20 research outputs found
Shell Filling and Trigonal Warping in Graphene Quantum Dots
Transport measurements through a few-electron circular quantum dot in bilayer
graphene display bunching of the conductance resonances in groups of four,
eight and twelve. This is in accordance with the spin and valley degeneracies
in bilayer graphene and an additional threefold 'minivalley degeneracy' caused
by trigonal warping. For small electron numbers, implying a small dot size and
a small displacement field, a two-dimensional s- and then a p-shell are
successively filled with four and eight electrons, respectively. For electron
numbers larger than twelve, as the dot size and the displacement field
increase, the single-particle ground state evolves into a three-fold degenerate
minivalley ground state. A transition between these regimes is observed in our
measurements and can be described by band-structure calculations. Measurements
in magnetic field confirm Hund's second rule for spin filling of the quantum
dot levels, emphasizing the importance of exchange interaction effects.Comment: 10 pages, 7 figure
Three-carrier spin blockade and coupling in bilayer graphene double quantum dots
The spin degree of freedom is crucial for the understanding of any condensed
matter system. Knowledge of spin-mixing mechanisms is not only essential for
successful control and manipulation of spin-qubits, but also uncovers
fundamental properties of investigated devices and material. For
electrostatically-defined bilayer graphene quantum dots, in which recent
studies report spin-relaxation times T1 up to 50ms with strong magnetic field
dependence, we study spin-blockade phenomena at charge configuration
. We examine the dependence of the spin-blockade
leakage current on interdot tunnel coupling and on the magnitude and
orientation of externally applied magnetic field. In out-of-plane magnetic
field, the observed zero-field current peak could arise from finite-temperature
co-tunneling with the leads; though involvement of additional spin- and
valley-mixing mechanisms are necessary for explaining the persistent sharp side
peaks observed. In in-plane magnetic field, we observe a zero-field current
dip, attributed to the competition between the spin Zeeman effect and the
Kane-Mele spin-orbit interaction. Details of the line shape of this current dip
however, suggest additional underlying mechanisms are at play
Pauli Blockade of Tunable Two-Electron Spin and Valley States in Graphene Quantum Dots
Pauli blockade mechanisms -- whereby carrier transport through quantum dots
(QDs) is blocked due to selection rules even when energetically allowed -- are
of both fundamental and technological interest, as a direct manifestation of
the Pauli exclusion principle and as a key mechanism for manipulating and
reading out spin qubits. Pauli spin blockade is well established for systems
such as GaAs QDs, where the two-electron spin-singlet ground state is separated
from the three triplet states higher in energy. However, Pauli blockade physics
remains largely unexplored for systems in which the Hilbert space is expanded
due to additional degrees of freedom, such as the valley quantum numbers in
carbon-based materials or silicon. Here we report experiments on coupled
graphene double QDs in which the spin and valley states can be precisely
controlled. We demonstrate that gate and magnetic-field tuning allows switching
between a spin-triplet--valley-singlet ground state with charge occupancy
(2,0), where valley-blockade is observed, and a spin-singlet--valley-triplet
ground state, where spin blockade is shown. These results demonstrate how the
complex two-particle Hilbert space of graphene quantum dots can be unravelled
experimentally, with implications for future spin and valley qubits
Spectroscopy of a single-carrier bilayer graphene quantum dot from time-resolved charge detection
We measured the spectrum of a single-carrier bilayer graphene quantum dot as
a function of both parallel and perpendicular magnetic fields, using a
time-resolved charge detection technique that gives access to individual tunnel
events. Thanks to our unprecedented energy resolution of 4eV, we could
distinguish all four levels of the dot's first orbital, in particular in the
range of magnetic fields where the first and second excited states cross
(mT). We thereby experimentally establish, the hitherto
extrapolated, single-charge carrier spectrum picture and provide a new upper
bound for the inter-valley mixing, equal to our energy resolution
Kondo effect and spin-orbit coupling in graphene quantum dots
The Kondo effect is a cornerstone in the study of strongly correlated
fermions. The coherent exchange coupling of conduction electrons to local
magnetic moments gives rise to a Kondo cloud that screens the impurity spin.
Whereas complete Kondo screening has been explored widely, realizations of the
underscreened scenario - where only some of several Kondo channels participate
in the screening - remain rare. Here we report the observation of fully
screened and underscreened Kondo effects in quantum dots in bilayer graphene.
More generally, we introduce a unique platform for studying Kondo physics. In
contrast to carbon nanotubes, whose curved surfaces give rise to strong
spin-orbit coupling breaking the SU(4) symmetry of the electronic states
relevant for the Kondo effect, we study a nominally flat carbon material with
small spin-orbit coupling. Moreover, the unusual two-electron triplet ground
state in bilayer graphene dots provides a route to exploring the underscreened
spin-1 Kondo effect
Author Correction:In-plane selective area InSb–Al nanowire quantum networks (Communications Physics, (2020), 3, 1, (59), 10.1038/s42005-020-0324-4)
The Data availability statement of this article has been modified to add the accession link to the raw data. The old Data availability statement read “Materials and data that support the findings of this research are available within the paper. All data are available from the corresponding author upon request”. This has been replaced by “Materials and data that support the findings of this research are available within the paper. The raw data have been deposited at https://zenodo.org/record/4589484#.YEoEOy1Y7Sd”. This has been corrected in both the HTML and PDF version of the article.</p
Automated Reconstruction of Bound States in Bilayer Graphene Quantum Dots
Bilayer graphene is a nanomaterial that allows for well-defined, separated quantum states to be defined by electrostatic gating and, therefore, provides an attractive platform to construct tunable quantum dots. When a magnetic field perpendicular to the graphene layers is applied, the graphene valley degeneracy is lifted, and splitting of the energy levels of the dot is observed. Although bilayer graphene quantum dots have recently been realized in experiments, it is critically important to devise robust methods that can identify the observed quantum states from accessible measurement data. Here, we develop an efficient algorithm for extracting the model parameters needed to characterize the states of a bilayer graphene quantum dot. Specifically, we put forward a Hamiltonian-guided random search method and demonstrate robust identification of quantum states on both simulated and experimental data. </p
Automated Reconstruction of Bound States in Bilayer Graphene Quantum Dots
Bilayer graphene is a nanomaterial that allows for well-defined, separated quantum states to be defined by electrostatic gating and, therefore, provides an attractive platform to construct tunable quantum dots. When a magnetic field perpendicular to the graphene layers is applied, the graphene valley degeneracy is lifted, and splitting of the energy levels of the dot is observed. Although bilayer graphene quantum dots have recently been realized in experiments, it is critically important to devise robust methods that can identify the observed quantum states from accessible measurement data. Here, we develop an efficient algorithm for extracting the model parameters needed to characterize the states of a bilayer graphene quantum dot. Specifically, we put forward a Hamiltonian-guided random search method and demonstrate robust identification of quantum states on both simulated and experimental data.ISSN:2331-701
Coulomb dominated cavities in bilayer graphene
Electrostatic confinement in bilayer graphene van der Waals heterostructures provides a versatile platform for studying electronic transport in bilayer graphene nanostructures. We study bilayer graphene cavities which we interpret in terms of quantum dots that are strongly coupled to the leads. We investigate how the transport signatures evolve when changing the size of the cavity for both electron and hole occupation. In addition, we analyze the interplay of single and double quantum dot physics in the regime where transport through the quantum dot system is almost pinched off.ISSN:2643-156