H. Kojima

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

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Publications (74)182.24 Total impact

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    ABSTRACT: We have experimentally studied the renormalized effective mass m* and Dingle temperature T_D in two spin subbands with essentially different electron populations. Firstly, we found that the product (m*T_D) that determines damping of quantum oscillations, to the first approximation, is the same in the majority and minority subbands even at the spin polarization degree as high as 66\%. This result confirms the theoretical predictions that the interaction takes place at high energies ~ E_F rather than within a narrow strip of energies E_F\pm k_BT. Secondly, to the next approximation, we revealed a difference in the damping factor of the two spin subbands, which causes skewness of the oscillation lineshape. In the absence of the in-plane magnetic field, the damping factor (m*T_D) is systematically smaller in the spin-majority subband. The difference, quantified with the skew factor \gamma = (T_{D\downarrow}-T_{D\uparrow})/2T_{D0} can be as large as 20%. The skew factor tends to decrease as B_\parallel or temperature grow, or perpendicular field decreases; for low electron densities and high in-plane fields the skew factor even changes sign. Finally, we compared the temperature and magnetic field dependencies of the magneto-oscillations amplitude with predictions of the interaction correction theory, and found, besides some qualitative similarities, several quantitative and qualitative differences. To explain qualitatively our results, we suggested an empirical model that assumes the existence of easily magnetized triplet scatterers on the Si/SiO_2 interface.
    08/2014;
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    ABSTRACT: Experimental exploration of highly spin-polarized states of liquid 3He by applying external magnetic field is limited by the availability of static magnetic field. In the “ferromagnetic” superfluid A1 phase of liquid 3He there is an alternate method for boosting spin-polarization by the process of spin pumping without requiring such high magnetic field. The spin pumping in the A1 phase takes advantage of a superleak (SL) acting simultaneously as a filter for both entropy and spin. The spin pump technique that uses the SL-spin filter and a mechanical actuator enables us to directly boost polarization of 3He. The amount of enhancement of spin polarization has been limited so far. We are now developing a new type of SL filter made of packed aluminum oxide powder (referred as PAP-SL), in order to achieve greater enhancement of spin polarization. Several kinds of the PAP-SL filter were constructed by pressing aluminum oxide powders into a cylinder holder. The packed structures were carefully characterized by a flow-rate-measurement, X-ray tomography, and mercury intrusion porosimetry. The preliminary result shows that the PAP-SL works as SL filter for the superfluid 3He.
    Journal of Low Temperature Physics 03/2014; · 1.18 Impact Factor
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    Pat Gumann, H. Kojima
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    ABSTRACT: Observations have been made to reveal unusual ageing behavior in solid He samples contained in a torsional oscillator. Oscillation of samples is initiated at a given oscillator drive amplitude near 100 mK. After the samples are cooled to a measurement temperature, they are “aged” for a waiting time, t w, between 15 min and 25 h. The drive amplitude is then halved and subsequent variation in the oscillator response amplitude, A(t), and frequency are monitored as time t elapses. When the measurement temperature is lower than Ts = 40 mK, A(t) shows unusual behavior: A(t) initially undershoots to less than half of the original value, partially recovers exponentially and eventually continues to increase logarithmically. The amount of undershoot, partial recovery magnitude and the rate of logarithmic increase all show aging effect with logarithmic dependence on tw. When the measurement temperature is greater than Ts , the above unusual behavior in A(t) disappears. If solid He cooled below Ts behaved analogously to spin glasses, A(t/tw) would be independent of tw. Such behavior of A(t/tw ) is not observed. Origin of the unusual aging behavior in solid He is not yet clear. Motion of dislocation lines is discussed as a possible origin.
