Browsing by Author "Kaikkonen, J. P."

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Quantum degeneracy in mesoscopic matter : Casimir effect and Bose-Einstein condensation
(2024-06-01) Todoshchenko, I.; Kamada, M.; Kaikkonen, J. P.; Liao, Y.; Savin, A.; Kauppinen, E.; Sergeicheva, E.; Hakonen, P. J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The ground-state phonon pressure is an analog to the famous Casimir pressure of vacuum produced by zero-point photons. The acoustic Casimir forces are, however, many orders of magnitude weaker than the electromagnetic Casimir forces, as the typical speed of sound is 100 000 times smaller than the speed of light. Because of its weakness, zero-point acoustic Casimir pressure was never observed, although the pressure of artificially introduced sound noise on a narrow aperture has been reported. However, the magnitude of Casimir pressure increases as 1/L3 with the decrease of the sample size L, and reaches piconewtons in the submicron scales. We demonstrate and measure the acoustic Casimir pressure induced by zero-point phonons in solid helium adsorbed on a carbon nanotube. We have also observed Casimir-like "pushing out"thermal phonons with the decreasing temperature or the length. We also show that all thermodynamic quantities are size dependent, and therefore in the mesoscopic range L≲h c/(kBT) quadruple points are possible on the phase diagram where four different phases coexist. Due to the smallness of solid helium sample, temperature of Bose-Einstein condensation (BEC) of vacancies is relatively high, 10-100mK. This allowed us to experimentally discover the BEC in a system of zero-point vacancies, predicted more than 50 years ago.
Quartz tuning fork as a probe of surface oscillations
(2017-02-13) Todoshchenko, I.; Savin, A.; Haataja, M.; Kaikkonen, J. P.; Hakonen, P. J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Quartz tuning forks are high-quality mechanical oscillators widely used in low temperature physics as viscometers, thermometers, and pressure sensors. We demonstrate that a fork placed in liquid helium near the surface of solid helium is very sensitive to the oscillations of the solid-liquid interface. We developed a double-resonance read-out technique, which allowed us to detect oscillations of the surface with an accuracy of 1 Å in 10 s. Using this technique, we have investigated crystallization waves in 4He down to 10 mK. In contrast to previous studies of crystallization waves, our measurement scheme has very low dissipation, on the order of 20 pW, which allows us to carry out experiments even at sub-mK temperatures. We propose to use this scheme in the search for crystallization waves in 3He, which exist only at temperatures well below 0.5 mK. The suggested technique can also be used for accurate displacement detection in a large variety of systems.
Topologically-imposed vacancies and mobile solid 3He on carbon nanotube
(2022-10-05) Todoshchenko, I.; Kamada, M.; Kaikkonen, J. P.; Liao, Y.; Savin, A.; Will, M.; Sergeicheva, E.; Abhilash, T. S.; Kauppinen, E.; Hakonen, P. J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Low dimensional fermionic quantum systems are exceptionally interesting because they reveal distinctive physical phenomena, including among others, topologically protected excitations, edge states, frustration, and fractionalization. Our aim was to confine 3He on a suspended carbon nanotube to form 2-dimensional Fermi-system. Here we report our measurements of the mechanical resonance of the nanotube with adsorbed sub-monolayer down to 10 mK. At intermediate coverages we have observed the famous 1/3 commensurate solid. However, at larger monolayer densities we have observed a quantum phase transition from 1/3 solid to an unknown, soft, and mobile solid phase. We interpret this mobile solid phase as a bosonic commensurate crystal consisting of helium dimers with topologically-induced zero-point vacancies which are delocalized at low temperatures. We thus demonstrate that 3He on a nanotube merges both fermionic and bosonic phenomena, with a quantum phase transition between fermionic solid 1/3 phase and the observed bosonic dimer solid.