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March 2009 - February 2015
Publications
Publications (45)
Coherent motion of electrons in Bloch states is one of the fundamental concepts of charge conduction in solid-state physics. In layered materials, however, such a condition often breaks down for the interlayer conduction, when the interlayer coupling is significantly reduced by, e.g., a large interlayer separation. We report that complete suppressi...
Van Hove singularities (VHSs) in the density of states play an outstanding and diverse role for the electronic and thermodynamic properties of crystalline solids1. At the critical point the Fermi surface connectivity changes and topological properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and...
An important development in the field of the fractional quantum Hall effect was the proposal that the 5/2 state observed in the Landau level with orbital index n = 1 of two-dimensional electrons in a GaAs quantum well¹ originates from a chiral p-wave paired state of composite fermions that are topological bound states of electrons and quantized vor...
The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low dimensional physics. For instance, the encapsulation of graphene or MoS2 between atomically flat hexagonal boron nitride (hBN) layers with strong a...
Proximity effects induced in the 2D Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene’s interaction with antiferromagnetic insulators. Here, we report a...
Hexagonal boron nitride (h‐BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h‐BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h‐BN encapsulation greatly removes the defect‐related gap states by stabilizing the chemisorbed oxygen m...
Due to Landau quantization, the conductance of two-dimensional electrons exposed to a perpendicular magnetic field exhibits oscillations that generate a fan of linear trajectories when plotted in the parameter space spanned by density and field. This fan looks identical, irrespective of the dispersion and field dependence of the Landau level energy...
We report the transport properties of α-RuCl3/bilayer graphene heterostructures, where carrier doping is induced by a work function difference, resulting in distinct electron and hole populations in α-RuCl3 and bilayer graphene, respectively. Through a comprehensive analysis of multi-channel transport signatures, including Hall measurements and qua...
The authors report on integer and fractional quantum Hall states in a stack of two twisted Bernal bilayer graphene sheets. By exploiting the momentum mismatch in reciprocal space, the single‐particle tunneling between both bilayers is suppressed. Since the bilayers are spatially separated by only 0.34 nm, the stack benefits from strong interlayer C...
We introduce a novel two-dimensional electronic system with ultrastrong interlayer interactions, namely, twisted bilayer graphene with a large twist angle, as an ideal ground for realizing interlayer-coherent excitonic condensates. In these systems, sub-nanometer atomic separation between the layers allows significant interlayer interactions, while...
Delafossite oxide CuAlO2 has received great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully synthesized with a reactive crucible melting method. The crystals are characterized by X-ray diffraction, scanning electron microscopy with energy-di...
Monolayer transition metal dichalcogenides (TMDs) have received great attention due to their fascinating physical properties and their potential for applications in novel semiconductor devices. However, the intrinsic physical properties of TMDs can be significantly altered by extrinsic disorders such as surface roughness and charge disorder induced...
Hexagonal boron nitride (h-BN) is a key ingredient for various two-dimensional (2D) van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, we report that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemiso...
Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal str...
Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal str...
Delafossite oxide CuAlO2 has engaged great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully achieved with a reactive crucible melting method. The crystals are characterized by X-ray diffraction, scanning electron microscopy with energy-disper...
A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials1–8. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic ge-factor of ~2 and zero-field splitting below...
We report on the quantum Hall effect in two stacked graphene layers rotated by 2°. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the density. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet...
We report on the quantum Hall effect in two stacked graphene layers rotated by 2 degree. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the density. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densitie...
In this chapter, we review the progress toward the ultimate quality of graphene devices. First, we discuss several approaches that indicate device quality. Then, three types of graphene production methods are introduced. These three methods have their unique advantages, however, in this chapter we only focus on their quality. Then, we review recent...
Quantum confinements, especially quantum in narrow wells, have been investigated because of their controllability over electrical parameters. For example, quantum dots can emit a variety of photon wavelengths even for the same material depending on their particle size. More recently, the research into two-dimensional (2D) materials has shown the av...
Systems in which interparticle interactions prevail can be described by hydrodynamics. This regime is typically difficult to access in the solid state for electrons. However, the high purity of encapsulated graphene combined with its advantageous phonon properties make it possible, and hydrodynamic corrections to the conductivity of graphene have b...
Although the recently predicted topological magnetoelectric effect1 and the response to an electric charge that mimics an induced mirror magnetic monopole2 are fundamental attributes of topological states of matter with broken time-reversal symmetry, so far they have not been directly observed in experiments. Using a SQUID-on-tip3, acting simultane...
The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low dimensional physics. For instance, the encapsulation of graphene or MoS2 between atomically flat hexagonal boron nitride (hBN) layers with strong a...
