149 research outputs found
Transport through graphene double dots
We present Coulomb blockade measurements in a graphene double dot system. The
coupling of the dots to the leads and between the dots can be tuned by graphene
in-plane gates. The coupling is a non-monotonic function of the gate voltage.
Using a purely capacitive model, we extract all relevant energy scales of the
double dot system
Competition of Mesoscales and Crossover to Tricriticality in Polymer Solutions
We show that the approach to asymptotic fluctuation-induced critical behavior
in polymer solutions is governed by a competition between a correlation length
diverging at the critical point and an additional mesoscopic length-scale, the
radius of gyration. Accurate light-scattering experiments on polystyrene
solutions in cyclohexane with polymer molecular weights ranging from 200,000 up
to 11.4 million clearly demonstrate a crossover between two universal regimes:
a regime with Ising asymptotic critical behavior, where the correlation length
prevails, and a regime with tricritical theta-point behavior determined by a
mesoscopic polymer-chain length.Comment: 4 pages in RevTeX with 4 figure
Coherent Electron-Phonon Coupling in Tailored Quantum Systems
The coupling between a two-level system and its environment leads to
decoherence. Within the context of coherent manipulation of electronic or
quasiparticle states in nanostructures, it is crucial to understand the sources
of decoherence. Here, we study the effect of electron-phonon coupling in a
graphene and an InAs nanowire double quantum dot. Our measurements reveal
oscillations of the double quantum dot current periodic in energy detuning
between the two levels. These periodic peaks are more pronounced in the
nanowire than in graphene, and disappear when the temperature is increased. We
attribute the oscillations to an interference effect between two alternative
inelastic decay paths involving acoustic phonons present in these materials.
This interpretation predicts the oscillations to wash out when temperature is
increased, as observed experimentally.Comment: 11 pages, 4 figure
Quantum dots and spin qubits in graphene
This is a review on graphene quantum dots and their use as a host for spin
qubits. We discuss the advantages but also the challenges to use graphene
quantum dots for spin qubits as compared to the more standard materials like
GaAs. We start with an overview of this young and fascinating field and will
then discuss gate-tunable quantum dots in detail. We calculate the bound states
for three different quantum dot architectures where a bulk gap allows for
confinement via electrostatic fields: (i) graphene nanoribbons with armchair
boundary, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer
graphene. In order for graphene quantum dots to be useful in the context of
spin qubits, one needs to find reliable ways to break the valley-degeneracy.
This is achieved here, either by a specific termination of graphene in (i) or
in (ii) and (iii) by a magnetic field, without the need of a specific boundary.
We further discuss how to manipulate spin in these quantum dots and explain the
mechanism of spin decoherence and relaxation caused by spin-orbit interaction
in combination with electron-phonon coupling, and by hyperfine interaction with
the nuclear spin system.Comment: 23 pages, 10 figures, topical review prepared for Nanotechnolog
Ultrasensitive force detection with a nanotube mechanical resonator
Since the advent of atomic force microscopy, mechanical resonators have been
used to study a wide variety of phenomena, such as the dynamics of individual
electron spins, persistent currents in normal metal rings, and the Casimir
force. Key to these experiments is the ability to measure weak forces. Here, we
report on force sensing experiments with a sensitivity of 12 zN Hz^(-1/2) at a
temperature of 1.2 K using a resonator made of a carbon nanotube. An
ultra-sensitive method based on cross-correlated electrical noise measurements,
in combination with parametric downconversion, is used to detect the
low-amplitude vibrations of the nanotube induced by weak forces. The force
sensitivity is quantified by applying a known capacitive force. This detection
method also allows us to measure the Brownian vibrations of the nanotube down
to cryogenic temperatures. Force sensing with nanotube resonators offers new
opportunities for detecting and manipulating individual nuclear spins as well
as for magnetometry measurements.Comment: Early version. To be published in Nature Nanotechnolog
The Stern-Gerlach Experiment Revisited
The Stern-Gerlach-Experiment (SGE) of 1922 is a seminal benchmark experiment
of quantum physics providing evidence for several fundamental properties of
quantum systems. Based on today's knowledge we illustrate the different
benchmark results of the SGE for the development of modern quantum physics and
chemistry.
The SGE provided the first direct experimental evidence for angular momentum
quantization in the quantum world and thus also for the existence of
directional quantization of all angular momenta in the process of measurement.
It measured for the first time a ground state property of an atom, it produced
for the first time a `spin-polarized' atomic beam, it almost revealed the
electron spin. The SGE was the first fully successful molecular beam experiment
with high momentum-resolution by beam measurements in vacuum. This technique
provided a new kinematic microscope with which inner atomic or nuclear
properties could be investigated.
The original SGE is described together with early attempts by Einstein,
Ehrenfest, Heisenberg, and others to understand directional quantization in the
SGE. Heisenberg's and Einstein's proposals of an improved multi-stage SGE are
presented. The first realization of these proposals by Stern, Phipps, Frisch
and Segr\`e is described. The set-up suggested by Einstein can be considered an
anticipation of a Rabi-apparatus. Recent theoretical work is mentioned in which
the directional quantization process and possible interference effects of the
two different spin states are investigated.
In full agreement with the results of the new quantum theory directional
quantization appears as a general and universal feature of quantum
measurements. One experimental example for such directional quantization in
scattering processes is shown. Last not least, the early history of the
`almost' discovery of the electron spin in the SGE is revisited.Comment: 50pp, 17 fig
Electrical and thermoelectrical transport in Dirac fermions through a quantum dot
We investigate the conductance and thermopower of massless Dirac fermions
through a quantum dot using a pseudogap Anderson model in the non-crossing
approximation. When the Fermi level is at the Dirac point, the conductance has
a cusp where the thermopower changes its sign. When the Fermi level is away
from the Dirac point, the Kondo temperature illustrates a quantum impurity
transition between an asymmetric strong coupling Kondo state and a localized
moment state. The conductance shows a peak near this transition and reaches the
unitary limit at low temperatures. The magnitude of the thermopower exceeds
, and the thermoelectric figure of merit exceeds unity.Comment: 5 pages, 4 figure
Statistical signatures of critical behavior in small systems
The cluster distributions of different systems are examined to search for
signatures of a continuous phase transition. In a system known to possess such
a phase transition, both sensitive and insensitive signatures are present;
while in systems known not to possess such a phase transition, only insensitive
signatures are present. It is shown that nuclear multifragmentation results in
cluster distributions belonging to the former category, suggesting that the
fragments are the result of a continuous phase transition.Comment: 31 pages, two columns with 30 figure
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