/Tag: hBN

Multiple EIT and Frequency Comb in a Mechanically Modulated Quantum Emitter. (arXiv:1806.06992v1 [quant-ph])

2018-06-20T04:30:21+00:00 June 20th, 2018|Categories: Publications|Tags: |

We propose an electromechanical scheme where the electronic degree of freedom of an embedded color center is coupled to the motion of the hosting resonator via dispersive forces. The localized emitters on a free-standing two-dimensional hexagonal boron nitride membrane are considered and studied, specifically. We show that the coupling of membrane vibrations to the electronic degree of freedom of the emitter can reach the strong regime. By suitable driving of a three-level $Lambda$-system composed of two spin degrees of freedom in the electronic ground state as well as an isolated excited state of the emitter a multiple electromagnetically induced transparency spectrum becomes available. The experimental feasibility of the efficient vibrational ground state cooling of the membrane via quantum interference effects in the two-color drive scheme is numerically confirmed. More interestingly, the emission spectrum of the defect exhibits a frequency comb with frequency spacings as small as the fundamental vibrational mode, which finds applications in high-precision spectroscopy.

Published : "arXiv Mesoscale and Nanoscale Physics".

Stable emission and fast optical modulation of quantum emitters in boron nitride nanotubes. (arXiv:1806.06146v1 [quant-ph])

2018-06-19T04:30:20+00:00 June 19th, 2018|Categories: Publications|Tags: |

Atom-like defects in two-dimensional (2D) hexagonal boron nitride (hBN) have recently emerged as a promising platform for quantum information science. Here we investigate single-photon emissions from atomic defects in boron nitride nanotubes (BNNTs). We demonstrate the first optical modulation of the quantum emission from BNNTs with a near-infrared laser. This one-dimensional system displays bright single-photon emission as well as high stability at room temperature and is an excellent candidate for optomechanics. The fast optical modulation of single-photon emission from BNNTs shows multiple electronic levels of the system and has potential applications in optical signal processing.

Published : "arXiv Mesoscale and Nanoscale Physics".

Direct and indirect excitons in boron nitride polymorphs: a story of atomic configuration and electronic correlation. (arXiv:1806.06201v1 [cond-mat.mtrl-sci])

2018-06-19T02:29:23+00:00 June 19th, 2018|Categories: Publications|Tags: , |

We compute and discuss the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual AA’ stacking, Bernal AB, and rhombohedral ABC). We focus on the changes in the electronic band structure and the exciton dispersion induced by the atomic configuration and the electron-hole interaction. Calculations are carried out at the level of the state-of-the-art textit{ab-initio} GW-Bethe-Salpeter equation and by means of an appropriate tight-binding model. We confirm the change from direct to indirect electronic gap when going from single layer to bulk systems and we give a detailed account of its origin by comparing the effect of different stacking sequences. We emphasize that the inclusion of the electron-hole interaction, which flattens the exciton dispersion with respect to the one obtained from the independent-particle picture, is crucial for the correct description of the momentum-dependence of the exciton binding energy. In the AB stacking this effect is particularly important as the lowest-lying exciton is predicted to be direct despite the indirect electronic band gap.

Published in: "arXiv Material Science".

Probing Tunneling Spin Injection into Graphene via Bias Dependence. (arXiv:1806.06526v1 [cond-mat.mes-hall])

2018-06-19T02:29:22+00:00 June 19th, 2018|Categories: Publications|Tags: , , |

The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative DC bias for both kinds of tunnel barriers. The analysis of the bias dependence for injector electrodes with a wide range of contact resistances suggests that the sign reversal correlates with bias voltage rather than current. We consider different mechanisms for nonlinear bias dependence and conclude that the energy-dependent spin-polarized electronic structure of the ferromagnetic electrodes, rather than the electrical field-induced spin drift effect or spin filtering effect of the tunnel barrier, is the most likely explanation of the experimental observations.

Published in: "arXiv Material Science".

Native point defects and impurities in hexagonal boron nitride

2018-06-18T14:33:55+00:00 June 18th, 2018|Categories: Publications|Tags: |

Author(s): L. Weston, D. Wickramaratne, M. Mackoit, A. Alkauskas, and C. G. Van de WalleHexagonal boron nitride (h-BN) is a technologically important electronic and dielectric material. Recently, research into this material has intensified due to the discovery of bright single-photon-emitting color centers in the form of point defects, with potential applications in advanced quantum technologies. Despite this technological promise, the defect physics of this material is still largely unexplored. Here, the authors have performed detailed first-principles investigations of the defect properties of h-BN, finding that the defect physics of this material is dictated by impurities, in particular carbon, oxygen, and hydrogen. They show how the defect properties of this material can be selectively engineered by growth or processing conditions. These insights will be fundamental to controlled generation of single-photon emitters and for general defect engineering of this material.[Phys. Rev. B 97, 214104] Published Mon Jun 18, 2018

Published in: "Physical Review B".

Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures

2018-06-15T18:36:32+00:00 June 15th, 2018|Categories: Publications|Tags: |

Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We

Published in: "Science Advances".

