Large magneto-optical effects and magnetic anisotropy energy in two-dimensional Cr$_2$Ge$_2$Te$_6$. (arXiv:1808.05086v1 [cond-mat.mes-hall])

2018-08-16T04:30:21+00:00 August 16th, 2018|Categories: Publications|Tags: |

By performing systematic $ab$ $initio$ density functional calculations, here we study two relativity-induced properties of atomically thin ferromagnetic (FM) Cr$_2$Ge$_2$Te$_6$ films [monolayer (ML), bilayer (BL) and trilayer (TL) as well as bulk], namely, magnetic anisotropy energy (MAE) and magneto-optical (MO) effects. Competing contributions of both magneto-crystalline anisotropy energy (C-MAE) and magnetic dipolar anisotropy energy (D-MAE) to the MAE, are computed. Calculated MAEs of these materials are large, being in the order of $sim$0.1 meV/Cr. Interestingly, we find that the out-of-plane magnetic anisotropy is preferred in all the systems except the ML where an in-plane magnetization is favored because here the D-MAE is larger than the C-MAE. Crucially, this explains why long-range FM order was observed in all the few-layer Cr$_2$Ge$_2$Te$_6$ except the ML because the out-of-plane magnetic anisotropy would open a spin-wave gap and thus suppress magnetic fluctuations so that long-range FM order could be stabilized at finite temperature. In the visible frequency range, large Kerr rotations up to $sim$1.0 deg in these materials are predicted and they are comparable to that observed in famous MO materials such as PtMnSb and Y$_3$Fe$_5$O$_{12}$. Moreover, they are $sim$100 times larger than that of 3$d$ transition metal MLs deposited on Au surfaces. Faraday rotation angles in these 2D materials are also large, being up to $sim$120 deg/$mu$m, and are thus comparable to the best-known MO semiconductor Bi$_3$Fe$_5$O$_{12}$. These findings thus suggest that with large MAE and MO effects, atomically thin Cr$_2$Ge$_2$Te$_6$ films would have potential applications in novel magnetic, MO and spintronic nanodevices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Collective excitations in two-component one-dimensional massless Dirac plasma. (arXiv:1808.05180v1 [cond-mat.mes-hall])

2018-08-16T04:30:19+00:00 August 16th, 2018|Categories: Publications|Tags: |

We study spectra of long wavelength plasma oscillations in a system of two energy splitted one-dimensional (1D) massless Dirac fermion subbands coupled by spin-orbit interaction. Such a system may be formed by edge subbands in semiconducting transition metal dichalcogenide monolayers. Intrasubband transitions of massless Dirac fermions give rise to optical and acoustic gapless branches of intrasubband 1D plasmons. We reveal that the optical branch is of quantum character with group velocity being inverse proportional to square root of the Planck constant, whereas the acoustic branch is classical one with group velocity proportional to geometric mean of the edge subband velocities. Spin-orbit interaction, allowing intersubband transitions in the system, results in emergence of two branches of intersubband 1D plasmons: upper and lower ones. The upper and lower branches are gapped at small wave vectors and evolve with positive and negative group velocities, respectively, from energy splitting of the edge subbands at Fermi-level. The both intersubband branches adjoin intersubband single particle excitation continuum from above, while in case of the edge subbands with unequal velocities the lower one experiences Landau damping at small wave vectors. In addition, the lower branch, attaining zero frequency at a non-zero wave vector, alters its group velocity from negative to positive one.

Published : "arXiv Mesoscale and Nanoscale Physics".

Picosecond transient thermoreflectance technique for measuring thermal conductivity in thin-films. (arXiv:1808.04972v1 [cond-mat.mtrl-sci])

2018-08-16T02:29:19+00:00 August 16th, 2018|Categories: Publications|Tags: |

We have developed a transient thermoreflectance technique using picosecond pulsed and cw laser to study thermal conductivity and interface conductance in both thin-films and bulk materials. A real time-resolved system observes a thermal transport along the cross-plane direction of the sample during a single pulse excitation. The suggested TTR technique can measure thermal conductivity in up to a few hundred nm of thin films with a reasonable uncertainty by carefully selecting metal transducer thickness. In this paper, we examine thermal conductivity in several substrates including Si, GaAs, Sapphire, and Glass after depositing Au thin film as metal transducer and compare with reported values to validate our technique. For further study on our method, MoS2 thin-films with different thicknesses are prepared via exfoliating, and their thermal conductivity are measured as average value of 3.4 W/mK. Compared to TDTR technique, TTR is a simpler and inexpensive method to study thermophysical properties and can also measure in-plane thermal property using a grating imaging technique. TTR can be one of the available options for observing thermal transport phenomena in both horizontal and vertical directions with a simple and inexpensive preparation.

