arXiv subjects

K. Watanabe

Publications and source records attributed to K. Watanabe.

At least 397 records · Page 22Linked to original sources

arXiv2013

Insulating behavior at the neutrality point in dual-gated, single-layer graphene

The fate of the low-temperature conductance at the charge-neutrality (Dirac) point in a single sheet of graphene is investigated down to 20 mK. As the temperature is lowered, the peak resistivity diverges with a power-law behavior and becomes as high as several Megohms per square at the lowest temperature, in contrast with the commonly observed saturation of the conductivity. As a perpendicular magnetic field is applied, our device remains insulating and directly transitions to the broken-valley-symmetry, nu=0 quantum Hall state, indicating that the insulating behavior we observe at zero magnetic field is a result of broken valley symmetry. Finally we discuss the possible origins of this effect.

cond-mat.mes-hall↗

arXiv2012

Hofstadter's butterfly in moire superlattices: A fractal quantum Hall effect

Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. In 1976 Douglas Hofstadter theoretically considered the intersection of these two problems and discovered that 2D electrons subjected to both a magnetic field and a periodic electrostatic potential exhibit a self-similar recursive energy spectrum. Known as Hofstadter's butterfly, this complex spectrum results from a delicate interplay between the characteristic lengths associated with the two quantizing fields, and represents one of the first quantum fractals discovered in physics. In the decades since, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical crystalline systems (<1 nm periodicity) require impossibly large magnetic fields to reach the commensurability condition, while in artificially engineered structures (>100 nm), the corresponding fields are too small to completely overcome disorder. Here we demonstrate that moire superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a nearly ideal-sized periodic modulation, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall effect features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of Hofstadter's spectrum in graphene provides the further opportunity to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.

cond-mat.mes-hall↗

arXiv2012

Quantum and classical confinement of resonant states in a trilayer graphene Fabry-Perot interferometer

The advent of few-layer graphenes has given rise to a new family of two-dimensional systems with emergent electronic properties governed by relativistic quantum mechanics. The multiple carbon sublattices endow the electronic wavefunctions with pseudospin, a lattice analog of the relativistic electron spin, while the multilayer structure leads to electric field effect tunable electronic bands. Here we use these properties to realize giant conductance oscillations in ballistic trilayer graphene Fabry-Perot interferometers, which result from phase coherent transport through resonant bound states beneath an electrostatic barrier. We cloak these states by selectively decoupling them from the leads, resulting in transport via non-resonant states and suppression of the giant oscillations. Cloaking is achieved both classically, by manipulating quasiparticle momenta with a magnetic field, and quantum mechanically, by locally varying the pseudospin character of the carrier wavefunctions. Our results illustrate the unique potential of trilayer graphene as a versatile platform for electron optics and pseudospintronics.

cond-mat.mes-hall↗

arXiv2012

Electron flow in split-gated bilayer graphene

We present transport measurements on a bilayer graphene sheet with homogeneous back gate and split top gate. The electronic transport data indicates the capability to direct electron flow through graphene nanostructures purely defined by electrostatic gating. By comparing the transconductance data recorded for different top gate geometries - continuous barrier and split-gate - the observed transport features for the split-gate can be attributed to interference effects inside the narrow opening.

cond-mat.mes-hall↗

arXiv2012

Strong Coulomb drag and broken symmetry in double-layer graphene

Spatially separated electron systems remain strongly coupled by electron-electron interactions even when they cannot exchange particles, provided that the layer separation d is comparable to a characteristic distance l between charge carriers within layers. One of the consequences of this remote coupling is a phenomenon called Coulomb drag, in which an electric current passed through one of the layers causes frictional charge flow in the other layer. Previously, only the regime of weak (d>>l) to intermediate (d ~ l) coupling could be studied experimentally. Here we use graphene-BN heterostructures with d down to 1 nm to probe interlayer interactions and Coulomb drag in the limit d<<l where the two Dirac liquids effectively nest within the same plane, but still can be tuned and measured independently. The strongly interacting regime reveals many unpredicted features that are qualitatively different from those observed previously. In particular, although drag vanishes because of electron-hole symmetry when either layer is neutral, we find that drag is at its strongest when /both/ layers are neutral. Under this circumstance, drag is positive in zero magnetic field but changes its sign and rapidly grows in strength with field. The drag remains strong at room temperature. The broken electron-hole symmetry is attributed to mutual polarization of closely-spaced interacting layers. The two-fluid Dirac system offers a large parameter space for further investigation of strong interlayer interaction phenomena.

