Evidence of charge-ordering and broken rotational symmetry in magic angle twisted bilayer graphene. (arXiv:1904.10153v1 [cond-mat.mes-hall])

2019-04-24T04:30:38+00:00April 24th, 2019|Categories: Publications|Tags: |

The discovery of correlated electronic phases, including a Mott-like insulator and superconductivity, in twisted bilayer graphene (TBLG) near the magic angle, and their intriguing similarity to high-temperature cuprate superconductors, has spurred a surge of research activity to uncover the underlying physical mechanism. Local spectroscopy which is capable of accessing the symmetry and spatial distribution of the spectral function can provide essential clues towards unraveling this puzzle. Here we use scanning tunneling microscopy and spectroscopy in TBLG near the magic angle to visualize the local density of states (DOS) and charge distribution. Doping the sample close to charge neutrality, where transport measurements revealed the emergence of correlated electronic phases, we find as a precursor to the superconducting state a charge-ordered phase that breaks the rotational symmetry of the underlying lattice. The charge modulation shows a d-wave local symmetry and a global charge ordered striped phase that resembles the charge ordering in hole-doped cuprates, providing new evidence which links the electronic structures of these two systems.

Published : "arXiv Mesoscale and Nanoscale Physics".

Graphene Molecule Compared With Fullerene C60 As Circumstellar Carbon Dust Of Planetary Nebula. (arXiv:1904.10140v1 [astro-ph.GA])

2019-04-24T04:30:35+00:00April 24th, 2019|Categories: Publications|Tags: |

It had been understood that astronomically observed infrared spectrum of carbon rich planetary nebula as like Tc 1 and Lin 49 comes from fullerene (C60). Also, it is well known that graphene is a raw material for synthesizing fullerene. This study seeks some capability of graphene based on the quantum-chemical DFT calculation. It was demonstrated that graphene plays major role rather than fullerene. We applied two astrophysical conditions, which are void creation by high speed proton and photo-ionization by the central star. Model molecule was ionized void-graphene (C23) having one carbon pentagon combined with hexagons. By molecular vibrational analysis, we could reproduce six major bands from 6 to 9 micrometer, large peak at 12.8, and largest peak at 19.0. Also, many minor bands could be reproduced from 6 to 38 micrometer. Also, deeply void induced molecules (C22) and (C21) could support observed bands.

Published : "arXiv Mesoscale and Nanoscale Physics".

Low-Temperature Annihilation Rate for Quasi-Localized Excitons in Monolayer MoS2. (arXiv:1904.10084v1 [cond-mat.mes-hall])

2019-04-24T04:30:33+00:00April 24th, 2019|Categories: Publications|Tags: |

The strong Coulomb forces in monolayer transition metal dichalcogenides ensure that optical excitation of band electrons gives rise to Wannier-Mott excitonic states, each of which can be conceptualized as a composite of a Gaussian wavepacket corresponding to center-of-mass motion and an orbital state corresponding to the motion of the electron and hole about the center-of-mass. Here, we show that at low temperature in monolayer MoS2, given quasi-localized excitons and consequently a significant inter-exciton spacing, the excitons undergo dipole-dipole interaction and annihilate one another in a manner analogous to Auger recombination. To construct our model, we assume that each exciton is localized in a region whose length is on the same scale as the excitonic diameter, thus causing the exciton to behave in a fermionic manner, while the distance between neighboring excitons is much larger than the exciton diameter. We construct the orbital ladder operators for each exciton and apply Fermi’s Golden Rule to derive the overall recombination rate as a function of exciton density.

Published : "arXiv Mesoscale and Nanoscale Physics".

Flat Bands in Buckled Graphene Superlattices. (arXiv:1904.10147v1 [cond-mat.mes-hall])

2019-04-24T04:30:31+00:00April 24th, 2019|Categories: Publications|Tags: |

Interactions between stacked two-dimensional (2D) atomic crystals can radically change their properties, leading to essentially new materials in terms of the electronic structure. Here we show that monolayers placed on an atomically flat substrate can be forced to undergo a buckling transition, which results in periodically strained superlattices. By using scanning tunneling microscopy and spectroscopy and support from numerical simulations, we show that such lateral superlattices in graphene lead to a periodically modulated pseudo-magnetic field, which in turn creates a post-graphene material with flat electronic bands. The described approach of controllable buckling of 2D crystals offers a venue for creating other superlattice systems and, in particular, for exploring interaction phenomena characteristic of flat bands.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electron accumulation-type Ohmic contact for MoS2 field-effect transistor. (arXiv:1904.10295v1 [cond-mat.mes-hall])