Published in: "arXiv Material Science".

Real time optical observation and control of atomically thin transition metal dichalcogenide synthesis. (arXiv:1808.05036v1 [cond-mat.mtrl-sci])

2018-08-16T02:29:16+00:00 August 16th, 2018|Categories: Publications|

Understanding the mechanisms involved in chemical vapour deposition (CVD) synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires the precise control of numerous growth parameters. All the proposed mechanisms and their relation to the growth conditions are inferred from characterising intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin crystals in salt assisted TMDC synthesis. Our real time observations reveal the reaction between the salt and the metallic precursor to form intermediate compounds which lead to the layered crystal formation. We identified that both the vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Furthermore, we demonstrate the role H$_{2}$ plays in the salt-assisted WSe$_{2}$ synthesis. Finally, we guided the crystal formation by directing the liquid intermediate compound through pre-patterned channels. The methods presented in this article can be extended to other materials that can be synthesized via CVD.

Published in: "arXiv Material Science".

Restoring self-limited growth of single-layer graphene on copper foil via backside coating. (arXiv:1808.05173v1 [cond-mat.mtrl-sci])

2018-08-16T02:29:12+00:00 August 16th, 2018|Categories: Publications|Tags: |

The growth of single-layer graphene (SLG) by chemical vapor deposition (CVD) on copper surfaces is very popular because of the self-limiting effect that prevents the growth of few-layer graphene (FLG). However, the reproducibility of the CVD growth of homogeneous SLG remains a major challenge, especially if one wants to avoid heavy surface treatments, monocrystalline substrates and expensive equipment to control the atmosphere inside the growth system. We demonstrate here that backside tungsten coating of copper foil allows the exclusive growth of SLG with full coverage by atmospheric pressure CVD implemented in a vacuum-free outfit. We show that the absence of FLG patches is related to the absence of decomposition of methane on the backside and consequently to the suppression of C diffusion through copper. In the perspective of large-scale production of graphene, this approach constitutes a significant improvement to the traditional CVD growth process since (1) a tight control of the hydrocarbon flow is no longer required to avoid FLG formation and, consequently, (2) the growth duration necessary to reach full coverage can be dramatically shortened.

Published in: "arXiv Material Science".

Field-effect-driven half-metallic multilayer graphene

2018-08-15T16:33:23+00:00 August 15th, 2018|Categories: Publications|Tags: |

Author(s): Jacopo Baima, Francesco Mauri, and Matteo CalandraRhombohedral stacked multilayer graphene displays the occurrence of a magnetic surface state at low temperatures. Recent angular resolved photoemission experiments demonstrate the robustness of the magnetic state in long sequences of ABC graphene. Here, by using first-principles calculations, we sho…[Phys. Rev. B 98, 075418] Published Wed Aug 15, 2018

Published in: "Physical Review B".

Femtosecond valley polarization and topological resonances in transition metal dichalcogenides

2018-08-15T16:33:21+00:00 August 15th, 2018|Categories: Publications|Tags: , |

Author(s): S. Azar Oliaei Motlagh, Jhih-Sheng Wu, Vadym Apalkov, and Mark I. StockmanWe theoretically introduce the fundamentally fastest induction of a significant population and valley polarization in a monolayer of a transition metal dichalcogenide (i.e., MoS2 and WS2). This may be extended to other two-dimensional materials with the same symmetry. This valley polarization can be…[Phys. Rev. B 98, 081406(R)] Published Wed Aug 15, 2018

Published in: "Physical Review B".

Electron spin inversion in gated silicene nanoribbons

2018-08-15T16:33:20+00:00 August 15th, 2018|Categories: Publications|Tags: , |

Author(s): Bartłomiej Rzeszotarski and Bartłomiej SzafranWe study locally gated silicene nanoribbons as spin active devices and we solve the quantum scattering problem in the atomistic tight-binding formalism. Particular attention is paid to the low energy range for which only four subbands appear at the Fermi level. We find that the gated segments of zig…[Phys. Rev. B 98, 075417] Published Wed Aug 15, 2018

Published in: "Physical Review B".

