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Search Results: 1 - 10 of 4719 matches for " Zhenhua Qiao "
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A peculiar composite M- and W-type REE tetrad effect: Evidence from the Shuiquangou alkaline syenite complex, Hebei Province, China
ZhenHua Zhao,ZhiWei Bao,YuLou Qiao
Chinese Science Bulletin , 2010, DOI: 10.1007/s11434-010-3231-3
Abstract: A new type of REE tetrad effect, a composite M- and W-type, was recognized in the K-feldsparthized and silicificated Shuiquangou alkaline syenites, Dongping, Hebei Province. Different analytical methods such as ICP-MS and isotopic dilution thermal ion mass spectrometer were exploited to verify the REE concentrations of the samples in three laboratories in China, France and Korea. The results are reliable and consistent within errors. In situ quantitative analysis of REE concentrations of individual zircons and apatites extracted from the very same sample has shown that fractional crystallization of magma and the superimposed hydrothermal alteration might have taken place in at least two-stage hydrothermal activities to generate the composite M- and W-type REE tetrad effect. The coexisting melt and aqueous phases, the superimposed alteration by volatile (Cl, CO2) and Si, K, Al-enriched high temperature hydrothermal fluids might be the important facts for this new MW-type of REE tetrad effect. In addition, the peculiar MW-type tetrad effect might be an indicator for Au mineralization of reworked plutons.
New universal conductance fluctuation of mesoscopic systems in the crossover regime from metal to insulator
Zhenhua Qiao,Yanxia Xing,Jian Wang
Physics , 2009,
Abstract: We report a theoretical investigation on conductance fluctuation of mesoscopic systems. Extensive numerical simulations on quasi-one dimensional, two dimensional, and quantum dot systems with different symmetries (COE, CUE, and CSE) indicate that the conductance fluctuation can reach a new universal value in the crossover regime for systems with CUE and CSE symmetries. The conductance fluctuation and higher order moments vs average conductance were found to be universal functions from diffusive to localized regimes that depend only on the dimensionality and symmetry. The numerical solution of DMPK equation agrees with our result in quasi-one dimension. Our numerical results in two dimensions suggest that this new universal conductance fluctuation is related to the metal-insulator transition.
Topological Phases in Two-Dimensional Materials: A Brief Review
Yafei Ren,Zhenhua Qiao,Qian Niu
Physics , 2015,
Abstract: Topological phases with insulating bulk and gapless surface or edge modes have attracted much attention because of their fundamental physics implications and potential applications in dissipationless electronics and spintronics. In this review, we mainly focus on the recent progress in the engineering of topologically nontrivial phases (such as $\mathbb{Z}_2$ topological insulators, quantum anomalous Hall effects, quantum valley Hall effects \textit{etc.}) in two-dimensional material systems, including quantum wells, atomic crystal layers of elements from group III to group VII, and the transition metal compounds.
The role of geometry and topological defects in the 1D zero-line modes of graphene
Xintao Bi,Jeil Jung,Zhenhua Qiao
Physics , 2015,
Abstract: Breaking inversion symmetry in chiral graphene systems, \textit{e.g.}, by applying a perpendicular electric field in chirally-stacked rhombohedral multilayer graphene or by introducing staggered sublattice potentials in monolayer graphene, opens up a bulk band gap that harbors a quantum valley-Hall state. When the gap size is allowed to vary and changes sign in space, a topologically-confined one-dimensional (1D) zero-line mode (ZLM) is formed along the zero lines of the local gap. Here we show that gapless ZLM with distinguishable valley degrees of freedom K and K$'$ exist for every propagation angle except for the armchair direction that exactly superpose the valleys. We further analyze the role of different geometries of top-bottom gated device setups that can be realized in experiments, discuss the effects of their edge misalignment, and analyze three common forms of topological defects that could influence the 1D ZLM transport properties in actual devices.
Stabilizing topological phases in graphene via random adsorption
Hua Jiang,Zhenhua Qiao,Haiwen Liu,Junren Shi,Qian Niu
Physics , 2012, DOI: 10.1103/PhysRevLett.109.116803
Abstract: We study the possibility of realizing topological phases in graphene with randomly distributed adsorbates. When graphene is subjected to periodically distributed adatoms, the enhanced spin-orbit couplings can result in various topological phases. However, at certain adatom coverages, the intervalley scattering renders the system a trivial insulator. By employing a finite-size scaling approach and Landauer-B\"{u}ttiker formula, we show that the randomization of adatom distribution greatly weakens the intervalley scattering, but plays a negligible role in spin-orbit couplings. Consequently, such a randomization turns graphene from a trivial insulator into a topological state.
