Graphene

/Tag: Graphene

Intercalated Rare-Earth Metals under Graphene on SiC. (arXiv:1811.06111v1 [cond-mat.mes-hall])

2018-11-16T04:30:25+00:00November 16th, 2018|Categories: Publications|Tags: |

Intercalation of rare earth metals ($RE$ = Eu, Dy, and Gd) is achieved by depositing the $RE$ metal on graphene that is grown on silicon-carbide (SiC) and by subsequent annealing at high temperatures to promote intercalation. STM images of the films reveal that the graphene layer is defect free and that each of the intercalated metals has a distinct nucleation pattern. Intercalated Eu forms nano-clusters that are situated on the vertices of a Moir{`e} pattern, while Dy and Gd form randomly distributed nano-clusters. X-ray magnetic circular dichroism (XMCD) measurements of intercalated films reveal the magnetic properties of these $RE$’s nano-clusters. Furthermore, field dependence and temperature dependence of the magnetic moments extracted from the XMCD show paramagnetic-like behaviors with moments that are generally smaller than those predicted by the Brillouin function. XMCD measurements of $RE$-oxides compared with those of the intercalated $RE$’s under graphene after exposure to air for months indicate that the graphene membranes protect these intercalants against oxidation.

Published : "arXiv Mesoscale and Nanoscale Physics".

Effect of Magnetic Field on Goos-H”anchen Shifts in Gaped Graphene Triangular Barrier. (arXiv:1811.06513v1 [cond-mat.mes-hall])

2018-11-16T04:30:21+00:00November 16th, 2018|Categories: Publications|Tags: , |

We study the effect of a magnetic field on Goos-H”anchen shifts in gaped graphene subjected to a double triangular barrier. Solving the wave equation separately in each region composing our system and using the required boundary conditions, we then compute explicitly the transmission probability for scattered fermions. These wavefunctions are then used to derive with the Goos-H”anchen shifts in terms of different physical parameters such as energy, electrostatic potential strength and magnetic field. Our numerical results show that the Goos-H”anchen shifts are affected by the presence of the magnetic field and depend on the geometrical structure of the triangular barrier.

Published : "arXiv Mesoscale and Nanoscale Physics".

Computational insights and the observation of SiC nanograin assembly: towards 2D silicon carbide. (arXiv:1701.07387v2 [cond-mat.mtrl-sci] UPDATED)

2018-11-16T02:29:21+00:00November 16th, 2018|Categories: Publications|Tags: , , , , |

While an increasing number of two-dimensional (2D) materials, including graphene and silicene, have already been realized, others have only been predicted. An interesting example is the two-dimensional form of silicon carbide (2D-SiC). Here, we present an observation of atomically thin and hexagonally bonded nanosized grains of SiC assembling temporarily in graphene oxide pores during an atomic resolution scanning transmission electron microscopy experiment. Even though these small grains do not fully represent the bulk crystal, simulations indicate that their electronic structure already approaches that of 2D-SiC. This is predicted to be flat, but some doubts have remained regarding the preference of Si for sp$^{3}$ hybridization. Exploring a number of corrugated morphologies, we find completely flat 2D-SiC to have the lowest energy. We further compute its phonon dispersion, with a Raman-active transverse optical mode, and estimate the core level binding energies. Finally, we study the chemical reactivity of 2D-SiC, suggesting it is like silicene unstable against molecular absorption or interlayer linking. Nonetheless, it can form stable van der Waals-bonded bilayers with either graphene or hexagonal boron nitride, promising to further enrich the family of two-dimensional materials once bulk synthesis is achieved.

Published in: "arXiv Material Science".

Anomalous quantum interference effects in graphene SNS junctions due to strain-induced gauge fields

2018-11-15T16:33:19+00:00November 15th, 2018|Categories: Publications|Tags: |

Author(s): Hadi Khanjani and Ali G. MoghaddamWe investigate the influence of gauge fields induced by strain on the supercurrent passing through the graphene-based Josephson junctions. We show that, in the presence of a constant pseudomagnetic field BS originating from an arc-shape elastic deformation, the Josephson current is monotonically enh…[Phys. Rev. B 98, 195421] Published Thu Nov 15, 2018

Published in: "Physical Review B".

Generalized Voigt broadening due to thermal fluctuations of electromechanical nanosensors and molecular electronic junctions. (arXiv:1811.05486v1 [cond-mat.mes-hall])

2018-11-15T04:30:23+00:00November 15th, 2018|Categories: Publications|Tags: |

Graphene and other 2D materials give a platform for electromechanical sensing of biomolecules in aqueous, room temperature environments. The electronic current changes in response to mechanical deflection, indicating the presence of forces due to interactions with, e.g., molecular species. We develop illustrative models of these sensors in order to give explicit, compact expressions for the current and signal-to-noise ratio. Electromechanical structures have an electron transmission function that follows a generalized Voigt profile, with thermal fluctuations giving a Gaussian smearing analogous to thermal Doppler broadening in solution/gas-phase spectroscopic applications. The Lorentzian component of the profile comes from the contact to the electrodes. After providing an accurate approximate form of this profile, we calculate the mechanical susceptibility for a representative two-level bridge and the current fluctuations for electromechanical detection. These results give the underlying mechanics of electromechanical sensing in more complex scenarios, such as graphene deflectometry.

Published : "arXiv Mesoscale and Nanoscale Physics".

