arXiv subjects

Z. Wang

Publications and source records attributed to Z. Wang.

At least 325 records · Page 18Linked to original sources

arXiv2005

Magnetic field asymmetry of nonlinear transport in carbon nanotubes

We demonstrate that nonlinear transport through a two-terminal nanoscale sample is not symmetric in magnetic field B. More specifically, we have measured the lowest order B-asymmetric terms in single-walled carbon nanotubes. Theoretically, the size of these terms can be used to infer both the strength of electron-electron interactions and the handedness of the nanotube. Consistent with theory, we find that at high temperatures the B-linear term is small and has a constant sign independent of Fermi energy, while at low temperatures it develops mesoscopic fluctuations. We also find significant magnetoresistance of nanotubes in the metallic regime which is unexplained.

cond-mat.mes-hall↗

arXiv2005

Evaporative Cooling of a Guided Rubidium Atomic Beam

We report on our recent progress in the manipulation and cooling of a magnetically guided, high flux beam of $^{87}{\rm Rb}$ atoms. Typically $7\times 10^9$ atoms per second propagate in a magnetic guide providing a transverse gradient of 800 G/cm, with a temperature $\sim550$ $μ$K, at an initial velocity of 90 cm/s. The atoms are subsequently slowed down to $\sim 60$ cm/s using an upward slope. The relatively high collision rate (5 s$^{-1}$) allows us to start forced evaporative cooling of the beam, leading to a reduction of the beam temperature by a factor of ~4, and a ten-fold increase of the on-axis phase-space density.

cond-mat.other↗

arXiv2005

Evidence for s-wave pairing from measurement on lower critical field in $MgCNi_3$

Magnetization measurements in the low field region have been carefully performed on a well-shaped cylindrical and an ellipsoidal sample of superconductor $MgCNi_3$. Data from both samples show almost the same results. The lower critical field $H_{c1}$ and the London penetration depth $λ$ are thus derived. It is found that the result of normalized superfluid density $λ^2(0)/λ^2(T)$ of $MgCNi_3$ can be well described by BCS prediction with the expectation for an isotropic s-wave superconductivity.

cond-mat.supr-con↗

arXiv2005

Spitzer Mid-Infrared Imaging of Nearby Ultraluminous Infrared Galaxies

We have observed 14 nearby (z<0.16) Ultraluminous Infrared Galaxies (ULIRGs) with Spitzer at 3.6-24 microns. The underlying host galaxies are well-detected, in addition to the luminous nuclear cores. While the spatial resolution of Spitzer is poor, the great sensitivity of the data reveals the underlying galaxy merger remnant, and provides the first look at off-nuclear mid-infrared activity.

astro-ph↗

arXiv2005

Deviations from plastic barriers in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ thin films

Resistive transitions of an epitaxial Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ thin film were measured in various magnetic fields ($H\parallel c$), ranging from 0 to 22.0 T. Rounded curvatures of low resistivity tails are observed in Arrhenius plot and considered to relate to deviations from plastic barriers. In order to characterize these deviations, an empirical barrier form is developed, which is found to be in good agreement with experimental data and coincide with the plastic barrier form in a limited magnetic field range. Using the plastic barrier predictions and the empirical barrier form, we successfully explain the observed deviations.

cond-mat.supr-con↗

arXiv2005

Comments on "Vortex phase diagram of HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ thin films from magnetoresistance measurements"

We make comments on the paper presented by Kim \textit{et al.} [Phys. Rev. B \textbf{61}, 11317 (2000)]. The authors analyzed activation energies of HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ thin films with a scaling relation, and defined four vortex regions for thin films. We find that the definitions of four vortex regions and the scaling relation are questionable when studying their definition of the resistivity range for thermally activated flux flow. Using the empirical activation energy form suggested by Zhang \textit{et al.} [Phys. Rev. B \textbf{71}, 052502 (2005)] for lower resistivity data, we find that the form is not only in good agreement with the resistivity $ρ(T,H)$, but also with the apparent activation energy $-\partial \ln ρ(T,H)/\partial T^{-1}$.

cond-mat.supr-con↗

arXiv2005

Fermi surface evolution and Luttinger theorem in Na$_x$CoO$_2$: a systematic photoemission study

We report a systematic angle-resolved photoemission study on Na$_x$CoO$_2$ for a wide range of Na concentrations ($0.3 \leq x \leq 0.72$). In all the metallic samples at different $x$, we observed (i) only a single hole-like Fermi surface centered around $Γ$ and (ii) its area changes with $x$ according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The $e_g^\prime$ band and the associated small Fermi surface pockets near the $K$ points predicted by band calculations are found to ``sink'' below the Fermi energy in a manner almost independent of the doping and temperature.