Transport Properties of Graphene Nanoroads in Boron-Nitride Sheets
Jeil Jung,Zhenhua Qiao,Qian Niu,Allan H. MacDonald
Physics , 2012, DOI: 10.1021/nl300610w
Abstract: We demonstrate that the one-dimensional (1D) transport channels that appear in the gap when graphene nanoroads are embedded in boron-nitride (BN) sheets are more robust when they are inserted at AB/BA grain boundaries. Our conclusions are based on ab-initio electronic structure calculations for a variety of different crystal orientations and bonding arrangements at the BN/C interfaces. This property is related to the valley-Hall conductivity present in the BN band structure and to the topologically protected kink states that appear in continuum Dirac models with position dependent masses.
Electronic Highways in Bilayer Graphene
Zhenhua Qiao,Jeil Jung,Qian Niu,Allan H. MacDonald
Physics , 2011, DOI: 10.1021/nl201941f
Abstract: Bilayer graphene with an interlayer potential difference has an energy gap and, when the potential difference varies spatially, topologically protected one-dimensional states localized along the difference's zero-lines. When disorder is absent, electronic travel directions along zero-line trajectories are fixed by valley Hall properties. Using the Landauer-B\"uttiker formula and the non-equilibrium Green's function technique we demonstrate numerically that collisions between electrons traveling in opposite directions, due to either disorder or changes in path direction, are strongly suppressed. We find that extremely long mean free paths of the order of hundreds of microns can be expected in relatively clean samples. This finding suggests the possibility of designing low power nanoscale electronic devices in which transport paths are controlled by gates which alter the inter-layer potential landscape.
Magnetic and Electronic Properties of Metal-Atom Adsorbed Graphene
Jun Ding,Zhenhua Qiao,Wanxiang Feng,Yugui Yao,Qian Niu
Physics , 2011, DOI: 10.1103/PhysRevB.84.195444
Abstract: We systematically investigate the magnetic and electronic properties of graphene adsorbed with diluted 3d-transition and noble metal atoms using first principles calculation methods. We find that most transition metal atoms (i.e. Sc, Ti, V, Mn, Fe) favor the hollow adsorption site, and the interaction between magnetic adatoms and \pi-orbital of graphene induces sizable exchange field and Rashba spin-orbit coupling, which together open a nontrivial bulk gap near the Dirac points leading to the quantum-anomalous Hall effect. We also find that the noble metal atoms (i.e. Cu, Ag, Au) prefer the top adsorption site, and the dominant inequality of the AB sublattice potential opens another kind of nontrivial bulk gap exhibiting the quantum-valley Hall effect.
Unbalanced edge modes and topological phase transition in gated trilayer graphene
Xiao Li,Zhenhua Qiao,Jeil Jung,Qian Niu
Physics , 2012, DOI: 10.1103/PhysRevB.85.201404
Abstract: Gapless edge modes hosted by chirally-stacked trilayer graphene display unique features when a bulk gap is opened by applying an interlayer potential difference. We show that trilayer graphene with half-integer valley Hall conductivity leads to unbalanced edge modes at opposite zigzag boundaries, resulting in a natural valley current polarizer. This unusual characteristic is preserved in the presence of Rashba spin-orbit coupling that turns a gated trilayer graphene into a ${Z}_2$ topological insulator with an odd number of helical edge mode pairs.
Low field phase diagram of spin-Hall effect in the mesoscopic regime
Zhenhua Qiao,Wei Ren,Jian Wang,Hong Guo
Physics , 2006, DOI: 10.1103/PhysRevLett.98.196402
Abstract: When a mesoscopic two dimensional four-terminal Hall cross-bar with Rashba and/or Dresselhaus spin-orbit interaction (SOI) is subjected to a perpendicular uniform magnetic field $B$, both integer quantum Hall effect (IQHE) and mesoscopic spin-Hall effect (MSHE) may exist when disorder strength $W$ in the sample is weak. We have calculated the low field "phase diagram" of MSHE in the $(B,W)$ plane for disordered samples in the IQHE regime. For weak disorder, MSHE conductance $G_{sH}$ and its fluctuations $rms(G_{SH})$ vanish identically on even numbered IQHE plateaus, they have finite values on those odd numbered plateaus induced by SOI, and they have values $G_{SH}=1/2$ and $rms(G_{SH})=0$ on those odd numbered plateaus induced by Zeeman energy. For moderate disorder, the system crosses over into a regime where both $G_{sH}$ and $rms(G_{SH})$ are finite. A larger disorder drives the system into a chaotic regime where $G_{sH}=0$ while $rms(G_{SH})$ is finite. Finally at large disorder both $G_{sH}$ and $rms(G_{SH})$ vanish. We present the physics behind this ``phase diagram".
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