Tunneling spin valves based on Fe$_3$GeTe$_2$/hBN/Fe$_3$GeTe$_2$ van der Waals heterostructures. (arXiv:1806.05411v1 [cond-mat.mes-hall])

2018-06-15T02:29:17+00:00 June 15th, 2018|Categories: Publications|Tags: , |

Thin van der Waals (vdW) layered magnetic materials disclose the possibility to realize vdW heterostructures with new functionalities. Here we report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic electrodes. Low-temperature anomalous Hall effect measurements show that thin Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to the layers, and a very sharp magnetization switching at magnetic field values that depend slightly on their geometry. In Fe3GeTe2/hBN/Fe3GeTe2 heterostructures, we observe a textbook behavior of the tunneling resistance, which is minimum (maximum) when the magnetization in the two electrodes is parallel (antiparallel) to each other. The magnetoresistance is 160% at low temperature, from which we determine the spin polarization of Fe3GeTe2 to be 0.66, corresponding to 83% and 17% of majority and minority carriers, respectively. The measurements also show that, with increasing temperature, the evolution of the spin polarization extracted from the tunneling magnetoresistance is proportional to the temperature dependence of the magnetization extracted from the analysis of the anomalous Hall conductivity. This suggests that the magnetic properties of the surface are representative of those of the bulk, as it may be expected for vdW materials.

Published in: "arXiv Material Science".

Polymer composites based on hexagonal boron nitride and their application in thermally conductive composites

2018-06-14T14:32:25+00:00 June 14th, 2018|Categories: Publications|Tags: |

RSC Adv., 2018, 8,21948-21967DOI: 10.1039/C8RA02685H, Review Article Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Cuiping Yu, Jun Zhang, Wei Tian, Xiaodong Fan, Yagang YaoThis review covers the fabrication of h-BN and its application for thermally conductive

Published in: "RSC Advances".

Measuring the Local Twist Angle and Layer Arrangement in Van der Waals Heterostructures. (arXiv:1806.05155v1 [cond-mat.mtrl-sci])

2018-06-14T02:29:15+00:00 June 14th, 2018|Categories: Publications|Tags: , , |

The properties of Van der Waals heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here we describe the use of spectroscopic Low Energy Electron Microscopy (LEEM) and micro Low Energy Electron Diffraction ({mu}LEED) methods to measure these properties locally. We present results on a MoS$_{2}$/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the lattice, we determine the cleaving history of the MoS$_{2}$ flake, i.e., which layer stems from where in the bulk.

Published in: "arXiv Material Science".

Functionalization of hexagonal boron nitride nanosheets and their copolymerized solid glasses

2018-06-13T18:34:08+00:00 June 13th, 2018|Categories: Publications|Tags: |

Hexagonal boron nitride nanosheets (h-BNNSs) with unique properties, including wide band gap, excellent thermal conductivity and thermal stability, are among the most promising 2D inorganic nanomaterials in recent years. Functionalization of h-BNNSs is necessary to prevent them from strong tendency to agglomeration and to improve their dispersity and interfacial properties in solvents and polymer composites. In this work, amino and silane functionalized h-BNNSs were obtained, respectively. The chemical-bonding amino and silane functionalization of h-BNNSs were verified by FTIR, XPS and TG-MS analyses. The functionalized h-BNNSs show higher concentration, improved compatibility and dispersity in solvents and gel matrices after doping them into organically-modified silicate gel glasses using a sol-gel method. The functionalized h-BNNSs have broadband absorption and their doped glasses show high transmittances in the visible and near-infrared regions. The nonlinear optical properti…

Published in: "2DMaterials".

Synthesis of large-area multilayer hexagonal boron nitride sheets on iron substrates and its use in resistive switching devices

2018-06-13T18:34:04+00:00 June 13th, 2018|Categories: Publications|Tags: , |

Hexagonal boron nitride ( h -BN) is an attractive insulating material for nanoelectronic devices due to its high reliability as dielectric and excellent compatibility with other two dimensional (2D) materials (e.g. graphene, MoS 2 ). Multilayer h -BN stacks have been readily grown on Cu and Pt substrates via chemical vapor deposition (CVD) approach, confirming its potential for wafer scale integration. However, the growth of h -BN on other substrates needs to be also achieved in order to expand the use of this material. Recently, the CVD growth of monolayer h -BN on Fe substrates was reported, but it just focused on material structure characterization. Here we present the first fabrication of electronic devices using multilayer h -BN dielectric stacks grown on Fe foils. We fabricate and characterize resistive switching (RS) devices based on Au/Ag/ h -BN/Fe nanojunctions, and observe the coexistence of both volatile and non-volatile RS d…

Published in: "2DMaterials".

