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One of the rate limiting steps in catalytic ammonia synthesis is a nitrogen dissociation on a catalyst surface, which can be promoted by vibrational excitation of nitrogen molecules. In this work, the efficiency of plasma assisted heterogeneous nitrogen activation via vibrational excitation on the Ru surface is investigated. The analysis consists of two parts: in the first, ab initio calculations are performed for the adsorption and dissociation energy pathways of $${\text{N}}_{{\text{2}}}$$ on terrace and step sites and heterogeneous vibrational energy relaxation time of adsorbed nitrogen molecule is calculated using ab initio molecular dynamics. The second part includes the solution of the chemical kinetic equations for vibrationally excited molecules both in the gas phase and on the surface, and an estimation of energy cost of heterogeneous plasma activation of nitrogen. It is shown that heterogeneous vibrational energy relaxation of nitrogen molecule on the Ru surface is rather fast, and results in relatively high energy cost of vibrational nitrogen activation, which is 16 eV/molec for atmospheric pressure and 5 eV/molec for P = 4 Torr according to our calculations.

The effect of atomic structural defects on the tribological properties of a system with structural superlubricity based on a graphene bilayer with twisted layers is studied using the density functional theory (DFT) method and the Kolmogorov–Crespi and Lebedev a semiempirical interatomic-interaction potentials. The amplitudes of corrugation the potential energy surface of the interlayer interaction and the barriers of relative motion of ideal graphene layers and layers with vacancies are estimated. Macroscopic ultralow friction is revealed for a graphene bilayer with ideal twisted layers. The DFT calculations show that the presence of vacancies in this graphene bilayer gives rise to a static friction force of 12–16 pN per vacancy. The adequacy of using semiempirical potentials of the interaction between graphene layers to study the tribological properties of a graphene bilayer with atomic structural defects is evaluated., The work of I.V. Lebedeva was supported by the European Union MaX Center of Excellence (grant EU-H2020 no. 824143).

The electrical conductivity of the polyimide R-BAPB polymer filled with single-wall carbon nanotubes (CNT) with chirality (5,5) is modeled using a multi-scale approach. The modeling starts with molecular dynamics simulations of time-dependent fluctuating atomic configurations of polymer filled CNTs junctions. Then the atomic positions obtained in the first step are used to perform fully first-principles microscopic calculations of the CNTs junctions contact resistances using the Green's function based quantum transport technique. And finally, those contact resistances are supplied as an input to a statistical calculation of a CNTs ensemble conductivity using a Monte Carlo percolation model. The results of the first-principles calculations show a very strong dependence of the polymer filled CNTs junctions contact resistance on the geometry of CNTs junctions, including an angle $\varphi$ between nanotubes axes and the positions of polymer atoms around CNTs. Incorporating into the percolation model this strong dependence as well as CNTs agglomeration, pushed the calculated values of electrical conductivity just above the percolation threshold below 0.01 S/m, which is within the experimental range for composites with various base polymers. Possible mechanisms for further reduction of composites conductivity are discussed., Comment: 12 pages, 9 figures

The elastic properties of a flat graphene layer calculated using the classical empirical Tersoff, Brenner, AIREBO, PPBE-G, and LCBOP potentials have been compared. It is shown that, although the popular Brenner and AIREBO potentials have been developed formally taking into account the elastic properties of graphene, they give significant discrepancies in the values of Young’s modulus and Poisson’s ratio. Among the potentials under consideration, the LCBOP potential yields the values of these parameters that are closest to experimental data and results of ab initio calculations in the limit of zero elongation. For the quantitative simulation of mechanical phenomena in graphene-based systems, the potential parameters should be fitted to reproduce elastic properties of graphene completely taking into account system deformations and dependences of these constants on the elongation.

The potential energy surface (PES) of interlayer interaction of twisted bilayer graphene with vacancies in one of the layers is investigated via density functional theory (DFT) calculations with van der Waals corrections. These calculations give a non-negligible magnitude of PES corrugation of 28 meV per vacancy and barriers for relative sliding of the layers of 7 - 8 meV per vacancy for the moir\'e pattern with coprime indices (2,1) (twist angle 21.8$^\circ$). At the same time, using the semiempirical potential fitted to the DFT results, we confirm that twisted bilayer graphene without defects exhibits superlubricity for the same moir\'e pattern and the magnitude of PES corrugation for the infinite bilayer is below the calculation accuracy. Our results imply that atomic-scale defects restrict the superlubricity of 2D layers and can determine static and dynamic tribological properties of these layers in a superlubric state. We also analyze computationally cheap approaches that can be used for modeling of tribological behavior of large-scale systems with defects. The adequacy of using state-of-the-art semiempirical potentials for interlayer interaction and approximations based on the first spatial Fourier harmonics for the description of interaction between graphene layers with defects is discussed., Comment: 11 pages, 4 figures

Spintronics is a new trend in the development of nanoelectronics and, therefore, requires the development of new software tools for modeling aimed at the development and further miniaturization of spintronic devices. In this paper, we present a new software package for the technology computer aided design modeling of spintronic devices based on magnetic tunnel junctions. The theoretical models and scenarios of the software package are described. Examples of application of the software package to solving the main problems arising in the design of magnetoresistive memory elements and consistent miniaturization of these devices are considered.

