arXiv subjects

J. Yang

Publications and source records attributed to J. Yang.

At least 199 records · Page 11Linked to original sources

arXiv2013

LLAGN and jet-scaling probed with the EVN

Accreting black holes on all mass scales (from stellar to supermassive) appear to follow a nonlinear relation between X-ray luminosity, radio luminosity and BH mass, indicating that similar physical processes drive the central engines in X-ray binaries and active galactic nuclei (AGN). However, in recent years an increasing number of BH systems have been identified that do not fit into this scheme. These outliers may be the key to understand how BH systems are powered by accretion. Here we present results from EVN observations of a sample of low-luminosity AGN (LLAGN) with known mass that have unusually high radio powers when compared with their X-ray luminosity.

astro-ph.CO↗

arXiv2013

Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2

A quantum critical point (QCP) is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently-discovered iron-pnictide high temperature superconductors, it is unknown whether a QCP exists or not in a carrier-doped system. Here we report transport and nuclear magnetic resonance (NMR) measurements on BaFe_{2-x}Ni_xAs_2 (0 =< x =< 0.17). We find two critical points at x_{c1} = 0.10 and x_{c2} = 0.14. The electrical resistivity follows ρ= ρ_0 + A*T^n, with n = 1 around x_{c1} and another minimal n = 1.1 at x_{c2}. By NMR measurements, we identity x_{c1} to be a magnetic QCP and suggest that x_{c2} is a new type of QCP associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.

cond-mat.supr-con↗

arXiv2013

Damped-Driven Granular Crystals: An Ideal Playground for Dark Breathers and Multibreathers

By applying an out-of-phase actuation at the boundaries of a uniform chain of granular particles, we demonstrate experimentally that time-periodic and spatially localized structures with a nonzero background (so-called dark breathers) emerge for a wide range of parameter values and initial conditions. Importantly, the number of ensuing breathers within the multibreather pattern produced can be "dialed in" by varying the frequency or amplitude of the actuation. The values of the frequency (resp. amplitude) where the transition between different multibreather states occurs are predicted accurately by the proposed theoretical model, which is numerically shown to support exact dark breather solutions. The existence, linear stability, and bifurcation structure of the theoretical dark breathers are also studied in detail. Moreover, the distributed sensing technologies developed herein enable a detailed space-time probing of the system and a systematic favorable comparison between theory, computation and experiments.

nlin.PS↗

arXiv2013

VLBI observations of the shortest orbital period black hole binary, MAXI J1659-152

The X-ray transient MAXI J1659-152 was discovered by Swift/BAT and it was initially identified as a GRB. Soon its Galactic origin and binary nature were established. There exists a wealth of multi-wavelength monitoring data for this source, providing a great coverage of the full X-ray transition in this candidate black hole binary system. We obtained two epochs of European VLBI Network (EVN) electronic-VLBI (e-VLBI) and four epochs of Very Long Baseline Array (VLBA) data of MAXI J1659-152 which show evidence for outflow in the early phases. The overall source properties (polarization, milliarcsecond-scale radio structure, flat radio spectrum) are described well with the presence of a compact jet in the system through the transition from the hard-intermediate to the soft X-ray spectral state. The apparent dependence of source size and the radio core position on the observed flux density (luminosity dependent core shift) support this interpretation as well. We see no evidence for major discrete ejecta during the outburst. For the source proper motion we derive 2 sigma upper limits of 115 microas/day in right ascension, and 37 microas/day in declination, over a time baseline of 12 days. These correspond to velocities of 1400 km/s and 440 km/s, respectively, assuming a source distance of 7 kpc.

astro-ph.HE↗

arXiv2013

Have we found conclusive evidence for dark matter through direct detection experiments?

