arXiv subjects

Z. Wang

Publications and source records attributed to Z. Wang.

At least 289 records · Page 16Linked to original sources

arXiv2010

Dissipationless Anomalous Hall Current in $Fe_{100-x}(SiO_2)_x$ Films

The observation of dissipationless anomalous Hall current is one of the experimental evidences to confirm the intrinsic origin of anomalous Hall effect. To study the origin of anomalous Hall effect in iron, Fe$_{100-x}$(SiO$_{2}$)$_{x}$ granular films with volume fraction of SiO$_{2}$ 0\le x \le 40.51 were fabricated using co-sputtering. Hall and longitudinal resistivities were measured in the temperature range 5 to 350 K with magnetic fields up to 5 Tesla. As x increased from 0 to 40.51, the anomalous Hall resistivity and longitudinal resistivity increased about 4 and 3 orders in magnitude, respectively. Analysis of the results revealed that the normalized anomalous Hall conductivity is a constant for all the samples, the evidence of dissipationless anomalous Hall current in Fe.

cond-mat.mes-hall↗

arXiv2010

Anomalous Hall Effect in Fe/Gd Bilayers

Non-monotonic dependence of anomalous Hall resistivity on temperature and magnetization, including a sign change, was observed in Fe/Gd bilayers. To understand the intriguing observations, we fabricated the Fe/Gd bilayers and single layers of Fe and Gd simultaneously. The temperature and field dependences of longitudinal resistivity, Hall resistivity and magnetization in these films have also been carefully measured. The analysis of these data reveals that these intriguing features are due to the opposite signs of Hall resistivity/or spin polarization and different Curie temperatures of Fe and Gd single-layer films.

cond-mat.mtrl-sci↗

arXiv2010

Operating quantum waveguide circuits with superconducting single-photon detectors

Advanced quantum information science and technology (QIST) applications place exacting de- mands on optical components. Quantum waveguide circuits offer a route to scalable QIST on a chip. Superconducting single-photon detectors (SSPDs) provide infrared single-photon sensitivity combined with low dark counts and picosecond timing resolution. In this study we bring these two technologies together. Using SSPDs we observe a two-photon interference visibility of 92.3\pm1.0% in a silica-on-silicon waveguide directional coupler at \lamda = 804 nm-higher than that measured with silicon detectors (89.9\pm0.3%). We further operated controlled-NOT gate and quantum metrology circuits with SSPDs. These demonstrations present a clear path to telecom-wavelength quantum waveguide circuits.

quant-ph↗

arXiv2009

CSIP - a Novel Photon-Counting Detector Applicable for the SPICA Far-Infrared Instrument

We describe a novel GaAs/AlGaAs double-quantum-well device for the infrared photon detection, called Charge-Sensitive Infrared Phototransistor (CSIP). The principle of CSIP detector is the photo-excitation of an intersubband transition in a QW as an charge integrating gate and the signal amplification by another QW as a channel with very high gain, which provides us with extremely high responsivity (10^4 -- 10^6 A/W). It has been demonstrated that the CSIP designed for the mid-infrared wavelength (14.7 um) has an excellent sensitivity; the noise equivalent power (NEP) of 7x10^-19 W/rHz with the quantum efficiency of ~2%. Advantages of the CSIP against the other highly sensitive detectors are, huge dynamic range of >10^6, low output impedance of 10^3 -- 10^4 Ohms, and relatively high operation temperature (>2K). We discuss possible applications of the CSIP to FIR photon detection covering 35 -- 60 um waveband, which is a gap uncovered with presently available photoconductors.

astro-ph.IM↗

arXiv2009

Dewetting-controlled binding of ligands to hydrophobic pockets

We report on a combined atomistic molecular dynamics simulation and implicit solvent analysis of a generic hydrophobic pocket-ligand (host-guest) system. The approaching ligand induces complex wetting/dewetting transitions in the weakly solvated pocket. The transitions lead to bimodal solvent fluctuations which govern magnitude and range of the pocket-ligand attraction. A recently developed implicit water model, based on the minimization of a geometric functional, captures the sensitive aqueous interface response to the concave-convex pocket-ligand configuration semi-quantitatively.

cond-mat.soft↗

arXiv2009

Tachyonic equations to reduce the divergent integral of QED

If the total integral including both 0<V<c and c<V<infinity is considered, divergence will be reduced.Firstly,we use the Cherenkov radiation in media as analogue to confirm the formulation (7)-(10) of a tachyon in vacuum.Then these equations are applied to calculate the Lamb shift and the cut-off frequency is unnecessary.

