159 Search Results

The interplay of Kondo screening and magnetic ordering in strongly correlated materials containing local moments is a subtle problem. Usually the number of conduction electrons per unit cell matches or exceeds the number of moments, and a Kondo-screened heavy Fermi liquid develops at low temperatures. Changing the pressure, magnetic field, or chemical doping can displace this heavy Fermi liquid in favor of a magnetically ordered state. Alternatively, Kondo singlet formation can be suppressed when the number of conduction electrons is small compared to the number of magnetic moments, known as the Kondo exhaustion scenario. Furthermore we report the discovery of such an “exhausted” Kondo lattice material, YbIr₃Si₇, where the bulk electrical conductivity tends to zero in the antiferromagnetic state below the Néel temperature T_N = 4.1 K, as all the free carriers are consumed in the formation of Kondo singlets. By contrast, the surface is conducting, as the Yb³⁺ ions relax into larger nonmagnetic Yb²⁺ in the presence of reduced chemical pressure, which shifts the chemical potential.

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics. High-T_c cuprates crystallize into a layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high-Tc cuprates are elongated along the c axis, leading to a 3dx²-y² orbital at the top of the band structure wherein the doped holes reside. This scenario gives rise to 2D characteristics in high-T_c cuprates that favor d-wave pairing symmetry. Here, we report superconductivity in a cuprate Ba₂CuO_4-y, wherein the local octahedron is in a very exceptional compressed version. The Ba₂CuO_4-y compound was synthesized at high pressure at high temperatures and shows bulk superconductivity with critical temperature (T_c) above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the T_c for the isostructural counterparts based on classical La₂CuO₄. X-ray absorption measurements indicate the heavily doped nature of the Ba₂CuO_4-y superconductor. In compressed octahedron, the 3d3z²-r² orbital will be lifted above the 3dx²-y² orbital, leading to significant 3D nature in addition to the conventional 3dx²-y² orbital. This work sheds important light on advancing our comprehensive understanding of the superconducting mechanism of high T_c in cuprate materials.

Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) requires low defect, large area, epitaxial coverage with ultra-clean interfaces. We have developed a chemical vapor deposition van der Waals epitaxy growth process where the metal and chalcogen sources are separated such that growth times can be extended significantly to yield high coverage while minimizing surface contamination. We also demonstrate the growth of high quality 2D WS₂ over large areas on graphene. The as-grown vertical heterostructures are exceptionally clean as demonstrated by ARPES, STM and spatially resolved photoluminescence mapping. With these correlated techniques we are able to relate defect density to electronic band structure and, ultimately, optical properties. We find that our synthetic approach provides ultra-clean, low defect density (~10¹² cm^-2), ~10 μm large WS₂ monolayer crystals, with an electronic band structure and valence band effective masses that perfectly match the theoretical prediction for pristine WS₂.

We identify sources with extremely hard X-ray spectra (i.e., with photon indices of Γ≲0.6) in the 13 deg{sup 2} NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at >10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5–24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; N{sub H}>1.5×10{sup 24} cm{sup −2}) AGNs at low redshift (z<0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray–mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10–40 keV luminosities of the extreme sources cover a broad range, from ≈5×10{sup 42} to 10{sup 45} erg s{sup −1}. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z<0.07), where we measure a high CT fraction of f{sub CT}{sup obs}=30{sub −12}{sup +16}%. For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of “normal” AGNs.

We investigate the dependence of black hole accretion rate (BHAR) on host-galaxy star formation rate (SFR) and stellar mass (M {sub *}) in the CANDELS/GOODS-South field in the redshift range of 0.5⩽z<2.0. Our sample consists of ≈18,000 galaxies, allowing us to probe galaxies with 0.1M{sub ⊙}yr{sup −1}≲SFR≲100 M{sub ⊙} yr{sup −1} and/or 10{sup 8}M{sub ⊙}≲M{sub ∗}≲10{sup 11} M{sub ⊙}. We use sample-mean BHAR to approximate long-term average BHAR. Our sample-mean BHARs are derived from the Chandra Deep Field-South 7 Ms observations, while the SFRs and M {sub *} have been estimated by the CANDELS team through spectral energy distribution fitting. The average BHAR is correlated positively with both SFR and M {sub *}, and the BHAR–SFR and BHAR–M {sub *} relations can both be described acceptably by linear models with a slope of unity. However, BHAR appears to be correlated more strongly with M {sub *} than SFR. This result indicates that M {sub *} is the primary host-galaxy property related to supermassive black hole (SMBH) growth, and the apparent BHAR–SFR relation is largely a secondary effect due to the star-forming main sequence. Among our sources, massive galaxies (M{sub ∗}≳10{sup 10}M{sub ⊙}) have significantly higher BHAR/SFR ratios than less massive galaxies, indicating that the former have higher SMBH fueling efficiency and/or higher SMBH occupation fraction than the latter. Our results can naturally explain the observed proportionality between M{sub BH} and M {sub *} for local giant ellipticals and suggest that their M{sub BH}/M{sub ∗} is higher than that of local star-forming galaxies. Among local star-forming galaxies, massive systems might have higher M{sub BH}/M{sub ∗} compared to dwarfs.

