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Microwave spectroscopy of few-carrier states in bilayer graphene quantum dots
Authors:
Max J. Ruckriegel,
Christoph Adam,
Rebecca Bolt,
Chuyao Tong,
David Kealhofer,
Artem O. Denisov,
Mohsen Bahrami Panah,
Kenji Watanabe,
Takashi Taniguchi,
Thomas Ihn,
Klaus Ensslin
Abstract:
Bilayer graphene is a maturing material platform for gate-defined quantum dots that hosts long-lived spin and valley states. Implementing solid-state qubits in bilayer graphene requires a fundamental understanding of such confined electronic systems. In particular, states of two and three carriers, for which the exchange interaction between particles plays a crucial role, are a cornerstone for qub…
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Bilayer graphene is a maturing material platform for gate-defined quantum dots that hosts long-lived spin and valley states. Implementing solid-state qubits in bilayer graphene requires a fundamental understanding of such confined electronic systems. In particular, states of two and three carriers, for which the exchange interaction between particles plays a crucial role, are a cornerstone for qubit readout and manipulation. Here we report on the spectroscopy of few-carrier states in bilayer graphene quantum dots, using circuit quantum electrodynamics (cQED) techniques that offer substantially improved energy resolution compared to standard transport techniques. Measurements using a superconducting high-impedance resonator capacitively coupled to the double quantum dot reveal dispersive features of two and three electron states, enabling the detection of Pauli spin and valley blockade and the characterization of the spin-orbit gap at zero magnetic field. The results deepen our understanding of few-carrier spin and valley states in bilayer graphene quantum dots and demonstrate that cQED techniques are a powerful state-selective probe for semiconductor nanostructures.
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Submitted 25 November, 2025;
originally announced November 2025.
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Tunable spin-orbit splitting in bilayer graphene/WSe$_2$ quantum devices
Authors:
Jonas D. Gerber,
Efe Ersoy,
Michele Masseroni,
Markus Niese,
Michael Laumer,
Artem O. Denisov,
Hadrien Duprez,
Wei Wister Huang,
Christoph Adam,
Lara Ostertag,
Chuyao Tong,
Takashi Taniguchi,
Kenji Watanabe,
Vladimir I. Fal'ko,
Thomas Ihn,
Klaus Ensslin,
Angelika Knothe
Abstract:
Bilayer graphene (BLG)-based quantum devices represent a promising platform for emerging technologies, such as quantum computing and spintronics. However, their intrinsically weak spin-orbit coupling (SOC) complicates spin and valley manipulation. Integrating BLG with transition metal dichalcogenides (TMDs) enhances the SOC via proximity effects. While this enhancement has been demonstrated in 2D-…
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Bilayer graphene (BLG)-based quantum devices represent a promising platform for emerging technologies, such as quantum computing and spintronics. However, their intrinsically weak spin-orbit coupling (SOC) complicates spin and valley manipulation. Integrating BLG with transition metal dichalcogenides (TMDs) enhances the SOC via proximity effects. While this enhancement has been demonstrated in 2D-layered structures, 1D and 0D nanostructures in BLG/TMD remain unrealized, with open questions regarding SOC strength and tunability. Here, we investigate quantum point contacts and quantum dots in two BLG/WSe$_2$ heterostructures with different stacking orders. Across multiple devices, we reproducibly demonstrate spin-orbit splitting up to 1.5 meV - more than 1 order of magnitude higher than in pristine BLG. Furthermore, we show that the induced SOC can be tuned in situ from its maximum value to near-complete suppression via the perpendicular electric field. This enhancement and in situ tunability establish the SOC as a control mechanism for dynamic spin and valley manipulation.
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Submitted 9 September, 2025; v1 submitted 8 April, 2025;
originally announced April 2025.
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Entropy spectroscopy of a bilayer graphene quantum dot
Authors:
Christoph Adam,
Hadrien Duprez,
Natalie Lehmann,
Antoni Yglesias,
Artem Olegovich Denisov,
Solenn Cances,
Max Josef Ruckriegel,
Michele Masseroni,
Chuyao Tong,
Wei Wister Huang,
David Kealhofer,
Rebekka Garreis,
Kenji Watanabe,
Takashi Taniguchi,
Klaus Ensslin,
Thomas Ihn
Abstract:
We measure the entropy change of charge transitions in an electrostatically defined quantum dot in bilayer graphene. Entropy provides insights into the equilibrium thermodynamic properties of both ground and excited states beyond transport measurements. For the one-carrier regime, the obtained entropy shows that the ground state has a two-fold degeneracy lifted by an out-of-plane magnetic field. T…
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We measure the entropy change of charge transitions in an electrostatically defined quantum dot in bilayer graphene. Entropy provides insights into the equilibrium thermodynamic properties of both ground and excited states beyond transport measurements. For the one-carrier regime, the obtained entropy shows that the ground state has a two-fold degeneracy lifted by an out-of-plane magnetic field. This observation is in agreement with previous direct transport measurements and confirms the applicability of this novel method. For the two-carrier regime, the extracted entropy indicates a non-degenerate ground state at zero magnetic field, contrary to previous studies suggesting a three-fold degeneracy. We attribute the degeneracy lifting to the effect of Kane-Mele type spin--orbit interaction on the two-carrier ground state, which has not been observed before. Our work demonstrates the validity and efficacy of entropy measurements as a unique, supplementary experimental tool to investigate the degeneracy of the ground state in quantum devices build in materials such as graphene. This technique, applied to exotic systems with fractional ground state entropies, will be a powerful tool in the study of quantum matter.
