819 research outputs found
Magnetic-field-dependent quasiparticle energy relaxation in mesoscopic wires
In order to find out if magnetic impurities can mediate interactions between
quasiparticles in metals, we have measured the effect of a magnetic field B on
the energy distribution function f(E) of quasiparticles in two silver wires
driven out-of-equilibrium by a bias voltage U. In a sample showing sharp
distributions at B=0, no magnetic field effect is found, whereas in the other
sample, rounded distributions at low magnetic field get sharper as B is
increased, with a characteristic field proportional to U. Comparison is made
with recent calculations of the effect of magnetic-impurities-mediated
interactions taking into account Kondo physics.Comment: 4 pages, 3 figures, to be published in Physical Review Letter
Influence of Magnetic Field on Effective Electron-Electron Interactions in a Copper Wire
We have measured in a copper wire the energy exchange rate between
quasiparticles as a function of the applied magnetic field. We find that the
effective electron-electron interaction is strongly modified by the magnetic
field, suggesting that magnetic impurities play a role on the interaction
processes.Comment: latex anthore.tex, 8 files, 6 figures, 7 pages in: Proceedings of the
XXXVIth Rencontres de Moriond `Electronic Correlations: From Meso- to
Nano-physics' Les Arcs, France January 20-27, 2001 [SPEC-S01/027
Phase controlled superconducting proximity effect probed by tunneling spectroscopy
Using a dual-mode STM-AFM microscope operating below 50mK we measured the
Local Density of States (LDoS) along small normal wires connected at both ends
to superconductors with different phases. We observe that a uniform minigap can
develop in the whole normal wire and in the superconductors near the
interfaces. The minigap depends periodically on the phase difference. The
quasiclassical theory of superconductivity applied to a simplified 1D model
geometry accounts well for the data.Comment: Accepted for publication in Physical Review Letter
Supercurrent Spectroscopy of Andreev States
We measure the excitation spectrum of a superconducting atomic contact. In
addition to the usual continuum above the superconducting gap, the single
particle excitation spectrum contains discrete, spin-degenerate Andreev levels
inside the gap. Quasiparticle excitations are induced by a broadband on-chip
microwave source and detected by measuring changes in the supercurrent flowing
through the atomic contact. Since microwave photons excite quasiparticles in
pairs, two types of transitions are observed: Andreev transitions, which
consists of putting two quasiparticles in an Andreev level, and transitions to
odd states with a single quasiparticle in an Andreev level and the other one in
the continuum. In contrast to absorption spectroscopy, supercurrent
spectroscopy allows detection of long-lived odd states.Comment: typos correcte
Exciting Andreev pairs in a superconducting atomic contact
The Josephson effect describes the flow of supercurrent in a weak link, such
as a tunnel junction, nanowire, or molecule, between two superconductors. It is
the basis for a variety of circuits and devices, with applications ranging from
medicine to quantum information. Currently, experiments using Josephson
circuits that behave like artificial atoms are revolutionizing the way we probe
and exploit the laws of quantum physics. Microscopically, the supercurrent is
carried by Andreev pair states, which are localized at the weak link. These
states come in doublets and have energies inside the superconducting gap.
Existing Josephson circuits are based on properties of just the ground state of
each doublet and so far the excited states have not been directly detected.
Here we establish their existence through spectroscopic measurements of
superconducting atomic contacts. The spectra, which depend on the atomic
configuration and on the phase difference between the superconductors, are in
complete agreement with theory. Andreev doublets could be exploited to encode
information in novel types of superconducting qubits.Comment: Submitted to Natur
Theory of microwave spectroscopy of Andreev bound states with a Josephson junction
We present a microscopic theory for the current through a tunnel Josephson
junction coupled to a non-linear environment, which consists of an Andreev
two-level system coupled to a harmonic oscillator. It models a recent
experiment [Bretheau, Girit, Pothier, Esteve, and Urbina, Nature (London) 499,
312 (2013)] on photon spectroscopy of Andreev bound states in a superconducting
atomic-size contact. We find the eigenenergies and eigenstates of the
environment and derive the current through the junction due to inelastic Cooper
pair tunneling. The current-voltage characteristic reveals the transitions
between the Andreev bound states, the excitation of the harmonic mode that
hybridizes with the Andreev bound states, as well as multi-photon processes.
The calculated spectra are in fair agreement with the experimental data.Comment: 8 pages, 6 figure
Manipulating the Quantum State of an Electrical Circuit
We have designed and operated a superconducting tunnel junction circuit that
behaves as a two-level atom: the ``quantronium''. An arbitrary evolution of its
quantum state can be programmed with a series of microwave pulses, and a
projective measurement of the state can be performed by a pulsed readout
sub-circuit. The measured quality factor of quantum coherence Qphi=25000 is
sufficiently high that a solid-state quantum processor based on this type of
circuit can be envisioned.Comment: 4 figures include
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