Published papers

4He confined in nanoporous Gelsil glass is a unique, strongly correlated Bose system exhibiting quantum phase transition (QPT) by controlling pressure. Previous studies revealed that the QPT occurs with four-dimensional (4D) XY criticality, which appears in the zero-temperature limit of the superfluid density. However, the P-T phase diagram also suggested that the 4D XY nature appears at finite temperatures. Here, we have determined the critical exponent of the superfluid density ρsof 4He in two Gelsil samples that have pore diameter to be about 3 nm, using a newly developed mechanical resonator technique. The critical exponent ζ in the powerlaw fitting ρsα |1 - T=Tc|ζ, where Tcis the superfluid transition temperature, was found to be 1.0 ± 0.1 for all pressures realized in this experiment, 0.1 > P > 2.4 MPa. This value of ζ gives decisive evidence that the finite-temperature superfluid transition belongs to the 4D XY universality class. The emergence of the 4D XY criticality is explained by the existence of many nanoscale superfluid droplets, the so-called localized Bose-Einstein condensates (LBECs), above Tc. Due to the large energy cost for 4He atoms to move between the LBECs, the phase of the LBEC order parameters fluctuates not only in spatial (3D) but imaginary time (+1D) dimensions, resulting in the 4D XY criticality by a temperature near Tc. Since the finite size of the system in the imaginary time dimension Ltis larger than the pore size, the 4D XY critical phenomenon is observed. In the very vicinity of Tcat which the correlation length exceeds Lt, there may be a crossover from 4D to 3D XY criticality. Below Tc, macroscopic superfluidity grows in the nanopores of Gelsil by the alignment of the phases of the LBEC order parameters. An excess dissipation peak observed below Tc is well explained by this phase-matching process.

Molecular hydrogen is a fascinating candidate for novel bosonic and fermionic superfluids. We have studied diffusion dynamics of thin films of H2, HD, and D2 adsorbed on a glass substrate by measurements of elasticity. The elasticity shows multiple anomalies well below the bulk triple point temperature. They are attributed to three different diffusion mechanisms of admolecules and their “freezing” into a localized state: classical thermal diffusion of vacancies, quantum tunneling of vacancies, and diffusion of molecules in the uppermost surface. The surface diffusion is active down to 1 K, below which the molecules become localized. This suggests that the surface layer of hydrogen films is on the verge of quantum phase transition to a superfluid state.

4He confined in nanoporous media is a model Bose system that exhibits quantum phase transition (QPT) by varying pressure. We have precisely determined the critical exponent of the superfluid density of liquid 4He in porous Gelsil glasses with pore size of 3.0 nm using the Helmholtz resonator technique. The critical exponent ζ of the superfluid density was found to be 1.0 ± 0.1 for the pressure range 0.1 < P < 2.4 MPa. This value provides decisive evidence that the finite-temperature superfluid transition belongs to the four-dimensional (4D) XY universality class, in contrast to the classical 3D XY one in bulk liquid 4He, in which ζ = 0.67. The quantum critical behavior at a finite temperature is understood by strong phase fluctuations in local Bose-Einstein condensates above the superfluid transition temperature. 4He in nanoporous media is a unique example in which quantum criticality emerges not only at 0K but at finite temperatures.

© 2017 The Physical Society of Japan. We investigated the magnetic properties of a Sr2RuO4-Ru eutectic microplate containing a single Ru-inclusion using micrometer-sized DC-SQUIDs (direct-current superconducting quantum interference devices). A phase frustration at the interface between chiral p-wave superconducting Sr2RuO4 and s-wave superconducting Ru is expected to cause novel magnetic vortex states such as the spontaneous Ru-center vortex under zero magnetic field [as reported by H. Kaneyasu and M. Sigrist, J. Phys. Soc. Jpn. 79, 053706 (2010)]. Our experimental results show no positive evidence for such a spontaneous vortex state. However, in an applied field, an abrupt change in the magnetic flux distribution was observed at a superconducting transition of Ru. The flux distribution is clarified by comparing our experimental results with electromagnetic field simulations in our sample geometry. We discuss the transition of the vortex states and the superconducting coupling at the Sr2RuO4=Ru interface

© 2016 IOP Publishing Ltd. We present the results of magnetic field imaging by scanning nano-superconducting quantum interference device (SQUID) microscopy on a tungsten carbide (W-C) film fabricated using focused-ion-beam chemical vapor deposition. We have investigated magnetic field change by a W-C film in an external magnetic field using a scanning nano-SQUID microscope system. We have found that the reduction of the magnetic field above the W-C film was 0.9%, indicating the penetration of vortices in the W-C at an external magnetic field of 0.171 mT.

