H. Kojima

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

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Publications (62)205.11 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.
    Full-text · Article · Aug 2014 · Physical Review B
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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.
    No preview · Article · Mar 2014 · Journal of Low Temperature Physics
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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.
    Full-text · Article · Mar 2013 · Low Temperature Physics
  • 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.
    No preview · Article · Dec 2012 · Journal of Physics Conference Series
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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.
    No preview · Article · Dec 2012 · Journal of Physics Conference Series
  • P. Gumann · M. C. Keiderling · H. Kojima
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    ABSTRACT: Measurements on hysteretic response of compound torsional oscillator containing annular-shaped solid 4He samples were carried out by varying the oscillator drive amplitude starting from high to low and then back up to the initial high value. Hysteresis in the oscillator frequency and amplitudes were observed only below an onset temperature. The hysteresis onset temperature (T H ) did not depend on the oscillator frequency, width of the sample annulus, annealing and refreezing after melting. A systematic increase in T H was observed as the 3He impurity concentration in solid 4He samples was increased. The dependence of T H on 3He impurity concentration followed approximately that of the dissipation peak temperatures. Possible relationships of the observed hysteresis phenomena with models of solid 4He dynamics based on freezing of a vortex liquid and dislocation motion are discussed.
    No preview · Article · Aug 2012 · Journal of Low Temperature Physics
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    ABSTRACT: The superfluid 3He A1 phase, containing a spin-polarized condensate allows us to explore the dynamics of superfluid spin current. In the mechano-spin effect (MSE), a mechanically applied pressure gradient and a superleak-spin filter enable one to directly boost spin polarization of 3He in a small chamber. We are developing new apparatus for achieving greater enhancement of spin density. A development of a new-type 3He-hydraulic actuator has been already reported. We present here the construction of new-type of superleak-spin-filter made of packed powder aluminum oxide (referred as PAP-SL). The PAP-SL is popular in the study of superfluid 4He, but has not been established for that of the superfluid 3He. The attempt to construct the PAP-SL for the spin pump experiment was made by using aluminum oxide powder with nominal 1 μm powder diameter and with packing fraction of 40 %. Before executing the experiment, the nuclear demagnetization cryostat of ISSP, Univ. Tokyo which has been used for this experimental activity, was heavily damaged by the 2011 Great East Japan (Higashi Nihon) Earthquake. The repair work and earthquake damage protection strengthening has just been accomplished.
    No preview · Article · May 2012 · Journal of Low Temperature Physics
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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.
    Full-text · Article · Feb 2011 · Physical review. B, Condensed matter
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    Y Aoki · A Yamaguchi · K Suzuki · H Ishimoto · H Kojima
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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
    Preview · Article · Aug 2010
  • Y. Aoki · A. Yamaguchi · K. Suzuki · H. Ishimoto · H. Kojima
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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 .
    No preview · Article · Aug 2010 · Physical review. B, Condensed matter
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    P. Gumann · D. Ruffner · M. Keiderling · H. Kojima
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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.
    Full-text · Article · Feb 2010 · Journal of Low Temperature Physics
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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.
    No preview · Article · Aug 2009 · Physical Review B
  • A Yamaguchi · Y Aoki · K Suzuki · H Ishimoto · H Kojima
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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.
    No preview · Article · Mar 2009 · Journal of Physics Conference Series
  • 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.
    No preview · Article · Feb 2009 · Journal of Physics Conference Series
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    ABSTRACT: We have measured the conductivity of high-mobility (001) Si metal-oxide-semiconductor field effect transistors (MOSFETs) over wide ranges of electron densities n=(1.8-15)x10^11cm^2, temperatures T=30mK-4.2K, and in-plane magnetic fields B=0-5T. The experimental data have been analyzed using the theory of interaction effects in the conductivity of disordered 2D systems. The parameters essential for comparison with the theory, such as the intervalley scattering time and valley splitting, have been measured or evaluated in independent experiments. The observed behavior of the conductivity, including its quasi-linear increase with decreasing T down to ~0.4K and its downturn at lower temperatures, is in agreement with the theory. The values of the Fermi- liquid parameter obtained from the comparison agree with the corresponding values extracted from the analysis of Shubnikov-de Haas oscillations based on the theory of magnetooscillations in interacting 2D systems.
