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Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 937 records · Page 52Linked to original sources
arXiv2023

Interference of chiral Andreev edge states

The search for topological excitations such as Majorana fermions has spurred interest in the boundaries between distinct quantum states. Here, we explore an interface between two prototypical phases of electrons with conceptually different ground states: the integer quantum Hall insulator and the s-wave superconductor. We find clear signatures of hybridized electron and hole states similar to chiral Majorana fermions, to which we refer as chiral Andreev edge states (CAES). They propagate along the interface in the direction determined by magnetic field and their interference can turn an incoming electron into an outgoing electron or a hole, depending on the phase accumulated by the CAES along their path. Our results demonstrate that these excitations can propagate and interfere over a significant length, opening future possibilities for their coherent manipulation.

cond-mat.mes-hall↗
arXiv2023

Loss and decoherence at the quantum Hall - superconductor interface

We perform a systematic study of Andreev conversion at the interface between a superconductor and graphene in the quantum Hall (QH) regime. We find that the probability of Andreev conversion from electrons to holes follows an unexpected but clear trend: the dependencies on temperature and magnetic field are nearly decoupled. We discuss these trends and the role of the superconducting vortices, whose normal cores could both absorb and dephase the individual electrons in a QH edge. Our study may pave the road to engineering future generation of hybrid devices for exploiting superconductivity proximity in chiral channels.

cond-mat.mes-hall↗
arXiv2023

Nanoscale confinement and control of excitonic complexes in a monolayer WSe2

Nanoscale control and observation of photophysical processes in semiconductors is critical for basic understanding and applications from optoelectronics to quantum information processing. In particular, there are open questions and opportunities in controlling excitonic complexes in two-dimensional materials such as excitons, trions or biexcitons. However, neither conventional diffraction-limited optical spectroscopy nor lithography-limited electric control provides a proper tool to investigate these quasiparticles at the nanometer-scale at cryogenic temperature. Here, we introduce a cryogenic capacitive confocal optical microscope (C3OM) as a tool to study quasiparticle dynamics at the nanometer scale. Using a conductive atomic force microscope (AFM) tip as a gate electrode, we can modulate the electronic doping at the nanometer scale in WSe2 at 4K. This tool allows us to modulate with nanometer-scale confinement the exciton and trion peaks, as well a distinct photoluminescence line associated with a larger excitonic complex that exhibits distinctive nonlinear optical response. Our results demonstrate nanoscale confinement and spectroscopy of exciton complexes at arbitrary positions, which should prove an important tool for quantitative understanding of complex optoelectronic properties in semiconductors as well as for applications ranging from quantum spin liquids to superresolution measurements to control of quantum emitters.

cond-mat.mes-hall↗
arXiv2023

Atomic engineering of interfacial polarization switching in van der Waals multilayers

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here, we show that introducing interlayer rotations in multilayer vdW heterostructures modulates both the spatial ordering and switching dynamics of polar domains, engendering unique tunability that is unparalleled in conventional bulk ferroelectrics or polar bilayers. Using operando transmission electron microscopy we show how changing the relative rotations of three WSe2 layers produces structural polytypes with distinct arrangements of polar domains, leading to either a global or localized switching response. Introducing uniaxial strain generates structural anisotropy that yields a range of switching behaviors, coercivities, and even tunable biased responses. We also provide evidence of physical coupling between the two interfaces of the trilayer, a key consideration for controlling switching dynamics in polar multilayer structures more broadly.

cond-mat.mtrl-sci↗
arXiv2023

Room-temperature correlated states in twisted bilayer MoS$_2$

Moiré superlattices have emerged as an exciting condensed-matter quantum simulator for exploring the exotic physics of strong electronic correlations. Notable progress has been witnessed, but such correlated states are achievable usually at low temperatures. Here, we report the transport evidences of room-temperature correlated electronic states and layer-hybridized SU(4) Hubbard model simulator in AB-stacked MoS$_2$ homo-bilayer moiré superlattices. Correlated insulating states at moiré band filling factors v = 1, 2, 3 are unambiguously established in twisted bilayer MoS$_2$. Remarkably, the correlated electronic states can persist up to a record-high critical temperature of over 285 K. The realization of room-temperature correlated states in twisted bilayer MoS$_2$ can be understood as the cooperation effects of the stacking-specific atomic reconstruction and the resonantly enhanced interlayer hybridization, which largely amplify the moiré superlattice effects on electronic correlations. Furthermore, extreme large non-linear Hall responses up to room-temperature are uncovered near correlated insulating states, demonstrating the quantum geometry of moiré flat conduction band.

cond-mat.mtrl-sci↗
arXiv2023

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, we combine lattice temperature, electron doping, and all-optical tuning of third-harmonic generation in a hBN-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. Our work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors and frequency converters.

cond-mat.mes-hall↗
arXiv2023

Non-linear Landau fan diagram for graphene electrons exposed to a moiré potential

