Metasurface-mediated anisotropic radiative heat transfer between nanoparticles. (arXiv:1903.08372v1 [cond-mat.mes-hall])

2019-03-21T04:30:32+00:00March 21st, 2019|Categories: Publications|Tags: , |

Metasurfaces, the two-dimensional (2D) counterpart of metamaterials, have recently attracted a great attention due to their amazing properties such as negative refraction, hyperbolic dispersion, manipulation of the evanescent spectrum. In this work, we propose a theory model for the near field radiative heat transfer (NFRHT) between two nanoparticles in the presence of an anisotropic metasurface. Specifically, we set the metasurface as an array of graphene strips (GS) since it is an ideal platform to implement any metasurface topology, ranging from isotropic to hyperbolic propagation. We show that the NFRHT between two nanoparticles can not only be significantly amplified when they are placed in proximity of the GS, but also be regulated over several orders of magnitude. In this configuration, the anisotropic surface plasmon polaritons (SPPs) supported by the GS are excited and provide a new channel for the near-field energy transport. We analyze how the conductance between two nanoparticles depends on the orientation, the structure parameters and the chemical potential of the GS, on the particle-surface or the particle-surface distances by clearly identifying the characteristics of the anisotropic SPPs such as dispersion relations, propagation length and decay length. Our findings provide a powerful way to regulate the energy transport in the particle systems, meanwhile in turn, open up a way to explore the anisotropic optical properties of the metasurface based on the measured heat transfer properties.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electric Field Tunable Correlated States and Magnetic Phase Transitions in Twisted Bilayer-Bilayer Graphene. (arXiv:1903.08596v1 [cond-mat.str-el])

2019-03-21T04:30:26+00:00March 21st, 2019|Categories: Publications|Tags: , |

The recent discovery of correlated insulator states and superconductivity in magic-angle twisted bilayer graphene has paved the way to the experimental investigation of electronic correlations in tunable flat band systems realized in twisted van der Waals heterostructures. This novel twist angle degree of freedom and control should be generalizable to other 2D systems, which may exhibit similar correlated physics behavior while at the same time enabling new techniques to tune and control the strength of electron-electron interactions. Here, we report on a new highly tunable correlated system based on small-angle twisted bilayer-bilayer graphene (TBBG), consisting of two rotated sheets of Bernal-stacked bilayer graphene. We find that TBBG exhibits a rich phase diagram, with tunable correlated insulators states that are highly sensitive to both twist angle and to the application of an electric displacement field, the latter reflecting the inherent polarizability of Bernal-stacked bilayer graphene. We find correlated insulator states that can be switched on and off by the displacement field at all integer electron fillings of the moir'{e} unit cell. The response of these correlated states to magnetic fields points towards evidence of electrically switchable magnetism. Moreover, the strong dependence of the resistance at low temperature and near the correlated insulator states indicates possible proximity to a superconducting phase. Furthermore, in the regime of lower twist angles, TBBG shows multiple sets of flat bands near charge neutrality, resulting in numerous correlated states corresponding to half-filling of each of these flat bands. Our results pave the way to the exploration of

Published : "arXiv Mesoscale and Nanoscale Physics".

Comprehensive tunneling spectroscopy of quasi-freestanding MoS$_2$ on graphene on Ir(111). (arXiv:1903.08601v1 [cond-mat.mes-hall])

2019-03-21T04:30:24+00:00March 21st, 2019|Categories: Publications|Tags: |

We apply scanning tunneling spectroscopy to determine the bandgaps of mono-, bi- and trilayer MoS$_2$ grown on a graphene single crystal on Ir(111). Besides the typical scanning tunneling spectroscopy at constant height, we employ two additional spectroscopic methods giving extra sensitivity and qualitative insight into the $k$-vector of the tunneling electrons. Employing this comprehensive set of spectroscopic methods in tandem, we deduce a bandgap of $2.53pm0.08$ eV for the monolayer. This is close to the predicted values for freestanding MoS$_2$ and larger than is measured for MoS$_2$ on other substrates. Through precise analysis of the `comprehensive’ tunneling spectroscopy we also identify critical point energies in the mono- and bilayer MoS$_2$ band structures. These compare well with their calculated freestanding equivalents, evidencing the graphene/Ir(111) substrate as an excellent environment upon which to study the many feted electronic phenomena of monolayer MoS$_2$ and similar materials. Additionally, this investigation serves to expand the fledgling field of the comprehensive tunneling spectroscopy technique itself.

