phys. 70, 4330 (1979)], whereas the 3He–3He interactions are modelled by a fictitious potential (FP) introduced to see what new features can be obtained. We are, so to speak, imagining and predicting the future experimental design of such synthetic potentials based on the fact that before the advent of the Feshbach resonance technique the manipulations of interatomic interactions was unthinkable. This work particularly examines the finite-temperature Matsubara Green's function (MGF) of the above system, and to the best of knowledge this has not been done before. It is found that the MGF displays strong signals arising from particle-hole (p–h) excitations in the 3He as well as the 4He layers. The main goal of this work is then to shed light on the role played by the 3He atoms in defining the strength of these excitations that are explained to occur as a result of 4He particle promotions to the 3He layer and the zero-point motion of 3He atoms. It is argued, that the scattering of the 4He atoms with the 3He particles is the major driving force for strong p–h excitations signalled by the MGF. For some reason, the statistical potential is able to yield values for the numerically obtained zero-point kinetic energy that is close to the one from ideal gas theory. In the absence of 3He atoms, no strong p–h excitations are detected. Our efforts are aimed at a rejuvination of this subject by an examination of the MGF, as the literature on this topic is rare. A value-added feature of this work is the use of the above mentioned FP for a qualitative description of the 3He–3He interactions since we are only interested in a qualitative study. The spatial positions of 3He and 4He atoms are displayed in a set of figures, with emphasis on a “fake” 3He clusterization. Numerical results for the energies are presented which are comparable to those reported by Krotscheck [Phys. Rev. B 32, 5713 (1985)] and Gasparini et al. [Phys. Rev. B 29, 4921 (1984)]. It should be noted, that the present simulations have been heavy-computational and required extensive CPU times.

Application of low frequency SQUID NMR to the ultra-low temperature study of atomically layered 3He films adsorbed on graphite

Low frequency pulsed NMR is a powerful technique for the investigation of the nuclear susceptibility and spin-dynamics of strongly correlated systems down to ultra-low temperatures. We describe a versatile broadband pulsed NMR spectrometer using a two- stage Superconducting Quantum Interference Device (SQUID). This instrument has enabled the investigation of the spin-dynamics of 3He films adsorbed on exfoliated graphite into the microkelvin temperature range. In prior work we reported a SQUID NMR study of twodimensional ferromagnetism on a triangular lattice, where 3He films adsorbed on graphite provide an ideal model system. Here we describe the detection of the much weaker signals from strongly correlated fluid films in the second layer of 3He on graphite, using a spectrometer with improved sensitivity. We have measured the low frequency spin-spin relaxation and spin-lattice relaxation of two-dimensional 3He into the microkelvin range. These show an unusual time and frequency dependence. We also describe the use of the 13C-signal from the exfoliated graphite for thermometry, and the unusual properties of the spin-lattice relaxation of that system.

The spin kinetics of 3He in contact with nanosized crystalline powders LaF3

The spin kinetics of 3He in contact with nanosized crystalline powders LaF3 has been studied by NMR methods at the temperature 1.5 K. The 3He longitudinal relaxation time increases proportionally to the magnitude of the external magnetic field and the transverse relaxation time does not depend on the magnetic field. Relaxation of the gaseous and liquid 3He in contact with nanosized crystalline powder LaF3 takes place by the 3He adsorbed layer. The nuclear magnetic relaxation of adsorbed 3He layer on the surface of LaF3 nanoparticles is due to the two-dimensional spin-diffusion motion.

Neutron reflection from the surface of a liquid 4He-3He mixture

We have used neutron reflection from the liquid surface at ultra-low temperatures to study the surface properties of liquid helium. We measured neutron scattering from the free surfaces of commercially pure 4He (0.3 ppm of 3He impurities) and of a 3He-4He mixture with a 3He concentration of 0.5% for temperatures in the range from 340mK to 2.2K We compare the reflected neutron intensity for different temperatures and we fit a model that describes the collected data. The data are described well by a diffusive 3He layer of a few hundred Angstrom thickness on the bulk 4He liquid surface. Even at high temperatures (∼2K) there is an increased concentration of 3He atoms near the liquid surface. The distribution does not change very much with temperature. At low temperatures the neutron absorption increases significantly, which might be an indication of the formation of Andreev states. However, the shapes of the curves do not change very much which seems to suggest that the layer formed by the 3He atoms in Andreev states is very thin ∼ 10Å. The experimental method, based on neutron reflectometry, opens up new opportunities for the study of the surfaces and interfaces of quantum fluids and solids.