Due to Landau quantization, the conductance of two-dimensional electrons exposed to a perpendicular magnetic field exhibits oscillations that generate a fan of linear trajectories when plotted in the parameter space spanned by density and magnetic field. This fan looks identical irrespective of the electron dispersion details that determines the field dependence of the Landau level energy. This is no surprise, since the position of conductance minima solely depends on the level degeneracy which is linear in flux. The fractal energy spectrum that emerges within each Landau band when electrons are also exposed to a two-dimensional superlattice potential produces numerous additional oscillations, but they too create just linear fans for the same reason. Here, we report on conductance oscillations of graphene electrons exposed to a moiré potential that defy this general rule of flux linearity and attribute the anomalous behavior to the simultaneous occupation of multiple minibands and magnetic breakdown.

cond-mat.mes-hall↗
arXiv2023

One-dimensional moire chains with partially-filled flat bands in two-dimensional twisted bilayer WSe2

Two-dimensional (2D) moire systems based on twisted bilayer graphene and transition metal dichalcogenides provide a promising platform to investigate emergent phenomena driven by strong electron-electron interactions in partially-filled flat bands1-11. A natural question arises: is it possible to expand the 2D correlated moire physics to one-dimensional (1D)? This requires selectively doping of 1D moire chain embedded in the 2D moire systems, which is an outstanding challenge in experiment and seems to be not within the grasp of today's technology. Therefore, an experimental demonstration of the 1D moire chain with partially-filled flat bands remains absent. Here we show that we can introduce 1D boundaries, separating two regions with different twist angles, in twisted bilayer WSe2 (tWSe2) by using scanning tunneling microscopy (STM), and demonstrate that the flat bands of moire sites along the 1D boundaries can be selectively filled. The charge and discharge states of correlated moire electrons in the 1D moire chain can be directly imaged and manipulated by combining a back-gate voltage with the STM bias. Our results open the door for realizing new correlated electronic states of the 1D moire chain in 2D systems.

cond-mat.mtrl-sci↗
arXiv2023

Coherent Charge Oscillations in a Bilayer Graphene Double Quantum Dot

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.

cond-mat.mes-hall↗
arXiv2023

Selective Manipulation and Tunneling Spectroscopy of Broken-Symmetry Quantum Hall States in a Hybrid-edge Quantum Point Contact

We present a device architecture of hybrid-edge and dual-gated quantum point contact. We demonstrate improved electrostatic control over the separation, position, and coupling of each broken-symmetry compressible strip in graphene. Via low-temperature magneto-transport measurement, we demonstrate selective manipulation over the evolution, hybridization, and transmission of arbitrarily chosen quantum Hall states in the channel. With gate-tunable tunneling spectroscopy, we characterize the energy gap of each symmetry-broken quantum Hall state with high resolution on the order of ~0.1 meV.

cond-mat.mes-hall↗
arXiv2023

Full-dry flipping transfer method for van der Waals heterostructure

We present a novel flipping transfer method for van der Waals heterostructures, offering a significant advancement over previous techniques by eliminating the need for polymers and solvents. Here, we utilize commercially available gel film and control its stickiness through oxygen plasma and UV-Ozone treatment, also effectively removing residues from the gel film surface. The cleanliness of the surface is verified through atomic force microscopy. We investigate the quality of our fabricated devices using magnetotransport measurements on graphene/hBN and graphene/α-RuCl3 heterostructures. Remarkably,graphene/hBN devices produced with the flipping method display quality similar to that of fully encapsulated devices. This is evidenced by the presence of a symmetry-broken state at 1 T. Additionally, features of the Hofstadter butterfly were also observed in the second devices. In the case of graphene/α-RuCl3, we observe quantum oscillations with a beating mode and two-channel conduction, consistent with fully encapsulated devices.

cond-mat.mtrl-sci↗
arXiv2023

Non-volatile electrical polarization switching via domain wall release in 3R-MoS$_2$ bilayer

Understanding the nature of sliding ferroelectricity is of fundamental importance for the discovery and application of two-dimensional ferroelectric materials. In this work, we investigate the phenomenon of switchable polarization in a bilayer MoS$_2$ with a natural rhombohedral stacking, where the spontaneous polarization is coupled with excitonic effects through an asymmetric interlayer coupling. Using optical spectroscopy and imaging techniques, we observe how a released domain wall switches the polarization of a large single domain. Our results highlight the importance of domain walls in the polarization switching of non-twisted rhombohedral transition metal dichalcogenides and open new opportunities for the non-volatile control of their optical response.

cond-mat.mes-hall↗
arXiv2023

Optically induced symmetry breaking due to nonequilibrium steady state formation in charge density wave material 1T-TiSe2

The strongly correlated charge density wave (CDW) phase of 1T-TiSe$_2$ is being extensively researched to verify the claims of a unique chiral order due to the presence of three equivalent Fermi wavevectors involved in the CDW formation. Characterization of the symmetries is therefore critical to understand the origin of their intriguing properties but can be complicated by the coupling of the electronic and lattice degrees of freedom. Here we use continuous wave laser excitation to probe the symmetries of TiSe$_2$ using the circular photogalvanic effect with very high sensitivity. We observe that the ground state of the CDW phase is achiral. However, laser excitation above a threshold intensity transforms TiSe$_2$ into a chiral phase in a nonequilibrium steady state, which changes the electronic correlations in the stacking direction of the layered material. The inherent sensitivity of the photogalvanic technique provides clear evidence of the different optically driven phases of 1T-TiSe$_2$, as well as emphasizes the interplay of electronic and lattice degrees of freedom in this system under optical excitation. Our work demonstrates that optically induced phase change can occur at extremely low optical intensities in strongly correlated materials, providing a pathway for future studies to engineer new phases using light.

