Table of contents

Experimental studies indicate that 3D crystalline metal nanoclusters (NCs) intercalated under the surface of graphite have flat-topped equilibrated shapes. We characterize the shapes of these facetted NCs sandwiched between a blanketing graphene layer and the underlying graphite substrate. Specifically, we focus on the cases of fcc Cu and hcp Fe NCs. The analysis involves numerical minimization of the system energy for a specified NC volume and NC height, the latter corresponding to the separation between parallel top and bottom facets. Our numerical analysis quantifies how the distance of the side facet planes from center of the nanocluster varies linearly with a natural characteristic linear dimension of the nanocluster. Calculated shapes of fcc Cu and hcp Fe NCs are consistent with the hexagonal footprints observed in scanning tunneling microscopy studies.

For surface-mediated processes in general, such as epitaxial growth and heterogeneous catalysis, a constant slope in the Arrhenius diagram of the rate of interest, R, against inverse temperature, log R vs 1/T, is traditionally interpreted as the existence of a bottleneck elementary reaction (or rate-determining step), whereby the constant slope (or apparent activation energy, ${E}_{\mathrm{a}\mathrm{p}\mathrm{p}}^{R}$) reflects the value of the energy barrier for that elementary reaction. In this study, we express ${E}_{\mathrm{a}\mathrm{p}\mathrm{p}}^{R}$ as a weighted average, where every term contains the traditional energy barrier for the corresponding elementary reaction plus an additional configurational term, while identifying each weight as the probability of executing the corresponding elementary reaction. Accordingly, the change in the leading (most probable) elementary reaction with the experimental conditions (e.g. temperature) is automatically captured and it is shown that a constant value of ${E}_{\mathrm{a}\mathrm{p}\mathrm{p}}^{R}$ is possible even if control shifts from one elementary reaction to another. To aid the presentation, we consider kinetic Monte Carlo simulations of submonolayer growth of Cu on Ni(111) and Ni on Cu(111) at constant deposition flux, including a large variety of single-atom, multi-atom and complete-island diffusion events. In addition to analysing the dominant contributions to the diffusion constant of the complete adparticle system (or tracer diffusivity) and its apparent activation energy as a function of both coverage and temperature for the two heteroepitaxial systems, their surface morphologies and island densities are also compared.

The mechanisms of H atoms interactions with single-layer MoS₂, a two-dimensional transition metal dichalcogenide, are studied by static and dynamic DFT (density functional theory) modeling. Adsorption energies for H atoms on MoS₂, barriers for H atoms migration and recombination on hydrogenated MoS₂ surface and effects of H atoms adsorptions on MoS₂ electronic properties and sulfur vacancy production were obtained by the static DFT calculations. The dynamic DFT calculations give insight into the dynamics of reactive interactions of incident H atoms with hydrogenated MoS₂ at H atoms energies in the range of 0.05–1 eV and elucidate the competitive mechanism of hydrogen adsorption and recombination that limits hydrogen surface coverage at the level of 30%. Various pathways of S-vacancies production and H atoms losses on MoS₂ are calculated and the effects of MoS₂ temperature on these processes are estimated and discussed.

Interfacial electronic properties are greatly significant to study the photoelectric properties of semiconductor heterostructures. The novel heterostructures are constructed using perovskite 3D CsPbX₃ (X = Cl, Br, I) and 2D PtSe₂, and the structural and photoelectrical properties are studied by density functional theory. The band levels transform and interfacial charge transfer have serious differences at the interface of the CsPbX₃–PtSe₂ heterostructures. The CsPbCl₃–PtSe₂ and CsPbBr₃–PtSe₂ heterostructures show the type-I band arrangement, however, the CsPbI₃–PtSe₂ heterostructure demonstrate the type-II band arrangement. The difference in work function of the two semiconductors causes electrons to flow spontaneously at the interface. Moreover, the monolayer PtSe₂ can broaden the absorption spectrum of the CsPbX₃–PtSe₂ heterostructures, that effectively enhance absorption capability of the heterostructures, especially the CsPbI₃–PtSe₂ heterostructure. These results demonstrate PtSe₂ semiconductor materials can effectively improve the photoelectric performance of all-inorganic metal halide perovskite.

