Layers Of 4He Research Articles

4He monolayer on graphene: a quantum Monte Carlo study

We revisit the problem of adsorption of a single 4He layer on graphene, focusing on the commensurate (C 1/3) crystalline phase, specifically on whether it may possess a nonzero superfluid response, and on the existence of superfluid phases, either (metastable) liquid or vacancy-doped crystalline. We make use of canonical quantum Monte Carlo simulations at zero and finite temperature, based on a realistic microscopic model of the system. Our results confirm the absence of any superfluid response in the commensurate crystal, and that no thermodynamically stable uniform phase exists at lower coverage. No evidence of a possibly long-lived, metastable superfluid phase at C 1/3 coverage is found. Altogether, the results of ground-state projection methods and finite-temperature simulations are entirely consistent.

Superfluid transition of the second layer of [formula omitted]He on graphite: Does substrate corrugation matter?

The second layer of 4He adsorbed on a graphite substrate is studied by Quantum Monte Carlo simulations. We make use of a microscopic model of the substrate fully accounting for its corrugation, and compare the results to those obtained with a smooth substrate. The only effect of corrugation is a ∼20% reduction of the value of the superfluid fraction of the top layer, in the limit of zero temperature. No evidence of any commensurate (7/12) crystalline and/or “supersolid” phase is found; the superfluid transition temperature is estimated to be ∼0.75 K. We discuss the implication of these findings on the interpretation of recent experiments.

Bound state of a 3He atom at the interface of crystal and superfluid 4He

The bound state of a 3He atom at the interface between coexisting crystalline and superfluid phases of 4He is studied theoretically by means of first principle Quantum Monte Carlo simulations. We consider both the case of a solid/superfluid 4He interface, as well as that of a few solid layers of 4He forming on an attractive substrate. It is observed that the 3He bound state is sharply localized in a well-defined, quasi-2D layer of 4He in the region intermediate between the solid and the liquid. The layer in which the 3He atom resides displays an intriguing interplay of atomic localization and quantum-mechanical exchanges; is not necessarily the first “non-solid” layer, as suggested in previous studies, and it is affected by the attractive strength of the substrate.

Superfluid and Supersolid Phases of ^{4}He on the Second Layer of Graphite.

We revisited the phase diagram of the second layer of ^{4}He on top of graphite using quantum MonteCarlo methods. Our aim was to explore the existence of the novel phases suggested recently in experimental works, and determine their properties and stability limits. We found evidence of a superfluid quantum phase with hexatic correlations, induced by the corrugation of the first Helium layer, and a quasi-two-dimensional supersolid corresponding to a 7/12 registered phase. The 4/7 commensurate solid was found to be unstable, while the triangular incommensurate crystals, stable at large densities, were normal.

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.

Decoupling of Solid 4He Layers under the Superfluid Overlayer

It has been reported that in a large oscillation amplitude, the mass decoupling of multilayer 4He films adsorbed on graphite results from the depinning of the second solid atomic layer. This decoupling suddenly vanishes below a certain low temperature TD due to the cancellation of mass decoupling by the superfluid counterflow of the the overylayer. We studied the relaxation of the depinned state at various temperatures, after reduction of oscillation amplitude below TD. It was found that above the superfluid transition temperature the mass decoupling revives with a relaxation time of several 100 s. It strongly supports that the depinned state of the second solid atomic layer remains underneath the superfluid overlayer.

Effect of Magnetic Boundary Conditions on Superfluid ^{3}He in Nematic Aerogel.

We report results of experiments with superfluid ^{3}He confined in aerogels with parallel strands which lead to anisotropic scattering of ^{3}He quasiparticles. We vary boundary conditions for the scattering by covering the strands with different numbers of atomic ^{4}He layers and observe that the superfluid phase diagram and the nature of superfluid phases strongly depend on the coverage. We assume the main reason for these phenomena is a magnetic channel of the scattering which becomes important at low coverages and can be essential in other Fermi systems with triplet pairing.

4He adsorption on a single C40 molecule: Path integral Monte Carlo study

Path-integral Monte Carlo calculations are performed to study the adsorption of 4He atoms on two different isomers of C40: one with the D2 symmetry and the other with the D5d symmetry. Here, we employ the 4He-C40 interaction described by the sum of 4He-C interatomic pair potentials, which allows us to examine the 4He corrugations on the fullerene molecular surfaces. We first observe the layer-by-layer growth of 4He on both C40 isomers, where the first layers are located at a distance of ~ 5.7 A from the molecular centers. From the investigation of the angular 4He density profiles, we find that the 4He layers display different quantum states depending on the number of 4He adatoms N. Between N = 22 and N = 40, the first 4He layer on the D2 isomer shows several different commensurate structures whereas the corresponding 4He layer on D5d undergoes a commensurate-to-incommensurate solid transition like the first 4He layer on graphite. As N increases, the superfluid responses of the first 4He layers on both isomers exhibit reentrant behavior with superfluidity being completely quenched at structurally-ordered states. In addition, the superfluid fraction of the first 4He layer is found to be about three times as large on D2 as on D5d for the same number of 4He atoms. The different structural and superfluid properties of 4He adatoms depending on the isomeric type could provide an experimental way of selecting a specific C40 isomer.

