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T. Taniguchi

Publications and source records attributed to T. Taniguchi.

At least 289 records · Page 16Linked to original sources
arXiv2018

Quantum Wires and Waveguides Formed in Graphene by Strain

Confinement of electrons in graphene to make devices has proven to be a challenging task. Electrostatic methods fail because of Klein tunneling, while etching into nanoribbons requires extreme control of edge terminations, and bottom-up approaches are limited in size to a few nanometers. Fortunately, its mechanical flexibility raises the possibility of using strain to alter graphene's properties and create novel straintronic devices. Here, we report transport studies of nanowires created by linearly-shaped strained regions resulting from individual folds formed by layer transfer onto hexagonal boron nitride. Conductance measurements across the folds reveal Coulomb blockade signatures, indicating confined charges within these structures, which act as quantum dots. Along folds, we observe sharp features in traverse resistivity measurements, attributed to an amplification of the dot conductance modulations by a resistance bridge incorporating the device. Our data indicates ballistic transport up to ~1 um along the folds. Calculations using the Dirac model including strain are consistent with measured bound state energies and predict the existence of valley-polarized currents. Our results show that graphene folds can act as straintronic quantum wires.

cond-mat.mes-hall↗
arXiv2018

Enhancing the response of NH3 graphene-sensors by using devices with different graphene-substrate distances

Graphene (G) is a two-dimensional material with exceptional sensing properties. In general, graphene gas sensors are produced in field effect transistor configuration on several substrates. The role of the substrates on the sensor characteristics has not yet been entirely established. To provide further insight on the interaction between ammonia molecules (NH3) and graphene devices, we report experimental and theoretical studies of NH3 graphene sensors with graphene supported on three substrates: SiO2, talc and hexagonal boron nitride (hBN). Our results indicate that the charge transfer from NH3 to graphene depends not only on extrinsic parameters like temperature and gas concentration, but also on the average distance between the graphene sheet and the substrate. We find that the average distance between graphene and hBN crystals is the smallest among the three substrates, and that graphene-ammonia gas sensors based on a G/hBN heterostructure exhibit the fastest recovery times for NH3 exposure and are slightly affected by wet or dry air environment. Moreover, the dependence of graphene-ammonia sensors on different substrates indicates that graphene sensors exhibit two different adsorption processes for NH3 molecules: one at the top of the graphene surface and another at its bottom side close to the substrate. Therefore, our findings show that substrate engineering is crucial to the development of graphene-based gas sensors and indicate additional routes for faster sensors.

cond-mat.mes-hall↗
arXiv2018

Coexisting localized and itinerant gapless excitations in a quantum spin liquid candidate 1T-TaS$_2$

To reveal the nature of elementary excitations in a quantum spin liquid (QSL), we measured low temperature thermal conductivity and specific heat of 1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient $γ$ and the finite residual linear term of the thermal conductivity in the zero temperature limit $κ_0/T=κ/T(T\rightarrow 0)$ are clearly resolved. This demonstrates the presence of highly mobile gapless excitations, which is consistent with fractionalized spinon excitations that form a Fermi surface. Remarkably, an external magnetic field strongly suppresses $γ$, whereas it enhances $κ_0/T$. This unusual contrasting behavior in the field dependence of specific heat and thermal conductivity can be accounted for by the presence of two types of gapless excitations with itinerant and localized characters, as recently predicted theoretically (I. Kimchi et al., arXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new insights into the influence of quenched disorder on the QSL.

cond-mat.str-el↗
arXiv2018

Cleaning Interfaces in Layered Materials Heterostructures

Heterostructures formed by stacking layered materials require atomically clean interfaces. However, contaminants are usually trapped between the layers, aggregating into blisters. We report a process to remove such blisters, resulting in clean interfaces. We fabricate blister-free regions of graphene encapsulated in hexagonal boron nitride of$\sim$5000$μ$m$^{2}$, limited only by the size of the exfoliated flakes. These have mobilities up to$\sim$180000cm$^2$V$^{-1}$s$^{-1}$ at room temperature, and$\sim$1.8$\times$10$^6$cm$^2$V$^{-1}$s$^{-1}$ at 9K. We further demonstrate the effectiveness of our approach by cleaning heterostructures assembled using graphene intentionally exposed to polymers and solvents. After cleaning, these samples reach similar high mobilities. We also showcase the general applicability of our approach to layered materials by cleaning blisters in other heterostructures based on MoS$_{2}$. This demonstrates that exposure of graphene to processing-related contaminants is compatible with the realization of high mobility samples, paving the way to the development of fab-based processes for the integration of layered materials in (opto)-electronic devices.

cond-mat.mes-hall↗
arXiv2018

Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure

We have combined spatially-resolved steady-state micro-photoluminescence ($μ$PL) with time-resolved photoluminescence (TRPL) to investigate the exciton diffusion in a WSe$_2$ monolayer encapsulated with hexagonal boron nitride (hBN). At 300 K, we extract an exciton diffusion length $L_X= 0.36\pm 0.02 \; μ$m and an exciton diffusion coefficient of $D_X=14.5 \pm 2\;\mbox{cm}^2$/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species : bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of $L_{X^D}=1.5\pm 0.02 \;μ$m.

