Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71

We report on archival Hubble Space Telescope (HST) observations of the globular cluster M71 (NGC 6838). These observations, covering the core of the globular cluster, were performed by the Advanced Camera for Surveys (ACS) and the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h = 1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29 Chandra X-ray sources while outside the half-mass radius, 6 possible optical counterparts to 4 X-ray sources are found. Based on the X-ray and optical properties of the identifications, we find 1 certain and 7 candidate cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain and potential chromospherically active binaries (ABs), respectively. The only star in the error circle of the known millisecond pulsar (MSP) is inconsistent with being the optical counterpart. The number of X-ray faint sources with L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations from other clusters on the basis of either collision frequency or mass. Since the core density of M71 is relatively low, we suggest that those CVs and ABs are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and Astrophysic

Electronic structure of Ni-Cu alloys : the d-electron charge distribution

[[abstract]]This work investigates charge redistribution in a series of Ni-Cu alloys using x-ray photoemission spectroscopy (XPS) and Ni/Cu L3,2- and K-edge x-ray-absorption near-edge structure (XANES). XPS results show that the constituent d bands are well separated and shifted to a slightly higher binding energy upon dilution into the other host, indicating that the atomic sites in the alloy are not as well screened relative to the pure metal. However, no significant d-band narrowing is observed, suggesting that there is modest d-d interaction in the alloys. In contrast to the XPS observation, XANES results show a reduction in white-line intensity at both edges relative to the pure metal suggesting that both Ni and Cu sites gain d charge. The unoccupied Ni d band is far from fully occupied even at infinite dilution. The discrepancy between the implications of the XPS and XANES results is dealt with using a charge redistribution model in which s-p-d rehybridization takes place at both sites within the framework of electroneutrality and electronegativity considerations. It appears that both Ni and Cu gain a small but measurable amount of d charge in alloy formation through rehybridization (loss of non-d conduction charge). Possible connection between these results and the disappearance of ferromagnetism in Ni1-xCux alloys at x>0.6 is discussed.[[incitationindex]]SCI[[booktype]]紙本[[booktype]]電子

Soft x-ray magnetic circular dichroism study on Gd-doped EuO thin films

We report on the growth and characterization of ferromagnetic Gd-doped EuO thin films. We prepared samples with Gd concentrations up to 11% by means of molecular beam epitaxy under distillation conditions, which allows a very precise control of the doping concentration and oxygen stoichiometry. Using soft x-ray magnetic circular dichroism at the Eu and Gd M4,5 edges, we found that the Curie temperature ranged from 69 K for pure stoichiometric EuO to about 170 K for the film with the optimal Gd doping of around 4%. We also show that the Gd magnetic moment couples ferromagnetically to that of Eu.Comment: 4 pages, 4 figure

Determining the crystal-field ground state in rare earth Heavy Fermion materials using soft-x-ray absorption spectroscopy

We infer that soft-x-ray absorption spectroscopy is a versatile method for the determination of the crystal-field ground state symmetry of rare earth Heavy Fermion systems, complementing neutron scattering. Using realistic and universal parameters, we provide a theoretical mapping between the polarization dependence of Ce $M_{4,5}$ spectra and the charge distribution of the Ce $4f$ states. The experimental resolution can be orders of magnitude larger than the $4f$ crystal field splitting itself. To demonstrate the experimental feasibility of the method, we investigated CePd$_2$Si$_2$, thereby settling an existing disagreement about its crystal-field ground state

X-ray-absorption studies of boron-doped diamond films

[[abstract]]X-ray-absorption near-edge structure (XANES) measurements have been performed for a variety of boron-doped and undoped diamond films at the C K edge using the sample drain current mode. The C K-edge XANES spectra of B-doped diamonds resemble that of the undoped diamond regardless of the B concentration, which suggests that the overall bonding configuration of the C atom is unaltered. B impurities are found to enhance both the sp3- and sp2-bond derived resonance features in the XANES spectra.[[notice]]補正完畢[[incitationindex]]SCI[[booktype]]紙

Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators

Three-dimensional (3D) topological insulators (TI) are novel quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The spin-helical Dirac surface states are expected to host exotic topological quantum effects and find applications in spintronics and quantum computation. The experimental realization of these ideas requires fabrication of versatile devices based on bulk-insulating TIs with tunable surface states. The main challenge facing the current TI materials exemplified by Bi2Se3 and Bi2Te3 is the significant bulk conduction, which remains unsolved despite extensive efforts involving nanostructuring, chemical doping and electrical gating. Here we report a novel approach for engineering the band structure of TIs by molecular beam epitaxy (MBE) growth of (Bi1-xSbx)2Te3 ternary compounds. Angle-resolved photoemission spectroscopy (ARPES) and transport measurements show that the topological surface states exist over the entire composition range of (Bi1-xSbx)2Te3 (x = 0 to 1), indicating the robustness of bulk Z2 topology. Most remarkably, the systematic band engineering leads to ideal TIs with truly insulating bulk and tunable surface state across the Dirac point that behave like one quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new TI devices with well-established semiconductor technology.Comment: Minor changes in title, text and figures. Supplementary information adde

Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr_2IrO_4 from time-resolved photocarrier dynamics

We perform a time-resolved optical study of Sr_2IrO_4 to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled J_(eff) = 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the Néel temperature T_N the system evolves continuously from a metal-like phase with fast (∼50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower (∼500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied J_(eff) = 1/2 band undergoes the sharpest upturn at TN, which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr_2IrO_4 to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at T_N in transport and thermodynamic measurements

A topological insulator surface under strong Coulomb, magnetic and disorder perturbations

Three dimensional topological insulators embody a newly discovered state of matter characterized by conducting spin-momentum locked surface states that span the bulk band gap as demonstrated via spin-resolved ARPES measurements . This highly unusual surface environment provides a rich ground for the discovery of novel physical phenomena. Here we present the first controlled study of the topological insulator surfaces under strong Coulomb, magnetic and disorder perturbations. We have used interaction of iron, with a large Coulomb state and significant magnetic moment as a probe to \textit{systematically test the robustness} of the topological surface states of the model topological insulator Bi$_2$Se$_3$. We observe that strong perturbation leads to the creation of odd multiples of Dirac fermions and that magnetic interactions break time reversal symmetry in the presence of band hybridization. We also present a theoretical model to account for the altered surface of Bi$_2$Se$_3$. Taken collectively, these results are a critical guide in manipulating topological surfaces for probing fundamental physics or developing device applications.Comment: 14 pages, 4 Figures. arXiv admin note: substantial text overlap with arXiv:1009.621