Intertwined superfluid and density wave order in two-dimensional 4He

A detailed analysis of low-temperature torsional oscillation measurements on two-dimensional 4He reveals evidence for intertwined superfluid and density wave order in this system. Superfluidity is a manifestation of the operation of the laws of quantum mechanics on a macroscopic scale. The conditions under which superfluidity becomes manifest have been extensively explored experimentally in both quantum liquids (liquid 4He being the canonical example) and ultracold atomic gases1,2, including as a function of dimensionality3,4. Of particular interest is the hitherto unresolved question of whether a solid can be superfluid5,6. Here we report the identification of a new state of quantum matter with intertwined superfluid and density wave order in a system of two-dimensional bosons subject to a triangular lattice potential. Using a torsional oscillator we have measured the superfluid response of the second atomic layer of 4He adsorbed on the surface of graphite, over a wide temperature range down to 2 mK. Superfluidity is observed over a narrow range of film densities, emerging suddenly and subsequently collapsing towards a quantum critical point. The unusual temperature dependence of the superfluid density in the limit of zero temperature and the absence of a clear superfluid onset temperature are explained, self-consistently, by an ansatz for the excitation spectrum, reflecting density wave order, and a quasi-condensate wavefunction breaking both gauge and translational symmetry.

Anisotropic superfluidity of (4)He on a C36 fullerene molecule.

We have performed path-integral Monte Carlo calculations to study the adsorption of (4)He atoms on two different C36 isomers with the D6h and the D2d symmetries. The radial (4)He density distributions reveal layer-by-layer growth with the first layer being located at a distance of ∼5.5 Å from the C36 molecular center and the second layer at ∼8.3 Å. From the angular density profiles of (4)He, we find different quantum states as the number of (4)He adatoms N varies. For N = 20, we observe commensurate solid structures on both D6h and D2d isomers, where each of 8 hexagon and 12 pentagon centers of the fullerene surfaces is occupied by a single (4)He atom. The second-layer promotion starts beyond N = 38 on both isomers, where a compressible incommensurate structure is observed on the D6h isomer and another commensurate structure on D2d. Between N = 20 and N = 38, the (4)He monolayer on D6h shows several distinct rings of delocalized (4)He atoms along with strongly anisotropic superfluid responses at low temperatures, while isotropic but weak superfluid responses are observed in the (4)He layer on D2d.

4He adsorption on a H(2)-plated C20 molecular surface: the formation of helium buckyballs.

We perform path-integral Monte Carlo calculations to study the adsorption of 4He atoms on a H2-plated C20 molecular surface. It is found that 32 H2 molecules form a complete solid layer on C20, where each H2 molecule is located either above one of the 12 pentagon centers or above one of the 20 carbon atoms. The angular density profiles of the first 4He layer on the (H2)32-C20 surface reveal different quantum states as the number of 4He atoms N varies. Especially, the helium layer exhibits an icosidodecahedron structure for N=30, where each 4He atom is located at one of the vertices of 20 corner-sharing triangles. While the 4He density peaks for N=60 constitute a truncated icosahedron with 12 pentagonal and 20 hexagonal faces, the additional atoms beyond N=60 are found to be placed at the hexagon centers of the truncated icosahedron to form a hexakis truncated icosahedron for N=80. The superfluid response of the 4He layer at a temperature of T=0.6 K is found to be completely quenched for N=30 and to be significantly suppressed for N=60 and 80, reflecting the formation of compact buckyball structures.

Temperature-dependent pinning or depinning of a 3He overlayer in a 3He-4He mixture film.

We report the results of a quartz crystal microbalance experiment at 100 MHz for a 3He-4He mixture film on a planar gold substrate. The results reveal temperature-dependent pinning or depinning of 3He overlayers above a critical oscillation velocity and indicate that the appearance of a macroscopic condensed state in the underlying 4He layer possibly affects the interfacial friction.

Preliminary Heat Capacity and Vapor Pressure Measurements of 2D 4He on ZYX Graphite

We report preliminary heat capacity and vapor pressure measurements of the first and second layers of 4He adsorbed on ZYX graphite. ZYX is known to have much better crystallinity than Grafoil, the most commonly-used exfoliated graphite substrate, such as a ten-times larger platelet size. This allows us to distinguish different phases in 2D 4He much more clearly and may provide qualitatively different insights into this system. We found a significantly asymmetric density-dependence of the heat-capacity peak associated with the $\sqrt{3}\times\sqrt{3}$ phase formation comparing with that obtained with Grafoil. The 2nd-layer promotion density is determined as 11.8±0.3 nm−2 from the heat-capacity measurement of low density samples in the 2nd layer and vapor pressure measurement.