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Submitted 18 February, 2026; v1 submitted 23 December, 2024;
originally announced December 2024.
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Receivers for the Black Hole Explorer (BHEX) Mission
Authors:
C. Edward Tong,
Kazunori Akiyama,
Paul Grimes,
Mareki Honma,
Janice Houston,
Michael D. Johnson,
Daniel P. Marrone,
Hannah Rana,
Yoshinori Uzawa
Abstract:
In this paper, we introduce the receiver architecture for the Black Hole Explorer (BHEX) Mission, designed to reveal the photon ring of black holes. The primary instrument is a dual-polarization receiver operating over the 240-320 GHz frequency range, utilizing a Superconductor-Insulator-Superconductor (SIS) mixer. This Double-Side-Band (DSB) receiver has an intermediate frequency (IF) range of 4-…
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In this paper, we introduce the receiver architecture for the Black Hole Explorer (BHEX) Mission, designed to reveal the photon ring of black holes. The primary instrument is a dual-polarization receiver operating over the 240-320 GHz frequency range, utilizing a Superconductor-Insulator-Superconductor (SIS) mixer. This Double-Side-Band (DSB) receiver has an intermediate frequency (IF) range of 4-12 GHz and operates at a bath temperature of 4.5 K, for optimal performance, which necessitates the integration of a cryocooler. Complementing the primary receiver is a secondary unit covering the 80-106 GHz spectrum, featuring a cryogenic low noise amplifier. This secondary receiver, affixed to the 20 K stage of the cryocooler, serves to augment the SIS receiver performance by employing the Frequency Phase Transfer technique to boost the signal-to-noise ratio at the correlator output. Together, this sophisticated receiver duo is engineered to achieve the quantum-limited sensitivity required to detect the photon ring of black holes, marking a breakthrough in astrophysical observation.
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Submitted 13 June, 2024;
originally announced June 2024.
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Spin-orbit proximity in MoS$_2$/bilayer graphene heterostructures
Authors:
M. Masseroni,
M. Gull,
A. Panigrahi,
N. Jacobsen,
F. Fischer,
C. Tong,
J. D. Gerber,
M. Niese,
T. Taniguchi,
K. Watanabe,
L. Levitov,
T. Ihn,
K. Ensslin,
H. Duprez
Abstract:
Van der Waals heterostructures provide a versatile platform for tailoring electronic properties through the integration of two-dimensional materials. Among these combinations, the interaction between bilayer graphene and transition metal dichalcogenides (TMDs) stands out due to its potential for inducing spin-orbit coupling (SOC) in graphene. Future devices concepts require the understanding the p…
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Van der Waals heterostructures provide a versatile platform for tailoring electronic properties through the integration of two-dimensional materials. Among these combinations, the interaction between bilayer graphene and transition metal dichalcogenides (TMDs) stands out due to its potential for inducing spin-orbit coupling (SOC) in graphene. Future devices concepts require the understanding the precise nature of SOC in TMD/bilayer graphene heterostructures and its influence on electronic transport phenomena. Here, we experimentally confirm the presence of two distinct types of SOC, Ising (1.55 meV) and Rashba (2.5 meV), in bilayer graphene when interfaced with molybdenum disulphide, recognized as one of the most stable TMDs. Furthermore, we reveal a non-monotonic trend in conductivity with respect to the electric displacement field at charge neutrality. This phenomenon is ascribed to the existence of single-particle gaps induced by the Ising SOC, which can be closed by a critical displacement field. Remarkably, our findings also unveil sharp peaks in the magnetoconductivity around the critical displacement field, challenging existing theoretical models.
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Submitted 25 March, 2024;
originally announced March 2024.
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Spin-Valley Protected Kramers Pair in Bilayer Graphene
Authors:
Artem O. Denisov,
Veronika Reckova,
Solenn Cances,
Max J. Ruckriegel,
Michele Masseroni,
Christoph Adam,
Chuyao Tong,
Jonas D. Gerber,
Wei Wister Huang,
Kenji Watanabe,
Takashi Taniguchi,
Thomas Ihn,
Klaus Ensslin,
Hadrien Duprez
Abstract:
The intrinsic valley degree of freedom makes bilayer graphene (BLG) a unique platform for semiconductor qubits. The single-carrier quantum dot (QD) ground state exhibits a two-fold degeneracy, where the two states that constitute a Kramers pair, have opposite spin and valley quantum numbers. Because of the valley-dependent Berry curvature, an out-of-plane magnetic field breaks the time-reversal sy…
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The intrinsic valley degree of freedom makes bilayer graphene (BLG) a unique platform for semiconductor qubits. The single-carrier quantum dot (QD) ground state exhibits a two-fold degeneracy, where the two states that constitute a Kramers pair, have opposite spin and valley quantum numbers. Because of the valley-dependent Berry curvature, an out-of-plane magnetic field breaks the time-reversal symmetry of this ground state and a qubit can be encoded in the spin-valley subspace. The Kramers states are protected against known spin- and valley-mixing mechanisms because mixing requires a simultaneous change of both quantum numbers. Here, we fabricate a tunable QD device in Bernal BLG and measure a spin-valley relaxation time for the Kramers states of ${38~\mathrm{s}}$, which is two orders of magnitude longer than the ${0.4~\mathrm{s}}$ measured for purely spin-blocked states. We also show that the intrinsic Kane-Mele spin-orbit splitting enables a Kramers doublet single-shot readout even at zero magnetic field with a fidelity above ${99\%}$. If these long-lived Kramers states also possess long coherence times and can be effectively manipulated, electrostatically defined QDs in BLG may serve as long-lived semiconductor qubits, extending beyond the spin qubit paradigm.
