Computer simulation of field ion images of nanoporous structure in the irradiated materials

Computer simulation and interpretation of field ion microscopy images of ion irradiated platinum are discussed. Field ion microscopy technique provides direct precise atomic scale investigation of crystal lattice defects of atomically pure surface of material; at the same time it allows to analyze the structural defects in volume by controlled and sequential removal of surface atoms by electric field. Defects identification includes the following steps: at the first stage the type of crystalline structure and spatial orientation of crystallographic directions were determined. Thus, we obtain the data about exact position of all atoms of the given volume, i.e. the model image of an ideal crystal. At the second stage, the ion image was processed used the program to obtain the data about real arrangement of atoms of the investigated sample. At the third stage the program compares these two data sets, with a split-hair accuracy revealing a site of all defects in a material. Results of the quantitative analysis show that shape of nanopores are spherical or cylindrical, diameter on nanopores was varied from 1 to 5 run, their depth was fond to be from 1 to 9 nm. It was observed that nearly 40% of nanopores are concentrated in the subsurface layer 10 nm thick, the concentration of nanopores decreased linearly with the distance from the irradiated surface

Tuning the properties of complex transparent conducting oxides: role of crystal symmetry, chemical composition and carrier generation

The electronic properties of single- and multi-cation transparent conducting oxides (TCOs) are investigated using first-principles density functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In$_2$O$_3$, $\alpha$- and $\beta$-Ga$_2$O$_3$, rock salt and wurtzite ZnO, and layered InGaZnO$_4$ reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electronic configuration of the cation(s), specifically, the type of orbital(s) -- $s$, $p$ or $d$ -- which form the conduction band; and (iii) the strength of the hybridization between the cation's states and the p-states of the neighboring oxygen atoms. The results not only explain the experimentally observed trends in the electrical conductivity in the single-cation TCO, but also demonstrate that multicomponent oxides may offer a way to overcome the electron localization bottleneck which limits the charge transport in wide-bandgap main-group metal oxides. Further, the advantages of aliovalent substitutional doping -- an alternative route to generate carriers in a TCO host -- are outlined based on the electronic band structure calculations of Sn, Ga, Ti and Zr-doped InGaZnO$_4$. We show that the transition metal dopants offer a possibility to improve conductivity without compromising the optical transmittance

Magnetically Mediated Transparent Conductors: In2_2O3_3 doped with Mo

First-principles band structure investigations of the electronic, optical and magnetic properties of Mo-doped In$_2$O$_3$ reveal the vital role of magnetic interactions in determining both the electrical conductivity and the Burstein-Moss shift which governs optical absorption. We demonstrate the advantages of the transition metal doping which results in smaller effective mass, larger fundamental band gap and better overall optical transmission in the visible -- as compared to commercial Sn-doped In$_2$O$_3$. Similar behavior is expected upon doping with other transition metals opening up an avenue for the family of efficient transparent conductors mediated by magnetic interactions

Combining high conductivity with complete optical transparency: A band-structure approach

A comparison of the structural, optical and electronic properties of the recently discovered transparent conducting oxide (TCO), nanoporous Ca12Al14O33, with those of the conventional TCO's (such as Sc-doped CdO) indicates that this material belongs conceptually to a new class of transparent conductors. For this class of materials, we formulate criteria for the successful combination of high electrical conductivity with complete transparency in the visible range. Our analysis suggests that this set of requirements can be met for a group of novel materials called electrides.Comment: 3 pages, 3 figures, submitted for publicatio

Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and Tc_c in MgB2_2

Results of FLMTO-GGA (full-potential linear muffin-tin orbital -- generalized gradient approximation) calculations of the band structure and boron electric field gradients (EFG) for the new medium-T$_c$ superconductor (MTSC), MgB$_2$, and related diborides MB$_2$, M=Be, Al, Sc, Ti, V, Cr, Mo and Ta are reported. The boron EFG variations are found to be related to specific features of their band structure and particularly to the M-B hybridization. The strong charge anisotropy at the B site in MgB$_2$ is completely defined by the valence electrons - a property which sets MgB$_2$ apart from other diborides. The boron EFG in MgB$_2$ is weakly dependent of applied pressure: the B p electron anisotropy increases with pressure, but it is partly compensated by the increase of core charge assymetry. The concentration of holes in bonding $\sigma$ bands is found to decrease slightly from 0.067 to 0.062 holes/B under a pressure of 10 GPa. Despite a small decrease of N(E$_F$), the Hopfield parameter increases with pressure and we believe that the main reason for the reduction under pressure of the superconducting transition temperature, T$_c$, is the strong pressure dependence of phonon frequencies, which is sufficient to compensate the electronic effects.Comment: 12 pages, 3 figure

