arXiv subjects

Guanghou Wang

Publications and source records attributed to Guanghou Wang.

At least 37 records · Page 2Linked to original sources

arXiv2013

Shubnikov de Haas quantum oscillation of the surface states in the metallic Bismuth Telluride sheets

Metallic Bi2Te3 crystalline sheets with the room-temperature resistivity of above 10 mΩ cm were prepared and their magnetoresistive transport was measured in a field of up to 9 Tesla. The Shubnikov de Haas oscillations were identified from the secondly-derived magnetoresistance curves. While changing the angle between the field and normal axis of the sheets, we find that the oscillation periods present a cosine dependence on the angle. This indicates a two-dimensional transport due to the surface state. The work reveals a resolvable surface contribution to the overall conduction even in a metallic topological insulator.

cond-mat.mes-hall↗

arXiv2012

Two-dimensional universal conductance fluctuations and the electron-phonon interaction of topological surface states in Bi2Te2Se nanoribbons

The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs). Even if one delicately suppresses the bulk conductance by improving the quality of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se nanoribbons. The solely-B\perp-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such survived UCFs of the topological surface states result from the limited dephasing length of the bulk carriers in ternary crystals. The electron-phonon interaction is addressed as a secondary source of the surface state dephasing based on the temperature-dependent scaling behavior.

cond-mat.mes-hall↗

arXiv2012

Visualizing plasmon coupling in closely-spaced chains of Ag nanoparticles by electron energy loss spectroscopy

Anisotropic plasmon coupling in closely-spaced chains of Ag nanoparticles was visualized using the electron energy loss spectroscopy in a scanning transmission electron microscope. For dimers as the simplest chain, mapping the plasmon excitations with nanometers' spatial resolution and 0.27 eV energy resolution intuitively identified two coupling plasmons. The in-phase mode redshifted from the ultraviolet region as the inter-particle spacing was reduced, reaching the visible range at 2.7 eV. Calculations based on the discrete dipole approximation confirmed its optical activeness, where the longitudinal direction was constructed as the path for light transportation. Two coupling paths were then observed in an inflexed 4-particle chain.

cond-mat.mes-hall↗

arXiv2012

Experimental evidence on the Altshuler-Aronov-Spivak interference of the topological surface states in the exfoliated Bi2Te3 nanoflakes

Here we demonstrate the Altshuler-Aronov-Spivak (AAS) interference of the topological surface states on the exfoliated Bi2Te3 microflakes by a flux period of h/2e in their magnetoresistance oscillations and its weak field character. Both the osillations with the period of h/e and h/2e are observed. The h/2e-period AAS oscillation gradually dominates with increasing the sample widths and the temperatures. This reveals the transition of the Dirac Fermions' transport to the diffusive regime.

cond-mat.mes-hall↗

arXiv2010

Visualizing topological insulating Bi2Te3 quintuple layers on SiO2-capped Si substrates and its contrast optimization

Thin Bi2Te3 flakes, with as few as 3 quintuple layers, are optically visualized on the SiO2-capped Si substrates. Their optical contrasts vary with the illumination wavelength, flake thickness and capping layers. The maximum contrast appears at the optimized light with the 570nm wavelength. The contrast turns reversed when the flake is reduced to less than 20 quintuple layers. A calculation based on the Fresnel law describes the above observation with the constructions of the layer number-wave length-contrast three-dimensional (3D) diagram and the cap thickness-wavelength-contrast 3D diagram, applicative in the current studies of topological insulating flakes.

cond-mat.mes-hall↗

arXiv2010

Calibrating the atomic balance by carbon nanoclusters

Carbon atoms are counted at near atomic-level precision using a scanning transmission electron microscope calibrated by carbon nanocluster mass standards. A linear calibration curve governs the working zone from a few carbon atoms up to 34,000 atoms. This linearity enables adequate averaging of the scattering cross sections, imparting the experiment with near atomic-level precision despite the use of a coarse mass reference. An example of this approach is provided for thin layers of stacked graphene sheets. Suspended sheets with a thickness below 100 nm are visualized, providing quantitative measurement in a regime inaccessible to optical and scanning probe methods.