Published : "arXiv Mesoscale and Nanoscale Physics".

Graphene–semiconductor heterojunction sheds light on emerging photovoltaics

2019-03-21T02:37:00+00:00March 21st, 2019|Categories: Publications|Tags: |

Graphene–semiconductor heterojunction sheds light on emerging photovoltaicsGraphene–semiconductor heterojunction sheds light on emerging photovoltaics, Published online: 20 March 2019; doi:10.1038/s41566-019-0391-9Graphene-on-bulk semiconductors may enable new directions for ultrathin optoelectronics and photovoltaics.

Published in: "Nature Photonics".

Combined molecular dynamics and phase-field modelling of crack propagation in defective graphene. (arXiv:1903.08390v1 [cond-mat.mtrl-sci])

2019-03-21T02:29:29+00:00March 21st, 2019|Categories: Publications|Tags: , |

In this work, a combined modelling approach for crack propagation in defective graphene is presented. Molecular dynamics (MD) simulations are used to obtain material parameters (Young’s modulus and Poisson ratio) and to determine the energy contributions during the crack evolution. The elastic properties are then applied in phase-field continuum simulations which are based on the Griffith energy criterion for fracture. In particular, the influence of point defects on elastic properties and the fracture toughness are investigated. For the latter, we obtain values consistent with recent experimental findings. Further, we discuss alternative definitions of an effective fracture toughness, which accounts for the conditions of crack propagation and establishes a link between dynamic, discrete and continuous, quasi-static fracture processes on MD level and continuum level, respectively. It is demonstrated that the combination of MD and phase-field simulations is a well-founded approach to identify defect-dependent material parameters.

Published in: "arXiv Material Science".

Confined step-flow growth of Cu intercalated between graphene and a Ru(0001) surface. (arXiv:1903.08397v1 [cond-mat.mtrl-sci])

2019-03-21T02:29:26+00:00March 21st, 2019|Categories: Publications|Tags: , |

By comparing the growth of Cu thin films on bare and graphene-covered Ru(0001) surfaces, we demonstrate the role of graphene as a surfactant allowing the formation of flat Cu films. Low-energy electron microscopy, X-ray photoemission electron microscopy and X-ray absorption spectroscopy reveal that depositing Cu at 580 K leads to distinct behaviors on both types of surfaces. On bare Ru, a Stranski-Krastanov growth is observed, with first the formation of an atomically flat and monolayer-thick wetting layer, followed by the nucleation of three-dimensional islands. In sharp contrast, when Cu is deposited on a graphene-covered Ru surface under the very same conditions, Cu intercalates below graphene and grows in a step-flow manner: atomically-high growth fronts of intercalated Cu form at the graphene edges, and extend towards the center of the flakes. Our findings suggest potential routes in metal heteroepitaxy for the control of thin film morphology.

Published in: "arXiv Material Science".

Van der Waals integration before and beyond two-dimensional materials

2019-03-20T18:37:00+00:00March 20th, 2019|Categories: Publications|

Van der Waals integration before and beyond two-dimensional materialsVan der Waals integration before and beyond two-dimensional materials, Published online: 20 March 2019; doi:10.1038/s41586-019-1013-xRecent advances and future directions in the use of van der Waals integration beyond two-dimensional materials are reviewed.

Published in: "Nature".

Twists in ferromagnetic monolayers with trigonal prismatic symmetry

2019-03-20T14:35:34+00:00March 20th, 2019|Categories: Publications|Tags: , |

Author(s): Kjetil M. D. Hals and Karin Everschor-SitteTwo-dimensional materials such as graphene or hexagonal boron nitride are indispensable in industry. The recently discovered 2D ferromagnetic materials also promise to be vital for applications. In this work, we develop a phenomenological description of noncentrosymmetric 2D ferromagnets with trigon…[Phys. Rev. B 99, 104422] Published Wed Mar 20, 2019

Published in: "Physical Review B".

