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Probing charge noise in bilayer graphene quantum dots by Landau-Zener-Stückelberg-Majorana spectroscopy
Authors:
Katrin Hecker,
Samuel Möller,
Tobias Deußen,
Hubert Dulisch,
Luca Banszerus,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
Charge noise is an important factor limiting qubit coherence and relaxation in solid-state devices. In bilayer graphene (BLG) quantum dots, recently established as a promising platform for spin- and valley-based qubits, both the origin and magnitude of charge noise remain largely unexplored. Here, we investigate high-frequency charge noise using Landau-Zener-Stückelberg-Majorana (LZSM) interferenc…
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Charge noise is an important factor limiting qubit coherence and relaxation in solid-state devices. In bilayer graphene (BLG) quantum dots, recently established as a promising platform for spin- and valley-based qubits, both the origin and magnitude of charge noise remain largely unexplored. Here, we investigate high-frequency charge noise using Landau-Zener-Stückelberg-Majorana (LZSM) interference spectroscopy. We study a single-particle charge qubit formed in a BLG double quantum dot at frequencies between 5 and 10 GHz and extract a noise spectral density $S_\varepsilon$ on the order of 0.5-0.9 neV$/\sqrt{\mathrm{Hz}}$. This is comparable to values reported for III-V semiconductor platforms and silicon. From the temperature and frequency dependence of the charge qubit decoherence, we conclude that thermal (Johnson) noise or electron-phonon coupling dominates over two-level fluctuators.
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Submitted 12 May, 2026;
originally announced May 2026.
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Radio-frequency charge detection on graphene electron-hole double quantum dots
Authors:
Katrin Hecker,
Samuel Möller,
Hubert Dulisch,
Şiyar Duman,
Leon Stecher,
Lucca Valerius,
Tobias Deußen,
Saketh Ravuri,
Kenji Watanabe,
Takashi Taniguchi,
Florian Libisch,
Christian Volk,
Christoph Stampfer
Abstract:
High-fidelity detection of charge transitions in quantum dots (QDs) is a key ingredient in solid state quantum computation. We demonstrate high-bandwidth radio-frequency charge detection in bilayer graphene quantum dots (QDs) using a capacitively coupled quantum point contact (QPC). The device design suppresses screening effects and enables sensitive QPC-based charge readout. The QPC is arranged t…
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High-fidelity detection of charge transitions in quantum dots (QDs) is a key ingredient in solid state quantum computation. We demonstrate high-bandwidth radio-frequency charge detection in bilayer graphene quantum dots (QDs) using a capacitively coupled quantum point contact (QPC). The device design suppresses screening effects and enables sensitive QPC-based charge readout. The QPC is arranged to maximize the readout contrast between two neighboring, coupled electron and hole QDs. We apply the readout scheme to a single-particle electron-hole double QD and demonstrate time-resolved detection of charge states as well as magnetic field dependent tunneling rates. This promises a high-fidelity readout scheme for individual spin and valley states, which is important for the operation of spin, valley or spin-valley qubits in bilayer graphene.
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Submitted 1 December, 2025; v1 submitted 15 September, 2025;
originally announced September 2025.
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Weak localization as probe of spin-orbit-induced spin-split bands in bilayer graphene proximity coupled to WSe$_2$
Authors:
E. Icking,
F. Wörtche,
A. W. Cummings,
A. Wörtche,
K. Watanabe,
T. Taniguchi,
C. Volk,
B. Beschoten,
C. Stampfer
Abstract:
Proximity coupling of bilayer graphene (BLG) to transition metal dichalcogenides (TMDs) offers a promising route to engineer gate-tunable spin-orbit coupling (SOC) while preserving BLG's exceptional electronic properties. This tunability arises from the layer-asymmetric electronic structure of gapped BLG, where SOC acts predominantly on the layer in contact with the TMD. Here, we present high-qual…
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Proximity coupling of bilayer graphene (BLG) to transition metal dichalcogenides (TMDs) offers a promising route to engineer gate-tunable spin-orbit coupling (SOC) while preserving BLG's exceptional electronic properties. This tunability arises from the layer-asymmetric electronic structure of gapped BLG, where SOC acts predominantly on the layer in contact with the TMD. Here, we present high-quality BLG/WSe$_2$ devices with a proximity-induced SOC gap and excellent electrostatic control. Operating in a quasi-ballistic regime, our double-gated heterostructures allow to form gate-defined p-n-p cavities and show clear weak anti-localization (WAL) features consistent with Rashba-type SOC. At lower hole densities, a transition to weak localization (WL) is observed, signaling transport through a single spin-split valence band. These findings - in agreement with calculations - provide direct spectroscopic evidence of proximity-induced spin-split band in BLG and underscore the potential of BLG/TMD heterostructures for spintronics and spin-based quantum technologies.
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Submitted 25 June, 2025; v1 submitted 30 May, 2025;
originally announced May 2025.
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Electric field tunable spin-orbit gap in a bilayer graphene/WSe$_{2}$ quantum dot
Authors:
Hubert Dulisch,
David Emmerich,
Eike Icking,
Katrin Hecker,
Samuel Möller,
Leonie Müller,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
We report on the investigation of proximity-induced spin-orbit coupling (SOC) in a heterostructure of bilayer graphene (BLG) and tungsten diselenide (WSe$_2$). A BLG quantum dot (QD) in the few-particle regime acts as a sensitive probe for induced SOC. Finite bias and magnetotransport spectroscopy measurements reveal a significantly enhanced SOC that decreases with the applied displacement field,…
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We report on the investigation of proximity-induced spin-orbit coupling (SOC) in a heterostructure of bilayer graphene (BLG) and tungsten diselenide (WSe$_2$). A BLG quantum dot (QD) in the few-particle regime acts as a sensitive probe for induced SOC. Finite bias and magnetotransport spectroscopy measurements reveal a significantly enhanced SOC that decreases with the applied displacement field, distinguishing it from pristine BLG. Furthermore, our measurements demonstrate a reduced valley $g$-factor at larger displacement fields, consistent with a weaker lateral confinement of the QD. Our findings show evidence of the influence of WSe$_2$ across BLG layers, driven by reduced real-space confinement and increased layer localization of the QD states on the BLG layer distant to the WSe$_2$ at higher displacement fields. This study demonstrates the electrostatic tunability of the spin-orbit gap in BLG/WSe$_2$ heterostructures, which is especially relevant for the field of spintronics and future spin qubit control in BLG QDs.
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Submitted 31 May, 2025; v1 submitted 16 April, 2025;
originally announced April 2025.