cond-mat.mes-hall↗
arXiv2018

RF-quantum capacitance of the topological insulator Bi2Se3 in the bulk depleted regime for field-effect transistors

A Metal-dielectric-topological insulator capacitor device based on hBN-encapsulated CVD grown Bi2Se3 is realized and investigated in the radio frequency regime. The RF quantum capacitance and device resistance are extracted for frequencies as a high as 10 GHz, and studied as a function of the applied gate voltage. The combination of the superior quality hBN dielectric gate with the optimized transport characteristics of CVD grown Bi2Se3 (n~10^18cm-3 in 8 nm) allow us to attain a bulk depleted regime by dielectric gating. A quantum capacitance minimum is observed revealing a purely Dirac regime, where the Dirac surface state in proximity to the gate reaches charge neutrality, but the bottom surface Dirac cone remains charged, and couples capacitively to the top surface via the insulating bulk. Our work paves the way towards implementation of topological materials in RF devices.

cond-mat.mes-hall↗
arXiv2018

Correlated Insulator Behaviour at Half-Filling in Magic Angle Graphene Superlattices

Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hexagonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted 'magic angle', the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids.

cond-mat.mes-hall↗
arXiv2018

Impact ionization and transport properties of hexagonal boron nitride in constant-voltage measurement

The electrical evaluation of the crystallinity of hexagonal boron nitride (h-BN) is still limited to the measurement of dielectric breakdown strength, in spite of its importance as the substrate for 2-dimensional van der Waals heterostructure devices. In this study, physical phenomena for degradation and failure in exfoliated single-crystal h-BN films were investigated using the constant-voltage stress test. At low electrical fields, the current gradually reduced and saturated with time, while the current increased at electrical fields higher than ~8 MV/cm and finally resulted in the catastrophic dielectric breakdown. These transient behaviors may be due to carrier trapping to the defect sites in h-BN because trapped carriers lower or enhance the electrical fields in h-BN depending on their polarities. The key finding is the current enhancement with time at the high electrical field, suggesting the accumulation of electrons generated by the impact ionization process. Therefore, a theoretical model including the electron generation rate by impact ionization process was developed. The experimental data support the expected degradation mechanism of h-BN. Moreover, the impact ionization coefficient was successfully extracted, which is comparable to that of SiO2, even though the fundamental band gap for h-BN is smaller than that for SiO2. Therefore, the dominant impact ionization in h-BN could be band-to-band excitation, not defect-assisted impact ionization.

cond-mat.mtrl-sci↗
arXiv2018

Many-particle effects in the cyclotron resonance of encapsulated monolayer graphene

We study the infrared cyclotron resonance of high mobility monolayer graphene encapsulated in hexagonal boron nitride, and simultaneously observe several narrow resonance lines due to interband Landau level transitions. By holding the magnetic field strength, $B$, constant while tuning the carrier density, $n$, we find the transition energies show a pronounced non-monotonic dependence on the Landau level filling factor, $ν\propto n/B$. This constitutes direct evidence that electron-electron interactions contribute to the Landau level transition energies in graphene, beyond the single-particle picture. Additionally, a splitting occurs in transitions to or from the lowest Landau level, which is interpreted as a Dirac mass arising from coupling of the graphene and boron nitride lattices.

cond-mat.mes-hall↗
arXiv2017

Even denominator fractional quantum Hall states at an isospin transition in monolayer graphene

Magnetic fields quench the kinetic energy of two dimensional electrons, confining them to highly degenerate Landau levels. In the absence of disorder, the ground state at partial Landau level filling is determined only by Coulomb interactions, leading to a variety of correlation-driven phenomena. Here, we realize a quantum Hall analog of the Neél-to-valence bond solid transition within the spin- and sublattice- degenerate monolayer graphene zero energy Landau level by experimentally controlling substrate-induced sublattice symmetry breaking. The transition is marked by unusual isospin transitions in odd denominator fractional quantum Hall states for filling factors $ν$ near charge neutrality, and the unexpected appearance of incompressible even denominator fractional quantum Hall states at $ν=\pm1/2$ and $\pm1/4$ associated with pairing between composite fermions on different carbon sublattices.

cond-mat.str-el↗
arXiv2017

Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

The optical properties of MoS2 monolayers are dominated by excitons, but for spectrally broad optical transitions in monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states is inaccessible. Encapsulation in hexagonal boron nitride (hBN) allows approaching the homogenous exciton linewidth, but interferences in the van der Waals heterostructures make direct comparison between transitions in optical spectra with different oscillator strength more challenging. Here we reveal in reflectivity and in photoluminescence excitation spectroscopy the presence of excited states of the A-exciton in MoS2 monolayers encapsulated in hBN layers of calibrated thickness, allowing to extrapolate an exciton binding energy of about 220 meV. We theoretically reproduce the energy separations and oscillator strengths measured in reflectivity by combining the exciton resonances calculated for a screened two-dimensional Coulomb potential with transfer matrix calculations of the reflectivity for the van der Waals structure. Our analysis shows a very different evolution of the exciton oscillator strength with principal quantum number for the screened Coulomb potential as compared to the ideal two-dimensional hydrogen model.

