arXiv subjects

Q. Zhang

Publications and source records attributed to Q. Zhang.

At least 145 records · Page 8Linked to original sources

arXiv2015

Measurement of the strong interaction induced shift and width of the 1s state of kaonic deuterium at J-PARC

The antikaon-nucleon interaction close to threshold provides crucial information on the interplay between spontaneous and explicit chiral symmetry breaking in low-energy QCD. In this context the importance of kaonic deuterium X-ray spectroscopy has been well recognized, but no experimental results have yet been obtained due to the difficulty of the measurement. We propose to measure the shift and width of the kaonic deuterium 1s state with an accuracy of 60 eV and 140 eV respectively at J-PARC. These results together with the kaonic hydrogen data (KpX at KEK, DEAR and SIDDHARTA at DAFNE) will then permit the determination of values of both the isospin I=0 and I=1 antikaon-nucleon scattering lengths and will provide the most stringent constraints on the antikaon-nucleon interaction, promising a breakthrough. Refined Monte Carlo studies were performed, including the investigation of background suppression factors for the described setup. These studies have demonstrated the feasibility of determining the shift and width of the kaonic deuterium atom 1s state with the desired accuracy of 60 eV and 140 eV.

nucl-ex↗

arXiv2014

G11.92-0.61-MM2: A Bonafide Massive Prestellar Core?

Core accretion models of massive star formation require the existence of stable massive starless cores, but robust observational examples of such objects have proven elusive. We report subarcsecond-resolution SMA 1.3 mm, 1.1 mm, and 0.88 mm and VLA 1.3 cm observations of an excellent massive starless core candidate, G11.92-0.61-MM2, initially identified in the course of studies of GLIMPSE Extended Green Objects (EGOs). Separated by ~7.2" from the nearby MM1 protostellar hot core, MM2 is a strong, compact dust continuum source (submillimeter spectral index alpha=2.6+/-0.1), but is devoid of star formation indicators. In contrast to MM1, MM2 has no masers, no centimeter continuum, and no (sub)millimeter wavelength line emission in ~24 GHz of bandwidth observed with the SMA, including N2H+(3-2), HCO+(3-2), and HCN(3-2). Additionally, there is no evidence for an outflow driven by MM2. The (sub)millimeter spectral energy distribution (SED) of MM2 is best fit with a dust temperature of ~17-19 K and luminosity of ~5-7 L_sun. The combined physical properties of MM2, as inferred from its dust continuum emission, are extreme: M>30 M_sun within a radius<1000 AU, N(H2)>10^25 cm^-2 and n(H2)>10^9 cm^-3. Comparison of the molecular abundance limits derived from our SMA observations with gas-grain chemical models indicates that extremely dense (n(H)>>10^8 cm^-3), cold (<20 K) conditions are required to explain the lack of observed (sub)millimeter line emission, consistent with the dust continuum results. Our data suggest that G11.92-0.61-MM2 is the best candidate for a bonafide massive prestellar core found to date, and a promising target for future, higher-sensitivity observations.

astro-ph.GA↗

arXiv2014

High pressure dynamics of hydrated protein in bio-protective trehalose environment

We present a pressure dependence study of the dynamics of lysozyme protein powder immersed in deuterated $α$,$α$-trehalose environment via quasi-elastic neutron scattering (QENS). The goal is to assess the baro-protective benefits of trehalose on bio-molecules by comparing the findings with those of a trehalose-free reference study. While the mean-square displacement of the trehalose-free protein (hydrated to $d_{D_2O}\simeq$40 w\%) as a whole, is reduced by increasing pressure, the actual observable relaxation dynamics in the pico-(ps) to nano-seconds (ns) time range remains largely unaffected by pressure - up to the maximum investigated pressure of 2.78(2) Kbar. Our observation is independent of whether or not the protein is mixed with the deuterated sugar. This suggests that the hydrated protein's conformational states at atmospheric pressure remain unaltered by hydrostatic pressures, below 2.78 Kbar. We also found the QENS response to be totally recoverable after ambient pressure conditions are restored. Circular dichroism and neutron diffraction measurements confirm that the protein structural integrity is conserved and remains intact, after pressure is released. We observe however a clear narrowing of the quasi-elastic neutron (QENS) response as the temperature is decreased from 290 K to 230 K in both cases, which we parametrize using the Kohlrausch-Williams-Watts (KWW) stretched exponential model. Only the fraction of protons that are immobile on the accessible time window of the instrument, referred to as the elastic incoherent structure factor or (EISF) is observably sensitive to pressure, increasing only marginally but systematically with increasing pressure.