cond-mat.mes-hall↗

arXiv2012

Electronic compressibility of layer polarized bilayer graphene

We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a 1-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For D > 0, the additional peak is observed only for electrons, while D < 0 the peak appears only for holes. This asymmetry that can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.

cond-mat.mes-hall↗

arXiv2012

Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride

The Schrödinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of band gaps near points of the reciprocal lattice known as Brillouin zone boundaries. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a band gap and instead new Dirac points appear in the electronic structure of the material. Graphene on hexagonal boron nitride (hBN) exhibits a rotation dependent Moiré pattern. In this letter, we show experimentally and theoretically that this Moiré pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy determined by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. The local density of states near these Dirac cones exhibits hexagonal modulations indicating an anisotropic Fermi velocity.

cond-mat.mes-hall↗

arXiv2011

Tunable metal-insulator transition in double-layer graphene heterostructures

We report a double-layer electronic system made of two closely-spaced but electrically isolated graphene monolayers sandwiched in boron nitride. For large carrier densities in one of the layers, the adjacent layer no longer exhibits a minimum metallic conductivity at the neutrality point, and its resistivity diverges at low temperatures. This divergence can be suppressed by magnetic field or by reducing the carrier density in the adjacent layer. We believe that the observed localization is intrinsic for neutral graphene with generic disorder if metallic electron-hole puddles are screened out.

cond-mat.mes-hall↗

arXiv2011

Mechanical cleaning of graphene

Contamination of graphene due to residues from nanofabrication often introduces background doping and reduces charge carrier mobility. For samples of high electronic quality, post-lithography cleaning treatments are therefore needed. We report that mechanical cleaning based on contact mode AFM removes residues and significantly improves the electronic properties. A mechanically cleaned dual-gated bilayer graphene transistor with hBN dielectrics exhibited a mobility of ~36,000 cm2/Vs at low temperature.

cond-mat.mtrl-sci↗

arXiv2011

Long wavelength local density of states oscillations near graphene step edges

Using scanning tunneling microscopy and spectroscopy, we have studied the local density of states (LDOS) of graphene over step edges in boron nitride. Long wavelength oscillations in the LDOS are observed with maxima parallel to the step edge. Their wavelength and amplitude are controlled by the energy of the quasiparticles allowing a direct probe of the graphene dispersion relation. We also observe a faster decay of the LDOS oscillations away from the step edge than in conventional metals. This is due to the chiral nature of the Dirac fermions in graphene.

cond-mat.mes-hall↗

arXiv2011

Tunneling Spectroscopy of Graphene-Boron Nitride Heterostructures

We report on the fabrication and measurement of a graphene tunnel junction using hexagonal-boron nitride as a tunnel barrier between graphene and a metal gate. The tunneling behavior into graphene is altered by the interactions with phonons and the presence of disorder. We extract prop- erties of graphene and observe multiple phonon-enhanced tunneling thresholds. Finally, differences in the measured properties of two devices are used to shed light on mutually-contrasting previous results of scanning tunneling microscopy in graphene.

cond-mat.mes-hall↗

arXiv2011

Micrometer-scale ballistic transport in encapsulated graphene at room temperature

Devices made from graphene encapsulated in hexagonal boron-nitride exhibit pronounced negative bend resistance and an anomalous Hall effect, which are a direct consequence of room-temperature ballistic transport on a micrometer scale for a wide range of carrier concentrations. The encapsulation makes graphene practically insusceptible to the ambient atmosphere and, simultaneously, allows the use of boron nitride as an ultrathin top gate dielectric.

cond-mat.mtrl-sci↗

arXiv2011

Boron Nitride Substrates for High Mobility Chemical Vapor Deposited Graphene

Chemical vapor deposited (CVD) graphene is often presented as a scalable solution to graphene device fabrication, but to date such graphene has exhibited lower mobility than that produced by exfoliation. Using a boron nitride underlayer, we achieve mobilities as high as 37,000 cm^2/Vs, an order of magnitude higher than commonly reported for CVD graphene and better than most exfoliated graphene. This result demonstrates that the barrier to scalable, high mobility CVD graphene is not the growth technique but rather the choice of a substrate that minimizes carrier scattering.

