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One of the courses we had in the 4th year at the Physics faculty in Magurele was "Experimental Methods", presented to us by Professor Mihai Vasile. It was at that time when I first met Professor Mihai Popescu, who was an associate professor to the faculty as senior researcher at the Institute of Materials Physics and Technology (IMPT), which is in the nearby building. Professor Mihai Popescu served as Editor in Chief of I JOAM i until 2016, when he passed this baton to Professor Adrian Petris from the National Institute of Laser, Plasma and Radiation Physics (NILPRP). [Extracted from the article]

Optical spectroscopy provides non-invasive, in-situ, real time (and sometimes femtosecond time-resolved) probing of surfaces and buried interfaces over macroscopic areas, features that complement atomic-scale probes such as scanning-tunneling microscopy. Since its inception back in 1995, the International Conference on Optics of Surfaces and Interfaces (OSI) has included cutting-edge experimental and theoretical work in ellipsometry, reflectance-anisotropy spectroscopy, magneto-optics, Raman scattering and surface second-harmonic generation. The seven peer-reviewed papers in this special section of I physica status solidi (b) i represent examples of both traditional and emerging OSI themes, as discussed at the 13 SP th sp International Conference on Optics of Surfaces and Interfaces (OSI-13), and include both theoretical and experimental work. [Extracted from the article]

Ping-Zhan Si et al. (article number 1900356) have prepared bulk MnAl nanocrystalline alloy with a highest room-temperature coercivity up to 0.59 T by using a high-pressure torsion process. - The cover article is part of the Special Section "Advanced Magnetic Oxides", comprising one Feature Article and 8 Original Papers (see the Preface, article number 2000058). [Extracted from the article]

Measuring the electrical surface resistance of 2D materials without contact can provide a method for obtaining their intrinsic characterization. Herein, the aim is to show that a rutile dielectric resonator (RDR) can be used to measure the electrical surface resistance of conducting coatings deposited on substrates, at the resonance frequency. Moreover, it is known that the substrate exerts a strong influence capable of intrinsically modify the properties of 2D materials, as found in graphene. The RDR method is used for different samples of metals (Cu, Mo, Ti, and brass), carbon nanotubes (bucky paper), a film of compacted graphene flakes, a film of compacted graphene oxide flakes, and graphene obtained by chemical vapor deposition (CVD) on different substrates (SiO2/Si, quartz, and polyethylene terephthalate [PET]). The results show that reasonable values can be obtained for thin conducting materials with a thickness of not less than a few micrometers. In the case of graphene grown on a substrate, the presence of graphene is clearly detected but the resistivity value cannot be extracted. [ABSTRACT FROM AUTHOR]

Physical features of titanium monoxide layers deposited on p-GaN(0001) substrate are reported in this paper. TiO was deposited under ultrahigh vacuum at room temperature. As revealed by scanning tunneling microscopy (STM) the deposit had grainy structure. The X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) studies have shown that, depending on the altered by annealing morphology of the TiO-based film, work function of the system underwent changes and electronic structure of the deposit displayed metallic character. The formation of specific TiN and ON bonds is confirmed by spectroscopic techniques. Electrical conductance of the TiO-based/p-GaN interface has been studied by current-sensing atomic force microscopy (CS-AFM) with Au-covered tip. Formation of ohmic nano-contacts on p-GaN surfaces was observed over whole probed surface. The contacts were homogeneous in contrast to the grainy structure of TiO-based film. The results suggest that TiO may be considered as an intermediate semiconducting layer (ISL) useful for p-GaN electrical contact engineering. [ABSTRACT FROM AUTHOR]

Variation of the H2 fraction in the carrier gas affects partial pressures and homogeneity of the species at the growth front. Changing the H2:N2 ratio thus requires readjustment of the flows of the reacting species to keep the V/III ratio at the growth front constant. In this paper the complex effect of the carrier gas composition, i.e., the H2 + N2 mixture on optical and structural properties of GaN films grown by hydride vapor phase epitaxy (HVPE) is studied. With constant input flows of the main reactants but different H2 fractions in each experiment, good morphology was observed only in a small parameter window. Partial pressures at the growth front were calculated using a commercial virtual reactor simulation tool. After readjustment of the growth species partial pressures to those which previously provided good morphology, it became possible to achieve higher growth rates, comparable morphologies, and widths of the rocking curves (FWHM) for a wide range of hydrogen fractions (0-70%). Further flow pattern correction enabled growth of 1 mm thick layers for 40-75% H2 in the carrier gas. The results of the study allow to find optimal conditions for HVPE growth of thick GaN layers when scaling up from 2 to 3 in. substrate diameter. [ABSTRACT FROM AUTHOR]