cond-mat.soft↗

arXiv2014

A new variable for SRS plan quality evaluation based on normal tissue sparing: The Effect of Prescription Isodose Levels

Objectives: A new dosimetric variable, dose dropping speed (DDS), was proposed and used to evaluate normal tissue sparing among stereotactic radiosurgery (SRS) plans with different prescription isodose lines. Methods: Forty plans were generated for 8 intracranial SRS cases, prescribing to isodose levels (IDLs) ranging from 50% to 90% in 10% increments. Whilst maintaining similar coverage and conformity, plans at different IDLs were evaluated in terms of normal tissue sparing using the proposed DDS. The DDS was defined as the greater decay coefficient in a double exponential decay fit of the dose drop-off outside the PTV, which models the steep portion of the drop-off. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. Results: Among all plans, the DDS was found the lowest for the prescription at 90% IDL and the highest for the prescription at 60% or 70%. Beam profile slope change in penumbra and its field size dependence were explored and given as the physical basis of the findings. Conclusions: A variable was proposed for SRS plan quality evaluation. Using this measure, prescriptions at 60% and 70% IDLs were found to provide best normal tissue sparing. Advances in knowledge: A new variable was proposed based on which normal tissue sparing was quantitatively evaluated, comparing different prescription IDLs in SRS.

physics.med-ph↗

arXiv2014

Impact of the various spin and orbital ordering processes on multiferroic properties of orthovanadate DyVO3

The orthovanadate DyVO3 crystal, known to exhibit multiple structural, spin and orbital ordering transitions, is presently investigated on the basis of magnetization, heat capacity, resistivity, dielectric and polarization measurements. Our main result is experimental evidence for the existence of multiferroicity below a high TC of 108 K over a wide temperature range including different spin-orbital ordered states. The onset of ferroelectricity is found to coincide with the antiferromagnetic C-type spin ordering transition taking place at 108 K, which indicates that DyVO3 belongs to type II multiferroics exhibiting a coupling between magnetism and ferroelectricity. Some anomalies detected on the temperature dependence of electric polarization are discussed with respect to the nature of the spin-orbital ordered states of the V sublattice and the degree of spin alignment in the Dy sublattice. The orthovanadates RVO3 (R = rare earth or Y) form an important new category for searching for high-TC multiferroics.

cond-mat.str-el↗

arXiv2014

Rapid Scanning Terahertz Time-Domain Magnetospectroscopy with a Table-Top Repetitive Pulsed Magnet

We have performed terahertz time-domain magnetospectroscopy by combining a rapid scanning terahertz time-domain spectrometer based on the electronically coupled optical sampling method with a table-top mini-coil pulsed magnet capable of producing magnetic fields up to 30 T. We demonstrate the capability of this system by measuring coherent cyclotron resonance oscillations in a high-mobility two-dimensional electron gas in GaAs and interference-induced terahertz transmittance modifications in a magnetoplasma in lightly doped n-InSb.

cond-mat.mtrl-sci↗

arXiv2014

A sub-arcsecond study of the hot molecular core in G023.01-00.41

(Abridged) METHODS: We performed SMA observations at 1.3 mm with both the most extended and compact array configurations, providing sub-arcsecond and high sensitivity maps of various molecular lines, including both hot-core and outflow tracers. We also reconstruct the spectral energy distribution of the region from millimeter to near infrared wavelengths, using the Herschel/Hi-GAL maps, as well as archival data. RESULTS: From the spectral energy distribution, we derive a bolometric luminosity of about 4x10^4 Lsun. Our interferometric observations reveal that the distribution of dense gas and dust in the HMC is significantly flattened and extends up to a radius of 8000 AU from the center of radio continuum and maser emission in the region. The equatorial plane of this HMC is strictly perpendicular to the elongation of the collimated bipolar outflow, as imaged on scales of about 0.1-0.5 pc in the main CO isotopomers as well as in the SiO(5-4) line. In the innermost HMC regions (ca. 1000 AU), the velocity field traced by the CH3CN(12_K-11_K) line emission shows that molecular gas is both expanding along the outflow direction following a Hubble-law, and rotating about the outflow axis, in agreement with the (3-D) velocity field traced by methanol masers. The velocity field associated with rotation indicates a dynamical mass of 19 Msun at the center of the core. The latter is likely to be concentrated in a single O9.5 ZAMS star, consistent with the estimated bolometric luminosity of G023.01-00.41. The physical properties of the CO(2-1) outflow emission, such as its momentum rate 6x10^-3 Msun km/s /yr and its outflow rate 2x10^-4 Msun/yr, support our estimates of the luminosity (and mass) of the embedded young stellar object.