physics.gen-ph↗

arXiv2009

Representation theory of a Class of Hopf algebras

The representations of the pointed Hopf algebras $U$ and $\su$ are described, where $U$ and $\su$ can be regarded as deformations of the usual quantized enveloping algebras $U_q(\mathfrak{sl}(3))$ and the small quantum groups respectively. It is illustrated that these representations have a close connection with those of the quantized enveloping algebras $U_q(\mathfrak{sl}(2))$ and those of the half quantum groups of $\mathfrak{sl}(3)$.

math.RA↗

arXiv2009

The half-quantum group U^>=0

Let U^>=0 denote the half quantum group for a fixed simple Lie algebra. We examine some properties and representation of U^>=0. We prove that the Hopf algebra U^>=0 is not quasi-cocommutative, and hence the category of left U^>=0-module is not a braided monoidal category. In the weight module category W, we describe all the simple objects and the projective objects. We also describe all simple Yetter-Drinfel'd U^>=0-weight modules.

math.RA↗

arXiv2009

Polarizibilities as a test of localized approximations to the self-interaction correction

We present applications of the recently introduced ``Generalized SIC-Slater'' scheme which provides a simple Self-Interaction Correction approximation in the framework of the Optimized Effective Potential. We focus on the computation of static polarizabilities which are known to constitute stringent tests for Density Functional Theory. We apply the new method to model H chains, but also to more realistic systems such as C4 (organic) chains, and less symmetrical systems such as a Na5 (metallic) cluster. Comparison is made with other SIC schemes, especially with the standard SIC-Slater one.

quant-ph↗

arXiv2009

Charge oscillation-induced light transmission through subwavelength slits and holes

We present a concrete picture of spoof surface plasmons (SSPs) combined with cavity resonance to clarify the basic mechanism underlying extraordinary light transmission through metal films with subwavelength slits or holes. This picture may indicate a general mechanism of metallic nanostructure optics: When light is incident on a non-planar conducting surface, the free electrons cannot move homogeneously in response to the incident electric field, i.e., their movement can be impeded at the rough parts, forming inhomogeneous charge distributions. The oscillating charges/dipoles then emit photons (similar to Thomson scattering of x rays by oscillating electrons), and the interference between the photons may give rise to anomalous transmission, reflection or scattering.

physics.optics↗

arXiv2009

Non-exponential London penetration depth in Ba$_{1-}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals

We have studied the in- and out-of-plane magnetic penetration depths in the hole- doped iron based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($T_{c}\approx$ 30K). The study was performed on single crystals grown from different fluxes and we find that the results are nearly the same. The in-plane London penetration depth $λ_{ab}$ does not show exponential saturation at low temperature, as would be expected from a fully gapped superconductor. Instead, $λ_{ab}(T)$ shows a power-law behavior, $λ\propto T^{n}$ ($n\approx 2$), down to $T\approx 0.02 T_{c}$, similar to the electron doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. The penetration depth anisotropy $γ_λ=λ_{c}(T)/λ_{ab}(T)$ increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, $γ_ξ=H_{c2}^{\perp c}(0)/H_{c2}^{\parallel c}(0)$. These are universal characteristics of both the electron and hole doped 122 systems, suggesting unconventional superconductivity. The behavior of the in-plane superfluid density $ρ_{ab}(T)$ is discussed in light of existing theoretical models proposed for the iron pnictides superconductors.

cond-mat.supr-con↗

arXiv2008

Electronic structure of optimally doped pnictide Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$: a comprehensive ARPES investigation

We have conducted a comprehensive angle-resolved photoemission study on the normal state electronic structure of the Fe-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. We have identified four dispersive bands which cross the Fermi level and form two hole-like Fermi surfaces around $Γ$ and two electron-like Fermi surfaces around M. There are two nearly nested Fermi surface pockets connected by an antiferromagnetic ($π$, $π$) wavevector. The observed Fermi surfaces show small $k_z$ dispersion and a total volume consistent with Luttinger theorem. Compared to band structure calculations, the overall bandwidth is reduced by a factor of 2. However, many low energy dispersions display stronger mass renormalization by a factor of $\sim$ 4, indicating possible orbital (energy) dependent correlation effects. Using an effective tight banding model, we fitted the band structure and the Fermi surfaces to obtain band parameters reliable for theoretical modeling and calculations of the important physical quantities, such as the specific heat coefficient.

cond-mat.supr-con↗

arXiv2008

Systematic characterization of upper critical fields for MgB$_2$ thin films using the two-band superconducting theory