It is demonstrated in this paper that 1.9-mm-thick gallium nitride grown by Hydride Vapor Phase Epitaxy (HVPE) on an ammonothermally grown GaN seed can reproduce the structural, in terms of defects, properties of the seed. The etch pit density and its correlation to the threading dislocation density in the ammonothermal GaN substrate and the HVPE-GaN layer is presented and analyzed. However, it has recently been observed that for HVPE-GaN thicker than 2 mm some additional defects are formed in the new grown material. Therefore, three HVPE growth runs were performed in the same experimental conditions, using three structurally identical ammonothermally grown GaN seeds of different thicknesses. The influence of the thickness of the seeds on the crystallization process and the properties of the HVPE-GaN layers is shown and discussed. [ABSTRACT FROM AUTHOR]

This paper reports the post-growth annealing effects of low-temperature grown Mg-doped InGaN. By using MOVPE, 1 μm-thick Mg-doped In xGa1- xN ( x ~ 0.36) films are grown at 570 °C. In order to activate the Mg acceptors, grown samples are treated by the conventional furnace annealing (FA) or the rapid thermal annealing (RTA). In the case of the FA at 650 °C for 20 min, the InGaN film is phase-separated. On the other hand, the RTA at a temperature higher than 700 °C enables us to get p-type samples. By using the RTA at 850 for 20 s, p-type samples with a hole concentration 1018-1019 cm−3 are successfully obtained without phase separation. [ABSTRACT FROM AUTHOR]

In this paper the results of experiments on terahertz generation from nitride light-emitting diode heterostructures under two-photon excitation by femtosecond laser pulses are reported. Dependencies of the photoluminescence and terahertz spectra on structural properties of the samples and intensity of laser pulses have been studied. It was found that the terahertz pulse amplitude increases and its spectrum shifts towards higher frequencies with an increasing number of quantum wells in the heterostructures. Photoluminescence spectral shape change at high excitation intensities was observed. The probable mechanisms explaining the observed experimental dependencies are discussed. [ABSTRACT FROM AUTHOR]

This paper reports magnetoresistance measurements in polycrystalline samples of K [ABSTRACT FROM AUTHOR]

In this paper atomic-layer deposition (ALD) Hf xAl1- xO y thin films with a wide range of metal components concentrations ( x = 0.2−0.8) for metal-insulator-metal (MIM) capacitors were investigated with emphasis on the chemical properties and phase composition. It was shown by X-ray diffraction (XRD) that all films are amorphous. X-ray photoelectron spectroscopy (XPS) revealed the same energy position with respect to the valence-band maximum of Al2p and Hf4f lines and a continuous shift of O1s and O-KLL lines toward the higher binding energy (0.45 and 2.2 eV, respectively) with increase of Al content. Both the shape and position of O1s and O-KLL lines indicated the inability to deconvolute it on sublines related to the binary oxides. The calculated modified Auger parameter indicated the change of the ionicity of the bonding in Hf xAl1- xO y films with change of the composition. Thus, it was proposed that Hf xAl1- xO y films in all the studied composition range represent amorphous solid solutions. The continuous change of energy bandgap measured by reflected electron energy loss spectroscopy (REELS) from ∼6.5 eV for Al2O3 to ∼5.4 eV for HfO2 and XRD data do not contradict this assumption. Correlation between structural properties of Hf xAl1- xO y films and electrical properties of MIM capacitors based on them was also shown, that is, continuous change in capacitance density (from 3.1 to 7.4fF/µm2), leakage current density and quadratic voltage coefficient of capacitance with composition change was observed. [ABSTRACT FROM AUTHOR]

THz emission from semiconductor surfaces is influenced by the sign and strength of the surface field as well as the relaxation dynamics of the photoinduced carriers. In this paper, an experimental study on argon implanted silicon surfaces is presented. Moderately doped silicon wafers were implanted with 100 keV argon ions with fluences ranging from [ABSTRACT FROM AUTHOR]

In this paper, we present results concerning ab initio calculations of the optical properties of group IV two-dimensional sheets and compare them with the prediction of an analytical model for excitons in two dimensions, which we explicitly derive. This comparison helps to understand the physics at the basis of the strong many-body effects present in the optical spectra of this class of materials. Although it demonstrates the need of full ab initio calculations for an accurate description of the two-dimensional excitons, it validates, at the same time, this simple approach to estimate, through a density functional theory calculation of the two-dimensional electronic and optical properties, the binding energy and the radius of the excitons in this class of materials. [ABSTRACT FROM AUTHOR]

Chemical vapor deposition (CVD) enables the mass synthesis of high-quality nanotubes. There is a large number of synthesis process parameters that have an influence on the type and form of the final product. The aim of the paper is the optimization of the synthesis process of multiwalled carbon nanotubes (MWCNTs) with the CVD method. Three synthesis parameters were selected (temperature, process time, and hydrogen flow rate) influencing the form and quality of the material produced. The experiments were carried out using our own approach to the conditions of carbon multiwalled nanotubes synthesis. The following research techniques were applied to examine the influence of selected parameters of the MWCNTs synthesis process: transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and Raman spectroscopy [ABSTRACT FROM AUTHOR]