astro-ph.GA↗

arXiv2014

Fragmentation of massive dense cores down to ~1000 AU: Relation between fragmentation and density structure

In order to shed light on the main physical processes controlling fragmentation of massive dense cores, we present a uniform study of the density structure of 19 massive dense cores, selected to be at similar evolutionary stages, for which their relative fragmentation level was assessed in a previous work. We inferred the density structure of the 19 cores through a simultaneous fit of the radial intensity profiles at 450 and 850 micron (or 1.2 mm in two cases) and the Spectral Energy Distribution, assuming spherical symmetry and that the density and temperature of the cores decrease with radius following power-laws. We find a weak (inverse) trend of fragmentation level and density power-law index, with steeper density profiles tending to show lower fragmentation, and vice versa. In addition, we find a trend of fragmentation increasing with density within a given radius, which arises from a combination of flat density profile and high central density and is consistent with Jeans fragmentation. We considered the effects of rotational-to-gravitational energy ratio, non-thermal velocity dispersion, and turbulence mode on the density structure of the cores, and found that compressive turbulence seems to yield higher central densities. Finally, a possible explanation for the origin of cores with concentrated density profiles, which are the cores showing no fragmentation, could be related with a strong magnetic field, consistent with the outcome of radiation magnetohydrodynamic simulations.

astro-ph.GA↗

arXiv2014

Hot Core, Outflows and Magnetic Fields in W43-MM1 (G30.79 FIR 10)

We present submillimeter spectral line and dust continuum polarization observations of a remarkable hot core and multiple outflows in the high-mass star-forming region W43-MM1 (G30.79 FIR 10), obtained using the Submillimeter Array (SMA). A temperature of $\sim$ 400 K is estimated for the hot-core using CH$_3$CN (J=19-18) lines, with detections of 11 K-ladder components. The high temperature and the mass estimates for the outflows indicate high-mass star-formation. The continuum polarization pattern shows an ordered distribution, and its orientation over the main outflow appears aligned to the outflow. The derived magnetic field indicates slightly super-critical conditions. While the magnetic and outflow energies are comparable, the B-field orientation appears to have changed from parsec scales to $\sim$ 0.1 pc scales during the core/star-formation process.

astro-ph.GA↗

arXiv2014

Tuning the critical temperature of cuprate superconductor films using self-assembled organic layers

Many of the electronic properties of high-temperature cuprate superconductors (HTSC) are strongly dependent on the number of charge carriers put into the CuO$_2$ planes (doping). Superconductivity appears over a dome-shaped region of the doping-temperature phase diagram. The highest critical temperature (Tc) is obtained for the so-called "optimum doping". The doping mechanism is usually chemical; it can be done by cationic substitution. This is the case, for example, in La$_{2-x}$Sr$_x$CuO$_4$ where La3+ is replaced by Sr2+ thus adding a hole to the CuO$_2$ planes. A similar effect is achieved by adding oxygen as in the case of YBa$_2$Cu$_3$O$_{6+δ}$ where $δ$ represents the excess oxygen in the sample. In this paper we report on a different mechanism, one that enables the addition or removal of carriers from the surface of the HTSC. This method utilizes a self-assembled monolayer (SAM) of polar molecules adsorbed on the cuprate surface. In the case of optically active molecules, the polarity of the SAM can be modulated by shining light on the coated surface. This results in a light-induced modulation of the superconducting phase transition of the sample. The ability to control the superconducting transition temperature with the use of SAMs makes these surfaces practical for various devices such as switches and detectors based on high-Tc superconductors.

cond-mat.supr-con↗

arXiv2014

Hybrid-cascade Coupled-Line Phasers for High-resolution Radio-Analog Signal Processing

A hybrid-cascade (HC) coupled-line phaser configuration is presented to synthesize enhanced group delay responses for high-resolution Radio-Analog Signal Processing (R-ASP). Using exact analytical transfer functions, the superiority of HC coupled-line phasers over conventional transversally cascaded C-section phasers is demonstrated and verified using full-wave simulations.

physics.ins-det↗

arXiv2014

MUSCLE W49 : A Multi-Scale Continuum and Line Exploration of the Most Luminous Star Formation Region in the Milky Way. I. Data and The Mass Structure of the Giant Molecular Cloud

