Electronic Structure of New LiFeAs High-Tc Superconductor

We present results of it ab initio LDA calculations of electronic structure of "next generation" layered ironpnictide High-Tc superconductor LiFeAs (Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely close to the Fermi level its electronic properties are also determined ma inly by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of the Fe-3d density o f states and Fermi surface.Comment: 4 pages, 5 figures; Electronic structure improved with respect to new experimental crystal structure dat

New model for surface fracture induced by dynamical stress

We introduce a model where an isotropic, dynamically-imposed stress induces fracture in a thin film. Using molecular dynamics simulations, we study how the integrated fragment distribution function depends on the rate of change and magnitude of the imposed stress, as well as on temperature. A mean-field argument shows that the system becomes unstable for a critical value of the stress. We find a striking invariance of the distribution of fragments for fixed ratio of temperature and rate of change of the stress; the interval over which this invariance holds is determined by the force fluctuations at the critical value of the stress.Comment: Revtex, 4 pages, 4 figures available upon reques

Electronic structure of Pr_{2-x}Ce_xCuO_4 studied via ARPES and LDA+DMFT+\Sigma_k

The electron-doped Pr(2-x)Ce(x)CuO(4) (PCCO) compound in the pseudogap regime (x~0.15) was investigated using angle-resolved photoemission spectroscopy (ARPES) and the generalized dynamical mean-field theory (DMFT) with the k-dependent self-energy (LDA+DMFT+\Sigma_k). Model parameters (hopping integral values and local Coulomb interaction strength) for the effective one-band Hubbard model were calculated by the local density approximation (LDA) with numerical renormalization group method (NRG) employed as an "impurity solver" in DMFT computations. An "external" k-dependent self-energy \Sigma_k was used to describe interaction of correlated conducting electrons with short-range antiferromagnetic (AFM) pseudogap fluctuations. Both experimental and theoretical spectral functions and Fermi surfaces (FS) were obtained and compared demonstrating good semiquantitative agreement. For both experiment and theory normal state spectra of nearly optimally doped PCCO show clear evidence for a pseudogap state with AFM-like nature. Namely, folding of quasiparticle bands as well as presence of the "hot spots" and "Fermi arcs" were observed.Comment: 4 pages, 4 figures, as accepted to PRB Rapid Communications. Title is changed by Editor

Local Ranking Problem on the BrowseGraph

The "Local Ranking Problem" (LRP) is related to the computation of a centrality-like rank on a local graph, where the scores of the nodes could significantly differ from the ones computed on the global graph. Previous work has studied LRP on the hyperlink graph but never on the BrowseGraph, namely a graph where nodes are webpages and edges are browsing transitions. Recently, this graph has received more and more attention in many different tasks such as ranking, prediction and recommendation. However, a web-server has only the browsing traffic performed on its pages (local BrowseGraph) and, as a consequence, the local computation can lead to estimation errors, which hinders the increasing number of applications in the state of the art. Also, although the divergence between the local and global ranks has been measured, the possibility of estimating such divergence using only local knowledge has been mainly overlooked. These aspects are of great interest for online service providers who want to: (i) gauge their ability to correctly assess the importance of their resources only based on their local knowledge, and (ii) take into account real user browsing fluxes that better capture the actual user interest than the static hyperlink network. We study the LRP problem on a BrowseGraph from a large news provider, considering as subgraphs the aggregations of browsing traces of users coming from different domains. We show that the distance between rankings can be accurately predicted based only on structural information of the local graph, being able to achieve an average rank correlation as high as 0.8

Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors

This work is provoked by recent discovery of new class prototype systems AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors (Tc=36K). Here we report ab initio LDA results for electronic structure of the AFFeAs systems. We provide detailed comparison between electronic properties of both new systems and reference LaOFeAs (La111) compound. In the vicinity of the Fermi level all three systems have essentially the same band dispersions. However for iron fluoride systems F(2p) states were found to be separated in energy from As(4p) ones in contrast to La111, where O(2p) states strongly overlaps with As(4p). Thus it should be more plausible to include only Fe(3d) and As(4p) orbitals into a realistic noninteracting model than for La111. Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in smaller value of the density of states at the Fermi level in the case of Ca compound. So to some extend Ca system reminds RE111 with later Rare Earths. However Fermi surface of new fluorides is found to be nearly perfect two-dimensional. Also we do not expect strong dependence of superconducting properties with respect to different types of A substitutes.Comment: 5 pages, 4 figure

Stability of C20 fullerene chains

The stability of (C20)N chains with N = 3 - 7 is analyzed by numerical simulation using a tight-binding potential and molecular dynamics. Various channels of losing the cluster-chain structure of the (C20)N complexes are observed, including the decay of C20 clusters, their coalescence, and the separation of one C20 fullerene from the chain.Comment: To appear in JETP Letter

Novel multiple-band superconductor SrPt2As2

We present LDA calculated electronic structure of recently discovered superconductor SrPt2As2 with Tc=5.2K. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt2As2 is very much different. Crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt2As2 is essentially three dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt2As2 with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of superconductivity in SrPt2As2 is also presented.Comment: 5 pages, 4 figure

Structure and Stability of Two-Dimensional Complexes of C_20 Fullerenes

Two-dimensional complexes of C_20 fullerenes connected to each other by covalent bonds have been studied. Several isomers with different types of intercluster bonds have been revealed. The lifetimes of the (C_20)_MxM systems with M = 2 and 3 have been directly calculated at T = 1800 - 3300 K making use of molecular dynamics. It has been shown that these complexes lose their periodic cluster structure due to either coalescence of two fullerenes C_20 or decay of C_20 fullerenes. The activation energies of these processes exceed 2 eV.Comment: 17 pages, 5 figure

Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P

We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt3P (A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling superconductivity, suggested by experimental data on specific heat of SrPt3P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease of superconducting transition temperature Tc from Sr to La compound can be explained by corresponding decrease of total density of states at the Fermi level N(E_F). The shape of the density of states near the Fermi level suggests that in SrPt3P electron doping (such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g. partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F) and possibly Tc.Comment: 5 pages, 5 figure

Cadherin-26 (CDH26) regulates airway epithelial cell cytoskeletal structure and polarity.

Polarization of the airway epithelial cells (AECs) in the airway lumen is critical to the proper function of the mucociliary escalator and maintenance of lung health, but the cellular requirements for polarization of AECs are poorly understood. Using human AECs and cell lines, we demonstrate that cadherin-26 (CDH26) is abundantly expressed in differentiated AECs, localizes to the cell apices near ciliary membranes, and has functional cadherin domains with homotypic binding. We find a unique and non-redundant role for CDH26, previously uncharacterized in AECs, in regulation of cell-cell contact and cell integrity through maintaining cytoskeletal structures. Overexpression of CDH26 in cells with a fibroblastoid phenotype increases contact inhibition and promotes monolayer formation and cortical actin structures. CDH26 expression is also important for localization of planar cell polarity proteins. Knockdown of CDH26 in AECs results in loss of cortical actin and disruption of CRB3 and other proteins associated with apical polarity. Together, our findings uncover previously unrecognized functions for CDH26 in the maintenance of actin cytoskeleton and apicobasal polarity of AECs