9,581 research outputs found
Elemental analysis in environmental land samples by stand-off laser-induced breakdown spectroscopy
The stand-off detection and analysis of environmental land samples have been demonstrated using laser-induced breakdown spectrometry. The samples of interest have included soils and vegetation powder. Elements Hg, As, Pb, Zn, Cd and Cr have been spectrally analysed with a focus on Hg as a trace contaminant in the samples. It is found that element Fe, usually contained in land samples, is a main source of spectral interference for Hg detection due to its ever present iron emission line at 253.68 nm that is closely adjacent to the strongest Hg emission line at 253.65 nm, and hence, a high resolution of spectral detection is necessary. The strong spectral signals from Bremsstrahlung emission in laser-induced plasma and atomic emission of Fe of high concentration caused a significant reduction in detection resolution in the use of image intensifier of an ICCD. The limit of detection at ~8 ppm for Hg detection in soil samples with iron as a minor constituent has been achieved, using an optical chopper and a CCD detector for laser-induced breakdown spectroscopy (LIBS) signal detection. Such detection method in LIBS system has shown a great advantage in determining trace elements from interfering elemental constituents in land sample matrixes
Helical edge and surface states in HgTe quantum wells and bulk insulators
The quantum spin Hall (QSH) effect is the property of a new state of matter
which preserves time-reversal, has an energy gap in the bulk, but has
topologically robust gapless states at the edge. Recently, it has been shown
that HgTe quantum wells realize this novel effect. In this work, we start from
realistic tight-binding models and demonstrate the existence of the helical
edge states in HgTe quantum wells and calculate their physical properties. We
also show that 3d HgTe is a topological insulator under uniaxial strain, and
show that the surface states are described by single-component massless
relativistic Dirac fermions in 2+1 dimensions. Experimental predictions are
made based on the quantitative results obtained from realistic calculations.Comment: 5 page
Quantum Anomalous Hall Effect in HgMnTe Quantum Wells
The quantum Hall effect is usually observed when the two-dimensional electron
gas is subjected to an external magnetic field, so that their quantum states
form Landau levels. In this work we predict that a new phenomenon, the quantum
anomalous Hall effect, can be realized in HgMnTe quantum wells,
without the external magnetic field and the associated Landau levels. This
effect arises purely from the spin polarization of the atoms, and the
quantized Hall conductance is predicted for a range of quantum well thickness
and the concentration of the atoms. This effect enables dissipationless
charge current in spintronics devices.Comment: 5 pages, 3 figures. For high resolution figures see final published
version when availabl
Intelligent Omni Surface-Assisted Self-Interference Cancellation for Full-Duplex MISO System
The full-duplex (FD) communication can achieve higher spectrum efficiency than conventional half-duplex (HD) communication; however, self-interference (SI) is the key hurdle. This paper is the first work to propose the intelligent omni surface (IOS)-assisted FD multi-input single-output (MISO) FD communication systems to mitigate SI, which solves the frequency-selectivity issue. In particular, two types of IOS are proposed, energy splitting (ES)-IOS and mode switching (MS)-IOS. We aim to maximize data rate and minimize SI power by optimizing the beamforming vectors, amplitudes and phase shifts for the ES-IOS and the mode selection and phase shifts for the MS-IOS. However, the formulated problems are non-convex and challenging to tackle directly. Thus, we design alternative optimization algorithms to solve the problems iteratively. Specifically, the quadratic constraint quadratic programming (QCQP) is employed for the beamforming optimizations, amplitudes and phase shifts optimizations for the ES-IOS and phase shifts optimizations for the MS-IOS. Nevertheless, the binary variables of the MS-IOS render the mode selection optimization intractable, and then we resort to semidefinite relaxation (SDR) and Gaussian randomization procedures to solve it. Simulation results validate the proposed algorithms’ efficacy and show the effectiveness of both the IOSs in mitigating SI compared to the case without an IOS
Magnetic Borophenes from an Evolutionary Search
A computational methodology based on ab initio evolutionary algorithms and spin-polarized density functional theory was developed to predict two-dimensional magnetic materials. Its application to a model system borophene reveals an unexpected rich magnetism and polymorphism. A metastable borophene with nonzero thickness is an antiferromagnetic semiconductor from first-principles calculations, and can be further tuned into a half-metal by finite electron doping. In this borophene, the buckling and coupling among three atomic layers are not only responsible for magnetism, but also result in an out-of-plane negative Poisson\u27s ratio under uniaxial tension, making it the first elemental material possessing auxetic and magnetic properties simultaneously
Model Hamiltonian for Topological Insulators
In this paper we give the full microscopic derivation of the model
Hamiltonian for the three dimensional topological insulators in the
family of materials (, and ). We first give a
physical picture to understand the electronic structure by analyzing atomic
orbitals and applying symmetry principles. Subsequently, we give the full
microscopic derivation of the model Hamiltonian introduced by Zhang {\it et al}
[\onlinecite{zhang2009}] based both on symmetry principles and the perturbation theory. Two different types of terms, which
break the in-plane full rotation symmetry down to three fold rotation symmetry,
are taken into account. Effective Hamiltonian is derived for the topological
surface states. Both the bulk and the surface models are investigated in the
presence of an external magnetic field, and the associated Landau level
structure is presented. For more quantitative fitting to the first principle
calculations, we also present a new model Hamiltonian including eight energy
bands.Comment: 18 pages, 9 figures, 5 table
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