Despite the high potential of endohedral metallofullerenes (EMFs) for application in biology, medicine and molecular electronics, and recent efforts in EMF synthesis, the variety of EMFs accessible by conventional synthetic methods remains limited and does not include, for example, EMFs of late transition metals. We propose a method in which EMF formation is initiated by electron irradiation in aberration-corrected high-resolution transmission electron spectroscopy (AC-HRTEM) of a metal cluster surrounded by amorphous carbon inside a carbon nanotube serving as a nanoreactor and apply this method for synthesis of nickel EMFs. The use of AC-HRTEM makes it possible not only to synthesize new, previously unattainable nanoobjects but also to study in situ the mechanism of structural transformations. Molecular dynamics simulations using the state-of-the-art approach for modeling the effect of electron irradiation are performed to rationalize the experimental observations and to link the observed processes with conditions of bulk EMF synthesis., A.S.S., I.V.L., A.A.K., and A.M.P. acknowledge the Russian Foundation of Basic Research (14-02-00739-a). I.V.L. acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and EU-H2020 project “MOSTOPHOS” (no. 646259). A.N.K., T.W.C., and R.L.M. acknowledge the ERC and EPSRC for financial support, and the Nanoscale & Microscale Research Centre (nmRC). T.Z., J.B., and U.K. gratefully acknowledge the funding by the DFG (German Research Foundation) and the Ministry of Science, Research and the Arts (MWK) of Baden Wuerttemberg in the framework of the SALVE (Sub Angstrom Low-Voltage Electron Microscopy) project.

The capacity of popular classical interatomic potentials to describe elastic properties of graphene is tested. The Tersoff potential, Brenner reactive bond-order potentials REBO-1990, REBO-2000, REBO-2002 and AIREBO as well as LCBOP, PPBE-G, ReaxFF-CHO and ReaxFF-C2013 are considered. Linear and non-linear elastic response of graphene under uniaxial stretching is investigated by static energy calculations. The Young's modulus, Poisson's ratio and high-order elastic moduli are verified against the reference data available from experimental measurements and ab initio studies. The density functional theory calculations are performed to complement the reference data on the effective Young's modulus and Poisson's ratio at small but finite elongations. It is observed that for all the potentials considered, the elastic energy deviates remarkably from the simple quadratic dependence already at elongations of several percent. Nevertheless, LCBOP provides the results consistent with the reference data and thus realistically describes in-plane deformations of graphene. Reasonable agreement is also observed for the computationally cheap PPBE-G potential. REBO-2000, AIREBO and REBO-2002 give a strongly non-linear elastic response with a wrong sign of the third-order elastic modulus and the corresponding results are very far from the reference data. The ReaxFF potentials drastically overestimate the Poisson's ratio. Furthermore, ReaxFF-C2013 shows a number of numerical artefacts at finite elongations. The bending rigidity of graphene is also obtained by static energy calculations for large-diameter carbon nanotubes. The best agreement with the experimental and ab initio data in this case is achieved using the REBO-2000, REBO-2002 and ReaxFF potentials. Therefore, none of the considered potentials adequately describes both in-plane and out-of-plane deformations of graphene., AMP and AAK acknowledge the Russian Foundation for Basic Research, Russia (Grant 18-02-00985). IVL acknowledges Grupos Consolidados del Gobierno Vasco (IT-578-13).

Thermally assisted magnetic random access memory (TAS-MRAM) nanodevices are promising candidates for future memory applications, because they provide non-volatile data storage at fast operation and low energy consumption. In this work, we analyze operating regimes of these devices using macrospin and micromagnetic modeling. It is shown that, for the successful performance of write and read operations in a TAS-MRAM device, the intrinsic magnetic anisotropy of ferromagnetic layers must not exceed some critical value. The relationship between the device parameters and the value of critical magnetic anisotropy is explained using analytical model and proved by the results of numerical simulation. This analysis is important for the future downscaling of TAS-MRAM nanodevices.

In this paper the results of validation of originally developed device-modeling tool on realistic organic-based devices are presented. Hole-only devices with α-NPD as a hole conductive layer are modeled. All material properties of α-NPD are obtained from first-principles calculations at atomistic level, and charge carrier mobilities are calculated using the Monte-Carlo method. A comparison of the simulation results with experimental data obtained for real devices shows a good correspondence. We also performed the sensitivity analysis of calculated results to model parameters. It is shown that the uncertainty in the dispersion of site energies σ makes the largest contribution to the uncertainty of predicted current-voltage characteristic of an organic electronic device.

The field dependence of critical voltages of switching of magnetic states of a synthetic multiferroic structure is studied based on a bifurcation analysis of Landau–Lifshitz–Gilbert equations with the torques caused by a tunneling spin transport, taking into account the voltage dependence of transferred spin momenta at the variations in the magnitude and direction of electric polarization. The voltage dependences of transferred spin momenta are determined based on the free electron model, taking into account exchange splitting of electron energy subbands in magnetic beaches and the effect of changing the tunnel barrier height at the variations in the polarization magnitude and state.

