Published papers

We report the investigation of vortex emission by a vibrating wire in superfluid 3He-B at a pressure of 28 bar. We used two vibrating wires with 47 Hz and 183 Hz resonance frequencies as a generator and a detector of vortices. The onset velocity of vortex emission for the 183 Hz vibrating wire is higher than a pair-breaking velocity, indicating that pair breaking causes vortex generation. In contrast, the onset velocity for the 47 Hz vibrating wire is lower than the pair-breaking velocity. This result suggests that another mechanism of vortex emission arises for the lower-frequency vibrating wire: pair breaking due to local flow enhanced by protuberances on the surface of the wire, or instability of remanent vortices attached to the wire by vibration. © 2009 IOP Publishing Ltd.

We report the turbulent transition in superfluid 4He generated by a vibrating wire as a function of its thickness. The response of a vibrating wire with a 3 μm diameter in superfluid 4He at 1.2 K reveals a hysteresis at the turbulent transition between an up sweep and a down sweep of driving force, while no hysteresis appears for wires with a thickness larger than 4.7 μm diameter. These results indicate that the 3 μm wire is efficient for reducing the number of vortex lines attached to it. A cover box and slow cooling also prevent vortex lines from attaching to a wire, resulting in a vortex-free vibrating wire. The effective mass of the vortex-free vibrating wire is almost constant in a wide range of velocities up to 400 mm/s; however, the wire density estimated from the resonance frequency is a half of the expected value of wire material, suggesting that a wire mass becomes lighter or a wire diameter becomes larger in the superfluid effectively. © Springer Science+Business Media, LLC 2009.

Recently, we revealed that the motion of the normal fluid component in the aerogel is well described by the frictional relaxation model (Higashitani et al. in Phys. Rev. B 71:134508, 2005). To clarify the origin of the friction between the quasiparitcles and the aerogel, we have performed the fourth sound resonance experiments at two different pressures. The fourth sound resonance experiment can derive both the static and the dynamic informations simultaneously, namely, the superfluid fraction and the energy loss. From the static part, we found that the superfluid fraction slightly changes with changing the pressure. We calculated the density of states in the impurity system by means of HSM and propose that the constituent of the normal fluid component is the quasiparticles at emerging levels in the energy gap, which we call the midgap states. From the dynamic part, we found that the energy loss depends on the pressure, in contrast to the superfluid fraction. The pressure dependence of the frictional relaxation time has been calculated, and we revealed that the response of the normal fluid component against the frictional force depends on the BCS coherence length. © 2009 Springer Science+Business Media, LLC.

We report the study on remanent vortices attached to rough boundaries in superfluid 4He after the turbulent transition. We used 2.6 μm vibrating wires with smooth surfaces and rough surfaces, a cover box and slow cooling method, in order to investigate the effect of surface roughness on the condition and the number of vortices attached to a wire. The responses of the wire with smooth surfaces show large hysteresis at the turbulent transition. This result indicates that remanent vortices attached between the wire and surrounding boundaries cause turbulence. At first sweep of driving force of the wire with rough surfaces, we also observed hysteresis as large as the case of the smooth wire: at the other sweeps, however, small hysteresis was observed. These results indicate that once turbulence is generated at a wire velocity during first sweep, vortex lines newly attach between rough surfaces of the wire, which easily cause turbulence at a low wire velocity. Therefore, we conclude that a smooth wire can reduce the number of vortices attached to a wire. © 2009 Springer Science+Business Media, LLC.

We report the time-of-flight of quantized vortex rings generated by a vibrating wire in superfluid 4He which contains normal fluid component. A cover box of vibrating wires and slow cooling of superfluid reduce the number of vortices attached to wire surfaces, enabling us to study vortex rings propagating from a turbulent region. Using two vibrating wires as a generator and a detector of vortices, the time-of-flight of vortices propagating a distance of 0.88 mm was measured at 1.25 K. We find that the time-of-flights distribute from 0.06 s to 27.4 s, much larger than the lifetimes of circular vortex rings limited in the size of a generator amplitude. These results imply that large vortex rings with non-circular shape or vortex tangles are created by the generator, propagating slowly and colliding with the detector before complete disappearance. © 2010 Springer Science+Business Media, LLC.

An oscillating obstacle generates quantum turbulence in superfluids, when vortices remained attached to obstacle surfaces or vortex rings collided with it during oscillation. Turbulence provides a source of vortices; however, the characteristics of these vortices are not clear. In the present work, we report the flight of vortices emitted from quantum turbulence in superfluid 4He at low temperatures, using vibrating wires as a generator and a detector of vortices. A vortex-free vibrating wire can detect only the first colliding vortex ring, though it will be refreshed after low vibration and be able to detect a vortex ring again. By measuring a period from the start of turbulence generation to the vortex detection repeatedly, we find an exponential distribution of time-of-flights with a non-detection period t 0 and a mean detection period t 1, suggesting a Poisson process. Both periods t 0 and t 1 increase with increasing distance between a generator and a detector. A vortex flight velocity estimated from period t 0 suggests that the sizes of the emitted vortex rings distribute to a range smaller than a generator thickness or a generator vibration amplitude. Vortices are emitted radially from a turbulence region, at least in the direction of oscillator vibration. © 2012 Springer Science+Business Media, LLC.