© 2016 American Physical Society. Phase-sensitive measurements of direct-current superconducting quantum interference devices (dc-SQUIDs) composed of Sr2RuO4-Ru eutectic crystals have been performed to temperatures below a bulk Ru superconducting transition temperature at 0.49 K. A SQUID with Nb/Ru/Sr2RuO4 junctions fabricated on one Ru inclusion exhibits two distinct transitions due to the Ru superconducting transition and competition of proximity-induced superconducting gaps at the junctions. At sufficiently low temperatures, the SQUID interference patterns start to collapse with large phase shifts of the Fraunhofer patterns. This result indicates the influence of magnetic fluxes induced by large bias currents flowing in a strongly asymmetric supercurrent path. Such a large change in supercurrent path suggests superconducting phase mismatch between the s-wave and chiral p-wave states at the Ru/Sr2RuO4 interface.

© 2018 American Physical Society. Helium films show various quantum phases that undergo quantum phase transitions by changing coverage n. We found anomalous elastic phenomena in bosonic He4 and fermionic He3 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 nc. The elastic anomaly is explained by thermal activation of helium atoms from the localized to extended states with a distributed energy gap. We determine the energy band structure of helium films from elasticity. The ground states of He4 and He3 at n

© 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.

An oscillating object can stretch quantized vortices attached to it in superfluids due to the relative superflow, steadily generating quantum turbulence, even in the zero-temperature limit. We report the emission and propagation of quantized vortices from quantum turbulence generated in superfluid 4He at low temperatures. A vortex-free vibrating wire enables us to detect the first collision of vortex rings and therefore to measure the time-of-flight of a vortex emitted from a generator to a detector. The detection times from the start of turbulence generation exhibit an exponential distribution, suggesting that the detection is a Poisson process. Vortices are emitted continuously, but each vortex has a random flight velocity and direction. We estimated the nondetection time and mean detection period from the distribution for two flight distances. By estimating the flight velocity, we find that only vortices with velocities lower than the detector velocity can be detected, even if the sizes of the emitted vortices are smaller than the wire thickness or the vibration amplitude. The ratio of the detection rate as a function of vortex velocity suggests anisotropic emission of vortices from the quantum turbulence. © 2013 American Physical Society.

We report the generation of a quantized vortex in superfluid 3He-B. Oscillating objects can generate vortices due to breaking of Cooper pairs in superfluid He, resulting that the threshold velocity for vortex generation is higher than the pair-breaking velocity. Using very-low-frequency vibrating wires with smooth surfaces, however, we find that the threshold velocity of vortex generation is lower than the pair-breaking velocity. The threshold velocity remains constant with decreasing 3He pressure until the pair-breaking velocity equals the vortex-generation threshold velocity at a low pressure. These results indicate that another mechanism that is independent of pressure generates vortices for the low-frequency vibrating wire in superfluid 3He. The threshold velocity observed in experiments turns out to be nearly equal to the velocity of the instability of the vortex lines attached to the wire, which is characterized by only the circulation quantum. ©2010 The American Physical Society.

We report the critical behavior of steady quantum turbulence in superfluid H4 e. By using a thin vibrating wire with no bridge vortices in the superfluid, we find that lifetime of turbulent state for a given driving force reveals an exponential distribution. The mean lifetime estimated from the distribution decreases exponentially but greatly below a critical injection power. We estimate the vortex line density in the turbulent state and find that this critical behavior arises when the interdistance between vortex lines becomes as large as the turbulent region. © 2010 The American Physical Society.

The superfluidity of liquid 3He in a high-porosity aerogel has been studied using a fourth-sound resonance technique. This technique has two significant advantages: it can directly determine the superfluid density and it can derive the transport properties of the viscous normal-fluid component. The temperature dependence of the resonance frequency revealed suppression of superfluidity and that a finite normal-fluid fraction exists even at T=0. The motion of the normal-fluid component has also been investigated. As T→0, the energy loss becomes very small, despite a finite amount of the normal-fluid component remaining. This implies that the normal-fluid component is highly constrained by the aerogel, and hence the dissipation mechanism cannot be described in terms of the conventional hydrodynamic model. We have succeeded to explain these results by introducing a frictional relaxation model to describe our observations, and found that the flow field changes from being parabolic (Hagen-Poiseuille viscous flow) to flat (Drude frictional flow) on introducing an aerogel. Numerical calculation of the relaxation time using the quasiclassical Green's-function method reproduces experimental results. © 2010 The American Physical Society.

