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Summary of a discussion session

of the dirty superconductor workshop that was closed yesterday. It’s also posted to the workshop blog. It’s been a marvelous workshop: thank you,Lorentz center and the organizers

Discussion Session I Summary: Quantum Phase slips

The discussion was mostly under the sign of the recent report of Oleg
Astafiev concerning the realization of phase-slip qubit with InO nanowires.
It was noted that a number of features in these devices do not correspond to
usual assumptions about what is good and what is bad for big values of
phase-slip amplitude. Big resistance per square in combination with
significant Tc is certainly thought to be needed for the effect. However,
the issue that required discussion is relatively big width and height of
the wires that by factors exceed coherence length expected for the material.
Another issue concerned the effect of uncompensated charged impurities
readily available in insulating subtrate. The charge induced by these
impurities could lead to a total compensation of the phase-slip magnitude.

It has been noted that a phase-slip in a wire that is wider than coherence
length can be seen as a tunneling of Abrikosov vorthex across the wire. The
fact that the tunneling amplitude is appreciable means that such a vorthex
bears a relatively small energy, perhaps at the scale of quasiparticle
energy, and can be regarded as an elementary excitantion of the dirty
superconductor. An analogy has been drawn with a vortex in Coulomb-blockaded
Josephson junction arrays where a vortex has no core and could have zero
energy. This suggestion is however different from earlier works and
hypotheses concerning dirty superconducting films and Josephson junction
arrays where the possibility of quantum tunneling of already created
Abrikosov vortices has been considered.

Lev Ioffe has outlined his many-body numerical simulations that lead him to
an estimate of phase-slip amplitude in InOx while using a few-site lattice
model. He also suggested that the phase-slip amplitude can be dominated just
by a single optimal path somewhere in the wire, this being in agreement with
other suggestions that weak links can determine the actual magnitude. Such
assumption may aslo explain why the charged impurities do not significantly
reduced the phase-slip amplitude.

It has been noted that the observation of Astafiev is very encouraging for
nanostructuring of the wires to produce more controllable phase-slip devices
where the Coulomb blockade effect can be readily and inambiguosly observed.
The simplest suggestion would be to make two constrictions in the wire
corresponding to two weak links.

It looks like that the report of Astafiev has moved the focus of attention
from the material-science issues to more practical questions. While it
remains to be seen if phase-slips can be observed in traditional candidate
materials like NbSi, TiN, MoGe; InO seems to work. Nano-fabrication and
nano-desing of InO-based devices will probably absorb the attention of
experimentalis for years coming.

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