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Glenn Wagner

Bio: Glenn Wagner is an academic researcher from University of Oxford. The author has contributed to research in topics: Bilayer graphene & Quantum Hall effect. The author has an hindex of 6, co-authored 16 publications receiving 238 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the properties of the XXL cluster catalogue are described in detail, as well as associated catalogues of more specific objects such as super-clusters and fossil groups, and the complete subset of clusters for which the selection function is well determined plus all X-ray clusters which are, to date, confirmed.
Abstract: Context. In the currently debated context of using clusters of galaxies as cosmological probes, the need for well-defined cluster samples is critical.Aims. The XXL Survey has been specifically designed to provide a well characterised sample of some 500 X-ray detected clusters suitable for cosmological studies. The main goal of present article is to make public and describe the properties of the cluster catalogue in its present state, as well as of associated catalogues of more specific objects such as super-clusters and fossil groups.Methods. Following from the publication of the hundred brightest XXL clusters, we now release a sample containing 365 clusters in total, down to a flux of a few 10−15 erg s−1 cm−2 in the [0.5–2] keV band and in a 1′ aperture. This release contains the complete subset of clusters for which the selection function is well determined plus all X-ray clusters which are, to date, spectroscopically confirmed. In this paper, we give the details of the follow-up observations and explain the procedure adopted to validate the cluster spectroscopic redshifts. Considering the whole XXL cluster sample, we have provided two types of selection, both complete in a particular sense: one based on flux-morphology criteria, and an alternative based on the [0.5–2] keV flux within 1 arcmin of the cluster centre. We have also provided X-ray temperature measurements for 80% of the clusters having a flux larger than 9 × 10−15 erg s−1 cm−2 .Results. Our cluster sample extends from z ~ 0 to z ~ 1.2, with one cluster at z ~ 2. Clusters were identified through a mean number of six spectroscopically confirmed cluster members. The largest number of confirmed spectroscopic members in a cluster is 41. Our updated luminosity function and luminosity–temperature relation are compatible with our previous determinations based on the 100 brightest clusters, but show smaller uncertainties. We also present an enlarged list of super-clusters and a sample of 18 possible fossil groups.Conclusions. This intermediate publication is the last before the final release of the complete XXL cluster catalogue when the ongoing C2 cluster spectroscopic follow-up is complete. It provides a unique inventory of medium-mass clusters over a 50 deg2 area out to z ~ 1.

102 citations

Journal ArticleDOI
TL;DR: In this article, the properties of the XXL cluster catalogue in its present state, as well as associated catalogues as super-clusters and fossil groups, are described in detail.
Abstract: In the currently debated context of using clusters of galaxies as cosmological probes, the need for well-defined cluster samples is critical. The XXL Survey has been specifically designed to provide a well characterised sample of some 500 X-ray detected clusters suitable for cosmological studies. The main goal of present article is to make public and describe the properties of the cluster catalogue in its present state, as well as of associated catalogues as super-clusters and fossil groups. We release a sample containing 365 clusters in total. We give the details of the follow-up observations and explain the procedure adopted to validate the cluster spectroscopic redshifts. Considering the whole XXL cluster sample, we have provided two types of selection, both complete in a particular sense: one based on flux-morphology criteria, and an alternative based on the [0.5-2] keV flux within one arcmin of the cluster centre. We have also provided X-ray temperature measurements for 80$\%$ of the clusters having a flux larger than 9$\times$10$^{-15}$$\rm \thinspace erg \, s^{-1} \, cm^{-2}$. Our cluster sample extends from z$\sim$0 to z$\sim$1.2, with one cluster at z$\sim$2. Clusters were identified through a mean number of six spectroscopically confirmed cluster members. Our updated luminosity function and luminosity-temperature relation are compatible with our previous determinations based on the 100 brightest clusters, but show smaller uncertainties. We also present an enlarged list of super-clusters and a sample of 18 possible fossil groups. This intermediate publication is the last before the final release of the complete XXL cluster catalogue when the ongoing C2 cluster spectroscopic follow-up is complete. It provides a unique inventory of medium-mass clusters over a 50~\dd\ area out to z$\sim$1.

94 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present the version of the point source catalogue of the XXL Survey that was used, in part, in the first series of XXL papers, and present the basic properties of the X-ray point sources and their counterparts.
Abstract: We present the version of the point source catalogue of the XXL Survey that was used, in part, in the first series of XXL papers. In this paper we release, in our database in Milan and at CDS: (i) the X-ray source catalogue with 26 056 objects in two areas of 25 deg2 with a flux limit (at 3σ) of ~10−15 erg s−1 cm−2 in [0.5–2] keV, and ~ 3 × 10−15 erg s−1 cm−2 in [2–10] keV, yielding a 90% completeness limit of 5.8 × 10−15 and 3.8 × 10−14 respectively;(ii) the associated multiwavelength catalogues with candidate counterparts of the X-ray sources in the infrared, near-infrared, optical, and ultraviolet (plus spectroscopic redshift when available); and (iii) a catalogue of spectroscopic redshifts recently obtained in the southern XXL area. We also present the basic properties of the X-ray point sources and their counterparts. Other catalogues described in the second series of XXL papers will be released contextually, and will constitute the second XXLdata release.

