arXiv subjects

K. Watanabe

Publications and source records attributed to K. Watanabe.

At least 253 records · Page 14Linked to original sources

arXiv2019

Interlayer Charge Transfer and Defect Creation in Type I van der Waals Heterostructures

Van der Waals heterostructures give access to a wide variety of new phenomena that emerge thanks to the combination of properties brought in by the constituent layered materials. We show here that owing to an enhanced interaction cross section with electrons in a type I van der Waals heterostructure, made of single layer molybdenum disulphide and thin boron nitride films, electrons and holes created in boron nitride can be transferred to the dichalcogenide where they form electron-hole pairs yielding luminescence. This cathodoluminescence can be mapped with a spatial resolution far exceeding what can be achieved in a typical photoluminescence experiment, and is highly valuable to understand the optoelectronic properties at the nanometer scale. We find that in heterostructures prepared following the mainstream dry transfer technique, cathodoluminescence is locally extinguished, and we show that this extinction is associated with the formation of defects, that are detected in Raman spectroscopy and photoluminescence. We establish that to avoid defect formation induced by low-energy electron beams and to ensure efficient transfer of electrons and holes at the interface between the layers, flat and uniform interlayer interfaces are needed, that are free of trapped species, airborne ones or contaminants associated with sample preparation. We show that heterostructure fabrication using a pick-up technique leads to superior, intimate interlayer contacts associated with significantly more homogeneous cathodoluminescence.

cond-mat.mes-hall↗

arXiv2019

Upconverted electroluminescence via Auger scattering of interlayer excitons in van der Waals heterostructures

The intriguing physics of carrier-carrier interactions, which likewise affect the operation of light emitting devices, stimulate the research on semiconductor structures at high densities of excited carriers, a limit reachable at large pumping rates or in systems with long-lived electron-hole pairs. By electrically injecting carriers into WSe$_2$/MoS$_2$ type-II heterostructures which are indirect in real and k-space, we establish a large population of typical optically silent interlayer excitons. Here, we reveal their emission spectra and show that the emission energy is tunable by an applied electric field. When the population is further increased by suppressing the radiative recombination rate with the introduction of an hBN spacer between WSe$_2$ and MoS$_2$, Auger-type and exciton-exciton annihilation processes become important. These processes are traced by the observation of an up-converted emission demonstrating that excitons gaining energy in non-radiative Auger processes can be recovered and recombine radiatively.

cond-mat.mes-hall↗

arXiv2019

Impact of intrinsic and extrinsic imperfections on the electronic and optical properties of MoS2

Intrinsic and extrinsic disorder from lattice imperfections, substrate and environment has a strong effect on the local electronic structure and hence the optical properties of atomically thin transition metal dichalcogenides that are determined by strong Coulomb interaction. Here, we examine the role of the substrate material and intrinsic defects in monolayer MoS2 crystals on SiO2 and hBN substrates using a combination of scanning tunneling spectroscopy, scanning tunneling microscopy, optical absorbance, and low-temperature photoluminescence measurements. We find that the different substrates significantly impact the optical properties and the local density of states near the conduction band edge observed in tunneling spectra. While the SiO2 substrates induce a large background doping with electrons and a substantial amount of band tail states near the conduction band edge of MoS2, such states as well as the high doping density are absent using high quality hBN substrates. By accounting for the substrate effects we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 480 meV. We identify several intrinsic lattice defects that are ubiquitious in MoS2, but we find that on hBN substrates the impact of these defects appears to be passivated. We conclude that the choice of substrate controls both the effects of intrinsic defects and extrinsic disorder, and thus the electronic and optical properties of MoS2. The correlation of substrate induced disorder and defects on the electronic and optical properties of MoS2 contributes to an in-depth understanding of the role of the substrates on the performance of 2D materials and will help to further improve the properties of 2D materials based quantum nanosystems.

cond-mat.mtrl-sci↗

arXiv2019

Chiral Quasiparticle Tunneling Between Quantum Hall Edges in Proximity with a Superconductor

We study a two-terminal graphene Josephson junction with contacts shaped to form a narrow constriction, less than 100nm in length. The contacts are made from type II superconducting contacts and able to withstand magnetic fields high enough to reach the quantum Hall (QH) regime in graphene. In this regime, the device conductance is determined by edge states, plus the contribution from the constricted region. In particular, the constriction area can support supercurrents up to fields of ~2.5T. Moreover, enhanced conductance is observed through a wide range of magnetic fields and gate voltages. This additional conductance and the appearance of supercurrent is attributed to the tunneling between counter-propagating quantum Hall edge states along opposite superconducting contacts.

cond-mat.mes-hall↗

arXiv2019

Excited states in bilayer graphene quantum dots

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.