Structured diamond films are required for several uses as photonic crystals, (bio-) sensors, biomedicine, etc. Often, these uses require fabrication of nano-sized features with the assistance of lithographic techniques. In this paper, we demonstrate the growth of diamond structures well ordered in periodic arrays. The technological process starts with ultrasonic seeding of Si substrates by ultra-dispersed detonation diamond nanoparticles. Then, a monolayer of polystyrene microspheres (PS) is deposited on seeded Si substrates by Langmuir-Blodgett method. The primary diameter of PS varies from 253 to 940 nm. The self-assembled PS are employed as a mechanical mask for reactive ion etching of the seeding layer via reducing the PS diameter down. The seeding structures remained under reduced PS were used for growing periodically ordered diamond arrays by microwave plasma-assisted chemical vapor deposition from a hydrogen rich gas mixture. Cross-section SEM images reveal that the diameter and size of the formed diamond structures increase both in vertical and lateral direction. At a certain deposition time, the structures start to coalesce into a continuous layer. [ABSTRACT FROM AUTHOR]

Curved graphene fragments can be considered as building blocks of carbon nanotubes. Thus, the size of graphene fragments may be considered as one of the most important characteristics of their structural disorder. In this paper, we have performed a comparative Raman study of CVD multi-walled carbon nanotubes (MWCNTs) and graphene flakes deposited on MWCNTs. Raman data have been considered in combination with HRTEM characterization of nanotubes. The basic attention has been paid to the behavior of D (disorder-induced), G (tangential mode), and 2D (two-phonon scattering) bands in Raman spectra in order to use them for MWCNT characterization. A ratio of intensities of 2D and D bands ( I2D/ ID) demonstrates almost a linear dependence on the mean diameter of MWCNTs produced with two different types of catalysts (see abstract figure). It should be mentioned that each type of catalyst provides the linear dependence with its own specific slope. The graphene fragments have been proposed to form a mosaic structure of nanotube walls. I2D/ ID ratio depends on the amount of graphene flakes deposited on nanotube surface via ethylene decomposition. A dependence of intensity ratio I2D/ ID on the diameter of nanotubes produced with different types of catalysts. [ABSTRACT FROM AUTHOR]

A simple formula describing the influence of the crystalline electric field free-ion excitations on the temperature dependence of the contribution of the s-f scattering to the thermal resistivity of normal rare-earth metals is presented. The corresponding formula for the electrical resistivity is also given and compared to the one being currently used. Theoretical electron-phonon scattering contributions derived in earlier papers and constant impurity scattering contributions are added to the derived s-f contribution formulae in order to fit the total electrical and thermal resistivity represented as functions of the temperature to experimental dependences on the temperature for DyIn [ABSTRACT FROM AUTHOR]

The behavior of radiation-induced complexes in MCZ silicon crystal after He irradiation was studied by highly sensitive and accurate infrared absorption spectroscopy. Samples from the same crystal that was used in the electron irradiation were employed. Many absorption lines with peak absorbance as small as 10−6 were successfully detected. Most of them were the same as those that had been observed in the electron irradiated samples and reported in the previous papers. Not only C-rich type complexes such as C iO i and IC iO i, but also C-lean type complexes such as I2O2 i and IO3 i were observed, in contrast to the electron case where only C-rich type had been observed in the same crystal. Peak absorbances for the He case are about half of those in the electron case. The He dose was 5 × 1013 cm−2, 1/200 of the electron dose of 1016 cm−2. Thus, the complex production rate per particle in the He case was about 100 times as large as that for electrons. The production range in the He case was restricted to within 20 microns from the surface. This range amounted to about 1/100 of that observed in the electron case where complexes had been formed uniformly throughout the 2 mm thick sample. Also, therefore, the complex concentration in the layer was about 100 times higher than that in the electron case. The annealing behavior of complexes was essentially the same as that in the electron case with minor differences. [ABSTRACT FROM AUTHOR]

Understanding anisotropic superconductivity has been one of the major theoretical challenges in the solid state physics. In this paper, we report a comparative study of chiral and non-chiral p-wave superconducting states by means of a generalized Hubbard Hamiltonian within the BCS formalism. The single-electron parameters were obtained by fitting ab initio band structure data supported by de Haas-van Alphen measurements in Sr2RuO4 and the electron correlation parameter was determined by the experimental critical temperature ( Tc). This study was carried out by looking at Tc, superconducting gap, Helmholtz free energy, and electronic specific heat. The results show that both chiral and non-chiral p-wave superconducting states possess the same Tc but different superconducting energy gaps. Moreover, both states have almost the same Helmholtz free energy, which leads to their possible coexistence. Finally, the calculated electronic specific heats without adjustable parameters for both p-wave superconducting states are compared with the experimental data obtained from Sr2RuO4, observing a better agreement for the non-chiral case for temperatures much lower than Tc. [ABSTRACT FROM AUTHOR]

