arXiv subjects

K. Watanabe

Publications and source records attributed to K. Watanabe.

At least 379 records · Page 21Linked to original sources

arXiv2014

Numerical evaluation of AC losses in an HTS insert coil for high field magnet during its energization

AC losses in a high temperature superconducting (HTS) insert coil for a 25-T cryogen-free superconducting magnet are numerically calculated during its energization, assuming slab approximation. The HTS insert coil consists of 68 single pancakes wound with coated conductors and generating a central magnetic field of 11.5 T, in addition to a contribution of 14.0 T from a set of low temperature superconducting (LTS) outsert coils. Both the HTS and LTS coils are cooled using cryocoolers, and energized simultaneously up to 25.5 T with a constant ramp rate for 60 min. The influences of the magnitudes and orientations of the locally applied magnetic fields, magnetic interactions between turns, and the transport currents flowing in the windings are taken into account in the AC loss calculations. The locally applied fields are separated into axial and radial components, and the individual contributions of these field components to the AC losses are summed simply to obtain the total losses. The contribution of the axial field component to the total AC loss is large at the early stages of the energization, whereas the total losses monotonically increase with time after the contribution of the radial field component becomes sufficiently large.

cond-mat.supr-con↗

arXiv2014

Electronic Properties of Graphene Encapsulated with Different Two-Dimensional Atomic Crystals

Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micron-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulphides and hBN are found to exhibit consistently high carrier mobilities of about 60,000 cm$^{2}$V$^{-1}$s$^{-1}$. In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ~ 1,000 cm$^{2}$ V$^{-1}$s$^{-1}$. We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.

cond-mat.mes-hall↗

arXiv2014

Reconfigurable p-n Junction Diodes and the Photovoltaic Effect in Exfoliated MoS2 Films

Realizing basic semiconductor devices such as p-n junctions are necessary for developing thin-film and optoelectronic technologies in emerging planar materials such as MoS2. In this work, electrostatic doping by buried gates is used to study the electronic and optoelectronic properties of p-n junctions in exfoliated MoS2 flakes. Creating a controllable doping gradient across the device leads to the observation of the photovoltaic effect in monolayer and bilayer MoS2 flakes. For thicker flakes, strong ambipolar conduction enables realization of fully reconfigurable p-n junction diodes with rectifying current-voltage characteristics, and diode ideality factors as low as 1.6. The spectral response of the photovoltaic effect shows signatures of the predicted band gap transitions. For the first excitonic transition, a shift of >4kBT is observed between monolayer and bulk devices, indicating a thickness-dependence of the excitonic coulomb interaction.

cond-mat.mes-hall↗

arXiv2014

Commensurate-incommensurate transition for graphene on hexagonal boron nitride

When a crystal is subjected to a periodic potential, under certain circumstances (such as when the period of the potential is close to the crystal periodicity; the potential is strong enough, etc.) it might adjust itself to follow the periodicity of the potential, resulting in a, so called, commensurate state. Such commensurate-incommensurate transitions are ubiquitous phenomena in many areas of condensed matter physics: from magnetism and dislocations in crystals, to vortices in superconductors, and atomic layers adsorbed on a crystalline surface. Of particular interest might be the properties of topological defects between the two commensurate phases: solitons, domain walls, and dislocation walls. Here we report a commensurate-incommensurate transition for graphene on top of hexagonal boron nitride (hBN). Depending on the rotational angle between the two hexagonal lattices, graphene can either stretch to adjust to a slightly different hBN periodicity (the commensurate state found for small rotational angles) or exhibit little adjustment (the incommensurate state). In the commensurate state, areas with matching lattice constants are separated by domain walls that accumulate the resulting strain. Such soliton-like objects present significant fundamental interest, and their presence might explain recent observations when the electronic, optical, Raman and other properties of graphene-hBN heterostructures have been notably altered.

cond-mat.mes-hall↗

arXiv2014

Selective equilibration of spin and valley polarized quantum Hall edge states in graphene

We report on transport measurements of dual-gated, single-layer graphene devices in the quantum Hall regime, allowing for independent control of the filling factors in adjoining regions. Progress in device quality allows us to study scattering between edge states when the four-fold degeneracy of the Landau level is lifted by electron correlations, causing edge states to be spin and/or valley polarized. In this new regime, we observe a dramatic departure from the equilibration seen in more disordered devices: edge states with opposite spins propagate without mixing. As a result, the degree of equilibration inferred from transport can reveal the spin polarization of the ground state at each filling factor. In particular, the first Landau level is shown to be spin-polarized at half-filling, providing an independent confirmation of a conclusion of Ref.[1]. The conductance in the bipolar regime is strongly suppressed, indicating that co-propagating edge states, even with the same spin, do not equilibrate along PN interfaces. We attribute this behavior to the formation of an insulating nu=0 stripe at the PN interface.

