論文

© 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.

ヘリウム薄膜は吸着量nを変えることで多彩な量子相転移を示すことが知られている。我々はガラス基盤上に吸着したヘリウム薄膜の弾性が、ボソンの4Heとフェルミオンの3He両方で異常な振る舞いを示すことを見出した。この振る舞いは、ヘリウム原子が局在した基底状態からエネルギーギャップを隔てた空間的に拡がった状態に熱励起されるという描像で説明される。この解析により、ヘリウム薄膜のエネルギー構造を明らかにした。4Heと3Heの基底状態は同等でギャップを持ち圧縮性を有する、モット絶縁体やモットグラスである可能性がある。

© 2018 American Physical Society. We report measurements of elastic moduli of hcp solid He4 down to 15 mK when the samples are rotated unidirectionally. Recent investigations have revealed that the elastic behavior of solid He4 is dominated by gliding of dislocations and pinning of them by He3 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 He4 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 He3 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 He3 on dislocation segment G decreases by several orders of magnitude. We propose that the motion of He3 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.

孔径3.5nmのナノ多孔質ガラスに吸着ないし加圧閉じこめした4Heがねじれ振り子の慣性モーメントに及ぼす影響を詳細に調べ、広範な圧力・温度範囲でヘリウムが振動に追随しない慣性異常が起こることを見出した。この現象は細孔壁近傍のアモルファスヘリウム層に起源を持つことがわかった。

グラファイト表面上では、固化圧力よりはるかに低圧で固体4Heの層状結晶成長が起こることが知られている。この機構に対する理解を得るためねじれ振り子を用いて0.1Kまでの極低温での結晶成長機構を研究した。結晶成長は単純な層状成長よりもずっと複雑で、かつ低温で起こりにくくなることが明らかになった。

ナノ多孔体中4Heの超流動特性を制御することを目的として、多孔質ガラスにKrを1原子層吸着させる前後での孔中4Heの超流動をねじれ振り子で調べ、吸着薄膜の超流動がKr吸着により抑制されることを発見した。この振る舞いはKr吸着によって細孔径分布が広がり、4He薄膜の超流動に対し量子局在効果をもたらしたものと解釈できる。

ナノ多孔体に閉じこめた4Heの量子相転移を理論的に考察し、極低温域で4次元XYユニバーサリティクラスに属する量子臨界現象を示し、実験結果をよく説明することを見出した。

ナノスケールでの摩擦現象を研究する目的で、極低温走査プローブ顕微鏡を用いて金属(NbSe2)および絶縁体(SrTiO3)表面での非接触摩擦を測定し、探針試料間距離が約2nmのとき摩擦力が極大を示すことを発見した。

ナノ多孔質ガラス中ヘリウム4の超音波実験を行った。孔中4Heが液体、固体両方の状態で音速が熱活性化型の緩和に起因して変化することを見出した。また、多孔ガラスの熱処理によって超流動性が影響を受けることがわかった。

ナノ多孔質ガラス中ヘリウム4の超流動特性の周波数依存性を調べるため、超音波(10MHz)とねじれ振り子(1kHz)の同時測定を行った。超流動特性が周波数依存性が無いことがわかった。