Dirac-Weyl equation on a hyperbolic graphene surface under perpendicular magnetic fields. (arXiv:1909.06831v1 [math-ph])

2019-09-17T04:30:31+00:00September 17th, 2019|Categories: Publications|Tags: , |

In this paper the Dirac-Weyl equation on a hyperbolic surface of graphene under magnetic fields is considered. In order to solve this equation analytically for some cases, we will deal with vector potentials symmetric under rotations around the z axis. Instead of using tetrads we will get this equation from a more intuitive point of view by restriction from the Dirac-Weyl equation of an ambient space. The eigenvalues and corresponding eigenfunctions for some magnetic fields are found by means of the factorization method. The existence of a zero energy ground level and its degeneracy is also analysed in relation to the Aharonov-Casher theorem valid for flat graphene.

Published : "arXiv Mesoscale and Nanoscale Physics".

Gate-tunable Strong Spin-orbit Interaction in Two-dimensional Tellurium Probed by Weak-antilocalization. (arXiv:1909.06659v1 [cond-mat.mes-hall])

2019-09-17T02:29:43+00:00September 17th, 2019|Categories: Publications|

Tellurium (Te) has attracted great research interest due to its unique crystal structure since 1970s. However, the conduction band of Te is rarely studied experimentally because of the intrinsic p-type nature of Te crystal. By atomic layer deposited dielectric doping technique, we are able to access the conduction band transport properties of Te in a controlled fashion. In this paper, we report on a systematic study of weak-antilocalization (WAL) effect in n-type two-dimensional (2D) Te films. We find that the WAL agrees well with Iordanskii, Lyanda-Geller, and Pikus (ILP) theory. The gate and temperature dependent WAL reveals that Dyakonov-Perel (DP) mechanism is dominant for spin relaxation and phase relaxation is governed by electron-electron (e-e) interaction. Large phase coherence length near 600nm at T=1K is obtained, together with gate tunable spin-orbit interaction (SOI). Transition from weak-localization (WL) to weak-antilocalization (WAL) depending on gate bias is also observed. These results demonstrate that newly developed solution-based synthesized Te films provide a new controllable strong SOI 2D semiconductor with high potential for spintronic applications.

Published in: "arXiv Material Science".

Photoemission Signatures of Non-Equilibrium Carrier Dynamics from First Principles. (arXiv:1909.06549v1 [cond-mat.mtrl-sci])

2019-09-17T02:29:40+00:00September 17th, 2019|Categories: Publications|Tags: |

Time- and angle-resolved photoemission spectroscopy (tr-ARPES) constitutes a powerful tool to inspect the dynamics and thermalization of hot carriers. The identification of the processes that drive the dynamics, however, is challenging even for the simplest systems owing to the coexistence of several relaxation mechanisms. Here, we devise a Green’s function formalism for predicting the tr-ARPES spectral function and establish the origin of carrier thermalization entirely from first principles. The predictive power of this approach is demonstrated by an excellent agreement with experiments for graphene over time scales ranging from a few tens of femtoseconds up to several picoseconds. Our work provides compelling evidence of a non-equilibrium dynamics dominated by the establishment of a hot-phonon regime.

Published in: "arXiv Material Science".

Pump-triple sum-frequency-probe spectroscopy of transition metal dichalcogenides. (arXiv:1909.06445v1 [physics.optics])

2019-09-17T02:29:38+00:00September 17th, 2019|Categories: Publications|Tags: , |

Triple sum-frequency (TSF) spectroscopy measures multidimensional spectra by resonantly exciting multiple quantum coherences of vibrational and electronic states. In this work we demonstrate pump-TSF-probe spectroscopy in which a pump excites a sample and some time later three additional electric fields generate a probe field which is measured. We demonstrate pump-TSF-probe spectroscopy on polycrystalline, smooth, thin films and spiral nanostructures of both MoS2 and WS2. The pump-TSF-probe spectra are qualitatively similar to the more conventional transient-reflectance spectra. While transient-reflectance sensitivity suffers under low surface coverage, pump-TSF-probe sensitivity is independent of the sample coverage and nanostructure morphologies. Our results demonstrate that pump-TSF-probe is a valuable methodology for studying microscopic material systems.

Published in: "arXiv Material Science".

