Electronic correlations in FeGa3 and the effect of hole doping on its magnetic properties

We investigate signatures of electronic correlations in the narrow-gap semiconductor FeGa 3 by means of electrical resistivity and thermodynamic measurements performed on single crystals of FeGa 3 , Fe 1−x Mn x Ga 3 , and FeGa 3−y Zn y , complemented by a study of the 4d analog material RuGa 3 . We find that the inclusion of sizable amounts of Mn and Zn dopants into FeGa 3 does not induce an insulator-to-metal transition. Our study indicates that both substitution of Zn onto the Ga site and replacement of Fe by Mn introduces states into the semiconducting gap that remain localized even at highest doping levels. Most importantly, using neutron powder diffraction measurements, we establish that FeGa 3 orders magnetically above room temperature in a complex structure, which is almost unaffected by the doping with Mn and Zn. Using realistic many-body calculations within the framework of dynamical mean field theory (DMFT), we argue that while the iron atoms in FeGa 3 are dominantly in an S=1 state, there are strong charge and spin fluctuations on short-time scales, which are independent of temperature. Further, the low magnitude of local contributions to the spin susceptibility advocates an itinerant mechanism for the spin response in FeGa 3 . Our joint experimental and theoretical investigations classify FeGa 3 as a correlated band insulator with only small dynamical correlation effects, in which nonlocal exchange interactions are responsible for the spin gap of 0.4 eV and the antiferromagnetic order. We show that hole doping of FeGa 3 leads, within DMFT, to a notable strengthening of many-body renormalizations

Composition dependence of bulk superconductivity in YFe2Ge2

In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ~100 mJ/molK^2 and a T^(3/2) temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2 k_B T_c/hbar. Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimised by precipitating from a slightly iron-rich melt, following by annealing

Magnetic structure of Yb2Pt2Pb: Ising moments on the Shastry-Sutherland lattice.

Neutron diffraction measurements were carried out on single crystals and powders of Yb2Pt2Pb, where Yb moments form two interpenetrating planar sublattices of orthogonal dimers, a geometry known as Shastry-Sutherland lattice, and are stacked along the c axis in a ladder geometry. Yb2Pt2Pb orders antiferromagnetically at TN=2.07K, and the magnetic structure determined from these measurements features the interleaving of two orthogonal sublattices into a 5×5×1 magnetic supercell that is based on stripes with moments perpendicular to the dimer bonds, which are along (110) and (−110). Magnetic fields applied along (110) or (−110) suppress the antiferromagnetic peaks from an individual sublattice, but leave the orthogonal sublattice unaffected, evidence for the Ising character of the Yb moments in Yb2Pt2Pb that is supported by point charge calculations. Specific heat, magnetic susceptibility, and electrical resistivity measurements concur with neutron elastic scattering results that the longitudinal critical fluctuations are gapped with ΔE≃0.07meV

Coexistence of magnetic order and valence fluctuations in the Kondo lattice system Ce2Rh3Sn5

We report on the electronic band structure, structural, magnetic, and thermal properties of Ce2Rh3Sn5. Ce LIII-edge XAS spectra give direct evidence for an intermediate valence behavior. Thermodynamic measurements reveal magnetic transitions at TN1≈2.9 K and TN2≈2.4 K. Electrical resistivity shows behavior typical for the Kondo lattices. The coexistence of magnetic order and valence fluctuations in a Kondo lattice system we attribute to a peculiar crystal structure in which Ce ions occupy two distinct lattice sites. Analysis of the structural features of Ce2Rh3Sn5, together with results of electronic band structure calculations and thermodynamic and spectroscopic data indicate that at low temperatures only Ce ions from the Ce1 sublattice adopt a stable trivalent electronic configuration and show local magnetic moments that give rise to the magnetic ordering. By contrast, our study suggests that Ce2 ions exhibit a nonmagnetic Kondo-singlet ground state. Furthermore, the valence of Ce2 ions estimated from the Ce LIII-edge XAS spectra varies between +3.18 at 6 K and +3.08 at room temperature. Thus our joined experimental and theoretical investigations classify Ce2Rh3Sn5 as a multivalent charge-ordered system

Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2.

One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition.This work is supported by the EPSRC through grant EP/K012894/1. SF acknowledges support by the ERC and the Alexander von Humboldt foundation. PR acknowledges funding from the Cusanuswerk and the EPSRC. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF DMR-1157490 and the State of Florida. SWT, WAC, and DEG were supported in part by DOE NNSA SSAA DE-NA0001979

