arXiv subjects

Y. Zhang

Publications and source records attributed to Y. Zhang.

At least 649 records · Page 36Linked to original sources

arXiv2007

Microstructure and charge-ordering transitions in LuFe2O4

Microstructure properties, phase transitions, and charge ordering in the LuFe2O4 materials have been extensively investigated by means of transmission electron microscopy (TEM). The (001) twins as a common defect frequently appear in the LuFe2O4 crystals along the c-axis direction, and the crystals across each boundary are rotated by 180 degree with respect to one another. The in-situ cooling TEM observations reveal remarkable temperature dependence of the superstructures in correlation with charger ordering (CO). The Fe2+, Fe3+, and Fe+2.5 ions are found to be crystallized in ordered stripes at the frustrated ground state characterized by a modulation with the wave vector of q1 =(1/3 1/3 2). In-situ heating TEM observations on LuFe2O4 clearly demonstrate that this modulation becomes evidently invisible above a critical temperature of about Tc=530K. These facts suggest that the CO should be the essential driving force for the structural transitions and ferroelectricity observed in present system.

cond-mat.str-el↗

arXiv2007

Phase Coherent Transport of Charges in Graphene Quantum Billiard

We experimentally investigate electrical transport properties of graphene, which is a two dimensional (2D) conductor with relativistic energy dispersion relation. By investigating single- and bi-layer graphene devices with different aspect ratios, we confirm experimentally that the minimum conductivity in wide and short graphene strips approaches the theoretical value of 4e^2/π\h. At low temperatures, quantum interference of multiply-reflected waves of electrons and holes in graphene give rise to periodic conductance oscillations with bias and gate voltages. Thus graphene acts as a quantum billiard, a 2D ballistic, phase coherent electron system with long phase coherence length that exceeds 5 microns. Additional features in differential conductance emerge when graphene is coupled to superconducting electrodes. We observe proximity-induced enhanced conductance at low bias, and conductance dips at energy scales far above the superconducting gap of the electrodes. The latter provides preliminary evidence for a novel superconducting material that consists of graphene coated with metallic atoms.

cond-mat.mes-hall↗

arXiv2007

Charge-stripe order in the electronic ferroelectric LuFe2O4

The structural features of the charge ordering states in LuFe2O4 are characterized by in-situ cooling TEM observations from 300K down to 20K. Two distinctive structural modulations, a major q1= (1/3, 1/3, 2) and a weak q2=q1/10 + (0, 0, 3/2), have been well determined at the temperature of 20K. Systematic analysis demonstrates that the charges at low temperatures are well crystallized in a charge stripe phase, in which the charge density wave behaviors in a non-sinusoidal fashion resulting in elemental electric dipoles for ferroelectricity. It is also noted that the charge ordering and ferroelectric domains often change markedly with lowering temperatures and yields a rich variety of structural phenomena.

cond-mat.mtrl-sci↗

arXiv2007

Room-Temperature Quantum Hall Effect in Graphene

The quantum Hall effect (QHE), one example of a quantum phenomenon that occur on a truly macroscopic scale, has been attracting intense interest since its discovery in 1980 and has helped elucidate many important aspects of quantum physics. It has also led to the establishment of a new metrological standard, the resistance quantum. Disappointingly, however, the QHE could only have been observed at liquid-helium temperatures. Here, we show that in graphene - a single atomic layer of carbon - the QHE can reliably be measured even at room temperature, which is not only surprising and inspirational but also promises QHE resistance standards becoming available to a broader community, outside a few national institutions.

cond-mat.mes-hall↗

arXiv2007

Temperature fluctuations in H II regions: t2 for the two-phase model

Aims: We investigate temperature fluctuations in H II regions in terms of a two-phase model, which assumes that the nebular gas consists of a hot and a cold phase. Methods: We derive general formulae for T([O III), the [O III] forbidden line temperature, and T(H I), the hydrogen Balmer jump temperature, in terms of the temperatures of the hot and cold phases, T_h and T_c. Results: For large temperature differences, the values of t2 required to account for the observed difference between T([O III]) and T(H I) are much lower than those deduced using the classical formulae that assume random and small amplitude temperature fluctuations. One should therefore be cautious when using a two-phase model to account for empirically derived $t^2$ values. We present a correction of a recent work by Giammanco & Beckman, who use a two-phase model to estimate the ionization rate of H II regions by cosmicrays. We show that a very small amount of cold gas is sufficient to account for t2 values typically inferred for H II regions.

