A. Loshak

University of California, Berkeley, Berkeley, California, United States

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Publications (13)165.66 Total impact

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    ABSTRACT: Backhaus et al. reply - Avenel et al. have suggested a mechanism that might explain the recently discovered metastable pi-state in a superfluid 3He weak-link array. We are pleased that our experiment is leading to new ideas that may extend the understanding of weak-link arrays. We agree with Avenel et al.'s comment that, when the individual apertures are in a (short coherence length, low temperature) hysteretic regime, collective phenomena quite distinct from single weak-link behaviour might be observed. Nevertheless, in the temperature regime in which the coherence length is comparable to the aperture dimensions, we have shown that the collective behaviour of the array is similar to that of a single weak link,.
    Nature 01/1999; 397. · 38.60 Impact Factor
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    ABSTRACT: We have discovered new features of the current-phase relation, Iphi, in superfluid 3He weak links. Firstly, we find that at any given temperature there are two distinct Iphi functions that characterize the weak link. Secondly, both functions continuously develop an unusual form that ultimately leads to the previously reported pi state. The observed form of Iphi has recently been predicted for unconventional quantum fluids such as 3He, high- Tc superconductors, and Bose-Einstein condensates. The two distinct states are likely to originate from the textural degree of freedom in superfluid 3He.
    Physical Review Letters 01/1999; 83(19):3860-3863. · 7.73 Impact Factor
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    ABSTRACT: Under certain circumstances
    Nature 04/1998; 392(6677):687-690. · 38.60 Impact Factor
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    ABSTRACT: Below 2.2 K, ^4He is a superfluid that exhibits nearly dissipationless flow for velocities less than a well defined critical velocity. For flow through submicron apertures, the superfluid begins to dissipate energy at the critical velocity by the nucleation and motion of individual quantized vortex lines across the aperture. In the simplest case, the application of a constant pressure drop (Delta P) across the aperture causes vortices to cross the aperture at an average Josephson frequency given by f_j=m_4Delta P/rho h. Without thermal fluctuations, the periodic vortex crossing would cause the superfluid velocity in the aperture to oscillate and emit acoustic radiation at f_j. Thermal fluctuations destroy the periodicity and spread the acoustic power into a large frequency band. The spectrum of this radiation has been measured using a 1/4 wave resonant detector and a novel cryogenic microphone. The spectrum is interpreted in terms of a shot noise-like vortex nucleation process.
    03/1998;
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    ABSTRACT: In studying the flow of superfluid ^3He-B through a weak link array, we have shown that the applied pressure head across the array generates quantum oscillations of the current through the array according to the AC-Josephson relation(Pereverzev S.V., Loshak A., Backhaus S., Davis J.C., Packard R.E. Quantum oscillations between two weakly coupled reservoirs of superfluid He-3. Nature) 388, 449-451 (1997). Here we report that when the frequency of these oscillations matches the resonance frequency of a coupled oscillator, the DC current through the array is enhanced. This experiment has striking similarity to the one where the superconducting Josephson junction interacts with a coupled microwave resonator.
    03/1998;
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    ABSTRACT: A weak link separating two reservoirs of superfluid ^3He has been shown to act as the superfluid analog of a superconducting Josephson junction for temperatures above 0.6 mK.(Pereverzev S.V., Loshak A., Backhaus S., Davis J.C., Packard R.E. Quantum oscillations between two weakly coupled reservoirs of superfluid He-3. Nature) 388, 449-451 (1997). We measure directly the current and quantum phase difference Deltaphi across the weak link and plot the current, and free energy stored, as a function of Deltaphi and temperature T. The current-phase relationship I(Deltaphi) changes with decreasing temperature. A new experiment at lower temperatures, which led to the discovery of a new branch of I(Deltaphi) will be described. This branch allows for the formation of a metastable state in which a quantum phase difference of pi is maintained across the weak link. We call this the pi-state.
    03/1998;
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    ABSTRACT: We have discovered that the dc mass current through a superfluid 3He weak link is substantially increased when the Josephson frequency matches the resonant frequency of a coupled mechanical oscillator. The phenomenon is the result of homodyne mixing between the Josephson oscillations and the oscillating pressure field associated with the resonant system. The measured sizes of the current enhancements are in excellent agreement with calculations based on this homodyne model. Similar observations in superconducting junctions, in which microwave radiation changes the dc electronic current, were used for the first confirmation of the dynamics of the superconducting Josephson effect.
    Physical Review Letters 01/1998; 81(6):1247-1250. · 7.73 Impact Factor
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    ABSTRACT: The correct interpretation of superfluid flow experiments relies on the knowledge of thermal and viscous effects that can cause deviations from ideal behavior. The previous paper presented a theoretical study of dissipative and reactive(nondissipative) thermoviscous effects in both steady and oscillating flow of an isotropic superfluid through small apertures and channels. Here, a detailed comparison is made between the theory and a wide array of experimental data. First, the calculated resistance to steady superflow is compared with measurements taken in a constant pressure-head flow cell. Second, the resonant frequency and Q of three different helmholtz oscillators are compared with predictions based on the calculated frequency response. The resonant frequency and Q are extracted numerically from the frequency response, and analytical results are given in experimentally important limits. Finally, the measured and calculated frequency response are compared at a temperature where the Helmholtz oscillator differs significantly from a simple harmonic oscillator. This difference is used to explain how the thermal properties of the oscillator affect its response. The quantitative agreement between the theory and experiment provide an excellent check of the previously derived equations. Also, the limiting expressions shown in this paper provide simple analytical expressions for calculating the effects of the various physical phenomena in a particular experimental situation.