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The role of antisymmetric orbitals and electron-electron interactions on the two-particle spin and valley blockade in graphene double quantum dots
Authors:
Samuel Möller,
Luca Banszerus,
Katrin Hecker,
Hubert Dulisch,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
We report on an experimental study of spin and valley blockade in two-electron bilayer graphene (BLG) double quantum dots (DQDs) and explore the limits set by asymmetric orbitals and electronelectron interactions. The results obtained from magnetotransport measurements on two-electron BLG DQDs, where the resonant tunneling transport involves both orbital symmetric and antisymmetric two-particle st…
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We report on an experimental study of spin and valley blockade in two-electron bilayer graphene (BLG) double quantum dots (DQDs) and explore the limits set by asymmetric orbitals and electronelectron interactions. The results obtained from magnetotransport measurements on two-electron BLG DQDs, where the resonant tunneling transport involves both orbital symmetric and antisymmetric two-particle states, show a rich level spectrum. We observe a magnetic field tunable spin and valley blockade, which is limited by the orbital splitting, the strength of the electron-electron interaction and the difference in the valley g-factors between the symmetric and antisymmetric twoparticle orbital states. Our conclusions are supported by simulations based on rate equations, which allow the identification of prominent interdot transitions associated with the transition from single to two-particle states observed in the experiment.
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Submitted 24 March, 2025; v1 submitted 11 January, 2025;
originally announced January 2025.
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Phonon-limited valley life times in single-particle bilayer graphene quantum dots
Authors:
Luca Banszerus,
Katrin Hecker,
Lin Wang,
Samuel Möller,
Kenji Watanabe,
Takashi Taniguchi,
Guido Burkard,
Christian Volk,
Christoph Stampfer
Abstract:
The valley degree of freedom in 2D semiconductors, such as gapped bilayer graphene (BLG) and transition metal dichalcogenides, is a promising carrier of quantum information in the emerging field of valleytronics. While valley dynamics have been extensively studied for moderate band gap 2D~semiconductors using optical spectroscopy techniques, very little is known about valley lifetimes in narrow ba…
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The valley degree of freedom in 2D semiconductors, such as gapped bilayer graphene (BLG) and transition metal dichalcogenides, is a promising carrier of quantum information in the emerging field of valleytronics. While valley dynamics have been extensively studied for moderate band gap 2D~semiconductors using optical spectroscopy techniques, very little is known about valley lifetimes in narrow band gap BLG, which is difficult to study using optical techniques. Here, we report single-particle valley relaxation times ($T_1$) exceeding several microseconds in electrostatically defined BLG quantum dots (QDs) using a pulse-gating technique. The observed dependence of $T_1$ on perpendicular magnetic field can be understood qualitatively and quantitatively by a model in which $T_1$ is limited by electron-phonon coupling. We identify the coupling to acoustic phonons via the bond length change and via the deformation potential as the limiting mechanisms.
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Submitted 4 June, 2025; v1 submitted 26 February, 2024;
originally announced February 2024.
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Impact of competing energy scales on the shell-filling sequence in elliptic bilayer graphene quantum dots
Authors:
Samuel Möller,
Luca Banszerus,
Angelika Knothe,
Lucca Valerius,
Katrin Hecker,
Eike Icking,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
We report on a detailed investigation of the shell-filling sequence in electrostatically defined elliptic bilayer graphene quantum dots (QDs) in the regime of low charge carrier occupation, $N \leq 12$, by means of magnetotransport spectroscopy and numerical calculations. We show the necessity of including both short-range electron-electron interaction and wavefunction-dependent valley g-factors f…
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We report on a detailed investigation of the shell-filling sequence in electrostatically defined elliptic bilayer graphene quantum dots (QDs) in the regime of low charge carrier occupation, $N \leq 12$, by means of magnetotransport spectroscopy and numerical calculations. We show the necessity of including both short-range electron-electron interaction and wavefunction-dependent valley g-factors for understanding the overall fourfold shell-filling sequence. These factors lead to an additional energy splitting at half-filling of each orbital state and different energy shifts in out-of-plane magnetic fields. Analysis of 31 different BLG QDs reveals that both valley g-factor and electron-electron interaction induced energy splitting increase with decreasing QD size, validating theory. However, we find that the electrostatic charging energy of such gate-defined QDs does not correlate consistently with their size, indicating complex electrostatics. These findings offer significant insights for future BLG QD devices and circuit designs.
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Submitted 25 August, 2023; v1 submitted 16 May, 2023;
originally announced May 2023.
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Particle-hole symmetry protects spin-valley blockade in graphene quantum dots
Authors:
Luca Banszerus,
Samuel Möller,
Katrin Hecker,
Eike Icking,
Kenji Watanabe,
Takashi Taniguchi,
Fabian Hassler,
Christian Volk,
Christoph Stampfer
Abstract:
Particle-hole symmetry plays an important role for the characterization of topological phases in solid-state systems. It is found, for example, in free-fermion systems at half filling, and it is closely related to the notion of antiparticles in relativistic field theories. In the low energy limit, graphene is a prime example of a gapless particle-hole symmetric system described by an effective Dir…
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Particle-hole symmetry plays an important role for the characterization of topological phases in solid-state systems. It is found, for example, in free-fermion systems at half filling, and it is closely related to the notion of antiparticles in relativistic field theories. In the low energy limit, graphene is a prime example of a gapless particle-hole symmetric system described by an effective Dirac equation, where topological phases can be understood by studying ways to open a gap by preserving (or breaking) symmetries. An important example is the intrinsic Kane-Mele spin-orbit gap of graphene, which leads to a lifting of the spin-valley degeneracy and renders graphene a topological insulator in a quantum spin Hall phase, while preserving particle-hole symmetry. Here, we show that bilayer graphene allows realizing electron-hole double quantum-dots that exhibit nearly perfect particle-hole symmetry, where transport occurs via the creation and annihilation of single electron-hole pairs with opposite quantum numbers. Moreover, we show that this particle-hole symmetry results in a protected single-particle spin-valley blockade. The latter will allow robust spin-to-charge conversion and valley-to-charge conversion, which is essential for the operation of spin and valley qubits.
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Submitted 17 March, 2023;
originally announced March 2023.
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Coherent Charge Oscillations in a Bilayer Graphene Double Quantum Dot
Authors:
Katrin Hecker,
Luca Banszerus,
Aaron Schäpers,
Samuel Möller,
Anton Peters,
Eike Icking,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
The coherent dynamics of a quantum mechanical two-level system passing through an anti-crossing of two energy levels can give rise to Landau-Zener-Stückelberg-Majorana (LZSM) interference. LZSM interference spectroscopy has proven to be a fruitful tool to investigate charge noise and charge decoherence in semiconductor quantum dots (QDs). Recently, bilayer graphene has developed as a promising pla…
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The coherent dynamics of a quantum mechanical two-level system passing through an anti-crossing of two energy levels can give rise to Landau-Zener-Stückelberg-Majorana (LZSM) interference. LZSM interference spectroscopy has proven to be a fruitful tool to investigate charge noise and charge decoherence in semiconductor quantum dots (QDs). Recently, bilayer graphene has developed as a promising platform to host highly tunable QDs potentially useful for hosting spin and valley qubits. So far, in this system no coherent oscillations have been observed and little is known about charge noise in this material. Here, we report coherent charge oscillations and $T_2^*$ charge decoherence times in a bilayer graphene double QD. The charge decoherence times are measured independently using LZSM interference and photon assisted tunneling. Both techniques yield $T_2^*$ average values in the range of 400 to 500 ps. The observation of charge coherence allows to study the origin and spectral distribution of charge noise in future experiments.