We employ a SUSY-model-independent method to examine the remaining evidence for the low mass dark matter. Using the XENON100's recent result of 224.6 live days $\times$ 34kg exposure and PICASSO's result published in 2009, we have obtained a constrain of couplings, $\| {a_n} \| < 0.6$ and $\| {a_p} \| < 1.0$, corresponding to the spin-dependent cross section of ${σ_n} < 5.6 \times {10^{- 38}}$cm$^{2}$ and ${σ_p} < 1.6 \times {10^{- 37}}$cm$^{2}$. Spin-independent isospin violating dark matter model also failed to reconcile the recent result from XENON100 with the positive results from DAMA and CoGeNT.

hep-ph↗

arXiv2013

AFGL 5157 NH$_3$: A new stellar cluster in the forming

We present the analysis of Spitzer/IRAC and NIR imaging observation of AFGL 5157, an active star forming region. In the IRAC images, this region shows strong PAH emissions in channel 4 and H$_2$ emissions in channel 2. Many of the H$_2$ features are aligned to form jet-like structures. Three bipolar jets in the NH$_3$ core region and a couple of jets northwest of the core have been identified. We identify the possible driving agents of the bipolar jets and show them to be very young. An embedded cluster has been detected in the NH$_3$ core; many members in the cluster show their SEDs increasing from JHK bands toward longer wavelength, indicative of their early evolutionary stages. Millimeter and sub-millimeter continuum emissions are found to coincide spatially with these presumable Class 0/I sources, in the NH$_3$ core and the NW subregion. The existence of H$_2$ bipolar jets and very young stellar objects suggests that star formation is still going on at present epoch in these subregions. Combining the information from previous studies, we propose a sequential star formation scenario in the whole AFGL 5157 region.

astro-ph.SR↗

arXiv2013

Finite temperature effects on anisotropic pressure and equation of state of dense neutron matter in an ultrastrong magnetic field

Spin polarized states in dense neutron matter with recently developed Skyrme effective interaction (BSk20 parametrization) are considered in the magnetic fields $H$ up to $10^{20}$ G at finite temperature. In a strong magnetic field, the total pressure in neutron matter is anisotropic, and the difference between the pressures parallel and perpendicular to the field direction becomes significant at $H>H_{th}\sim10^{18}$ G. The longitudinal pressure decreases with the magnetic field and vanishes in the critical field $10^{18}<H_c\lesssim10^{19}$ G, resulting in the longitudinal instability of neutron matter. With increasing the temperature, the threshold $H_{th}$ and critical $H_c$ magnetic fields also increase. The appearance of the longitudinal instability prevents the formation of a fully spin polarized state in neutron matter and only the states with moderate spin polarization are accessible. The anisotropic equation of state is determined at densities and temperatures relevant for the interiors of magnetars. The entropy of strongly magnetized neutron matter turns out to be larger than the entropy of the nonpolarized matter. This is caused by some specific details in the dependence of the entropy on the effective masses of neutrons with spin up and spin down in a polarized state.

nucl-th↗

arXiv2013

Anisotropic pressure in strange quark matter under the presence of a strong magnetic field

Thermodynamic properties of strange quark matter in strong magnetic fields $H$ up to $10^{20}$ G are considered within the MIT bag model at zero temperature implying the constraints of total baryon number conservation, charge neutrality and chemical equilibrium. The pressure anisotropy, exhibiting in the difference between the pressures along and perpendicular to the field direction, becomes essential at $H>H_{th}$, with the estimate $10^{17}<H_{th}\lesssim10^{18}$ G. The longitudinal pressure vanishes in the critical field $H_c$, which can be somewhat less or larger than $10^{18}$ G, depending on the total baryon number density and bag pressure. As a result, the longitudinal instability occurs in strange quark matter, which precludes: (1) a significant drop in the content of $s$ quarks, which, otherwise, could happen at $H\sim10^{20}$ G; (2) the appearance of positrons in weak processes in a narrow interval near $H\sim2\cdot10^{19}$ G (replacing electrons). The occurrence of the longitudinal instability leaves the possibility only for electrons to reach a fully polarized state, while for all quark flavors the polarization remains mild even for the fields near $H_c$. The anisotropic equation of state is determined under the conditions relevant to the interiors of magnetars.

hep-ph↗

arXiv2012

Microscopic coexistence of antiferromagnetic order and superconductivity in Ba$_{0.77}$K$_{0.23}$Fe$_{2}$As$_{2}$

