Quartz Microbalance Study of Thick 4He Film Near the Superfluid Transition

We present the resugts of a quartz microbalance measurement of a ～440 Å thick ⁴He film adsorbed on Au. This measurement confirms the resugt of an earlier capacitance measurement that showed a dip in the film thickness just below Ta due to the critical Casimir force. The magnitude of the dip in the two measurements agrees within 15% . In addition to the dip, we see a signal due to the onset of superfluidity in the film. An apparent peak in the film thickness, not observed previously, is also found coinciding closely with the bulk T _λ. This extra feature may be due to a sound resonance in the cell associated with the bulk fluid.     

Superfluid Density of 4He Films Adsorbed in Porous MCM-41 Ceramic

No Heading The superfluid transition of ⁴He films adsorbed in MCM-41 ceramic with 40-Â-diameter, micron-length cylindrical channels is studied with a torsion oscillator technique. For film coverages above 1.7 layers a finite-size Kosterlitz-Thouless transition becomes apparent in the data, with considerable broadening of the KT jump of the superfluid fraction. With decreasing helium coverage the extent of the broadening increases, indicating that the vortex core size is increasing. The data is consistent with the Machta-Guyer theory of the KT transition in a cylindrical channel, but with a coverage-dependent vortex core size. At low temperatures a linear decrease of the superfluid fraction with temperature is observed, indicating a zero-dimensional excitation.PACS numbers: 67.40.Db, 67.40.Rp, 67.40.Vs, 67.40.Hf     

Competition Between the Superfluid Overlayer and the Mobile Solid Layer of 3He-4He Mixture Films on Porous Gold

In the previous quartz crystal microbalance measurements for ⁴He films adsorbed on porous gold, we have observed a competition between superfluidity and slippage: In low areal densities, the resonance frequency increases gradually below T _S due to the slippage of solid layer, while at high areal densities the slippage disappears and the superfluid onset of liquid overlayer is observed at T _C . In the crossover region, the slippage below T _S is suddenly suppressed at T _D , which is much lower than T _C . In the present work, we introduced a small amount of ³He onto ⁴He films, and studied the competition as a function of ³He areal density ρ ₃. As ρ ₃ is increased, T _C is monotonically lowered. In contrast, T _D increases up to a certain value of ρ ₃, and then turns over to decrease in parallel to T _C .     

The Superfluid Transition and Vortex Dynamics in Submonolayer Superfluid 4He Films in Porous Glass Under Rotation

A combination of a rotating dilution refrigerator and high-Q torsional oscillator technique has been used to study dynamics of vortices in thin ⁴ He films adsorbed on the porous glass (d=1m pore size). Under rotation an additional dissipation peak with the amplitude proportional to the angular velocity is seen at the middle of the superfluid transition, on the low temperature side of the stationary peak which is present even at =0. We attribute this peak to the 3D Type vortices created in multiply connected ⁴ He film by the rotation. Peak shape of the rotation-induced dissipation could be interpreted as a freezing of the 3D vortices well below T _c     

石英晶体微天平质量检测量研究

基于石英晶体微天平质量检测原理,通过分析石英谐振器表面的振动位移,得出决定其电极表面每一点处质量灵敏度的物理量为该点处的振动加速度。在此基础上,基于石英谐振器的巴特沃斯-范·戴克等效电路模型提出一种计算石英晶体谐振器表面振荡幅度的方法,最终得到石英晶体微天平在整个检测过程中能检测到的平均质量。用10MHz的石英晶体微天平进行了数值验证,计算值与其理论上能达到的pg级检测限基本相吻合。     

Quartz Oscillator Study of Monolayer 4He and 3He - 4He Mixture Films Adsorbed on Solid H2

The superfluid transition in submonolayer and monolayer ⁴ He films and ³ He - ⁴ He mixture films on solid H ₂ has been studied using a quartz crystal microbalance technique. Kosterlitz-Thouless (KT) transitions were observed in submonolayer ⁴ He films with density greater than 0.062 ± 0.002 Å ^–2 . We determine a binding energy of ⁴ He to 0.241 Å ^–2 H ₂ of –15.7 K in the. presence of 1 monolayer of ⁴ He. At several ⁴ He coverages, a range of submonolayer ³ He coverages was studied (n ₃ 0.0567 Å ^–2 ). With each increase in the ³ He coverage, the KT transition temperature decreased. For the higher coverage mixture films studied (n ₄ 0.0726 Å ^–2 ) we observed an apparent second decoupling of the film from the quartz oscillator frequency in addition to the KT transition. We have studied the. coverage dependence of this new feature.     

