arXiv subjects

Z. Wang

Publications and source records attributed to Z. Wang.

At least 307 records · Page 17Linked to original sources

arXiv2008

Micromachined Polycrystalline Sige-Based Thermopiles for Micropower Generation on Human Body

This paper presents a polycrystalline silicon germanium (poly-SiGe) thermopile specially designed for thermoelectric generators used on human body. Both the design of the single thermocouple and the arrangement of the thermocouple array have been described. A rim structure has been introduced in order to increase the temperature difference across the thermocouple junctions. The modeling of the thermocouple and the thermopile has been performed analytically and numerically. An output power of about 1 $μ$W at an output voltage of more than 1 V is expected from the current design of thermopiles in a watch-size generator. The key material properties of the poly-SiGe have been measured. The thermopile has been fabricated and tested. Experimental results clearly demonstrate the advantage of the rim structure in increasing output voltage. In presence of forced convection, the output voltage of a non-released thermopile can increase from about 53 mV/K/cm2 to about 130 mV/K/cm2 after the rim structure is formed. A larger output voltage from the thermopile is expected upon process completion.

cs.OH↗

arXiv2008

Optical/Infrared Observations of the Anomalous X-ray Pulsar 1E 1048.1-5937 During Its 2007 X-Ray Flare

We report on optical and infrared observations of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937, made during its ongoing X-ray flare which started in 2007 March. We detected the source in the optical I and near-infrared Ks bands in two ground-based observations and obtained deep flux upper limits from four observations, including one with the Spitzer Space Telescope at 4.5 and 8.0 microns. The detections indicate that the source was approximately 1.3--1.6 magnitudes brighter than in 2003--2006, when it was at the tail of a previous similar X-ray flare. Similar related flux variations have been seen in two other AXPs during their X-ray outbursts, suggesting common behavior for large X-ray flux variation events in AXPs. The Spitzer flux 1E 1048.1-5937 limits are sufficiently deep that we can exclude mid-infrared emission similar to that from the AXP 4U 0142+61, which has been interpreted as arising from a dust disk around the AXP. The optical/near-infrared emission from probably has a magnetospheric origin. The similarity in the flux spectra of 4U 0142+61 and 1E 1048.1-5937 challenges the dust disk model proposed for the latter.

astro-ph↗

arXiv2008

Anomalous Hall effect in Fe/Cu bilayers

The scaling of anomalous Hall resistivity on the longitudinal resistivity has been intensively studied in the different magnetic systems, including multilayers and granular films, to examine which mechanism, skew scattering or side-jump, dominates. The basis of the scaling law is that both the resistivities are due to the electron scattering at the imperfections in the materials. By studying of anomalous Hall effect (AHE) in the simple Fe/Cu bilayers, we demonstrate that the measured anomalous Hall effect should not follow the scaling laws derived from skew scattering or side-jump mechanism due to the short-circuit and shunting effects of the non-magnetic layers.

cond-mat.other↗

arXiv2008

The Infrared Detection of the Pulsar Wind Nebula in the Galactic Supernova Remnant 3C 58

We present infrared observations of 3C 58 with the Spitzer Space Telescope and the Canada-France-Hawaii Telescope. Using the IRAC camera, we have imaged the entire source resulting in clear detections of the nebula at 3.6 and 4.5 microns. The derived flux values are consistent with extrapolation of the X-ray spectrum to the infrared band, demonstrating that any cooling break in the synchrotron spectrum must occur near the soft X-ray band. We also detect the torus surrounding PSR J0205+6449, the 65 ms pulsar that powers 3C 58. The torus spectrum requires a break between the infrared and X-ray bands, and perhaps multiple breaks. This complex spectrum, which is an imprint of the particles injected into the nebula, has considerable consequences for the evolution of the broadband spectrum of 3C 58. We illustrate these effects and discuss the impact of these observations on the modeling of broadband spectra of pulsar wind nebulae.

