206 research outputs found
Excited state spectroscopy in carbon nanotube double quantum dots
We report on low temperature measurements in a fully tunable carbon nanotube
double quantum dot. A new fabrication technique has been used for the top-gates
in order to avoid covering the whole nanotube with an oxide layer as in
previous experiments. The top-gates allow us to form single dots, control the
coupling between them and we observe four-fold shell filling. We perform
inelastic transport spectroscopy via the excited states in the double quantum
dot, a necessary step towards the implementation of new microwave-based
experiments.Comment: 16 pages, 6 figures, submitted to nanoletter
Singular conductance of a spin 1 quantum dot
We interpret the recent observation of a zero-bias anomaly in spin-1 quantum
dots in terms of an underscreened Kondo effect. Although a spin-1 quantum dots
are expected to undergo a two-stage quenching effect, in practice the log
normal distribution of Kondo temperatures leads to a broad temperature region
dominated by underscreened Kondo physics. General arguments, based on the
asymptotic decoupling between the partially screened moment and the leads,
predict a singular temperature and voltage dependence of the conductance
and differential conductance , resulting in and . Using a Schwinger boson approach, we show how these qualitative
expectations are borne out in a detailed many body calculation.Comment: Four pages, four figures. Paper revised with additional references
added in response to feedback from reader
Diameter-dependent conductance of InAs nanowires
Electrical conductance through InAs nanowires is relevant for electronic
applications as well as for fundamental quantum experiments. Here we employ
nominally undoped, slightly tapered InAs nanowires to study the diameter
dependence of their conductance. Contacting multiple sections of each wire, we
can study the diameter dependence within individual wires without the need to
compare different nanowire batches. At room temperature we find a
diameter-independent conductivity for diameters larger than 40 nm, indicative
of three-dimensional diffusive transport. For smaller diameters, the resistance
increases considerably, in coincidence with a strong suppression of the
mobility. From an analysis of the effective charge carrier density, we find
indications for a surface accumulation layer.Comment: 9 pages, 5 figure
Observation of Fano-Resonances in Single-Wall Carbon Nanotubes
We have explored the low-temperature linear and non-linear electrical
conductance of metallic carbon nanotubes (CNTs), which were grown by the
chemical-vapor deposition method. The high transparency of the contacts allows
to study these two-terminal devices in the high conductance regime. We observe
the expected four-fold shell pattern together with Kondo physics at
intermediate transparency {G\alt 2e^2/h} and a transition to the open regime
in which the maximum conductance is doubled and bound by . In
the high- regime, at the transition from a quantum dot to a weak link, the
CNT levels are strongly broadened. Nonetheless, sharp resonances appear
superimposed on the background which varies slowly with gate voltage. The
resonances are identified by their lineshape as Fano resonances. The origin of
Fano resonances is discussed along the modelling.Comment: pdf including figures, see:
http://www.unibas.ch/phys-meso/Research/Papers/2004/Fano-CVD-SWNT.pd
Inelastic tunneling in a double quantum dot coupled to a bosonic environment
Coupling a quantum system to a bosonic environment always give rise to
inelastic processes, which reduce the coherency of the system. We measure
energy dependent rates for inelastic tunneling processes in a fully
controllable two-level system of a double quantum dot. The emission and
absorption rates are well repro-duced by Einstein's coefficients, which relate
to the spontaneous emission rate. The inelastic tunneling rate can be
comparable to the elastic tunneling rate if the boson occupation number becomes
large. In the specific semiconductor double dot, the energy dependence of the
inelastic rate suggests that acoustic phonons are coupled to the double dot
piezoelectrically.Comment: 6 pages, 4 figure
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