The Multi-Scale Continuum and Line Exploration of W49 (MUSCLE W49) is a comprehensive gas and dust survey of the giant molecular cloud (GMC) of W49A, the most luminous star-formation region in the Milky Way. It covers the entire GMC at different scales and angular resolutions. In this paper we present: 1) an all-configuration SMA mosaic in the 230-GHz band covering the central 3 arcmin (10 pc, known as W49N), with most of the embedded massive stars; and 2) PMO 14m telescope observations in the 90-GHz band, covering the entire GMC with maps up to 35 arcmin in size, or 113 pc. We also make use of archival data from the VLA, JCMT-SCUBA, IRAM 30m, and the CSO BOLOCAM GPS. Our main findings are: 1) The W49 GMC is one of the most massive in the Galaxy, with a total mass ~1.1x10^6 Msun within a radius of 60 pc. Within a radius of 6 pc, the total gas mass is ~2x10^5 Msun. At these scales only 1% of the material is photoionized. The mass reservoir is sufficient to form several young massive clusters (YMCs) as massive as a globular cluster. 2) The mass of the GMC is distributed in a hierarchical network of filaments. At scales <10 pc, a triple, centrally condensed structure peaks toward the ring of HC HII regions in W49N. This structure extends to scales from ~10 to 100 pc. The W49A starburst most likely formed from global gravitational contraction with localized collapse in a "hub-filament" geometry. 3) Currently, feedback from the central YMCs (with a present mass Mcl > 5x10^4 Msun) is still not enough to entirely disrupt the GMC, but further stellar mass growth could be enough to allow radiation pressure to clear the cloud and halt star formation. 4) The resulting stellar content will probably remain as a gravitationally bound massive star cluster, or a small system of bound clusters. (ABRIDGED)

astro-ph.GA↗

arXiv2013

Plasmonic Nature of the Terahertz Conductivity Peak in Single-Wall Carbon Nanotubes

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and DC transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

cond-mat.mes-hall↗

arXiv2013

Dynamics of lysozyme and its hydration water under electric field

The effects of static electric field on the dynamics of lysozyme and its hydration water have been investigated by means of incoherent quasi-elastic neutron scattering (QENS). Measurements were performed on lysozyme samples, hydrated respectively with heavy water (D2O) to capture the protein dynamics, and with light water (H2O), to probe the dynamics of the hydration shell, in the temperature range from 210 $<$ T $<$ 260 K. The hydration fraction in both cases was about $\sim$ 0.38 gram of water per gram of dry protein. The field strengths investigated were respectively 0 kV/mm and 2 kV/mm (2 10$^6$ V/m) for the protein hydrated with D2O and 0 kV and 1 kV/mm for the H2O hydrated counterpart. While the overall internal protons dynamics of the protein appears to be unaffected by the application of electric field up to 2 kV/mm, likely due to the stronger intra-molecular interactions, there is also no appreciable quantitative enhancement of the diffusive dynamics of the hydration water, as would be anticipated based on our recent observations in water confined in silica pores under field values of 2.5 kV/mm. This may be due to the difference in surface interactions between water and the two adsorption hosts (silica and protein), or to the existence of a critical threshold field value Ec $\sim$ 2-3 kV/mm for increased molecular diffusion, for which electrical breakdown is a limitation for our sample.

cond-mat.soft↗

arXiv2013

Submillimeter polarisation and magnetic field properties in the envelopes of proto-planetary nebulae CRL 618 and OH 231.8+4.2

We have carried out continuum and line polarisation observations of two Proto-planetary nebulae (PPNe), CRL 618 and OH 231.8+4.2, using the Submillimeter Array (SMA) in its compact configuration. The frequency range of observations, 330-345 GHz, includes the CO(J=3-2) line emission. CRL 618 and OH 231.8+4.2 show quadrupolar and bipolar optical lobes, respectively, surrounded by a dusty envelope reminiscent of their AGB phase. We report a detection of dust continuum polarised emission in both PPNe above 4 sigma but no molecular line polarisation detection above a 3 sigma limit. OH 231.8+4.2 is slightly more polarised on average than CRL 618 with a mean fractional polarisation of 4.3 and 0.3 per cent, respectively. This agrees with the previous finding that silicate dust shows higher polarisation than carbonaceous dust. In both objects, an anti-correlation between the fractional polarisation and the intensity is observed. Neither PPNe show a well defined toroidal equatorial field, rather the field is generally well aligned and organised along the polar direction. This is clearly seen in CRL 618 while in the case of OH 231.8+4.2, the geometry indicates an X-shaped structure coinciding overall with a dipole/polar configuration. However in the later case, the presence of a fragmented and weak toroidal field should not be discarded. Finally, in both PPNe, we observed that the well organised magnetic field is parallel with the major axis of the 12CO outflow. This alignment could indicate the presence of a magnetic outflow launching mechanism. Based on our new high resolution data we propose two scenarios to explain the evolution of the magnetic field in evolved stars.