astro-ph↗

arXiv2007

Search for Mid-IR Flux Variations from The Anomalous X-ray Pulsar 4U 0142+61

We report on our Spitzer observations of the anomalous X-ray pulsar 4U 0142+61, made following a large X-ray burst that occurred on 2007 February 7. To search for mid-infrared flux variations, four imaging observations were carried out at 4.5 and 8.0 $μ$m with the Infrared Array Camera from February 14 to 21. No significant flux variations were detected, and the average fluxes were 32.1$\pm$2.0 $μ$Jy at 4.5 $μ$m and 59.8$\pm8.5$ $μ$Jy at 8.0 $μ$m, consistent with those obtained in 2005. The non-detection of variability is interesting in light of reported rapid variability from this source in the near-infrared, but consistent with the fact that the source already went back to its quiescent state before our observations began, as indicated by contemporaneous X-ray observations. In order to understand the origin of the near-infrared variability, frequent, simultaneous multi-wavelength observations are needed.

astro-ph↗

arXiv2007

Efficient spatially-resolved multimode quantum memory

We propose a method that enables efficient storage and retrieval of a photonic excitation stored in an ensemble quantum memory consisting of Lambda-type absorbers with non-zero Stokes shift. We show that this can be used to implement a multimode quantum memory storing multiple frequency-encoded qubits in a single ensemble, and allowing their selective retrieval. The read-out scheme applies to memory setups based on both electromagnetically-induced transparency and stimulated Raman scattering, and spatially separates the output signal field from the control fields.

quant-ph↗

arXiv2007

Strong saturation absorption imaging of dense clouds of ultracold atoms

We report on a far above saturation absorption imaging technique to investigate the characteristics of dense packets of ultracold atoms. The transparency of the cloud is controlled by the incident light intensity as a result of the non-linear response of the atoms to the probe beam. We detail our experimental procedure to calibrate the imaging system for reliable quantitative measurements, and demonstrate the use of this technique to extract the profile and its spatial extent of an optically thick atomic cloud.

physics.optics↗

arXiv2007

Spitzer Mid-infrared Upper Limits on Anomalous X-Ray Pulsars 1E 1048.1-5937, 1RXS J170849-400910, and XTE J1810-197

We report on mid-infrared imaging observations of the anomalous X-ray pulsars (AXPs) 1E 1048.1-5937, 1RXS J170849-400910, and XTE J1810-197. The observations were carried out at 4.5 and 8.0 microns with the Infrared Array Camera and at 24 microns with the Multiband Imaging Photometer on the Spitzer Space Telescope. No mid-infrared counterparts were detected. As infrared emission from AXPs may be related to their X-ray emission either via the magnetosphere or via a dust disk, we compare the derived upper limits on the infrared/X-ray flux ratios of the AXPs to the same ratio for 4U 0142+61, an AXP previously detected in the mid-infrared range. The comparison indicates that our three non-detections are consistent with their relatively low X-ray fluxes. For XTE J1810-197, our upper limits set a constraint on its rising radio/millimeter energy spectrum, suggesting a spectral break between 1.5$\times 10^{11}$--6$\times10^{13}$ Hz.

astro-ph↗

arXiv2007

The Local Galaxy 8 micron Luminosity Function

A SST survey in the NOAO Deep-Wide Field in Boötes provides a complete, 8-micron-selected sample of galaxies to a limiting (Vega) magnitude of 13.5. In the 6.88 deg$^2$ field sampled, 79% of the 4867 galaxies have spectroscopic redshifts, allowing an accurate determination of the local (z<0.3) galaxy luminosity function. Stellar and dust emission can be separated on the basis of observed galaxy colors. Dust emission (mostly PAH) accounts for 80% of the 8 micron luminosity, stellar photospheres account for 19%, and AGN emission accounts for roughly 1 %. A sub-sample of the 8 micron-selected galaxies have blue, early-type colors, but even most of these have significant PAH emission. The luminosity functions for the total 8 micron luminosity and for the dust emission alone are both well fit by Schechter functions. For the 8 micron luminosity function, the characteristic luminosity is νL_ν^*(8.0 \micron) = 1.8 \times 10^{10}$ \Lsun while for the dust emission alone it is 1.6 x 10^{10}$ \Lsun\null. The average 8 \micron luminosity density at z<0.3 is 3.1 x 10^7 \Lsun Mpc^{-3}, and the average luminosity density from dust alone is 2.5 x 10^7 \Lsun Mpc^{-3}. This luminos ity arises predominantly from galaxies with 8 \micron luminosities ($νL_ν$) between $2\times 10^9$ and $2 x 10^{10}$ \Lsun, i.e., normal galaxies, not LIRGs or ULIRGs.

