Yuki Aoki

Rutgers, The State University of New Jersey, New Brunswick, New Jersey, United States

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Publications (5)1.02 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We have constructed a compound torsional oscillator which could be operated at two resonant modes (the first at 496 and the second at 1172 Hz). This device allowed us to study the non-classical rotation inertia of the identical solid ^4He at the two oscillator modes driven separately. We present here recent studies of NCRI when the two modes are simultaneously excited. The idea was to drive the first mode at variable high amplitude and to detect its effect on NCRI fraction by the second mode. We expected that when the solid ^4He was driven at high amplitudes with the first mode to produce significant reduction in NCRI fraction, the same reduction would be measured with the second mode driven simultaneously at very low amplitude. On the contrary, the observed reduction in NCRI fraction by the second mode was much smaller than that expected from the first mode. If the driver/detector roles of the first and second modes were reversed, the amount of reduction of NCRI fraction detected and induced by a high drive amplitude of the second mode became greater in the first mode driven at a low amplitude. The critical drive amplitude effects of NCRI induced in one mode are not entirely ``seen'' by the other mode in our oscillator.
  • Yuki Aoki · Xi Lin · Harry Kojima
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    ABSTRACT: Heat pulse propagation in the solid 4He has been studied between 40 and 500 mK. Response to heat pulses are detected by a titanium film superconducting transition edge bolometer. Crossover behavior from second sound in normal solid above 500 mK to ballistic propagation below 200 mK is observed. Detailed study is made to search for possible modification of this propagation behavior by the appearance of supersolidity. It is found, that the ballistic phonon propagation velocity remains constant, within 0.3% scatter of data, below 100 mK at all pressures measured between 25 and 56 bar. The temporal evolution of the detected pulse shape has not revealed any anomaly below 200 mK.
    Journal of Low Temperature Physics 08/2007; 148(5-6):659-664. DOI:10.1007/s10909-007-9489-4 · 1.02 Impact Factor
  • Yuki Aoki · Harry Kojima
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    ABSTRACT: Phonon propagations in hcp solid ^4He are being studied down to 30 mK including the recently discovered supersolid phase range. Phonons are generated with 2 mus long heat pulses by applying current into a metal film resistor (of area 5.2 mm^2) deposited onto a planar silicon substrate. The power applied to the heater is varied between 20 and 200 muW/mm^2. The phonons travel through 4.3 mm of solid ^4He and are detected by superconducting edge sensor of 60 nm thick titanium film. In agreement with earlier studies, the main propagation peak detected by the sensor is the transverse sound propagation. Crossover from second sound in normal solid to ballistic propagation is observed between 100 mK and 300 mK at 25 bars. Measurements of ballistic phonon propagation are extended up to 50 bars for the first time. The velocity of transverse phonon propagation increases from 220 m/s at 25 bars to 250 m/s at 50 bars. It is expected that supersolid transition modifies the velocity of transverse sound propagation. It is found, within ±1.5 % scatter of data, the ballistic phonon propagation velocity(measured at 200 muW/mm^2) remains constant below 100 mK at both 25 and 50 bars. At 30 mK, the velocity is independent of heater power within ±1 %.
  • Joseph Graves · Yuki Aoki · Harry Kojima
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    ABSTRACT: In order to observe supersolid behavior in solid ^4He at multiple frequencies while keeping all other parameters constant, an oscillator with two torsional modes has been constructed. The torsion rod is made of beryllium copper and the cylindrical sample chamber is made of Stycast 1266. The two modes have resonant frequencies of 500 and 1200 Hz. Preliminary studies have shown fairly high quality factors of 10^4 at 300 K and 7x10^4 at 77 K. We plan to measure the changes, at the two frequencies in the identical solid ^4He sample, of the resonant frequency, dissipation and critical velocity associated with the supersolid phase at temperatures below 200 mK.
  • Yuki Aoki · Harry Kojima
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    ABSTRACT: The phonon propagation generated by heat pulse has been studied in solid ^4He under pressure between 25 and 56 bar to search for a sign of supersolid behavior at temperatures down to 40 mK. Response to input heat pulses are detected by a titanium film superconducting edge bolometer separated by a 4.5 mm thick solid ^4He from the heater. According to theoretical studies, a new fourth sound-like mode is expected to emerge in the supersolid state. The sensitivity of our bolometer has been improved from an earlier version by an order of magnitude by changing the film structure for the purpose of searching for a small temperature deviation signal accompanying the fourth sound-like propagation mode. The response of the bolometer to heat pulse was measured in different quality solid samples which had been grown with different cooling rates during solidification. The detected response signal has not revealed any identifiable signature of a new mode within a temperature excursion of about deltaT = 5 muK from the background signal shape. An estimated superflow velocity corresponding to the temperature excursion is greater than the critical velocity observed by Kim and Chan. Our detection sensitivity must be further increased before a definitive conclusion on the fourth sound-like mode can be made.

Publication Stats

5 Citations
1.02 Total Impact Points


  • 2007
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, New Jersey, United States