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Watanabe, Kenji

Publications and source records attributed to Watanabe, Kenji.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe 2 -WS 2 -WSe 2 trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe 2 valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe 2 layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe 2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Polaritonic Probe of an Emergent 2D Dipole Interface

The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using α-RuCl 3 as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of α-RuCl 3 adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energetically with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Finally, our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrahigh supercurrent density in a two-dimensional topological material

Ongoing advances in superconductors continue to revolutionize technology thanks to the increasingly versatile and robust availability of lossless supercurrents. In particular, high supercurrent density can lead to more efficient and compact power transmission lines, high-field magnets, as well as high-performance nanoscale radiation detectors and superconducting spintronics. In this work, we report the discovery of an unprecedentedly high superconducting critical current density (17 MA / cm 2 at 0 T and 7 MA / cm 2 at 8 T) in 1T'–WS 2 , exceeding those of all reported two-dimensional superconductors to date. 1T'–WS 2 features a strongly anisotropic (both in- and out-of-plane) superconducting state that violates the Pauli paramagnetic limit signaling the presence of unconventional superconductivity. Spectroscopic imaging of the vortices further substantiates the anisotropic nature of the superconducting state. More intriguingly, the normal state of 1T'–WS 2 carries topological properties. The band structure obtained via angle-resolved photoemission spectroscopy and first-principles calculations points to a Z 2 topological invariant. The concomitance of topology and superconductivity in 1T'–WS 2 establishes it as a topological superconductor candidate, which is promising for the development of quantum computing technology.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Programming correlated magnetic states with gate-controlled moiré geometry

The ability to control the underlying lattice geometry of a system may enable transitions between emergent quantum ground states. We report in situ gate switching between honeycomb and triangular lattice geometries of an electron many-body Hamiltonian in rhombohedral (R)–stacked molybdenum ditelluride (MoTe 2 ) moiré bilayers, resulting in switchable magnetic exchange interactions. At zero electric field, we observed a correlated ferromagnetic insulator near one hole per moiré unit cell with a widely tunable Curie temperature up to 14 K. Applying an electric field switched the system into a half-filled triangular lattice with antiferromagnetic interactions; further doping this layer-polarized superlattice tuned the antiferromagnetic exchange interaction back to ferromagnetic. Our work demonstrates R-stacked MoTe 2 moirés to be a laboratory for engineering correlated states with nontrivial topology.

Science & Technology - Other Topics↗
DOE OSTI2023

Magnetic Field-Stabilized Wigner Crystal States in a Graphene Moiré Superlattice

ABC-stacked trilayer graphene on boron nitride (ABC-TLG/hBN) moiré superlattices provides a tunable platform for exploring Wigner crystal states in which the electron correlation can be controlled by electric and magnetic fields. Here, in this work, we report the observation of magnetic field-stabilized Wigner crystal states in a ABC-TLG/hBN. We show that correlated insulating states emerge at multiple fractional and integer fillings corresponding to ν = 1/3, 2/3, 1, 4/3, 5/3, and 2 electrons per moiré lattice site under a magnetic field. These correlated insulating states can be attributed to generalized Mott states for the integer fillings and generalized Wigner crystal states for the fractional fillings. The generalized Wigner crystal states are stabilized by a vertical magnetic field and are strongest at one magnetic flux quantum per three moiré superlattices. The ν = 2 insulating state persists up to 30 T, which can be described by a Mott–Hofstadter transition at a high magnetic field.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Broken symmetries and excitation spectra of interacting electrons in partially filled Landau levels

Interacting electrons in flat bands give rise to a variety of quantum phases. One fundamental aspect of such states is the ordering of the various flavours – such as spin or valley – that the electrons can undergo and the excitation spectrum of the broken symmetry states that they form. These properties cannot be probed directly with electrical transport measurements. The zeroth Landau level of monolayer graphene with four-fold spin-valley degeneracy is a model system for such investigations, but the nature of its broken symmetry states – particularly at partial fillings – is still not understood. Here, in this study, we demonstrate a non-invasive spectroscopic technique with a scanning tunneling microscope and use it to perform measurements of the valley polarization of the electronic wavefunctions and their excitation spectrum in the partially filled zeroth Landau level of graphene. We can extract information such as the strength of Haldane pseudopotentials that characterize the repulsive interactions underlying the fractional quantum states. Our experiments also demonstrate that fractional quantum Hall phases are built upon broken symmetry states that persist at partial filling. Our experimental approach quantifies the valley phase diagram of the partially filled Landau level as a model flat band platform which is applicable to other graphene-based electronic systems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2023