Friedel-Crafts reaction of fluorinated graphene for high-yield arylation of graphene

2018-08-15T16:32:42+00:00 August 15th, 2018|Categories: Publications|Tags: |

Chem. Commun., 2018, Accepted ManuscriptDOI: 10.1039/C8CC05762A, CommunicationWenchuan Lai, Jiaxiang Liu, Longbo Luo, Xu Wang, Taijun He, Kun Fan, Xiangyang LiuHerein, we report the Friedel-Crafts reaction of fluorinated graphene with aryl molecules including methylbenzene, chlorobenzene and polystyrene. The reaction achieved the high-yield arylation functionalization of graphene under mild reaction conditions…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Chemical Communications".

Reduced Graphene Oxide Nanosheet Modified NiMn-LDH nanoflake arrays for High-Performance Supercapacitors

2018-08-15T12:33:09+00:00 August 15th, 2018|Categories: Publications|Tags: , |

Chem. Commun., 2018, Accepted ManuscriptDOI: 10.1039/C8CC05745A, CommunicationLi Sun, Yuanxing Zhang, Yu Zhang, Haochen Si, Wenpeng Qin, Yihe ZhangReduced graphene oxide (rGO) nanosheets are exploited as a modification layer on Ni foam-supported NiMn-LDH nanoflake array electrode via a facile dip-coating method. rGO serves as a second electron collector,…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Chemical Communications".

A facile and clean process for exfoliating MoS 2 nanosheets assisted by a surface active agent in aqueous solution

2018-08-15T10:33:50+00:00 August 15th, 2018|Categories: Publications|

A facile, efficient and environmentally friendly process to exfoliate MoS 2 is essentially critical to apply the obtained mono- and few-layer nanosheets in various electronic devices and sensors. Here we report a liquid phase exfoliation method for exfoliation of MoS 2 , which employs a surfactant of sodium dodecyl benzene sulfonate (SDBS) in water. The nonpolar benzene rings in SDBS can firmly bind to the MoS 2 layer, facilitating the effective exfoliation of nanosheets in aqueous solution. It is found that the exfoliation efficiency and thickness of MoS 2 nanosheets are related to the concentration of SDBS, and the mechanism was investigated. Defect free mono- and few-layer MoS 2 nanosheets are obtained by controlling the amount of SDBS in solution, which exhibit stable dispersion in water over months, and it renders them as having great potential for solution-based device fabrication.

Published in: "Nanotechnology".

Impact of a van der Waals interface on intrinsic and extrinsic defects in an MoSe 2 monolayer

2018-08-15T10:33:48+00:00 August 15th, 2018|Categories: Publications|Tags: , |

In this work, we study growth and migration of atomic defects in MoSe 2 on graphene using multiple advanced transmission electron microscopy techniques to explore defect behavior in vdW heterostructures. A MoSe 2 /graphene vdW heterostructure is prepared by a direct growth of both monolayers, thereby attaining an ideal vdW interface between the monolayers. We investigate the intrinsic defects (inversion domains and grain boundaries) in synthesized MoSe 2 , their evolution amid growth processing steps, and their influence on the formation and movement of extrinsic defects. Electron diffraction identifies a preferential interlayer orientation of 2° between MoSe 2 and graphene, which is caused by the presence of intrinsic IBD defects. Extrinsic defects (point and line defects) are generated by in situ electron irradiation in the MoSe 2 layer. Our results shed light on how to independently modify the MoSe 2 atomic stru…

Published in: "Nanotechnology".

Tuning the topological insulator states of artificial graphene. (arXiv:1808.04548v1 [cond-mat.mes-hall])

2018-08-15T04:30:30+00:00 August 15th, 2018|Categories: Publications|Tags: , |

We develop a robust, non-perturbative approach to study the band structure of artificial graphene. Artificial graphene, as considered here, is generated by imposing a superlattice structure on top of a two dimensional hole gas in a semiconductor heterostructure, where the hole gas naturally possesses large spin-orbit coupling. Via tuning of the system parameters we demonstrate how best to exploit the spin-orbit coupling to generate time reversal symmetry-protected topological insulator phases. Our major conclusion is the identification of a second set of topological Dirac bands in the band structure (with spin Chern number $C=3$), which were not reliably obtainable in previous perturbative approaches to artificial graphene. Importantly, the second Dirac bands host more desirable features than the previously studied first set of Dirac bands (with $C=1$). Moreover, we find that upon tuning of the system parameters, we can drive the system to the highly desirable regime of the topological flat band. We discuss the possibilities this opens up for exotic, strongly correlated phases.

Published : "arXiv Mesoscale and Nanoscale Physics".