2019-04-24T04:30:27+00:00April 24th, 2019|Categories: Publications|Tags: , |

The formation of an Ohmic contact at a metal/two-dimensional (2D) semiconductor interface is a critical step for the future development of high-performance and energy-efficient electronic and optoelectronic applications based on semiconducting transition-metal dichalcogenides. The deposition process of metals at high thermal energy introduces crystalline defects in 2D semiconducting layers, leading to an uncontrollable Schottky barrier height regardless of work function of a metal and high contact resistance. Here, we report the fabrication of Ohmic contacts by evaporation of indium (In) at a relatively low thermal energy onto molybdenum disulfide (MoS2), resulting in a van der Waals In MoS2 accumulation-type contact with a metal-induced electron doping density as ~10^12/cm^2. We show that the transport at the In/accumulation-type contact is dominated by the field-emission mechanism over a wide temperature range from 2.4 to 300 K and at a carrier density as low as ~10^12/cm^2 for a few-layered MoS2 device. In this case, the contact resistance reaches 0.6 kOhm um at cryogenic temperatures. These results pave a practically available path for fabricating Ohmic MoS2 contacts for high-performance electronic and optoelectronic applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

$k cdot p$ theory for phosphorene: effective g-factors, Landau levels, excitons. (arXiv:1904.10328v1 [cond-mat.mes-hall])

2019-04-24T04:30:24+00:00April 24th, 2019|Categories: Publications|Tags: , |

Phosphorene, a single layer of black phosphorous, is a direct-band gap two-dimensional semiconductor with promising charge and spin transport properties. The electronic band structure of phosphorene is strongly affected by the structural anisotropy of the underlying crystal lattice. We describe the relevant conduction and valence bands close to the $Gamma$ point by four- and six-band (including spin) $k cdot p$ models, including the previously overlooked interband spin-orbit coupling which is essential for studying anisotropic crystals. All the $k cdot p$ parameters are obtained by a robust fit to {it ab initio} data, by taking into account not only the nominal band structure but also the $k$-dependence of the effective mass. The inclusion of interband spin-orbit coupling allows us to determine dipole transitions along both armchair and zigzag directions. The interband coupling is also key to determine the effective g-factors and Zeeman splittings of the Landau levels. We predict the electron g-factor $g_{c1} approx 2.17$, rather close to the hole $g_{v1}approx 2.20$ for which there exists a range of xperimental data (our value falls within the range). Furthermore, we investigate excitonic effects using the $k cdot p$ models and find exciton binding energy (0.82 eV) and exciton diameters consistent with experiments and {it ab initio} based calculations. The proposed $k cdot p$ Hamiltonians should be useful for investigating magnetic, spin, transport, and optical properties of phosphorene.

Published : "arXiv Mesoscale and Nanoscale Physics".

Landau Level Degeneracy in Twisted Bilayer Graphene: Role of Symmetry Breaking. (arXiv:1904.10452v1 [cond-mat.str-el])

2019-04-24T04:30:22+00:00April 24th, 2019|Categories: Publications|Tags: |

The degeneracy of Landau levels flanking charge neutrality in twisted bilayer graphene is known to change from eight-fold to four-fold when the twist angle is reduced to values near the magic angle of $approx 1.05^circ$. This degeneracy lifting has been reproduced in experiments by multiple groups, and is known to occur even in devices which do not harbor the correlated insulators and superconductors. We propose $C_3$ symmetry breaking as an explanation of such robust degeneracy lifting, and support our proposal by numerical results on the Landau level spectrum in near-magic-angle twisted bilayer graphene. Motivated by recent experiments, we further consider the effect of $C_2$ symmetry breaking on the Landau levels.

Published : "arXiv Mesoscale and Nanoscale Physics".