Tailoring the mechanical properties of 2D materials and heterostructures

2018-06-13T18:34:03+00:00 June 13th, 2018|Categories: Publications|Tags: , , |

The mechanical behavior of high quality two-dimensional (2D) crystals offers exciting opportunities for new material design, such as the combination of extremely high in-plane stiffness and bending flexibility, compared to existing three-dimensional (3D) material forms. By combining different 2D crystals vertically or by in-plane stitching, unusual properties can arise due to nonlinear mechanical interactions between them. Van der Waals forces between vertically stacked crystals give rise to a wide range of useful phenomena such as layer-number dependent friction, superlubricity, creasing, and spatial modulation of elastic properties through MoirĂ© structures. In the present article, we review and explain the mechanical behavior of 2D materials and heterostructures (graphene, hexagonal boron nitride, transition metal dichalcogenides). Linear elastic properties of these 2D crystalline monolayers are well-studied using membrane nanoindentation towards their application in nano-elec…

Published in: "2DMaterials".

Valley Physics in Non-Hermitian Artificial Acoustic Boron Nitride

2018-06-12T20:34:13+00:00 June 12th, 2018|Categories: Publications|Tags: |

Author(s): Mudi Wang, Liping Ye, J. Christensen, and Zhengyou LiuThe valley can serve as a new degree of freedom in the manipulation of particles or waves in condensed matter physics, whereas systems containing combinations of gain and loss elements constitute rich building units that can mimic non-Hermitian properties. By introducing gain and loss in artificial …[Phys. Rev. Lett. 120, 246601] Published Tue Jun 12, 2018

Published in: "Physical Review Letters".

Lateral heterostructures of hexagonal boron nitride and graphene: BCN alloy formation and microstructuring mechanism. (arXiv:1806.03892v1 [cond-mat.mes-hall])

2018-06-12T04:30:29+00:00 June 12th, 2018|Categories: Publications|Tags: , , , , |

Integration of individual two-dimensional materials into heterostructures is a crucial step which enables development of new and technologically interesting functional systems of reduced dimensionality. Here, well-defined lateral heterostructures of hexagonal boron nitride and graphene are synthesized on Ir(111) by performing sequential chemical vapor deposition from borazine and ethylene in ultra-high vacuum. Low-energy electron microscopy (LEEM) and selected-area electron diffraction ({mu}-LEED) show that the heterostructures do not consist only of hexagonal boron nitride (an insulator) and graphene (a conductor), but that also a 2D alloy made up of B, C, and N atoms (a semiconductor) is formed. Composition and spatial extension of the alloy can be tuned by controlling the parameters of the synthesis. A new method for in situ fabrication of micro and nanostructures based on decomposition of hexagonal boron nitride is experimentally demonstrated and modeled analytically, which establishes a new route for production of BCN and graphene elements of various shapes. In this way, atomically-thin conducting and semiconducting components can be fabricated, serving as a basis for manufacturing more complex devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Missing links towards understanding equilibrium shapes of hexagonal boron nitride: algorithm, hydrogen passivation, and temperature effects. (arXiv:1806.03799v1 [cond-mat.mtrl-sci])

2018-06-12T02:29:19+00:00 June 12th, 2018|Categories: Publications|Tags: , |

There is a large discrepancy between the experimental observations and the theoretical predictions in the morphology of hexagonal boron nitride (h-BN) nanosheets. Theoretically-predicted hexagons terminated by armchair edges are not observed in experiments; and experimentally-observed triangles terminated by zigzag edges are found theoretically unstable. There are two key issues in theoretical investigations, namely, an efficient and accurate algorithm of absolute formation energy of h-BN edges, and a good understanding of the role of hydrogen passivation during h-BN growth. Here, we first proposed an efficient algorithm to calculate asymmetric edges with a self-consistent accuracy of about 0.0014 eV/{AA}. This method can also potentially serve as a standard approach for other two-dimensional (2D) compound materials. Then, by using this method, we discovered that only when edges are passivated by hydrogen atoms and temperature effects are taken into account can experimental morphology be explained. We further employed Wulff construction to obtain the equilibrium shapes of H-passivated h-BN nanosheets under its typical growth conditions at T = 1300 K and p = 1 bar, and found out that the equilibrium shapes are sensitive to hydrogen passivation and the growth conditions. Our results resolved long-standing discrepancies between experimental observations and theoretical analysis, explaining the thermodynamic driving force of the triangular, truncated triangular, and hexagonal shapes, and revealing the key role of hydrogen in h-BN growth. These discoveries and the advancement in algorithm may open the gateway towards the realization of 2D electronic and spintronic devices based on h-BN.

Published in: "arXiv Material Science".

Probing nonlocal effects in metals with graphene plasmons

2018-06-11T16:33:34+00:00 June 11th, 2018|Categories: Publications|Tags: , |

Author(s): Eduardo J. C. Dias, David Alcaraz Iranzo, P. A. D. Gonçalves, Yaser Hajati, Yuliy V. Bludov, Antti-Pekka Jauho, N. Asger Mortensen, Frank H. L. Koppens, and N. M. R. PeresIn this paper, we analyze the effects of nonlocality on the optical properties of a system consisting of a thin metallic film separated from a graphene sheet by a hexagonal boron nitride (hBN) layer. We show that nonlocal effects in the metal have a strong impact on the spectrum of the surface plasm…[Phys. Rev. B 97, 245405] Published Mon Jun 11, 2018

Published in: "Physical Review B".

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