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Submitted 24 November, 2023; v1 submitted 17 March, 2023;
originally announced March 2023.
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Transport spectroscopy of ultraclean tunable band gaps in bilayer graphene
Authors:
E. Icking,
L. Banszerus,
F. Wörtche,
F. Volmer,
P. Schmidt,
C. Steiner,
S. Engels,
J. Hesselmann,
M. Goldsche,
K. Watanabe,
T. Taniguchi,
C. Volk,
B. Beschoten,
C. Stampfer
Abstract:
The importance of controlling both the charge carrier density and the band gap of a semiconductor cannot be overstated, as it opens the doors to a wide range of applications, including, e.g., highly-tunable transistors, photodetectors, and lasers. Bernal-stacked bilayer graphene is a unique van-der-Waals material that allows tuning the band gap by an out-of-plane electric field. Although the first…
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The importance of controlling both the charge carrier density and the band gap of a semiconductor cannot be overstated, as it opens the doors to a wide range of applications, including, e.g., highly-tunable transistors, photodetectors, and lasers. Bernal-stacked bilayer graphene is a unique van-der-Waals material that allows tuning the band gap by an out-of-plane electric field. Although the first evidence of the tunable gap was already found ten years ago, it took until recent to fabricate sufficiently clean heterostructures where the electrically induced gap could be used to fully suppress transport or confine charge carriers. Here, we present a detailed study of the tunable band gap in gated bilayer graphene characterized by temperature-activated transport and finite-bias spectroscopy measurements. The latter method allows comparing different gate materials and device technologies, which directly affects the disorder potential in bilayer graphene. We show that graphite-gated bilayer graphene exhibits extremely low disorder and as good as no subgap states resulting in ultraclean tunable band gaps up to 120 meV. The size of the band gaps are in good agreement with theory and allow complete current suppression making a wide range of semiconductor applications possible.
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Submitted 7 July, 2022; v1 submitted 4 June, 2022;
originally announced June 2022.
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Universal control of a six-qubit quantum processor in silicon
Authors:
Stephan G. J. Philips,
Mateusz T. Mądzik,
Sergey V. Amitonov,
Sander L. de Snoo,
Maximilian Russ,
Nima Kalhor,
Christian Volk,
William I. L. Lawrie,
Delphine Brousse,
Larysa Tryputen,
Brian Paquelet Wuetz,
Amir Sammak,
Menno Veldhorst,
Giordano Scappucci,
Lieven M. K. Vandersypen
Abstract:
Future quantum computers capable of solving relevant problems will require a large number of qubits that can be operated reliably. However, the requirements of having a large qubit count and operating with high-fidelity are typically conflicting. Spins in semiconductor quantum dots show long-term promise but demonstrations so far use between one and four qubits and typically optimize the fidelity…
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Future quantum computers capable of solving relevant problems will require a large number of qubits that can be operated reliably. However, the requirements of having a large qubit count and operating with high-fidelity are typically conflicting. Spins in semiconductor quantum dots show long-term promise but demonstrations so far use between one and four qubits and typically optimize the fidelity of either single- or two-qubit operations, or initialization and readout. Here we increase the number of qubits and simultaneously achieve respectable fidelities for universal operation, state preparation and measurement. We design, fabricate and operate a six-qubit processor with a focus on careful Hamiltonian engineering, on a high level of abstraction to program the quantum circuits and on efficient background calibration, all of which are essential to achieve high fidelities on this extended system. State preparation combines initialization by measurement and real-time feedback with quantum-non-demolition measurements. These advances will allow for testing of increasingly meaningful quantum protocols and constitute a major stepping stone towards large-scale quantum computers.
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Submitted 18 February, 2022;
originally announced February 2022.
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Spin relaxation in a single-electron graphene quantum dot
Authors:
L. Banszerus,
K. Hecker,
S. Möller,
E. Icking,
K. Watanabe,
T. Taniguchi,
C. Volk,
C. Stampfer
Abstract:
The relaxation time of a single-electron spin is an important parameter for solid-state spin qubits, as it directly limits the lifetime of the encoded information. Thanks to the low spin-orbit interaction and low hyperfine coupling, graphene and bilayer graphene (BLG) have long been considered promising platforms for spin qubits. Only recently, it has become possible to control single-electrons in…
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The relaxation time of a single-electron spin is an important parameter for solid-state spin qubits, as it directly limits the lifetime of the encoded information. Thanks to the low spin-orbit interaction and low hyperfine coupling, graphene and bilayer graphene (BLG) have long been considered promising platforms for spin qubits. Only recently, it has become possible to control single-electrons in BLG quantum dots (QDs) and to understand their spin-valley texture, while the relaxation dynamics have remained mostly unexplored. Here, we report spin relaxation times ($T_1$) of single-electron states in BLG QDs. Using pulsed-gate spectroscopy, we extract relaxation times exceeding 200 $μ$s at a magnetic field of 1.9 T. The $T_1$ values show a strong dependence on the spin splitting, promising even longer $T_1$ at lower magnetic fields, where our measurements are limited by the signal-to-noise ratio. The relaxation times are more than two orders of magnitude larger than those previously reported for carbon-based QDs, suggesting that graphene is a potentially promising host material for scalable spin qubits.
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Submitted 28 June, 2022; v1 submitted 25 October, 2021;
originally announced October 2021.
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Probing two-electron multiplets in bilayer graphene quantum dots
Authors:
Samuel Möller,
Luca Banszerus,
Angelika Knothe,
Corinne Steiner,
Eike Icking,
Stefan Trellenkamp,
Florian Lentz,
Kenji Watanabe,
Takashi Taniguchi,
Leonid Glazman,
Vladimir Fal'ko,
Christian Volk,
Christoph Stampfer
Abstract:
We report on finite bias spectroscopy measurements of the two-electron spectrum in a gate defined bilayer graphene (BLG) quantum dot for varying magnetic fields. The spin and valley degree of freedom in BLG give rise to multiplets of 6 orbital symmetric and 10 orbital anti-symmetric states. We find that orbital symmetric states are lower in energy and separated by $\approx 0.4 - 0.8$ meV from orbi…
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We report on finite bias spectroscopy measurements of the two-electron spectrum in a gate defined bilayer graphene (BLG) quantum dot for varying magnetic fields. The spin and valley degree of freedom in BLG give rise to multiplets of 6 orbital symmetric and 10 orbital anti-symmetric states. We find that orbital symmetric states are lower in energy and separated by $\approx 0.4 - 0.8$ meV from orbital anti-symmetric states. The symmetric multiplet exhibits an additional energy splitting of its 6 states of $\approx 0.15 - 0.5$ meV due to lattice scale interactions. The experimental observations are supported by theoretical calculations, which allow to determine that inter-valley scattering and 'current-current' interaction constants are of the same magnitude in BLG.