astro-ph.SR↗

arXiv2013

Bulk evidence for a time reversal symmetry broken superconducting state in URu2Si2

URu$_2$Si$_2$ is claimed to be a chiral \emph{d}-wave superconductor with a $k_z (k_x \pm ik_y)$ time-reversal symmetry broken orbital component for the Cooper pair wave-function, which contains both nodal points and lines of nodes \cite{kasahara, kasahara2}. To study the magnetic response of such an unconventional state through a bulk, thermodynamic probe, we measured the magnetic torque $τ$ in very high-quality, well-characterized URu$_2$Si$_2$ single-crystals \cite{altarawneh,altarawneh2} at high magnetic-fields $H$ and at very low temperatures $T$. The magnetization $M(H) \propto τ(H) /H$ of URu$_2$Si$_2$, in its superconducting state and for angles within $15^{\circ}$ from the \emph{ab}-plane, reveals a change in its sign for $H$ approaching $H_{c2}$: from a clear diamagnetic response dominated by the pinning of vortices to a state with a smaller but "paramagnetic-like" hysteretic response which \emph{disappears} at $H_{c2}$, thus implying that it is intrinsically related to the superconducting state. We argue that this anomalous, angular-dependent behavior is evidence for a time-reversal symmetry broken superconducting state in URu$_2$Si$_2$, although not necessarily for the $k_z (k_x \pm ik_y)$ state.

cond-mat.supr-con↗

arXiv2013

Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4 spinel

Nuclear magnetic resonance (NMR), neutron diffaction (ND), x-ray diffraction, magnetic susceptibility and specific heat measurements on the frustrated A-site spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below Tn = 9.8(2) K. A high quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x = 0.057(20) in (Co1-xAlx)[Al2-xCox]O4, shows a broad maximum around 15 K in magnetic susceptibility and a sharp peak at Tn in heat capacity. The average ordered magnetic moment of Co^2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1) Bohr magneton from NMR and 1.9(5) Bohr magneton from ND which are smaller than the expected value of 3 Bohr magneton for S = 3/2 and g = 2. Antiferromagnetic spin uctuations and correlations in the paramagnetic state are revealed from the magnetic susceptibility, NMR and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice Tn.

cond-mat.str-el↗

arXiv2013

Anomalous metallic state and anisotropic multiband superconductivity in Nb3Pd0.7Se7

We report the discovery of superconductivity in Nb$_3$Pd$_{x}$Se$_7$ with a $x$-dependent superconducting transition-temperature as high as $T_c \simeq 2.1 $ K for $x \simeq0.7$ (middle point of the resistive transition). Needle-like single crystals display anisotropic upper-critical fields with an anisotropy $γ= H^{b}_{c2}/H^{a}_{c2}$ as large as 6 between fields applied along their needle axis (or $b-$axis) or along the $a-$axis. As for the Fe based superconductors $γ$ is temperature-dependent suggesting that Nb$_3$Pd$_{0.7}$Se$_7$ is a multi-band superconductor. This is supported by band structure calculations which reveal a Fermi surface composed of quasi-one-dimensional and quasi-two-dimensional sheets of hole character, as well as three-dimensional sheets of both hole- and electron-character. Remarkably, $H^{b}_{c2}$ is observed to saturate at $H^{b}_{c2}(T \rightarrow 0 \text{K}) \simeq 14.1$ T which is $4.26 \times H_p$ where $H_p$ is the Pauli-limiting field in the weak-coupling regime. The synthesis procedure yields additional crystals belonging to the Nb$_2$Pd$_{x}$Se$_5$ phase which also becomes superconducting when the fraction of Pd is varied. For both phases we find that superconductivity condenses out of an anomalous metallic state, i.e. displaying $\partial ρ/ \partial T < 0$ above $T_c$ similarly to what is observed in the pseudogap-phase of the underdoped cuprates. An anomalous metallic state, low-dimensionality, multi-band character, extremely high and anisotropic $H_{c2}$s, are all ingredients for unconventional superconductivity.

cond-mat.supr-con↗