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Submitted 11 February, 2025; v1 submitted 12 March, 2024;
originally announced March 2024.
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Dipole coupling of a bilayer graphene quantum dot to a high-impedance microwave resonator
Authors:
Max J. Ruckriegel,
Lisa M. Gächter,
David Kealhofer,
Mohsen Bahrami Panah,
Chuyao Tong,
Christoph Adam,
Michele Masseroni,
Hadrien Duprez,
Rebekka Garreis,
Kenji Watanabe,
Takashi Taniguchi,
Andreas Wallraff,
Thomas Ihn,
Klaus Ensslin,
Wei Wister Huang
Abstract:
We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform for semiconductor qubits that can host long-lived spin and valley states. The presented device combines a high-impedance ($Z_\mathrm{r} \approx 1 \mathrm{kΩ}$) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waal…
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We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform for semiconductor qubits that can host long-lived spin and valley states. The presented device combines a high-impedance ($Z_\mathrm{r} \approx 1 \mathrm{kΩ}$) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection with a signal-to-noise ratio of 3.5 within 1 $μ\mathrm{s}$ integration time. The charge-photon interaction is quantified in the dispersive and resonant regimes by comparing the coupling-induced change in the resonator response to input-output theory, yielding a maximal coupling strength of $g/2π = 49.7 \mathrm{MHz}$. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge-photon coupling with bilayer graphene quantum dots.
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Submitted 29 January, 2024; v1 submitted 22 December, 2023;
originally announced December 2023.
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Spin-valley locked excited states spectroscopy in a one-particle bilayer graphene quantum dot
Authors:
Hadrien Duprez,
Solenn Cances,
Andraz Omahen,
Michele Masseroni,
Max J. Ruckriegel,
Christoph Adam,
Chuyao Tong,
Jonas Gerber,
Rebekka Garreis,
Wister Huang,
Lisa Gächter,
Takashi Taniguchi,
Kenji Watanabe,
Thomas Ihn,
Klaus Ensslin
Abstract:
Current semiconductor qubits rely either on the spin or on the charge degree of freedom to encode quantum information. By contrast, in bilayer graphene the valley degree of freedom, stemming from the crystal lattice symmetry, is a robust quantum number that can therefore be harnessed for this purpose. The simplest implementation of a valley qubit would rely on two states with opposite valleys as i…
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Current semiconductor qubits rely either on the spin or on the charge degree of freedom to encode quantum information. By contrast, in bilayer graphene the valley degree of freedom, stemming from the crystal lattice symmetry, is a robust quantum number that can therefore be harnessed for this purpose. The simplest implementation of a valley qubit would rely on two states with opposite valleys as in the case of a single-carrier bilayer graphene quantum dot immersed in a small perpendicular magnetic field ($B_\perp\lesssim 100$mT). However, the single-carrier quantum dot excited states spectrum has not been resolved to date in the relevant magnetic field range. Here, we fill this gap, by measuring the parallel and perpendicular magnetic field dependence of this spectrum with an unprecedented resolution of $4μ$eV. We use a time-resolved charge detection technique that gives us access to individual tunnel events. Our results come as a direct verification of the predicted spectrum and establish a new upper-bound on inter-valley mixing, equal to our energy resolution. Our charge detection technique opens the door to measuring the relaxation time of a valley qubit in a single-carrier bilayer graphene quantum dot.
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Submitted 11 February, 2025; v1 submitted 21 November, 2023;
originally announced November 2023.
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Long distance electron-electron scattering detected with point contacts
Authors:
Lev V. Ginzburg,
Yuze Wu,
Marc P. Röösli,
Pedro Rosso Gomez,
Rebekka Garreis,
Chuyao Tong,
Veronika Stará,
Carolin Gold,
Khachatur Nazaryan,
Serhii Kryhin,
Hiske Overweg,
Christian Reichl,
Matthias Berl,
Takashi Taniguchi,
Kenji Watanabe,
Werner Wegscheider,
Thomas Ihn,
Klaus Ensslin
Abstract:
We measure electron transport through point contacts in an electron gas in AlGaAs/GaAs heterostructures and graphene for a range of temperatures, magnetic fields and electron densities. We find a magnetoconductance peak around B = 0. With increasing temperature, the width of the peak increases monotonically, while its amplitude first increases and then decreases. For GaAs point contacts the peak i…
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We measure electron transport through point contacts in an electron gas in AlGaAs/GaAs heterostructures and graphene for a range of temperatures, magnetic fields and electron densities. We find a magnetoconductance peak around B = 0. With increasing temperature, the width of the peak increases monotonically, while its amplitude first increases and then decreases. For GaAs point contacts the peak is particularly sharp at relatively low temperatures $T\approx$1.5 K: the curve rounds on a scale of few tens of $μ$T hinting at length scales of several millimeters for the corresponding scattering processes. We propose a model based on the transition between different transport regimes with increasing temperature: from ballistic transport to few electron-electron scatterings to hydrodynamic superballistic flow to hydrodynamic Poiseuille-like flow. The model is in qualitative and, in many cases, quantitative agreement with the experimental observations.
