High-order harmonic generation in gapped bilayer graphene. (arXiv:1905.08016v1 [cond-mat.mes-hall])

2019-05-21T04:30:21+00:00May 21st, 2019|Categories: Publications|Tags: |

Microscopic nonlinear quantum theory of interaction of coherent electromagnetic radiation with gapped bilayer graphene is developed. The Liouville-von Neumann equation for the density matrix is solved numerically at the multiphoton excitation regime. The developed theory of interaction of charged carriers with strong driving wave field is valid near the Dirac points of the Brillouin zone. We consider the harmonic generation process in the nonadiabatic regime of interaction when the Keldysh parameter is of the order of unity. On the basis of numerical solutions, we examine the rates of odd and even high-harmonics at the particle-hole annihilation in the field of a strong pump wave of arbitrary polarization. Obtained results show that the gapped bilayer graphene can serve as an effective medium for generation of even and odd high harmonics in the THz and far infrared domains of frequencies.

Published : "arXiv Mesoscale and Nanoscale Physics".

Nematic topological semimetal and insulator in magic angle bilayer graphene at charge neutrality. (arXiv:1905.07409v1 [cond-mat.str-el])

2019-05-21T04:30:16+00:00May 21st, 2019|Categories: Publications|Tags: , |

We report on a fully self-consistent momentum space Hartree-Fock calculation of interaction effects on the Moir’e flat bands of twisted bilayer graphene, tuned near the magic angle. We focus on the charge neutrality point, where experiments have variously reported either insulating or semimetallic behavior. We find three types of self-consistent solutions with competitive ground state energy (i) inversion $C_2$ breaking insulators with valley Chern number (ii) spin or valley polarized insulators and (iii) rotation $C_3$ symmetry breaking semimetals whose gaplessness is protected by the topology of the Moir’e flat bands. We find that the relative stability of these states can be tuned by weak strains that break rotation. Both the nematic semimetal and also, somewhat unexpectedly, the valley-Chern insulator, are stabilized by weak strain. These ground states can explain the semimetallic and insulating behaviors seen at charge neutrality, and the sample variability of their observation. We also find agreement with the results of STM measurements and quantum oscillations near charge neutrality.

Published : "arXiv Mesoscale and Nanoscale Physics".

Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides [Applied Physical Sciences]

2019-05-21T02:31:59+00:00May 21st, 2019|Categories: Publications|

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to…

Published in: "PNAS (Ahead)".

The New Nitrides: Layered, Ferroelectric, Magnetic, Metallic and Superconducting Nitrides to Boost the GaN Photonics and Electronics Eco-System. (arXiv:1905.07627v1 [cond-mat.mtrl-sci])

2019-05-21T02:29:35+00:00May 21st, 2019|Categories: Publications|Tags: , |

The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of new nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III-V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductor NbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors.

Published in: "arXiv Material Science".

Electrically-tunable flat bands and magnetism in twisted bilayer graphene. (arXiv:1905.07651v1 [cond-mat.mes-hall])

2019-05-21T02:29:32+00:00May 21st, 2019|Categories: Publications|Tags: |

Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moir'{e} superlattice. Such superlattices are known to host bulk valley currents at tiny angles ($alphaapprox 0.3 ^circ$) and flat bands at magic angles ($alpha approx 1^circ$). We show that tuning the twist angle to $alpha^*approx 0.8^circ$ generates flat bands with triangular superlattice periodicity. When doped with $pm 6$ electrons per moir’e cell, these bands are half-filled and electronic interactions produce a symmetry-broken ground state (Stoner instability) with spin-polarized regions that order ferromagnetically. Application of an interlayer electric field breaks inversion symmetry and introduces valley-dependent dispersion that quenches the magnetic order. With these results, we propose a solid-state platform that realizes electrically tunable strong correlations.

Published in: "arXiv Material Science".

Spontaneous emission of color centers at 4eV in hexagonal boron nitride under hydrostatic pressure. (arXiv:1905.07985v1 [cond-mat.mtrl-sci])

2019-05-21T02:29:26+00:00May 21st, 2019|Categories: Publications|Tags: , |

The light emission properties of color centers emitting in 3.3-4 eV region are investigated for hydrostatic pressures ranging up to 5GPa at liquid helium temperature. The light emission energy decreases with pressure less sensitively than the bandgap. This behavior at variance from the shift of the bandgap is typical of deep traps. Interestingly, hydrostatic pressure reveals the existence of levels that vary differently under pressure (smaller increase of the emission wavelength compared to the rest of the levels in this energy region or even decrease of it) with pressure. This discovery enriches the physics of the color centers operating in the UV in hBN.

Published in: "arXiv Material Science".