Incomplete stacking dislocations are predicted to form at edges of the shorter upper layer in two-dimensional hexagonal bilayers upon stretching the longer bottom layer. A concept of the edge Burgers vector is introduced to describe such dislocations by analogy with the Burgers vector of standard bulk dislocations. Analytical solutions for the structure and energy of edge stacking dislocations in bilayer graphene are obtained depending on the magnitude of elongation and angles between the edge Burgers vector, direction of elongation and edge. The barrier for penetration of stacking dislocations inside the bilayer is estimated. The possibilities to measure the barrier to relative motion of graphene layers and strain of graphene on a substrate by observation of edge stacking dislocations are discussed., The authors acknowledge the Russian Foundation of Basic Research (14-02-00739-a). IL acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and H2020-NMP-2014 project “MOSTOPHOS” (n. 646259).

Exchange-correlation functionals with corrections for van der Waals interactions (PBE-D2, PBE-D3, PBE-D3(BJ), PBE-TS, optPBE-vdW and vdW-DF2) are tested for graphene and hexagonal boron nitride, both in the form of bulk and bilayer. The characteristics of the potential energy surface, such as the barrier to relative sliding of the layers and magnitude of corrugation, and physically measurable properties associated with relative in-plane and out-of-plane motion of the layers including the shear modulus and modulus for axial compression, shear mode frequency and frequency of out-of-plane vibrations are considered. The PBE-D3(BJ) functional gives the best results for the stackings of hexagonal boron nitride and graphite that are known to be ground-state from the experimental studies. However, it fails to describe the order of metastable states of boron nitride in energy. The PBE-D3 and vdW-DF2 functionals, which reproduce this order correctly, are identified as the optimal choice for general studies. The vdW-DF2 functional is preferred for evaluation of the modulus for axial compression and frequency of out-of-plane vibrations, while the PBE-D3 functional is somewhat more accurate in calculations of the shear modulus and shear mode frequency. The best description of the latter properties, however, is achieved also using the vdW-DF2 functional combined with consideration of the experimental interlayer distance. In the specific case of graphene, the PBE-D2 functional works very well and can be further improved by adjustment of the parameters., AL and AP acknowledge the Russian Foundation for Basic Research (Grant 16-52-00181). IL acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and EU-H2020 project “MOSTOPHOS” (n. 646259).

The interlayer interaction in graphene/boron-nitride heterostructures is studied using density functional theory calculations with the correction for van der Waals interactions. It is shown that the use of the experimental interlayer distance allows one to describe the potential energy surface at the level of more accurate but expensive computational methods. On the other hand, it is also demonstrated that the dependence of the interlayer interaction energy on the relative in-plane position of the layers can be fitted with high accuracy by a simple expression determined by the system symmetry. The use of only two independent parameters in such an approximation suggests that various physical properties of flat graphene/boron-nitride systems are interrelated and can be expressed through these two parameters. Here we estimate some of the corresponding physical properties that can be accessed experimentally, including the correction to the period of the moiré superstructure for the highly incommensurate ground state of the graphene/boron-nitride bilayer coming from the interlayer interaction, the width of stacking dislocations in slightly incommensurate systems of boron nitride on stretched graphene, and shear mode frequencies for commensurate graphene/boron-nitride systems, such as a flake on a layer. We propose that the commensurate-incommensurate phase transition can be observed in boron nitride on stretched graphene and that experimental measurements of the corresponding critical strain can be also used to gain insight into graphene/boron-nitride interactions., A.V.L. and A.M.P. acknowledge the Russian Foundation of Basic Research (Grant No. 16-52-00181) and computational time on the Multipurpose Computing Complex NRC “Kurchatov Institute”. I.V.L. acknowledges financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13) and H2020-NMP-2014 Project “MOSTOPHOS” (No. 646259).

The possibility to control the commensurability and distance between graphene layers separated by a dielectric spacer is considered by the example of a heterostructure consisting of double-layer graphene separated by atomic layers of argon. Van der Waals-corrected density functional theory (DFT-D) is applied to study structural, energetic, and tribological characteristics of this heterostructure. It is found that, in the ground state, monolayer and bilayer argon spacers are incommensurate with the graphene layers, whereas sub-monolayer argon spacers which are commensurate with the graphene layers can exist only as metastable states. The calculations show that this incommensurability provides negligibly small static friction for relative motion of argon-separated graphene layers; therefore, such a heterostructure holds great promise for use in nanoelectromechanical systems. A scheme and operational principles of the nanorelay based on the revealed tribological properties of the heterostructure are proposed.