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Submitted 10 January, 2022; v1 submitted 15 June, 2021;
originally announced June 2021.
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Spin-valley coupling in single-electron bilayer graphene quantum dots
Authors:
Luca Banszerus,
Samuel Möller,
Corinne Steiner,
Eike Icking,
Stefan Trellenkamp,
Florian Lentz,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
Understanding how the electron spin is coupled to orbital degrees of freedom, such as a valley degree of freedom in solid-state systems is central to applications in spin-based electronics and quantum computation. Recent developments in the preparation of electrostatically-confined quantum dots in gapped bilayer graphene (BLG) enables to study the low-energy single-electron spectra in BLG quantum…
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Understanding how the electron spin is coupled to orbital degrees of freedom, such as a valley degree of freedom in solid-state systems is central to applications in spin-based electronics and quantum computation. Recent developments in the preparation of electrostatically-confined quantum dots in gapped bilayer graphene (BLG) enables to study the low-energy single-electron spectra in BLG quantum dots, which is crucial for potential spin and spin-valley qubit operations. Here, we present the observation of the spin-valley coupling in a bilayer graphene quantum dot in the single-electron regime. By making use of a highly-tunable double quantum dot device we achieve an energy resolution allowing us to resolve the lifting of the fourfold spin and valley degeneracy by a Kane-Mele type spin-orbit coupling of $\approx 65~μ$eV. Also, we find an upper limit of a potentially disorder-induced mixing of the $K$ and $K'$ states below $20~μ$eV.
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Submitted 19 June, 2021; v1 submitted 8 March, 2021;
originally announced March 2021.
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Dispersive sensing of charge states in a bilayer graphene quantum dot
Authors:
Luca Banszerus,
Samuel Möller,
Eike Icking,
Corinne Steiner,
Daniel Neumaier,
Martin Otto,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
We demonstrate dispersive readout of individual charge states in a gate-defined few-electron quantum dot in bilayer graphene. We employ a radio frequency reflectometry circuit, where an LC resonator with a resonance frequency close to 280 MHz is directly coupled to an ohmic contact of the quantum dot device. The detection scheme based on changes in the quantum capacitance operates over a wide gate…
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We demonstrate dispersive readout of individual charge states in a gate-defined few-electron quantum dot in bilayer graphene. We employ a radio frequency reflectometry circuit, where an LC resonator with a resonance frequency close to 280 MHz is directly coupled to an ohmic contact of the quantum dot device. The detection scheme based on changes in the quantum capacitance operates over a wide gate-voltage range and allows to probe excited states down to the single-electron regime. Crucially, the presented sensing technique avoids the use of an additional, capacitively coupled quantum device such as a quantum point contact or single electron transistor, making dispersive sensing particularly interesting for gate-defined graphene quantum dots.
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Submitted 13 February, 2021; v1 submitted 11 December, 2020;
originally announced December 2020.
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Pulsed-gate spectroscopy of single-electron spin states in bilayer graphene quantum dots
Authors:
Luca Banszerus,
Katrin Hecker,
Eike Icking,
Stefan Trellenkamp,
Florian Lentz,
Daniel Neumaier,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
Graphene and bilayer graphene quantum dots are promising hosts for spin qubits with long coherence times. Although recent technological improvements make it possible to confine single electrons electrostatically in bilayer graphene quantum dots, and their spin and valley texture of the single particle spectrum has been studied in detail, their relaxation dynamics remains still unexplored. Here, we…
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Graphene and bilayer graphene quantum dots are promising hosts for spin qubits with long coherence times. Although recent technological improvements make it possible to confine single electrons electrostatically in bilayer graphene quantum dots, and their spin and valley texture of the single particle spectrum has been studied in detail, their relaxation dynamics remains still unexplored. Here, we report on transport through a high-frequency gate controlled single-electron bilayer graphene quantum dot. By transient current spectroscopy of single-electron spin states, we extract a lower bound of the spin relaxation time of 0.5~$μ$s. This result represents an important step towards the investigation of spin coherence times in graphene-based quantum dots and the implementation of spin-qubits.
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Submitted 27 January, 2021; v1 submitted 4 December, 2020;
originally announced December 2020.
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Tunable interdot coupling in few-electron bilayer graphene double quantum dots
Authors:
Luca Banszerus,
Alexander Rothstein,
Eike Icking,
Samuel Möller,
Kenji Watanabe,
Takashi Taniguchi,
Christoph Stampfer,
Christian Volk
Abstract:
We present a highly controllable double quantum dot device based on bilayer graphene. Using a device architecture of interdigitated gate fingers, we can control the interdot tunnel coupling between 1 to 4 GHz and the mutual capacitive coupling between 0.2 and 0.6 meV, independently of the charge occupation of the quantum dots. The charging energy and hence the dot size remains nearly unchanged. Th…
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We present a highly controllable double quantum dot device based on bilayer graphene. Using a device architecture of interdigitated gate fingers, we can control the interdot tunnel coupling between 1 to 4 GHz and the mutual capacitive coupling between 0.2 and 0.6 meV, independently of the charge occupation of the quantum dots. The charging energy and hence the dot size remains nearly unchanged. The tuning range of the tunnel coupling covers the operating regime of typical silicon and GaAs spin qubit devices.
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Submitted 23 December, 2020; v1 submitted 27 October, 2020;
originally announced October 2020.