The transition to turbulence in the boundary flow of superfluid He4 is investigated using a vortex-free vibrating wire. At high wire vibration velocities, we found that stable alternating flow around the wire enters a turbulent phase triggered by free vortex rings. Numerical simulations of vortex dynamics demonstrate that vortex rings can attach to the surface of an oscillating obstacle and expand unstably due to the boundary flow of the superfluid, forming turbulence. Experimental investigations indicate that the turbulent phase continues even after stopping the injection of vortex rings, which is also confirmed by the simulations. © 2008 The American Physical Society.

We review our study of critical phenomena in superfluid 4He confined in nanoporous glasses. 4He in nanoporous media is an ideal ground to survey the quantum phase transition of bosons. In the present work, critical phenomena were examined using a newly developed hydrodynamic mechanical resonator. The critical exponent of superfluid density ζ was found to be 1.0, in contrast to 0.67 in bulk 4He. We also demonstrate that the superfluid density is proportional to |P-Pc|ζp with ζp= 1 at any finite temperatures. These are the decisive pieces of evidence for the 4D XY criticality, which should have been observed only at 0 K, at finite temperatures. We propose a mechanism of the quantum criticality at finite temperatures in terms of phase alignment among the nanoscale localized Bose condensates (LBECs) in nanopores. The proposed mechanism is discussed in the consideration of the correlation length compared with the quantum effect.

The fourth sound resonance experiment has been done on liquid 3He in 98.5% porosity aerogel. Aerogel was grown inside the pores among the sintered silver powder to avoid the vibration of the aerogel strands by the sound experiment. The measurement was performed at zero magnetic field and 27 bar. We observed the phase transition between the A-like and B-like phases and also their coexistent state. The A-like to B-like phase transition occurs not at a temperature but within a temperature band. In this band, the A-like phase gradually converts to the B-like phase. Possible picture of the coexistent state is discussed. © Springer Science+Business Media, LLC 2007.

We performed a high-sensitivity fourth sound resonance experiment for pure superfluid 3He at 29.0 bar in sintered silver powder cell to clarify the hydrodynamic property of 3He in the aerogel-sintered silver system. We discuss the energy loss of fourth sound. An anomaly in the energy loss at the AB phase transition as found in the aerogel system was not observed. Our analysis shows that the energy loss Q -1, which monotonically decreases with decreasing temperature, can be understood by the hydrodynamic theory in the B phase qualitatively and quantitatively. We estimated the effective pore radius R in the powder cell. © Springer Science+Business Media, LLC 2007.

The transition to turbulence in boundary flow of superfluid 4He is investigated using a vibrating wire. An object in a superfluid can easily generate the turbulence by oscillation even at very low temperatures, because remanent vortices are attached to it from the beginning. However, these vortices affect the transition to turbulence, occasionally causing intermittent switchings between laminar and turbulent flows. Recently, we have successfully devised a condition of vortex-free superfluid, in which a vibrating wire cannot generate turbulence. This condition prevents an oscillator from the switchings, enabling us to study the transition from turbulent to laminar flows. Using a vortex-free vibrating wire and a generator of vortex rings, we find that a turbulent state has a lifetime at driving forces near the transition, while it seems to be permanent at high driving forces. Lifetime data scatter widely, suggesting that the transition from turbulent to laminar flows is a probability event. © 2009 IOP Publishing Ltd.

While the shear viscous motion of the normal fluid component causes the energy loss of the fourth sound resonance in superfluid 3He confined in narrow pores, it is not yet clear what causes the energy loss of the fourth sound in the aerogel system. We prepared two aerogel samples of 99.0% and 97.5% porosities, which were directly grown in sintered silver sponges and performed the fourth sound resonance experiments. We found that the energy loss of the fourth sound resonance in aerogel is much smaller than that in pure liquid 3He, showing that the mean free path of 3He quasiparticles is strongly suppressed by aerogel strands. Such behavior is explained with the frictional flow. The temperature dependence of the frictional relaxation time will be discussed. © 2009 IOP Publishing Ltd.