Strain Enhanced Superconductivity in Li-Doped Graphene

We present a new way to enhance the electron-phonon coupling constant and the critical superconducting temperature of graphene, significantly beyond all reported values. Using density functional theory, we explore the application effects of the tensile biaxial strain on the lithium intercalated graphene. Both effects together, the presence of adatom and the strain, trigger enhancement of critical temperature, up to 300\%, compared to non-strained lithium intercalated graphene.Comment: 6 pages, 6 figures, 1 tabl

Kristalizacija i optičke osobine monokristala LiNbO3

LiNbO3 single crystals were grown by the Czochralski technique in an air atmosphere. The critical crystal diameter Dc = 1.5 cm and the critical rate of rotation wc = 35 rpm were calculated from the dynamic of fluids equations for buoyancy-driven and forced convections under which the shape of the melt/crystal interface changed. The domain inversion was carried out at 1473 K using a 10 min 3.75 V/cm electric field. The obtained crystals were cut, polished and etched to determine the presence of dislocations and single domain structures. The lattice parameters a = 0.51494 nm, c = 1.38620 nm and V = 0.3186 nm 3 were determined by X-ray powder diffraction. The optical properties were studied by infrared spectroscopy in the wave number range 20 5000 cm -1 . With decreasing temperature, an atypical behaviour of the phonon modes, due to the ferroelectric properties of LiNbO3 single crystal, could be seen. The optical constants were calculated by Kramers-Kronig analysis and the value of the critical temperature was estimated. The obtained results are discussed and compared with published data.Monokristali LiNbO3 su dobijeni metodom rasta kristala po Čohralskom na vazduhu.Iz jednačina dinamike fluida za termičku i forsiranu konvekciju izračunati su kritični prečnik kristala Dc = 1,5 cm i vrednost kritične brzine rotacije wc = 35 o/min pri kojima dolazi do promene oblika fronta kristalizacije. Monodomenizacija je obavljena na temperaturi od 1473 K, pri električnom polju od 3,75 V/cm u trajanju od 10 minuta. Dobijeni kristali su sečeni, polirani i nagrizani da bi se odredilo prisustvo dislokacija i dobijanje monodomenske strukture. Rendgenskom difrakcijom praha odredjeni su parametri jedinične ćelije a = 0,51494 nm , c = 1,38620 nm i zapremina V = 0,3186 nm 3 .Optičke osobine su proučavane u infracrvenoj oblasti spektra u opsegu talasnih brojeva od 20–5000 cm –1 . Refleksioni spektri su snimani na različitim temperaturama da bi se posmatrao položaj fononskih modova. Sa opadanjem temperature može da se vidi atipično ponašanje fononskih modova što se pripisuje feroelektričnim osobinama monokristala LiNbO3. Optičke konstante monokristala LiNbO3 su dobijene korišćenjem Kramers-Kronig analize i procenjena je vrednost kritične temperature.Dobijeni rezultati su diskutovani i uporedjivani sa podacima iz literature.Dedicated to Professor Slobodan Ribnikar on the occasion of his 70th birthda

Enhancement of Electron-Phonon Coupling in Alkali-Doped Graphene and Thin MgB2 Layers

We present first principle investigation of enhancement of the electron-phonon coupling of superconducting alkali-doped graphene (e.g. LiC6-mono) and thin layers of MgB2 using density functional theory (DFT). These systems resemble in many ways, not only in geometry but as well as in electronic structure. Namely, like in alkali-C6, where we have carbon hexagons with alkali adatom in the hollow site, in MgB2 there are boron hexagons with Mg in hollow site. For instance, both theoretical and experimental studies of LiC6-mono proved a possibility of superconductivity in doped graphene. Continuing this investigation, we explored an enhancement of electron-phonon coupling and the raise of critical temperature in LiC6-mono. Inspired by similarity of intercalated graphite with MgB2, we studied the electron-phonon coupling in MgB2 thin films. We used ab-initio calculations of phonon dispersion and electron-phonon-coupling within framework of DFT using Quantum Espresso. Our research included dopant and strain effects on an enhancement of electron-phonon coupling. We demonstrated an increase of electron-phonon coupling and critical temperatures

Coordinate transformation based design of confined metamaterial structures

The coordinate transformation method is applied to bounded domains to design metamaterial devices for steering spatially confined electromagnetic fields. Both waveguide and free-space beam applications are considered as these are analogous within the present approach. In particular, we describe devices that bend the propagation direction and squeeze confined electromagnetic fields. Two approaches in non-magnetic realization of these structures are examined. The first is based on using a reduced set of material parameters, and the second on finding non-magnetic transformation media. It is shown that transverse-magnetic fields can be bent or squeezed to an arbitrary extent and without reflection using only dielectric structures.Comment: The previous version has been revised and considerably expande