Gate-tunable graphene-based Hall sensors on flexible substrates with increased sensitivity. (arXiv:1909.07058v1 [physics.app-ph])

2019-09-17T02:29:35+00:00September 17th, 2019|Categories: Publications|Tags: |

We demonstrate a novel concept for operating graphene-based Hall sensors using an alternating current (AC) modulated gate voltage, which provides three important advantages compared to Hall sensors under static operation: 1) The sensor sensitivity can be doubled by utilizing both n- and p-type conductance. 2) A static magnetic field can be read out at frequencies in the kHz range, where the 1/f noise is lower compared to the static case. 3) The off-set voltage in the Hall signal can be reduced. This significantly increases the signal-to-noise ratio compared to Hall sensors without a gate electrode. A minimal detectable magnetic field Bmin down to 290 nT/sqrt(Hz) and sensitivity up to 0.55 V/VT was found for Hall sensors fabricated on flexible foil. This clearly outperforms state-of-the-art flexible Hall sensors and is comparable to the values obtained by the best rigid III/V semiconductor Hall sensors.

Published in: "arXiv Material Science".

CuAu, a hexagonal two-dimensional metal. (arXiv:1909.06372v1 [cond-mat.mtrl-sci])

2019-09-17T02:29:29+00:00September 17th, 2019|Categories: Publications|Tags: , , |

Growth of two-dimensional metal structures has eluded materials scientists since the discovery of the atomically thin graphene and other covalently bound 2D structures. Here we report a two-atom-thick hexagonal copper-gold alloy, grown through thermal evaporation on freestanding graphene and hexagonal boron nitride. Islands with sizes up to tens of nanometers were grown, but there should be no fundamental limit to their size. The structures are imaged at atomic resolution with scanning transmission electron microscopy and further characterized with spectroscopic techniques. Electron irradiation in the microscope provides sufficient energy for the crumpling of the 2D structure—atoms are released from their lattice sites with the gold atoms eventually forming face-centered cubic nanoclusters on top of 2D regions during observation. The presence of copper in the alloy enhances sticking of gold to the substrate, which has clear implications for creating atomically thin electrodes for applications utilizing 2D materials. Its practically infinite surface-to-bulk ratio also makes the 2D CuAu particularly interesting for catalysis applications.

Published in: "arXiv Material Science".

Dangling Bonds in Hexagonal Boron Nitride as Single-Photon Emitters

2019-09-16T18:39:53+00:00September 16th, 2019|Categories: Publications|Tags: |

Author(s): Mark E. Turiansky, Audrius Alkauskas, Lee C. Bassett, and Chris G. Van de WalleHexagonal boron nitride has been found to host color centers that exhibit single-photon emission, but the microscopic origin of these emitters is unknown. We propose boron dangling bonds as the likely source of the observed single-photon emission around 2 eV. An optical transition where an electron …[Phys. Rev. Lett. 123, 127401] Published Mon Sep 16, 2019

Published in: "Physical Review Letters".

Phonon sidebands of color centers in hexagonal boron nitride

2019-09-16T16:38:00+00:00September 16th, 2019|Categories: Publications|Tags: |

Author(s): P. Khatri, I. J. Luxmoore, and A. J. RamsayLow temperature photoluminescence spectra of the phonon sidebands of a color center in hexagonal boron nitride are compared to an independent boson model. We infer that the LA-phonon sideband is described by deformation coupling proportional to in-plane strain, resulting in a phonon bath that is eff…[Phys. Rev. B 100, 125305] Published Mon Sep 16, 2019

Published in: "Physical Review B".

Quantum Monte Carlo study of magnetic ordering and superconducting pairing symmetry in twisted bilayer graphene

2019-09-16T16:37:58+00:00September 16th, 2019|Categories: Publications|Tags: |

Author(s): Shi-Chao Fang, Guang-Kun Liu, Hai-Qing Lin, and Zhong-Bing HuangTo understand correlated insulating and unconventional superconducting states in twisted bilayer graphene, we perform a systematic study of spin and pairing correlations in an effective two-orbital Hubbard model, by using the ground-state constrained-path quantum Monte Carlo method. Our numerical si…[Phys. Rev. B 100, 115135] Published Mon Sep 16, 2019

Published in: "Physical Review B".