Dysfunction of the meibomian glands in patients with diabetes mellitus

The prevalence of diabetes mellitus has increased in recent decades. The most common ophthalmic manifestations of diabetes mellitus are retinopathy, epitheliopathies, corneal erosion and dry eye syndrome, the symptoms of which are more pronounced than in people without diabetes. The meibomian glands, which are the producer of various lipids, participate in the formation of the lipid layer of the tear film, which prevents the evaporation of the water-mucin phase and ensures the preservation of homeostasis of the ocular surface. Meibum is a lipid-rich secret produced by fully differentiated meibocytes. Impaired insulin secretion, insulin resistance, absolute insulin deficiency, as well as hyperglycemia, potentiate the development of oxidative stress and a cascade of metabolic changes, leading to a change in the anatomical and functional state of the meibomian glands, which affects the qualitative and quantitative secretion. Cytological abnormalities, as well as the structure of the excretory ducts of the meibomian glands in patients with long-term diabetes mellitus, were established using the method of laser scanning microscopy. Using the method of mass spectrometry, it is possible to determine the patterns of changes in the chemical composition of meibum in patients with diabetes mellitus. The data obtained can become one of the criteria for predicting the course, reflect the degree of compensation and / or progression of diabetes mellitus

Structural and superconducting properties of MgB2−x_{2-x}Bex_x

We prepared MgB$_{2-x}$Be$_{x}$ ($x=0$, 0.2, 0.3, 0.4, and 0.6) samples where B is substituted with Be. MgB$_{2}$ structure is maintained up to $x=0.6$. In-plane and inter-plane lattice constants were found to decrease and increase, respectively. Superconducting transition temperature $T_{c}$ decreases with $x$. We found that the $T_{c}$ decrease is correlated with in-plane contraction but is insensitive to carrier doping, which is consistent with other substitution studies such as Mg$_{1-x}$Al$_{x}$B$_{2}$ and MgB$_{2-x}$C$_{x}$. Implication of this work is discussed in terms of the 2D nature of $\sigma$-band.Comment: 3 pages,4 figures, to be published in Phys. Rev.

Electronic structure properties and BCS superconductivity in beta-pyrochlore oxides: KOs_2O_6

We report a first-principles density-functional calculation of the electronic structure and properties of the recently discovered superconducting beta-pyrochlore oxide KOs_2O_6. We find that the electronic structure near the Fermi energy E_F is dominated by strongly hybridized Os-5d and O-2p states. A van Hove singularity very close to E_F leads to a relatively large density of states at E_F, and the Fermi surface exhibits strong nesting along several directions. These features could provide the scattering processes leading to the observed anomalous temperature dependence of the resistivity and to the rather large specific heat mass enhancement we obtain from the calculated density of states and the observed specific heat coefficient. An estimate of T_c within the framework of the BCS theory of superconductivity taking into account the possible effects of spin fluctuations arising from nesting yields the experimental value.Comment: 5 pages, 4 figures; submitted for publicatio

Dielectric functions and collective excitations in MgB_2

The frequency- and momentum-dependent dielectric function $\epsilon{(\bf q,\omega)}$ as well as the energy loss function Im[-$\epsilon^{-1}{(\bf q,\omega)}$\protect{]} are calculated for intermetallic superconductor $MgB_2$ by using two {\it ab initio} methods: the plane-wave pseudopotential method and the tight-binding version of the LMTO method. We find two plasmon modes dispersing at energies $\sim 2$-8 eV and $\sim 18$-22 eV. The high energy plasmon results from a free electron like plasmon mode while the low energy collective excitation has its origin in a peculiar character of the band structure. Both plasmon modes demonstrate clearly anisotropic behaviour of both the peak position and the peak width. In particular, the low energy collective excitation has practically zero width in the direction perpendicular to boron layers and broadens in other directions.Comment: 3 pages with 10 postscript figures. Submitted to PRB on May 14 200

Electronic structure and light-induced conductivity in a transparent refractory oxide

Combined first-principles and experimental investigations reveal the underlying mechanism responsible for a drastic change of the conductivity (by 10 orders of magnitude) following hydrogen annealing and UV-irradiation in a transparent oxide, 12CaO.7Al2O3, found by Hayashi et al. The charge transport associated with photo-excitation of an electron from H, occurs by electron hopping. We identify the atoms participating in the hops, determine the exact paths for the carrier migration, estimate the temperature behavior of the hopping transport and predict a way to enhance the conductivity by specific doping.Comment: 4 pages including 4 figure