211 research outputs found
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
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
Transport through Zero-Dimensional States in a Quantum Dot
We have studied the electron transport through zero-dimensional (0D) states. 0D states are formed when one-dimensional edge channels are confined in a quantum dot. The quantum dot is defined in a two-dimensional electron gas with a split gate technique. To allow electronic transport, connection to the dot is arranged via two quantum point contacts, which have adjustable selective transmission properties for edge channels. The 0D states show up as pronounced oscillations in the conductance (up to 40% of e2/h), when the flux enclosed by the confined edge channel is varied, either by changing the magnetic field or the gate voltage. A prerequisite for the appearance of 0D states is that the transport through the entire device is adiabatic (i.e. with conservation of quantum numbers), which will be shown to occur at high magnetic field. The experimental results are in good agreement with theory and show that in the ballistic quantum Hall regime the current is carried entirely by edge channels.
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
A Tunable Kondo Effect in Quantum Dots
We demonstrate a tunable Kondo effect realized in small quantum dots. We can
switch our dot from a Kondo impurity to a non-Kondo system as the number of
electrons on the dot is changed from odd to even. We show that the Kondo
temperature can be tuned by means of a gate voltage as a single-particle energy
state nears the Fermi energy. Measurements of the temperature and magnetic
field dependence of a Coulomb-blockaded dot show good agreement with
predictions of both equilibrium and non-equilibrium Kondo effects.Comment: 8 pages, 4 figure
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
- …