Dislocations corresponding to a change of stacking in two-dimensional hexagonal bilayers, graphene and boron nitride, and associated with boundaries between commensurate domains are investigated using the two-chain Frenkel-Kontorova model on top of ab initio calculations. Structural transformations of bilayers in which the bottom layer is stretched and the upper one is left to relax freely are considered for gradually increased elongation of the bottom layer. Formation energies of dislocations, dislocation width, and orientation of the boundary between commensurate domains are analyzed depending on the magnitude and direction of elongation. The second-order phase transition from the commensurate phase to the incommensurate one with multiple dislocations is predicted to take place at some critical elongation. The order parameter for this transition corresponds to the density of dislocations, which grows continuously upon increasing the elongation of the bottom layer above the critical value. In graphene and metastable boron nitride with the layers aligned in the same direction, where elementary dislocations are partial, this transition, however, is preceded by formation of the first dislocation at the elongation smaller than the critical one. The phase diagrams including this intermediate state are plotted in coordinates of the magnitude and direction of elongation of the bottom layer., The authors also acknowledge the Russian Foundation of Basic Research (Grant No. 14-02-00739-a) and computational time on the Supercomputing Center of Lomonosov Moscow State University and the Multipurpose Computing Complex NRC “Kurchatov Institute.” I.L. acknowledges the financial support from Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13) and H2020-NMP-2014 project “MOSTOPHOS” (Project No. 646259).

Principal characteristics of interlayer interaction and relative motion of hexagonal boron nitride (h-BN) layers are investigated by the first-principles method taking into account van der Waals interactions. Dependences of the interlayer interaction energy on relative translational displacement of h-BN layers (potential energy surfaces) are calculated for two relative orientations of the layers, namely, for the layers aligned in the same direction and in the opposite directions upon relative rotation of the layers by 180 degrees. It is shown that the potential energy surfaces of bilayer h-BN can be approximated by the first Fourier components determined by symmetry. As a result, a wide set of physical qualintities describing relative motion of h-BN layers aligned in the same direction including barriers to their relative sliding and rotation, shear mode frequency and shear modulus are determined by a single parameter corresponding to the roughness of the potential energy surface, similar to bilayer graphene. The properties of h-BN layers aligned in the opposite directions are described by two such parameters. The possibility of partial and full dislocations in stacking of the layers is predicted for h-BN layers aligned in the same and opposite directions, respectively. The extended two-chain Frenkel-Kontorova model is used to estimate the width and formation energy of these dislocations on the basis of the calculated potential energy surfaces., 13 pages, 6 figures

Modern methods of predictive modeling and progress in understanding the mechanisms of self-assembly of carbon atoms into perfect carbon nanostructures achieved using these methods are described. The concept of multiscale modeling which makes it possible to model the formation of carbon nanostructures based on the first principles and in which the role of experimental data is restricted to verification of the models developed is discussed.

The possibility of designing nanoelectromechanical systems (NEMS) based on relative motion or vibrations of graphene layers is analyzed. Ab initio and empirical calculations of the potential relief of interlayer interaction energy in bilayer graphene are performed. A new potential based on the density functional theory calculations with the dispersion correction is developed to reliably reproduce the potential relief of interlayer interaction energy in bilayer graphene. Telescopic oscillations and small relative vibrations of graphene layers are investigated using molecular dynamics simulations. It is shown that these vibrations are characterized with small Q-factor values. The perspectives of nanoelectromechanical systems based on relative motion or vibrations of graphene layers are discussed., Comment: 19 pages, 4 figures

The structure and properties of the (001) surface of indium oxide (In2O3) and indium tin oxide (ITO) are investigated theoretically. Ultrasoft pseudopotentials and the PBE exchange-correlation functional are used in our calculations. The calculated band structure of bulk In2O3, ITO, and their slabs with various surface oxidation levels are compared. Band structure features associated with surface states are detected. The dependence of the calculated electron work function for the In2O3 (001) and ITO (001) surfaces on the surface oxidation level is analyzed. It is found that the work function increases with the oxidation level of the surfaces.

According to experimental investigations, carbon nanotubes and fullerenes can be formed from graphite sheets. We present here the results of the molecular dynamics simulation of the folding of a single graphite sheet. The role of defects in this process is examined. Furthermore, the possible ways of reducing the number of defects in an imperfect fullerene, such as Stone–Wales rearrangements and the addition of carbon atoms and dimers, are investigated. Based on these investigations, we estimated the temperature range within which perfect nanostructures can be obtained.

The selection of a proper metal for replacement of polycrystalline silicon as the metal gate in future generation transistors has been hampered by pinning of the metal Fermi level at the metal/dielectric interface. Using monoclinic hafnia and zirconia as the gate dielectric we compare two different metal gate/gate dielectric interface structures where the oxygen affinity of the metal gate is either low or high under normal processing conditions. We show that for the metal gate with high oxygen affinity (molybdenum) the metal Fermi pinning is weaker than for the metal gate with low oxygen affinity (platinum). Through first principles calculations this behavior is explained as the result of an oxygen-rich interface for the former and a reduced interface for the latter. Based on these observations we propose a general model for Fermi pinning at the metal/metal oxide interface as mainly caused by low levels of interfacial oxygen. Such red uced interfaces may result from processing conditions as well as by the metal gate low affinity for oxygen.