cond-mat.str-el↗

arXiv2004

Two dimensional scaling of resistance in flux flow region in $Tl_2Ba_2CaCu_2O_8$ thin films

The resistance of $Tl_2Ba_2CaCu_2O_8$ thin films has been measured when the angle between the applied fields and $ab$-plane of the film is changed continuously at various temperatures. Under various magnetic fields, the resistance can be well scaled in terms of the c-axis component of the applied fields at the same temperature in the whole angle range. Meanwhile, we show that the measurement of resistance in this way is a complementary method to determine the growth orientation of the anisotropic high-$T_c$ superconductors.

cond-mat.supr-con↗

arXiv2004

The Orbital Period of the Ultracompact Low-Mass X-Ray Binary 4U 1543--624

We report the discovery of the orbital period of the ultracompact low-mass X-ray binary (LMXB) 4U 1543-624 using time-resolved optical photometry taken with the 6.5-m Clay (Magellan II) telescope in Chile. The light curve in the Sloan r' band clearly shows a periodic, sinusoidal modulation at 18.2+-0.1 min with a fractional semiamplitude of 8%, which we identify as the binary period. This is the second shortest orbital period among all the known LMXBs, and it verifies the earlier suggestion of 4U 1543-624 as an ultracompact binary based on X-ray spectroscopic properties. The sinusoidal shape of the optical modulation suggests that it arises from X-ray heating of the mass donor in a relatively low-inclination binary, although it could also be a superhump oscillation in which case the orbital period is slightly shorter. If the donor is a C-O white dwarf as previously suggested, its likely mass and radius are around 0.03 M_sun and 0.03 R_sun, respectively. For conservative mass transfer onto a neutron star and driven by gravitational radiation, this implies an X-ray luminosity of 6.5X10^36 erg/s and a source distance of 7 kpc. We also discuss optical photometry of another LMXB, the candidate ultracompact binary 4U 1822-000. We detected significant optical variability on a time scale of about 90 min, but it is not yet clear whether this was due to a periodic modulation.

astro-ph↗

arXiv2004

Fermi surface topology of Ca_{1.5}Sr_{0.5}RuO_{4} determined by ARPES

We report ARPES results of the Fermi surface of Ca_{1.5}Sr_{0.5}RuO$_{4}, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t_{2g} energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr_{2}RuO$_{4}. We find that while the Fermi surface topology of the alpha, beta (d_{yz, zx}) sheets remains almost the same in these two materials, the gamma (d_{xy}) sheet exhibits a hole-like Fermi surface in Ca_{1.5}Sr_{0.5}RuO_{4} in contrast to being electron-like in Sr_{2}RuO_{4}. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca_{1.5}Sr_{0.5}RuO_{4}.

cond-mat.str-el↗

arXiv2004

The Off-nuclear Starbursts In NGC 4038/4039 (The Antennae Galaxies)

Imaging of the Antennae galaxies (NGC 4038/4039) with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope reveals large concentrations of star forming activity away from both nuclei of the two merging galaxies. These images confirm earlier findings based on ISO data with lower angular resolution. The short wavelength emission shows numerous compact sources identified as stellar clusters. At the longer wavelengths, bright, more amorphous and filamentary features correlate well with the known distributions of denser gas, warm dust, and HII regions. There are also fainter, more diffuse components at all wavelengths that permeate the entire region and extend into the two tidal tails. Non-stellar dust emission dominates the 5.8 and 8.0 micron images, accounting for as much as 79% of the light at 5.8 micron and 95% at 8 micron, averaged over the entire galaxy. Assuming that the non-stellar emission traces star formation, the IRAC data provide a view into the total underlying star forming activities unaffected by obscuration. Using the flux ratio of non-stellar to stellar emission as a guide, we map the local star formation rate in the Antennae and compare that to similar measurements in both normal and infrared-luminous galaxies. This rate in the active regions is found to be as high as those seen in starburst and some ultra-luminous infrared galaxies on ``per unit mass'' basis. The two galactic centers actually have lower star forming rates than the off-nuclear regions despite the presence of abundant dense gas and dust, suggesting that the latter is a necessary but not sufficient condition for on-going star formation.