Slippage of Nonsuperfluid 3He-4He Mixture Film on Gold

We report the result of a QCM experiment at 100 MHz for a coverage of 3He-4He mixture film (n3 = 0.75 and n4 = 2.90 bulk-density layers) on a planar gold substrate. This study is done by taking the temperature scan of both warming and cooling at various constant oscillation velocities v from 19 to 59 μm/s. At minimum v = 19 μm/s, a frequency shift of about 5 Hz due to the superfluid transition is observed below the onset temperature To = 0.16 K Surprisingly, at v = 35 μm/s, the observation is dramatically changed and an extra large frequency shift with a hysteresis loop between warming and cooling is found. Upon increasing v further, the extra shift evolves into much larger shifts. These observations suggest that a part of 3He on underlying 4He layer slips by a pinning-depinning mechanism above v = 35 μm/s. At maximum v = 59 μm/s, the decoupling ratio of slippage reaches 90% of the whole 3He population.

Phase Diagrams of 4He on Flat and Curved Environments

By means of diffusion Monte Carlo calculations, we obtained the phase diagrams of a first and second layer of 4He on graphene and on the outside of different isolated armchair carbon nanotubes with radii in the range 3.42 to 10.85 A. That corresponds to tubes between the (5, 5) and (16, 16) in standard nomenclature. In both cases, the ground state is either a liquid (second layer on graphene and on nanotubes whose radii is greater than ∼7 A) or an incommensurate solid (for thinner tubes). In the former case, upon a density increase, the system undergoes a first-order phase transition to another incommensurate solid. A study of the influence of the C–He potential (isotropic or anisotropic) on the phase diagrams is also presented.

Anomalous Response of 4He Confined in Nanoporous Media to Torsional Oscillation

The existence of “Non-Classical Rotational Inertia (NCRI)” in solid 4He below 0.2 K has been controversial and interpreted by a number of different theories. We report on torsional oscillator measurements for 4He in a nanoporous Gelsil glass, which has a network of nanopores with 3.5 nm in diameter. In addition to the usual “low-T NCRI” with an onset temperature 0.15 K, we find a larger decrease in rotational moment of inertia in a broad range of temperature from 0.2 to 1.9 K. This “high-T inertial anomaly” is accompanied with multiple dissipation peaks, but has no dependence on torsional oscillation velocity unlike the low-T NCRI. Since the high-T anomaly is observed also in confined liquid states, it originates in amorphous solid 4He layer near the pore wall. Our result shows that different types of supersolid—like phenomena, i.e. inertial anomalies, can coexist in a single 4He sample, even with genuine superfluidity of liquid 4He.

Interlayer exchange coupling and local superfluidity in (4He) N around C20

The path-integral Monte Carlo method has been employed to study local superfluidity of 4He layers adsorbed on a C20 molecular surface. Strong inhomogeneity in the superfluid response between the first two helium layers around the fullerene molecule is found. Furthermore we find that even the strongly-bound first layer exhibiting a clear structural order has a finite superfluid fraction when the second layer is filled with itinerant helium atoms. This first-layer superfluidity is understood to be due to exchange coupling mediated by the second-layer helium atoms.

Surface Andreev bound states of the superfluid 3He B phase

Surface Andreev bound states of superfluid 3He have been detected by transverse acoustic impedance measurement. In the diffusive limit of the quasiparticle scattering off a wall, a singularity in the temperature dependence of the acoustic impedance appears at a particular temperature. At this temperature, pair excitations of the surface Andreev bound state quasiparticle and the propagating Bogoliubov quasiparticle drastically change their nature due to a very sharp band edge of the surface Andreev bound states, resulting in the singularity of the acoustic impedance. This temperature dependence was well reproduced by a theory which properly takes into account the surface Andreev bound states. In a partially specular wall boundary condition achieved by coating the wall with thin 4He layers, the singularity shifted to a higher temperature since the bandwidth became broader as the specularity increased. The coating with 4He influences only the states in the vicinity of the wall and thus the shift is further evidence that the transverse acoustic impedance senses the surface Andreev bound states.

New Anomaly in the Transverse Acoustic Impedance of SuperfluidHe3−Bwith a Wall Coated by Several Layers ofHe4

We measured the transverse acoustic impedance of superfluid 3He-B with a wall coated by several layers of 4He. The coating is known to enhance the specularity in quasiparticle scattering by the wall. We found a new anomaly, a bump and a peak, in the temperature dependence of the transverse acoustic impedance. This agrees with a theoretical calculation using a partially specular wall boundary condition. The new anomaly is shown to arise from a change in the surface density of states by coating and the scattering of thermally occupied surface bound states to other states. The change is towards the density of states of Majorana cone in the specular limit.

He4on a Single Graphene Sheet

The phase diagram of the first layer of 4He adsorbed on a single graphene sheet has been calculated by a series of diffusion Monte Carlo calculations including corrugation effects. Since the number of C-He interactions is smaller than in graphite, the binding energy of 4He atoms to graphene is reduced approximately 13.4 K per helium atom. Our results indicate that the phase diagram is qualitatively similar to that of helium on top of graphite. A two-dimensional liquid film on graphene is predicted to be metastable with respect to the commensurate solid but the difference in energy between both phases is very small, opening the possibility of such a liquid film to be experimentally observed.