Electronic Structure of Transition-Metal Dicyanamides Me[N(CN)2_2]2_2 (Me = Mn, Fe, Co, Ni, Cu)

The electronic structure of Me[N(CN)$_2$]$_2$ (Me=Mn, Fe, Co, Ni, Cu) molecular magnets has been investigated using x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) as well as theoretical density-functional-based methods. Both theory and experiments show that the top of the valence band is dominated by Me 3d bands, while a strong hybridization between C 2p and N 2p states determines the valence band electronic structure away from the top. The 2p contributions from non-equivalent nitrogen sites have been identified using resonant inelastic x-ray scattering spectroscopy with the excitation energy tuned near the N 1s threshold. The binding energy of the Me 3d bands and the hybridization between N 2p and Me 3d states both increase in going across the row from Me = Mn to Me = Cu. Localization of the Cu 3d states also leads to weak screening of Cu 2p and 3s states, which accounts for shifts in the core 2p and 3s spectra of the transition metal atoms. Calculations indicate that the ground-state magnetic ordering, which varies across the series is largely dependent on the occupation of the metal 3d shell and that structural differences in the superexchange pathways for different compounds play a secondary role.Comment: 20 pages, 11 figures, 2 table

Number--conserving model for boson pairing

An independent pair ansatz is developed for the many body wavefunction of dilute Bose systems. The pair correlation is optimized by minimizing the expectation value of the full hamiltonian (rather than the truncated Bogoliubov one) providing a rigorous energy upper bound. In contrast with the Jastrow model, hypernetted chain theory provides closed-form exactly solvable equations for the optimized pair correlation. The model involves both condensate and coherent pairing with number conservation and kinetic energy sum rules satisfied exactly and the compressibility sum rule obeyed at low density. We compute, for bulk boson matter at a given density and zero temperature, (i) the two--body distribution function, (ii) the energy per particle, (iii) the sound velocity, (iv) the chemical potential, (v) the momentum distribution and its condensate fraction and (vi) the pairing function, which quantifies the ODLRO resulting from the structural properties of the two--particle density matrix. The connections with the low--density expansion and Bogoliubov theory are analyzed at different density values, including the density and scattering length regime of interest of trapped-atoms Bose--Einstein condensates. Comparison with the available Diffusion Monte Carlo results is also made.Comment: 21 pages, 12 figure

Fermion Condensation Quantum Phase Transition versus Conventional Quantum Phase Transitions

The main features of fermion condensation quantum phase transition (FCQPT), which are distinctive in several aspects from that of conventional quantum phase transition (CQPT), are considered. We show that in contrast to CQPT, whose physics in quantum critical region is dominated by thermal and quantum fluctuations and characterized by the absence of quasiparticles, the physics of a Fermi system near FCQPT or undergone FCQPT is controlled by the system of quasiparticles resembling the Landau quasiparticles. Contrary to the Landau quasiparticles, the effective mass of these quasiparticles strongly depends on the temperature, magnetic fields, density, etc. This system of quasiparticles having general properties determines the universal behavior of the Fermi system in question. As a result, the universal behavior persists up to relatively high temperatures comparatively to the case when such a behavior is determined by CQPT. We analyze striking recent measurements of specific heat, charge and heat transport used to study the nature of magnetic field-induced QCP in heavy-fermion metal CeCoIn$_5$ and show that the observed facts are in good agreement with our scenario based on FCQPT and certainly seem to rule out the critical fluctuations related with CQPT. Our general consideration suggests that FCQPT and the emergence of novel quasiparticles near and behind FCQPT and resembling the Landau quasiparticles are distinctive features intrinsic to strongly correlated substances.Comment: 10 pages, Revtex, new references and facts are adde

Crossovers in Unitary Fermi Systems

Universality and crossover is described for attractive and repulsive interactions where, respectively, the BCS-BEC crossover takes place and a ferromagnetic phase transition is claimed. Crossovers are also described for optical lattices and multicomponent systems. The crossovers, universal parameters and phase transitions are described within the Leggett and NSR models and calculated in detail within the Jastrow-Slater approximation. The physics of ultracold Fermi atoms is applied to neutron, nuclear and quark matter, nuclei and electrons in solids whenever possible. Specifically, the differences between optical lattices and cuprates is discussed w.r.t. antiferromagnetic, d-wave superfluid phases and phase separation.Comment: 50 pages, 15 figures. Contribution to Lecture Notes in Physics "BCS-BEC crossover and the Unitary Fermi Gas" edited by W. Zwerge

The local adsorption geometry of benzenethiolate on Cu(1 0 0)

