EGAS41 - Swansea University

41 st EGAS CP 178 Gdańsk 2009
Aggregation of metal atoms on quantized vortices in superfluid
4 He and nanowire formation
P. Moroshkin 1∗ , V. Lebedev 1 , A. Weis 1 , E.B. Gordon 2 , J.P. Toennies 3
1 Department of Physics, University of Fribourg,
Chemin du Musée 3, CH–1700 Fribourg, Switzerland
2 Institute of Problems of Chemical Physics RAS, 142432 Chernogolovka, Russia
3 Max-Planck-Institut für Dynamik und Selbstorganisation,
Bunsenstrasse 10, D–37073, Göttingen, Germany
∗ Corresponding author: peter.moroshkin@unfr.ch,
Quantized vortex lines in superfluid 4 He are known to bind electrons, atoms, and ions. One
therefore expects the growth of nanoscopic (possible monoatomic) quasi one-dimen-sional
filaments by the coalescence of atoms and nano-clusters along vortex lines. The formation
of filament-like structures by impurity particles in superfluid helium has indeed been
reported by several authors, whereas in normal fluid only spherical clusters were observed
[1]. All previous observations were only visual, and could thus only set a lower limit of
a few microns for the filament thickness. Here we report the first direct experimental
evidence that the filaments formed by the coalescence of impurity atoms and/or clusters
in superfluid He indeed have transverse dimensions in the nanometer range.
Figure 1: SEM images of gold nanowires produced by laser ablation in superfluid 4 He.
Gold atoms were introduced into superfluid 4 He (T = 1.5 K) by pulsed laser ablation
from a gold target [2]. The gold filaments – collected after warming up the cryostat
– were analyzed by a scanning electron (SEM) microscope, which reveals a very dense
“spider web” structure (Fig. 1.a) made from a large number of thin fibers with diameters
of 2-20 nm (Fig. 1.b). These nanowires are twisted together and form thicker “ropes”
that are visible to the naked eye. The characteristic length of an individual nanowire
is larger than 3µm, whereas the “ropes” have lengths comparable to the cryostat cell
dimensions (≃6 cm). It is thus conceivable that the length of a nanowire could reach
several centimeters when produced under better controlled conditions.
Acknowledgment
Work funded by the Swiss National Science Foundation, #200020–119786.
References
[1] G.P. Bewley, D. Lathrop, K.R. Sreenivasan, Nature 441, 588 (2006)
[2] P. Moroshkin, A. Hofer, A. Weis, Physics Reports 469, 1 (2008)
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