Visualizing Encapsulated Graphene, its Defects and its Charge Environment by Sub-Micrometer Resolution Electrical Imaging. (arXiv:1811.05912v1 [cond-mat.mes-hall])

2018-11-15T04:30:20+00:00November 15th, 2018|Categories: Publications|Tags: , , |

Devices made from two-dimensional (2D) materials such as graphene or transition metal dichalcogenides possess interesting electronic properties that can become accessible to experimental probes when the samples are protected from deleterious environmental effects by encapsulating them between hexagonal boron nitride (hBN) layers. While the encapsulated flakes can be detected through post-processing of optical images or confocal Raman mapping, these techniques lack the sub-micrometer scale resolution to identify tears, structural defects or impurities, which is crucial for the fabrication of high-quality devices. Here we demonstrate a simple method to visualize such buried flakes with sub-micrometer resolution, by combining Kelvin force probe microscopy (KPFM) with electrostatic force microscopy (EFM). KPFM, which measures surface potential fluctuations, is extremely effective in spotting charged contaminants within and on top of the heterostructure, making it possible to distinguish contaminated regions in the buried flake. When applying a tip bias larger than the surface potential fluctuations, EFM becomes extremely efficient in highlighting encapsulated flakes and their sub-micron structural defects. We show that these imaging modes, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for locating encapsulated conductors, for visualizing nanometer scale defects and bubbles, and for characterizing their local charge environment.

Published : "arXiv Mesoscale and Nanoscale Physics".

Anderson-Bogoliubov and Carlson-Goldman modes in counterflow superconductors. The case study of a double monolayer graphene. (arXiv:1811.05899v1 [cond-mat.mes-hall])

2018-11-15T04:30:16+00:00November 15th, 2018|Categories: Publications|Tags: |

The impact of electron-hole pairing on the spectrum of plasma excitations of double layer systems is investigated. The approach accounts coupling of the scalar potential oscillations with fluctuations of the order parameter $Delta$. The theory is developed with reference to a double monolayer graphene. We find that the spectrum of antisymmetric (acoustic) plasma excitations contains a weakly damped mode below the gap $2Delta$ and a strongly damped mode above the gap. The lower mode can be interpreted as an analog of the Carlson-Goldman mode. This mode has an acoustic dispersion relation at small wave vectors and it saturates at the level $2Delta$ at large wave vectors. Its velocity is larger than the velocity of the Anderson-Bogoliubov mode $v_{AB}=v_F$/$sqrt{2}$, and it can be smaller as well as larger than the Fermi velocity $v_F$. The damping rate of this mode strongly increases under increase in temperature. Out-of-phase oscillations of the order parameters of two spin components are also analyzed. This part of the spectrum contains two modes one of which is recognized as the Anderson-Bogoliubov (phase) mode and the other, as the Schmid (amplitude) mode. With minor modifications the theory describes collective modes in the double bilayer graphene system as well.

Published : "arXiv Mesoscale and Nanoscale Physics".

Unique gap structure and symmetry of the charge density wave in single-layer VSe$_2$. (arXiv:1811.05690v1 [cond-mat.mes-hall])

2018-11-15T02:29:20+00:00November 15th, 2018|Categories: Publications|Tags: |

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe$_2$, which shows a ($sqrt7 times sqrt3$) CDW in contrast to the (4 $times$ 4) CDW for the layers in bulk VSe$_2$. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable ($sqrt7 times sqrt3$) CDW gap of $sim$100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

Published in: "arXiv Material Science".

Composition and Stacking Dependent Topology in Bilayers from the Graphene Family. (arXiv:1811.05525v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:17+00:00November 15th, 2018|Categories: Publications|Tags: , , |

We present a compositional and structural investigation of silicene, germanene, and stanene bilayers from first-principles. Due to the staggering of the individual layers, several stacking patterns are possible, most of which are not available to the bilayer graphene. This structural variety, in conjunction with the presence of the spin-orbit coupling, unveil a diversity of the electronic properties, with the appearance of distinct band features, including orbital hybridization and band inversion. We show that for particular cases, the intrinsic spin Hall response exhibits signatures of non-trivial electronic band topology, making these structures promising candidates to probe Dirac-like physics.

Published in: "arXiv Material Science".

Narrow photoluminescence and Raman peaks of epitaxial MoS 2 on graphene/Ir(1 1 1)

2018-11-14T18:33:53+00:00November 14th, 2018|Categories: Publications|Tags: |

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown ##IMG## [http://ej.iop.org/images/2053-1583/6/1/011006/tdmaaebd3ieqn001.gif] on graphene/Ir(1 1 1). This observation is explained in terms of a weak graphene-MoS 2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS 2 with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS 2 as well as a different thermal expansion of both materials. Molecular beam epitaxy grown MoS 2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties.

Published in: "2DMaterials".

Screening of long-range Coulomb interaction in graphene nanoribbons: Armchair versus zigzag edges

2018-11-14T16:33:30+00:00November 14th, 2018|Categories: Publications|Tags: |

Author(s): H. Hadipour, E. Şaşıoğlu, F. Bagherpour, C. Friedrich, S. Blügel, and I. MertigWe study the electronic screening of the long-range Coulomb interaction in graphene nanoribbons (GNRs) with armchair and zigzag edges as a function of the ribbon width by employing ab initio calculations in conjunction with the random-phase approximation. We find that in GNRs with armchair edges qua…[Phys. Rev. B 98, 205123] Published Wed Nov 14, 2018

Published in: "Physical Review B".

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