cond-mat.mes-hall↗

arXiv2010

Two-step splitting the expandable graphite for few-layer graphene

Few-layer graphene sheets are prepared by splitting the expanded graphites using a high-power sonication. Atomic-level quantitative scanning transmission electron microscopy (Q-STEM) is employed to carry out the efficient layer statisticsm, enabling global optimization of the experimental conditions. A two-step splitting mechanism is thus revealed, in which the mean layer number was firstly reduced to less than 20 by heating to 1100°C and then tuned to the few-layer region by a 5-minute 104W/litre sonication. Raman spectroscopic analysis confirms the above mechanism and demonstrates that the sheets are largely free of defects and oxides.

cond-mat.mes-hall↗

arXiv2010

Experimental observation of nanojets formed by heating the PbO-coated Pb clusters

In this article, we will present the first experimental observation of nanojets formed by heating PbO-coated Pb clusters, which has been predicted theoretically by M. Moseler and U Landman. During the heating, the hot liquid ejects through the broken orifice into vacuum, and forms a condensed trail in the tadpole shape as shown in the TEM micrographs. The temperature-variable Raman spectra indicates that the nanojet formation is closely related to the heating temperature and thus essentially to the internal pressure in the coated clusters. The pressure inside the shell, which rises from the inner core's melting and its confined volume expansion and then drops after the final explosion, dominates the whole nanojetting process.

cond-mat.mes-hall↗

arXiv2010

Free-standing Graphene as Gold Standard

We demonstrate using graphene sheets as a novel mass standard in the scanning transmission electron microscopy (STEM) based mass spectrometry. Here, free-standing graphene sheets are investigated by STEM. The discrete number of graphene layers enables an accurate calibration of STEM intensity, as performed over an extended thickness and with single atomic layer sensitivity. This leads to the direct and accurate measurement of the electron mean free path. As an application, we gain the insight of carbon nanoparticles of complex structure.

cond-mat.mes-hall↗

arXiv2010

Films with the discrete nano-DLC-particles as the field emission cascade

The films with the discrete diamond-like-carbon nanoparticles were prepared by the deposition of the carbon nanoparticle beam. Their morphologies were imaged by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The nanoparticles were found distributed on the silicon (100) substrate discretely. The semisphere shapes of the nanoparticles were demonstrated by the AFM line profile. EELS was measured and the sp3 ratio as high as 86% was found. The field-induced electron emission of the as-prepared cascade (nanoDLC/ Si) was tested and the current density of 1mA/cm2 was achieved at 10.2V/μm.

cond-mat.mes-hall↗

arXiv2010

Low energy cluster beam deposited BN films as the cascade for Field Emission

The atomic deposited BN films with the thickness of nanometers (ABN) were prepared by radio frequency magnetron sputtering method and the nanostructured BN films (CBN) were prepared by Low Energy Cluster Beam Deposition. UV-Vis Absorption measurement proves the band gap of 4.27eV and field emission of the BN films were carried out. F-N plots of all the samples give a good fitting and demonstrate the F-N tunneling of the emission process. The emission of ABN begins at the electric field of 14.6 V/μm while that of CBN starts at 5.10V/μm. Emission current density of 1mA/cm2 for ABN needs the field of 20V/μm while that of CBN needs only 12.1V/μm. The cluster-deposited BN on n-type Silicon substrate proves a good performance in terms of the lower gauge voltage, more emission sites and higher electron intensity and seems a promising substitute for the cascade of Field Emission.

cond-mat.mes-hall↗

arXiv2003

Electronic and magnetic properties of multishell Co nanowires coated with Cu

The structural, electronic, and magnetic properties of ultrathin Cu-coated Co nanowires have been studied by using empirical genetic algorithm simulations and a tight-binding $spd$ model Hamiltonian in the unrestricted Hartree-Hock approximation. For some specific stoichiometric compositions, Cu atoms occupy the surface, while Co atoms prefer to stay in the interior, forming the perfect coated multishell structures. The outer Cu layers lead to substantial variations of the magnetic moment of interior Co atoms, depending on the structure and thickness of Cu layers. In particular, single Co atom row in the center of nanowire is found to be nonmagnetic when coated with two Cu layers. All the other Co nanowires in the coated Cu shell are still magnetic but the magnetic moments are reduced as compared with Co nanowires without Cu coating. The interaction between Cu and Co atoms induces nonzero magnetic moment for Cu atoms.