The local adsorption geometry of benzenethiolate in the ordered c(2 × 6) phase on Cu(1 0 0) has been investigated by a combination of S K-edge near-edge X-ray absorption fine structure (NEXAFS), normal incidence X-ray standing waves (NIXSW) and S 1s scanned-energy mode photoelectron diffraction (PhD). NEXAFS and PhD show that the molecular plane is tilted from the surface normal by 20 ± 15°, while NIXSW clearly identifies the S head-group as occupying the four-fold coordinated hollow sites. PhD shows the S atoms lies 1.34 ± 0.04 Å above the outermost Cu atomic layer, leading to a Cu–S bondlength of 2.25 ± 0.02 Å. The combination of the PhD and NIXSW results shows the Cu surface layer has an outward relaxation of 0.15 ± 0.06 Å. Possible origins for this large adsorbate-induced relaxation are discussed

On the purification of α-cellulose from resinous wood for stable isotope (H, C and O) analysis

α-Cellulose was isolated from four samples of Scots pine (Pinus sylvestris L.). Each sample was divided into two portions. One portion had the resins removed by solvent extraction prior to removal of lignins by treatment with acidic sodium chlorite solution and treatment with sodium hydroxide solution to remove hemicelluloses. The other portion was processed in the same way apart from the solvent extraction step. The isolated wood constituents were characterised by attenuated total reflectance Fourier transform infrared (ATR/FT-IR) spectroscopy. The infrared spectra of the resulting α-cellulose samples were identical indicating that treatment with acidic sodium chlorite and sodium hydroxide was sufficient to remove resins. The values of the stable isotope ratios (carbon, oxygen and hydrogen) for each pair of α-cellulose sub-samples also showed no significant differences within the reproducibility of the methods. The implication of these studies demonstrate that the customary step of resin extraction from pine is unnecessary if sodium chlorite and sodium hydroxide are used for the isolation of α-cellulose following the technique described in this paper. In addition, the study demonstrates that the oxygen isotope ratio of the water used for cellulose extraction does not influence the stable isotope values in the α-cellulose obtained. The importance of isotopic homogeneity within the cellulose sample is also highlighted

Magnetism in Dense Quark Matter

We review the mechanisms via which an external magnetic field can affect the ground state of cold and dense quark matter. In the absence of a magnetic field, at asymptotically high densities, cold quark matter is in the Color-Flavor-Locked (CFL) phase of color superconductivity characterized by three scales: the superconducting gap, the gluon Meissner mass, and the baryonic chemical potential. When an applied magnetic field becomes comparable with each of these scales, new phases and/or condensates may emerge. They include the magnetic CFL (MCFL) phase that becomes relevant for fields of the order of the gap scale; the paramagnetic CFL, important when the field is of the order of the Meissner mass, and a spin-one condensate associated to the magnetic moment of the Cooper pairs, significant at fields of the order of the chemical potential. We discuss the equation of state (EoS) of MCFL matter for a large range of field values and consider possible applications of the magnetic effects on dense quark matter to the astrophysics of compact stars.Comment: To appear in Lect. Notes Phys. "Strongly interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Ye

A Solvable Regime of Disorder and Interactions in Ballistic Nanostructures, Part I: Consequences for Coulomb Blockade

We provide a framework for analyzing the problem of interacting electrons in a ballistic quantum dot with chaotic boundary conditions within an energy $E_T$ (the Thouless energy) of the Fermi energy. Within this window we show that the interactions can be characterized by Landau Fermi liquid parameters. When $g$, the dimensionless conductance of the dot, is large, we find that the disordered interacting problem can be solved in a saddle-point approximation which becomes exact as $g\to\infty$ (as in a large-N theory). The infinite $g$ theory shows a transition to a strong-coupling phase characterized by the same order parameter as in the Pomeranchuk transition in clean systems (a spontaneous interaction-induced Fermi surface distortion), but smeared and pinned by disorder. At finite $g$, the two phases and critical point evolve into three regimes in the $u_m-1/g$ plane -- weak- and strong-coupling regimes separated by crossover lines from a quantum-critical regime controlled by the quantum critical point. In the strong-coupling and quantum-critical regions, the quasiparticle acquires a width of the same order as the level spacing $\Delta$ within a few $\Delta$'s of the Fermi energy due to coupling to collective excitations. In the strong coupling regime if $m$ is odd, the dot will (if isolated) cross over from the orthogonal to unitary ensemble for an exponentially small external flux, or will (if strongly coupled to leads) break time-reversal symmetry spontaneously.Comment: 33 pages, 14 figures. Very minor changes. We have clarified that we are treating charge-channel instabilities in spinful systems, leaving spin-channel instabilities for future work. No substantive results are change

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results