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Submitted 11 August, 2023;
originally announced August 2023.
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Pauli blockade catalogue and three- and four-particle Kondo effect in bilayer graphene quantum dots
Authors:
Chuyao Tong,
Annika Kurzmann,
Rebekka Garreis,
Kenji Watanabe,
Takashi Taniguchi,
Thomas Ihn,
Klaus Ensslin
Abstract:
Pauli blockade is a fundamental quantum phenomenon that also serves as a powerful tool for qubit manipulation and read-out. While most systems exhibit a simple even-odd pattern of double-dot Pauli spin blockade due to the preferred singlet pairing of spins, the additional valley degree of freedom offered by bilayer graphene greatly alters this pattern. Inspecting bias-triangle measurements at doub…
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Pauli blockade is a fundamental quantum phenomenon that also serves as a powerful tool for qubit manipulation and read-out. While most systems exhibit a simple even-odd pattern of double-dot Pauli spin blockade due to the preferred singlet pairing of spins, the additional valley degree of freedom offered by bilayer graphene greatly alters this pattern. Inspecting bias-triangle measurements at double-dot charge degeneracies with up to four electrons in each dot reveals a much richer double-dot Pauli blockade catalogue with both spin and/or valley blockade. In addition, we use single-dot Kondo effect measurements to substantiate our understanding of the three- and four-particle state spectra by analyzing their magnetic field dependence. With high controllability and reported long valley- and spin-relaxation times, bilayer graphene is a rising platform for hosting semiconductor quantum dot qubits. A thorough understanding of state spectra is crucial for qubit design and manipulation, and the rich Pauli blockade catalogue provides an abundance of novel qubit operational possibilities and opportunities to explore intriguing spin and valley physics.
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Submitted 10 April, 2024; v1 submitted 5 May, 2023;
originally announced May 2023.
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Long-lived valley states in bilayer graphene quantum dots
Authors:
Rebekka Garreis,
Chuyao Tong,
Jocelyn Terle,
Max Josef Ruckriegel,
Jonas Daniel Gerber,
Lisa Maria Gächter,
Kenji Watanabe,
Takashi Taniguchi,
Thomas Ihn,
Klaus Ensslin,
Wei Wister Huang
Abstract:
Bilayer graphene is a promising platform for electrically controllable qubits in a two-dimensional material. Of particular interest is the ability to encode quantum information in the so-called valley degree of freedom, a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure. The use of valleys could be advantageous, as known spin- and orbital-mixing mechanis…
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Bilayer graphene is a promising platform for electrically controllable qubits in a two-dimensional material. Of particular interest is the ability to encode quantum information in the so-called valley degree of freedom, a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure. The use of valleys could be advantageous, as known spin- and orbital-mixing mechanisms are unlikely to be at work for valleys, promising more robust qubits. The Berry curvature associated with valley states allows for electrical control of their energies, suggesting routes for coherent qubit manipulation. However, the relaxation time of valley states -- which ultimately limits these qubits' coherence properties and therefore their suitability as practical qubits -- is not yet known. Here, we measure the characteristic relaxation times of these spin and valley states in gate-defined bilayer graphene quantum dot devices. Different valley states can be distinguished from each other with a fidelity of over 99%. The relaxation time between valley triplets and singlets exceeds 500ms, and is more than one order of magnitude longer than for spin states. This work facilitates future measurements on valley-qubit coherence, demonstrating bilayer graphene as a practical platform hosting electrically controlled long-lived valley qubits.
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Submitted 23 January, 2024; v1 submitted 3 April, 2023;
originally announced April 2023.
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Three-carrier spin blockade and coupling in bilayer graphene double quantum dots
Authors:
Chuyao Tong,
Florian Ginzel,
Wei Wister Huang,
Annika Kurzmann,
Rebekka Garreis,
Kenji Watanabe,
Takashi Taniguchi,
Guido Burkard,
Jeroen Danon,
Thomas Ihn,
Klaus Ensslin
Abstract:
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 T…
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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 $(1,2)\leftrightarrow(0,3)$. 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.
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Submitted 9 November, 2022;
originally announced November 2022.