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Electron-hole crossover in gate-controlled bilayer graphene quantum dots
Authors:
Luca Banszerus,
Alexander Rothstein,
Thomas Fabian,
Samuel Möller,
Eike Icking,
Stefan Trellenkamp,
Florian Lentz,
Daniel Neumaier,
Kenji Watanabe,
Takashi Taniguchi,
Florian Libisch,
Christian Volk,
Christoph Stampfer
Abstract:
Electron and hole Bloch states in gapped bilayer graphene exhibit topological orbital magnetic moments with opposite signs near the band edges, which allows for tunable valley-polarization in an out-of-plane magnetic field. This intrinsic property makes electron and hole quantum dots (QDs) in bilayer graphene interesting for valley and spin-valley qubits. Here we show measurements of the electron-…
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Electron and hole Bloch states in gapped bilayer graphene exhibit topological orbital magnetic moments with opposite signs near the band edges, which allows for tunable valley-polarization in an out-of-plane magnetic field. This intrinsic property makes electron and hole quantum dots (QDs) in bilayer graphene interesting for valley and spin-valley qubits. Here we show measurements of the electron-hole crossover in a bilayer graphene QD, demonstrating the opposite sign of the orbital magnetic moments associated with the Berry curvature. Using three layers of metallic top gates, we independently control the tunneling barriers of the QD while tuning the occupation from the few-hole regime to the few-electron regime, crossing the displacement-field controlled band gap. The band gap is around 25 meV, while the charging energies of the electron and hole dots are between 3-5 meV. The extracted valley g-factor is around 17 and leads to opposite valley polarization for electron and hole states at moderate B-fields. Our measurements agree well with tight-binding calculations for our device.
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Submitted 19 September, 2020; v1 submitted 6 August, 2020;
originally announced August 2020.
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Electrostatic detection of Shubnikov-de-Haas oscillations in bilayer graphene by Coulomb resonances in gate-defined quantum dots
Authors:
Luca Banszerus,
Thomas Fabian,
Samuel Möller,
Eike Icking,
Henning Heiming,
Stefan Trellenkamp,
Florian Lentz,
Daniel Neumaier,
Martin Otto,
Kenji Watanabe,
Takashi Taniguchi,
Florian Libisch,
Christian Volk,
Christoph Stampfer
Abstract:
A gate-defined quantum dot in bilayer graphene is utilized as a sensitive electrometer for probing the charge density of its environment. Under the influence of a perpendicular magnetic field, the charge carrier density of the channel region next to the quantum dot oscillates due to the formation of Landau levels. This is experimentally observed as oscillations in the gate-voltage positions of the…
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A gate-defined quantum dot in bilayer graphene is utilized as a sensitive electrometer for probing the charge density of its environment. Under the influence of a perpendicular magnetic field, the charge carrier density of the channel region next to the quantum dot oscillates due to the formation of Landau levels. This is experimentally observed as oscillations in the gate-voltage positions of the Coulomb resonances of the nearby quantum dot. From the frequency of the oscillations, we extract the charge carrier density in the channel and from the amplitude the shift of the quantum dot potential. We compare these experimental results with an electrostatic simulation of the device and find good agreement.
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Submitted 9 September, 2020; v1 submitted 23 June, 2020;
originally announced June 2020.
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Single-electron double quantum dots in bilayer graphene
Authors:
Luca Banszerus,
Samuel Möller,
Eike Icking,
Kenji Watanabe,
Takashi Taniguchi,
Christian Volk,
Christoph Stampfer
Abstract:
We present transport measurements through an electrostatically defined bilayer graphene double quantum dot in the single electron regime. With the help of a back gate, two split gates and two finger gates we are able to control the number of charge carriers on two gate-defined quantum dot independently between zero and five. The high tunability of the device meets requirements to make such a devic…
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We present transport measurements through an electrostatically defined bilayer graphene double quantum dot in the single electron regime. With the help of a back gate, two split gates and two finger gates we are able to control the number of charge carriers on two gate-defined quantum dot independently between zero and five. The high tunability of the device meets requirements to make such a device a suitable building block for spin-qubits. In the single electron regime, we determine interdot tunnel rates on the order of 2~GHz. Both, the interdot tunnel coupling, as well as the capacitive interdot coupling increase with dot occupation, leading to the transition to a single quantum dot. Finite bias magneto-spectroscopy measurements allow to resolve the excited state spectra of the first electrons in the double quantum dot; being in agreement with spin and valley conserving interdot tunneling processes.
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Submitted 24 December, 2019;
originally announced December 2019.
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Quantum Dot Arrays in Silicon and Germanium
Authors:
W. I. L. Lawrie,
H. G. J. Eenink,
N. W. Hendrickx,
J. M. Boter,
L. Petit,
S. V. Amitonov,
M. Lodari,
B. Paquelet Wuetz,
C. Volk,
S. Philips,
G. Droulers,
N. Kalhor,
F. van Riggelen,
D. Brousse,
A. Sammak,
L. M. K. Vandersypen,
G. Scappucci,
M. Veldhorst
Abstract:
Electrons and holes confined in quantum dots define an excellent building block for quantum emergence, simulation, and computation. In order for quantum electronics to become practical, large numbers of quantum dots will be required, necessitating the fabrication of scaled structures such as linear and 2D arrays. Group IV semiconductors contain stable isotopes with zero nuclear spin and can thereb…
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Electrons and holes confined in quantum dots define an excellent building block for quantum emergence, simulation, and computation. In order for quantum electronics to become practical, large numbers of quantum dots will be required, necessitating the fabrication of scaled structures such as linear and 2D arrays. Group IV semiconductors contain stable isotopes with zero nuclear spin and can thereby serve as excellent host for spins with long quantum coherence. Here we demonstrate group IV quantum dot arrays in silicon metal-oxide-semiconductor (SiMOS), strained silicon (Si/SiGe) and strained germanium (Ge/SiGe). We fabricate using a multi-layer technique to achieve tightly confined quantum dots and compare integration processes. While SiMOS can benefit from a larger temperature budget and Ge/SiGe can make ohmic contact to metals, the overlapping gate structure to define the quantum dots can be based on a nearly identical integration. We realize charge sensing in each platform, for the first time in Ge/SiGe, and demonstrate fully functional linear and two-dimensional arrays where all quantum dots can be depleted to the last charge state. In Si/SiGe, we tune a quintuple quantum dot using the N+1 method to simultaneously reach the few electron regime for each quantum dot. We compare capacitive cross talk and find it to be the smallest in SiMOS, relevant for the tuning of quantum dot arrays. These results constitute an excellent base for quantum computation with quantum dots and provide opportunities for each platform to be integrated with standard semiconductor manufacturing.
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Submitted 14 September, 2019;
originally announced September 2019.
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Insulating state in low-disorder graphene nanoribbons
Authors:
A. Epping,
C. Volk,
F. Buckstegge,
K. Watanabe,
T. Taniguchi,
C. Stampfer
Abstract:
We report on quantum transport measurements on etched graphene nanoribbons encapsulated in hexagonal boron nitride (hBN). At zero magnetic field our devices behave qualitatively very similar to what has been reported for graphene nanoribbons on $\text{SiO}_2$ or hBN, but exhibit a considerable smaller transport gap. At magnetic fields of around $3~$T the transport behavior changes considerably and…
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We report on quantum transport measurements on etched graphene nanoribbons encapsulated in hexagonal boron nitride (hBN). At zero magnetic field our devices behave qualitatively very similar to what has been reported for graphene nanoribbons on $\text{SiO}_2$ or hBN, but exhibit a considerable smaller transport gap. At magnetic fields of around $3~$T the transport behavior changes considerably and is dominated by a much larger energy gap induced by electron-electron interactions completely suppressing transport. This energy gap increases with a slope on the order of $3-4~ $meV/T reaching values of up to $ 30~\mathrm{meV} $ at $ 9~ $T.