Controversies on the "strain glass" concept are discussed in the present paper. Both ferroic glassy theory and traditional anelasticity theory can explain the "strain glassy behavior". However, due to the limited frequency range of internal friction measurements, it is difficult to distinguish whether "strain glass" is a nanodomain frozen transition or thermally activated process. [ABSTRACT FROM AUTHOR]

In this paper, we review the theory of spin switching for systems of different sizes, from a quantum mechanical master equation approach for small atomic clusters to the quasiclassical description of spin dynamics by a stochastic Landau-Lifshits-Gilbert equation in metallic nanoscale magnetic systems. We present characteristic results emphasizing the role of the quantum character of spin at different scales. Comparing the quantum mechanical and the quasiclassical approach to spin switching, we draw analogies between the factors affecting quantum and classical spin dynamics. We analyze assumptions used in each approach and emphasize the limits of applicability of the quantum mechanical and of the quasiclassical descriptions. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the magnetic properties of Gd-doped GaN. This diluted magnetic semiconductor shows hysteretic magnetization behavior at room temperature, which is attributed to ferromagnetism with a Curie temperature well above 300 K. However, the experimental results regarding the magnetic properties are not completely consistent and the microscopic origin for the reported magnetic properties is still unclear. We discuss the role of the growth method of the GaN comparing molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) including GaN doped during the growth process with Gd as well as Gd-implanted material. It seems that in general it is easier to obtain hysteretic magnetization behavior for MBE-grown material probably due to the higher oxygen and lower hydrogen content. An exception is Gd-implanted GaN MBE-grown on Si(111) where we observe no ferromagnetism. We will present experiments where by oxygen implantation and annealing the impurity concentration was manipulated. The role of native defects is addressed and new experiments where additional defects have been introduced by nitrogen implantation in MBE-grown GaN:Gd are discussed. We present our results on anomalous Hall effect observed in a Gd-implanted GaN/AlGaN heterostructure. [ABSTRACT FROM AUTHOR]

In recent years, the spin dynamics and spin-orbit interaction in GaAs-based two-dimensional hole systems (2DHS) have created a lot of attention. This is due to the complex structure of the valence band, with its p-like character, which leads to strong spin-orbit coupling. In this paper, we review our recent studies on hole spin dynamics and valence-band spin excitations in GaAs-based, p-modulation-doped quantum wells (QWs). In 2DHS with low carrier concentration, we demonstrate that maximizing the heavy-hole-light-hole band splitting by changing the QW width leads to long hole spin dephasing times at low temperatures. Different mechanisms for initializing a resident hole spin polarization by optical excitation are presented. To accurately determine hole spin dynamics parameters, the resonant spin amplification technique is utilized. The strong anisotropy of the hole g factor, and electrical g factor control are investigated, using this technique. In highly doped 2DHS, we use resonant inelastic light scattering (RILS) to study the spin splitting of the valence band. We observe a low-energy spin-density excitation (SDE), which is a measure of the spin splitting of the hole ground state. By varying the laser energy in the RILS experiment, we can resonantly probe the k dependence of the spin splitting. The spectral shape of the SDE depends on the orientation of the light polarizations relative to the crystal axes and reflects the in-plane anisotropy of the valence-band spin splitting. [ABSTRACT FROM AUTHOR]

In this paper, the first investigation of stimulated Raman scattering (SRS) effects in the tetragonal crystal LiLuF4 is presented. It is known as a promising host-material for trivalent lasant lanthanides (Ln3+). We discovered its three 'original' and one 'combined' SRS-promoting modes at room temperature under picosecond laser excitation in the visible and near-IR spectral range. Apart from multi-phonon Stokes and anti-Stokes generation, we observed cross-cascaded χ(3) ↔ χ(3) processes involving different pairs of SRS-active phonons. Furthermore, a comparative estimation of the stead-state Raman gain coefficient for first Stokes generation in LiLuF4 related to the most active SRS-phonon mode with ωSRS1 ≈ 323 cm−1 has been carried out for both the visible and near-IR. A brief review of the pioneering publications on laser action in Ln3+-ions of all known fluoride SRS-active single crystals is given in tabular form as well. [ABSTRACT FROM AUTHOR]

The compositions in the ternary chalcogenide systems from the demarcation region between the glass-formation domain (GFD) and the partially or fully crystalline formation domain seem to exhibit outstanding properties. We have shown in this paper that the compositions with the best memory-switching properties are situated at the border of the GFD in many systems. One can use this correlation to find new phase-change materials with better switching properties or to discover GFDs that were not observed yet. [ABSTRACT FROM AUTHOR]

In this paper, Cerium-doped lutetium yttrium oxyorthosilicate crystal (Lu,Y)2SiO5:Ce (LYSO:Ce) samples were used to thoroughly investigate the structural and optical anisotropy of the crystal. Results presented demonstrate that there is insignificant anisotropy in most of the properties of LYSO:Ce crystal - structural integrity, refractive index, transmittance, excitation and emission spectra, and photoluminescence decay time, which is a comparative advantage for various applications including preparation of optical ceramics. Besides, anisotropy phenomena in fine-structure of LYSO:Ce, afterglow, thermoluminescence and light output of samples with small size were observed. A mechanism of anisotropy in defect distribution is discussed. [ABSTRACT FROM AUTHOR]