We present experimental results of the upper critical fields $H_{\rm c2}$ of various MgB$_2$ thin films prepared by the molecular beam epitaxy, multiple-targets sputtering, and co-evaporation deposition apparatus. Experimental data of the $H_{\rm c2}(T)$ are successfully analyzed by applying the Gurevich theory of dirty two-band superconductivity in the case of $D_π/D_σ>1$, where $D_π$ and $D_σ$ are the intraband electron diffusivities for $π$ and $σ$ bands, respectively. We find that the parameters obtained from the analysis are strongly correlated to the superconducting transition temperature $T_{\rm c}$ of the films. We also discuss the anormalous narrowing of the transition width at intermediate temperatures confirmed by the magnetoresistance measurements.

cond-mat.supr-con↗

arXiv2008

A Catalog of X-ray Point Sources from Two Megaseconds of Chandra Observations of the Galactic Center

We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 by 0.8 degree field around the Galactic center. We increase the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Msec of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities >4e32 erg/s (0.5-8.0 keV; 90% confidence) over an area of one square degree, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Msec) around Sgr A*. The positions of 60% of our sources are accurate to <1" (95% confidence), and 20% have positions accurate to <0.5". We search for variable sources, and find that 3% exhibit flux variations within an observation, 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7-285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically-accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8 sigma evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l~0.1 degree.

astro-ph↗

arXiv2008

Observation of a novel orbital selective Mott transition in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$

Electrons in a simple correlated system behave either as itinerant charge carriers or as localized moments. However, there is growing evidence for the coexistence of itinerant electrons and local moments in transition metals with nearly degenerate $d$-orbitals. It demands one or more selective electron orbitals undergo the Mott transition while the others remain itinerant. Here we report the first observation of such an orbital selective Mott transition (OSMT) in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ by angle-resolved photoemission spectroscopy (ARPES). While we observed two sets of dispersing bands and Fermi surface associated with the doubly-degenerate $d_{yz}$ and $d_{zx}$ orbitals, the Fermi surface associated with the wider $d_{xy}$ band is missing, a consequence of selective Mott localization. Our theoretical calculations demonstrate that this novel OSMT is mainly driven by the combined effects of interorbital carrier transfer, superlattice potential, and orbital degeneracy, whereas the bandwidth difference plays a less important role.

cond-mat.str-el↗

arXiv2008

Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2

The recent discovery of superconductivity in iron-arsenic compounds below a transition temperature (Tc) as high as 55K ended the monopoly of copper oxides (cuprates) in the family of high-Tc superconductors. A critical issue in understanding this new superconductor, as in the case of cuprates, is the nature, in particular the symmetry and orbital dependence, of the superconducting gap. There are conflicting experimental results, mostly from indirect measurements of the low energy excitation gap, ranging from one gap to two gaps, from line nodes to nodeless gap function in momentum space. Here we report a direct observation of the superconducting gap, including its momentum, temperature, and Fermi surface (FS) dependence in Ba0.6K0.4Fe2As2 (Tc = 37 K) using angle-resolved photoelectron spectroscopy. We find two superconducting gaps with different values: a large gap (~ 12 meV) on the two small hole-like and electron-like FS sheets, and a small gap (~ 6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk Tc, are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Delta/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FS, which are connected by the (pi, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.

cond-mat.supr-con↗

arXiv2008

On Picture (2+1)-TQFTs

The goal of the paper is an exposition of the simplest $(2+1)$-TQFTs in a sense following a pictorial approach. In the end, we fell short on details in the later sections where new results are stated and proofs are outlined. Comments are welcome and should be sent to the 4th author.

math.QA↗

arXiv2008

Evolution of Fermi surface and normal-state gap in chemically substituted cuprates Bi$_{2}$Sr$_{2-x}$Bi$_{x}$CuO$_{6+δ}$

We have performed a systematic angle-resolved photoemission study of chemically substituted cuprates Bi$_{2}$Sr$_{2-x}$Bi$_{x}$CuO$_{6+δ}$. We observed that the Fermi surface area shrinks linearly with Bi substitution content $x$, reflecting the electron doping nature of this chemical substitution. In addition, the spectral linewidth broadens rapidly with increasing $x$, and becomes completely incoherent at the superconducting-insulating boundary. The d-wave-like normal-state gap observed in the lightly underdoped region gradually evolves into a large soft gap, which suppresses antinodal spectral weight linearly in both the excitation energy and temperature. Combining with the bulk resistivity data obtained on the same samples, we establish the emergence of the Coulomb gap behavior in the very underdoped regime. Our results reveal the dual roles, doping and disorder, of off-plane chemical substitutions in high-$T_c$ cuprates and elucidate the nature of the quantum electronic states due to strong correlation and disorder.

cond-mat.str-el↗