astro-ph↗

arXiv2007

Antiferromagnetism-superconductivity competition in electron-doped cuprates triggered by oxygen reduction

We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{1-x}$LaCe$_{x}$CuO$_4$ electron-doped cuprates. In contrast to the common belief, neither the band filling nor the band parameters are significantly affected by the oxygen reduction process. Instead, we show that the main electronic role of the reduction process is to remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.

cond-mat.supr-con↗

arXiv2007

Entanglement fidelity of quantum memories

We introduce a figure of merit for a quantum memory which measures the preservation of entanglement between a qubit stored in and retrieved from the memory and an auxiliary qubit. We consider a general quantum memory system consisting of a medium of two level absorbers, with the qubit to be stored encoded in a single photon. We derive an analytic expression for our figure of merit taking into account Gaussian fluctuations in the Hamiltonian parameters, which for example model inhomogeneous broadening and storage time dephasing. Finally we specialize to the case of an atomic quantum memory where fluctuations arise predominantly from Doppler broadening and motional dephasing.

quant-ph↗

arXiv2007

High Resolution Imaging of Warm and Dense Molecular Gas in the Nuclear Region of the Luminous Infrared Galaxy NGC6240

We present ~2'' resolution CO(3-2), HCO+(4-3) and 880micron continuum images of the luminous infrared galaxy NGC6240 obtained at the Submillimeter Array. We find that the spatially resolved CO(3-2), HCO+(4-3) and the 880micron emission peaks between the two nuclear components that are both known to harbor AGNs. Our Large Velocity Gradient (LVG) analysis performed on each velocity channel suggests that the peak of the molecular gas emission traced in our observations is warm (T = 20 - 100K), dense (nH2 = 10^(5.0 - 5.4) cm^-3) and moderately optically thin (tau = 0.2 - 2) in the central 1 kpc. We also find large column densities of ~10^(23) cm^(-2). Such extreme conditions are observed over ~300 km s^(-1) centered around the CO derived systemic velocity. The derived molecular gas mass from the CO(3-2) emission and a CO-to-H2 conversion factor commonly used for ULIRGs is (6.9 +/- 1.7) x 10^9 Msun, and this is consistent with the mass derived from previous CO(2-1) observations. The gas is highly turbulent in the central kpc (Delta v_(FWZI) ~ 1175 km s^(-1)). Furthermore, possible inflow or outflow activity is suggested from the CO(3-2) velocity distribution. We tentatively state that 3.5 x 10^8 Msun of isolated CO(3-2) emission seen west of the northern disk may be associated with outflows from starburst superwinds, but the gas outflow scenario from one of the central AGN is not completely ruled out. Piecing all of the information together, the central region of NGC 6240 harbors 2 AGNs, ~10^(10) Msun of molecular gas mass, 5 x 10^7 Msun of dust mass, and has possible evidence of inflow and outflow activity.

astro-ph↗

arXiv2006

Universal quasiparticle decoherence in hole- and electron-doped high-Tc cuprates

We use angle-resolved photoemission to unravel the quasiparticle decoherence process in the high-$T_c$ cuprates. The coherent band is highly renormalized, and the incoherent part manifests itself as a nearly vertical ``dive'' in the $E$-$k$ intensity plot that approaches the bare band bottom. We find that the coherence-incoherence crossover energies in the hole- and electron-doped cuprates are quite different, but scale to their corresponding bare bandwidth. This rules out antiferromagnetic fluctuations as the main source for decoherence. We also observe the coherent band bottom at the zone center, whose intensity is strongly suppressed by the decoherence process. Consequently, the coherent band dispersion for both hole- and electron-doped cuprates is obtained, and is qualitatively consistent with the framework of Gutzwiller projection.