In this paper, based on first-principles calculations, we have carried out a comprehensive study of substitutional oxygen defects in hexagonal silicon nitride (β-Si3N4). First of all, it is found that substitutional oxygens tend to form clusters at three different sites due to the intensive attractive interaction. By analyzing modified Bader charge and trap energy, we next discuss retention characteristic of the three clusters. The results manifest that all of the clusters are amphoteric defects with the ability to trap both electrons and holes. Moreover, every cluster is more powerful to trap holes, which indicates that oxygen clusters have a higher stability to hold holes than electrons. With regard to endurance characteristic, our studies reveal that the three clusters present differences after program/erase cycles, and then we explore the mechanism of endurance degeneration by nudged elastic band method (NEB). Taking full account of retention and endurance, we deem holes rather than electrons to be optimal to act as operational charge carriers for oxygen defects in Si3N4-based charge trapping memories. [ABSTRACT FROM AUTHOR]

In this paper, we report the results of investigation of 9.5 µm AlGaAs/GaAs and strain compensated 4.7 µm AlInAs/InGaAs/InP QCLs. We also show the results for 9.5 µm lasers based on lattice matched AlInAs/InGaAs/InP structures. The developed GaAs/AlGaAs lasers show the record pulse powers of 6 W at 77 K and up to 50 mW at 300 K. This has been achieved by careful optimization of the MBE growth process and by applying a high reflectivity metallic coating to the back facet of the laser. The 9.5 µm AlInAs/InGaAs/InP lasers utilize AlInAs waveguide and were grown exclusively by MBE without MOCVD regrowth. The short wavelength, strain compensated QCLs were grown by MOCVD. They represent state-of-the-art parameters for the devices of their design. For epitaxial process control, the atomic-force microscopy (AFM), high resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) were used to characterize the morphological and structural properties of the layers. The basic electro-optical characterization of the lasers is provided. We also present results of Green's function modeling of mid-IR QCLs and demonstrate the capability of non-equilibrium Green's function (NEGF) approach for sophisticated but still computationally effective simulation of laser's characteristics. [ABSTRACT FROM AUTHOR]

Stents are scaffold structures that are implanted in body vessels to keep them open. As the image on the front cover illustrates, the use of these biomedical devices can range from supporting blocked arteries in patients suffering from artery stenosis to supporting gullets in patients suffering from oesophageal cancer. The ability of these biomedical devices to fulfil their purpose relies heavily, amongst other things, on how they mechanically behave when they are infl ated and bent to fi t within the vessels. Some of these mechanical aspects are explored in the work by Attard, Gatt and Grima et al. using techniques normally applied to model auxetic structures. The paper on pp. 321–327 shows that with appropriate corrections, the existent models based on simple rotating rigid units can be used to predict the properties of more complex auxetic systems such as the oesophageal stent devices proposed by Ali and Rehman based on the rotating squares model by Grima and Evans. In the paper on pp. 328–337, stents based on hexagonal honeycomb geometries are analysed in terms of their expandability, foreshortening, dogboning and conformability. These effects may have a huge impact on the performance of a stent. The work highlights that, although particular geometries may perform very well in certain aspects, they may perform less well in others. This work has been funded by the Malta Council for Science and Technology through the R&I‐2011‐024 (Smart Stents) project awarded to the University of Malta, HM RD Ltd. part of the Velsud Group, and Tek‐Moulds Precision Engineering Ltd. [ABSTRACT FROM AUTHOR]

The CSW2020 should have been a joint venue for the 47th International Symposium on Compound Semiconductors (ISCS) and the 32nd International Conference on Indium Phosphide and Related Materials (IPRM). She has demonstrated the first ultrafast switchable terahertz polarisation modulators, which are based on arrays of GaAs nanowires, and have potential application in ultrafast terahertz-band communications. His work opened the doors to the use of these devices in THz applications, and to consider III-V compound semiconductor alloys as serious alternatives to Si in deep nanoscale CMOS applications. [Extracted from the article]