Vortex Diffusivity and Core Diameter of 2D Superfluid in 4He Films on Gold and H2 Substrates

The two-dimensional (2D) ⁴He fluid films show the Kosterlitz-Thouless (KT) transition where pairing and unpairing of the 2D vortices play an important role. However, the vortex properties (the diffusion constant D, the core diameter a ₀) have not been precisely obtained for various conditions. Here, we accurately determined the parameter D/a ₀ ² by the high frequency dependence of the superfluid onset up to 180 MHz for the submonolayer ⁴He fluid films adsorbed on gold and H₂ (3.3 layers) preplated on gold, respectively. The superfluid onset coverage changes from 1.6 (gold) to 0.5 layers (H₂), which clearly indicates the large difference of the adsorption potential. The parameter D/a ₀ ² , on the other hand, has the same value for the coverages with the same KT temperature T _KT. This suggests that the vortex diffusions on both substrates have the largest value D～?/m in the quantum limit. The core diameter a ₀ was estimated to be the same magnitude as the de Broglie wavelength at T _KT between 0.1 and 0.9 K.     

Substrate Dependence of the Superfluid Onset of 4He Films

We have performed torsional oscillator measurements of ⁴ He films adsorbed on porous gold and gold preplated with Ar, Ne, D ₂ , HD, and H ₂ . On all substrates, the superfluid response is similar. The thickness of the nonsuperfluid layer as function of the substrate, however, increases monotonically with the well depth of the atom-substrate interaction potential.     

Experimental Survey of Wetting and Superfluid Onset of 4He on Alkali Metal Surfaces

We present a survey of the wetting behavior of ⁴ He on evaporated films of rubidium, potassium and sodium. ⁴ He wets these surfaces at all temperatures. Off coexistence on rubidium and potassium there is a prewetting transition that is closely coupled with superfluid onset and possibly a surface tricritical point where the prewetting transition and superfluid onset intersect. Sodium has a prewetting critical temperature well below 1 K. Wetting and superfluid onset phase diagrams for ⁴ He on rubidium and potassium are presented and compared with that of ⁴ He on cesium.     

Adiabatic Melting of 4He Crystal in Superfluid 3He at Sub-millikelvin Temperatures

Adiabatic melting of ⁴He crystal to phase separated ³He–⁴He solution (at T< 2 mK) is probably the most promising method to cool the dilute phase down to temperatures substantially below 0.1 mK. When started well below the superfluid transition temperature T _c of pure ³He, this process allows, in principle, to get the final temperature (T _f ) several orders of magnitude less than the initial one (T _i ). This work is the first practical implementation of the method below the T _c of ³He. The observed cooling factor was T _i /T _f =1.4 at 0.9 mK, being mainly limited by the bad performance of the superleak filling line, by incomplete solidification of ⁴He in the cell, and by the improper thermal contact between the cell wall and the liquid.     

Universal Features of the Superfluid Transition of Thin 4He Films on Mylar

Data from a series of torsional oscillator experiments performed on ⁴ He films on Mylar have been available for some time. However, up until now it was thought that disorder in the substrate leads to a wide variation in the results. Here the data is analysed in a new way which captures for the first time the universal features of the results. Comparisons are made with recent theoretical work which predicts universal behaviour in the superfluid film transition and with the results of similar experiments performed on a Grafoil substrate.     

Third Sound and Quartz Crystal Microbalance Measurements of Helium on Rubidium

We present preliminary results from an experiment in progress which seeks to study third sound propagation on clean glass and rubidiated glass for several ⁴ He coverages. The third sound is driven thermally and detected with thin-film zinc bolometers over the temperature range 0.100T0.670 K. Measurements of the frequency of a quartz crystal microbalance which has rubidium deposited onto one side are made through the temperature range 0.100T1.5 K for each helium coverage studied. Results of measurements for pure ⁴ He films, primarily for third sound, are presented for coverages8.52 atomic layers.     

Antibody Immobilization on Functional Monolayers Using a Quartz Crystal Microbalance

This paper evaluated immobilization of anti-C-reactive protein (CRP) monoclonal antibody on a quartz crystal microbalance (QCM) when 2-aminoethanethiol (AET), 4,4'-dithiodibutyric acid (DDA), and 11-mercaptoundecanoic acid (MUA) were deposited on the gold surface of QCM. In all monolayers, anti-CRP antibodies were immobilized such as Langmuir types because it had been introduced with a corresponding active group. According to the Langmuir isotherm equation$a_max$, the maximum immobilized amounts of anti-CRP antibody were 4.27, 2.72, and 3.74$hboxpmol/hboxcm^2$, respectively. Although the immobilized amount of anti-CRP antibody was highest on the AET monolayer, the amount of antigen–antibody binding between the anti-CRP antibody and the CRP was highest on the MUA monolayer. CRP was detected from negative to positive levels when the calibration curve was achieved using MUA monolayer.     

