128 research outputs found
A low-temperature dynamic mode scanning force microscope operating in high magnetic fields
A scanning force microscope was implemented operating at temperatures below
4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were
employed for non optical tip-sample distance control in the dynamic operation
mode. Fast response was achieved by using a phase-locked loop for driving the
mechanical oscillator. Possible applications of this setup for various scanning
probe techniques are discussed.Comment: 5 pages, 5 figures, submitted to "Review of Scientific Instruments
Quantum capacitance and density of states of graphene
We report on measurements of the quantum capacitance in graphene as a
function of charge carrier density. A resonant LC-circuit giving high
sensitivity to small capacitance changes is employed. The density of states,
which is directly proportional to the quantum capacitance, is found to be
significantly larger than zero at and around the charge neutrality point. This
finding is interpreted to be a result of potential fluctuations with amplitudes
of the order of 100 meV in good agreement with scanning single-electron
transistor measurements on bulk graphene and transport studies on nanoribbons
Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages
Nanolithography based on local oxidation with a scanning force microscope has
been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an
undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide
growth and the resulting electronic properties of the patterned structures are
compared for constant and modulated voltage applied to the conductive tip of
the scanning force microscope. All the lithography has been performed in
non-contact mode. Modulating the applied voltage enhances the aspect ratio of
the oxide lines, which significantly strengthens the insulating properties of
the lines on GaAs. In addition, the oxidation process is found to be more
reliable and reproducible. Using this technique, a quantum point contact and a
quantum wire have been defined and the electronic stability, the confinement
potential and the electrical tunability are demonstrated to be similar to the
oxidation with constant voltage.Comment: 7 pages, 7 figures, accepted by J. Appl. Phy
Operation characteristics of piezoelectric quartz tuning forks in high magnetic fields at liquid helium temperatures
Piezoelectric quartz tuning forks are investigated in view of their use as
force sensors in dynamic mode scanning probe microscopy at temperatures down to
1.5 K and in magnetic fields up to 8 T. The mechanical properties of the forks
are extracted from the frequency dependent admittance and simultaneous
interferometric measurements. The performance of the forks in a cryogenic
environment is investigated. Force-distance studies performed with these
sensors at low temperatures are presented
Counting statistics and super-Poissonian noise in a quantum dot
We present time-resolved measurements of electron transport through a quantum
dot. The measurements were performed using a nearby quantum point contact as a
charge detector. The rates for tunneling through the two barriers connecting
the dot to source and drain contacts could be determined individually. In the
high bias regime, the method was used to probe excited states of the dot.
Furthermore, we have detected bunching of electrons, leading to
super-Poissonian noise. We have used the framework of the full counting
statistics (FCS) to model the experimental data. The existence of
super-Poissonian noise suggests a long relaxation time for the involved excited
state, which could be related to the spin relaxation time
An Integrand Reconstruction Method for Three-Loop Amplitudes
We consider the maximal cut of a three-loop four point function with massless
kinematics. By applying Groebner bases and primary decomposition we develop a
method which extracts all ten propagator master integral coefficients for an
arbitrary triple-box configuration via generalized unitarity cuts. As an
example we present analytic results for the three loop triple-box contribution
to gluon-gluon scattering in Yang-Mills with adjoint fermions and scalars in
terms of three master integrals.Comment: 15 pages, 1 figur
Quantum capacitance and density of states of graphene
Abstract We report capacitance measurements in top-gated graphene sheets as a function of charge carrier density. A measurement method using an LC-circuit provides high sensitivity to small capacitance changes and hence allows the observation of the quantum part of the capacitance. The extracted density of states has a finite value of 1 × 10 17 m −2 eV −1 in the vicinity of the Dirac point, which is in contrast to the theoretical prediction for ideal graphene. We attribute this discrepancy to fluctuations of the electrostatic potential with a typical amplitude of 100 meV in our device
A New Algorithm For The Generation Of Unitarity-Compatible Integration By Parts Relations
Many multi-loop calculations make use of integration by parts relations to
reduce the large number of complicated Feynman integrals that arise in such
calculations to a simpler basis of master integrals. Recently, Gluza, Kajda,
and Kosower argued that the reduction to master integrals is complicated by the
presence of integrals with doubled propagator denominators in the integration
by parts relations and they introduced a novel reduction procedure which
eliminates all such integrals from the start. Their approach has the advantage
that it automatically produces integral bases which mesh well with generalized
unitarity. The heart of their procedure is an algorithm which utilizes the
weighty machinery of computational commutative algebra to produce complete sets
of unitarity-compatible integration by parts relations. In this paper, we
propose a conceptually simpler algorithm for the generation of complete sets of
unitarity-compatible integration by parts relations based on recent results in
the mathematical literature. A striking feature of our algorithm is that it can
be described entirely in terms of straightforward linear algebra.Comment: 20 pages; My apologies to Krzysztof Kajda for misspelling his name in
v1; in v3: the labeling of the variables in (4.5) and eqs. (4.20) and (4.21)
was adjusted to match the notation used in the rest of Section 4. I thank
York Schroeder for pointing out the notational inconsistenc
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