An analytical study on controlling quantum breathers in one-dimensional ferromagnetic XXZ spin chains through an oblique magnetic field is presented in this paper. Based on the time-dependent Hartree approximation and the semidiscrete multiple-scale method, we prove the existence of quantum breathers, analyze conditions for their appearance, and discuss their properties. Our results show that the appearance and features of quantum breathers can be controlled by varying the value of the oblique angle θ. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the efficiency of excitonic and activator recombination channels in activated alkali halides depending on the activator concentration. Both theoretical considerations and experimental verification of competition of these channels were carried out. For a model system a CsI-based scintillator was chosen. It is shown that at low temperature (20 K) the excitonic luminescence significantly drops in a hyperbolic law with increase of activator (Tl or In) emission. A corresponding increase of the activator-related recombination channel at 20 K was found to be only 20% of the excitonic yield of pure crystals, reaching a saturation at about 0.04 mol% of the activator. The energy loss observed is attributed to trapping of electron excitations at different centers. This is connected with the competition of electron-hole recombination and trapping of electrons on activators. Estimation of recombination efficiency was done based on the generalized Onsager model. Activator-induced screening, broad spatial distribution of thermalized electrons (mean thermalization length in CsI is about 180 nm) and strongly inhomogeneous electron track structure were taken into account in the model. Energy transfer from the excitons to the activator was suggested to be the main cause of the activator emission observed in doped CsI crystals at 20 K. [ABSTRACT FROM AUTHOR]

Vertically-oriented graphene (VG) with predominantly edge plane structure is considered as a promising catalyst support candidate for the methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). Inspired by the fact that catalyst nanoparticles are spontaneously attached to the exposed graphene edges, a novel highly-branched vertically-oriented graphene (HBVG) based catalyst support is proposed in the current work. With employing atmospheric normal glow discharge (ANGD) and microwave plasma-enhanced chemical vapor deposition (PECVD) methods, HBVG and maze-like VG (MVG) are synthesized on the surface of carbon paper (CP), respectively. Pt nanoparticles are deposited on pristine CP, HBVG-CP, and MVG-CP with the same route of repeated double-potential pulse electrodeposition (RDPE). Experimental results show that HBVG-CP with dense open graphitic edge planes can desirably provide a considerable number of nuclei sites for the fast nucleation and well dispersion of Pt nanoparticles. Meanwhile, a certain amount of oxygen-containing functional groups on the HBVG surface will benefit the rapid removal of CO and accumulated carbonaceous species. Based on the electrochemical measurements on Pt utilization efficiency, catalytic activity, CO tolerance, and long-term stability, Pt/HBVG-CP is demonstrated to be able to present enhanced catalytic performance over the Pt/CP and Pt/MVG-CP counterparts, holding a great potential for fuel cell applications. [ABSTRACT FROM AUTHOR]

Gigantic dielectric response (GDR) in polar insulators attracts significant scientific interest due its remarkable physics and the bright future of GDR materials in energy storage and memory devices. So far, the physical mechanism of extremely high complex dielectric permittivity (with the real part up to 106) is not established convincingly. Moreover, the application area of GDR materials is currently restricted due to unacceptable losses. In this paper, the polaronic approach for elucidation the GDR in solid polar insulators is developed based on a classical Fröhlich polaron model and polaronic phase transition criteria, pioneered by Fratini and Quémerais [Eur. Phys. J. B 14, 99 (2000)]. The dielectric functions of isolated Fröhlich polarons, a polaronic Wigner crystal, a 'polaronic liquid' and an 'electron liquid' exhibit surprising diversity. In particular, sufficient for GDR 'dipolar' permittivity and a moderate level of dielectric loss are expected for a 'polaronic liquid,' while extremely high metal-like negative real part of the complex permittivity and its excessive imaginary part are customary for the 'electron liquid.' Natural explanations for so-called 'low-temperature anomalies' in the GDR response observed in codoped nickel oxide, CaCu3Ti4O12 and other quaternary compounds as well as a realistic model for the spectra of optical conductivity and dielectric permittivity obtained from those materials in the far- and midinfrared regions at different temperatures are provided based on the proposed polaronic approach and the Fano resonance model. Furthermore, this polaronic approach provides physically transparent guidelines for the design of new GDR materials, i.e., selection of appropriate host insulators and dopants (codopants) to them, estimations of their critical concentrations and temperature ranges corresponding to a GDR with an acceptable loss level. Simulated effect of polaron concentration n on static dielectric permittivity of CaCu3Ti4O12 (CCTO) and NiO, codoped with Li, Ti (LTNO). [ABSTRACT FROM AUTHOR]

Chalcogenide glasses are interesting materials for their infrared transmitting properties and photo-induced effects. This paper reports the influence of light on the optical properties of Sb10S40Se50 thin films. The amorphous nature and chemical composition of the deposited film was studied by X-ray diffraction and energy dispersive X-ray analysis (EDAX). The optical constants, i.e., refractive index, extinction coefficient, and optical band gap as well as film thickness are determined from the measured transmission spectra using the Swanepoel method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The dispersion energy parameter was found to be less for the laser-irradiated film, which indicates the laser-irradiated film is more microstructurally disordered as compared to the as-prepared film. It is observed that laser-irradiation of the films leads to decrease in optical band gap (photo-darkening) while increase in refractive index. The decrease in the optical band gap is explained on the basis of change in nature of films due to chemical disorderness and the increase in refractive index may be due to the densification of films with improved grain structure because of microstructural disorderness in the films. The optical changes are supported by X-ray photoelectron spectroscopy data. [ABSTRACT FROM AUTHOR]