Generalized Brewster-angle effect in thin-film optical absorbers and its application for graphene hydrogen sensing. (arXiv:1904.10075v1 [physics.optics])

2019-04-24T02:29:33+00:00April 24th, 2019|Categories: Publications|Tags: |

Generalized Brewster angle (GBA) is the incidence angle at which polarization by reflection for p- and s-polarized light takes place. Realizing s-polarization Brewster effect requires a material with magnetic response which is challenging at optical frequencies since the magnetic response of materials at these frequencies is extremely weak. Here, we experimentally realize GBA effect in the visible using a thin-film absorber system consisting of a dielectric film on an absorbing substrate. Polarization by reflection is realized for both p- and s- polarized light at different angles of incidence and multiple wavelengths. We provide a theoretical framework for the generalized Brewster effect in thin-film light absorbers. We demonstrate hydrogen gas sensing using a single layer graphene film transferred on a thin-film absorber at the GBA with ~1 fg/mm2 aerial mass sensitivity. The ultrahigh sensitivity stems from the strong phase sensitivity near point of darkness, particularly at the GBA, and the strong light-matter interaction in planar nanocavities. These findings depart from the traditional domain of thin-films as mere interference optical coatings and highlight its many potential applications including gas sensing and biosensing.

Published in: "arXiv Material Science".

Large-area synthesis of continuous two-dimensional MoTexSe2-x alloy films by chemical vapor deposition. (arXiv:1904.10218v1 [cond-mat.mtrl-sci])

2019-04-24T02:29:29+00:00April 24th, 2019|Categories: Publications|Tags: |

Great achievements have been made in alloying of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), which can allow tunable band gaps for practical applications in optoelectronic devices. However, telluride-based TMDs alloys were less studied due to the difficulties of sample synthesis. Here, in this work we report the large-area synthesis of 2D MoTexSe2-x alloy films with controllable Te composition by a modified alkali metal halides assisted chemical vapor deposition method. The as-prepared films have millimeter-scale transverse size. Raman spectra experiments combining calculated Raman spectra and vibrational images obtained by density functional theory (DFT) confirmed the 2H-phase of the MoTexSe2-x alloys. The A1g mode of MoSe2 shows a significant downshift accompanied by asymmetric broadening to lower wavenumber with increasing value of x, while E12g mode seems unchanged, which were well explained by a phonon confinement model. Our work provides a simple method to synthesize large-scale 2H phase Te-based 2D TMDs alloys for their further applications.

Published in: "arXiv Material Science".

Waveguide-integrated van der Waals heterostructure photodetector at telecom band with high speed and high responsivity. (arXiv:1904.10287v1 [cond-mat.mes-hall])

2019-04-24T02:29:26+00:00April 24th, 2019|Categories: Publications|Tags: , , |

Intensive efforts have been devoted to exploit novel optoelectronic devices based on two-dimensional (2D) transition-metal dichalcogenides (TMDCs) owing to their strong light-matter interaction and distinctive material properties. In particular, photodetectors featuring both high-speed and high-responsivity performance are of great interest for a vast number of applications such as high-data-rate interconnects operated at standardized telecom wavelengths. Yet, the intrinsically small carrier mobilities of TMDCs become a bottleneck for high-speed application use. Here, we present high-performance vertical van der Waals heterostructure-based photodetectors integrated on a silicon photonics platform. Our vertical MoTe2/graphene heterostructure design minimizes the carrier transit path length in TMDCs and enables a record-high measured bandwidth of at least 24GHz under a moderate bias voltage of -3 volts. Applying a higher bias or employing thinner MoTe2 flakes boosts the bandwidth even to 50GHz. Simultaneously, our device reaches a high external responsivity of 0.2A/W for incident light at 1300nm, benefiting from the integrated waveguide design. Our studies shed light on performance trade-offs and present design guidelines for fast and efficient devices. The combination of 2D heterostructures and integrated guided-wave nano photonics defines an attractive platform to realize high-performance optoelectronic devices, such as photodetectors, light-emitting devices and electro-optic modulators.

Published in: "arXiv Material Science".

Two-dimensional second-order topological insulator in graphdiyne. (arXiv:1904.09985v1 [cond-mat.mes-hall])

2019-04-24T02:29:21+00:00April 24th, 2019|Categories: Publications|Tags: |

A second-order topological insulator (SOTI) in $d$ spatial dimensions features topologically protected gapless states at its $(d-2)$-dimensional boundary at the intersection of two crystal faces, but is gapped otherwise. As a novel topological state, it has been attracting great interest, but it remains a challenge to identify a realistic SOTI material in two dimensions (2D). Here, based on first-principles calculations and theoretical analysis, we reveal the already experimentally synthesized large gap semiconductor graphdiyne as the first realistic example of a 2D SOTI, with topologically protected 0D corner states. The role of crystalline symmetry, the robustness against symmetry-breaking, and the possible experimental characterization are discussed. Our results uncover a hidden topological character of graphdiyne and promote it as a concrete material platform for exploring the intriguing physics of higher-order topological phases.

Published in: "arXiv Material Science".

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