    Low Temperature Physics 03/2013; 39(10):1. · 0.82 Impact Factor
  • Harry Kojima, Izumi Iwasa, John Goodkind
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    ABSTRACT: The interpretation of observed anomalous increases in the frequencies of torsional oscillators (TO) containing solid ^4He confined in Vycor nanopores as evidence for emergence of a supersolid phase has been met recently by conflicting experiments. Yet questions remain on the origin of the observed TO anomalies in bulk solid ^4He samples. To search for the origin, we are carrying out simultaneous measurements of 10 MHz longitudinal ultrasound and TOs (250 ˜ 1100 Hz) on identical solid ^4He samples. Temperature dependence of velocity and attenuation of ultrasound and that of amplitude and frequency of TO are measured. At the temperatures, where TO anomalies occur, anomalies in sound velocity and attenuation also appear. When solid ^4He is doped with 20 ppm ^3He, the tempeature of TO anomaly tracks that of ultrasound. Interpretation of these observations in terms of the motion of dislocation lines will be presented.
    03/2013;
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    ABSTRACT: The superfluid 3He A1 phase contains a spin-polarized condensate. This property allows novel superfluid spin current experiments. In the mechano-spin effect of the A1 phase a mechanically applied pressure gradient and a superleak-spin filter enable to directly boost spin polarization of 3He in a small chamber. Using a flexible membrane as an electrostatically actuated pump, we carried out such experiments and observed 50% enhancement of spin density. Here we report on a new 3He-hydraulic actuator for achieving greater enhancement of spin density. The actuator consists of two liquid 3He chambers located at a 4.2 K plate and in the interior of the cell. The pressure in the 4.2 K chamber is heater-controlled and it transmits a force onto a membrane in the cell. The motion of the membrane induces spin-polarized current into an accumulation chamber.
    Journal of Physics Conference Series 12/2012; 400(1):2081-.
  • M. C. Keiderling, H. Kojima
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    ABSTRACT: A compound torsion oscillator (TO), having two resonant mode frequencies (492 Hz and 1163 Hz) and high Q (0.5~1 × 106), has become an important tool in studying the supersolid properties of 4He loaded into the main TO bob. By simultaneously exciting both modes of this TO, observations could be made on how changes in the drive amplitude of one mode affect the sample response measured by the other mode. In order to separate out the effects arising from the sample, “background” characteristics due to the unloaded TO itself were studied. The resonant frequency and response amplitude of both modes were measured at 10 mK as the drive amplitude of one mode was varied while the other was held fixed at a low drive amplitude. Unexpectedly, the resonant frequency and the response amplitude of the fixed mode decreased as the drive amplitude of the varied mode was increased. The two modes interact and affect each other. The decrease in frequency could be due to a decrease in the stiffness of the torsion rods, while the decrease in amplitude could indicate an increase in internal friction.
    Journal of Physics Conference Series 12/2012; 400(5):2011-.
  • Bettina Hein, John Goodkind, Harry Kojima
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    ABSTRACT: Kim and Chan[1] were motivated in part by an anomalous ultra sound propagation observed by Ho et al.[2] in solid ^4He near 200 mK to carry out torsional oscillator(TO) experiments which led to the discovery of supersolid phenomena. We constructed a 270 Hz TO oscillator which incorporated quartz transducers for simultaneously observing 10 MHz longitudinal ultra sound and torsional oscillation of solid ^4He samples. We are searching for correlation between behaviors in ultra sound propagation and TO response. The length and density of dislocations extracted from ultra sound and the frequency shifts of TO measured in some half a dozen solid ^4He samples have not shown clear correlation.[4pt] [1] E. Kim and M. Chan, Nature 427, 225(2004).[0pt] [2] P. Ho, I. Bindloss and J. Goodkind, JLTP 109, 409(1997).
