464 research outputs found
Superconducting Nanowires as Nonlinear Inductive Elements for Qubits
We report microwave transmission measurements of superconducting Fabry-Perot
resonators (SFPR), having a superconducting nanowire placed at a supercurrent
antinode. As the plasma oscillation is excited, the supercurrent is forced to
flow through the nanowire. The microwave transmission of the resonator-nanowire
device shows a nonlinear resonance behavior, significantly dependent on the
amplitude of the supercurrent oscillation. We show that such
amplitude-dependent response is due to the nonlinearity of the current-phase
relationship (CPR) of the nanowire. The results are explained within a
nonlinear oscillator model of the Duffing oscillator, in which the nanowire
acts as a purely inductive element, in the limit of low temperatures and low
amplitudes. The low quality factor sample exhibits a "crater" at the resonance
peak at higher driving power, which is due to dissipation. We observe a
hysteretic bifurcation behavior of the transmission response to frequency sweep
in a sample with a higher quality factor. The Duffing model is used to explain
the Duffing bistability diagram. We also propose a concept of a nanowire-based
qubit that relies on the current dependence of the kinetic inductance of a
superconducting nanowire.Comment: 28 pages, 7 figure
Nucleation of Superconductivity in a Mesoscopic Loop of Finite Width
The normal/superconducting phase boundary Tc has been calculated for
mesoscopic loops, as a function of an applied perpendicular magnetic field H.
While for thin-wire loops and filled disks the Tc(H) curves are well known, the
intermediate case, namely mesoscopic loops of finite wire width, have been
studied much less. The linearized first Ginzburg-Landau equation is solved with
the proper normal/vacuum boundary conditions both at the internal and at the
external loop radius. For thin-wire loops the Tc(H) oscillations are perfectly
periodic, and the Tc(H) background is parabolic (this is the usual Little-Parks
effect). For loops of thicker wire width, there is a crossover magnetic field
above which Tc(H) becomes quasi-linear, with the period identical to the Tc(H)
of a filled disk (i.e. pseudoperiodic oscillations). This dimensional
transition is similar to the 2D-3D transition for thin films in a parallel
field, where vortices start penetrating the material as soon as the film
thickness exceeds the temperature dependent coherence length by a factor 1.8.
For the presently studied loops, the crossover point is controlled by a similar
condition. In the high field '3D' regime, a giant vortex state establishes,
where only a surface superconducting sheath near the sample's outer radius is
present.Comment: 7 pages text, 2 EPS figures, uses LaTeX's elsart.sty, proceedings of
the First Euroconference on "Vortex Matter in Superconductors", held in Crete
(18-24 september 1999
Formation of Quantum Phase Slip Pairs in Superconducting Nanowires
Macroscopic quantum tunneling (MQT) is a fundamental phenomenon of quantum
mechanics related to the actively debated topic of quantum-to-classical
transition. The ability to realize MQT affects implementation of qubit-based
quantum computing schemes and their protection against decoherence. Decoherence
in qubits can be reduced by means of topological protection, e.g. by exploiting
various parity effects. In particular, paired phase slips can provide such
protection for superconducting qubits. Here, we report on the direct
observation of quantum paired phase slips in thin-wire superconducting loops.
We show that in addition to conventional single phase slips that change
superconducting order parameter phase by , there are quantum transitions
changing the phase by . Quantum paired phase slips represent a
synchronized occurrence of two macroscopic quantum tunneling events, i.e.
cotunneling. We demonstrate the existence of a remarkable regime in which
paired phase slips are exponentially more probable than single ones
Quantitative analysis of quantum phase slips in superconducting MoGe nanowires revealed by switching-current statistics
We measure quantum and thermal phase-slip rates using the standard deviation
of the switching current in superconducting nanowires at high bias current. Our
rigorous quantitative analysis provides firm evidence for the presence of
quantum phase slips (QPS) in homogeneous nanowires. We observe that as
temperature is lowered, thermal fluctuations freeze at a characteristic
crossover temperature Tq, below which the dispersion of the switching current
saturates to a constant value, indicating the presence of QPS. The scaling of
the crossover temperature Tq with the critical temperature Tc is linear, which
is consistent with the theory of macroscopic quantum tunneling. We can convert
the wires from the initial amorphous phase to a single crystal phase, in situ,
by applying calibrated voltage pulses. This technique allows us to probe
directly the effects of the wire resistance, critical temperature and
morphology on thermal and quantum phase slips.Comment: 7 pages, 7 figures, 1 tabl
Superconducting properties of polycrystalline Nb nanowires templated by carbon nanotubes
Journal ArticleContinuous Nb wires, 7-15 nm in diameter, have been fabricated by sputter-coating single fluorinated carbon nanotubes. Transmission electron microscopy revealed that the wires are polycrystalline, having grain sizes of about 5 nm. The critical current of wires thicker than ~12 nm is very high (107 A/cm2) and comparable to the expected depairing current. The resistance versus temperature curves measured down to 0.3 K are well described by the Langer-Ambegaokar-McCumber-Halperin theory of thermally activated phase slips. Quantum phase slips are suppressed
Vortex matter in mesoscopic superconductors
Superconducting mesoscopic devices in magnetic fields present novel
properties which can only be accounted for by both the quantum confinement of
the Cooper pairs and by the interaction between the magnetic-field-induced
vortices. Sub-micrometer disks, much the same as their semiconductor
counterparts known as quantum dots, are being subject to experimental
investigation by measuring their conducting properties and, more recently,
their magnetization by using state-of-the-art ballistic Hall magnetometry. In
this work I review the main results obtained in these two types of experiments
as well as the current theoretical developments which are contributing to our
understanding of the superconducting condensate in these systems.Comment: 16 pages, 4 figures. Invited presentation at the 13th International
Conference on High Magnetic Fields in Semiconductor Physics to appear in
Physica
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