We report ^{75}As nuclear magnetic resonance studies on an underdoped single-crystal Ba0.77K0.23Fe2As2 with Tc = 16.5 K. Below T_N = 46 K, the NMR peaks for H//c split and those for H //a shift to higher frequencies, which indicates that an internal magnetic field along the c axis develops below T_N. The spin-lattice relaxation rate 1/T_1 measured at the shifted peak with H//a, which experiences the internal field, shows a distinct decrease below Tc(μ_0H = 12 T) = 16 K, following a T^3 relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted.

cond-mat.supr-con↗

arXiv2012

Conduction mechanisms of epitaxial EuTiO3 thin films

To investigate leakage current density versus electric field characteristics, epitaxial EuTiO3 thin films were deposited on (001) SrTiO3 substrates by pulsed laser deposition and were post-annealed in a reducing atmosphere. This investigation found that conduction mechanisms are strongly related to temperature and voltage polarity. It was determined that from 50 to 150 K the dominant conduction mechanism was a space-charge-limited current under both negative and positive biases. From 200 to 300 K, the conduction mechanism shows Schottky emission and Fowler-Nordheim tunneling behaviors for the negative and positive biases, respectively. This work demonstrates that Eu3+ is one source of leakage current in EuTiO3 thin films.

cond-mat.mtrl-sci↗

arXiv2012

Epitaxial Ferromagnetic Nanoislands of Cubic GdN in Hexagonal GaN

Periodic structures of GdN particles encapsulated in a single crystalline GaN matrix were prepared by plasma assisted molecular beam epitaxy. High resolution X-ray diffractometery shows that GdN islands, with rock salt structure are epitaxially oriented to the wurtzite GaN matrix. Scanning transmission electron microscopy combined with in-situ reflection high energy electron diffraction allows for the study of island formation dynamics, which occurs after 1.2 monolayers of GdN coverage. Magnetometry reveals two ferromagnetic phases, one due to GdN particles with Curie temperature of 70K and a second, anomalous room temperature phase.

cond-mat.mtrl-sci↗

arXiv2011

Is a Field-Induced Ferromagnetic Phase Transition in the Magnetar Core Actually Possible?

Spin polarized states in dense neutron matter with BSk20 Skyrme force are considered in magnetic fields up to $10^{20}$ G. It is shown that the appearance of the longitudinal instability in a strong magnetic field prevents the formation of a fully spin polarized state in neutron matter, and only the states with moderate spin polarization can be developed.

astro-ph.HE↗

arXiv2011

Anisotropic pressure in dense neutron matter under the presence of a strong magnetic field

Dense neutron matter with recently developed BSk19 and BSk21 Skyrme effective forces is considered in magnetic fields up to $10^{20}$ G at zero temperature. The breaking of the rotational symmetry by the magnetic field leads to the differentiation between the pressures along and perpendicular to the field direction which becomes significant in the fields $H>H_{th}\sim10^{18}$ G. The longitudinal pressure vanishes in the critical field $10^{18}<H_c\lesssim10^{19}$ G, resulting in the longitudinal instability of neutron matter. For the Skyrme force fitted to the stiffer underlying equation of state (BSk21 vs. BSk19) the threshold $H_{th}$ and critical $H_c$ magnetic fields become larger. The longitudinal and transverse pressures as well as the anisotropic equation of state of neutron matter are determined under the conditions relevant for the cores of magnetars.

