Unusual Behavior of a MEMS Resonator in Superfluid 4 He
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Unusual Behavior of a MEMS Resonator in Superfluid 4 He M. Gonzalez · P. Zheng · B.H. Moon · E. Garcell · Y. Lee · H.B. Chan
Received: 6 July 2012 / Accepted: 2 August 2012 / Published online: 23 August 2012 © Springer Science+Business Media, LLC 2012
Abstract Novel mechanical resonators based on micro-electro-mechanical systems (MEMS) technology were developed for the study of superfluid 4 He. The MEMS device is composed of two parallel plates, the movable plate suspended by four serpentine springs above the substrate, forming a shear mechanical oscillator. A specific device with a 1.25 µm gap was tested in the superfluid phase of 4 He. At temperatures below 400 mK the device exhibits nonlinear and hysteretic behavior when the excitation exceeds a threshold. The anomalies are reminiscent of quantum turbulence and vorticity effects observed in other mechanical oscillators such as tuning forks or vibrating grids. Keywords Superfluid 4 He · MEMS 1 Introduction Since the introduction of the torsional oscillator, first used by Andronikashvili in 1946 to quantify the normal density component of helium II [1], there has been a constant search for novel mechanical oscillators to probe the physical properties of liquid helium. Vibrating wires [2] and more recently quartz tuning forks [3, 4] have been implemented in helium II for studies of its hydrodynamic and non-hydrodynamic properties. As pure liquid 4 He is cooled down below its superfluid transition (2.17 K), the normal fluid component (ρn ) in helium II decreases continuously as described by the M. Gonzalez · P. Zheng · B.H. Moon · E. Garcell · Y. Lee () Department of Physics, University of Florida, Gainesville, FL 32611, USA e-mail: [email protected] H.B. Chan Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
J Low Temp Phys (2013) 171:200–206
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two fluid model [5]. This decrease in the normal fluid fraction leads to a decrease in viscosity. If an oscillating object is introduced in the liquid at very low temperatures (close to 1 K) where the normal component becomes very small, the creation of low energy excitations, rotons and phonons, provides a damping mechanism to the object’s motion. Below 1 K the main contribution to the viscosity comes from the dilute phonon gas [6], which renders ρn ∝ T 4 . Due to the long mean free path of the phonon excitations in this temperature regime (can be of the order of 1 mm), it is possible to see a transition to a ballistic or Knudsen regime even for macroscopic vibrating objects such as tuning forks. As a consequence of these phenomena, a peak in the damping or viscosity, η, has been reported (∼0.7 K), followed by a T 4 behaviour at lower temperatures [7–10]. An important area of research in fluid dynamics is the generation, detection, and quantitative measurement of turbulence. Due to its vanishingly small viscosity and high attainable Reynolds number, superfluid 4 He is very well suited for this purpose. When an oscillating body immersed in helium II is agitated a
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