Stacking-dependent excitonic properties of bilayer blue phosphorene

2019-09-16T16:37:55+00:00September 16th, 2019|Categories: Publications|Tags: |

Author(s): F. Iyikanat, E. Torun, R. T. Senger, and H. SahinAb initio calculations in the framework of many-body perturbation theory (MBPT) are performed to calculate the electronic and optical properties of monolayer and bilayer blue phosphorene with different stacking configurations. It is found that the stacking configuration of bilayer blue phosphorene s…[Phys. Rev. B 100, 125423] Published Mon Sep 16, 2019

Published in: "Physical Review B".

Selected graphenelike zigzag nanoribbons with chemically functionalized edges: Implications for electronic and magnetic properties

2019-09-16T16:37:53+00:00September 16th, 2019|Categories: Publications|Tags: , , |

Author(s): S. KrompiewskiIt is known that there is a wide class of quasi-two-dimensional graphenelike nanomaterials which in many respects can outperform graphene. So, here in addition to graphene, the attention is directed to stanene (buckled honeycomb structure) and phosphorene (puckered honeycomb structure). It is shown …[Phys. Rev. B 100, 125421] Published Mon Sep 16, 2019

Published in: "Physical Review B".

Tuning topology in thin films of topological insulators by strain gradients

2019-09-16T16:37:49+00:00September 16th, 2019|Categories: Publications|

Author(s): Raffaele Battilomo, Niccoló Scopigno, and Carmine OrtixWe theoretically show that the coupling of inhomogeneous strains to the Dirac fermions of three-dimensional topological insulators (3DTI) in thin film geometries results in the occurrence of phase transitions between topologically distinct insulating phases. By means of minimal k·p models for strong…[Phys. Rev. B 100, 115131] Published Mon Sep 16, 2019

Published in: "Physical Review B".

Semimetallic features in quantum transport through a gate-defined point contact in bilayer graphene

2019-09-16T16:37:42+00:00September 16th, 2019|Categories: Publications|Tags: |

Author(s): T. L. M. Lane, A. Knothe, and V. I. Fal’koWe demonstrate that, at the onset of conduction, an electrostatically defined quantum wire in bilayer graphene with an interlayer asymmetry gap may act as a one-dimensional semimetal, due to the multiple-minivalley dispersion of its lowest subband. Formation of a nonmonotonic subband coincides with …[Phys. Rev. B 100, 115427] Published Mon Sep 16, 2019

Published in: "Physical Review B".

Robust graphene-based molecular devices

2019-09-16T16:37:18+00:00September 16th, 2019|Categories: Publications|Tags: |

Nature Nanotechnology, Published online: 16 September 2019; doi:10.1038/s41565-019-0533-8Mechanically and electronically stable graphene/molecule/graphene devices can be fabricated by combining a covalent binding of the molecules to the substrate with an optimized intermolecular π–π interaction.

Published in: "Nature Nanotechnology".

Type-II Ising Superconductivity in Two-Dimensional Materials with Spin-Orbit Coupling

2019-09-16T14:38:18+00:00September 16th, 2019|Categories: Publications|

Author(s): Chong Wang, Biao Lian, Xiaomi Guo, Jiahao Mao, Zetao Zhang, Ding Zhang, Bing-Lin Gu, Yong Xu, and Wenhui DuanCentrosymmetric materials with spin-degenerate bands are generally considered to be trivial for spintronics and related physics. In two-dimensional (2D) materials with multiple degenerate orbitals, we find that the spin-orbit coupling can induce spin-orbital locking, generate out-of-plane Zeeman-lik…[Phys. Rev. Lett. 123, 126402] Published Mon Sep 16, 2019

Published in: "Physical Review Letters".

Spin-orbit coupling in elemental two-dimensional materials

2019-09-16T14:38:07+00:00September 16th, 2019|Categories: Publications|Tags: , , , , , |

Author(s): Marcin Kurpas, Paulo E. Faria Junior, Martin Gmitra, and Jaroslav FabianThe fundamental spin-orbit coupling and spin mixing in graphene and rippled honeycomb lattice materials silicene, germanene, stanene, blue phosphorene, arsenene, antimonene, and bismuthene is investigated from first principles. The intrinsic spin-orbit coupling in graphene is revisited using multiba…[Phys. Rev. B 100, 125422] Published Mon Sep 16, 2019

Published in: "Physical Review B".

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