Nonlinear magnetotransport shaped by Fermi surface topology and convexity

2019-03-20T10:34:26+00:00March 20th, 2019|Categories: Publications|Tags: |

Nonlinear magnetotransport shaped by Fermi surface topology and convexityNonlinear magnetotransport shaped by Fermi surface topology and convexity, Published online: 20 March 2019; doi:10.1038/s41467-019-09208-8The nature of non-saturating magnetoresistance (MR) in topological materials is an important issue in condensed matter research but remains elusive. The authors here report the nonlinear MR at room temperature in WTe2 with temperature-driven inversion due to the temperature-induced changes in Fermi surface convexity.

Published in: "Nature Communications".

Tunneling and Fluctuating Electron-Hole Cooper Pairs in Double Bilayer Graphene. (arXiv:1903.07739v1 [cond-mat.mes-hall])

2019-03-20T04:30:31+00:00March 20th, 2019|Categories: Publications|Tags: |

A strong low-temperature enhancement of the tunneling conductance between graphene bilayers has been reported recently, and interpreted as a signature of electron-hole pairing. The pairing in electron-hole double layers was first predicted more than forty years ago but has since avoided observation. Here we provide a detailed theory of conductance enhanced by electron-hole Cooper pair fluctuations, which are a precursor to equilibrium pairing, that reflects specific details of double graphene bilayer systems. Above the equilibrium condensation temperature, the pairs have finite temporal coherence and do not support dissipationless tunneling. Instead they strongly boost the tunneling conductivity via a fluctuational internal Josephson effect. We find dependences of the zero-bias peak in the differential tunneling conductance on temperature and electron-hole density imbalance that are in an enough good agreement with experiment.

Published : "arXiv Mesoscale and Nanoscale Physics".

Spin-polarized Correlated Insulator and Superconductor in Twisted Double Bilayer Graphene. (arXiv:1903.08130v1 [cond-mat.mes-hall])

2019-03-20T04:30:22+00:00March 20th, 2019|Categories: Publications|Tags: , , |

Ferromagnetism and superconductivity typically compete with each other since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors. Only a handful of ferromagnetic superconductors are known in heavy fermion systems, where many-body electron interactions promoted by the narrow energy bands play a key role in stabilizing these emergent states. Recently, interaction-driven superconductivity and ferromagnetism have been demonstrated in different density regimes of flat bands enabled by graphene moire superlattices, as separate phenomena. Combining superconductivity and magnetism in a single ground state may lead to more exotic quantum phases. Here, employing van der Waals heterostructures of twisted double bilayer graphene (TDBG), we realize a flat electron band that is tunable by perpendicular electric fields. Similar to the magic angle twisted bilayer graphene, TDBG exhibits energy gaps at the half and quarter filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the ferromagnetic half-filled insulator, superconductivity appears with a critical temperature controlled by both density and electric fields. We observe that the in-plane magnetic field enhances the superconductivity in the low field regime, suggesting spin-polarized electron pairing. Spin-polarized superconducting states discovered in TDBG provide a new route to engineering interaction-driven topological superconductivity.

Published : "arXiv Mesoscale and Nanoscale Physics".

The Strength of Mechanically-Exfoliated Monolayer Graphene. (arXiv:1903.07695v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:41+00:00March 20th, 2019|Categories: Publications|Tags: |

The deformation and fracture behaviour of one-atom-thick mechanically exfoliated graphene has been studied in detail. Monolayer graphene flakes with different lengths, widths and shapes were successfully prepared by mechanical exfoliation and deposited onto poly(methyl methacrylate) (PMMA) beams. The fracture behaviour of the monolayer graphene was followed by deforming the PMMA beams. Through in-situ Raman mapping at different strain levels, the distributions of strain over the graphene flakes were determined from the shift of the graphene Raman 2D band. The failure mechanisms of the exfoliated graphene were either by flake fracture or failure of the graphene/polymer interface. The fracture of the flakes was observed from the formation of cracks identified from the appearance of lines of zero strain in the strain contour maps. It was found that the strength of the monolayer graphene flakes decreased with increasing flake width. The strength dropped to less than 10 GPa for large flakes, much lower than the reported value of 130 GPa for monolayer graphene, thought to be due to the presence of defects. It is shown that a pair of topological defects in monolayer graphene will form a pseudo crack and the effect of such defects upon the strength of monolayer graphene has been modelled using molecular mechanical simulations.

