Published papers

© 2019 American Physical Society. We respond to the Comment by Pushkarov. We find that, in solid He4, He3 impurity atoms that pin dislocation segments are not impuritons, but randomly walking particles. The interpretation for the rotation effect, which was based on the impuriton model, does have to be totally reconsidered, taking into account the motion of randomly walking He3 in the strong elastic potential near the core of edge dislocations.

Helium films show various quantum phases that undergo quantum phase transitions by changing coverage n. We found anomalous elastic phenomena in bosonic 4He and fermionic 3He films adsorbed on a glass substrate. The films stiffen under AC strain at low temperature with an excess dissipation. The onset temperature of the stiffening decreases to 0 K as n approaches a critical coverage n_c . The elastic anomaly is explained by thermal activation of helium atoms from the localized to extended states with a distributed energy gap. We determine for the first time the energy band structure of helium films from elasticity. The ground states of 4He and 3He at n < n_c are identically gapped and compressible, which are possibly a sort of Mott insulator or Mott glass.

We report measurements of elastic moduli of hcp solid 4He down to 15 mK when the samples are rotated unidirectionally. Recent investigations have revealed that the elastic behavior of solid 4He is dominated by gliding of dislocations and pinning of them by 3He impurities, which move in the solidlike Bloch waves (impuritons). Motivated by the recent controversy of torsional oscillator studies, we have performed direct measurements of shear and Young's moduli of annular solid 4He using pairs of quarter-circle-shape piezoelectric transducers (PZTs) while the whole apparatus is rotated with angular velocity Ω up to 4 rad/s. We have found that shear modulus μ is suppressed by rotation below 80 mK, when shear strain applied by PZT exceeds a critical value, above which μ decreases because the shear strain unbinds dislocations from 3He impurities. The rotation-induced decrement of μ at Ω=4 rad/s is about 14.7(12.3)% of the total change of temperature dependent μ for solid samples of pressure 3.6(5.4) MPa. The decrements indicate that the probability of pinning of 3He on dislocation segment G decreases by several orders of magnitude. We propose that the motion of 3He impuritons under rotation becomes strongly anisotropic by the Coriolis force, resulting a decrease in G for dislocation lines aligning parallel to the rotation axis.

© 2019 The Physical Society of Japan. Adsorbed molecular films provide two-dimensional systems that show various emergent phenomena that are not observed in bulk counterparts. We have measured the elasticity of thin neon films adsorbed on porous glass down to 1 K by the torsional oscillator technique. The shear modulus of a neon film anomalously increases at low temperatures with excess dissipation. This behavior indicates a crossover from a soft (fluidlike) state at high temperatures to a stiff (solidlike) state at low temperatures. The temperature dependence of the anomaly is qualitatively similar to that of the elastic anomaly of helium films found in our recent study. The dissipation peak temperature, however, becomes constant at about 5 K, contrary to the case of helium, in which it decreases to 0 K at a critical coverage of a quantum phase transition between a gapped localized phase and a mobile (superfluid) phase. It is concluded that neon films behave as a classical system that does not show a quantum phase transition or superfluidity, although the films may be strongly supercooled to temperatures much lower than the bulk triple point, 24.6 K. Our results suggest that the elastic anomaly is a universal phenomenon of atomic or molecular films adsorbed on disordered substrates.

© 2018 American Physical Society. We reexamined the rotation-induced effect observed in solid He4 by using a rigid two-frequency torsional oscillator (TO). The previous rotation experiments reported the rotation-induced suppression of the "nonclassical" TO response that was interpreted as evidence of irrotational bulk superfluidity in solid He4. However, the experiment employed a nonrigid TO that could amplify the elastic contribution in the TO response. Thus, it is important to clarify if the rotation-induced suppression of the TO response could be attributed to an unavoidable elastic effect. In our rigid TO, complicated nonlinear viscoelastic contributions are systematically eliminated. In addition, the TO operating at two different resonant frequencies allows us to decompose a possible superfluidlike frequency-independent contribution on period drop from that of the linear elastic overshoot effect. We found no substantial rotation-induced effect in the out-of-phase resonant mode unlike that found in the previous rotation experiments. It indicates that the previous rotation effect in the nonrigid TO cannot be attributed to the genuine supersolidity. According to the frequency analysis of the TO response, the frequency-dependent period drop, which can be attributed to the elastic overshoot effect, remains unaffected upon application of dc rotation. However, the frequency-independent superfluidlike contribution exhibits a strikingly different rotation effect that is currently inexplicable.