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Counting Statistics of Single Electron Transport in Bilayer Graphene Quantum Dots
Authors:
Rebekka Garreis,
Jonas Daniel Gerber,
Veronika Stará,
Chuyao Tong,
Carolin Gold,
Marc Röösli,
Kenji Watanabe,
Takashi Taniguchi,
Klaus Ensslin,
Thomas Ihn,
Annika Kurzmann
Abstract:
We measure telegraph noise of current fluctuations in an electrostatically defined quantum dot in bilayer graphene by real-time detection of single electron tunneling with a capacitively coupled neighboring quantum dot. Suppression of the second and third cumulant (related to shot noise) in a tunable graphene quantum dot is demonstrated experimentally. With this method we demonstrate the ability t…
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We measure telegraph noise of current fluctuations in an electrostatically defined quantum dot in bilayer graphene by real-time detection of single electron tunneling with a capacitively coupled neighboring quantum dot. Suppression of the second and third cumulant (related to shot noise) in a tunable graphene quantum dot is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels. Furthermore, we use this method to investigate the first spin excited state, an essential prerequisite to measure spin relaxation.
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Submitted 24 January, 2023; v1 submitted 14 October, 2022;
originally announced October 2022.
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Automated reconstruction of bound states in bilayer graphene quantum dots
Authors:
Jozef Bucko,
Frank Schäfer,
František Herman,
Rebekka Garreis,
Chuyao Tong,
Annika Kurzmann,
Thomas Ihn,
Eliska Greplova
Abstract:
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. Given the experim…
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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. Given the experimental ability to engineer this energy valley splitting, bilayer graphene quantum dots have a great potential for hosting robust qubits. Although bilayer graphene quantum dots have been recently 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 completely. We introduce a Hamiltonian-guided random search method and demonstrate robust identification of quantum states on both simulated and experimental data.
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Submitted 22 December, 2022; v1 submitted 1 March, 2022;
originally announced March 2022.
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Single-shot readout in graphene quantum dots
Authors:
Lisa Maria Gächter,
Rebekka Garreis,
Chuyao Tong,
Max Josef Ruckriegel,
Benedikt Kratochwil,
Folkert Kornelis de Vries,
Annika Kurzmann,
Kenji Watanabe,
Takashi Taniguchi,
Thomas Ihn,
Klaus Ensslin,
Wister Wei Huang
Abstract:
Electrostatically defined quantum dots in bilayer graphene offer a promising platform for spin qubits with presumably long coherence times due to low spin-orbit coupling and low nuclear spin density. We demonstrate two different experimental approaches to measure the decay times of excited states. The first is based on direct current measurements through the quantum device. Pulse sequences are app…
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Electrostatically defined quantum dots in bilayer graphene offer a promising platform for spin qubits with presumably long coherence times due to low spin-orbit coupling and low nuclear spin density. We demonstrate two different experimental approaches to measure the decay times of excited states. The first is based on direct current measurements through the quantum device. Pulse sequences are applied to control the occupation of ground and excited states. We observe a lower bound for the excited state decay on the order of hundred microseconds. The second approach employs a capacitively coupled charge sensor to study the time dynamics of the excited state using the Elzerman technique. We find that the relaxation time of the excited state is of the order of milliseconds. We perform single-shot readout of our two-level system with a visibility of $87.1\%$, which is an important step for developing a quantum information processor in graphene.
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Submitted 22 December, 2021;
originally announced December 2021.
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Pauli Blockade of Tunable Two-Electron Spin and Valley States in Graphene Quantum Dots
Authors:
Chuyao Tong,
Annika Kurzmann,
Rebekka Garreis,
Wei Wister Huang,
Samuel Jele,
Marius Eich,
Lev Ginzburg,
Christopher Mittag,
Kenji Watanabe,
Takashi Taniguchi,
Klaus Ensslin,
Thomas Ihn
Abstract:
Pauli blockade mechanisms -- whereby carrier transport through quantum dots is blocked due to selection rules even when energetically allowed -- are a direct manifestation of the Pauli exclusion principle, as well as a key mechanism for manipulating and reading out spin qubits. Pauli spin blockade is well established for systems such as GaAs QDs, but is to be further explored for systems with addi…
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Pauli blockade mechanisms -- whereby carrier transport through quantum dots is blocked due to selection rules even when energetically allowed -- are a direct manifestation of the Pauli exclusion principle, as well as a key mechanism for manipulating and reading out spin qubits. Pauli spin blockade is well established for systems such as GaAs QDs, but is to be further explored for systems with additional degrees of freedom, such as the valley quantum numbers in carbon-based materials or silicon. Here we report experiments on coupled bilayer graphene double quantum dots, in which the spin and valley states are precisely controlled, enabling the observation of the two-electron combined blockade physics. We demonstrate that the doubly occupied single dot switches between two different ground states with gate and magnetic-field tuning, allowing for the switching of selection rules: with a spin-triplet--valley-singlet ground state, valley-blockade is observed; and with the spin-singlet--valley-triplet ground state, robust spin blockade is shown.
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Submitted 10 February, 2022; v1 submitted 8 June, 2021;
originally announced June 2021.
