We study superconductivity in the two-dimensional 16-band d-p model extracted from a tightbinding fit to the band structure of LaFeAsO, using the random phase approximation. When the intraorbital repulsion U is larger than the interorbital one U ′ , an extended s-wave (s±-wave) pairing with sign reversal of order parameter is mediated by antiferromagnetic spin fluctuations, while when U < U ′ another kind of s-wave (s++-wave) pairing without sign reversal is mediated by ferro-orbital fluctuations. The s++-wave pairing is enhanced due to the electron-phonon coupling and then can be expanded over the realistic parameter region with U > U ′ .
PACS. 71.10.Fd -Lattice fermion models (Hubbard model, etc.). PACS. 71.30.+h -Metal-insulator transitions and other electronic transitions . PACS. 71.38.-k -Polarons and electron-phonon interactions (see also 63.20.Kr Phonon-electron interactions in lattices).Abstract. -We investigate metal-insulator transitions in the half-filled Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant the transition is to a Mott-insulator; when the electron-phonon interaction dominates, the transition is to a localised bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U . We also present results for the quasiparticle weight and the double-occupancy as function of U and g.c EDP Sciences
We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below TQ which induces the tetragonal-orthorhombic structural transition at Ts = TQ, together with the stripe-type antiferromagnetic order below TN . Near the phase transitions, the system shows the s++-wave superconductivity due to the orbital fluctuation for a large g case with TQ > TN , while the s±-wave due to the magnetic fluctuation for a small g case with TQ ≤ TN . The former case is consistent with the phase diagram of doped iron pnictides with Ts > TN .KEYWORDS: iron pnictides, superconductivity, elastic softening, structural transition, orbital orderThe recently discovered iron pnictide superconductors 1, 2) RFeP nO 1−x F x (R=Rare Earth, P n=As, P) with a transition temperature T c exceeding 50K have attracted much attention. The parent compounds with x = 0 show the tetragonal-orthorhombic structural transition at T s and the stripe-type antiferromagnetic (AFM) transition at T N . The carrier doping x suppresses both of the transition temperatures T s and T N and induces the superconductivity. In RFeP nO 1−x F x , T s is always higher than T N , while in Ba(Fe 1−x Co x ) 2 As 2 , the simultaneous first-order transition for nondoped case splits into two transitions with doping x where T s > T N . 3)Theoretically, the s-wave pairing with sign change of the order parameter between the hole and electron Fermi surfaces (FSs), so called s ± -wave, mediated by the AFM fluctuation was proposed as a possible pairing state in the iron pnictides.4-6) The s ± -wave state with a full superconducting gap seems to be consistent with most of the experiments. 7) As for the impurity effects, however, the small T c -suppression against nonmagnetic impurities 8, 9) is not consistent with the s ± -wave state where T c is considered to rapidly decrease with the nonmagnetic impurities.10) Therefore, the s-wave state without sign change of the order parameter, so called s ++ -wave, mediated by the orbital fluctuation which is enhanced due to the effects of the inter-orbital Coulomb interaction was proposed on the basis of the one-dimensional two-band Hubbard model 11,12) and the two-dimensional 16-band d-p model. 13)Remarkably, drastic softenings of the elastic constants have been observed in recent ultrasonic experiments.14-16) As the elastic constant C ε is given by the second derivative of the total energy w.r.t. the strain field ε and includes the contribution such as −g 2 η χ η with the susceptibility χ η for the electric operatorη linearly coupled with the strain field as g ηη ε, the enhancement of χ η is responsible for the softening of C ε . The detailed ultrasonic measurements 16) revealed that nondoped and underdoped Ba(Fe 1−x Co x ) 2 As 2 shows drastic softenings of the elastic constants with decreasing T down to T s * E-mail address: yanagi@p...
It has long been assumed that the red cell membrane is highly permeable to gases because the molecules of gases are small, uncharged, and soluble in lipids, such as those of a bilayer. as expected, but also appeared to reduce intracellular A, although separate experiments showed it has no effect on CA activity in homogenous solution. A decrease in P m,CO 2 would explain this finding. With a more advanced computational model, which solves for CA activity and membrane permeabilities to both CO 2 and HCO 3 ؊ , we found that DIDS inhibited both P m,HCO ؊ 3 and P m,CO 2 , whereas intracellular CA activity remained unchanged. The mechanism by which DIDS reduces CO 2 permeability may not be through an action on the lipid bilayer itself, but rather on a membrane transport protein, implying that this is a normal route for at least part of red cell CO 2 exchange. Ϫ and H 2 16 O in a red cell suspension at chemical equilibrium can be used in principle to measure the proportional intracellular acceleration (A) of CO 2 hydration produced inside intact red blood cells by carbonic anhydrase (CA) compared with the uncatalyzed rate (1) and the self-exchange permeability of the membrane to HCO 3 (P m,HCO Ϫ 3 ). As predicted, A was found to be the same in intact erythrocytes as in hemolysate, both under normal conditions and when exposed to varying concentrations of a membrane-permeable CA sulfonamide inhibitor (ethoxzolamide), whereas P m,HCO Ϫ 3 was not affected (2). Validation of the ability of the technique to differentiate between CA activity and HCO 3 Ϫ permeability in intact red cells was extended by exposing them to phlorizin (3), an established Band III inhibitor (1); this drug decreased P m,HCORecently we exposed red cells to a more specific inhibitor of Band III protein, 4,4Ј-diisothiocyanato-stilbene-2,2Ј-disulfonate (DIDS), to decrease P m,HCO Ϫ 3 without inhibiting CA. However, we found that whereas DIDS lowered HCO 3 Ϫ transport, it also lowered the membrane permeability to CO 2 . METHODSTwo groups of experiments were carried out, one in Philadelphia and one in Hannover, Germany. Blood from healthy adults, freshly drawn into heparin, was washed three times with 145 mM NaCl at 4°C; the red cells were diluted to approximately 40% hematocrit and used the same day. Hemolysates were prepared by freezing and thawing the cell suspension.The fractional water volume, v, of the red cells in the reaction suspension was calculated as the hematocrit of the stock cell suspension ϫ water content of red cells [0.61 (1) for the Philadelphia experiments and 0.65 (4) for the Hannover experiments] ϫ its dilution in the reaction mixture. In later experiments, v was calculated as 55.5 ϫ [cyanmethemoglobin] in mM in the reactant mixture. NaHCO 3 was prepared by incubating 2% and 5% 18O labeled HOH with unlabeled NaHCO 3 at 150°C in a pressure bomb for several days, after which the water was removed by lyophylization. DIDS was obtained from Research Organics and phlorizin (phloridzin) was obtained from Sigma.The technique of the ...
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