Massless and Zero-Energy States in Condensed Matter Systems. (arXiv:1808.04568v1 [cond-mat.mes-hall])

2018-08-15T04:30:28+00:00 August 15th, 2018|Categories: Publications|Tags: , |

We present a Hamiltonian which can be employed to explain a condensed matter system, such as Graphene, with effectively massless and zero energy states. We analyze the 2D tunneling problem and derive the transmission and reflection coefficients for the massless and zero energy states. We find that the transmission coefficient of the massless and zero-energy particles in this case is consistent with non-chiral tunneling of quasiparticles which is in contrast to the Klein tunneling known for electrons described by the Dirac equation. Our analysis predicts that the massless electron can be reflected as a hole-like state whereas this transition does not occur for the zero-energy state.

Published : "arXiv Mesoscale and Nanoscale Physics".

Spin-wave susceptibility properties of graphene-phosphorene van der Waals heterolayer: The consequence of response functions on field effect transmission. (arXiv:1808.04418v1 [cond-mat.mes-hall])

2018-08-15T04:30:26+00:00 August 15th, 2018|Categories: Publications|Tags: , , |

We study the spin-wave susceptibility behaviour tuned via electronic structure in chosen two dimensional (2D) graphene-phosphorene (Grp-P) van der Waals (vdWs) heterostructure model combining renormalization group approach and density functional theory simulations. First, we apply Aharonov-Bohm-Coulomb-Dirac (ABCD) formulated eld-theoretic renormalization group approach for the model interacting Fermi system with nearest-neighbour hopping amplitudes. Next, the renormalization group is further extended for spin wave dependent susceptibility calculations. The analytical findings are corroborated with ab initio electronic structure calculations of the chosen heterostructure system in local spin density approximation (LSDA) framework. We notice a Giant Stark Effect from the calculations in the Grp-P heterolayers, where a phase transition from insulator to metal occurs in the semiconducting transport channels at low bias voltage. Lastly, a dual-gate effect transistor is strategically proposed having on-off ratio of 100 based on the electric field effect transmission.

Published : "arXiv Mesoscale and Nanoscale Physics".

On the exact quantum scale invariance of three-dimensional reduced QED theories. (arXiv:1808.04709v1 [hep-th])

2018-08-15T04:30:24+00:00 August 15th, 2018|Categories: Publications|Tags: |

An effective quantum field theory description of graphene in the ultra-relativistic regime is given by reduced QED aka. pseudo QED aka. mixed-dimensional QED. It has been speculated in the literature that reduced QED constitutes an example of a specific class of hard-to-find theories: an interacting CFT in more than two dimensions. This speculation was based on two-loop perturbation theory. Here, we give a proof of this feature, namely the exact vanishing of the $beta$-function, thereby showing that reduced QED can effectively be considered as an interacting (boundary) CFT, underpinning recent work in this area. The argument, valid for both two- and four-component spinors, also naturally extends to an exactly marginal deformation of reduced QED, thence resulting in a non-supersymmetric conformal manifold.

Published : "arXiv Mesoscale and Nanoscale Physics".

Intrinsic Point Defects in Ultrathin 1T-PtSe2 Layers. (arXiv:1808.04719v1 [cond-mat.mes-hall])

2018-08-15T04:30:22+00:00 August 15th, 2018|Categories: Publications|Tags: , , |

Among two dimensional (2D) transition metal dichalcogenides (TMDs), platinum diselenide (PtSe2) stands at a unique place in the sense that it undergoes a phase transition from type-II Dirac semimetal to indirect-gap semiconductor as thickness decreases. Defects in 2D TMDs are ubiquitous and they play crucial roles in understanding electronic, optical, and magnetic properties and tailoring them for desirable applications. Here we investigate intrinsic point defects in ultrathin 1T-PtSe2 layers grown on mica through the chemical vapor transport method, using scanning tunneling microscopy (STM) and first-principles calculations. We found five distinct defects from STM topography images and obtained the local density of states of the defects. By combining the STM results with the first-principles calculations, we identified the types and characteristics of these defects, which are Pt vacancies at the topmost and next monolayers, Se vacancies in the topmost monolayer, and Se antisites at Pt sites within the topmost monolayer. Interestingly, our study shows that the Se antisite defects are the most abundant with the lowest formation energy in a Se-rich growth condition, in contrast to cases of 2D TMD MoS2 family. Our findings will directly influence tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering or growth condition in thin semiconductor PtSe2 layers.

Published : "arXiv Mesoscale and Nanoscale Physics".

Some say, that 2D Research is the best website in the world.