cond-mat.mes-hall↗

arXiv2011

Giant Nonlocality near the Dirac Point in Graphene

Transport measurements have been a powerful tool for uncovering new electronic phenomena in graphene. We report nonlocal measurements performed in the Hall bar geometry with voltage probes far away from the classical path of charge flow. We observe a large nonlocal response near the Dirac point in fields as low as 0.1T, which persists up to room temperature. The nonlocality is consistent with the long-range flavor currents induced by lifting of spin/valley degeneracy. The effect is expected to contribute strongly to all magnetotransport phenomena near the neutrality point.

cond-mat.mes-hall↗

arXiv2011

A Reaction Plane Detector for PHENIX at RHIC

A plastic scintillator paddle detector with embedded fiber light guides and photomultiplier tube readout, referred to as the Reaction Plane Detector (RXNP), was designed and installed in the PHENIX experiment prior to the 2007 run of the Relativistic Heavy Ion Collider (RHIC). The RXNP's design is optimized to accurately measure the reaction plane (RP) angle of heavy-ion collisions, where, for mid-central $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions, it achieved a $2^{nd}$ harmonic RP resolution of $\sim$0.75, which is a factor of $\sim$2 greater than PHENIX's previous capabilities. This improvement was accomplished by locating the RXNP in the central region of the PHENIX experiment, where, due to its large coverage in pseudorapidity ($1.0<|η|<2.8$) and $ϕ$ (2$π$), it is exposed to the high particle multiplicities needed for an accurate RP measurement. To enhance the observed signal, a 2-cm Pb converter is located between the nominal collision region and the scintillator paddles, allowing neutral particles produced in the heavy-ion collisions to contribute to the signal through conversion electrons. This paper discusses the design, operation and performance of the RXNP during the 2007 RHIC run.

nucl-ex↗

arXiv2011

STM Spectroscopy of ultra-flat graphene on hexagonal boron nitride

Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy-momentum dispersion relations which cross at the Dirac point. However, accessing the physics of the low density region at the Dirac point has been difficult because of the presence of disorder which leaves the graphene with local microscopic electron and hole puddles, resulting in a finite density of carriers even at the charge neutrality point. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult. Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance. In this letter, we use scanning tunneling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moire patterns in the topographic images. However, contrary to recent predictions, this conformation does not lead to a sizable band gap due to the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron-hole charge fluctuations are reduced by two orders of magnitude as compared to those on silicon oxide. This leads to charge fluctuations which are as small as in suspended graphene, opening up Dirac point physics to more diverse experiments than are possible on freestanding devices.

cond-mat.mtrl-sci↗

arXiv2010

Fabrication and some properties of biaxially aligned Sr0.6K0.4Fe2As2 superconductors by processing in high magnetic field

We fabricated the c axis and ab-plane biaxially aligned Sr0.6K0.4Fe2As2 superconductor using a two-step magnetic field procedure. The effect of magnetic fields on the structure and superconducting properties of Sr0.6K0.4Fe2As2 has been investigated by using X-ray diffraction and magnetic measurements. The degree of orientation of the samples was about 0.39 for the c axis and 0.51 for ab-plane orientation, as evaluated from the Lotgering factor of X-ray diffraction. This technology might be useful in a variety of potential applications, including preparing iron based superconducting bulks and wires with high critical currents.

cond-mat.supr-con↗

arXiv2010

Multicomponent fractional quantum Hall effect in graphene

We report observation of the fractional quantum Hall effect (FQHE) in high mobility multi-terminal graphene devices, fabricated on a single crystal boron nitride substrate. We observe an unexpected hierarchy in the emergent FQHE states that may be explained by strongly interacting composite Fermions with full SU(4) symmetric underlying degrees of freedom. The FQHE gaps are measured from temperature dependent transport to be up 10 times larger than in any other semiconductor system. The remarkable strength and unusual hierarcy of the FQHE described here provides a unique opportunity to probe correlated behavior in the presence of expanded quantum degrees of freedom.

cond-mat.mes-hall↗