Cd1− xZn xS solid solutions have drawn much attention for the extensive potential applications. In this paper, we systematically computed the lattice constant, mixing enthalpy and bandgap for each configuration of zinc-blende (ZB) and wurtzite (WZ) type Cd1− xZn xS ( x = 0, 0.25, 0.5, 0.75, 1) solid solutions using the first-principles method, and quantitatively revealed that the Zn-S bond was the most critical factor in determining the relative stability among different configurations with the same composition, as well as the order of the bandgap. Using the configuration-averaged method, the average values of these properties, as well as the optical bowing coefficient, were obtained. We estimated the influence of the configuration average on these quantities, indicating that the configuration average significantly affected the mixing enthalpy, which explicitly determined the phase stability of the solid solutions. [ABSTRACT FROM AUTHOR]

In this paper, we report a theoretical prediction of the zero-temperature pressure-composition phase diagram for the binary Ti-C system using ab initio evolutionary methodology. Our ground state search successfully identifies the known stable Ti2C, Ti3C2, Ti6C5, and TiC structures at 0 GPa. Our study uncovers two new titanium carbides that are thermodynamically stable at high pressures: tetragonal Ti2C with I4/m symmetry and tetragonal Ti3C2 with P4/mbm symmetry. Both carbides are quenchable to ambient pressure and remain mechanically stable, and are potential candidates for hard materials. The I4/m and P4/mbm structures are also expected to be stable in the isoelectronic Zr-C and Hf-C systems at high pressures. Experimental synthesis of these novel carbides will be of great interest. [ABSTRACT FROM AUTHOR]

Auxetic fabrics have gained an increasing attention in the auxetic material area in recent years. Especially, auxetic fabrics fabricated based on both weft-knitted and warp-knitted structures have made some important progress. However, all the knitted auxetic fabrics reported so far are fabricated based on 2D knitted structures, although some of the weft-knitted auxetic fabrics are finally formed into 3D forms. This paper reports a novel kind of 3D auxetic fabric fabricated based on a warp-knitted spacer structure with adoption of a special geometrical configuration which is formed with parallelograms. The auxetic behavior of these fabrics is discussed in terms of tensile direction, geometrical parameter, and repeated extension. The shape fitting ability on a spherical surface is also demonstrated. The results shows that the 3D auxetic fabrics have auxetic behavior in all the fabric plane directions and the highest auxetic effect is obtained when stretched in the weft direction of the fabric. The 3D auxetic fabrics also display an excellent shape fitting ability, which makes them very attractive for various applications where fitting to the human body's shape is highly required, such as bra cups, protectors for knees and elbows, shoes covers, etc. [ABSTRACT FROM AUTHOR]

This paper presents the theoretical analysis of anisotropic low-field electron diffusion coefficient and mobility in wurtzite (WZ) indium nitride (InN). The electron diffusion coefficient and mobility as functions of temperature and doping concentration are investigated in detail. For low concentration, the diffusion coefficient and mobility is higher at low temperature. However, the results are opposite for high concentration. The anisotropy of the band structure is also taken into consideration by a Herring-Volt transform. The diffusion coefficient and mobility in the Γ- A direction ( c-direction) is higher than that in the Γ- M direction (basal plane) for a smaller effective mass. [ABSTRACT FROM AUTHOR]

The conduction electrons can deliver an indirect exchange interaction, namely the RKKY interaction, between impurities dissolved in nonmagnetic metal hosts by the interaction between the conduction electron spins and localized impurity spins. The effect of the laser field on the RKKY interaction between two localized spins in two-dimensional metals is studied by a quantum theory in this paper. The calculation results indicate an intense laser field can effectively manipulate this kind of indirect exchange interaction. The threshold of field strength for achieving this effect is given, and the exchange integrals at different impurity separations and laser field strengths are exhibited. It is discussed that both the finite spatial extension of the interaction potential and thermal energy of irradiation with a laser can influence the RKKY interaction. [ABSTRACT FROM AUTHOR]

This paper provides a comprehensive overview of state-of-the-art computational techniques to thermodynamically model magnetic and chemical order-disorder transitions. Recent advances as well as limitations of various approaches to these so-called lambda transitions are examined in detail, focussing on calphad models and first-principles methods based on density functional theory (DFT). On the one hand empirical implementations -based on the Inden-Hillert-Jarl formalism -are investigated, including a detailed interpretation of the relevant parameters, physical limiting cases and potential extensions. In addition, Bragg-Williams-based approaches as well as cluster-variation methods of chemical order-disorder transitions are discussed. On the other hand, it is shown how magnetic contributions can be introduced based on various microscopic model Hamiltonians (Hubbard model, Heisenberg model and beyond) in combination with DFT-computed parameters. As a result of the investigation we were able to indicate similarities between the treatment of chemical and magnetic degrees of freedom as well as the treatment within the calphad and DFT approaches. Potential synergy effects resulting from this overlap have been derived and alternative approaches have been suggested, in order to improve future thermodynamic modelling of lambda transitions. [ABSTRACT FROM AUTHOR]