The electronic and optical properties of graphene quantum dots can be significantly tailored by doping it with heteroatoms, thus extending its potential applications. In this work, we have employed time-dependent density functional theory to systematically explore the effect of introduction of nitrogen atoms in varying concentration at pyridinic and graphitic configuration in armchair and zigzag-edged triangular shaped graphene quantum dots (TQDs) of different sizes. Our results indicate that the electronic band-gap in these N-doped systems can be effectively tuned by varying the configuration as well as concentration of dopants and nature of edge-termination. The variation of electronic band-gap is critically determined by the localized/delocalized nature of molecular orbitals and presence of additional energy levels due to dopant nitrogen atoms. However, the significance of these extra energy levels in modulating the optical properties (appearance of characteristic N-dopant absorption peaks) becomes conspicuous only for specific configuration and concentration of nitrogen atoms. In addition, our studies have attributed the strong dependence of blue/red-shift of absorption spectra and variation in the peak profile to position as well as concentration of dopant atoms and edge-termination pattern. Further, it is observed that the effect of increasing size of TQDs on the strength of most intense absorption peak of pyridinic N-doped TQDs is remarkably different from graphitic N-doped systems. This selective manipulation of optical properties in TQDs due to different N-doping pattern can open up new frontiers for rational design of novel optoelectronic devices.

Lattice dynamic properties of the tetragonal modification of ZnP₂ and CdP₂ crystals (space group P4₁2₁2, no 92) are calculated within the density functional theory. Theoretical results are shown to compare favorably with available Raman scattering and infrared reflection/transmission experimental data, which allows assignment of Raman-and infrared-active modes to the specific lattice eigenmodes. It is confirmed that several distinct features of vibrational spectra of these compounds steam from the presence of four phosphorous spiraling chains within crystallographic unit cell.

The effect of Sr doping in BaTiO₃ (BTO) with nominal compositions Ba_0.80Sr_0.20TiO₃ (BSTO) have been explored on its structural, lattice vibration, dielectric, ferroelectric and electrocaloric properties. The temperature dependent dielectric results elucidate the enhancement in dielectric constant and exhibit three frequency independent transitions around 335, 250 and 185 K, which are related to different structural transitions. All these transitions occur at lower temperature as compared with pristine BTO, however; remnant electric polarization (P_r) of BSTO is much higher than in BTO. The value of P_r is ∼5 μC cm⁻² at room temperature and the maximum P_r ∼ 8 μC cm⁻² is observed at tetragonal to orthorhombic and orthorhombic to rhombohedral transitions. The electro-caloric effect shows the maximum adiabatic change in temperature ΔT ∼ 0.24 K at cubic to tetragonal transition. The temperature dependent synchrotron x-ray diffraction and Raman results show correlations between P_r, crystal structure and lattice vibrations. Our results demonstrate the enhancement in ferroelectric properties of BTO with Sr doping. The origin of the enhancement in ferroelectric property is also discussed in correlations with the appearance of superlattice peak around room temperature due to TiO₆ octahedral distortion. These enhanced properties would be useful to design lead free high quality ferroelectric and piezoelectric materials.

Theoretical studies using the state-of-the-art density functional theory and dynamicalmean-field theory (DFT + DMFT) method show that weak electronic correlation effects are crucial for reproducing the experimentally observed pressure-induced phase transitions of calcium from β-tin to Cmmm and then to the simple cubic structure. The formation of an electride state in calcium leads to the emergence of partially filled and localized electronic states under compression. The electride state was described using a basis containing molecular orbitals centered on the interstitial site and Ca-d states. We investigate the influence of Coulomb correlations on the structural properties of elemental Ca, noting that approaches based on the Hartree–Fock method (DFT + U or hybrid functional schemes) are poorly suited for describing correlated metals. We find that only the DFT + DMFT method reproduces the correct sequence of high-pressure phase transitions of Ca at low temperatures.