We report the nonlinear and unsteady response of the finite amplitude pressure wave in superfluid 4He, using a classical vibrating liquid column standing wave resonator. The frequency was kept at the resonance frequency of the liquid column, so that the maximum of the pressure field always came to the surface of the driver. As a result, in the low driving-amplitude regime, I/O relations were linear. However, in the high driving-amplitude regime, anomalous responses were found; The amplitude of the signals suddenly reduced and gradually recovered; they were repeated with random intervals. Moreover, in the highly suppressed region in the waveform, we found a generation of new type of turbulence. These absorptions are expected to be the result of the vapor bubble ejection from the surface of the driver transducer. © Published under licence by IOP Publishing Ltd.

Motions of vortices are fundamental characteristics of quantum turbulence. These motions are expected to be governed only by quantized circulations in superfluids at the zero temperature limit. In the present paper, we report the motions of vortex rings emitted from a quantum turbulence in superfluid 4He, by detecting vortex rings using a vortex-free vibrating wire as a detector. The time of flights of vortex rings are distributed, because vortex rings are emitted in any direction from a turbulent region and the detector can respond only to a reachable vortex ring. By measuring time-of-flights many times, we find an exponential distribution of time-of-flights with a non-detection period, which corresponds to the fastest time of flights of vortex rings. For a larger generation power of vortex rings, a distribution of time-of-flights still shows a single exponential distribution, but a non-detection period becomes shorter. This result implies that sizes of emitted vortex rings are distributed dependently on the generation power of turbulence. The observed exponential distributions are confirmed by numerical simulations of the dynamics of vortex rings. © Published under licence by IOP Publishing Ltd.

© 2016, Springer Science+Business Media New York. We developed high-sensitivity magnetization measurement devices composed of micro-dc-SQUIDs and a superconducting Sr (Formula presented.) RuO (Formula presented.) microplate, aiming to investigate novel magnetic properties related to a spin-triplet chiral p-wave superconductor with a mesoscopic size. Micron-sized dc-SQUID was fabricated by thin Al electrodes, and the SQUID structure was improved to prevent magnetic fluxes from intruding into SQUID electrodes. A Sr (Formula presented.) RuO (Formula presented.) superconducting microplate was fabricated into the size as small as the SQUID loop using a focused ion beam and directly mounted on the SQUID with precise positioning for high-sensitivity magnetization measurements. In the preliminary magnetization measurements of this device, we observed vortices trapped into the plate and thus the lower critical field. The improved magnetization measurement device developed to exclude undesirable flux intrusion successfully enabled high-sensitivity detection of quantized vortex.

© Published under licence by IOP Publishing Ltd. We report magnetization measurements of Sr2RuO4-Ru eutectic microplates using micro-dc-SQUIDs. Sr2RuO4 is considered as a chiral p-wave superconductor and hence Sr2RuO4-Ru eutectic becomes in an unstable state with a superconducting phase frustration between a chiral p-wave state of Sr2RuO4 and a s-wave state of Ru. To compensate the frustration, a single quantum vortex is spontaneously formed at the center of the Ru inclusion at sufficiently low temperatures. However, such a spontaneous vortex state has not been experimentally observed yet. In this study, we prepared a micro-dc-SQUID and a Sr2RuO4-Ru eutectic microplate containing a single Ru-inclusion at the center of the microplate. We performed magnetization measurements down below the superconducting transition temperature of the Ru inclusion to investigate the spontaneous Ru-center vortex state.

© Published under licence by IOP Publishing Ltd. We have measured transport properties and magnetic responses of direct current superconducting quantum interference devices(dc-SQUIDs) with Nb/Ru/Sr2RuO4 junctions on The same Ru inclusion of a Sr2RuO4-Ru eutectic crystal. We observed a superconducting transition of The Ru at 0.50 K and another transition with anomalous interference patterns of The SQUID at 0.40 K. The second transition led to change of The SQUID structure due to energy competition of alternative supercurrent paths. Phase shifts of The peak of The Fraunhofer diffraction pattern were observed below The transition temperatures, implying topological effects due to a chiral p-wave state or its competition with a s-wave state.

© Published under licence by IOP Publishing Ltd. Superconducting quantum interference devices (SQUIDs) with nano (micro)-meter dimensions are called nano (micro)-SQUIDs. The high sensitivity for flux and position of nano (micro)-SQUIDs can be applied to detect local magnetic fields induced by vortices and The magnetization of mesoscopic superconductors. Nano-SQUIDs based on carbon-nanotube junctions and niobium weak junctions are well known. However, such nano-SQUIDs are not suitable for large-scale integrated circuits and mass production. Therefore, we employ a combination of lithography using The Niemeyer-Dolan technique and The inductively coupled plasma reactive-ion etching technique to fabricate nano-SQUIDs. Here, we report The fabrication of nano (micro)-SQUIDs based on superconducting aluminum tunnel junctions and their application for vortex formation into mesoscopic chiral superconducting Sr2RuO4[1-3].

© 2016, Springer Science+Business Media New York. We developed a two-dimensional array of superconducting quantum interference devices (SQUIDs) for investigation of fine spatial distribution of magnetization in superconducting Sr (Formula presented.) RuO (Formula presented.). Micrometer-sized SQUIDs based on homogeneously formed Al/AlO (Formula presented.) /Al tunnel-type Josephson junctions were fabricated using shadow evaporation technique. Unnecessary electrodes formed by the shadow evaporation were removed by inductively coupled plasma reactive ion etching, in order to realize a dense array of SQUIDs. We measured the magnetic modulation of the maximum Josephson current of each SQUID in the array and evaluated the interaction among the SQUIDs.