astro-ph↗

arXiv2006

BBN, ADTree and MLP Comparison in Separating Quasars from Large Survey Catalogues

We compare the performance of Bayesian Belief Networks (BBN), Multilayer Perceptron (MLP) networks and Alternating Decision Trees (ADtree) on separating quasars from stars with the database from the 2MASS and FIRST survey catalogs. Having a training sample of sources of known object types, the classifiers are trained to separate quasars from stars. By the statistical properties of the sample, the features important for classification are selected. We compare the classification results with and without feature selection. Experiments show that the results with feature selection are better than those without feature selection. From the high accuracy, it is concluded that these automated methods are robust and effective to classify point sources, moreover they all may be applied for large survey projects (e.g. selecting input catalogs) and for other astronomical issues, such as the parameter measurement of stars and the redshift estimation of galaxies and quasars.

astro-ph↗

arXiv2006

The central nature of the Hidden Subgroup problem

We show that several problems that figure prominently in quantum computing, including Hidden Coset, Hidden Shift, and Orbit Coset, are equivalent or reducible to Hidden Subgroup for a large variety of groups. We also show that, over permutation groups, the decision version and search version of Hidden Subgroup are polynomial-time equivalent. For Hidden Subgroup over dihedral groups, such an equivalence can be obtained if the order of the group is smooth. Finally, we give nonadaptive program checkers for Hidden Subgroup and its decision version.

cs.CC↗

arXiv2006

Identify bottom contribution in non-photonic electron spectra and \vv\ from \AuAu collisions at RHIC

We present a study on the spectra and elliptic flow v2 for heavy flavor (charm and bottom) decayed electrons provided the relative contributions of charm and bottom hadrons from the PYTHIA calculations. We made a simultaneous fit to both measured non-photonic electron spectra and v2 distributions. The results suggest that the bottom contribution is not dominant for electron pt<5 GeV/c in the 200 GeV Au+Au collisions.

hep-ph↗

arXiv2006

Yang-Mills condensate dark energy coupled with matter and radiation

The coincidence problem is studied for the dark energy model of effective Yang-Mills condensate in a flat expanding universe during the matter-dominated stage. The YMC energy $ρ_y(t)$ is taken to represent the dark energy, which is coupled either with the matter, or with both the matter and the radiation components. The effective YM Lagrangian is completely determined by quantum field theory up to 1-loop order. It is found that under very generic initial conditions and for a variety of forms of coupling, the existence of the scaling solution during the early stages and the subsequent exit from the scaling regime are inevitable. The transition to the accelerating stage always occurs around a redshift $z\simeq (0.3\sim 0.5)$. Moreover, when the Yang-Mills condensate transfers energy into matter or into both matter and radiation, the equation of state $w_y$ of the Yang-Mills condensate can cross over -1 around $z\sim 2$, and takes on a current value $\simeq -1.1$. This is consistent with the recent preliminary observations on supernovae Ia. Therefore, the coincidence problem can be naturally solved in the effective YMC dark energy models.

gr-qc↗

arXiv2006

Tunable Noise Cross-Correlations in a Double Quantum Dot

We report measurements of the cross-correlation between current noise fluctuations in two capacitively coupled quantum dots in the Coulomb blockade regime. The sign of the cross-spectral density is found to be tunable by gate voltage and source-drain bias. Good agreement is found with a model of sequential tunneling through the dots in the presence of inter-dot capacitive coupling.

cond-mat.mes-hall↗

arXiv2006

A system for measuring auto- and cross-correlation of current noise at low temperatures

We describe the construction and operation of a two-channel noise detection system for measuring power and cross spectral densities of current fluctuations near 2 MHz in electronic devices at low temperatures. The system employs cryogenic amplification and fast-Fourier-transform based spectral measurement. The gain and electron temperature are calibrated using Johnson noise thermometry. Full shot noise of 100 pA can be resolved with an integration time of 10 s. We report a demonstration measurement of bias-dependent current noise in a gate defined GaAs/AlGaAs quantum point contact.