Vapour sensing properties of graphene-covered gold nanoparticles. (arXiv:1905.08045v1 [cond-mat.mes-hall])

2019-05-21T02:29:23+00:00May 21st, 2019|Categories: Publications|Tags: |

We investigated the vapour sensing properties of different graphene-gold hybrid nanostructures. We observed the shifts in the optical spectra near the local surface plasmon resonance of the gold nanoparticles by changing the concentration and nature of the analytes (ethanol, 2-propanol, and toluene). The smaller, dome-like gold nanoparticles proved to be more sensitive to these vapours compared to slightly larger, flat nanoparticles. We investigated how the optical response of the gold nanoparticles can be tuned with a corrugated graphene overlayer. We showed that the presence of graphene increased the sensitivity to ethanol and 2-propanol, while it decreased it towards toluene exposure (at concentrations higher than 30%). The slope changes observed on the optical response curves were discussed in the framework of capillary condensation. These results can have potential impact on the development of new sensors based on graphene-gold hybrids.

Published in: "arXiv Material Science".

Valley optomechanics in a monolayer semiconductor

2019-05-20T16:33:13+00:00May 20th, 2019|Categories: Publications|Tags: |

Nature Photonics, Published online: 20 May 2019; doi:10.1038/s41566-019-0428-0Transduction of valley information to mechanical states in a monolayer MoS2 resonator can be realized by optically pumping the valley carriers and applying an out-of-plane magnetic field gradient to induce a displacement-dependent valley splitting.

Published in: "Nature Photonics".

Excitation of SPPs in graphene by a waveguide mode

2019-05-20T16:33:07+00:00May 20th, 2019|Categories: Publications|Tags: , |

We present a semi-analytical model that predicts the excitation of surface-plasmon polaritons (SPPs) on a graphene sheet located in front of a sub-wavelength slit drilled in a thick metal screen. We identify the signature of the SPP in the transmission, reflection, and absorption curves. Following the previous literature on noble-metal plasmonics, we characterize the efficiency of excitation of SPPs in graphene computing a spatial probability density. This quantity shows the presence of plasmonics resonances dispersing with the Fermi energy, E F , as ##IMG## [http://ej.iop.org/images/0295-5075/126/2/27001/epl19656ieqn1.gif] {$sqrt{E_F}$} an unambiguous signature of graphene plasmons.

Published in: "EPL".

Continuum models for twisted bilayer graphene: Effect of lattice deformation and hopping parameters

2019-05-20T14:33:32+00:00May 20th, 2019|Categories: Publications|Tags: |

Author(s): Francisco Guinea and Niels R. WaletTwisted bilayer graphene is one of the most promising low-dimensional materials, showing both superconducting and insulating behavior. This is understood to be due to the occurrence of extremely flat bands at the magic angles, which enhances the residual interaction effects. Here, the authors show that the most common understanding of this problem neglects two important effects, the deformation of the graphene layers and the modification of electronic hopping between the atoms of the layers. A method to extract a continuum model in the presence of these effects is presented.[Phys. Rev. B 99, 205134] Published Mon May 20, 2019

Published in: "Physical Review B".

Deconfined criticality in the ${mathrm{QED}}_{3}$ Gross-Neveu-Yukawa model: The $1/N$ expansion revisited

2019-05-20T14:33:29+00:00May 20th, 2019|Categories: Publications|

Author(s): Rufus Boyack, Ahmed Rayyan, and Joseph MaciejkoThe critical properties of the QED3 Gross-Neveu-Yukawa (GNY) model in 2 + 1 dimensions with N flavors of two-component Dirac fermions are computed to first order in the 1/N expansion. For the specific case of N=2, the critical point is conjectured to be dual to the Néel-to-valence-bond-solid (VBS) d…[Phys. Rev. B 99, 195135] Published Mon May 20, 2019

Published in: "Physical Review B".

Three-dimensional graphene networks and RGO-based counter electrode for DSSCs

2019-05-20T08:32:51+00:00May 20th, 2019|Categories: Publications|Tags: |

RSC Adv., 2019, 9,15678-15685DOI: 10.1039/C9RA02792K, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Bo Tang, Haogang Yu, Weiqiu Huang, Yunfei Sun, Xufei Li, Sen Li, Tingting MaGraphene is considered to be a potential replacement for the

Published in: "RSC Advances".

In-plane magnetoelectric response in bilayer graphene. (arXiv:1905.07093v1 [cond-mat.mes-hall])

2019-05-20T04:30:44+00:00May 20th, 2019|Categories: Publications|Tags: , |

A graphene bilayer shows an unusual magnetoelectric response whose magnitude is controlled by the valley-isospin density, making it possible to link magnetoelectric behavior to valleytronics. Complementary to previous studies, we consider the effect of static homogeneous electric and magnetic fields that are oriented parallel to the bilayer’s plane. Starting from a tight-binding description and using quasi-degenerate perturbation theory, the low-energy Hamiltonian is derived including all relevant magnetoelectric terms whose prefactors are expressed in terms of tight-binding parameters. We confirm the existence of an expected axion-type pseudoscalar term, which turns out to have the same sign and about twice the magnitude of the previously obtained out-of-plane counterpart. Additionally, small anisotropic corrections to the magnetoelectric tensor are found that are fundamentally related to the skew interlayer hopping parameter $gamma_4$. We discuss possible ways to identify magnetoelectric effects by distinctive features in the optical conductivity.