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Submitted 29 August, 2019;
originally announced August 2019.
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Fast charge sensing of Si/SiGe quantum dots via a high-frequency accumulation gate
Authors:
Christian Volk,
Anasua Chatterjee,
Fabio Ansaloni,
Charles M. Marcus,
Ferdinand Kuemmeth
Abstract:
Quantum dot arrays are a versatile platform for the implementation of spin qubits, as high-bandwidth sensor dots can be integrated with single-, double- and triple-dot qubits yielding fast and high-fidelity qubit readout. However, for undoped silicon devices, reflectometry off sensor ohmics suffers from the finite resistivity of the two-dimensional electron gas (2DEG), and alternative readout meth…
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Quantum dot arrays are a versatile platform for the implementation of spin qubits, as high-bandwidth sensor dots can be integrated with single-, double- and triple-dot qubits yielding fast and high-fidelity qubit readout. However, for undoped silicon devices, reflectometry off sensor ohmics suffers from the finite resistivity of the two-dimensional electron gas (2DEG), and alternative readout methods are limited to measuring qubit capacitance, rather than qubit charge. By coupling a surface-mount resonant circuit to the plunger gate of a high-impedance sensor, we realized a fast charge sensing technique that is compatible with resistive 2DEGs. We demonstrate this by acquiring at high speed charge stability diagrams of double- and triple-dot arrays in Si/SiGe heterostructures as well as pulsed-gate single-shot charge and spin readout with integration times as low as 2.4 $μ$s.
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Submitted 11 November, 2019; v1 submitted 25 June, 2019;
originally announced June 2019.
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Loading a quantum-dot based "Qubyte" register
Authors:
C. Volk,
A. M. J. Zwerver,
U. Mukhopadhyay,
P. T. Eendebak,
C. J. van Diepen,
J. P. Dehollain,
T. Hensgens,
T. Fujita,
C. Reichl,
W. Wegscheider,
L. M. K. Vandersypen
Abstract:
Electrostatically defined quantum dot arrays offer a compelling platform for quantum computation and simulation. However, tuning up such arrays with existing techniques becomes impractical when going beyond a handful of quantum dots. Here, we present a method for systematically adding quantum dots to an array one dot at a time, in such a way that the number of electrons on previously formed dots i…
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Electrostatically defined quantum dot arrays offer a compelling platform for quantum computation and simulation. However, tuning up such arrays with existing techniques becomes impractical when going beyond a handful of quantum dots. Here, we present a method for systematically adding quantum dots to an array one dot at a time, in such a way that the number of electrons on previously formed dots is unaffected. The method allows individual control of the number of electrons on each of the dots, as well as of the interdot tunnel rates. We use this technique to tune up a linear array of eight GaAs quantum dots such that they are occupied by one electron each. This new method overcomes a critical bottleneck in scaling up quantum-dot based qubit registers.
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Submitted 2 January, 2019;
originally announced January 2019.
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Back action of graphene charge detectors on graphene and carbon nanotube quantum dots
Authors:
C. Volk,
S. Engels,
C. Neumann,
C. Stampfer
Abstract:
We report on devices based on graphene charge detectors (CDs) capacitively coupled to graphene and carbon nanotube quantum dots (QDs). We focus on back action effects of the CD on the probed QD. A strong influence of the bias voltage applied to the CD on the current through the QD is observed. Depending on the charge state of the QD the current through the QD can either strongly increase or comple…
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We report on devices based on graphene charge detectors (CDs) capacitively coupled to graphene and carbon nanotube quantum dots (QDs). We focus on back action effects of the CD on the probed QD. A strong influence of the bias voltage applied to the CD on the current through the QD is observed. Depending on the charge state of the QD the current through the QD can either strongly increase or completely reverse as a response to the applied voltage on the CD. To describe the observed behavior we employ two simple models based on single electron transport in QDs with asymmetrically broadened energy distributions of the source and the drain leads. The models successfully explain the back action effects. The extracted distribution broadening shows a linear dependency on the bias voltage applied to the CD. We discuss possible mechanisms mediating the energy transfer between the CD and QD and give an explanation for the origin of the observed asymmetry.
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Submitted 21 August, 2015;
originally announced August 2015.
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Correlations of mutual positions of charge density waves nodes in side-by-side placed InAs wires measured with scanning gate microscopy
Authors:
A. A. Zhukov,
Ch. Volk,
A. Winden,
H. Hardtdegen,
Th. Schaepers
Abstract:
We investigate the correlations of mutual positions of charge density waves nodes in side-by-side placed InAs nanowires in presence of a conductive atomic force microscope tip served as a mobile gate at helium temperatures. Scanning gate microscopy scans demonstrate mutual correlation of positions of charge density waves nodes of two wires. A general mutual shift of the nodes positions and "crysta…
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We investigate the correlations of mutual positions of charge density waves nodes in side-by-side placed InAs nanowires in presence of a conductive atomic force microscope tip served as a mobile gate at helium temperatures. Scanning gate microscopy scans demonstrate mutual correlation of positions of charge density waves nodes of two wires. A general mutual shift of the nodes positions and "crystal lattice mismatch" defect were observed. These observations demonstrate the crucial role of Coulomb interaction in formation of charge density waves in InAs nanowires.
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Submitted 24 April, 2015;
originally announced April 2015.
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Investigations of local electronic transport in InAs nanowires by scanning gate microscopy at helium temperatures
Authors:
A. A. Zhukov,
Ch. Volk,
A. Winden,
H. Hardtdegen,
Th. Schaepers
Abstract:
In the current paper a set of experiments dedicated to investigations of local electronic transport in undoped InAs nanowires at helium temperatures in the presence of a charged atomic-force microscope tip is presented. Both nanowires without defects and with internal tunneling barriers were studied. The measurements were performed at various carrier concentrations in the systems and opacity of co…
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In the current paper a set of experiments dedicated to investigations of local electronic transport in undoped InAs nanowires at helium temperatures in the presence of a charged atomic-force microscope tip is presented. Both nanowires without defects and with internal tunneling barriers were studied. The measurements were performed at various carrier concentrations in the systems and opacity of contact-to-wire interfaces. The regime of Coulomb blockade is investigated in detail including negative differential conductivity of the whole system. The situation with open contacts with one tunneling barrier and undivided wire is also addressed. Special attention is devoted to recently observed quasi-periodic standing waves.