We present the first full catalog and science results for the Nuclear Spectroscopic Telescope Array (NuSTAR) serendipitous survey. The catalog incorporates data taken during the first 40 months of NuSTAR operation, which provide ≈20 Ms of effective exposure time over 331 fields, with an areal coverage of 13 deg{sup 2}, and 497 sources detected in total over the 3–24 keV energy range. There are 276 sources with spectroscopic redshifts and classifications, largely resulting from our extensive campaign of ground-based spectroscopic follow-up. We characterize the overall sample in terms of the X-ray, optical, and infrared source properties. The sample is primarily composed of active galactic nuclei (AGNs), detected over a large range in redshift from z = 0.002 to 3.4 (median of 〈z〉=0.56), but also includes 16 spectroscopically confirmed Galactic sources. There is a large range in X-ray flux, from log(f{sub 3−24keV}/erg s{sup −1} cm{sup −2})≈−14 to −11, and in rest-frame 10–40 keV luminosity, from log(L{sub 10−40keV}/erg s{sup −1})≈39 to 46, with a median of 44.1. Approximately 79% of the NuSTAR sources have lower-energy (<10 keV) X-ray counterparts from XMM-Newton, Chandra, and Swift XRT. The mid-infrared (MIR) analysis, using WISE all-sky survey data, shows that MIR AGN color selections miss a large fraction of the NuSTAR-selected AGN population, from ≈15% at the highest luminosities (L{sub X}>10{sup 44} erg s{sup −1}) to ≈80% at the lowest luminosities (L{sub X}<10{sup 43} erg s{sup −1}). Our optical spectroscopic analysis finds that the observed fraction of optically obscured AGNs (i.e., the type 2 fraction) is F{sub Type2}=53{sub −15}{sup +14}%, for a well-defined subset of the 8–24 keV selected sample. This is higher, albeit at a low significance level, than the type 2 fraction measured for redshift- and luminosity-matched AGNs selected by <10 keV X-ray missions.

Finding solutions to improve the performance of semiconductor light absorbers and catalyst materials remains an outstanding issue that prevents the realization of solar fuel generators. Nanostructuring approaches of photoelectrocatalytic materials have the potential to reduce bulk recombination and improve electron-hole pair separation in semiconductor light absorbers, as well as to increase the active surface area and influence the activity in catalytic systems. Herein, we propose a versatile approach for the synthesis of reproducible, highly homogeneous, large scale nanoporous (photo)electrocatalytic materials for artificial photosynthesis. By identifying and carefully analyzing critical parameters for forming opal templates from solutions of colloidal polystyrene beads (PS), we are able to reproducibly fabricate large area ( > cm²) PS films with high optical quality over a wide diameter range (170-600 nm). Using these PS bead opal films as templates, we demonstrate that electrodeposition is a suitable bottom-up infilling technique to produce scalable, homogeneous, and highly ordered nanoporous (photo)electrocatalytic materials, namely Cu₂O, BiVO₄, CuBi₂O₄, and Cu. Here, we provide morphological, structural, and optical characterization of the resulting opal replicas. Finally, we demonstrate preliminary integration of the Cu₂O inverse opal film into a working photocathode under CO₂ reduction conditions.

Hot dust-obscured galaxies (hot DOGs), selected from Wide-Field Infrared Survey Explorer’s all-sky infrared survey, host some of the most powerful active galactic nuclei known and may represent an important stage in the evolution of galaxies. Most known hot DOGs are located at z>1.5, due in part to a strong bias against identifying them at lower redshift related to the selection criteria. We present a new selection method that identifies 153 hot DOG candidates at z∼1, where they are significantly brighter and easier to study. We validate this approach by measuring a redshift z = 1.009 and finding a spectral energy distribution similar to that of higher-redshift hot DOGs for one of these objects, WISE J1036+0449 (L{sub Bol}≃8×10{sup 46} erg s{sup −1}). We find evidence of a broadened component in Mg ii, which would imply a black hole mass of M{sub BH}≃2×10{sup 8} M{sub ⊙} and an Eddington ratio of λ{sub Edd}≃2.7. WISE J1036+0449 is the first hot DOG detected by the Nuclear Spectroscopic Telescope Array, and observations show that the source is heavily obscured, with a column density of N{sub H}≃(2--15)×10{sup 23} cm{sup −2}. The source has an intrinsic 2–10 keV luminosity of ∼6×10{sup 44} erg s{sup −1}, a value significantly lower than that expected from the mid-infrared/X-ray correlation. We also find that other hot DOGs observed by X-ray facilities show a similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness and the absorption properties of hot DOGs. Hot DOGs at z≲1 could be excellent laboratories to probe the characteristics of the accretion flow and of the X-ray emitting plasma at extreme values of the Eddington ratio.

Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGNs) is difficult to measure. A statistically significant correlation between the Eddington ratio, λ {sub Edd}, and the X-ray power-law index, Γ, observed in unobscured AGNs offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line of sight to the central engine is heavily obscured, the recovery of the intrinsic Γ is challenging. Here we study a sample of local, predominantly CT megamaser AGNs, where the black hole mass, and thus Eddington luminosity, are well known. We compile results of the X-ray spectral fitting of these sources with sensitive high-energy ( E > 10 keV) NuSTAR data, where X-ray torus models, which take into account the reprocessing effects have been used to recover the intrinsic Γ values and X-ray luminosities, L {sub X}. With a simple bolometric correction to L {sub X} to calculate λ {sub Edd}, we find a statistically significant correlation between Γ and λ {sub Edd} ( p = 0.007). A linear fit to the data yields Γ = (0.41 ± 0.18)log{sub 10} λ {sub Edd} + (2.38 ± 0.20), which is statistically consistent with results for unobscured AGNs. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes ( M {sub BH} ≈ 10{sup 6}–10{sup 7} M {sub ⊙}) and are highly inclined, our results extend the Γ– λ {sub Edd} relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of Γ as a growth-rate indicator for accreting black holes, even for CT AGNs.

We used resonant inelastic x-ray scattering to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO₃-LaNiO₃-3×(LaAlO₃), a system with exceptionally large polarization, as a model system. Furthermore, we find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This also provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.