Peter Rickhaus

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Rickhaus

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Peter

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0000-0003-3828-8153

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Publications 1 - 10 of 16

Electron transport in dual-gated three-layer
Masseroni, Michele; Davatz, Tim; Pisoni, Riccardo; et al. (2021)
Physical Review Research
The low-energy band structure of few-layer MoS2 is relevant for a large variety of experiments ranging from optics to electronic transport. Its characterization remains challenging due to complex multiband behavior. We investigate the conduction band of dual-gated three-layer MoS2 by means of magnetotransport experiments. The total carrier density is tuned by voltages applied between MoS2 and both top and bottom gate electrodes. For asymmetrically biased top and bottom gates, electrons accumulate in the layer closest to the positively biased electrode. In this way, the three-layer MoS2 can be tuned to behave electronically like a monolayer. In contrast, applying a positive voltage on both gates leads to the occupation of all three layers. Our analysis of the Shubnikov–de Haas oscillations originating from different bands lets us attribute the corresponding carrier densities in the top and bottom layers. We find a twofold Landau level degeneracy for each band, suggesting that the minima of the conduction band lie at the ±K points of the first Brillouin zone. This is in contrast to band structure calculations for zero layer asymmetry, which report minima at the Q points. Even though the interlayer tunnel coupling seems to leave the low-energy conduction band unaffected, we observe scattering of electrons between the outermost layers for zero layer asymmetry. The middle layer remains decoupled due to the spin-valley symmetry, which is inverted for neighboring layers. When the bands of the outermost layers are energetically in resonance, interlayer scattering takes place, leading to an enhanced resistance and to magneto-interband oscillations.
Quantum capacitive coupling between large-angle twisted graphene layers
Mrenca-Kolasinska, Alina; Rickhaus, Peter; Zheng, Giulia; et al. (2022)
2D Materials
Large-angle twisted bilayer graphene (tBLG) is known to be electronically decoupled due to the spatial separation of the Dirac cones corresponding to individual graphene layers in the reciprocal space. The close spacing between the layers causes strong capacitive coupling, opening possibilities for applications in atomically thin devices. Here, we present a self-consistent quantum capacitance model for the electrostatics of decoupled graphene layers, and further generalize it to deal with decoupled tBLG at finite magnetic field and large-angle twisted double bilayer graphene at zero magnetic field. We probe the capacitive coupling through the conductance, showing good agreement between simulations and experiments for all the systems considered. We also propose a new experiment utilizing the decoupling effect to induce a huge and tunable bandgap in bilayer graphene by applying a moderately low bias. Our model can be extended to systems composed of decoupled graphene multilayers as well as non-graphene systems, opening a new realm of quantum-capacitively coupled materials.
Combined Minivalley and Layer Control in Twisted Double Bilayer Graphene
de Vries, Folkert Kornelis; Zhu, Jihang; Portolés, Elías; et al. (2020)
Physical Review Letters
Control over minivalley polarization and interlayer coupling is demonstrated in double bilayer graphene twisted with an angle of 2.37°. This intermediate angle is small enough for the minibands to form and large enough such that the charge carrier gases in the layers can be tuned independently. Using a dual-gated geometry we identify and control all possible combinations of minivalley polarization via the population of the two bilayers. An applied displacement field opens a band gap in either of the two bilayers, allowing us to even obtain full minivalley polarization. In addition, the carriers, formerly separated by their minivalley character, are mixed by tuning through a Lifshitz transition, where the Fermi surface topology changes. The high degree of control over the minivalley character of the bulk charge transport in twisted double bilayer graphene offers new opportunities for realizing valleytronics devices such as valley valves, filters, and logic gates.
Kagome Quantum Oscillations in Graphene Superlattices
de Vries, Folkert K.; Slizovskiy, Sergey; Tomic, Petar; et al. (2024)
Nano Letters
Electronic spectra of solids subjected to a magnetic field are often discussed in terms of Landau levels and Hofstadter-butterfly-style Brown-Zak minibands manifested by magneto-oscillations in two-dimensional electron systems. Here, we present the semiclassical precursors of these quantum magneto-oscillations which appear in graphene superlattices at low magnetic field near the Lifshitz transitions and persist at elevated temperatures. These oscillations originate from Aharonov-Bohm interference of electron waves following open trajectories that belong to a kagome-shaped network of paths characteristic for Lifshitz transitions in the moire superlattice minibands of twistronic graphenes.