37 citations

Journal ArticleDOI
TL;DR: In this article, the authors consider magic-angle twisted bilayer graphene (TBG) at filling ρ = + 3, where experiments have observed a robust quantized anomalous Hall effect, attributed to the formation of a valley and spin-polarized ground state that spontaneously breaks time-reversal symmetry.
Abstract: We consider magic-angle twisted bilayer graphene (TBG) at filling $ u=+3$, where experiments have observed a robust quantized anomalous Hall effect. This has been attributed to the formation of a valley- and spin-polarized Chern insulating ground state that spontaneously breaks time-reversal symmetry, and is stabilized by a hexagonal boron nitride (hBN) substrate. We identify three different types of domain wall, and study their properties and energetic selection mechanisms via theoretical arguments and Hartree-Fock calculations adapted to deal with inhomogeneous moir\'e systems. We comment on the implications of these results for transport and scanning probe experiments.

18 citations

Journal ArticleDOI
TL;DR: This work numerically calculates the dc transport properties of bilayer graphene near charge neutrality using the semiclassical quantum Boltzmann equation and shows that the two-fluid model produces quantitatively accurate results for conductivity, thermopower, and thermal conductivity.
Abstract: Using the semiclassical quantum Boltzmann equation (QBE), we numerically calculate the dc transport properties of bilayer graphene near charge neutrality. We find, in contrast to prior discussions, that phonon scattering is crucial even at temperatures below 40 K. Nonetheless, electron-electron scattering still dominates over phonon collisions allowing a hydrodynamic approach. We introduce a simple two-fluid hydrodynamic model of electrons and holes interacting via Coulomb drag and compare our results to the full QBE calculation. We show that the two-fluid model produces quantitatively accurate results for conductivity, thermopower, and thermal conductivity.

15 citations


Cited by
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15 Mar 1979
TL;DR: In this article, the experimental estimation of parameters for models can be solved through use of the likelihood ratio test, with particular attention to photon counting experiments, and procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply.
Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

1,748 citations

01 Jan 2018
TL;DR: In this paper, the authors studied electron transport through graphene constrictions and showed that their conductance below 150 K increases with increasing temperature, in stark contrast to the metallic character of doped graphene.
Abstract: Graphene systems are clean platforms for studying electron–electron (e–e) collisions. Electron transport in graphene constrictions is now found to behave anomalously due to e–e interactions: conductance values exceed the maximum free-electron value. Electron–electron (e–e) collisions can impact transport in a variety of surprising and sometimes counterintuitive ways1,2,3,4,5,6. Despite strong interest, experiments on the subject proved challenging because of the simultaneous presence of different scattering mechanisms that suppress or obscure consequences of e–e scattering7,8,9,10,11. Only recently, sufficiently clean electron systems with transport dominated by e–e collisions have become available, showing behaviour characteristic of highly viscous fluids12,13,14. Here we study electron transport through graphene constrictions and show that their conductance below 150 K increases with increasing temperature, in stark contrast to the metallic character of doped graphene15. Notably, the measured conductance exceeds the maximum conductance possible for free electrons16,17. This anomalous behaviour is attributed to collective movement of interacting electrons, which ‘shields’ individual carriers from momentum loss at sample boundaries18,19. The measurements allow us to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, our work shows that viscous effects can facilitate high-mobility transport at elevated temperatures, a potentially useful behaviour for designing graphene-based devices.

207 citations

Journal Article
TL;DR: The quantum anomalous Hall (QAH) effect combines topology and magnetism to produce precisely quantized Hall resistance at zero magnetic field as discussed by the authors, which is driven by intrinsic strong interactions, which polarize the electrons into a single spin and valley-resolved moiré miniband with Chern number C = 1.
Abstract: Quantum anomalous Hall goes intrinsic Quantum anomalous Hall effect—the appearance of quantized Hall conductance at zero magnetic field—has been observed in thin films of the topological insulator Bi2Se3 doped with magnetic atoms. The doping, however, introduces inhomogeneity, reducing the temperature at which the effect occurs. Two groups have now observed quantum anomalous Hall effect in intrinsically magnetic materials (see the Perspective by Wakefield and Checkelsky). Serlin et al. did so in twisted bilayer graphene aligned to hexagonal boron nitride, where the effect enabled the switching of magnetization with tiny currents. In a complementary work, Deng et al. observed quantum anomalous Hall effect in the antiferromagnetic layered topological insulator MnBi2Te4. Science, this issue p. 900, p. 895; see also p. 848 Transport measurements indicate quantized Hall conductance without a magnetic field. The quantum anomalous Hall (QAH) effect combines topology and magnetism to produce precisely quantized Hall resistance at zero magnetic field. We report the observation of a QAH effect in twisted bilayer graphene aligned to hexagonal boron nitride. The effect is driven by intrinsic strong interactions, which polarize the electrons into a single spin- and valley-resolved moiré miniband with Chern number C = 1. In contrast to magnetically doped systems, the measured transport energy gap is larger than the Curie temperature for magnetic ordering, and quantization to within 0.1% of the von Klitzing constant persists to temperatures of several kelvin at zero magnetic field. Electrical currents as small as 1 nanoampere controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory.