cond-mat.mtrl-sci↗
arXiv2017

Out-of-plane heat transfer in van der Waals stacks: electron-hyperbolic phonon coupling

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.

cond-mat.mes-hall↗
arXiv2017

Tunneling Devices with Perpendicular Magnetic Anisotropy Electrodes on Atomically Thin van der Waals Heterostructures

We report the fabrication of perpendicular ferromagnetic electrodes for tunneling devices consist of van der Waals heterostructure. We found MgO/Co/Pt films on Hexagonal BN shows perpendicular magnetic anisotropy (PMA) with the easy axis perpendicular to the substrate. Vacuum annealing enhances the perpendicular anisotropy. The easy axis along the perpendicular direction persists up to room temperature with the Pt layer thickness ranges from 1.5 to 5 nm. We employed the PMA electrodes to construct tunneling devices on graphene and monolayer WSe2, where spin injection characteristics and field effect transistor behavior were demonstrated without strong Schottky barrier formation.

cond-mat.mes-hall↗
arXiv2017

Approaching quantum anomalous Hall effect in proximity-coupled YIG/graphene/h-BN sandwich structure

Quantum anomalous Hall state is expected to emerge in Dirac electron systems such as graphene under both sufficiently strong exchange and spin-orbit interactions. In pristine graphene, neither interaction exists; however, both interactions can be acquired by coupling graphene to a magnetic insulator (MI) as revealed by the anomalous Hall effect. Here, we show enhanced magnetic proximity coupling by sandwiching graphene between a ferrimagnetic insulator yttrium iron garnet (YIG) and hexagonal-boron nitride (h-BN) which also serves as a top gate dielectric. By sweeping the top-gate voltage, we observe Fermi level-dependent anomalous Hall conductance. As the Dirac point is approached from both electron and hole sides, the anomalous Hall conductance reaches 1/4 of the quantum anomalous Hall conductance 2e2/h. The exchange coupling strength is determined to be as high as 27 meV from the transition temperature of the induced magnetic phase. YIG/graphene/h-BN is an excellent heterostructure for demonstrating proximity-induced interactions in two-dimensional electron systems.

cond-mat.mes-hall↗
arXiv2017

Dynamic band structure tuning of graphene moiré superlattices with pressure

Heterostructures of atomically-thin materials have attracted significant interest owing to their ability to host novel electronic properties fundamentally distinct from their constituent layers. In the case of graphene on boron nitride, the closely-matched lattices yield a moiré superlattice that modifies the graphene electron dispersion and opens gaps both at the primary Dirac point (DP) and the moiré-induced secondary Dirac point (SDP) in the valence band. While significant effort has focused on controlling the superlattice period via the rotational stacking order, the role played by the magnitude of the interlayer coupling has received comparatively little attention. Here, we modify the interaction between graphene and boron nitride by tuning their separation with hydrostatic pressure. We observe a dramatic enhancement of the DP gap with increasing pressure, but little change in the SDP gap. Our surprising results identify the critical role played by atomic-scale structural deformations of the graphene lattice and reveal new opportunities for band structure engineering in van der Waals heterostructures.

cond-mat.mes-hall↗
arXiv2017

Manipulation and steering of hyperbolic surface polaritons in hexagonal boron nitride

Hexagonal boron nitride (hBN) is a natural hyperbolic material that supports both volume-confined hyperbolic polaritons (HPs) and sidewall-confined hyperbolic surface polaritons (HSPs). In this work, we demonstrate effective excitation, control and steering of HSPs in hBN through engineering the geometry and orientation of hBN sidewalls. By combining infrared (IR) nano-imaging and numerical simulations, we investigate the reflection, transmission and scattering of HSPs at the hBN corners with various apex angles. We show that the sidewall-confined nature of HSPs enables a high degree of control over their propagation by designing the geometry of hBN nanostructures.

cond-mat.mes-hall↗
arXiv2017

Superballistic flow of viscous electron fluid through graphene constrictions

Electron-electron (e-e) collisions can impact transport in a variety of surprising and sometimes counterintuitive ways. Despite strong interest, experiments on the subject proved challenging because of the simultaneous presence of different scattering mechanisms that suppress or obscure consequences of e-e scattering. Only recently, sufficiently clean electron systems with transport dominated by e-e collisions have become available, showing behavior characteristic of highly viscous fluids. Here we study electron transport through graphene constrictions and show that their conductance below 150 K increases with increasing temperature, in stark contrast to the metallic character of doped graphene. Notably, the measured conductance exceeds the maximum conductance possible for free electrons. This anomalous behavior is attributed to collective movement of interacting electrons, which 'shields' individual carriers from momentum loss at sample boundaries. The measurements allow us to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, our work shows that viscous effects can facilitate high-mobility transport at elevated temperatures, a potentially useful behavior for designing graphene-based devices.

cond-mat.mes-hall↗