Present-day computational techniques provide a possibility of evaluating properties of macrosystems using ab initio quantum chemistry and theories of elementary processes. Physical and chemical phenomena on very different timescales have to be taken into account (excitation, emission, chemical reactions, diffusion) at different levels of refining. This refining covers a very wide region of parameters starting from the structure of species up to the macro chemical mechanism of their conversion. This multilevel approach is described in detail in the paper and includes interaction and data transfer between different levels of phenomena description. In the framework of the approach, unknown properties of molecules, ions and atoms (structure, potential energy curves, transition dipole moments) are calculated based on quantum-chemical methods. The calculation results are used to evaluate rate characteristics of physical and chemical processes. The developed kinetic state-to-state scheme is then used to calculate the macro properties of the system under investigation. As an example of the multilevel approach, the emission properties of the Ar–GaI3 positive column discharge plasma were calculated using the Chemical Work Bench computational environment. The calculations yield the electron energy balance and emission efficiency as functions of plasma parameters.

Using charge transport in sintered ceramic samples it is observed that at all doping, including non superconducting overdoped samples, there exists a temperature in which below it d R /d T 2 plane is never overdoped. Data relating experimental Cooper pair density, conductivity and T c suggest that Homes’ relation might need a more specific definition of the conductivity σ .

The electron-density-functional approach is used to calculate the band structure of the CdGeAs2 semiconductor and the diluted magnetic semiconductor MnxCd1−xGeAs2, which has a ferromagnetic structure at x = 0.06. The results indicate that the incorporation of Mn increases the band gap at its center and leads to the formation of a band derived from Mn levels, whose energy is a weak function of wave vector. The calculation results agree with experimental data.

The well known phenomenon of the increase of Tc of YBCO after slow cooling or low temperature annealing without change of the oxygen content, was found also for the YBCO like tetragonal superconductors of (CaxLa1−x)(LauBa1−u)2Cu3Oy (this compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO). It has been observed at 150 and 100 °C for oxygen underdoped, optimally- and overdoped ceramics. The products retain their tetragonal unit cells. The possible reasons of this phenomenon are discussed.

Work with ceramic samples of (CaxLa1−x)(LauBa1−u)2Cu3Oy (these compounds have previously been denoted as CLBLCO, CLBCO or CaLaBaCuO) has shown that ordinary cooling rates used after annealing at high oxygen pressures may result in samples having higher (overestimated) Tc values when compared to the fast cooled ones having the same y. Thus, exposure of a sample of x = 0.1 and u = 0.175 to temperatures lower than 200 °C, which occur during cooling, leads (without oxygen absorption) to an overestimation of the Tc value. Increase of the cooling rate by dropping the pellet into the cold part of the autoclave suppresses this effect. Another effect is oxygen absorption during cooling when it is performed after high temperatures of annealing. The products of this absorption have an inhomogeneous distribution of oxygen ions in the ceramic grain. Their Tc values are also overestimated.

Using first-principles calculations we investigated the electronic properties of niobium and molybdenum mononitride (NbN and MoN, respectively) surfaces and their dependence on the surface orientation and termination. Work functions calculated for polar surfaces strongly depend on the surface termination, with nitrogen-terminated surfaces yielding the highest value, up to 6.6 eV for the fully N-covered MoN(0 0 1) surface. The dependence of the work function on coverage for the polar surface is monotonic for nitrogen termination, but does not follow the same trend in the case of metal termination. The work function decreases by ∼2 eV for MoN from a 100% metal-terminated surface to at least 25% metal-terminated surface, and then increases rapidly between 25% and 0% metal-terminated surface to recover its nitrogen termination result. The same trend was obtained for NbN. We observed a significant increase in the charge of the surface metal atom, up to its bulk value, with decreasing metal surface coverage. Electron transfer from the metal surface atoms to the subsurface atoms can explain these submonolayer metal coverage results. Finally we found that for the non-polar surfaces, the mononitrides work functions are generally lower than the work functions of the corresponding simple metal surfaces.

Different values of Tc were found for the 1:2:3 tetragonal superconductors (CaxLa1?x)(La0.25+xBa1.75?x)Cu3Oy (these compounds have previously been denoted as CLBLCO, CLBCO or CaLaBaCuO) prepared under various conditions but having the same x and y. The x = 0.1 ceramic samples prepared using various grindings differ in Tc by 8?K and the x = 0.4 ones prepared at various temperatures differ in Tc by 4?K. The different ratios between the unit cells and the various cation distributions might be the causes of this phenomenon.

The effect of a substitute ion on the lattice constants and T c of the optimal oxygen doped (M x La 1 - x )(La u Ba 1 - u ) 2 Cu 3 O y has been studied. This series is variously called CLBLCO, CLBCO or CaLaBaCuO when M = Ca 2 + , or YLBLCO when M = Y 3 + . We found that the lattice constants depend linearly on x and u. The size of M is the main contributor that affects the lattice constant a, while its valence mainly influences the lattice constant c. The size and valence of the ions located on the Ba-site have no significant effect on the lattice constant a, in contrast to the lattice constant c, for which the effect of the substitute on the Ba-site is significant. The Cu-O bond covalence increases, which is probably the reason for the increase of T c , m a x with the content of M. However, the decrease of the valence of M affects the increase of T c , m a x to a greater extent than the character of the Cu-O bond.