    Journal of Low Temperature Physics 11/1997; 109(3):527-546. · 1.18 Impact Factor
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    ABSTRACT: Direct measurements of the current-phase relation, I versus Deltaphi, for a weak link coupling two reservoirs of B-phase superfluid helium-3 (3He-B) were made over a wide range of temperatures. The weak link consists of a square array of 100-nanometer-diameter apertures. For temperatures T such that T/Tc >/= 0.6 (where Tc is the superfluid transition temperature), I approximately sin(Deltaphi). At lower temperatures, I(Deltaphi) approaches a straight line. Several remarkable phenomena heretofore inaccessible to superconducting Josephson junctions, including direct observation of quantum oscillations and continuous knowledge of Deltaphi, were also observed.
    Science 11/1997; 278(5342):1435-8. · 31.20 Impact Factor
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    ABSTRACT: 1-3 that if macroscopic quantum systems are weakly coupled together, particle currents should oscillate between the two systems. The conditions for these quantum oscillations to occur are that the two systems must both have a well defined quantum phase, f, and a different average energy per particle, m: the term 'weakly coupled' means that the wavefunctions describing the systems must overlap slightly. The frequency of the resulting oscillations is then given by f =( m 2 - m 1)/h, where h is Planck's constant. To date, the only observed example of this phenomenon is the oscillation of electric current between two superconductors coupled by a Josephson tunnelling weak link 4 . Here we report the observation of oscillating mass currents between two reservoirs of superfluid 3 He, the weak link being provided by an array of submicrometre apertures in a membrane separating the reservoirs. An applied pressure difference creates mass-current oscillations, which are detected as sound in a nearby microphone. The sound frequency (typically 6,000-200 Hz) is precisely proportional to the applied pressure difference, in accordance with the above equation. These superfluid quantum oscillations were first detected while monitoring an amplified microphone signal with the human ear. The theory underlying the above equation was developed in the context of generalizing the ideas of Josephson4. He predicted that in a superconducting tunnel junction, the quantum phase difference, Df, across the function is related to the electrical current, I, through it, by the equation I º Ic sin ﷿Df fi﷿ 1 fi
    Nature 07/1997; 388(6641). · 38.60 Impact Factor
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    [Show abstract] [Hide abstract]
    ABSTRACT: The correct interpretation of superfluid flow experiments relies on the knowledge of thermal and viscous effects that can cause deviations from ideal behavior. The previous paper presented a theoretical study of dissipative and reactive(nondissipative) thermoviscous effects in both steady and oscillating flow of an isotropic superfluid through small apertures and channels. Here, a detailed comparison is made between the theory and a wide array of experimental data. First, the calculated resistance to steady superflow is compared with measurements taken in a constant pressure-head flow cell. Second, the resonant frequency and Q of three different Helmholtz oscillators are compared with predictions based on the calculated frequency response. The resonant frequency and Q are extracted numerically from the frequency response, and analytical results are given in experimentally important limits. Finally, the measured and calculated frequency response are compared at a temperature where the Helmholtz oscillator differs significantly from a simple harmonic oscillator. This difference is used to explain how the thermal properties of the oscillator affect its response. The quantitative agreement between the theory and experiment provide an excellent check of the previously derived equations. Also, the limiting expressions shown in this paper provide simple analytical expressions for calculating the effects of the various physical phenomena in a particular experimental situation.
    Journal of Low Temperature Physics 01/1997; 109(3):527-546. · 1.18 Impact Factor
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    ABSTRACT: We present preliminary measurements of the intrinsic critical velocity for vortex production due to flow through a submicron aperture at temperatures from 1.65 K to T lambda- T=3×10-3 K and fluid drive pressures (Josephson frequencies) from 0.5 mPa ( f j=35 Hz) to 250 mPa ( f j=17 kHz). The critical velocity is probed using a constant pressure drive technique which maintains a constant phase slip nucleation rate for several minutes. The measured critical velocity qualitatively agrees with a model of thermally activated vortex half rings.
    Czechoslovak Journal of Physics 01/1996; 46:127-128. · 0.42 Impact Factor
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    ABSTRACT: An array of 1126 identical microapertures is used as the hydrodynamic inductance in a superfluid3He Helmholtz resonator. Each aperture is 0.27 micron square, fabricated by e-beam lithography in a 0.1 micron thick SiN membrane. These dimensions are not too much larger than the superfluid coherence length and therefore, particularly at higher temperatures, an aperture might be though of as a Josephson weak link (in the sense of a Dayem bridge). The superfluid3He critical mass current, Ic, through this array has been measured over a wide range of temperatures. The results indicate that the critical current has a Ginzburg-Landau form of Ic=Ico(1-T/Tc)3/2. This temperature dependence is consistent with phase slippage (not necessarily involving vortices), at the pair breaking velocity, as the critical mechanism.
    Czechoslovak Journal of Physics 12/1995; 46:115-116. · 0.42 Impact Factor