All-angle left-handed negative refraction in Kagome and honeycomb lattice photonic crystals

Possibilities of all-angle left-handed negative refraction in 2D honeycomb and Kagome lattices made of dielectric rods in air are discussed for the refractive indices 3.1 and 3.6. In contrast to triangular lattice photonic crystals made of rods in air, both the honeycomb and Kagome lattices show all-angle left-handed negative refraction in the case of the TM2 band for low normalized frequencies. Certain advantages of the honeycomb and Kagome structures over the triangular lattice are emphasized. This specially concerns the honeycomb lattice with its circle-like equifrequency contours where the effective indices are close to -1 for a wide range of incident angles and frequencies.Comment: 7 pages, 8 figures, pd

Surface enhanced Raman spectroscopy of thiacyanine dye J-aggregates on single silver nanoaggregates

Dye-coated colloidal metal nanoparticles (NPs) exhibit interesting optical properties originating from the interaction between metal core and dye shell. Depending on the interaction mechanism between the two, optical properties of dyes or NPs can be changed separately or jointly within the dye-NP assembly [1]. Many of the recent studies are focused on dyes which are able to self-assemble in highly oriented structures called Jaggregates on the surface of metallic NPs [2,3]. Owing to the variety of mechanisms by which dyes and their J-aggregates can interact with metallic NPs, dye-NP assemblies can lead to applications ranging from nanoscale sensing [4] to advanced composite materials for novel active and nonlinear optical devices [5]. Here we study the influence of TC concentration on its J-aggregation on the surface of AgNPs assemblies using Raman mapping and atomic force microscopy (AFM). Aqueous solutions (colloids) of citrate stabilized AgNPs with an average diameter of ~10 nm are mixed with TC dye solution and then deposited onto freshly cleaved highly oriented pyrolytic graphite and mica surfaces. The spectral signature of citrate ions is identified by (i) the O-H band around 220 cm-1, (ii) the C-H band around 2950 cm-1 and (iii) pronounced blinking in the 1000-1800 cm-1 range. In contrast, dye molecules adsorbed on nanoparticles are recognized by several stable Raman bands between 200 and 1600 cm-1. In situ AFM measurements show that SERS 'hot spots' are formed either on large single nanoparticles (diameter > 100 nm) or within assemblies of small nanoparticles (with diameters in the 10 - 50 nm range). However, only the latter are found to yield a citrate or TC dye SERS signal. We find that the TC dye adsorbed on the surface of AgNP nanoassemblies always forms J-aggregates when the dye concentration in the TC-AgNP solution is varied between 0.5μM and 17μM. Even though, a clear SERS spectra of dye Jaggregates can be acquired for high dye concentration (17μM) the citrate ions always exist on the AgNP surface and so does their SERS signature in form of O-H (220 cm-1) and C-H (2960 cm-1) bands. Assemblies with low TC concentration (0.5μM) do not have a clear dye SERS spectra, but rather spectra similar to the one of citrate ions meaning that either not all AgNPs are dye coated, or rather that the amount of TC molecules adsorbed on the surface of the nanoparticle is small and hence not detectable. We are grateful to the Serbian Ministry of Education, Science and Technological Development for financial support through projects Nos. OI 171005, OI 172023. This work was performed in the context of the European COST Action MP1302 Nanospectroscopy.V International School and Conference on Photonics and COST actions: MP1204, BM1205 and MP1205 and the Second international workshop "Control of light and matter waves propagation and localization in photonic lattices" : PHOTONICA2015 : book of abstracts; August 24-28, 2015; Belgrad

Comparative studies on osmosis based encapsulation of sodium diclofenac in porcine and outdated human erythrocyte ghosts

The objective of our study was to develop controlled drug delivery system based on erythrocyte ghosts for amphiphilic compound sodium diclofenac considering the differences between erythrocytes derived from two readily available materials - porcine slaughterhouse and outdated transfusion human blood. Starting erythrocytes, empty erythrocyte ghosts and diclofenac loaded ghosts were compared in terms of the encapsulation efficiency, drug releasing profiles, size distribution, surface charge, conductivity, surface roughness and morphology. The encapsulation of sodium diclofenac was performed by an osmosis based process - gradual hemolysis. During this process sodium diclofenac exerted mild and delayed antihemolytic effect and increased potassium efflux in porcine but not in outdated human erythrocytes. FTIR spectra revealed lack of any membrane lipid disorder and chemical reaction with sodium diclofenac in encapsulated ghosts. Outdated human erythrocyte ghosts with detected nanoscale damages and reduced ability to shrink had encapsulation efficiency of only 8%. On the other hand, porcine erythrocyte ghosts had encapsulation efficiency of 37% and relatively slow drug release rate. More preserved structure and functional properties of porcine erythrocytes related to their superior encapsulation and release performances, define them as more appropriate for the usage in sodium diclofenac encapsulation process