Aggregation energy of cholesterol with selected oxy- and peroxysterols

Cholesterol oraz jego pochodne: oksysterole i peroksysterole, to związki pełniące wiele różnorodnych funkcji w komórkach zwierzęcych. Oprócz niewątpliwie pozytywnych skutków ich działań, wśród których można wyróżnić stabilizację błon komórkowych czy regulację metabolizmu, mogą one także indukować szereg niezbyt korzystnych procesów. Na szczególną uwagę zasługuje ich rola w powstawaniu miażdżycy, choroby powszechnie występującej w populacji ludzkiej. Wielokrotnie potwierdzono eksperymentalnie zdolność cholesterolu do tworzenia agregatów w środowisku wodnym. Celem pracy było zbadanie energii agregacji cholesterolu z cholesterolem oraz z jego pochodnymi: 7-α-hydroksysterolem, 7-ß-hydroksysterolem, 7-α-hydroperoksysterolem i 7-ß-hydroperoksysterolem. Potwierdzono tendencję cholesterolu do dimeryzacji, a wynikający z niej zysk energetyczny wyniósł ok. -3 kcal/mol. Podobną różnicę energii uzyskano dla agregacji tego związku z 7-ß-hydroksysterolem oraz 7-ß-hydroperoksysterolem. W przypadku 7-α-hydroksysterolu różnica ta wyniosła ok. -1,8 kcal/mol, a 7-α-hydroperoksysterolu ok. -1,5 kcal/mol. Dzięki zastosowaniu metody “umbrella sampling” wygenerowano pełne profile energetyczne powyższych procesów, co pozwoliło na zaobserwowanie minimum lokalnego w agregacji cholesterolu z cholesterolem oraz z 7-α-hydroksysterolem.Zaobserwowano, że w przypadku produktów utleniania cholesterolu agregacja w środowisku wodnym jest dla nich korzystna energetycznie. Ponadto ustalono, że dla steroli posiadających grupę funkcyjną na pozycji ß różnica energii między układem pojedynczych cząstek, a agregatem będzie dużo większa niż dla analogicznych związków z grupą funkcyjną na pozycji α, w przypadku tych drugich wspomniana grupa będzie stanowiła zawadę steryczną przy ich zbliżaniu się do siebie.“Aggregation energy of cholesterol with selected oxy- and peroxysterols”Cholesterol and its derivatives: oxysterols and peroxysterols have various functions in animal cells. Among many they tend to participate in stabilizing cell membranes and to control metabolism reactions. On the other hand they are able to have an negative impact on animal body, such as a creation of atherosclerotic plaque, which is common in human population.Previously, it was experimentally proven that cholesterol tend to aggregate in aqueous environment.The main goal of this thesis was to examine cholesterol-cholesterol dimerization as well as its aggregation with 7-α-hydroxysterol, 7-ß-hydroxysterol, 7-α-hydroperoxysterol, 7-ß-hydroperoxysterol. It was confirmed that indeed cholesterol forms dimers in water with an energetic gain around -3 kcal/mol. The same result was given by simulations with 7-ß-hydroxysterol and 7-ß-hydroperoxysterol. Aggregation of cholesterol with 7-α-hydroxysterol gave an energy difference of about -1,8 kcal/mol and with 7-α-hydroperoxysterol of about -1,5 kcal/mol. Umbrella sampling method enabled to obtain full energy profile in each case and to find local energy minima in cholesterol-cholesterol and cholesterol-7-α-hydroxysterol reaction.Therefore it was proven that also products of cholesterol oxygenation tend to make aggregates in aqueous solution. Moreover it was observed that oxy- and peroxysterols with functional group in a ß position would connect with cholesterol more willingly than sterols with this group in an α position

Electronic structure of CeRhX (X=Sn, In)

79.60.-i Photoemission and photoelectron spectra, 71.20.LP Intermetallic compounds, 71.27.+a Strongly correlated electron systems; heavy fermions,

Electronic structure of Ce 5 Rh 4 Sn 10 from XPS and band structure calculations

We present a combined experimental and theoretical study of the electronic structure for the heavy-fermion antiferromagnet Ce 5Rh 4Sn 10 based on X-ray photoemission spectroscopy (XPS) data and ab initio band structure calculations. The Ce core-level XPS spectra point to a stable trivalent configuration of Ce atoms in Ce 5Rh 4Sn 10 , consistently with both the magnetic susceptibility data and the results of computational structure optimization. The band structure calculations confirm a magnetic ground state with significant magnetic moments only at the Ce atoms. The qualitatively correct description of Ce 3+ in Ce 5Rh 4Sn 10 has been achieved using the LSDA+U approach for the Ce 4f states. The comparison of the theoretical results with experimental XPS valence band spectrum supports their validity. The calculated partial densities of states suggest that there is a variation in binding energy of the occupied 4f states between Ce atoms in nonequivalent crystallographic positions, which is related to the hybridization with Sn states. Finally, the band structure and charge density maps point to the formation of zig-zag chains of the strongly bounded Sn(2), Sn(3) and Rh atoms along the tetragonal axis, whereas Sn(1) shows nearly dispersionless 5s bands. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200879.60.-i Photoemission and photoelectron spectra, 71.20.Lp Intermetallic compounds, 71.27.+a Strongly correlated electron systems; heavy fermions,