Twisted cuprate van der Waals heterostructures with controlled Josephson coupling

Twisted van derWaals (vdW) heterostructures offer a unique platform for engineering the efficient Josephson coupling between cuprate thin crystals harboring the nodal superconducting order parameter. Preparing the vdW heterostructures-based Josephson junction comprising stacked cuprates requires maintaining an ordered interface with preserved surface superconductivity. Here, we report the preparation of the Josephson junction out of the stacked Bi 2 Sr 2 CaCu 2 O 8+d crystals using the cryogenic dry transfer technique and encapsulating the junction with an insulating layer, that protects the interface during the electrical contacts evaporation at the 1×10 -6 mbar base pressure. We find that the Josephson critical current $I$ c has a maximum at low twist angles, comparable to that of the bulk intrinsic Josephson junctions, and is reduced by two orders of magnitude at twist angles close to 45°. In conclusion, the reduction of $I$ c occurs due to a mismatch between superconducting $d$-wave order parameters, which suppresses the direct Cooper pair tunneling.

2D materials↗
DOE OSTI2023

Nanoscale electrostatic control in ultraclean van der Waals heterostructures by local anodic oxidation of graphite gates

In an all-van der Waals heterostructure, the active layer, gate dielectrics and gate electrodes are assembled from two-dimensional crystals that have a low density of atomic defects. This design allows two-dimensional electron systems with very low disorder to be created, particularly in heterostructures where the active layer also has intrinsically low disorder, such as crystalline graphene layers or metal dichalcogenide heterobilayers. A key missing ingredient has been nanoscale electrostatic control, with existing methods for fabricated local gates typically introducing unwanted contamination. Here we describe a resist-free local anodic oxidation process for patterning sub-100 nm features in graphite gates, and their subsequent integration into an all-van der Waals heterostructure. We define a quantum point contact in the fractional quantum Hall regime as a benchmark device and observe signatures of chiral Luttinger liquid behaviour, indicating an absence of extrinsic scattering centres in the vicinity of the point contact. In the integer quantum Hall regime, we demonstrate in situ control of the edge confinement potential, a key requirement for the precision control of chiral edge states. In conclusion, this technique may enable the fabrication of devices capable of single anyon control and coherent edge-state interferometry in the fractional quantum Hall regime.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Observation of half-integer Shapiro steps in graphene Josephson junctions

We study quantum transport and AC Josephson effect of hexagonal boron nitride encapsulated graphene (BGB) Josephson junctions (JJs). Our experiments reveal the emergence of the half-integer Shapiro steps in the n-type regime with high electron carrier densities. We attribute this observation to the gate-tunable transmission probability of the graphene junction. Furthermore, our numerical simulations are consistent with the appearance of half-integer Shapiro steps at high transparency, which suggests a skewed current phase relationship in the graphene JJ.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Imaging Field–Driven Melting of a Molecular Solid at the Atomic Scale

Solid–liquid phase transitions are basic physical processes, but atomically resolved microscopy has yet to capture their full dynamics. A new technique is developed for controlling the melting and freezing of self–assembled molecular structures on a graphene field–effect transistor (FET) that allows phase–transition behavior to be imaged using atomically resolved scanning tunneling microscopy. This is achieved by applying electric fields to 2,3,5,6–tetrafluoro–7,7,8,8–tetracyanoquinodimethane–decorated FETs to induce reversible transitions between molecular solid and liquid phases at the FET surface. Nonequilibrium melting dynamics are visualized by rapidly heating the graphene substrate with an electrical current and imaging the resulting evolution toward new 2D equilibrium states. An analytical model is developed that explains observed mixed–state phases based on spectroscopic measurement of solid and liquid molecular energy levels. The observed nonequilibrium melting dynamics are consistent with Monte Carlo simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure

Quantum geometry in condensed matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in 2D electron gases and the anomalous Hall effect (AHE) in ferromagnets, quantum metric has rarely been explored. Here, in this paper, we report a nonlinear Hall effect induced by quantum metric dipole by interfacing even-layered MnBi 2 Te 4 with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the AFM spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Signatures of fractional quantum anomalous Hall states in twisted MoTe 2

The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) state, which exhibits an integer quantum Hall effect at zero magnetic field owing to intrinsic ferromagnetism. In the presence of strong electron–electron interactions, fractional QAH (FQAH) states at zero magnetic field can emerge. These states could host fractional excitations, including non-Abelian anyons—crucial building blocks for topological quantum computation9. Here we report experimental signatures of FQAH states in a twisted molybdenum ditelluride (MoTe 2 ) bilayer. Magnetic circular dichroism measurements reveal robust ferromagnetic states at fractionally hole-filled moiré minibands. Using trion photoluminescence as a sensor, we obtain a Landau fan diagram showing linear shifts in carrier densities corresponding to filling factor v = −2/3 and v = −3/5 ferromagnetic states with applied magnetic field. These shifts match the Streda formula dispersion of FQAH states with fractionally quantized Hall conductance of ${\sigma }_{xy}=-\,\frac{2}{3}\frac{{e}^{2}}{h}$ and ${\sigma }_{xy}=-\,\frac{3}{5}\frac{{e}^{2}}{h}$ , respectively. Moreover, the v = −1 state exhibits a dispersion corresponding to Chern number −1, consistent with the predicted QAH state. In comparison, several non-ferromagnetic states on the electron-doping side do not disperse, that is, they are trivial correlated insulators. The observed topological states can be electrically driven into topologically trivial states. Our findings provide evidence of the long-sought FQAH states, demonstrating MoTe 2 moiré superlattices as a platform for exploring fractional excitations.

Quantum Hall↗
DOE OSTI2023

Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower - to - Higher Bandgap 2D Material

High light absorption (~15%) and strong photoluminescence (PL) emission in monolayer (1L) transition metal dichalcogenides (TMDs) make them ideal candidates for optoelectronic device applications. Competing interlayer charge transfer (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in TMD heterostructures (HSs). In TMDs, long-distance ET can survive up to several tens of nm, unlike the CT process. Our experiment shows that an efficient ET occurs from the 1Ls WSe 2 -to-MoS 2 with an interlayer hexagonal boron nitride (hBN), due to the resonant overlapping of the high-lying excitonic states between the two TMDs, resulting in enhanced HS MoS 2 PL emission. This type of unconventional ET from the lower-to-higher optical bandgap material is not typical in the TMD HSs. With increasing temperature, the ET process becomes weaker due to the increased electron–phonon scattering, destroying the enhanced MoS 2 emission. Our work provides new insight into the long-distance ET process and its effect on the photocarrier relaxation pathways.

2D material↗
DOE OSTI2023

Measurement of the conduction band spin-orbit splitting in $\mathrm{WSe_2}$ and $\mathrm{WS_2}$ monolayers

We have investigated charge tunable devices based on $\mathrm{WSe_2}$ and $\mathrm{WS_2}$monolayers encapsulated in hexagonal boron nitride. In addition to the well-known radiative recombination of neutral and charged excitons, stationary photoluminescence measurements highlight a weaker-intensity optical transition that appears when the monolayer is electrostatically doped with electrons. We have carried out a detailed characterization of this photoluminescence line based on its doping dependence, its variation as a function of the optical excitation power, its polarization characteristics, and its g factor measured under longitudinal magnetic field. We interpret this luminescence peak as an impurity-assisted radiative recombination of the intervalley negatively charged exciton (triplet trion). In addition to the standard direct recombination, the triplet trion has a second recombination channel triggered by elastic scattering off the short-range impurity potential. The energy difference between the emitted photons from these two possible recombination processes of the same triplet trion is simply the single-particle conduction band spin-orbit splitting Δ c . We measure Δ c = 12 ± 0.5 meV for the $\mathrm{WSe_2}$ monolayer and Δ c = 12 ± 1 meV for the $\mathrm{WS_2}$ monolayer. Here, these results demonstrate the importance of second-order exciton recombination processes in transition-metal dichalcogenide structures.