206 citations

Journal ArticleDOI
TL;DR: In this paper , the eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory combines a large field of view and collecting area in the energy range of 0.2-2.3 keV with the capability to perform uniform scanning observations of large sky areas.
Abstract: Context. The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory combines a large field of view and collecting area in the energy range $\sim$0.2 to $\sim$8.0 keV with the capability to perform uniform scanning observations of large sky areas. Aims. SRG/eROSITA performed scanning observations of the $\sim$140 square degrees eROSITA Final Equatorial Depth Survey (eFEDS) field as part of its performance verification phase. The observing time was chosen to slightly exceed the depth of equatorial fields after the completion of the eROSITA all-sky survey. We present a catalog of detected X-ray sources in the eFEDS field providing source positions and extent information, as well as fluxes in multiple energy bands and document a suite of tools and procedures developed for eROSITA data processing and analysis, validated and optimized by the eFEDS work. Methods. A multi-stage source detection procedure was optimized and calibrated by performing realistic simulations of the eROSITA eFEDS observations. We cross-matched the eROSITA eFEDS source catalog with previous XMM-ATLAS observations, confirming excellent agreement of the eROSITA and XMM-ATLAS source fluxes. Result. We present a primary catalog of 27910 X-ray sources, including 542 with significant spatial extent, detected in the 0.2-2.3 keV energy range with detection likelihoods $\ge 6$, corresponding to a (point source) flux limit of $\approx 6.5 \times 10^{-15}$ erg/cm$^2$/s in the 0.5-2.0 keV energy band (80% completeness). A supplementary catalog contains 4774 low-significance source candidates with detection likelihoods between 5 and 6. In addition, a hard band sample of 246 sources detected in the energy range 2.3-5.0 keV above a detection likelihood of 10 is provided. The dedicated data analysis software package, calibration database, and calibrated data products are described in an appendix.

109 citations

Journal ArticleDOI
TL;DR: In this paper, the properties of the XXL cluster catalogue are described in detail, as well as associated catalogues of more specific objects such as super-clusters and fossil groups, and the complete subset of clusters for which the selection function is well determined plus all X-ray clusters which are, to date, confirmed.
Abstract: Context. In the currently debated context of using clusters of galaxies as cosmological probes, the need for well-defined cluster samples is critical.Aims. The XXL Survey has been specifically designed to provide a well characterised sample of some 500 X-ray detected clusters suitable for cosmological studies. The main goal of present article is to make public and describe the properties of the cluster catalogue in its present state, as well as of associated catalogues of more specific objects such as super-clusters and fossil groups.Methods. Following from the publication of the hundred brightest XXL clusters, we now release a sample containing 365 clusters in total, down to a flux of a few 10−15 erg s−1 cm−2 in the [0.5–2] keV band and in a 1′ aperture. This release contains the complete subset of clusters for which the selection function is well determined plus all X-ray clusters which are, to date, spectroscopically confirmed. In this paper, we give the details of the follow-up observations and explain the procedure adopted to validate the cluster spectroscopic redshifts. Considering the whole XXL cluster sample, we have provided two types of selection, both complete in a particular sense: one based on flux-morphology criteria, and an alternative based on the [0.5–2] keV flux within 1 arcmin of the cluster centre. We have also provided X-ray temperature measurements for 80% of the clusters having a flux larger than 9 × 10−15 erg s−1 cm−2 .Results. Our cluster sample extends from z ~ 0 to z ~ 1.2, with one cluster at z ~ 2. Clusters were identified through a mean number of six spectroscopically confirmed cluster members. The largest number of confirmed spectroscopic members in a cluster is 41. Our updated luminosity function and luminosity–temperature relation are compatible with our previous determinations based on the 100 brightest clusters, but show smaller uncertainties. We also present an enlarged list of super-clusters and a sample of 18 possible fossil groups.Conclusions. This intermediate publication is the last before the final release of the complete XXL cluster catalogue when the ongoing C2 cluster spectroscopic follow-up is complete. It provides a unique inventory of medium-mass clusters over a 50 deg2 area out to z ~ 1.

102 citations