cond-mat.mes-hall↗

arXiv2019

High-mobility, wet-transferred graphene grown by chemical vapor deposition

We report high room-temperature mobility in single layer graphene grown by Chemical Vapor Deposition (CVD) after wet transfer on SiO$_2$ and hexagonal boron nitride (hBN) encapsulation. By removing contaminations trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to$\sim70000cm^2 V^{-1} s^{-1}$ at room temperature and$\sim120000cm^2 V^{-1} s^{-1}$ at 9K. These are over twice those of previous wet transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room temperature mobilities$\sim30000 cm^2 V^{-1} s^{-1}$. These results show that, with appropriate encapsulation and cleaning, room temperature mobilities well above $10000cm^2 V^{-1} s^{-1}$ can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

cond-mat.mes-hall↗

arXiv2019

Tunable crystal symmetry in graphene-boron nitride heterostructures with coexisting moiré superlattices

In heterostructures consisting of atomically thin crystals layered on top of one another, lattice mismatch or rotation between the layers results in long-wavelength moiré superlattices. These moiré patterns can drive significant band structure reconstruction of the composite material, leading to a wide range of emergent phenomena including superconductivity, magnetism, fractional Chern insulating states, and moiré excitons. Here, we investigate monolayer graphene encapsulated between two crystals of boron nitride (BN), where the rotational alignment between all three components can be varied. We find that band gaps in the graphene arising from perfect rotational alignment with both BN layers can be modified substantially depending on whether the relative orientation of the two BN layers is 0 or 60 degrees, suggesting a tunable transition between the absence or presence of inversion symmetry in the heterostructure. Small deviations ($<1^{\circ}$) from perfect alignment of all three layers leads to coexisting long-wavelength moiré potentials, resulting in a highly reconstructed graphene band structure featuring multiple secondary Dirac points. Our results demonstrate that the interplay between multiple moiré patterns can be utilized to controllably modify the electronic properties of the composite heterostructure.

cond-mat.mes-hall↗

arXiv2019

Observation of a Be double-Lambda hypernucleus in the J-PARC E07 experiment

A double-$Λ$ hypernucleus, ${}_{ΛΛ}\mathrm{Be}$, was observed by the J-PARC E07 collaboration in nuclear emulsions tagged by the $(K^{-},K^{+})$ reaction. This event was interpreted as a production and decay of $ {}_{ΛΛ}^{\;10}\mathrm{Be}$, ${}_{ΛΛ}^{\;11}\mathrm{Be}$, or ${}_{ΛΛ}^{\;12}\mathrm{Be}^{*}$ via $Ξ^{-}$ capture in ${}^{16}\mathrm{O}$. By assuming the capture in the atomic 3D state, the binding energy of two $Λ$ hyperons$\,$($B_{ΛΛ}$) of these double-$Λ$ hypernuclei are obtained to be $15.05 \pm 0.11\,\mathrm{MeV}$, $19.07 \pm 0.11\,\mathrm{MeV}$, and $13.68 \pm 0.11\,\mathrm{MeV}$, respectively. Based on the kinematic fitting, ${}_{ΛΛ}^{\;11}\mathrm{Be}$ is the most likely explanation for the observed event.

nucl-ex↗

arXiv2019

Universal quantized thermal conductance in graphene

The universal quantization of thermal conductance provides information on the topological order of a state beyond electrical conductance. Such measurements have become possible only recently, and have discovered, in particular, that the value of the observed thermal conductance of the 5/2 state is not consistent with either the Pfaffian or the anti-Pfaffian model, motivating several theoretical articles. The analysis of the experiments has been made complicated by the presence of counter-propagating edge channels arising from edge reconstruction, an inevitable consequence of separating the dopant layer from the GaAs quantum well. In particular, it has been found that the universal quantization requires thermalization of downstream and upstream edge channels. Here we measure the thermal conductance in hexagonal boron nitride encapsulated graphene devices of sizes much smaller than the thermal relaxation length of the edge states. We find the quantization of thermal conductance within 5% accuracy for ν = 1, 4/3, 2 and 6 plateaus and our results strongly suggest the absence of edge reconstruction for fractional quantum Hall in graphene, making it uniquely suitable for interference phenomena exploiting paths of exotic quasiparticles along the edge.