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Submitted 30 May, 2014;
originally announced May 2014.
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Reducing disorder in graphene nanoribbons by chemical edge modification
Authors:
Jan Dauber,
Bernat Terrés,
Christian Volk,
Stefan Trellenkamp,
Christoph Stampfer
Abstract:
We present electronic transport measurements on etched graphene nanoribbons on silicon dioxide before and after a short hydrouoric acid (HF) treatment. We report on changes in the transport properties, in particular, in terms of a decreasing transport gap and a reduced doping level after HF dipping. Interestingly, the effective energy gap is nearly unaffected by the HF treatment. Additional measur…
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We present electronic transport measurements on etched graphene nanoribbons on silicon dioxide before and after a short hydrouoric acid (HF) treatment. We report on changes in the transport properties, in particular, in terms of a decreasing transport gap and a reduced doping level after HF dipping. Interestingly, the effective energy gap is nearly unaffected by the HF treatment. Additional measurements on a graphene nanoribbon with lateral graphene gates support strong indications that the HF significantly modifies the edges of the investigated nanoribbons leading to a significantly reduced disorder potential in these graphene nanostructures.
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Submitted 21 November, 2013; v1 submitted 20 November, 2013;
originally announced November 2013.
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Etched graphene single electron transistors on hexagonal boron nitride in high magnetic fields
Authors:
A. Epping,
S. Engels,
C. Volk,
K. Watanabe,
T. Taniguchi,
S. Trellenkamp,
C. Stampfer
Abstract:
We report on the fabrication and electrical characterisation of etched graphene single electron transistors (SETs) of various sizes on hexagonal boron nitride (hBN) in high magnetic fields. The electronic transport measurements show a slight improvement compared to graphene SETs on SiO2. In particular, SETs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T in con…
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We report on the fabrication and electrical characterisation of etched graphene single electron transistors (SETs) of various sizes on hexagonal boron nitride (hBN) in high magnetic fields. The electronic transport measurements show a slight improvement compared to graphene SETs on SiO2. In particular, SETs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T in contrast to measurements reported on SETs on SiO2. This result indicates a reduced surface disorder potential in SETs on hBN which might be an important step towards clean and more controllable graphene QDs.
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Submitted 19 November, 2013;
originally announced November 2013.
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The electronic transport of top subband and disordered sea in InAs nanowire in presence of a mobile gate
Authors:
A. A. Zhukov,
Ch. Volk,
A. Winden,
H. Hardtdegen,
Th. Schaepers
Abstract:
We performed measurements at helium temperatures of the electronic transport in an InAs quantum wire ($R_{wire} \sim 30$\,k$Ω$) in the presence of a charged tip of an atomic force microscope serving as a mobile gate. The period and the amplitude of the observed quasiperiodic oscillations are investigated in detail as a function of electron concentration in the linear and non-linear regime. We demo…
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We performed measurements at helium temperatures of the electronic transport in an InAs quantum wire ($R_{wire} \sim 30$\,k$Ω$) in the presence of a charged tip of an atomic force microscope serving as a mobile gate. The period and the amplitude of the observed quasiperiodic oscillations are investigated in detail as a function of electron concentration in the linear and non-linear regime. We demonstrate the influence of the tip-to-sample distance on the ability to locally affect the top subband electrons as well as the electrons in the disordered sea. Furthermore, we introduce a new method of detection of the subband occupation in an InAs wire, which allows us to evaluate the number of the electrons in the conductive band of the wire.
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Submitted 14 September, 2013; v1 submitted 10 September, 2013;
originally announced September 2013.
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Etched graphene quantum dots on hexagonal boron nitride
Authors:
S. Engels,
A. Epping,
C. Volk,
S. Korte,
B. Voigtländer,
K. Watanabe,
T. Taniguchi,
S. Trellenkamp,
C. Stampfer
Abstract:
We report on the fabrication and characterization of etched graphene quantum dots (QDs) on hexagonal boron nitride (hBN) and SiO2 with different island diameters. We perform a statistical analysis of Coulomb peak spacings over a wide energy range. For graphene QDs on hBN, the standard deviation of the normalized peak spacing distribution decreases with increasing QD diameter, whereas for QDs on Si…
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We report on the fabrication and characterization of etched graphene quantum dots (QDs) on hexagonal boron nitride (hBN) and SiO2 with different island diameters. We perform a statistical analysis of Coulomb peak spacings over a wide energy range. For graphene QDs on hBN, the standard deviation of the normalized peak spacing distribution decreases with increasing QD diameter, whereas for QDs on SiO2 no diameter dependency is observed. In addition, QDs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T. Both results indicate a substantially reduced substrate induced disorder potential in graphene QDs on hBN.
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Submitted 9 August, 2013;
originally announced August 2013.
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Graphene-based charge sensors
Authors:
C. Neumann,
C. Volk,
S. Engels,
C. Stampfer
Abstract:
We discuss graphene nanoribbon-based charge sensors and focus on their functionality in the presence of external magnetic fields and high frequency pulses applied to a nearby gate electrode. The charge detectors work well with in-plane magnetic fields of up to 7 T and pulse frequencies of up to 20 MHz. By analyzing the step height in the charge detector's current at individual charging events in a…
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We discuss graphene nanoribbon-based charge sensors and focus on their functionality in the presence of external magnetic fields and high frequency pulses applied to a nearby gate electrode. The charge detectors work well with in-plane magnetic fields of up to 7 T and pulse frequencies of up to 20 MHz. By analyzing the step height in the charge detector's current at individual charging events in a nearby quantum dot, we determine the ideal operation conditions with respect to the applied charge detector bias. Average charge sensitivities of 1.3*10^-3 e/sqrt{Hz} can be achieved. Additionally, we investigate the back action of the charge detector current on the quantum transport through a nearby quantum dot. By setting the charge detector bias from 0 to 4.5 mV, we can increase the Coulomb peak currents measured at the quantum dot by a factor of around 400. Furthermore, we can completely lift the Coulomb blockade in the quantum dot.
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Submitted 2 April, 2013; v1 submitted 29 March, 2013;
originally announced April 2013.
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Probing relaxation times in graphene quantum dots
Authors:
Christian Volk,
Christoph Neumann,
Sebastian Kazarski,
Stefan Fringes,
Stephan Engels,
Federica Haupt,
André Müller,
Christoph Stampfer
Abstract:
Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dot devices have been extensively investigated with respect to their excitation spectrum, spin-filling sequence, and electron-hole crossover. However their relaxation dynamics remain largely un…
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Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dot devices have been extensively investigated with respect to their excitation spectrum, spin-filling sequence, and electron-hole crossover. However their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular regarding the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here, we report on pulsed-gate transient spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design, allowing to tune the tunnelling barriers individually down to the low MHz regime and to monitor their asymmetry with integrated charge sensors. Measuring the transient currents through electronic excited states, we estimate lower limit of charge relaxation times on the order of 60-100 ns.
