83 research outputs found
Evidence for long-lived quasiparticles trapped in superconducting point contacts
We have observed that the supercurrent across phase-biased, highly
transmitting atomic size contacts is strongly reduced within a broad phase
interval around {\pi}. We attribute this effect to quasiparticle trapping in
one of the discrete sub-gap Andreev bound states formed at the contact.
Trapping occurs essentially when the Andreev energy is smaller than half the
superconducting gap {\Delta}, a situation in which the lifetime of trapped
quasiparticles is found to exceed 100 \mus. The origin of this sharp energy
threshold is presently not understood.Comment: Article (5 pages) AND Supplemental material (14 pages). To be
published in Physical Review Letter
Quantum phase slip phenomenon in superconducting nanowires with low-Ohmic environment
In a number of recent experiments it has been demonstrated that in
ultra-narrow superconducting channels quantum fluctuations of the order
parameter, alternatively called quantum phase slips, are responsible for the
finite resistance well below the critical temperature. The acceptable agreement
between those experiments and the models describing quantum fluctuations in
quasi-one-dimensional superconductors has been established. However the very
concept of the phase slip is justified when these fluctuations are the
relatively rare events, meaning that the effective resistance of the system
should be much smaller than the normal state equivalent. In this paper we study
the limit of the strong quantum fluctuations where the existing models are not
applicable. In particular case of ultra-thin titanium nanowires it is
demonstrated that below the expected critical temperature the resistance does
not demonstrate any trend towards the conventional for a superconductor
zero-resistivity state even at negligibly small measuring currents. Application
of a small magnetic field leads to an unusual negative magnetoresistance, which
becomes more pronounced at lower temperatures. The origin of the negative
magnetoresistance effect is not clear
Ion beam shaping and downsizing of nanostructures
We report a new approach for progressive and well-controlled downsizing of
nanostructures below the 10 nm scale. Low energetic ion beam (Ar+) is used for
gentle surface erosion, progressively shrinking the dimensions with ~ 1 nm
accuracy. The method enables shaping of nanostructure geometry and polishing
the surface. The process is clean room / high vacuum compatible being suitable
for various applications. Apart from technological advantages, the method
enables study of various size phenomena on the same sample between sessions of
ion beam treatment.Comment: 14 pages, 6 figure
Quantum fluctuations in ultranarrow superconducting nanowires
Progressive reduction of the effective diameter of a nanowire is applied to
trace evolution of the shape of superconducting transition in
quasi-one-dimensional aluminum structures. In nanowires with effective diameter
15 nm the dependences are much wider than predicted by the model
of thermally activated phase slips. The effect can be explained by quantum
fluctuations of the order parameter. Negative magnetoresistance is observed in
the thinest samples. Experimental results are in reasonable agreement with
existing theoretical models. The effect should have a universal validity
indicating a breakdown of zero resistance state in a superconductor below a
certain scale.Comment: 18 pages, 5 figure
Superconducting nanowires: quantum confinement and spatially dependent Hartree-Fock potential
It is well-known that in bulk, the solution of the Bogoliubov-de Gennes
equations is the same whether or not the Hartree-Fock term is included. Here
the Hartree-Fock potential is position independent and, so, gives the same
contribution to both the single-electron energies and the Fermi level (the
chemical potential). Thus, the single-electron energies measured from the Fermi
level (they control the solution) stay the same. It is not the case for
nanostructured superconductors, where quantum confinement breaks the
translational symmetry and results in a position dependent Hartree-Fock
potential. In this case its contribution to the single-electron energies
depends on the relevant quantum numbers. We numerically solved the
Bogoliubov-de Gennes equations with the Hartree-Fock term for a clean
superconducting nanocylinder and found a shift of the curve representing the
thickness-dependent oscillations of the critical superconducting temperature to
larger diameters
Quantum phase slip phenomenon in ultra-narrow superconducting nanorings
The smaller the system, typically - the higher is the impact of fluctuations.
In narrow superconducting wires sufficiently close to the critical temperature
Tc thermal fluctuations are responsible for the experimentally observable
finite resistance. Quite recently it became possible to fabricate sub-10 nm
superconducting structures, where the finite resistivity was reported within
the whole range of experimentally obtainable temperatures. The observation has
been associated with quantum fluctuations capable to quench zero resistivity in
superconducting nanowires even at temperatures T-->0. Here we demonstrate that
in tiny superconducting nanorings the same phenomenon is responsible for
suppression of another basic attribute of superconductivity - persistent
currents - dramatically affecting their magnitude, the period and the shape of
the current-phase relation. The effect is of fundamental importance
demonstrating the impact of quantum fluctuations on the ground state of a
macroscopically coherent system, and should be taken into consideration in
various nanoelectronic applications.Comment: 20 pages, 4 figure
- …