The paper reviews recent advances in the application of graphene and related materials, viz. graphene nanoplatelets (GNP), functionalized graphene sheets (FGS), graphene oxide (GO), and reduced graphene oxide (rGO) as electrode building materials in dye-sensitized solar cell (DSC). Graphene promises multifaceted use in DSC: as prospective sensitizer, photoanode additive, current collector, and electrocatalyst for the redox-shuttle turnover. Of particular significance is the use of graphene as cathode catalyst in the state of art Co-mediated solar cells. Here, it can successfully replace platinum without any compromise on the electrode's catalytic activity, associated with good optical transparency. [ABSTRACT FROM AUTHOR]

The visualization of graphs describing molecular structures or other atomic arrangements is necessary in theoretical studying or examining nanostructures of several atoms. In the present paper, we review first the previous results obtained by drawing graphs with the help of various matrices as the adjacency matrix, the Laplacian matrix and the Colin de Verdiére matrix. We explain why they are applicable on if the atoms are on spherical surfaces. We have found recently a matrix [ABSTRACT FROM AUTHOR]

The 'Quo Vadis?' meeting in Bremen (March 2013) was a spectacular opportunity for people involved in molecular electronics to catch up on the latest, to think back, and to project into the future. This manuscript is divided into two halves. In the first half, we address some of the history and where the field has advanced in the areas of measuring, modeling, making, and understanding materials. We review some big ideas that have animated the field of molecular electronics since its beginning, and are at the height of interest and accomplishment at the moment. Then, we discuss six major areas where the field is evolving, and in which we expect to see very exciting work in the years and decades ahead. As a representative of one of the neer themes, the second half of the paper is devoted to molecular thermoelectrics. It contains some formalism, some results, some experimental comparison, and some intriguing conceptual questions, both for pure science and for device applications. An artist's rendition of a self-assembled monolayer of polyphenylether molecules on Au contacted by a Au STM. [ABSTRACT FROM AUTHOR]

Time-dependent quantum transport parameters for graphene nanoribbons (GNR) are calculated by the hierarchical equation of motion (HEOM) method based on the nonequilibrium Green's function (NEGF) theory [Xie et al., J. Chem. Phys. 137, 044113 (2012)]. In this paper, a new initial-state calculation technique is introduced and accelerated by the contour integration for large systems. Some Lorentzian fitting schemes for the self-energy matrices are developed to effectively reduce the number of Lorentzians and maintain good fitting results. With these two developments in HEOM, we have calculated the transient quantum transport parameters in GNR. We find a new type of surface state with delta-function-like density of states in many semi-infinite armchair-type GNR. For zigzag-type GNR, a large overshooting current and slowly decaying transient charge are observed, which is due to the sharp lead spectra and the 'even-odd' effect. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel semi-analytical approach to calculate first-order electron-vibration (EV) coupling constants within the framework of density functional theory. It combines analytical expressions for the first-order derivative of the Kohn-Sham operator with respect to nuclear displacements with coupled-perturbed Kohn-Sham theory to determine the derivative of the electronic density matrix. This allows us to efficiently compute accurate EV coupling constants.We apply our approach to describe inelastic electron tunneling (IET) spectra of metallic and molecular junctions. A gold junction bridged by an atomic chain is used to validate the developed method, reproducing established experimental and theoretical results. For octanedithiol and octanediamine single-molecule junctions, we discuss the influence of the anchoring group and mechanical stretching on the IET spectra. [ABSTRACT FROM AUTHOR]

Chiral systems are a class of structures, which may exhibit the anomalous property of a negative Poisson's ratio. Proposed by Wojciechowski and implemented later by Lakes, these structures have aroused interest due to their remarkable mechanical properties and numerous potential applications. In view of this, this paper investigates the on-axis mechanical properties of the general forms of the flexing anti-tetrachiral system through analytical and finite element models. The results suggest that these are highly dependent on the geometry (the ratio of ligament lengths, thicknesses, and radius of nodes) and material properties of the constituent materials. We also show that the rigidity of an anti-tetrachiral system can be changed without altering the Poisson's ratio. The anti-tetrachiral system, with the unit cell shown in red. [ABSTRACT FROM AUTHOR]

Auxetics are a modern class of material fabricated by altering the material microstructure. Unlike conventional materials, auxetics exhibit a negative Poisson's ratio when subjected to tensile loading. These materials have gained popularity within the research community because of their enhanced properties, such as density, stiffness, fracture toughness and dampening. This paper provides a critical oversight of the auxetic field with particular emphasis to the auxetic foams, due to their low price, easy availability and desirable mechanical properties. Key areas discussed include the fabrication method, the effects played by different parameters (temperature, heating time, cell shape and size and volumetric compression ratio), microstructural models, mechanical properties and potential applications. [ABSTRACT FROM AUTHOR]