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Submitted 21 March, 2013;
originally announced March 2013.
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Transport in coupled graphene-nanotube quantum devices
Authors:
S. Engels,
P. Weber,
B. Terrés,
J. Dauber,
C. Meyer,
C. Volk,
S. Trellenkamp,
U. Wichmann,
C. Stampfer
Abstract:
We report on the fabrication and characterization of all-carbon hybrid quantum devices based on graphene and single-walled carbon nanotubes. We discuss both, carbon nanotube quantum dot devices with graphene charge detectors and nanotube quantum dots with graphene leads. The devices are fabricated by chemical vapor deposition growth of carbon nanotubes and subsequent structuring of mechanically ex…
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We report on the fabrication and characterization of all-carbon hybrid quantum devices based on graphene and single-walled carbon nanotubes. We discuss both, carbon nanotube quantum dot devices with graphene charge detectors and nanotube quantum dots with graphene leads. The devices are fabricated by chemical vapor deposition growth of carbon nanotubes and subsequent structuring of mechanically exfoliated graphene. We study the detection of individual charging events in the carbon nanotube quantum dot by a nearby graphene nanoribbon and show that they lead to changes of up to 20% of the conductance maxima in the graphene nanoribbon acting as a good performing charge detector. Moreover, we discuss an electrically coupled graphene-nanotube junction, which exhibits a tunneling barrier with tunneling rates in the low GHz regime. This allows to observe Coulomb blockade on a carbon nanotube quantum dot with graphene source and drain leads.
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Submitted 9 August, 2013; v1 submitted 6 April, 2012;
originally announced April 2012.
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Charge detection in a bilayer graphene quantum dot
Authors:
Stefan Fringes,
Christian Volk,
Caroline Norda,
Bernat Terrés,
Jan Dauber,
Stephan Engels,
Stefan Trellenkamp,
Christoph Stampfer
Abstract:
We show measurements on a bilayer graphene quantum dot with an integrated charge detector. The focus lies on enabling charge detection with a 30 nm wide bilayer graphene nanoribbon located approximately 35 nm next to a bilayer graphene quantum dot with an island diameter of about 100 nm. Local resonances in the nanoribbon can be successfully used to detect individual charging events in the dot eve…
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We show measurements on a bilayer graphene quantum dot with an integrated charge detector. The focus lies on enabling charge detection with a 30 nm wide bilayer graphene nanoribbon located approximately 35 nm next to a bilayer graphene quantum dot with an island diameter of about 100 nm. Local resonances in the nanoribbon can be successfully used to detect individual charging events in the dot even in regimes where the quantum dot Coulomb peaks cannot be measured by conventional techniques.
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Submitted 26 October, 2011;
originally announced October 2011.
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Tunable capacitive inter-dot coupling in a bilayer graphene double quantum dot
Authors:
Stefan Fringes,
Christian Volk,
Bernat Terrés,
Jan Dauber,
Stephan Engels,
Stefan Trellenkamp,
Christoph Stampfer
Abstract:
We report on a double quantum dot which is formed in a width-modulated etched bilayer graphene nanoribbon. A number of lateral graphene gates enable us to tune the quantum dot energy levels and the tunneling barriers of the device over a wide energy range. Charge stability diagrams and in particular individual triple point pairs allow to study the tunable capacitive inter-dot coupling energy as we…
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We report on a double quantum dot which is formed in a width-modulated etched bilayer graphene nanoribbon. A number of lateral graphene gates enable us to tune the quantum dot energy levels and the tunneling barriers of the device over a wide energy range. Charge stability diagrams and in particular individual triple point pairs allow to study the tunable capacitive inter-dot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points. We extract a mutual capacitive inter-dot coupling in the range of 2 - 6 meV and an inter-dot tunnel coupling on the order of 1.5 μeV.
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Submitted 26 October, 2011;
originally announced October 2011.
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Electronic Excited States in Bilayer Graphene Double Quantum Dots
Authors:
Christian Volk,
Stefan Fringes,
Bernat Terrés,
Jan Dauber,
Stephan Engels,
Stefan Trellenkamp,
Christoph Stampfer
Abstract:
We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features addition energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable int…
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We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features addition energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points over a large energy range. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected single-particle energy spacing in bilayer graphene quantum dots. Finally, we investigate the evolution of the electronic excited states in a parallel magnetic field.
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Submitted 18 March, 2013; v1 submitted 10 May, 2011;
originally announced May 2011.
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Disorder induced Coulomb gaps in graphene constrictions with different aspect ratios
Authors:
B. Terrés,
J. Dauber,
C. Volk,
S. Trellenkamp,
U. Wichmann,
C. Stampfer
Abstract:
We present electron transport measurements on lithographically defined and etched graphene nanoconstrictions with different aspect ratios including different lengths (L) and widths (W). A roughly length-independent disorder induced effective energy gap can be observed around the charge neutrality point. This energy gap scales inversely with the width even in regimes where the length of the constri…
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We present electron transport measurements on lithographically defined and etched graphene nanoconstrictions with different aspect ratios including different lengths (L) and widths (W). A roughly length-independent disorder induced effective energy gap can be observed around the charge neutrality point. This energy gap scales inversely with the width even in regimes where the length of the constriction is smaller than its width (L<W). In very short constrictions, we observe both resonances due to localized states or charged islands and an elevated overall conductance level (0.1-1e2/h), which is strongly length-dependent in the gap region. This makes very short graphene constrictions interesting for highly transparent graphene tunneling barriers.
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Submitted 15 February, 2011; v1 submitted 9 November, 2010;
originally announced November 2010.
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Spin-orbit coupling and phase-coherence in InAs nanowires
Authors:
S. Estévez Hernández,
M. Akabori,
K. Sladek,
Ch. Volk,
S. Alagha,
H. Hardtdegen,
N. Demarina,
D. Grützmacher,
Th. Schäpers,
M. G. Pala
Abstract:
We investigated the magnetotransport of InAs nanowires grown by selective area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation of the magnetic field or the back-gate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the…
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We investigated the magnetotransport of InAs nanowires grown by selective area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation of the magnetic field or the back-gate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length l_phi is determined. At the lowest temperatures l_phi is found to be at least 300 nm, while for temperatures exceeding 2 K a monotonous decrease of l_phi with temperature is observed. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctuations. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the experimental data to simulations based on a recursive two-dimensional Green's function approach a spin-orbit scattering length of approximately 70 nm was extracted, indicating the presence of strong spin-orbit coupling.
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Submitted 6 November, 2010;
originally announced November 2010.