基于单片机的石英晶体微天平气体检测系统设计

在对石英晶体微天平(QCM)的工作原理和检测方法进行了研究之后,本文提出采用参比QCM补偿的方法来克服温湿度等非质量因素对测量结果的影响,并研制了以MSP430F112单片机为核心的低成本的QCM气体检测系统.详细阐述了该系统的硬件结构和软件设计.为验证系统设计的合理性,用聚苯胺作为敏感材料制备了QCM氨传感器,通过并使用该系统对它性能进行检测.结果表明,与QCM传感器一起配合,该系统可以用来检测环境中气体的浓度.     

Hysteresis,Switching and Anomalous Behaviour of a Quartz Tuning Fork in Superfluid 4He

We have been studying the behaviour of commercial quartz tuning forks immersed in superfluid ⁴He and driven at resonance. For one of the forks we have observed hysteresis and switching between linear and non-linear damping regimes at temperatures below 10 mK. We associate linear damping with pure potential flow around the prongs of the fork, and non-linear damping with the production of vortex lines in a turbulent regime. At appropriate prong velocities, we have observed metastability of both the linear and the turbulent flow states, and a region of intermittency where the flow switched back and forth between each state. For the same fork, we have also observed anomalous behaviour in the linear regime, with large excursions in both damping, resonant frequency, and the tip velocity as a function of driving force.     

Superfluid State of 4He Films Adsorbed on 27 Å Pores in HMM-2

We have measured the superfluid density ρ _s of ⁴He films adsorbed on the porous material HMM-2 by a torsional oscillator. The substrate has pores of 27 Å in diameter with ordered three-dimensional connectivity. ρ _s increases as T decreases, without any feature of the universal jump and the associated dissipation peak characteristic of the two-dimensional Kosterlitz–Thouless transition. We also observed resonances of the standing wave modes, from which we can estimate the sound velocity. Assuming linear dependence on (ρ _s/ρ _s(0))^1/2 and ignoring refractions, the sound velocities are extrapolated to about 20 m/s at T=0.     

Superfluid Phase Transition of 3He-4He Mixture Films Adsorbed on Alumina Powder

The superfluid phase transition of ³ He- ⁴ He mixture films adsorbed on 500 Å alumina powder has been studied for mixture films whose superfluid thickness is less than a monolayer. The transitions are found to be controlled by the Kosterlitz-Thouless critical line, but a strong broadening of the transition is observed as the ³ He concentration is increased. Analyzing the broadening in terms of a KT vortex-pair theory modified for the the finite powder size yields a vortex core parameter which increases nearly linearly with added ³ He. Also observed in these measurements is a temperature-dependent and ³ He-dependent depletion of the superfluid density at low temperatures, which is thought to arise from the high-frequency ripplon/third sound excitations of the film.     

Capacitive Generation and Detection of Third Sound Resonances in Saturated Superfluid 4He Films

We report on the operation of a third sound resonator for use with saturated superfluid helium films. Measurements of superfluid ⁴ He third sound velocities as a function of film thickness are described for films ranging in thickness from 26 nm to 85 nm. The film resides on the gold-covered surface of a 38 mm diameter copper disk, which has been machined with a diamond-tipped cutting tool, and has an rms roughness of about 10 nm.     

基于石英晶体微天平的聚醚酰亚胺湿度传感器

研制了基于石英晶体微天平方法的聚醚酰亚胺湿度传感器,研究了它对相对湿度的响应特性和灵敏度以及这种湿度传感器在高湿度对比下的循环特性和长期老化性能。结果表明:在较宽的湿度范围内,这种湿度传感器表现出良好的湿敏特性。当相对湿度从11%变化至95%时,频率变化达2000Hz。高湿度差对比下的响应时间和恢复时间分别为30和20s。在循环测试和长时间老化测试中,也表现出优良的重复性和稳定性。     

QCM Measurements of Superfluid Response in 4He Films up to 180 MHz

At finite frequencies, a dynamic Kosterlitz–Thouless (KT) theory predicts a frequency dependence of the superfluid transition in ⁴He films on planar surfaces. We report results of QCM measurements to study the superfluid response on planar gold surfaces for very high frequencies up to 180 MHz in the temperature range of 0.6–1.0 K. As the frequency is increased, we observed the expected KT behavior that the superfluid transition shifts to a higher temperature from the static transition temperature T _KT and the transition temperature region broadens. The frequency dependence of the dissipation peak temperature at the transition agrees with a simple equation of the frequency dependence based on the dynamic KT theory. The microscopic parameter for the dynamic transition, the ratio of the diffusion constant to the square of the vortex core radius D/r ₀ ², is estimated to be on the order of 10¹⁰ s⁻¹.