    02/2012;
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    ABSTRACT: High sensitivity of supersolid phenomenon in solid ^4He to low levels of ^3He impurity(x3) is a puzzle not yet understood. We have analyzed the data taken using our compound torsional oscillator on the variation of dissipation as x3 was varied between 0.3 and 25 ppm. The compound oscillator allows studies of the dissipation at two oscillator mode frequencies(0.5 and 1.2 kHz). Arrhenius plots of temperatures, where peaks in dissipation occur, vs. frequency allow extracting the activation energy and the characteristic time. The data are consistent with distributions of activation energy whose widths increase with x3 but the mean value of ˜ 430 mK independent of x3. The characteristic time varies approximately as x3^2/3. Temperature dependence of the dissipation is consistent with Debye model but frequency dependence is not. We give an interpretation of the characteristic time in terms of diffusion of ^3He along dislocation lines.
    03/2011;
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    ABSTRACT: Frequency shifts and dissipation of a compound torsional oscillator induced by solid He4 samples containing He3 impurity concentrations (x3 = 0.3, 3, 6, 12, and 25 in units of 10-6) have been measured at two resonant mode frequencies (f1 = 493 and f2 = 1164 Hz) at temperatures (T) between 0.02 and 1.1 K. The fractional frequency shifts of the f1 mode were much smaller than those of the f2 mode. The observed frequency shifts continued to decrease as T was increased above 0.3 K, and the conventional nonclassical rotation inertia fraction was not well defined in all samples with x3=> 3 ppm. Temperatures where peaks in dissipation of the f2 mode occurred were higher than those of the f1 mode in all samples. The peak dissipation magnitudes of the f1 mode were greater than those of the f2 mode in all samples. The activation energy and the characteristic time (tau0) were extracted for each sample from an Arrhenius plot between mode frequencies and inverse peak temperatures. The average activation energy among all samples was 430 mK, and tau0 ranged from 2×10-7 s to 5×10-5 s in samples with x3 = 0.3-25 ppm. The characteristic time increased with increasing x3. Observed temperature dependencies of dissipation were consistent with those expected from a simple Debye relaxation model if the dissipation peak magnitude was separately adjusted for each mode. Observed frequency shifts were greater than those expected from the model. The discrepancies between the observed and the model frequency shifts increased at the higher frequency mode.
    Physical review. B, Condensed matter 02/2011; 83. · 3.77 Impact Factor
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    ABSTRACT: Systematic observations of the magnetically generated fountain pressure in the superfluid $^3$He A$_1$ have been carried out in a newly built apparatus designed to reduce the effect of thermal gradients. In the same apparatus, mechanical pumping and filtering of polarized nuclear spins were realized by the pneumatic pumping action of an electrostatically actuated membrane. In both experiments, the measured induced pressure was observed to decay at all temperatures where the A$_1$ phase appeared in magnetic fields up to 13 T and liquid pressures between 1 and 29 bar. The inferred spin relaxation rate tended to increase as the low temperature phase boundary with the A$_2$ phase (T$_{C2}$) was approached. The increase in spin relaxation rate near T$_{C2}$ can be explained by the presence of a minority spin condensate in the A$_1$ phase as predicted by Monien and Tewordt and by the application of the Leggett-Takagi theory of spin relaxation in superfluid $^3$He. Comment: 12 pages, 15 figures, to appear in Physical Review B
    08/2010;
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    ABSTRACT: We have carried out a series of torsional oscillator experiments on solid ^4He containing ^3He-concentrations (with concentration x3 = 0.3, 3, 6, 12, 25 ppm) using our compound torsional oscillator (TO). Our studies of the supersolid phenomenon focused on the energy dissipation change (˜delta Q-1) as x3 was varied. The unique compound TO design allows observations of identical samples at two distinct frequencies (f1=493 and f2=1165 Hz). When normalized as deltaQ-1/deltaQ-1max(x3) the data collapse onto a universal bell-shaped function of T/Tmax(x3) except for the f1 mode in the 12 and 25 ppm samples and the f2 mode in the 25 ppm sample. A clear deviation of deltaQ-1 in both modes from the other values of x3 suggests some additional mechanism for the extra dissipation present in the 25 ppm sample.