cond-mat.mes-hall↗

arXiv2014

Photo-induced Doping in Graphene/Boron Nitride Heterostructures

The design of stacks of layered materials in which adjacent layers interact by van der Waals forces[1] has enabled the combination of various two-dimensional crystals with different electrical, optical and mechanical properties, and the emergence of novel physical phenomena and device functionality[2-8]. Here we report photo-induced doping in van der Waals heterostructures (VDHs) consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photo-induced doping maintains the high carrier mobility of the graphene-boron nitride (G/BN) heterostructure, which resembles the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially-varying doping profiles such as p-n junctions. We show that this photo-induced doping arises from microscopically coupled optical and electrical responses of G/BN heterostructures, which includes optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.

cond-mat.mes-hall↗

arXiv2014

Electric Field Control of Soliton Motion and Stacking in Trilayer Graphene

The crystal structure of a material plays an important role in determining its electronic properties. Changing from one crystal structure to another involves a phase transition which is usually controlled by a state variable such as temperature or pressure. In the case of trilayer graphene, there are two common stacking configurations (Bernal and rhombohedral) which exhibit very different electronic properties. In graphene flakes with both stacking configurations, the region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon-carbon bond distance. Here we show the ability to move this strain soliton with a perpendicular electric field and hence control the stacking configuration of trilayer graphene with only an external voltage. Moreover, we find that the free energy difference between the two stacking configurations scales quadratically with electric field, and thus rhombohedral stacking is favored as the electric field increases. This ability to control the stacking order in graphene opens the way to novel devices which combine structural and electrical properties.

cond-mat.mtrl-sci↗

arXiv2014

Ballistic transport in graphene grown by chemical vapor deposition

In this letter we report the observation of ballistic transport on micron length scales in graphene synthesised by chemical vapour deposition (CVD). Transport measurements were done on Hall bar geometries in a liquid He cryostat. Using non-local measurements we show that electrons can be ballistically directed by a magnetic field (transverse magnetic focussing) over length scales of ~μm. Comparison with atomic force microscope measurements suggests a correlation between the absence of wrinkles and the presence of ballistic transport in CVD graphene.

cond-mat.mes-hall↗

arXiv2014

Kondo-like mass enhancement of Dirac fermion in iron pnictides Ba(Fe$_{1-x}$Mn$_x$As)$_2$

The effect of Mn substitution, acting as a magnetic impurity for Fe, on the Dirac cone was investigated in Ba(Fe$_{1-x}$Mn$_x$As)$_2$. Both magnetoresistance and Hall resistivity studies clearly indicate that the cyclotron effective mass ($m^{\ast}$) of the Dirac cone is anomalously enhanced at low temperatures by the impurity, although its evolution as a function of carrier number proceeds in a conventional manner at higher temperatures. Kondo-like band renormalization induced by the magnetic impurity scattering is suggested as an explanation for this, and the anomalous mass enhancement of the Dirac fermions is discussed.

cond-mat.supr-con↗

arXiv2013

Etched graphene single electron transistors on hexagonal boron nitride in high magnetic fields

We report on the fabrication and electrical characterisation of etched graphene single electron transistors (SETs) of various sizes on hexagonal boron nitride (hBN) in high magnetic fields. The electronic transport measurements show a slight improvement compared to graphene SETs on SiO2. In particular, SETs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T in contrast to measurements reported on SETs on SiO2. This result indicates a reduced surface disorder potential in SETs on hBN which might be an important step towards clean and more controllable graphene QDs.

cond-mat.mes-hall↗

arXiv2013

Screening Charged Impurities and Lifting the Orbital Degeneracy in Graphene by Populating Landau Levels

We report the observation of an isolated charged impurity in graphene and present direct evidence of the close connection between the screening properties of a 2D electron system and the influence of the impurity on its electronic environment. Using scanning tunneling microscopy and Landau level spectroscopy we demonstrate that in the presence of a magnetic field the strength of the impurity can be tuned by controlling the occupation of Landau-level states with a gate-voltage. At low occupation the impurity is screened becoming essentially invisible. Screening diminishes as states are filled until, for fully occupied Landau-levels, the unscreened impurity significantly perturbs the spectrum in its vicinity. In this regime we report the first observation of Landau-level splitting into discrete states due to lifting the orbital degeneracy.

