22 research outputs found
Probe-Configuration-Dependent Decoherence in an Aharonov-Bohm Ring
We have measured transport through mesoscopic Aharonov-Bohm (AB) rings with
two different four-terminal configurations. While the amplitude and the phase
of the AB oscillations are well explained within the framework of the
Landaur-B\"uttiker formalism, it is found that the probe configuration strongly
affects the coherence time of the electrons, i.e., the decoherence is much
reduced in the configuration of so-called nonlocal resistance. This result
should provide an important clue in clarifying the mechanism of quantum
decoherence in solids.Comment: 4 pages, 4 figures, RevTe
Dephasing of Electrons in Mesoscopic Metal Wires
We have extracted the phase coherence time of electronic
quasiparticles from the low field magnetoresistance of weakly disordered wires
made of silver, copper and gold. In samples fabricated using our purest silver
and gold sources, increases as when the temperature
is reduced, as predicted by the theory of electron-electron interactions in
diffusive wires. In contrast, samples made of a silver source material of
lesser purity or of copper exhibit an apparent saturation of
starting between 0.1 and 1 K down to our base temperature of 40 mK. By
implanting manganese impurities in silver wires, we show that even a minute
concentration of magnetic impurities having a small Kondo temperature can lead
to a quasi saturation of over a broad temperature range, while
the resistance increase expected from the Kondo effect remains hidden by a
large background. We also measured the conductance of Aharonov-Bohm rings
fabricated using a very pure copper source and found that the amplitude of the
conductance oscillations increases strongly with magnetic field. This set
of experiments suggests that the frequently observed ``saturation'' of
in weakly disordered metallic thin films can be attributed to
spin-flip scattering from extremely dilute magnetic impurities, at a level
undetectable by other means.Comment: 16 pages, 11 figures, to be published in Physical Review
Geometry dependent dephasing in small metallic wires
Temperature dependent weak localization is measured in metallic nanowires in
a previously unexplored size regime down to width nm. The dephasing time,
, shows a low temperature dependence close to quasi-1D
theoretical expectations () in the narrowest wires,
but exhibits a relative saturation as for wide samples of the same
material, as observed previously. As only sample geometry is varied to exhibit
both suppression and divergence of , this finding provides a new
constraint on models of dephasing phenomena.Comment: 6 pages, 3 figure
Electron dephasing near zero temperature: an experimental review
The behavior of the electron dephasing time near zero temperature,
, has recently attracted vigorous attention. This renewed interest
is primarily concerned with whether should reach a finite or an
infinite value as 0. While it is accepted that should
diverge if there exists only electron-electron (electron-phonon) scattering,
several recent measurements have found that depends only very
weakly on temperature, if at all, when is sufficiently low. This article
discusses the current experimental status of "the saturation problem", and
concludes that the origin(s) for this widely observed saturation are still
unresolved
Magnetic field effects in energy relaxation mediated by Kondo impurities
We study the energy distribution function of quasiparticles in voltage biased
mesoscopic wires in presence of magnetic impurities and applied magnetic field.
The system is described by a Boltzmann equation where the collision integral is
determined by coupling to spin 1/2 impurities. We derive an effective coupling
to a dissipative spin system which is valid well above Kondo temperature in
equilibrium or for sufficiently smeared distribution functions in
non-equilibrium. For low magnetic field an enhancement of energy relaxation is
found whereas for larger magnetic fields the energy relaxation decreases again
meeting qualitatively the experimental findings by Anthore et al.
(cond-mat/0109297). This gives a strong indication that magnetic impurities are
in fact responsible for the enhanced energy relaxation in copper wires. The
quantitative comparison, however, shows strong deviations for energy relaxation
with small energy transfer whereas the large energy transfer regime is in
agreement with our findings.Comment: 14 pages, 8 figure
Dephasing of Electrons by Two-Level Defects in Quantum Dots
The electron dephasing time in a diffusive quantum dot is
calculated by considering the interaction between the electron and dynamical
defects, modelled as two-level system. Using the standard tunneling model of
glasses, we obtain a linear temperature dependence of ,
consistent with the experimental observation. However, we find that, in order
to obtain dephasing times on the order of nanoseconds, the number of two-level
defects needs to be substantially larger than the typical concentration in
glasses. We also find a finite system-size dependence of , which
can be used to probe the effectiveness of surface-aggregated defects.Comment: two-column 9 page
Direct demonstration of circulating currents in a controllable -SQUID generated by a 0 to transition of the weak links
A controllable -SQUID is a DC SQUID with two controllable
-junctions as weak links. A controllable -junction consists of a
superconducting - normal metal - superconducting Josephson junction with two
additional contacts to the normal region of the junction. By applying a voltage
over these contacts it is possible to control the sate of the junction,
i.e. a conventional (0) state or a -state, depending on the magnitude of
. We demonstrate experimentally that, by putting one junction into a
-state, a screening current is generated around the SQUID loop at integer
external flux. To be able to do this, we have fabricated controllable
-junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at
1.4 K only the Nb-Ag device shows the transition to a -state as a function
of consistent with theoretical predictions. In a controllable SQUID
based on Nb-Ag we observe, a part from a screening current at integer external
flux, a phase shift of of the oscillations under suitable
current bias, depending on the magnitude of .Comment: 11 pages, 12 figures, subm. to Phys. Rev.
Electron Dephasing in Mesoscopic Metal Wires
The low-temperature behavior of the electron phase coherence time,
, in mesoscopic metal wires has been a subject of controversy
recently. Whereas theory predicts that in narrow wires should
increase as as the temperature is lowered, many samples exhibit
a saturation of below about 1 K. We review here the experiments
we have performed recently to address this issue. In particular we emphasize
that in sufficiently pure Ag and Au samples we observe no saturation of
down to our base temperature of 40 mK. In addition, the measured
magnitude of is in excellent quantitative agreement with the
prediction of the perturbative theory of Altshuler, Aronov and Khmelnitskii. We
discuss possible explanations why saturation of is observed in
many other samples measured in our laboratory and elsewhere, and answer the
criticisms raised recently by Mohanty and Webb regarding our work.Comment: 14 pages, 3 figures; to appear in proceedings of conference
"Fundamental Problems of Mesoscopic Physics", Granada, Spain, 6-11 September,
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