127 research outputs found
Coulomb oscillations in three-layer graphene nanostructures
We present transport measurements on a tunable three-layer graphene single
electron transistor (SET). The device consists of an etched three-layer
graphene flake with two narrow constrictions separating the island from source
and drain contacts. Three lateral graphene gates are used to electrostatically
tune the device. An individual three-layer graphene constriction has been
investigated separately showing a transport gap near the charge neutrality
point. The graphene tunneling barriers show a strongly nonmonotonic coupling as
function of gate voltage indicating the presence of localized states in the
constrictions. We show Coulomb oscillations and Coulomb diamond measurements
proving the functionality of the graphene SET. A charging energy of meV is extracted.Comment: 10 pages, 6 figure
Transport through a strongly coupled graphene quantum dot in perpendicular magnetic field
We present transport measurements on a strongly coupled graphene quantum dot
in a perpendicular magnetic field. The device consists of an etched
single-layer graphene flake with two narrow constrictions separating a 140 nm
diameter island from source and drain graphene contacts. Lateral graphene gates
are used to electrostatically tune the device. Measurements of Coulomb
resonances, including constriction resonances and Coulomb diamonds prove the
functionality of the graphene quantum dot with a charging energy of around 4.5
meV. We show the evolution of Coulomb resonances as a function of perpendicular
magnetic field, which provides indications of the formation of the graphene
specific 0th Landau level. Finally, we demonstrate that the complex pattern
superimposing the quantum dot energy spectra is due to the formation of
additional localized states with increasing magnetic field.Comment: 6 pages, 4 figure
Observation of excited states in a graphene quantum dot
We demonstrate that excited states in single-layer graphene quantum dots can
be detected via direct transport experiments. Coulomb diamond measurements show
distinct features of sequential tunneling through an excited state. Moreover,
the onset of inelastic cotunneling in the diamond region could be detected. For
low magnetic fields, the positions of the single-particle energy levels
fluctuate on the scale of a flux quantum penetrating the dot area. For higher
magnetic fields, the transition to the formation of Landau levels is observed.
Estimates based on the linear energy-momentum relation of graphene give carrier
numbers of the order of 10 for our device.Comment: 3 pages, 3 figure
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
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
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
Geometric phases and Bloch sphere constructions for SU(N), with a complete description of SU(4)
A two-sphere ("Bloch" or "Poincare") is familiar for describing the dynamics
of a spin-1/2 particle or light polarization. Analogous objects are derived for
unitary groups larger than SU(2) through an iterative procedure that constructs
evolution operators for higher-dimensional SU in terms of lower-dimensional
ones. We focus, in particular, on the SU(4) of two qubits which describes all
possible logic gates in quantum computation. For a general Hamiltonian of SU(4)
with 15 parameters, and for Hamiltonians of its various sub-groups so that
fewer parameters suffice, we derive Bloch-like rotation of unit vectors
analogous to the one familiar for a single spin in a magnetic field. The
unitary evolution of a quantal spin pair is thereby expressed as rotations of
real vectors. Correspondingly, the manifolds involved are Bloch two-spheres
along with higher dimensional manifolds such as a four-sphere for the SO(5)
sub-group and an eight-dimensional Grassmannian manifold for the general SU(4).
This latter may also be viewed as two, mutually orthogonal, real
six-dimensional unit vectors moving on a five-sphere with an additional phase
constraint.Comment: 9 page
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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