cond-mat.mes-hall↗

arXiv2019

Phonon scattering dominated electron transport in twisted bilayer graphene

Twisted bilayer graphene (tBLG) has recently emerged as a platform for hosting correlated phenomena, owing to the exceptionally flat band dispersion that results near interlayer twist angle $θ\approx1.1^\circ$. At low temperature a variety of phases are observed that appear to be driven by electron interactions including insulating states, superconductivity, and magnetism. Electrical transport in the high temperature regime has received less attention but is also highly anomalous, exhibiting gigantic resistance enhancement and non-monotonic temperature dependence. Here we report on the evolution of the scattering mechanisms in tBLG over a wide range of temperature and for twist angle varying from 0.75$^\circ$ - 2$^\circ$. We find that the resistivity, $ρ$, exhibits three distinct phenomenological regimes as a function of temperature, $T$. At low $T$ the response is dominated by correlation and disorder physics; at high $T$ by thermal activation to higher moiré subbands; and at intermediate temperatures $ρ$ varies linearly with $T$. The $T$-linear response is much larger than in monolayer graphenefor all measured twist angles, and increases by more than three orders of magnitude for $θ$ near the flat-band condition. Our results point to the dominant role of electron-phonon scattering in twisted layer systems, with possible implications for the origin of the observed superconductivity.

cond-mat.str-el↗

arXiv2019

Absence of dissipationless transport in clean 2D superconductors

Dissipationless charge transport is one of the defining properties of superconductors (SC). The interplay between dimensionality and disorder in determining the onset of dissipation in SCs remains an open theoretical and experimental problem. In this work, we present measurements of the dissipation phase diagrams of SCs in the two dimensional (2D) limit, layer by layer, down to a monolayer in the presence of temperature (T), magnetic field (B), and current (I) in 2H-NbSe2. Our results show that the phase-diagram strongly depends on the SC thickness even in the 2D limit. At four layers we can define a finite region in the I-B phase diagram where dissipationless transport exists at T=0. At even smaller thicknesses, this region shrinks in area. In a monolayer, we find that the region of dissipationless transport shrinks towards a single point, defined by T=B=I=0. In applied field, we show that time-dependent-Ginzburg-Landau (TDGL) simulations that describe dissipation by vortex motion, qualitatively reproduce our experimental I-B phase diagram. Last, we show that by using non-local transport and TDGL calculations that we can engineer charge flow and create phase boundaries between dissipative and dissipationless transport regions in a single sample, demonstrating control over non-equilibrium states of matter.

cond-mat.mes-hall↗

arXiv2019

Topological valley transport at the curved boundary of a folded bilayer graphene

The development of valleytronics demands long-range electronic transport with preserved valley index, a degree of freedom similar to electron spin. A promising structure for this end is a topological one-dimensional (1D) channel formed in bilayer graphene (BLG) under special electrostatic conditions or specific stacking configuration, called domain wall (DW). In these 1D channels, the valley-index defines the propagation direction of the charge carriers and the chiral edge states (kink states) are robust over many kinds of disorder. However, the fabrication of DWs is challenging, requiring the design of complex multi-gate structures or have been producing on rough substrates, showing a limited mean free path. Here, we report on a high-quality DW formed at the curved boundary of folded bilayer graphene (folded-BLG). At such 1D conducting channel we measured a two-terminal resistance close to the quantum resistance $R = e^2/4h$ at zero magnetic field, a signature of kink states. Our experiments reveal a long-range ballistic transport regime that occurs only at the DW of the folded-BLG, while the other regions behave like semiconductors with tunable band gap.

cond-mat.mes-hall↗

arXiv2019

Photonic crystals for nano-light in moiré graphene superlattices

Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moiré superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus obtained twisted bilayer graphene (TBG). We studied propagation of plasmon polaritons in TBG by infrared nano-imaging. We discovered that the atomic reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway towards controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials eliminating need for arduous top-down nanofabrication.

cond-mat.mes-hall↗

arXiv2019

Evidence for pairing states of composite fermions in double-layer graphene

Pairing interaction between fermionic particles leads to composite Bosons that condense at low temperature. Such condensate gives rise to long range order and phase coherence in superconductivity, superfluidity, and other exotic states of matter in the quantum limit. In graphene double-layers separated by an ultra-thin insulator, strong interlayer Coulomb interaction introduces electron-hole pairing across the two layers, resulting in a unique superfluid phase of interlayer excitons. In this work, we report a series of emergent fractional quantum Hall ground states in a graphene double-layer structure, which is compared to an expanded composite fermion model with two-component correlation. The ground state hierarchy from bulk conductance measurement and Hall resistance plateau from Coulomb drag measurement provide strong experimental evidence for a sequence of effective integer quantum Hall effect states for the novel two-component composite fermions (CFs), where CFs fill integer number of effective LLs (Lambda-level). Most remarkably, a sequence of incompressible states with interlayer correlation are observed at half-filled Lambda-levels, which represents a new type of order involving pairing states of CFs that is unique to graphene double-layer structure and beyond the conventional CF model.

