Probing superfluid He4 with high-frequency nanomechanical resonators down to millikelvin temperatures

Abstract

Superfluids, such as superfluid 3 He and 4 He, exhibit a broad range of quantum phenomena and excitations which are unique to these systems. Nanoscale mechanical resonators are sensitive and versatile force detectors with the ability to operate over many orders of magnitude in damping. Using nanomechanical-doubly clamped beams of extremely high quality factors (Q > 10 6 ), we probe superfluid 4 He from the superfluid transition temperature down to mK temperatures at frequencies up to 11.6 MHz. Our studies show that nanobeam damping is dominated by hydrodynamic viscosity of the normal component of 4 He above 1 K. In the temperature range 0.3 − 0.8 K, the ballistic quasiparticles (phonons and rotons) determine the beams' behavior. At lower temperatures, damping saturates and is determined either by magnetomotive losses or acoustic emission into helium. It is remarkable that all these distinct regimes can be extracted with just a single device, despite damping changing over six orders of magnitude. arXiv:1907.00970v1 [cond-mat.mes-hall] 1 Jul 2019

Nanoscale Real-Time Detection of Quantum Vortices at Millikelvin Temperatures

“…Secondly, the damping of the beam hardly differs from that of the vortex-free or vacuum state, as expected, since the capture of a single vortex should not significantly change the acoustic emission 4 , nor should it introduce any new dissipation mechanism. Thirdly, the frequency of the upper plateau is almost always the same (3 kHz above the default state), supporting the idea of the capture of a singly-quantized vortex.…”

“…That said, we have hitherto lacked detectors capable of the real-time, non-invasive probing of the wide range of length scales involved in quantum turbulence. However, we demonstrate here the real-time detection of quantum vortices by a nanoscale resonant beam in superfluid 4 He at 10 mK. The basic idea is that we can trap a single vortex along the length of a nanobeam and observe the transitions as a vortex is either trapped or released, which we observe through the shift in the resonant frequency of the beam.…”

“…The nanobeams we use for detecting single vortex events in real time have a characteristic dimension less than 1 µm and response times faster than 1 ms. Such devices have recently emerged as highly sensitive probes of hydrodynamic 2,3 and ballistic 4 He 4,5 . We present here events demonstrating single-vortex capture, its interaction with the surrounding vortex tangle, and subsequent release via reconnection with a nearby vortex in the surrounding tangle.…”

Amplitude of Waves in the Kelvin-Wave Cascade

“…Such situation never occurs in a typical experiment on quantum turbulence. Recently, progress in experimental techniques [15][16][17] enables controllable excitation of waves on straight or nearly straight vortices, see Fig. 1 for possible setups.…”

Acoustic damping of quartz tuning forks in normal and superfluid He3

“…We can use the fork prong width W as an effective cylinder diameter. We ignore the roton damping contribution as it is negligible compared to the phonons at 450 mK [50,52] where our ballistic 4 He measurements were conducted. In superfluid 3 He, the oscillator damping arises from its interaction with broken Cooper pairs, the so-called quasiparticles [35,36].…”