We reexamined the rotation-induced effect observed in solid 4He by using a rigid two-frequency torsional oscillator (TO). The previous rotation experiments reported the rotation-induced suppression of the “nonclassical” TO response that was interpreted as evidence of irrotational bulk superfluidity in solid 4He. However, the experiment employed a nonrigid TO that could amplify the elastic contribution in the TO response. Thus, it is important to clarify if the rotation-induced suppression of the TO response could be attributed to an unavoidable elastic effect. In our rigid TO, complicated nonlinear viscoelastic contributions are systematically eliminated. In addition, the TO operating at two different resonant frequencies allows us to decompose a possible superfluidlike frequency-independent contribution on period drop from that of the linear elastic overshoot effect. We found no substantial rotation-induced effect in the out-of-phase resonant mode unlike that found in the previous rotation experiments. It indicates that the previous rotation effect in the nonrigid TO cannot be attributed to the genuine supersolidity. According to the frequency analysis of the TO response, the frequency-dependent period drop, which can be attributed to the elastic overshoot effect, remains unaffected upon application of dc rotation. However, the frequency-independent superfluidlike contribution exhibits a strikingly different rotation effect that is currently inexplicable.

著者が発見したナノ多孔体中加圧4Heの量子相転移に関するレビュー。

著者が発見したナノ多孔体中加圧4Heの量子相転移に関する日本語のレビュー。

We have studied the growth of solid 4He on multilayer graphene immersed in the superfluid well below the bulk freezing pressure using the torsional oscillator method at temperatures from 1.65 to 0.1 K. Below 1.2 K, the growth of one solid layer is observed as a series of discontinuous steps, each accompanied by energy dissipation. We speculate that the discontinuities result from a series of two-dimensional structural phase transitions of the uppermost solid layer, between several commensurate and incommensurate states.

We performed torsional oscillator measurements for 4He in a nanoporous Gelsil glass, which has a network of nanopores with 3.5 nm in diameter. We find a large 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. It originates in amorphous solid 4He layer near the pore wall.

The growth of solid 4He on graphite from the superfluid phase is known to occur at pressures well below the bulk solidification point. The number of adsorbed layers increases with pressure and the solid growth undergoes non-continuous layer-by-layer growth at low temperature. We have studied this growth using the torsional oscillator method for isotherms down to 0.1 K. In contrast with simple layer-by-layer growth scenarios, our evidence suggests that the growth of adsorbed solid 4He is more complex and less solid is present on the graphite substrate at low temperature.

We have examined how a change in porous structure by preplating a monolayer of krypton affects the superfluid properties of 4He adsorbed or confined in a nanoporous Gelsil glass (pore size: 5.8 nm). The superfluid transition temperature and the superfluid density of adsorbed films decrease about 10% by Kr preplating. The suppression of film superfluidity is attributed to the quantum localization of 4He atoms by the randomness in the substrate potential, which is caused by the preplating-induced broadening of the pore size distribution.

The superfluid transition in liquid 4He filled in Gelsil glass observed in recent experiments is discussed in the framework of quantum critical phenomena. We show that quantum fluctuations of phase are indeed important in the experimentally studied temperature range owing to the small pore size of Gelsil, in contrast to 4He filled in previously studied porous media such as Vycor glass. As a consequence of an effective particle-hole symmetry, the quantum critical phenomena of the system are described by the four-dimensional XY universality class, except in the immediate vicinity of the quantum critical point, and the resulting simple scaling agrees with the experimental data remarkably well.

We report measurements of noncontact friction between surfaces of NbSe2 and SrTiO3 and a sharp Pt-Ir tip that is oscillated laterally by a quartz tuning fork cantilever. At 4.2 K, the friction coefficients on both the metallic and insulating materials show a giant maximum at the tip-surface distance of several nanometers. The maximum is strongly correlated with an increase in the spring constant of the cantilever. These features can be understood phenomenologically by a distance-dependent relaxation mechanism with distributed time scales.

We have carried out ultrasound measurements for 4He confined in a nano-porous glass, Gelsil. The sound velocity of Gelsil glass in a vacuum is determined by the two-level system (TLS) contribution. Regardless of whether 4He in the pore is liquid or solid, there exists an additional sound velocity change due to the relaxation of ultrasound stresses by a thermal activation mechanism. It was found that the different heat treatment of Gelsil glass affects both this relaxation and the superfluidity of 4He in the pore.

We have performed simultaneous measurements with a torsional oscillator and with ultrasound for pressurized 4He in Gelsil. Tc is in agreement between the two techniques at all pressures, and the superfluid component above 0.5 K shows most of the same temperature dependence. Furthermore, it was found that the chi-factor (the fraction of superfluid which remains locked to the substrate) is independent of measuring frequency between the torsional oscillator (kHz) and the ultrasound (MHz) ranges.