2-dimensional systems↗
DOE OSTI2023

Dipole ladders with large Hubbard interaction in a moiré exciton lattice

Two-dimensional semiconductor moiré superlattices have emerged as a powerful platform for engineering correlated electronic phenomena. On the other hand, optical excitation creates charge neutral interlayer excitons with an out-of-plane electric dipole. Strong onsite dipole–dipole interaction promises the formation of correlated bosonic states, akin to the Mott states of electrons, but has not yet been demonstrated. Here we report a large interaction between excitons occupying the same moiré lattice site—characterized by the Hubbard U parameter—and consequent dipole ladders with spin- and electron-filling dependence in WSe 2 /WS 2 moiré superlattices. Photoluminescence measurements show successive peaks emerging with an energy separation of around 34 meV above the ground state as the exciton density is increased. This corresponds to the sequential injection of excitons into a single site with an energy cost to overcome the large exciton Hubbard U, forming a dipole ladder. Based on findings of local magnetic moments at two holes per moiré cell, we show that excitons can also fill a second moiré orbital, establishing the two-orbital nature of the moiré potential landscape. Our results show that the Bose–Hubbard model with possible exciton crystal phases can be investigated in interacting opto-moiré quantum matter.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Insulators at fractional fillings in twisted bilayer graphene partially aligned to hexagonal boron nitride

At partial fillings of its flat electronic bands, magic-angle twisted bilayer graphene (MATBG) hosts a rich variety of competing correlated phases that show sample-to-sample variations. Divergent phase diagrams in MATBG are often attributed to the sublattice polarization energy scale, tuned by the degree of alignment of the hexagonal boron nitride (hBN) substrates typically used in van der Waals devices. Unaligned MATBG exhibits unconventional superconductor and correlated insulator phases, while nearly perfectly aligned MATBG/hBN exhibits zero-field Chern insulating phases and lacks superconductivity. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) to observe gapped phases at partial fillings of the flat bands of MATBG in a new intermediate regime of sublattice polarization, observed when MATBG is only partially aligned (θ Gr-hBN ≈ 1.65°) to the underlying hBN substrate. Under this condition, MATBG hosts not only phenomena that naturally interpolate between the two sublattice potential limits, but also unexpected gapped phases absent in either of these limits. At charge neutrality, we observe an insulating phase with a small energy gap (Δ < 5 meV) likely related to weak sublattice symmetry breaking from the hBN substrate. In addition, we observe new gapped phases near fractional fillings ν = ±1/3 and ν = ±1/6, which have not been previously observed in MATBG. Importantly, energy-resolved STS unambiguously identifies these fractional filling states to be of single-particle origin, possibly a result of the super-superlattice formed by two moiré superlattices. Finally, our observations emphasize the power of STS in distinguishing single-particle gapped phases from many-body gapped phases in situations that could be easily confused in electrical transport measurements, and demonstrate the use of substrate engineering for modifying the electronic structure of a moiré flat-band material.

Physics↗
DOE OSTI2023

Andreev reflection between aluminum and graphene across van der Waals barriers

We present planar aluminum superconductor–graphene junctions whose hybrid interface is engineered for couplings ranging from tunneling to the strongly coupled regime by employing an atomically thin van der Waals tunneling barrier. Without the vdW barrier, we find Al makes strongly coupled contacts with the fully proximities graphene channel underneath. Using a large band gap hexagonal boron nitride (hBN) barrier, we find the junctions always remain in the weak coupling regime, exhibiting tunneling characteristics. Using monolayer semi-conducting transition metal dichalcogenides (TMDs) such as MoS2, we realize intermediate coupling with enhanced junction conductance due to the Andreev process. In this intermediate regime, we find that junction resistance changes in discrete steps when sweeping a perpendicular magnetic field. The period of the resistance steps in the magnetic field is inversely proportional to the junction area, suggesting the physical origin of our observations is due to magnetic-field-induced vortex formation in the planar junction.

Physics↗