The recently predicted topological magnetoelectric effect and the response to an electric charge that mimics an induced mirror magnetic monopole are fundamental attributes of topological states of matter with broken time reversal symmetry. Using a SQUID-on-tip, acting simultaneously as a tunable scanning electric charge and as ultrasensitive nanosc...
Proximity effects induced in the 2D Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene's interaction with antiferromagnetic insulators. Here, we report a...
Spins constitute a group of quantum objects forming a key resource in modern quantum technology. Two-dimensional (2D) van der Waals materials are of fundamental interest for studying nanoscale magnetic phenomena. However, isolating singular paramagnetic spins in 2D systems is challenging. We report here on a quantum emitting source embedded within...
Tunneling field-effect transistors (TFETs) are of considerable interest owing to their capability of low-power operation. Here, we demonstrate a novel type of TFET which is composed of a thin black phosphorus–tin diselenide (BP–SnSe2) heterostructure. This combination of 2D semiconductor thin sheets enables device operation either as an Esaki diode...
A remarkable development in the field of the fractional quantum Hall effect has been the proposal that the 5/2 state observed in the Landau level with orbital index $n = 1$ of two-dimensional electrons in a GaAs quantum well originates from a chiral $p$-wave paired state of composite fermions which are topological bound states of electrons and quan...
Multi-channel Bi2Se3 thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi2Se3 grew on amorphous Al2O3 even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar....
Combining low-dimensional materials and a wide bandgap semiconductor opens a new playing field for quantum optics using sub-bandgap excitation. 2D van der Waals stacked hexagonal boron nitride (h-BN) has gained considerable attention as it hosts single quantum emitters (QEs) for wavelengths above the UV spectral range. Previously, QEs density in h-...
Twisted bilayer graphene offers a unique bilayer two-dimensional-electron system where the layer separation is only in sub-nanometer scale. Unlike Bernal-stacked bilayer, the layer degree of freedom is disentangled from spin and valley, providing eight-fold degeneracy in the low energy states. We have investigated broken-symmetry quantum Hall (QH)...
Newly discovered van der Waals materials like MoS$_2$, WSe$_2$, hexagonal boron nitride (h-BN) and recently C$_2$N have sparked intensive research to unveil the quantum behavior associated with their 2D topology. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band-gap of 5.95 eV, has been shown to host single qua...
Atomically-thin nanosheets, as recently realized using van der Waals layered materials, offer a versatile platform for studying the stability and tunability of the correlated electron phases in the reduced dimension. Here we investigate a thickness-dependent excitonic insulating (EI) phase on a layered ternary chalcogenide Ta2NiSe5. Using Raman spe...
We performed angle-resolved photoemission experiments on CaC6 and measured kz-dependent electronic structures to investigate the interlayer states. The results reveal a spherical interlayer Fermi surface centered at the Γ point. We also find that the graphene-driven band possesses a weak kz dispersion. The overall electronic structure shows a pecul...
Among various candidates of atomically thin crystals, many of them are known to be chemically unstable against elevated temperatures or moisture/oxygen in ambient conditions. Using a mechanical transfer method combined with regular electron-beam lithography, we developed a simple procedure for fabricating the electronic devices based on air-sensiti...
We have investigated fractional quantum Hall (QH) states in Bernal-stacked bilayer graphene using transconductance fluctuation measurements. A variety of odd-denominator fractional QH states with νQH→ νQH + 2 symmetry, as previously reported, are observed. However, surprising is that also particle-hole symmetric states are clearly resolved in the s...
We employed graphene as a patternable template to protect the intrinsic surface states of thin films of topological insulators (TIs) from environment. Here we find that the graphene provides high-quality interface so that the Shubnikov de Haas (SdH) oscillation associated with a topological surface state could be observed at the interface of a meta...
Topological insulators (TIs) are exotic materials which have topologically protected states on the surface due to strong spin-orbit coupling. However, a lack of ordered growth of TI thin films on amorphous dielectrics and/or insulators presents a challenge for applications of TI-junctions. We report the growth of topological insulator Bi2Se3 thin f...
We report that high-quality single-layer graphene (SLG) has been successfully synthesized directly on various dielectric substrates including amorphous SiO2/Si by a Cu-vapor-assisted chemical vapor deposition (CVD) process. The Cu vapors produced by the sublimation of Cu foil that is suspended above target substrates without physical contact cataly...
We report that high quality single layer graphene (SLG) has been successfully synthesized directly on various dielectric substrates including amorphous SiO2/Si by Cu vapor-assisted chemical vapor deposition (CVD) process. The Cu vapors produced by the sublimation of Cu foil that is suspended above target substrates without physical contact catalyze...
We fabricated graphene pnp devices, by embedding pre-defined local gates in an oxidized surface layer of a silicon substrate. With neither deposition of dielectric material on the graphene nor electron-beam irradiation, we obtained high-quality graphene pnp devices without degradation of the carrier mobility even in the local-gate region. The corre...