cond-mat.mes-hall↗

arXiv2019

A corner reflector of graphene Dirac fermions as a phonon-scattering sensor

Dirac fermion optics exploits the refraction of chiral fermions across optics-inspired Klein-tunneling barriers defined by high-transparency p-n junctions. We consider the corner reflector (CR) geometry introduced in optics or radars. We fabricate Dirac fermion CRs using bottom-gate-defined barriers in hBN-encapsulated graphene. By suppressing transmission upon multiple internal reflections, CRs are sensitive to minute phonon scattering rates. We report on doping-independent CR transmission in quantitative agreement with a simple scattering model including thermal phonon scattering. As a new signature of CRs, we observe Fabry-Pérot oscillations at low temperature, consistent with single-path reflections. Finally, we demonstrate high-frequency operation which promotes CRs as fast phonon detectors. Our work establishes the relevance of Dirac fermion optics in graphene and opens a route for its implementation in topological Dirac matter.

cond-mat.mes-hall↗

arXiv2019

Spin-orbit driven band inversion in bilayer graphene by van der Waals proximity effect

Spin orbit coupling (SOC) is the key to realizing time-reversal invariant topological phases of matter. Famously, SOC was predicted by Kane and Mele to stabilize a quantum spin Hall insulator; however, the weak intrinsic SOC in monolayer graphene has precluded experimental observation. Here, we exploit a layer-selective proximity effect---achieved via van der Waals contact to a semiconducting transition metal dichalcogenide--to engineer Kane-Mele SOC in ultra-clean \textit{bilayer} graphene. Using high-resolution capacitance measurements to probe the bulk electronic compressibility, we find that SOC leads to the formation of a distinct incompressible, gapped phase at charge neutrality. The experimental data agrees quantitatively with a simple theoretical model in which the new phase results from SOC-driven band inversion. In contrast to Kane-Mele SOC in monolayer graphene, the inverted phase is not expected to be a time reversal invariant topological insulator, despite being separated from conventional band insulators by electric field tuned phase transitions where crystal symmetry mandates that the bulk gap must close. Electrical transport measurements, conspicuously, reveal that the inverted phase has a conductivity $\sim e^2/h$, which is suppressed by exceptionally small in-plane magnetic fields. The high conductivity and anomalous magnetoresistance are consistent with theoretical models that predict helical edge states within the inversted phase, that are protected from backscattering by an emergent spin symmetry that remains robust even for large Rashba SOC. Our results pave the way for proximity engineering of strong topological insulators as well as correlated quantum phases in the strong spin-orbit regime in graphene heterostructures.

cond-mat.mes-hall↗

arXiv2018

Magic Angle Spectroscopy

The electronic properties of heterostructures of atomically-thin van der Waals (vdW) crystals can be modified substantially by Moiré superlattice potentials arising from an interlayer twist between crystals. Moiré-tuning of the band structure has led to the recent discovery of superconductivity and correlated insulating phases in twisted bilayer graphene (TBLG) near the so-called "magic angle" of $\sim$1.1°, with a phase diagram reminiscent of high T$_c$ superconductors. However, lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the Moiré pattern has so far precluded a coherent theoretical understanding of the correlated states. Here, we directly map the atomic-scale structural and electronic properties of TBLG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). We observe two distinct van Hove singularities (vHs) in the LDOS which decrease in separation monotonically through 1.1° with the bandwidth (t) of each vHs minimized near the magic angle. When doped near half Moiré band filling, the conduction vHs shifts to the Fermi level and an additional correlation-induced gap splits the vHs with a maximum size of 7.5 meV. We also find that three-fold (C$_3$) rotational symmetry of the LDOS is broken in doped TBLG with a maximum symmetry breaking observed for states near the Fermi level, suggestive of nematic electronic interactions. The main features of our doping and angle dependent spectroscopy are captured by a tight-binding model with on-site (U) and nearest neighbor Coulomb interactions. We find that the ratio U/t is of order unity, indicating that electron correlations are significant in magic angle TBLG. Rather than a simple maximization of the DOS, superconductivity arises in TBLG at angles where the ratio U/t is largest, suggesting a pairing mechanism based on electron-electron interactions.

cond-mat.mes-hall↗

arXiv2018

Shubnikov-de Haas oscillations in optical conductivity of monolayer MoSe$_2$

We report polarization-resolved resonant reflection spectroscopy of a charge-tunable atomically-thin valley semiconductor hosting tightly bound excitons coupled to a dilute system of fully spin- and valley-polarized holes in the presence of a strong magnetic field. We find that exciton-hole interactions manifest themselves in hole-density dependent, Shubnikov-de Haas-like oscillations in the energy and line broadening of the excitonic resonances. These oscillations are evidenced to be precisely correlated with the occupation of Landau levels, thus demonstrating that strong interactions between the excitons and Landau-quantized itinerant carriers enable optical investigation of quantum-Hall physics in transition metal dichalcogenides.

cond-mat.mes-hall↗