nucl-th↗

arXiv2011

The radio core and jet in the broad absorption line quasar PG 1700+518

The blue-shifted broad absorption lines (BAL) or troughs are observed in Active Galactic Nuclei (AGNs) when our line of sight is intercepted by a high speed outflow (wind), likely originating in the accretion disc. The outflow or wind can shed light on the internal structure obscured by the AGN torus. Recently, it has been shown that this outflow is rotating in the BAL quasar PG 1700+518, further supporting the accretion disc origin of the wind. With the purpose of giving independent constraints on the wind geometry, we performed high-resolution European VLBI Network (EVN) observations at 1.6 GHz in 2010. Combining the VLBI (Very Long Baseline Interferometry) results with the Very Large Array (VLA) archival data at 8.4 GHz, we present its jet structure on scales of parsec (pc) to kiloparsec (kpc) for the first time. The source shows two distinct jet features in East-West direction with a separation of around 4 kpc. The Eastern feature, which has so far been assumed to hide the core, is a kpc-scale hot spot, which is completely resolved out in the EVN image. In the western jet feature, we find a compact jet component, which pinpoints the position of the central black hole in the galaxy. Jet components on both sides of the core are additionally detected in the Northwest-Southeast direction, and they show a symmetric morphology on scale of <1 kpc. This two-sided jet feature is not common in the known BAL quasars and indicates that the jet axis is far away from the line of sight. Furthermore, it is nearly parallel to the scattering plane revealed earlier by optical polarimetry. By analogy to polar-scattered Seyfert 1 galaxies, we conclude that the jet likely has a viewing angle around 45 degree. The analogy is further supported by the recent report of significant cold absorption in the soft X-rays, a nearly unique feature to polar-scattered Seyfert galaxies.

astro-ph.HE↗

arXiv2011

VLBI observations of SN2011dh: imaging of the youngest radio supernova

We report on the VLBI detection of supernova SN2011dh at 22GHz using a subset of the EVN array. The observations took place 14 days after the discovery of the supernova, thus resulting in a VLBI image of the youngest radio-loud supernova ever. We provide revised coordinates for the supernova with milli-arcsecond precision, linked to the ICRF. The recovered flux density is a factor 2 below the EVLA flux density reported by other authors at the same frequency and epoch of our observations. This discrepancy could be due to extended emission detected with the EVLA or to calibration problems in the VLBI and/or EVLA observations.

astro-ph.HE↗

arXiv2011

Transient Relativistic Ejections and Stationary Core in XTE J1752-223

The Galactic X-ray transient XTE J1752-223 was shown to have properties of black hole binary candidates. As reported in our previous paper, we identified transient and decelerating ejecta in multi-epoch Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) and the NRAO Very Long Baseline Array (VLBA). Here we present new EVN and VLBA data in which a new transient ejection event and later a stationary component are identified. The latter is interpreted as a reappearance of the radio core/compact jet during the transition from soft to hard X-ray state. This component appears to be highly variable in brightness although effects of tropospheric instabilities might play a role too. We also re-analyze the earlier VLBI data and find that the transient ejecta closer to the core position has significantly higher proper motion, further strengthening the case for strongly decelerating ejecta on the scale of several hundred milli-arcsecond, never observed in X-ray binaries before. Although the distance of the source is not well constrained, it is clear that these ejectas are at least mildly relativistic at the early stages. Moreover, we show the large scale environment of the transient from the Westerbork synthesis array data recorded in parallel during the EVN run.

astro-ph.HE↗

arXiv2011

Theory of fishnet negative-index optical metamaterials

We theoretically study fishnet metamaterials at optical frequencies. In contrast to earlier works, we provide a microscopic description by tracking the transversal and longitudinal flows of energy through the fishnet mesh composed of intersecting subwavelength plasmonic waveguides. The analysis is supported by a semi-analytical model based on surface-plasmon coupled-mode equations, which provides accurate formulas for the fishnet refractive index, including the real-negative and imaginary parts. The model simply explains how the surface plasmons couple at the waveguide intersections and it shines new light on the fishnet negative-index paradigm at optical frequencies. Extension of the theory for loss-compensated metamaterials with gain media is also presented.

physics.optics↗

arXiv2011

Doping and temperature-dependent optical properties of oxygen-reduced BaTiO3-d

We report on optical properties of reduced BaTiO3-d at different doping levels including insulating and metallic samples. In all the samples, including metallic one, we observe structural phase transitions from the changes in the infrared-active phonon modes. Metallic ground state is confirmed by the Drude-type lowfrequency optical reflectance. Similar to SrTiO3-d we find that the midinfrared-absorption band in BaTiO3-d appears and grows with an increase in the oxygen-vacancy concentration. Upon decrease in temperature from 300 K, the midinfrared band shifts slightly to higher frequency and evolves into two bands: the existing band and a new and smaller band at lower frequency. The appearance of the new and smaller band seems to be correlated with the structural phase transitions

cond-mat.str-el↗