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In-plane selective area InSb-Al nanowire quantum networks
Authors:
Roy L. M. Op het Veld,
Di Xu,
Vanessa Schaller,
Marcel A. Verheijen,
Stan M. E. Peters,
Jason Jung,
Chuyao Tong,
Qingzhen Wang,
Michiel W. A. de Moor,
Bart Hesselmann,
Kiefer Vermeulen,
Jouri D. S. Bommer,
Joon Sue Lee,
Andrey Sarikov,
Mihir Pendharkar,
Anna Marzegalli,
Sebastian Koelling,
Leo P. Kouwenhoven,
Leo Miglio,
Chris J. Palmstrøm,
Hao Zhang,
Erik P. A. M. Bakkers
Abstract:
Strong spin-orbit semiconductor nanowires coupled to a superconductor are predicted to host Majorana zero modes. Exchange (braiding) operations of Majorana modes form the logical gates of a topological quantum computer and require a network of nanowires. Here, we develop an in-plane selective-area growth technique for InSb-Al semiconductor-superconductor nanowire networks with excellent quantum tr…
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Strong spin-orbit semiconductor nanowires coupled to a superconductor are predicted to host Majorana zero modes. Exchange (braiding) operations of Majorana modes form the logical gates of a topological quantum computer and require a network of nanowires. Here, we develop an in-plane selective-area growth technique for InSb-Al semiconductor-superconductor nanowire networks with excellent quantum transport properties. Defect-free transport channels in InSb nanowire networks are realized on insulating, but heavily mismatched InP substrates by 1) full relaxation of the lattice mismatch at the nanowire/substrate interface on a (111)B substrate orientation, 2) nucleation of a complete network from a single nucleation site, which is accomplished by optimizing the surface diffusion length of the adatoms. Essential quantum transport phenomena for topological quantum computing are demonstrated in these structures including phase-coherent transport up to 10 $μ$m and a hard superconducting gap accompanied by 2$e$-periodic Coulomb oscillations with an Al-based Cooper pair island integrated in the nanowire network.
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Submitted 11 March, 2021;
originally announced March 2021.
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Kondo effect and spin-orbit coupling in graphene quantum dots
Authors:
Annika Kurzmann,
Yaakov Kleeorin,
Chuyao Tong,
Rebekka Garreis,
Angelika Knothe,
Marius Eich,
Christopher Mittag,
Carolin Gold,
Folkert K. de Vries,
Kenji Watanabe,
Takashi Taniguchi,
Vladimir Fal'ko,
Yigal Meir,
Thomas Ihn,
Klaus Ensslin
Abstract:
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 - re…
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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.
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Submitted 8 March, 2021;
originally announced March 2021.
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Shell Filling and Trigonal Warping in Graphene Quantum Dots
Authors:
Rebekka Garreis,
Angelika Knothe,
Chuyao Tong,
Marius Eich,
Carolin Gold,
Kenji Watanabe,
Takashi Taniguchi,
Vladimir Fal'ko,
Thomas Ihn,
Klaus Ensslin,
Annika Kurzmann
Abstract:
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 dis…
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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.
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Submitted 12 April, 2021; v1 submitted 16 November, 2020;
originally announced November 2020.
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Tunable valley splitting and bipolar operation in graphene quantum dots
Authors:
Chuyao Tong,
Rebekka Garreis,
Angelika Knothe,
Marius Eich,
Agnese Sacchi,
Kenji Watanabe,
Takashi Taniguchi,
Vladimir Fal'ko,
Thomas Ihn,
Klaus Ensslin,
Annika Kurzmann
Abstract:
Quantum states in graphene are four-fold degenerate: two fold in spins, and two fold in valleys.Both degrees of freedom can be utilized for qubit preparations. In our bilayer graphene quantumdots, we demonstrate that the valley g-factorgv, defined analogously as the spin g-factorgsforvalley splitting in perpendicular magnetic field, is tunable by over a factor of 4 from 20 to 90. Wefind that large…
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Quantum states in graphene are four-fold degenerate: two fold in spins, and two fold in valleys.Both degrees of freedom can be utilized for qubit preparations. In our bilayer graphene quantumdots, we demonstrate that the valley g-factorgv, defined analogously as the spin g-factorgsforvalley splitting in perpendicular magnetic field, is tunable by over a factor of 4 from 20 to 90. Wefind that largergvresults from larger electronic dot sizes, determined from the charging energy.This control is achieved by adjusting voltages on merely two gates, which also allows for tuning ofthe dot-lead tunnel coupling. On our versatile device, bipolar operation, charging our quantum dotwith charge carriers of the same or the opposite polarity as the leads, can be performed. Dots ofboth polarity are tunable to the first charge carrier by action of the plunger gate, such that thetransition from an electron to a hole dot can be observed. By adding more gates, this system caneasily be extended to host double dots.
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Submitted 9 September, 2020;
originally announced September 2020.