We report here that Fermi pinning at the polysilicon/metal-oxide interface causes high threshold voltages in MOSFET devices. In Part I, we investigated the different gatestack regions and determined that the polysilicon/metal oxide interface plays a key role on the threshold voltages. Now in Part II, the effects of the interfacial bonding are examined by experiments with submonolayer atomic-layer deposition (ALD) metal oxides and atomistic simulation. Results indicate that pinning occurs due to the interfacial Si-Hf and Si-O-Al bonds for HfO/sub 2/ and Al/sub 2/O/sub 3/, respectively. Oxygen vacancies at polysilicon/HfO/sub 2/ interfaces also lead to Fermi pinning. This fundamental characteristic affects the observed polysilicon depletion.

The conditions (temperature, pressure and time) for attaining equilibrium with oxygen for different YBCO materials are reported. The equilibrium oxygen content was used in order to determine more exact values for the enthalpies and entropies of interaction of YBa2Cu3Oy with oxygen. Internal equilibrium in YBCO is proposed and is able to explain the effects of ageing and low temperature annealing on Tc. Homogeneity and transitions to superconductivity of ceramics having equilibrium and non-equilibrium oxygen contents are discussed.

A mechanism of HfO2 and ZrO2 film growth in an ALD reactor from metal chlorides and water vapor is proposed to explain the experimentally observed features of the process: the formation of less than one monolayer per cycle and the dependence of the film growth rate (mass or thickness increment per cycle) and the residual chorine concentration on the process temperature. Energy parameters of the relevant gas-surface reactions are estimated from quantum-chemical density functional theory calculations. The rate constants of the elementary reactions are calculated using RRKM theory. ALD process simulations, based on the proposed mechanism and a transient plug-flow reactor model, are consistent with the available experimental data, indicating a decrease in deposition rate with increasing temperature. The reduction in deposition rate is attributed to the increased dehydroxylation of the film surface as the temperature is increased. The H2O adsorption energy was found to increase with increasing dehydroxylation from 33 to 53 kcal/mol for ZrO2 and from 35 to 51 kcal/mol for HfO2. A kinetic Monte Carlo model of film growth, based on the proposed mechanism, describes the observed temperature dependence of the residual chlorine concentration in the film in terms of the steric repulsion between chemisorbed surface groups and adsorbed MCl4 molecules (M=Zr, Hf).

YBa2Cu3Oy ceramics having different morphologies: low porosity, composed of well shaped and closely connected large crystal grains, high porosity, composed of badly packed irregularly shaped small grains, and of intermediate microstructure,were investigated with respect to reactions that lead to the deviation of oxygen content from its equilibrium value and loss of homogeneity of oxygen ion distribution. These reactions, which occur during quenching performed after the equilibrium of YBCO with oxygen has been attained, are the oxygen absorption during quenching in liquid oxygen or in common liquid nitrogen (containing 2–3% dissolved oxygen) and the oxygen release during quenching in pure liquid argon. In these reactions and in the reaction with CO2 impurities during attaining equilibrium (which also leads to an inhomogeneous sample), the reactivity of the investigated ceramics varied from high (in high porosity ceramic) to almost inert (in low porosity ceramic), while even an almost inert ceramic attains equilibrium under 0.01–1 atm O2 and 450–920 °C during less than 35 h. Therefore this ceramic is most suitable for preparation of the samples having any oxygen content. The interrelations found make it easier to search for the proper conditions of ceramic sintering, equilibration with oxygen and quenching for preparation of both homogeneous and inhomogeneous samples having the desired morphology and properties.

The absolute thermopower (S) of superconducting is p-type. It increases superlinearly with temperature up to K; at higher temperatures its rate of change decreases and saturation at is observed above 150 K. S(T) for resembles that of anhydrous NaxCoO2 (), but is lower by a factor of and its saturation is more pronounced. These results are consistent with electronic transport by strongly correlated electrons in a very narrow band.

We use a combination of first-principles density functional theory and non-perturbative scattering theory to investigate the effect of point defects on the hole leakage current through ultra thin oxide films. In particular, we consider O vacancies and B interstitial in monoclinic HfO2; five different defects are considered: (1) O vacancy at a three- and (2) a four-coordinated O site located in the HfO2 region, (3) O vacancy along a Hf–O–Si bond and (4) along a Si–O–Si bond at the Si/HfO2 interface, and (5) an interstitial Batom in the HfO2 region. We find that the neutral bulk vacancies and an interface vacancy along theSi–O–Si bond have little impact on the leakage current. On the contrary, an interface vacancy along the Hf–O–Si bridge and an interstitial B atom in the HfO2 region introduce states in the Si band gap and in the region just below the Si valence band edge thus strongly enhancing the leakage current at a low bias. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principle calculations demonstrate that the adsorption energies of water on the (001) and (101) surfaces of tetragonal zirconia (t-ZrO2) and on the (001) surface of monoclinic zirconia and hafnia (m-ZrO2 and m-HfO2) strongly depend on the surface hydroxylation degree. It is found that the fully hydroxylated t-ZrO2(001) surface undergoes a 2×2 reconstruction. The influence of surface dipole-dipole interactions on the calculated adsorption energy is discussed.