cond-mat.mes-hall↗

arXiv2013

Gate dependent Raman spectroscopy of graphene on hexagonal boron nitride

Raman spectroscopy, a fast and nondestructive imaging method, can be used to monitor the doping level in graphene devices. We fabricated chemical vapor deposition (CVD) grown graphene on atomically flat hexagonal boron nitride (hBN) flakes and SiO$_2$ substrates. We compared their Raman response as a function of charge carrier density using an ion gel as a top gate. The G peak position, 2D peak position, 2D peak width and the ratio of the 2D peak area to the G peak area show a dependence on carrier density that differs for hBN compared to SiO$_2$. Histograms of two-dimensional mapping are used to compare the fluctuations in the Raman peak properties between the two substrates. The hBN substrate has been found to produce fewer fluctuations at the same charge density owing to its atomically flat surface and reduced charged impurities.

cond-mat.mtrl-sci↗

arXiv2013

Extending spin coherence times of diamond qubits by high temperature annealing

Spins of negatively charged nitrogen-vacancy (NV$^-$) defects in diamond are among the most promising candidates for solid-state qubits. The fabrication of quantum devices containing these spin-carrying defects requires position-controlled introduction of NV$^-$ defects having excellent properties such as spectral stability, long spin coherence time, and stable negative charge state. Nitrogen ion implantation and annealing enable the positioning of NV$^-$ spin qubits with high precision, but to date, the coherence times of qubits produced this way are short, presumably because of the presence of residual radiation damage. In the present work, we demonstrate that a high temperature annealing at 1000$^\circ$C allows 2 millisecond coherence times to be achieved at room temperature. These results were obtained for implantation-produced NV$^-$ defects in a high-purity, 99.99% $^{12}$C enriched single crystal chemical vapor deposited diamond. We discuss these remarkably long coherence times in the context of the thermal behavior of residual defect spins. [Published in Physical Review B {\bf{88}}, 075206 (2013)]

quant-ph↗

arXiv2013

Dielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride

Kohn anomalies in three-dimensional metallic crystals are dips in the phonon dispersion that are caused by abrupt changes in the screening of the ion-cores by the surrounding electron-gas. These anomalies are also present at the high-symmetry points Γand K in the phonon dispersion of two-dimensional graphene, where the phonon wave-vector connects two points on the Fermi surface. The linear slope around the kinks in the highest optical branch is proportional to the electron-phonon coupling. Here, we present a combined theoretical and experimental study of the influence of the dielectric substrate on the vibrational properties of graphene. We show that screening by the dielectric substrate reduces the electron-phonon coupling at the high-symmetry point K and leads to an up-shift of the Raman 2D-line. This results in the observation of a Kohn anomaly that can be tuned by screening. The exact position of the 2D-line can thus be taken also as a signature for changes in the (electron-phonon limited) conductivity of graphene.

cond-mat.mes-hall↗

arXiv2013

Etched graphene quantum dots on hexagonal boron nitride

We report on the fabrication and characterization of etched graphene quantum dots (QDs) on hexagonal boron nitride (hBN) and SiO2 with different island diameters. We perform a statistical analysis of Coulomb peak spacings over a wide energy range. For graphene QDs on hBN, the standard deviation of the normalized peak spacing distribution decreases with increasing QD diameter, whereas for QDs on SiO2 no diameter dependency is observed. In addition, QDs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T. Both results indicate a substantially reduced substrate induced disorder potential in graphene QDs on hBN.

cond-mat.mes-hall↗

arXiv2013

A Possible Detection of Solar Gamma-Rays by the Ground Level Detector

On March 7, 2011 from 19:48:00 to 20:03:00 UT, the solar neutron telescope located at Mt. Sierra Negra, Mexico (4,600m) observed a 8.8sigma enhancement. In this paper, we would like to try to explain this enhancement by a hypothesis that a few GeV gamma-rays arrived at the top of the mountain produced by the Sun. We postulate that protons were accelerated at the shock front. They precipitate at the solar surface and produced those gamma-rays. If hypothesis is confirmed, this enhancement is the first sample of GeV gamma-rays observed by a ground level detector.

astro-ph.HE↗

arXiv2013

Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires, and graphene. Recently, a new paradigm has emerged with the advent of symmetry-protected surface states on the boundary of topological insulators, enabling the creation of electronic systems with novel properties. For example, time reversal symmetry (TRS) endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, locking the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations of the one-dimensional boundary states of a two-dimensional topological insulator are also possible, but have yet to be observed in the leading candidate materials. Here, we demonstrate experimentally that charge neutral monolayer graphene displays a new type of quantum spin Hall (QSH) effect, previously thought to exist only in TRS topological insulators, when it is subjected to a very large magnetic field angled with respect to the graphene plane. Unlike in the TRS case, the QSH presented here is protected by a spin-rotation symmetry that emerges as electron spins in a half-filled Landau level are polarized by the large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic (CAF) state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture.

cond-mat.mes-hall↗

arXiv2013

Massive Dirac fermions and Hofstadter butterfly in a van der Waals heterostructure

Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally-aligned hexagonal boron nitride substrate. The spatially-varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moiré superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.

cond-mat.mes-hall↗