cond-mat.mes-hall↗

arXiv2006

Shot-Noise Signatures of 0.7 Structure and Spin in a Quantum Point Contact

We report simultaneous measurement of shot noise and dc transport in a quantum point contact as a function of source-drain bias, gate voltage, and in-plane magnetic field. Shot noise at zero field exhibits an asymmetry related to the 0.7 structure in conductance. The asymmetry in noise evolves smoothly into the symmetric signature of spin-resolved electron transmission at high field. Comparison to a phenomenological model with density-dependent level splitting yields good quantitative agreement.

cond-mat.mes-hall↗

arXiv2006

Fe/Ni ratio in the Ant Nebula Mz 3

We have analyzed the [Fe II] and [Ni II] emission lines in the bipolar planetary nebula Mz~3. We find that the [Fe II] and [Ni II] lines arise exclusively from the central regions. Fluorescence excitation in the formation process of these lines is negligible for this low-excitation nebula. From the [Fe II]/[Ni II] ratio, we obtain a higher Fe/Ni abundance ratio with respect to the solar value. The current result provides further supporting evidence for Mz 3 as a symbiotic Mira.

astro-ph↗

arXiv2006

Abundances of s-process elements in planetary nebulae: Br, Kr & Xe

We identify emission lines of post-iron peak elements in very high signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement with the theories of s-process in AGB star. Surprisingly, we did not detect lines from Br even though s-process calculations indicate that it should be produced with Kr at detectable levels.

astro-ph↗

arXiv2006

An antisymmetric plasmon resonance in coupled gold nanoparticles as a sensitive tool for detection of local index of refraction

A nanofabricated regular array of coupled gold nano-pillars is employed to detect local indices of refraction of different liquids using a shift of an antisymmetric plasmon resonance peak observed in the reflection spectra. The peak spectral position is found to be a unique function of the local refractive index for a wide range of indices. We discuss possible applications of the fabricated nanostructured arrays in bio and chemical sensors.

physics.optics↗

arXiv2006

Nanometric laser trapping based on nanostructured substrates

Laser trapping near the surface of a nanostructured substrate is demonstrated. Stable microbubbles with radii of 1-20micrometers have been created and manipulated with sub-micron precision by a focused laser beam in an immersion oil covering arrays of pairs of gold nanopillars deposited on a glass substrate. The threshold for bubble creation and trapping characteristics depended on near-field coupling of nanopillars. The nanometric laser tweezers showed giant trapping efficiency of Q~50 for the trapped microbubbles.

physics.optics↗

arXiv2006

Chemical abundances for Hf 2-2, a planetary nebula with the strongest known heavy element recombination lines

We present high quality optical spectroscopic observations of the planetary nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination lines (ORLs) from heavy element ions. Analysis of the H {\sc i} and He {\sc i} recombination spectrum yields an electron temperature of $\sim 900$ K, a factor of ten lower than given by the collisionally excited [O {\sc iii}] forbidden lines. The ionic abundances of heavy elements relative to hydrogen derived from ORLs are about a factor of 70 higher than those deduced from collisionally excited lines (CELs) from the same ions, the largest abundance discrepancy factor (adf) ever measured for a PN. By comparing the observed O {\sc ii} $λ$4089/$λ$4649 ORL ratio to theoretical value as a function of electron temperature, we show that the O {\sc ii} ORLs arise from ionized regions with an electron temperature of only $\sim 630$ K. The current observations thus provide the strongest evidence that the nebula contains another previously unknown component of cold, high metallicity gas, which is too cool to excite any significant optical or UV CELs and is thus invisible via such lines. The existence of such a plasma component in PNe provides a natural solution to the long-standing dichotomy between nebular plasma diagnostics and abundance determinations using CELs on the one hand and ORLs on the other.

astro-ph↗

arXiv2006

Landau Level Splitting in Graphene in High Magnetic Fields

The quantum Hall (QH) effect in two-dimensional (2D) electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors $ν=0,\pm 1,\pm 4$ are discovered at magnetic fields $B>$20 T, indicating the lifting of the four-fold degeneracy of the previously observed QH states at $ν=\pm(|n|+1/2)$, where $n$ is the Landau level index. In particular, the presence of the $ν=0, \pm 1$ QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at $ν=\pm 4$ is investigated in tilted magnetic field and can be attributed to lifting of the spin-degeneracy of the $n=1$ Landau level.

cond-mat.mes-hall↗