cond-mat.str-el↗
arXiv2023

Tunable Inter-Moiré Physics in Consecutively-Twisted Trilayer Graphene

We fabricate a twisted trilayer graphene device with consecutive twist angles of 1.33 and 1.64 degrees, in which we electrostatically tune the electronic states from each of the two co-existing moiré superlattices and the interactions between them. When both moiré superlattices contribute equally to electrical transport, we report a new type of inter-moiré Hofstadter butterfly. Its Brown-Zak oscillation corresponds to one of the intermediate quasicrystal length scales of the reconstructed moiré of moiré (MoM) superlattice, shedding new light on emergent physics from competing atomic orders.

cond-mat.mes-hall↗
arXiv2023

Probing the tunable multi-cone bandstructure in Bernal bilayer graphene

Controlling the bandstructure of Dirac materials is of wide interest in current research but has remained an outstanding challenge for systems such as monolayer graphene. In contrast, Bernal bilayer graphene (BLG) offers a highly flexible platform for tuning the bandstructure, featuring two distinct regimes. In one regime, which is well established and widely used, a tunable bandgap is induced by a large enough transverse displacement field. Another is a gapless metallic band occurring near charge neutrality and at not too strong fields, featuring rich 'fine structure' consisting of four linearly-dispersing Dirac cones with opposite chiralities in each valley and van Hove singularities. Even though BLG was extensively studied experimentally in the last two decades, the evidence of this exotic bandstructure is still elusive, likely due to insufficient energy resolution. Here, rather than probing the bandstructure by direct spectroscopy, we use Landau levels as markers of the energy dispersion and carefully analyze the Landau level spectrum in a regime where the cyclotron orbits of electrons or holes in momentum space are small enough to resolve the distinct mini Dirac cones. We identify the presence of four distinct Dirac cones and map out complex topological transitions induced by electric displacement field. These findings introduce a valuable addition to the toolkit for graphene electronics.

cond-mat.mes-hall↗
arXiv2023

Valley-hybridized gate-tunable 1D exciton confinement in MoSe2

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the fields of quantum photonics and optoelectronics. Achieving this control holds great potential for unlocking strong exciton-exciton interaction regimes, enabling exciton-based logic operations, exploring exotic quantum phases of matter, facilitating deterministic positioning and tuning of quantum emitters, and designing advanced optoelectronic devices. Monolayers of transition metal dichalcogenides (TMDs) offer inherent two-dimensional confinement and possess significant binding energies, making them particularly promising candidates for achieving electric-field-based confinement of excitons without dissociation. While previous exciton engineering strategies have predominantly focused on local strain gradients, the recent emergence of electrically confined states in TMDs has paved the way for novel approaches. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering. Here, we show electric control of light polarization emitted from one-dimensional (1D) quantum confined states in MoSe2. By employing non-uniform in-plane electric fields, we demonstrate the in-situ tuning of the trapping potential and reveal how gate-tunable valley-hybridization gives rise to linearly polarized emission from these localized states. Remarkably, the polarization of the localized states can be entirely engineered through either the spatial geometry of the 1D confinement potential or the application of an out-of-plane magnetic field.

cond-mat.mes-hall↗
arXiv2023

Imaging Self-aligned Moiré Crystals and Quasicrystals in Magic-angle Bilayer Graphene on hBN Heterostructures

Using scanning-tunneling-microscopy and theoretical modeling on heterostructures of twisted bilayer graphene and hexagonal Boron-Nitride, we show that the emergent super-moire structures display a rich landscape of moire-crystals and quasicrystals. We reveal a phase-diagram comprised of commensurate moire-crystals embedded in swaths of moire quasicrystals. The 1:1 commensurate crystal, expected to be a Chern insulator, should only exist at one point on the phase-diagram, implying that it ought to be practically undetectable. Surprisingly we find that the commensurate crystals exist over a much wider than predicted range, providing evidence of an unexpected self-alignment mechanism that is explained using an elastic-network model. The remainder of the phase-diagram, where we observe tunable quasicrystals, affords a new platform for exploring the unique electronic-properties of these rarely found in nature structures.

cond-mat.mes-hall↗
arXiv2023

Engineering 2D material exciton lineshape with graphene/h-BN encapsulation

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Remarkable advances have been achieved through alloying, chemical and electrical doping, and applied strain. However, the integration of TMDs with other 2D materials in van der Waals heterostructures (vdWHs) to tailor novel functionalities remains largely unexplored. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton lineshape and charge state. Fano-like asymmetric spectral features are produced in WS$_{2}$, MoSe$_{2}$ and WSe$_{2}$ vdWHs combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe$_{2}$/graphene with a neutral exciton redshift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron-beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

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