astro-ph↗

arXiv2004

The IRAC Shallow Survey

The IRAC shallow survey covers 8.5 square degrees in the NOAO Deep Wide-Field Survey in Bootes with 3 or more 30 second exposures per position. An overview of the survey design, reduction, calibration, star-galaxy separation, and initial results is provided. The survey includes approximately 370,000, 280,000, 38,000, and 34,000 sources brighter than the 5 sigma limits of 6.4, 8.8, 51, and 50 microJy at 3.6, 4.5, 5.8, and 8 microns respectively, including some with unusual spectral energy distributions.

astro-ph↗

arXiv2004

Deep mid-infrared observations of Lyman-break galaxies

As part of the In-Orbit Checkout activities for the Spitzer Space Telescope, the IRAC team carried out a deep observation (average integration time ~8 hours) of a field surrounding the bright QSO HS 1700+6416. This field contains several hundred z~3 Lyman-break galaxy candidates, and we report here on their mid-infrared properties, including the IRAC detection rate, flux densities and colors, and the results of fitting population synthesis models to the optical, near-infrared, and IRAC magnitudes. The results of the model-fitting show that previous optical/near-infrared studies of LBGs were not missing large, hidden old stellar populations. The LBG candidates' properties are consistent with those of massive, star-forming galaxies at z~3. Other IRAC sources in the same field have similar properties, so IRAC selection may prove a promising method of finding additional high-redshift galaxies.

astro-ph↗

arXiv2004

Number Counts At 3 < lambda < 10 um From the Spitzer Space Telescope

Infrared source counts at wavelengths 3 < lambda < 10 um cover more than 10 magnitudes in source brightness, four orders of magnitude in surface density, and reach an integrated surface density of 10^5 sources/deg^2. At m<14 mag, most of the sources are Galactic stars, in agreement with models. After removal of Galactic stars, galaxy counts are consistent with what few measurements exist at nearby wavelengths. At 3.6 and 4.5 um, the galaxy counts follow the expectations of a Euclidean world model down to ~16 mag and drop below the Euclidean curve for fainter magnitudes. Counts at these wavelengths begin to show decreasing completeness around magnitude 19.5. At 5.8 and 8 um, the counts relative to a Euclidean world model show a large excess at bright magnitudes. This is probably because local galaxies emit strongly in the aromatic dust (``PAH'') features. The counts at 3.6 um resolve <50% of the Cosmic Infrared Background at that wavelength.

astro-ph↗

arXiv2004

Realization of a magnetically guided atomic beam in the collisional regime

We describe the realization of a magnetically guided beam of cold rubidium atoms, with a flux of $7\times 10^9$ atoms/s, a temperature of 400 $μ$K and a mean velocity of 1 m/s. The rate of elastic collisions within the beam is sufficient to ensure thermalization. We show that the evaporation induced by a radio-frequency wave leads to appreciable cooling and increase in phase space density. We discuss the perspectives to reach the quantum degenerate regime using evaporative cooling.

cond-mat.other↗

arXiv2004

Nanosecond Dynamics of Single-Molecule Fluorescence Resonance Energy Transfer

Motivated by recent experiments on photon statistics from individual dye pairs planted on biomolecules and coupled by fluorescence resonance energy transfer (FRET), we show here that the FRET dynamics can be modelled by Gaussian random processes with colored noise. Using Monte-Carlo numerical simulations, the photon intensity correlations from the FRET pairs are calculated, and are turned out to be very close to those observed in experiment. The proposed stochastic description of FRET is consistent with existing theories for microscopic dynamics of the biomolecule that carries the FRET coupled dye pairs.

quant-ph↗

arXiv2003

ARPES on Na$_{0.6}$CoO$_{2}$: Fermi surface, extended flat dispersion, and unusual band splitting

The electronic structure of single crystals Na$_{0.6}$CoO$_2$, which are closely related to the superconducting Na$_{0.3}$CoO$_2$.$y$H$_2$O ($T_c \sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($Γ$-$K$, $Γ$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $Γ$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $Γ$-$M$ direction, which differs from the predicted bilayer splitting.

cond-mat.supr-con↗

arXiv2003

A search for the optical and near-infrared counterpart of the accreting millisecond X-ray pulsar XTE J1751-305

We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751-305. There are no stars in the 0.7'' error circle (0.7'' is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of R>23.1, I>21.6, Z>20.6, J>19.6, K>19.2. We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disk subject to the reddening observed in X-rays for XTE J1751-305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disk or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultra-compact system.

astro-ph↗