Published : "arXiv Mesoscale and Nanoscale Physics".

Colossal infrared and terahertz magneto-optical activity in a two-dimensional Dirac material. (arXiv:1905.07159v1 [cond-mat.str-el])

2019-05-20T04:30:41+00:00May 20th, 2019|Categories: Publications|Tags: |

When two-dimensional electron gases (2DEGs) are exposed to magnetic field, they resonantly absorb electromagnetic radiation via electronic transitions between Landau levels (LLs). In 2DEGs with a Dirac spectrum, such as graphene, theory predicts an exceptionally high infrared magneto-absorption, even at zero doping. However, the measured LL magneto-optical effects in graphene have been much weaker than expected because of imperfections in the samples available so far for such experiments. Here we measure magneto-transmission and Faraday rotation in high-mobility encapsulated monolayer graphene using a custom designed setup for magneto-infrared microspectroscopy. Our results show a strongly enhanced magneto-optical activity in the infrared and terahertz ranges characterized by a maximum allowed (50%) absorption of light, a 100% magnetic circular dichroism as well as a record high Faraday rotation. Considering that sizeable effects have been already observed at routinely achievable magnetic fields, our findings demonstrate a new potential of magnetic tuning in 2D Dirac materials for long-wavelength optoelectronics and plasmonics.

Published : "arXiv Mesoscale and Nanoscale Physics".

Twisted bilayer graphene with Kekule distortion: isolated flat band. (arXiv:1905.07316v1 [cond-mat.mes-hall])

2019-05-20T04:30:38+00:00May 20th, 2019|Categories: Publications|Tags: |

Twisted bilayer graphenes with magical angle exhibit strongly correlated electronic properties because of the isolated flat band at the Fermi level. We studied the twisted bilayer graphene with substrates on both layers. The substrate induce Kekule distortion for each graphene layer. Our tight binding calculation shows that isolated flat band with small band width and large gap could be engineered.

Published : "arXiv Mesoscale and Nanoscale Physics".

Exciton-exciton annihilation in hBN. (arXiv:1905.07133v1 [cond-mat.mes-hall])

2019-05-20T02:30:20+00:00May 20th, 2019|Categories: Publications|Tags: |

Known as a prominent recombination path at high excitation densities, exciton-exciton annihilation (EEA) is evidenced in bulk hexagonal boron nitride (hBN) by cathodoluminescence at low temperature. Thanks to a careful tune of the the exciton density by varying either the current or the focus of the incident electron beam, we could estimate an EEA rate of 2$times$10$^{-6}$ cm$^{3}$.s$^{-1}$ at $T=10$ K, the highest reported so far for a bulk semiconductor. Expected to be even stronger in nanotubes or atomic layers, EEA probablly contributes to the luminescence quenching observed in low-dimensionality BN materials.

Published in: "arXiv Material Science".

Novel MAB phases and insights into their exfoliation into 2D MBenes. (arXiv:1905.07310v1 [cond-mat.mtrl-sci])

2019-05-20T02:30:16+00:00May 20th, 2019|Categories: Publications|Tags: , |

Considering the recent breakthroughs in the synthesis of novel two-dimensional (2D) materials from layered bulk structures, ternary layered transition metal borides, known as MAB phases, have come under scrutiny as a means of obtaining novel 2D transition metal borides, so-called MBene. Here, based on a set of phonon calculations, we show the dynamic stability of many Al-containing MAB phases, MAlB (M = Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc), M$_2$AlB$_2$ (Sc, Ti, Zr, Hf, V, Cr, Mo, W, Mn, Tc, Fe, Rh, Ni), M$_3$Al$_2$B$_2$ (M = Sc, T, Zr, Hf, Cr, Mn, Tc, Fe, Ru, Ni), M$_3$AlB$_4$ (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe), and M$_4$AlB$_6$ (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo). By comparing the formation energies of these MAB phases with those of their available competing binary M$-$B and M$-$Al, and ternary M$-$Al$-$B phases, we find that some of the Sc-, Ti-, V-, Cr-, Mo-, W-, Mn-, Tc-, and Fe-based MAB phases could be favorably synthesized in an appropriate experimental condition. In addition, by examining the strengths of various bonds in MAB phases via crystal orbital Hamilton population and spring constant calculations, we find that the B$-$B and then M$-$B bonds are stiffer than the M$-$Al and Al$-$B bonds. The different strength between these bonds implies the etching possibility of Al atoms from MAB phases, consequently forming various 2D MB, M$_2$B$_3$, and M$_3$B$_4$ MBenes. Furthermore, we employ the nudged elastic band method to investigate

Published in: "arXiv Material Science".

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