    03/2010;
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    ABSTRACT: We studied the effects of ^3He-impurity on the hysteresis phenomenon in solid ^4He grown in an annular container as measured by a compound torsional oscillator (TO). The sample is cooled while oscillating at an initial high (˜ 500 mum/s) velocity from a high temperature to a regulated target temperature where the Non-Classical Rotational Inertia fraction(NCRIf) is measured. The velocity is decreased to below a critical velocity of about 15 mum/s and then increased back to the original velocity where the NCRIf is measured again. The difference in the two NCRIf represents hysteresis. The difference is found to vanish (i.e. reversible) when the temperature is above a temperature Th(x3) which depends on the ^3He impurity concentration x3. Th was measured for x3 = 0.3, 6, 25 ppm. Surprisingly, Th coincides with the phase separation temperature evaluated according to Edwards and Balibar theory [1]. [1] D. O. Edwards and S. Balibar, Phys. Rev. B 39, 4083 (1989)
    03/2010;
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    ABSTRACT: Systematic observations of the magnetically generated fountain pressure in the superfluid H3e A1 have been carried out in a newly built apparatus designed to reduce the effect of thermal gradients. In the same apparatus, mechanical pumping and filtering of polarized nuclear spins were realized by the pneumatic pumping action of an electrostatically actuated membrane. In both experiments, the measured induced pressure was observed to decay at all temperatures where the A1 phase appeared in magnetic fields up to 13 T and liquid pressures between 1 and 29 bar. The inferred spin-relaxation rate tended to increase as the low-temperature phase boundary with the A2 phase (TC2) was approached. The increase in spin-relaxation rate near TC2 can be explained by the presence of a minority spin condensate in the A1 phase as predicted by Monien and Tewordt and by the application of the Leggett-Takagi theory of spin relaxation in superfluid H3e .
    Physical review. B, Condensed matter 01/2010; 82(5). · 3.77 Impact Factor
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    ABSTRACT: We have carried out systematic measurements of the 3He-impurity effect using a compound torsional pendulum, which allows probing “non-classical rotational inertia” (NCRI) of the identical solid 4He sample at two different frequencies at 495 Hz and 1172 Hz. The measurements have shown significant differences in the temperature dependence of the NCRI signal as well as the energy dissipation peak positions between the two frequencies. The NCRI fraction derived from the higher frequency mode is greater than that derived from the lower frequency mode at all temperatures. The normalized NCRI fraction shows that T 50 (≡temperature at which NCRI fraction decreases to 50% of the maximum) is greater in the higher frequency mode by ∼9, 18, 30 and 35 mK when the nominal 3He impurity added is 0.3, 6, 12 and 25 ppm, respectively. The systematic changes in the inverse quality factor (Q −1) were also measured at the same 3He-impurity concentrations.
    Journal of Low Temperature Physics 01/2010; 158(3):567-571. · 1.18 Impact Factor
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    ABSTRACT: Mechanical pumping and filtering of spin-polarized condensate were realized in the superfluid He3 A1 phase by the pneumatic pumping action of an electrostatically actuated diaphragm. Spin pumping increased the net spin polarization by 20-50% as measured by the induced pressure change during spin pumping. The observed spin relaxation time was consistent with the increased spin polarization. These observations demonstrate the feasibility of using spin pumping to substantially increase the effective magnetic field to which the A1 phase is exposed.
    Physical Review B 08/2009; 80(5). · 3.66 Impact Factor
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    ABSTRACT: The magnetic fountain effect occurring in superfluid 3He A1 phase is a unique phenomenon in which the pressure and magnetic field gradients in the chemical potential are balanced. The effect has been applied extensively to investigate the intrinsic spin relaxation. We constructed a new improved sample cell. The new cell includes an inner detector and an outer reservoir chamber made of "Macor" which was helpful to reduce the heat release possibly arising from proton nuclei under high magnetic fields. The measured temperature difference between the two chambers was cut to less than 5 μK. The measured relaxation time τ of the fountain pressure decreases monotonically and smoothly as the temperature is decreased from Tc1 (normal- A1 boundary) towards Tc2 (A1- A2 boundary). As the temperature approaches Tc2, τ tends to vanish smoothly.