Published in: "arXiv Material Science".

Physical origin of giant excitonic and magneto-optical responses in two-dimensional ferromagnetic insulators. (arXiv:1903.07787v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:37+00:00March 20th, 2019|Categories: Publications|

The magneto-optical (MO) effects, such as the magneto-optical Kerr effect (MOKE) and the Faraday effect (FE), have been intensively investigated experimentally in a variety of magnetic materials serving as a highly sensitive probe for electronic and magnetic properties. Recent measurements using MOKE have led to the discovery of two-dimensional (2D) magnets, and demonstrated their rich magnetic behaviors. In particular, a giant Kerr response has been measured in mono- and few-layer CrI$_3$. However, the exact microscopic origin of such large MO signals in 2D materials is still unknown, because treating accurately sizable spin-orbit coupling (SOC) and excitonic effects that are essential for such an understanding is beyond the capability of existing methods. With a newly developed full-spinor GW and Bethe-Salpeter equation (GW-BSE) method, we show, for the first time from first principles, that the exceedingly large optical and MO responses in ferromagnetic monolayer CrI$_3$ arise from strongly bound exciton states consisting of spin-polarized electron-hole pairs. These exciton states are shown to have distinct characteristics compared with either the Frenkel excitons in ionic crystals and polymers, or Wannier excitons in other 2D semiconductors. By simulating a realistic experimental setup, we further find that substrate configuration and excitation frequency of the photon strongly shape the MO signals. Our results provide a new conceptual mechanism, explaining quantitatively the recent experiments on CrI$_3$. In addition, comparison between bulk and monolayer CrI$_3$ reveals the pivotal role of quantum confinement in enhancing the MO signals.

Published in: "arXiv Material Science".

Dirac Cones, Can Be Realized Generally in Bilayered Perovskites. (arXiv:1903.07853v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:34+00:00March 20th, 2019|Categories: Publications|Tags: |

We propose that the Dirac cone electronic states can be generally realized in those hexagonal perovskite bilayers sliced out directly from their cubic bulk phases. This is based on the simple analyses that the A-site atoms of these perovskites contribute neither valence nor conduction electrons and after the quasi-atom simplification their hexagonal bilayers indeed possess the honeycomb symmetry. Taking the CsPbBr$_3$ (111) bilayer as an example, with the density functional theory (DFT) calculations, we demonstrate their Dirac cones around the Fermi level and find the corresponding Fermi velocity, $sim$ 2$times$10$^6$m/s, can be almost twice as large as that of graphene. While under ambient conditions the solid ionic compounds normally do not conduct charge carriers very well, such high-velocity ballistic carrier transport in the new layered Dirac materials may revitalize the conventional perovskites and thereby bring new ultra-fast ionic electronics.

Published in: "arXiv Material Science".

Metal to insulator transition in Conducting Polyaniline/Graphene Oxide composites. (arXiv:1903.07954v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:31+00:00March 20th, 2019|Categories: Publications|Tags: , |

Broadband Dielectric Spectroscopy (BDS) measurements of P{omicron}lyaniline/Graphene oxide composites were conducted for an as-prepared and a thermally annealed specimen, respectively, from 15K to room temperature. Electrical conductivity values of the annealed composite display a very modest rise denoting the important contributions of the GO component to achieving electrical stability of the polymer. Patterns of the dc conductivity as a function of temperature also reveal a metal to insulator transition around 75K. The transition is dominated by two key factors; temperature and annealing process. Metal-like and insulating features are subsequently detected, as well, and accordingly described to provide a qualitative inspection of the charge transfer mechanisms involved

Published in: "arXiv Material Science".

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