    Full-text · Article · Sep 2008 · Physical review. B, Condensed matter
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    Y Aoki · M C Keiderling · H Kojima
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    ABSTRACT: We describe the first observations on the time-dependent dissipation when the drive level of a torsional oscillator containing solid (4)He is abruptly changed. The relaxation of dissipation in solid (4)He shows rich dynamical behavior including exponential and logarithmic time-dependent decays, hysteresis, and memory effects.
    Preview · Article · May 2008 · Physical Review Letters
  • Y. Aoki · H. Kojima · X. Lin
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    ABSTRACT: A systematic study is carried out to search for fourth sound propagation solid 4He samples below 500 mK down to 40 mK between 25 and 56 bar using the techniques of heat pulse generator and titanium superconducting transition edge bolometer. If solid 4He is endowed with superfluidity below 200 mK, as indicated by recent torsional oscillator experiments, theories predict fourth sound propagation in such a supersolid state. If found, fourth sound would provide convincing evidence for superfluidity and a new tool for studying the new phase. The search for a fourth sound-like mode is based on the response of the bolometers to heat pulses traveling through cylindrical samples of solids grown with different crystal qualities. Bolometers with increasing sensitivity are constructed. The heater generator amplitude is reduced to the sensitivity limit to search for any critical velocity effects. The fourth sound velocity is expected to vary as ~rhos/rho. Searches for a signature in the bolometer response with such a characteristic temperature dependence are made. The measured response signal has not so far revealed any signature of a new propagating mode within a temperature excursion of 5 muK from the background signal shape. Possible reasons for this negative result are discussed. Prior to the fourth sound search, the temperature dependence of heat pulse propagation was studied as it transformed from ``second sound'' in the normal solid 4He to transverse ballistic phonon propagation. Our work extends the studies of [V. Narayanamurti and R. C. Dynes, Phys. Rev. B 12, 1731 (1975)] to higher pressures and to lower temperatures. The measured transverse ballistic phonon propagation velocity is found to remain constant (within the 0.3% scatter of the data) below 100 mK at all pressures and reveals no indication of an onset of supersolidity. The overall dynamic thermal response of solid to heat input is found to depend strongly on the sample preparation procedure.
    No preview · Article · Apr 2008 · Low Temperature Physics
  • Y. Aoki · J. C. Graves · H. Kojima
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    ABSTRACT: We have developed a double resonance compound torsional oscillator for studying non-classical rotational inertia (NCRI) of solid 4He at two different frequencies. The torsional oscillator consists of two beryllium copper torsion members, and two masses. The two masses rotate in phase in the first mode and out of phase in the second mode at resonance frequencies, 496 Hz and 1173 Hz, respectively. Samples of solid 4He (commercial grade with 3He impurity level less than 1 ppm) are grown with the blocked capillary method at pressures between 27 and 42 bar. Temperature dependences of NCRI signals at two different frequencies are observed in identical solid 4He at 37 bar.
    No preview · Article · Jan 2008 · Journal of Low Temperature Physics
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    ABSTRACT: A fountain effect is a common phenomenon in both 3He and 4He superfluids. Unique to superfluid 3He is the magnetic fountain effect, which has been used to determine the spin direction of the condensate in 3He A1 phase. Here we present a pressure driven fountain effect in A1 phase. The experimental cell is composed of a large reservoir connected to a small detector chamber through superleak channels of width of 20μm. One wall of the detector chamber houses a movable circular 6μm thick membrane which serves as a sensitive capacitive pressure sensor and also acts as a spin pump. In A1 phase, a DC voltage applied on the capacitor induces a simultaneous mass and spin superfluid current into the small chamber. After equilibration, removal of the DC voltage causes a sudden pressure drop followed by a slow relaxation. The sudden drop is a consequence of reversed superfluid flow through the superleak. The observed decay times during the slow relaxation agree with those obtained in magnetically induced spin flow experiment. These observations show that the slow relaxation stems from spin relaxation in the absence of applied field gradient.
    No preview · Article · Aug 2007 · Journal of Low Temperature Physics
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    Y Aoki · J C Graves · H Kojima
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    ABSTRACT: The nonclassical rotational inertia fraction of the identical cylindrical solid 4He below 300 mK is studied at 496 and 1173 Hz by a double resonance torsional oscillator. Below 35 mK, the fractions are the same at sufficiently low rim velocities. Above 35 mK, the fraction is greater for the higher than the lower mode. The dissipation peak of the lower mode occurs at a temperature approximately 4 mK lower than that of the higher mode. The drive dependence of the two modes shows that the reduction of the fraction is characterized by critical velocity, not amplitude or acceleration.
    Preview · Article · Aug 2007 · Physical Review Letters