Gap Opening in Twisted Double Bilayer Graphene by Crystal Fields
Rickhaus, Peter; Zheng, Giulia; Lado, Jose L.; et al. (2019)
Nano Letters
Gate-defined superconducting channel in magic-angle twisted bilayer graphene
Zheng, Giulia; Portolés, Elías; Mestre-Torà, Alexandra; et al. (2024)
Physical Review Research
Magic-angle twisted bilayer graphene (MATBG) combines in one single material different phases such as insulating, metallic, and superconducting. These phases and their in situ tunability make MATBG an important platform for the fabrication of superconducting devices. We realize a split-gate-defined geometry which enables us to tune the width of a superconducting channel formed in MATBG. We observe a smooth transition from superconductivity to highly resistive transport by progressively reducing the channel width using the split gates or by reducing the density in the channel. Using the gate-defined constriction, we control the flow of the supercurrent, either guiding it through the constriction or throughout the whole device or even blocking its passage completely. This serves as a foundation for developing quantum constriction devices such as superconducting quantum point contacts, quantum dots, and Cooper-pair boxes in MATBG.
Gate-defined Josephson junctions in magic-angle twisted bilayer graphene
de Vries, Folkert K.; Portolés, Elías; Zheng, Giulia; et al. (2021)
Nature Nanotechnology
In situ electrostatic control of two-dimensional superconductivity1 is commonly limited due to large charge carrier densities, and gate-defined Josephson junctions are therefore rare2,3. Magic-angle twisted bilayer graphene (MATBG)4,5,6,7,8 has recently emerged as a versatile platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal9,10,11,12,13,14. Although MATBG appears to be an ideal two-dimensional platform for gate-tunable superconductivity9,11,13, progress towards practical implementations has been hindered by the need for well-defined gated regions. Here we use multilayer gate technology to create a device based on two distinct phases in adjustable regions of MATBG. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable d.c. and a.c. Josephson effects15,16. The ability to tune the superconducting state within a single material circumvents interface and fabrication challenges, which are common in multimaterial nanostructures. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics17,18 and quantum information technology19,20.
Quasiparticle and superfluid dynamics in Magic-Angle Graphene
Portolés, Elías; Perego, Marta; Volkov, Pavel A.; et al. (2025)
Nature Communications
Magic-Angle Twisted Bilayer Graphene (MATBG) shows a wide range of correlated phases which are electrostatically tunable. Despite a growing knowledge of the material, there is yet no consensus on the microscopic mechanisms driving its superconducting phase. A major obstacle to progress in this direction is that key thermodynamic properties, such as specific heat, electron-phonon coupling and superfluid stiffness, are challenging to measure due to the 2D nature of the material and its relatively low energy scales. Here, we use a gate-defined, radio frequency-biased, Josephson junction to probe the electronic dynamics of MATBG. We demonstrate evidence for two processes determining the low-frequency dynamics across the phase diagram: thermalization of electronic quasiparticles through phonon scattering and inductive response of the superconducting condensate. A phenomenological approach allows us to relate the experimentally observed dynamics to several thermodynamic properties of MATBG, including electron-phonon coupling and superfluid stiffness. Our findings support anisotropic or nodal superconductivity in MATBG and demonstrate a broadly applicable method for studying properties of 2D materials with out-of-equilibrium nanodevice dynamics.
Tunable Valley Splitting due to Topological Orbital Magnetic Moment in Bilayer Graphene Quantum Point Contacts
Lee, Yongjin; Knothe, Angelika; Overweg, Hiske; et al. (2020)
Physical Review Letters
Correlated electron-hole state in twisted double-bilayer graphene
Rickhaus, Peter; de Vries, Folkert K.; Zhu, Jihang; et al. (2021)
Science
When twisted to angles near 1°, graphene multilayers provide a window on electron correlation physics. Here, we report the discovery of a correlated electron-hole state in double-bilayer graphene twisted to 2.37°. At this angle, the moiré states retain much of their isolated bilayer character, allowing their bilayer projections to be separately controlled by gates. We use this property to generate an energetic overlap between narrow isolated electron and hole bands with good nesting properties. Our measurements reveal the formation of ordered states with reconstructed Fermi surfaces, consistent with a density-wave state. This state can be tuned without introducing chemical dopants, enabling studies of correlated electron-hole states and their interplay with superconductivity.

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Peter Rickhaus

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