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Gap Opening in Twisted Double Bilayer Graphene by Crystal fields
Authors:
Peter Rickhaus,
Giulia Zheng,
Jose L. Lado,
Yongjin Lee,
Annika Kurzmann,
Marius Eich,
Riccardo Pisoni,
Chuyao Tong,
Rebekka Garreis,
Carolin Gold,
Michele Masseroni,
Takashi Taniguchi,
Kenji Wantanabe,
Thomas Ihn,
and Klaus Ensslin
Abstract:
Crystal fields occur due to a potential difference between chemically different atomic species. In Van-der-Waals heterostructures such fields are naturally present perpendicular to the planes. It has been realized recently that twisted graphene multilayers provide powerful playgrounds to engineer electronic properties by the number of layers, the twist angle, applied electric biases, electronic in…
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Crystal fields occur due to a potential difference between chemically different atomic species. In Van-der-Waals heterostructures such fields are naturally present perpendicular to the planes. It has been realized recently that twisted graphene multilayers provide powerful playgrounds to engineer electronic properties by the number of layers, the twist angle, applied electric biases, electronic interactions and elastic relaxations, but crystal fields have not received the attention they deserve. Here we show that the bandstructure of large-angle twisted double bilayer graphene is strongly modified by crystal fields. In particular, we experimentally demonstrate that twisted double bilayer graphene, encapsulated between hBN layers, exhibits an intrinsic bandgap. By the application of an external field, the gaps in the individual bilayers can be closed, allowing to determine the crystal fields. We find that crystal fields point from the outer to the inner layers with strengths in the bottom (top) bilayer of -0.13 V/nm (0.12 V/nm). We show both by means of first principles calculations and low energy models that crystal fields open a band gap in the groundstate. Our results put forward a physical scenario in which a crystal field effect in carbon substantially impacts the low energy properties of twisted double bilayer graphene, suggesting that such contributions must be taken into account in other regimes to faithfully predict the electronic properties of twisted graphene multilayers.
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Submitted 4 November, 2019; v1 submitted 23 October, 2019;
originally announced October 2019.
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Excited states in bilayer graphene quantum dots
Authors:
A. Kurzmann,
M. Eich,
H. Overweg,
M. Mangold,
F. Herman,
P. Rickhaus,
R. Pisoni,
Y. Lee,
R. Garreis,
C. Tong,
K. Watanabe,
T. Taniguchi,
K. Ensslin,
T. Ihn
Abstract:
We report on ground- and excited state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole g…
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We report on ground- and excited state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole ground-state to be a spin-triplet and valley-singlet state. This spin alignment can be seen as Hund's rule for a valley-degenerate system, which is fundamentally different to quantum dots in carbon nano tubes and GaAs-based quantum dots. The spin-singlet excited states are found to be valley-triplet states by tilting the magnetic field with respect to the sample plane. We quantify the exchange energy to be 0.35meV and measure a valley and spin g-factor of 36 and 2, respectively.
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Submitted 15 April, 2019;
originally announced April 2019.
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Tunneling induced dark states and controllable fluorescence spectrum in quantum-dot molecules
Authors:
Si-Cong Tian,
Ren-Gang Wan,
Cun-Zhu Tong,
Yong-Qiang Ning,
Li-Jun Wang
Abstract:
We theoretically investigate the spectrum of the fluorescence from triple quantum-dot molecules and demonstrate that it is possible to use tunneling to induce dark states. Unlike the atomic system, in quantum-dot molecules we can use tunneling to create the dark states and control fluorescence emission, requiring no coupling lasers. And interesting features such as quenching and narrowing of the f…
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We theoretically investigate the spectrum of the fluorescence from triple quantum-dot molecules and demonstrate that it is possible to use tunneling to induce dark states. Unlike the atomic system, in quantum-dot molecules we can use tunneling to create the dark states and control fluorescence emission, requiring no coupling lasers. And interesting features such as quenching and narrowing of the fluorescence can be obtained. We also explain the spectrum with the transition properties of the dressed states generated by the coupling of the laser and the two tunneling. The quenching of the fluorescence is due to the tunneling induced dark states, while the narrowing of the central peak is due to the slow decay rate of the dressed levels.
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Submitted 12 November, 2013;
originally announced November 2013.
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Effects of spontaneously generated coherence on resonance fluorescence from lateral triple quantum -dot molecules
Authors:
Si-Cong Tian,
Cun-Zhu Tong,
Chun-Liang Wang,
Yong-Qiang Ning,
Li-Jun Wang
Abstract:
We investigate the spectrum of the resonance fluorescence from the lateral triple quantum dots controlled by voltage and obtain some interesting features such as controllable triple narrow peaks. In our system we use tunneling instead of coupling lasers, and the positions, widths and heights of the resonance fluorescence peaks can be controlled by tuning the tunneling couplings. We explain the obs…
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We investigate the spectrum of the resonance fluorescence from the lateral triple quantum dots controlled by voltage and obtain some interesting features such as controllable triple narrow peaks. In our system we use tunneling instead of coupling lasers, and the positions, widths and heights of the resonance fluorescence peaks can be controlled by tuning the tunneling couplings. We explain the observed spectrum with the transition properties of the dressed states generated by the coupling of the two tunneling and the laser field. These features can also be viewed as the effects of Spontaneously Generated Coherence between the close-lying levels in the dressed state picture of the tunneling couplings. And the scheme proposed here can permit the observation of Spontaneously Generated Coherence.
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Submitted 17 October, 2013;
originally announced October 2013.