We investigate the effect of O vacancies and B interstitial on the leakage current through monoclinic HfO2. Transport is calculated from a combination of first-principles molecular dynamics simulation using local-orbital density functional theory, and non-perturbative scattering theory. Five different defects were considered: (1) O vacancy at a three- and (2) a four-coordinated O site located in the HfO2 region, (3) O vacancy along a Hf-O-Si bond and (4) along a Si-O-Si bond at the Si/HfO2 interface, and (5) an interstitial B atom in the HfO2 region. Bulk vacancies decrease the leakage current because they act as hole, not electron traps, while an interface vacancy along the Si-O-Si bond has a minor effect on the leakage. On the other hand, a vacancy along the Hf-O-Si bond creates states in the Si band gap that strongly enhance the leakage current at a low bias. The presence of an interstitial B atom in bulk HfO2 enhances the leakage current possibly through a resonant tunneling mechanism.

Equilibrium of 1:2:3 superconductors (Ca x La 1− x )(Ba 1.75− x La 0.25+ x )Cu 3 O y (this compound has in the past variously denoted as CLBLCO, CLBCO or CaLaBaCuO) with oxygen was studied for x =0.1 and x =0.4 in the temperature range of 150–950 °C under 1 atm. O 2 . The main process is the reversible reaction −Cu 3 2+ O 6.625 +0.25O 2 =−Cu 2 2+ Cu 3+ O 7.125 which is completed with the formation of one Cu 3+ . The enthalpy (in kJ/mol CLBLCO) and entropy (in J(mol CLBLCO) −1 K −1 ) of this reaction were calculated from the temperature dependence of the equilibrium constant. The values are Δ H =−33.1 and Δ S =−29.9 for x =0.1 and Δ H =−49.4 and Δ S =−42.7 for x =0.4. It was found that the equilibrium of ceramic pellet of CLBLCO with oxygen cannot be practically achieved below 300 °C while the equilibrium for powder is achieved even at 200 °C. Low rate of reaction of CLBLCO with oxygen causes the problem in low temperature equilibration. In contrast, diffusion of oxygen ions in the ceramics is observed even at 200 °C. This diffusion proceeds without the change of the oxygen content and may be applied in order to improve the homogeneity of the distribution of oxygen ions.

Liquid nitrogen, liquid oxygen and liquid argon were tested as coolers for quenching performed after equilibrium of (Ca x La 1− x )(Ba 1.75− x La 0.25+ x )Cu 3 O y ( x =0.1 or 0.4) with oxygen has been attained. This compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO. Absorption of O 2 during quenching in liquid oxygen was found and measured. Such samples are oxygen inhomogeneous. The transition to superconductivity is wide and begins 20 K higher than for a homogeneous sample having the same oxygen content. Liquid nitrogen, which is usually used as an external cooling agent containing 2–3% of oxygen, also leads to notable oxygen absorption. Only quenching in oxygen free liquid argon or in oxygen free liquid nitrogen does not cause oxygen absorption and may be used for the preparation of homogeneous samples of CLBLCO after equilibration at any temperature in the range from 300 to 950 °C.

The temperature of superconductive transitions T c was measured for a variety of compositions ( x , c ) of the 1:2:3 tetragonal superconductors (Ca x La 1− x )(Ba c− x La 2−c+ x )Cu 3 O y (so called CLBLCO or CLBCO or Ca–La–Ba–Cu–O) both in oxygen underdoped and overdoped region. The maximal values of T c ( T c,max ) were calculated directly from these T c vs y graphs. We found that instead of the 1:2:3 superconductors without calcium where substitution of Ba for La effectively decreases T c,max , Ca rich compositions of CLBLCO do not change T c,max over the wide range of this substitution. This fact and also the previously found increasing of T c,max with the Ca-content ( x ) are explained by the cluster component method; we found that the compositions with unit cells containing La on both the Y and Ba-sites, are nonsuperconductive, while all compositions containing unit cells with Ca on the Y-site have the highest T c,max =81 K . All the experimentally found T c,max ( x , c ) may be described by the varying of the ratios of these unit cells in the sample. The role of Ca in stabilizing of T c,max at substitution of Ba for La, and also in the increasing of T c,max with increasing the Ca-content is because of the much higher ability of Ca to bind La (from the Ba site) than the ability of La on the Y site for the same binding. This prevents the formation of La(LaBa)Cu 3 O y unit cells, the presence of which in CLBLCO decreases T c,max .