cond-mat.mtrl-sci↗

arXiv2002

Melting behavior of ultrathin titanium nanowires

The thermal stability and melting behavior of ultrathin titanium nanowires with multi-shell cylindrical structures are studied using molecular dynamic simulation. The melting temperatures of titanium nanowires show remarkable dependence on wire sizes and structures. For the nanowire thinner than 1.2 nm, there is no clear characteristic of first-order phase transition during the melting, implying a coexistence of solid and liquid phases due to finite size effect. An interesting structural transformation from helical multi-shell cylindrical to bulk-like rectangular is observed in the melting process of a thicker hexagonal nanowire with 1.7 nm diameter.

cond-mat.mtrl-sci↗

arXiv2002

Magic structures of helical multi-shell zirconium nanowires

The structures of free-standing zirconium nanowires with 0.6$-$2.8 nm in diameter are systematically studied by using genetic algorithm simulations with a tight-binding many body potential. Several multi-shell growth sequences with cylindrical structures are obtained. These multi-shell structures are composed of coaxial atomic shells with the three- and four-strands helical, centered pentagonal and hexagonal, and parallel double-chain-core curved surface epitaxy. Under the same growth sequence, the numbers of atomic strands in inner- and outer-shell show even-odd coupling and usually differ by five. The size and structure dependence of angular correlation functions and vibrational properties of zirconium nanowire are also discussed.

cond-mat.mtrl-sci↗

arXiv2002

Does the melting behavior in ultrathin metallic nanowires begin from the surface ?

Surface melting temperature is well-known significantly lower than the bulk melting point. But we find that the interior melting temperature in ultrathin nanowires is lower than that of the surface melting . The thermal stability of helical multi-walled cylindrical gold nanowires is studied using molecular dynamics simulations. The melting temperature of gold nanowires is lower than the bulk value, but higher than that of gold nanoclusters. An interesting interior melting is revealed in the gold nanowires and the thermodynamics is closely related to interior structures. The melting starts from the interior atoms, while the surface melting occurs at relatively higher temperature. We propose that the surface melting represents the overall melting in ultrathin metallic nanowires.

cond-mat.mtrl-sci↗

arXiv2001

Novel structures of Co-Cu bimetallic clusters and their magnetic properties

The structural and magnetic properties of Co$_{18-m}$Cu$_m$ ($0\leq m\leq 18$) clusters are investigated with a genetic algorithm and a $spd$-band model Hamiltonian in the unrestricted Hartree-Hock approximation respectively. In general, Cu atoms tend to occupy the surface, while Co atoms prefer to the interior of the clusters. Layered structures appear in some clusters with given stoichiometric compositions. The introduction of Cu atoms leads to large increase of the magnetic moment of Co-rich circumstance and nearly zero magnetism of the Cu-rich ambient. The interaction between Cu and Co atoms induces nonzero magnetic moment for Cu atoms. The total magnetic moments tend to decrease with the increase of Cu atoms. However, some particular large magnetic moment are found to be closely related to the structures. The environment of Cu and Co atoms have a dominant effect on the magnetism of the cluster.

physics.atm-clus↗

arXiv2001

Polarizabilities of germanium clusters

Polarizabilities of Ge$_n$ clusters with 2 to 25 atoms are calculated using coupled-perturbation Hartree-Fock (CPHF) and finite field (FF) method within density functional theory. The polarizabilities of the Ge$_n$ clusters increase rapidly in the size range of 2 to 5 atoms and then fluctuate around the bulk value. The polarizabilities are sensitively dependent on the cluster geometries and electronic structures. The large HOMO-LUMO gap may lead to the small polarizability. As compared with the compact structure and diamond structure, the prolate cluster structure corresponds to a larger polarizability.

physics.atm-clus↗

arXiv2001

Density functional study of Au$_n$ (n=2-20) clusters: lowest-energy structures and electronic properties

We have investigated the lowest-energy structures and electronic properties of the Au$_n$(n=2-20) clusters based on density functional theory (DFT) with local density approximation. The small Au$_n$ clusters adopt planar structures up to n=6. Tabular cage structures are preferred in the range of n=10-14 and a structural transition from tabular cage-like structure to compact near-spherical structure is found around n=15. The most stable configurations obtained for Au$_{13}$ and Au$_{19}$ clusters are amorphous instead of icosahedral or fcc-like, while the electronic density of states sensitively depend on the cluster geometry. Dramatic odd-even alternative behaviors are obtained in the relative stability, HOMO-LUMO gaps and ionization potentials of gold clusters. The size evolution of electronic properties is discussed and the theoretical ionization potentials of Au$_n$ clusters compare well with experiments.

physics.atm-clus↗