Andrew D Nicholson

Interband pairing in multiorbital systems

The discovery of high-

Spectral density in a nematic state of iron pnictides

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Pairing symmetries of a hole-doped extended two-orbital model for the pnictides

superconductivity in the pnictides, materials with a Fermi surface determined by several bands, highlights the need to understand how superconductivity arises in multiband systems. In this effort, using symmetry considerations and mean-field approximations, we discuss how strong hybridization among orbitals may lead to both intraband and interband pairings, and we present calculations of the spectral functions to guide the experimental search for this kind of state.

Multiagency Urban Search Experiment Detector and Algorithm Test Bed

Using cluster-perturbation theory, we calculate the spectral density A(k, ) for a nematic phase of models describing pnictide superconductors, where very short-range magnetic correlations choose the ordering vector ( ,0) over the equivalent (0, ) and thus, break the fourfold rotation symmetry of the underlying lattice without inducing long-range magnetic order. In excellent agreement with angle-resolved photoemission spectroscopy (ARPES), we find that the yz bands at X move to higher energies. When on-site Coulomb repulsion brings the system close to a spin-density wave (SDW) and renormalizes the bandwidth by a factor of 2, even small anisotropic couplings of 10 15 meV strongly distort the bands, splitting the formerly degenerate states at X and Y by 70 meV and shifting the yz states at X above the chemical potential. This similarity to the SDW bands is in excellent agreement with ARPES. An important difference to the SDW bands is that the yz bands still cross the Fermi level, again in agreement with experiments. We find that orbital weights near the Fermi surface provide a better characterization than overall orbital densities and orbital polarization.

Constraints imposed by symmetry on pairing operators for the iron pnictides

The hole-doped ground state of a recently introduced extended t-U-J two-orbital Hubbard model for the Fe-based superconductors is studied via exact diagonalization methods on small clusters. Similarly as in the previously studied case of electron doping [A. Nicholson et al., Phys. Rev. Lett. 106, 217002 (2011)], upon hole doping it is observed that there are several competing pairing symmetries, including A1g , B1g , and B2g . However, contrary to the electron-doped case, the ground state of the hole-doped state has pseudocrystal momentum k = ( , ) in the unfolded Brillouin zone. In the two Fe-atom per unit cell representation, this indicates that the ground state involves antibonding, rather than bonding, combinations of the orbitals of the two Fe atoms in the unit cell. The lowest state with k = (0,0) has only a slightly higher energy. These results indicate that this simple two-orbital model may be useful to capture some subtle aspects of the hole-doped pnictides, since calculations for the five-orbital model have unveiled a hole pocket centered at M [k = ( , )] in the unfolded Brillouin zone.

A Methodology for Determining the Concentration of Naturally Occurring Radioactive Materials in an Urban Environment

In order to provide benchmark data sets for radiation detector and algorithm development, a particle transport test bed has been created using experimental data as model input and validation. A detailed radiation measurement campaign at the Combined Arms Collective Training Facility in Fort Indiantown Gap, PA (FTIG), USA, provides sample background radiation levels for a variety of materials present at the site (including cinder block, gravel, asphalt, and soil) using long dwell high-purity germanium (HPGe) measurements. In addition, detailed light detection and ranging data and ground-truth measurements inform model geometry. This paper describes the collected data and the application of these data to create background and injected source synthetic data for an arbitrary gamma-ray detection system using particle transport model detector response calculations and statistical sampling. In the methodology presented here, HPGe measurements inform model source terms while detector response calculations are validated via long dwell measurements using 2”

Systematic Assessment of Neutron and Gamma Backgrounds Relevant to Operational Modeling and Detection Technology Implementation

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Magnetic state of K0.8Fe1.6Se2 from a five-orbital Hubbard model in the Hartree-Fock approximation

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Characterization of gamma-ray background outside of the High Flux Isotope Reactor

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Uncertainty Quantification for Nuclear Safeguards and Non-Destructive Assay - Fact Sheet

” NaI(Tl) detectors at a variety of measurement points. A collection of responses, along with sampling methods and interpolation, can be used to create data sets to gauge radiation detector and algorithm (including detection, identification, and localization) performance under a variety of scenarios. Data collected at the FTIG site are available for query, filtering, visualization, and download at muse.lbl.gov.