Magnetic properties and concurrence for fluid 3He on kagome lattice

We present the results of magnetic properties and entanglement for kagome lattice using Heisenberg model with two- and three-site exchange interactions in strong magnetic field. Kagome lattice correspond to the third layer of fluid 3He absorbed on the surface of graphite. The magnetic properties and concurrence as a measure of pairwise thermal entanglement are studied by means of variational mean-field like treatment based on Gibbs-Bogoliubov inequality. The system exhibits different magnetic behaviors depending on the values of the exchange parameters (J 2, J 3). We have obtained the magnetization plateaus at low temperatures. The central theme of the paper is comparing the entanglement and magnetic behavior for kagome lattice. We have found that in the antiferromagnetic region behavior of the concurrence coincides with the magnetic susceptibility one.

Magnetization of 3He layers in ferromagnetic regime: Cluster size effects

Abstract Magnetization of pure 3He monolayers on graphite and solid 3He on 4He-preplated graphite in the ferromagnetic regime is investigated theoretically within a two-dimensional spin-1/2 Heisenberg model in an external magnetic field. We develop an analytical approach based on a second-order two-time Green function formalism which describes the thermodynamic functions for finite as well as for infinite spin systems in the whole temperature range at arbitrary fields h. In particular, the present theory provides a proper description of the magnetization in the intermediate temperature region h T J at h well below the exchange J, that is in the region where the measurements for solid 3He monolayers are usually made. It is known from the experiment that only a part of 3He atoms is involved into ferromagnetic exchange. The ferromagnetic spins form clusters whose size is a function of coverage. The coverage dependences of the exchange constant, saturation magnetization, and average cluster size are found and analyzed. Nonmonotonic variation of the exchange constant with coverage is discussed. The effect of the cluster size on the temperature dependence of the magnetization is studied. To clarify how the cluster shape and boundary conditions affect magnetic properties of nanoclusters we calculate numerically the magnetization of small magnetic systems using the exact diagonalization method.

Fisher zeros on the Husimi lattice

Fisher zeros for the partition function with respect to a temperature-dependent parameter are studied. The Ising approximation for Heisenberg model with two- and three-site exchange interactions on the Husimi lattice was used. This model approximates the third layer of 3He, absorbed on the surface of graphite (kagome lattice). Using dynamic approach, we have found an exact recursion relation for the partition function. The presence of a phase transition, both in the real and complex regions on the temperature plane was shown.

Magnetic Phase Transition in a Nanonetwork of SolidHe3in Aerogel

When immersed in liquid 3He, the nanometer strands of aerogel are coated with a thin layer of solid 3He, forming a network of irregular nanotubes. Owing to its high purity and weak interactions, this system is ideal for studying fundamental processes. We report the first experiments on solid 3He in aerogel at ultralow temperatures, cooled by direct adiabatic demagnetization. Simultaneous nuclear magnetic susceptibility and heat capacity measurements indicate a magnetic phase transition.

Magnetization and Lyapunov exponents on a kagome chain with multi-site exchange interaction

The Ising approximation of the Heisenberg model in a strong magnetic field, with two, three and six spin exchange interactions is studied on a kagome chain. The kagome chain can be considered as an approximation of the third layer of 3He absorbed on the surface of graphite (kagome lattice). By using dynamical approach we have found one- and multi-dimensional mappings (recursion relations) for the partition function. The magnetization diagrams are plotted and they show that the kagome chain is separating into four sublattices with different magnetizations. Magnetization curves of two sublattices exhibit plateaus at zero and 2/3 of the saturation field. The maximal Lyapunov exponent for multi-dimensional mapping is considered and it is shown that near the magnetization plateaus the maximal Lyapunov exponent also exhibits plateaus.

Magnetic Susceptibility of Heavy Fermion 3He-Bilayers

A 3He bilayer film adsorbed on graphite preplated with a bilayer of solid 4He has recently been shown to be a new heavy fermion system that can be tuned towards a quantum critical point (QCP) by varying the 3He coverage. We report new measurements of the temperature dependence of the magnetic susceptibility of this system over the temperature range 0.3 to 300 mK. The magnetisation of the 3He film is measured using pulsed NMR at a frequency of 60 kHz, on a SQUID detector setup with an untuned input circuit. The signal sensitivity attainable with this setup enables us to collect high-precision data with reasonable acquisition times, despite the low surface area of the sample (2 m2) and the small liquid susceptibility. The detailed NMR response provides an essential diagnostic tool to precisely adjust the 4He preplating. On cooling, the magnetisation evolves from Curie-law, through a distinct Kondo-like maximum at a temperature T*, to a weakly rising magnetisation far below the Curie law. This behaviour signifies the formation of the heavy fermion state of the coupled 3He layers. As the 3He coverage is increased towards the putative QCP, T* is tuned to lower temperature. These preliminary observations demonstrate the power of the technique, confirm previous results obtained in a different sample cell by a different method, and pave the way for a detailed study close to the QCP.