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Tunneling induced transparency and controllable group velocity in triple and multiple quantum-dot molecules
Authors:
Si-Cong Tian,
Cun-Zhu Tong,
Ren-Gang Wan,
Yong-Qiang Ning,
Li-Jun Wang
Abstract:
We analyze the interaction of a triple quantum dot molecules controlled by the tunneling coupling instead of coupling laser. A general analytic expression for the steady-state linear susceptibility for a probe-laser field is obtained and we show that the system can exhibit two transparency windows. The group velocity of the probe-laser pulse is also analyzed. By changing the tunneling couplings, t…
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We analyze the interaction of a triple quantum dot molecules controlled by the tunneling coupling instead of coupling laser. A general analytic expression for the steady-state linear susceptibility for a probe-laser field is obtained and we show that the system can exhibit two transparency windows. The group velocity of the probe-laser pulse is also analyzed. By changing the tunneling couplings, two laser pulses with different central frequency can propagate with the same group velocity. And the group velocity can be as low as 300 m/s in our system. We extend our analysis to the case of multiple quantum dot molecules (the number of the quantum dots is N) and show that the system can exhibit at most N-1 transparency windows. And at most N-1 laser pulses with different central frequencies can be slowed down.
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Submitted 17 October, 2013;
originally announced October 2013.
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Phase field modeling of wetting on structured surfaces
Authors:
Kaifu Luo,
Mikko-Pekka Kuittu,
Chaohui Tong,
Sami Majaniemi,
Tapio Ala-Nissila
Abstract:
We study the dynamics and equilibrium profile shapes of contact lines for wetting in the case of a spatially inhomogeneous solid wall with stripe defects. Using a phase-field model with conserved dynamics, we first numerically determine the contact line behavior in the case of a stripe defect of varying width. For narrow defects, we find that the maximum distortion of the contact line and the he…
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We study the dynamics and equilibrium profile shapes of contact lines for wetting in the case of a spatially inhomogeneous solid wall with stripe defects. Using a phase-field model with conserved dynamics, we first numerically determine the contact line behavior in the case of a stripe defect of varying width. For narrow defects, we find that the maximum distortion of the contact line and the healing length are related to the defect width, while for wide defects, they saturate to constant values. This behavior is in quantitative agreement with experimental data. In addition, we examine the shape of the contact line between two stripe defects as a function of their separation. Using the phase-field model, we also analytically estimate the contact line configuration, and find good qualitative agreement with the numerical results.
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Submitted 21 February, 2006;
originally announced February 2006.
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Interface Equations for Capillary Rise in Random Environment
Authors:
T. Laurila,
C. Tong,
S. Majaniemi,
T. Ala-Nissila
Abstract:
We consider the influence of quenched noise upon interface dynamics in 2D and 3D capillary rise with rough walls by using phase-field approach, where the local conservation of mass in the bulk is explicitly included. In the 2D case the disorder is assumed to be in the effective mobility coefficient, while in the 3D case we explicitly consider the influence of locally fluctuating geometry along a…
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We consider the influence of quenched noise upon interface dynamics in 2D and 3D capillary rise with rough walls by using phase-field approach, where the local conservation of mass in the bulk is explicitly included. In the 2D case the disorder is assumed to be in the effective mobility coefficient, while in the 3D case we explicitly consider the influence of locally fluctuating geometry along a solid wall using a generalized curvilinear coordinate transformation. To obtain the equations of motion for meniscus and contact lines, we develop a systematic projection formalism which allows inclusion of disorder. Using this formalism, we derive linearized equations of motion for the meniscus and contact line variables, which become local in the Fourier space representation. These dispersion relations contain effective noise that is linearly proportional to the velocity. The deterministic parts of our dispersion relations agree with results obtained from other similar studies in the proper limits. However, the forms of the noise terms derived here are quantitatively different from the other studies.
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Submitted 2 May, 2006; v1 submitted 20 January, 2006;
originally announced January 2006.
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Dynamics and Kinetic Roughening of Interfaces in Two-Dimensional Forced Wetting
Authors:
T. Laurila,
C. Tong,
I. Huopaniemi,
S. Majaniemi,
T. Ala-Nissila
Abstract:
We consider the dynamics and kinetic roughening of wetting fronts in the case of forced wetting driven by a constant mass flux into a 2D disordered medium. We employ a coarse-grained phase field model with local conservation of density, which has been developed earlier for spontaneous imbibition driven by a capillary forces. The forced flow creates interfaces that propagate at a constant average…
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We consider the dynamics and kinetic roughening of wetting fronts in the case of forced wetting driven by a constant mass flux into a 2D disordered medium. We employ a coarse-grained phase field model with local conservation of density, which has been developed earlier for spontaneous imbibition driven by a capillary forces. The forced flow creates interfaces that propagate at a constant average velocity. We first derive a linearized equation of motion for the interface fluctuations using projection methods. From this we extract a time-independent crossover length $ξ_\times$, which separates two regimes of dissipative behavior and governs the kinetic roughening of the interfaces by giving an upper cutoff for the extent of the fluctuations. By numerically integrating the phase field model, we find that the interfaces are superrough with a roughness exponent of $χ= 1.35 \pm 0.05$, a growth exponent of $β= 0.50 \pm 0.02$, and $ξ_\times \sim v^{-1/2}$ as a function of the velocity. These results are in good agreement with recent experiments on Hele-Shaw cells. We also make a direct numerical comparison between the solutions of the full phase field model and the corresponding linearized interface equation. Good agreement is found in spatial correlations, while the temporal correlations in the two models are somewhat different.
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Submitted 27 April, 2005;
originally announced April 2005.