    Journal of Physics Conference Series 03/2009; 150(3):032122.
  • Michael C. Keiderling, Harry Kojima
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    ABSTRACT: We have extended our studies on the non-classical behavior of solid ^4He contained in compound torsional oscillator (TO) cell below 1 K. Our unique TO design allows observations on the identical sample at two distinct frequencies(f1=493 and f2=1165 Hz). The sample was grown by blocked capillary method in an annular cell(id = 8.0 mm, od = 10.0 mm, height = 9.0 mm). We focus here on experiments in which the two modes are excited simultaneously. While keeping the drive of f2 mode at a very low level, the drive of f1 mode was varied from high to low levels to produce substantial variations in the non-classical rotation inertia fraction (NCRIf). When the NCRIf seen by f1 mode is reduced by 89, 91 and 94 % at 9.7, 23.5 and 56.5 mK, respectively, the NCRIf seen by f2 mode (driven at low level) is reduced by 62, 68 and 80 %. The discrepancies and their temperature dependence in the observed reductions in NCRIf are not yet understood. Similar Measurements with the roles of the drive levels of the modes reversed as well as the changes in the dissipation of the torsional oscillator during the simultaneous drive will be reported.
    03/2009;
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    ABSTRACT: An intriguing observation related to the discovery of non-classical rotational inertia(NCRI) of solid ^4He at low temperatures is the extreme sensitivity to ^3He concentration(x3). Both the magnitude and temperature dependence of the NCRI are affected by relatively low x3 introduced into solid ^4He samples.[1] We are exploring the ^3He impurity effect using our compound torsional pendulum which allows probing the NCRI of the identical solid ^4He sample at two different frequencies (˜493 and ˜1165 Hz). The NCRI fractions were derived from the measured shifts in the oscillator frequency of the two modes as functions of temperature. The NCRI fraction derived from the higher frequency mode is greater than that derived from the lower frequency mode at all temperatures. If the NCRI fractions of both modes are normalized at their maximums, the temperatures at which they decrease to 50 % of the maximum are greater in the higher mode by ˜9, 31 and 56 mK when the nominal x3 added is 0.3, 3 and 10 ppm, respectively. Greater values of x3 are currently being studied.[1]Kim, et al., Phys. Rev. Lett. 100 065301(2008).
    03/2009;
  • M C Keiderling, Y Aoki, H Kojima
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    ABSTRACT: The response of oscillating hcp solid 4He samples was studied with a unique compound torsional oscillator a dummy mass and a sample (cylindrical or annular) container connected by two torsion rods. Identical solid sample could be probed within the same apparatus at two different frequencies (~ 0.5 and 1.2 kHz) separately or simultaneously. The apparent onset of the non-classical rotational inertia (NCRI) occurred at a higher temperature in the higher frequency mode. The peak in dissipation of the higher mode also occurred at higher temperature. Surprisingly, the mechanical dissipation was significantly greater in the lower mode. When the lower mode was driven at high levels to induce "critical state" in the sample and the higher mode was simultaneously driven at a low level for probing, the critical state seen in the lower mode did not entirely appear. Conversely, if a critical state was induced by the higher mode, it also did not appear in the lower mode. These preliminary results are contrary to the simple expectation from identifying the critical state as indication of suppressed superfluid density.
    Journal of Physics Conference Series 01/2009; 150(3).

Publication Stats

490 Citations
182.24 Total Impact Points

Institutions

  • 1995–2014
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, New Jersey, United States
  • 2007
    • The University of Tokyo
      • Institute for Solid State Physics
      Tokyo, Tokyo-to, Japan
  • 2004
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1998
    • Tokyo Institute of Technology
      Edo, Tōkyō, Japan