arXiv subjects
Publications and source records attributed to Z. Ren.
List of proceedings from the HAWC Collaboration presented at the 34th International Cosmic Ray Conference, 30 July - 6 August 2015, The Hague, The Netherlands.
A survey of the inner Galaxy region of Galactic longitude l in [+15, +50] degree and latitude b in [-4,+4] degree is performed using one-third of the High Altitude Water Cherenkov (HAWC) Observatory operated during its construction phase. To address the ambiguities arising from unresolved sources in the data, we use a maximum likelihood technique to identify point source candidates. Ten sources and candidate sources are identified in this analysis. Eight of these are associated with known TeV sources but not all have differential fluxes compatible with previous measurements. Three sources are detected with significances $>5\,σ$ after accounting for statistical trials, and are associated with known TeV sources.
A kind of novel thermal history nanosensors were theoretically designed and experimentally demonstrated to permanently record thermal events. The photoluminescence spectrum of core-shelled quantum dots CdSe/ZnS irreversibly shifted with heating histories (temperature and duration) of thermal events. The induced photoluminescence shift of the quantum dots CdSe/ZnS was employed to permanently record thermal histories. We further modeled a kind of thermal history nanosensor based on the thermal induced phenomena of core-shelled quantum dots to permanently record thermal histories at microscale and demonstrated to reconstruct temperature and duration of heating events simultaneously from photoluminescence spectra of the quantum dots. The physical mechanism of the sensors was discussed.
CeAu2Si2 is a newly discovered pressure-induced heavy fermion superconductor which shows very unusual interplay between superconductivity and magnetism under pressure. Here we compare the results of high-pressure measurements on single crystalline CeAu2Si2 samples with different levels of disorder. It is found that while the magnetic properties are essentially sample independent, superconductivity is rapidly suppressed when the residual resistivity of the sample increases. We show that the depression of bulk Tc can be well understood in terms of pair breaking by nonmagnetic disorder, which strongly suggests an unconventional pairing state in pressurized CeAu2Si2. Furthermore, increasing the level of disorder leads to the emergence of another phase transition at T* within the magnetic phase, which might be in competition with superconductivity.
High pressure provides a powerful means for exploring unconventional superconductivity which appears mostly on the border of magnetism. Here we report the discovery of pressure-induced heavy fermion superconductivity up to 2.5 K in the antiferromanget CeAu2Si2 (TN ~ 10 K). Remarkably, the magnetic and superconducting phases are found to overlap across an unprecedentedly wide pressure interval from 11.8 to 22.3 GPa. Moreover, both the bulk Tc and TM are strongly enhanced when increasing the pressure from 16.7 to 20.2 GPa. Tc reaches a maximum at a pressure slightly below pc ~ 22.5 GPa, at which magnetic order disappears. Furthermore, the scaling behavior of the resistivity provides evidence for a continuous delocalization of the Ce 4f-electrons associated with a critical endpoint lying just above pc. We show that the maximum Tc of CeAu2Si2 actually occurs at almost the same unit-cell volume as that of CeCu2Si2 and CeCu2Ge2, and when the Kondo and crystal field splitting energies becomes comparable. Dynamical mean-filed theory calculations suggest that the peculiar behavior in pressurized CeAu2Si2 might be related to its Ce 4f-orbital occupancy. Our results not only provide a unique example of the interplay between superconductivity and magnetism, but also underline the role of orbital physics in understanding Ce-based heavy fermion systems.
The luminosity in the ZEUS detector was measured using photons from electron bremsstrahlung. In 2001 the HERA collider was upgraded for operation at higher luminosity. At the same time the luminosity-measuring system of the ZEUS experiment was modified to tackle the expected higher photon rate and synchrotron radiation. The existing lead-scintillator calorimeter was equipped with radiation hard scintillator tiles and shielded against synchrotron radiation. In addition, a magnetic spectrometer was installed to measure the luminosity independently using photons converted in the beam-pipe exit window. The redundancy provided a reliable and robust luminosity determination with a systematic uncertainty of 1.7%. The experimental setup, the techniques used for luminosity determination and the estimate of the systematic uncertainty are reported.
We have performed angle-resolved photoemission spectroscopy of the strongly spin-orbit coupled low-carrier density superconductor Sn1-xInxTe (x = 0.045) to elucidate the electronic states relevant to the possible occurrence of topological superconductivity recently reported for this compound from point-contact spectroscopy. The obtained energy-band structure reveals a small holelike Fermi surface centered at the L point of the bulk Brillouin zone, together with a signature of a topological surface state which indicates that this superconductor is essentially a doped topological crystalline insulator characterized by band inversion and mirror symmetry. A comparison of the electronic states with a band-non-inverted superconductor possessing a similar Fermi surface structure, Pb1-xTlxTe, suggests that the anomalous behavior in the superconducting state of Sn1-xInxTe is likely to be related to the peculiar orbital characteristics of the bulk valence band and/or the presence of a topological surface state.
The magnetic, transport and optical properties of EuFe$_2$As$_2$ single crystals have been investigated parallel and perpendicular to the $ab$-plane. The anisotropy $ρ_c/ρ_{ab}\approx 8$ depends only slightly on temperature. In both orientations, the spin-density wave transition at $T_{\rm SDW}=189$ K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu$^{2+}$ moments at $T_N=19$ K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only $4.2\times 10^{21} {\rm cm}^{-3}$ and a bandmass of $m_b=2m_0$. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below $T_{\rm SDW}$, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes of the electronic properties at $T_{\rm SDW}$. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below $T_{\rm SDW}$, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.
Here we report pressure effect on superconducting transition temperature (Tc) of ReFeAsO0.85 (Re= Sm and Nd) system without fluorine doping. In-situ measurements under high pressure showed that Tc of the two compounds decrease monotonously over the pressure range investigated. The pressure coefficients dTc/dP in SmFeAsO0.85 and Nd FeAsO0.85 were different, revealing the important influence of the deformation in layers on Tc. Theoretical calculations suggested that the electron density of states decrease with increasing pressure, following the same trend of experimental data.
We report on the observation of a K$_s$p resonance signal at a mass of 1765$\pm$5 MeV/c$^2$, with intrinsic width $Γ= 108\pm 22$ MeV/c$^2$, produced inclusively in $Σ^-$-nucleus interactions at 340 GeV/c in the hyperon beam experiment WA89 at CERN. The signal was observed in the kinematic region $x_F>0.7$, in this region its production cross section rises approximately linearly with $(1-x_F)$, reaching $BR(X\to K_S p)\cdot dσ/dx_F = (5.2\pm 2.3) μb $ per nucleon at $x_F=0.8$. The hard \xf spectrum suggests the presence of a strong leading particle effect in the production and hence the identification as a $Σ^{*+}$ state. No corresponding peaks were observed in the $K^- p$ and $Λπ^{\pm}$ mass spectra.
The upgrade of the HERA accelerator has provided much increased collider luminosity. In turn, the improvements have necessitated a new design for the ZEUS luminosity measurements. The intense synchrotron radiation field, as well as the high probability of a bremsstrahlung photon in each bunch crossing, posed new experimental constraints. In this report, we describe how these challenges were met with the ZEUS luminosity spectrometer system. The design, testing and commissioning of the device are described, and the results from the initial operational experience are reported.