Hydrostatic pressure studies on the tetragonal 123 superconductor (La1-xCax) (Ba1.75-xLa0.25+x)Cu3Oy for x =0.1 and variable oxygen content y show that Tc increases rapidly under pressure (+5 K/GPa) for underdoped, optimally doped and overdoped samples. This points to a common cause for the pressure-induced changes in the superconductivity at all levels of doping, with negligible effects from charge transfer. Weak relaxation behavior in Tc is observed only for the most underdoped sample.

We report on a comparative study of the transport properties of lightly La-doped layered CaO $({\mathrm{CaMnO}}_{3}{)}_{m}$ $(m=1,2,$ and $\ensuremath{\infty})$ and similar ${\mathrm{Mn}}^{3+}{/\mathrm{M}\mathrm{n}}^{4+}$ ratios. In contrast to the large and rather nonsystematic variations of the resistivity $\ensuremath{\rho}(T)$ for samples with the same doping and various m, the absolute thermopower $(S)$ exhibits remarkable systematics. In the paramagnetic (PM) state, S varies linearly with temperature and its extrapolation to $T=0$ has a finite intercept. The intercept and the slope of these lines vary systematically with $m.$ In particular, within the experimental error, the inverse slopes of the straight lines increase linearly with $1/m,$ which in its turn varies linearly with the average number of Mn-Mn near neighbors. The implications of this simple systematics are discussed using a model developed by Cutler and Mott [Phys. Rev. 181, 1336 (1968)] for transport by activated hopping of carriers obeying nondegenerate statistics. This model (or a possible extension into the semidegenerate regime) leads also to a simple, qualitative interpretation of $S(T)$ of these materials on crossing the N\'eel temperature ${(T}_{N})$ from the PM state and below ${T}_{N}.$

“CLBLCO” is a family of 1-2-3 superconductors, represented by the chemical formula (Ca x La 1− x )(Ba c − x La 2− c + x )Cu 3 O y . By annealing in oxygen at various pressures, up to 4300 bar, at a temperature of 300°C and above, we have prepared specimens whose oxygen content covers a wide range, from y =6.4 (a totally “underdoped” insulating phase) to a highly overdoped phase with y ≳7.354. In contrast to YBCO, the specimens remain tetragonal for all values of x , c , and y . Measurements of the superconducting transition temperature, the resistivity, and the thermoelectric power, over a range of y values, 6.84≤ y ≤7.30 are presented, for samples with calcium content from x =0.1 to 0.4, and with c =1.75. We also report some transport data for CLBLCO with nickel impurities. Some significant differences were found between the properties of the underdoped and overdoped regimes. In contrast to YBCO, the thermoelectric power characteristics behave like those of most other high- T c cuprates. We found that a proposed universal relation between the thermoelectric power and the ratio between T c and its maximum value T c,max is not truly universal; it holds in some cases, but fails in others. The temperature-dependence of the resistivity shows a departure from linearity in the overdoped region, which is perhaps the signature of the appearance of a normal-state gap. The temperature at which the non-linearity sets in appears to increase as the doping level falls.

${\mathrm{BaCoS}}_{2}$ is a Mott insulator with a Ne\'el temperature ${(T}_{N})$ close to room temperature. In the ordered antiferromagnetic phase, the magnetic moments are confined in the ${\mathrm{Co}}_{2}{\mathrm{S}}_{2}$ planes, and arranged in alternatingly directed ferromagnetic chains. We report on resistivity and thermopower measurements and on room-temperature cell constants of samples of ${\mathrm{BaCoS}}_{z}$ with nominal sulfur content $1.9l~zl~2.1.$ We discuss the effect of sulfur content on cell constants and band filling. The main observation is a crossover from activated electronic transport at high temperatures, to high resistivity almost independent of temperature, and accompanied by a vanishingly small thermopower, at low temperatures (far below ${T}_{N}$). We interpret the transport mechanism in the low-temperature regime as tunneling in the ${\mathrm{Co}}_{2}{\mathrm{S}}_{2}$ planes, to nearest neighbors along the ferromagnetic chains and to next-nearest neighbors in the perpendicular direction, or alternatively to nearest neighbors along the ferromagnetic chains with conducting inclusions acting as shorts.

A characteristic feature of the high-temperature superconductors is the existence of a chemical composition that gives a maximum transition temperature, Tc, separating the so-called under-doped and over-doped regimes1, 2. This behaviour is thought to be universal for high-temperature superconductors. In practice, there are only a few high- Tc compounds for which the composition can be varied continuously throughout the entire doping range. Here we report a study of correlations between structure and Tc in a compound with the ‘123’ structure in which both the under-doped and over-doped regimes can be accessed. We observe a clear scaling between Tc and the buckling of the copper oxide planes; both go through a maximum at the same oxygen composition (and hence doping level), so implying a common origin. Previous work has shown that, for a fixed chemical composition, increased CuO2 plane buckling lowers the transition temperature3,4,5,6,7,8,9,10,11. Thus the observation of a maximum in the buckling at the maximum Tc indicates that, as the composition is changed to increase Tc, there is a structural response that competes with superconductivity.