cond-mat.supr-con↗

arXiv2006

Determination of vortex characteristics of YBa$_2$Cu$_3$O$_{7-δ}$ thin films with two different assumptions

This paper has been withdrawn by the authors.

cond-mat.supr-con↗

arXiv2006

Modematching an optical quantum memory

We analyse the off-resonant Raman interaction of a single broadband photon, copropagating with a classical `control' pulse, with an atomic ensemble. It is shown that the classical electrodynamical structure of the interaction guarantees canonical evolution of the quantum mechanical field operators. This allows the interaction to be decomposed as a beamsplitter transformation between optical and material excitations on a mode-by-mode basis. A single, dominant modefunction describes the dynamics for arbitrary control pulse shapes. Complete transfer of the quantum state of the incident photon to a collective dark state within the ensemble can be achieved by shaping the control pulse so as to match the dominant mode to the temporal mode of the photon. Readout of the material excitation, back to the optical field, is considered in the context of the symmetry connecting the input and output modes. Finally, we show that the transverse spatial structure of the interaction is characterised by the same mode decomposition.

quant-ph↗

arXiv2006

Unusual photoemission resonances of oxygen-dopant induced states in Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+x}$

We have performed an angular-resolved photoemission study of underdoped, optimally doped and overdoped Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+x}$ samples using a wide photon energy range (15 - 100 eV). We report a small and broad non-dispersive A$_{1g}$ peak in the energy distribution curves whose intensity scales with doping. We attribute it to a local impurity state similar to the one observed recently by scanning tunneling spectroscopy and identified as the oxygen dopants. Detailed analysis of the resonance profile and comparison with the single-layered Bi$_{2}$Sr$_2$CuO$_{6+x}$ suggest a mixing of this local state with Cu via the apical oxygens.

cond-mat.supr-con↗

arXiv2006

Spitzer Imaging of nearby ULIRGs and their Progeny: Fine-Structure Elliptical Galaxies

We present results from two mid-infrared imaging programs of ultraluminous infrared galaxies (ULIRGs) and fine-structure elliptical galaxies. The former are known to nearly all be recent mergers between two gas-rich spiral galaxies, while the latter are also believed to be even more aged merger remnants. An examination of these two classes of objects, which may represent different stages of the same putative merger sequence, should reveal similarities in the distribution of their stellar populations and dust content consistent with that expected for time-evolution of the merger. The data reveal for the first time extended dust emission in the ULIRG galaxy bodies and along their tidal features. However, contrary to expectation, we find that the vast majority of the fine-structure elliptical galaxies lack such structured emission. This likely results from the optical selection of the elliptical sample, resulting in a population that reflects the IR-activity (or lack thereof) of optically selected interacting galaxies. Alternately, this may reflect an evolutionary process in the distribution of the dust content of the galaxy bodies.

astro-ph↗

arXiv2005

Manipulating vortex motion by thermal and Lorentz force in high temperature superconductors

By using thermal and Lorentz force, the vortex motion is successfully manipulated in the mixed state of underdoped La$_{2 - x}$Sr$_{x}$CuO$_{4}$ single crystals and optimally doped YBa$_{2}$Cu$_{3}$O$_{7 - δ}$ thin films. A conclusion is drawn that the strong Nernst signal above $T_{c}$ is induced by vortex motion. In the normal state, in order to reduce the dissipative contribution from the quasiparticle scattering and enhance the signal due to the possible vortex motion, a new measurement configuration is proposed. It is found that the in-plane Nernst signal ($H$ $| | $ $c$) can be measurable up to a high temperature in the pseudogap region, while the Abrikosov flux flow dissipation can only be measured up to $T_{c}$. This may point to different vortices below and above $T_{c}$ if we attribute the strong Nernst signal in the pseudogap region to the vortex motion. Below $T_{c}$ the dissipation is induced by the motion of the Abrikosov vortices. Above $T_{c}$ the dissipation may be caused by the motion of the spontaneously generated unbinded vortex-antivortex pairs.

cond-mat.supr-con↗