Patterns and heat transfer in Rayleigh-Bénard thermal gravitational convection in helium (3He) near the critical point

Thermal gravitational convection in a bottom-heated layer of near-critical 3He is considered. The range of criteria determining the convection parameters beyond the stability threshold is discussed. The specific features of 2D and 3D supercritical structures, the adiabatic compression effect, and heat transfer are considered.

Pulse NMR of 3He in aerogel at temperature 1.5 K

Experimental data of 3He pulse NMR in contact with aerogel (95%) in a nuclear Larmor frequency range of 5 to 20 MHz at T=1.5 K are reported. For the first time spin kinetics of 3He (adsorbed, gaseous and liquid) has been studied systematically at temperatures higher than degeneracy temperature of this quantum liquid. It has been shown, that in all cases the nuclear spin-lattice relaxation time T1 is linearly proportional to the Larmor frequency, while the spin-spin relaxation time T2 is frequency independent. The magnetic relaxation times T1 and T2 are linearly proportional to the ratio of the magnetization of whole spin system to magnetization of the adsorbed layer of 3He.

Cw-NMR study of liquid 3He in nanometer-sized porous alumina

Continuous-wave NMR measurements were performed for liquid 3He in porous alumina discs, which have many small pores of 200 nm in nominal diameter regularly aligned in the same direction. The experimental cell is composed of a stack of 85 anopore discs with C-shaped disc spacers between them. The spacers make the sample space well defined and make it easier to discriminate between liquid NMR signals from different spaces. To eliminate solid 3He layer adsorbed on the alumina's surface, 4 atomic layers of 4He were pre-coated on the surface in the experimental cell. Experiments were made at pressure of 17.9 bar and temperatures down to about 0.5 mK. The obtained NMR signals are composed of two contributions below superfluid transition temperatures. One seems to come from the liquid inside pores and the other between each anopore discs.

Nuclear magnetic relaxation of 3He in contact with an aerogel above the Fermi temperature

The spin kinetics of 3He in an aerogel has been studied above the Fermi temperature. The magnetic relaxation times T 1 and T 2 of adsorbed, gaseous, and liquid 3He in a 95% silica aerogel at a temperature of 1.5 K have been determined as functions of frequency by means of pulse nuclear magnetic resonance. It has been found that the time T 1 is linear in frequency in all three cases, whereas T 2 is independent of frequency. To explain the observed behavior of the longitudinal relaxation rate, a theoretical model of relaxation in the adsorbed layer of 3He taking into account the filamentary structure of the aerogel is proposed.

Large Boundary Magnetism and Superfluidity of Liquid 3He in Nanometer-sized Porous Alumina

Continuous-wave NMR measurements were performed for liquid 3He in porous alumina with nominal pore size of 20 nm in diameter, at temperatures down to 0.3 mK. The signal is composed of two contributions: from the liquid and from the boundary solid layer of 3He on the alumina’s surface. The latter shows a well-known ferromagnetic tendency and signal intensities can be fitted to a Curie-Weiss law in the high temperature region. The obtained Weiss temperatures are 0.18 and 0.50 mK at 7.5 and 28 bar, respectively. 4He coverage (4 monolayers) completely eliminates boundary signal between 7.5 bar and 32.5 bar. The residual liquid signal shows frequency shift and broadening below superfluid transition temperatures depending on liquid pressures. The obtained P-T phase diagram well resembles that of bulk liquid 3He in spite of the very narrow pore-size comparable to the coherence length of superfluid 3He.

Growth of third ferromagnetic solid 3He layer on graphite with and without 4He pre-plating

We have measured the nuclear susceptibility of 3He in Grafoil filled with pure liquid 3He over the pressure region between 0.6 and 31.38 bar and at temperatures down to 0.5 mK with a cw NMR method. The nuclear magnetization corresponding to the adsorbed 3He layers on the Grafoil surface shows a strong ferromagnetic tendency with a periodic behavior as a function of liquid pressure. This observation is attributable to the growth of third and fourth solid 3He layer with the liquid pressure increase. The pressure dependence of the Weiss temperature indicates that the third layer is completed at 19 bar and the fourth probably at 28 bar. The results for Grafoil pre-plated by 2.5 and 3.5 layers of 4He are consistent with this scenario.

Solidification of the third helium layer on graphite

We report heat capacity and magnetization measurements of 3He adsorbed on graphite pre-plated by a solid bilayer of 4He. The first 3He layer adsorbed on this composite substrate remains fluid up to and beyond layer promotion. However it is observed to solidify significantly before the second 3He layer is completed. Remarkably, at fluid coverages on the verge of solidification, we observe a distinctive heat capacity and magnetisation maximum, which occurs at progressively lower temperatures as the coverage is increased. Possible magnetic instabilities in the fluid and analogues with heavy fermion systems are discussed.