We present a comprehensive, analytical treatment of the finite Kitaev chain for arbitrary chemical potential and chain length. By means of an exact analytical diagonalization in the real space, we derive the momentum quantization conditions and present exact analytical formulas for the resulting energy spectrum and eigenstate wave functions, encompassing boundary and bulk states. In accordance with an analysis based on the winding number topological invariant, and as expected from the bulk-edge correspondence, the boundary states are topological in nature. They can have zero, exponentially small or even finite energy. Further, for a fixed value of the chemical potential, their properties are ruled by the ratio of the decay length to the chain length. A numerical analysis confirms the robustness of the topological states against disorder.

A simple variational argument is presented which indicates that the spin–orbit coupling in itinerant systems can be enhanced by strong electronic correlations. The importance of the enhancement in the formation of the giant magnetic anisotropy found in the metallic paramagnetic and magnetically ordered states of compounds containing transition metal and light actinide elements (such as tetragonal Sr₂RhO₄, Sr₂IrO₄, the cubic uranium monochalcogenides and tetragonal URu₂Si₂) is discussed.

The graphitic carbon nitride (g-C₃N₄) is a promising layered two-dimension material with an opened bandgap. It is of interest to explore the tunability of the bandgap together with the magnetism by doping transition metal atoms. In this work, we investigated the transition metals (Mn, Fe, Co, Ni) and their hydroxides doped g-C₃N₄ monolayers. The electron correlations between the 3d electrons of the doped transition metal atoms are self-consistently calculated and analyzed based on the density functional theory. The magnetism, electronic band structures and optical properties are systematically investigated. It reveals that the transition metal doped g-C₃N₄ is ferromagnetic (FM) state at small doping concentration, where the two spins show different bandgaps. When the doping is high enough, it turns to metallic antiferromagnetic (AFM) state except that Mn doped g-C₃N₄ is metallic FM state. On another hand, the system shows variable absorption spectra at different doping level. When the vacancy sites are fully occupied, a large absorption peak appears around 1.5 eV suitable for visible light. Moreover, within the transition metal hydroxides doped g-C₃N₄, the global ground state shows as AFM, and the absorption spectra within low energy range is distinct due to the presence of hydroxyl group. Therefore, doping with transition metal atoms and hydroxides can effectively tune the bandgap, magnetism and optical properties of g-C₃N₄ so as to promote its applications.

The electron correlation and spin–orbit coupling (SOC) effects are investigated for body-centered-cubic tungsten and intrinsic & irradiative impurities using first principles calculations based upon the density functional theory. It is found that the electron correlation between the localized 5d electrons and the SOC effect are significant in modifying the band structures and the formation energies of defects. For the latter one, the involving of electron correlation always makes the defects stabler than the Perdew–Burke–Ernzerhof results, while the SOC contributes diversely for different defects. Moreover, the migration barrier of single tungsten vacancy moving in ⟨111⟩ direction is explored, where the inclusion of electron correlations remarkably decreases the migration barrier, while the influence of SOC is almost negligible. This study can help to validate the previous studies on irradiative defects in tungsten and improve the further investigations.

We consider an elastic helical medium formed by uniformly rotating a triclinic crystal around a given axis to constitute a helical medium giving rise to a material whose tensor stiffness rotates uniformly and varies along the helix axis. A detailed analysis of its elastic properties has been done previously. Here, we are concerned in analyzing the role of thermal coupling with heat flow through the dilatation tensor. Starting from a general dynamic description of the thermoelastic phenomena which takes into account the finite speed of propagation of thermal waves, we establish a set of equations for the strains, stresses, temperature and heat flow. These equations allow to calculate the band structure and the logarithmic ratio between longitudinal and transverse strains. We express our results for different values of the thermoelastic coupling and period of the helix which show remarkable modifications when compared with the case in which no thermoelastic coupling is present.

La based Co–Fe combined double perovskite (La_1.8Pr_0.2CoFeO₆) was synthesized and the dielectric (zero-field and in-field), magnetic, x-ray absorption and Raman spectroscopy measurements have been investigated for La_1.8Pr_0.2CoFeO₆ double perovskite. The existence of re-entrant cluster glass state is observed. The magneto–dielectric (MD) is found in two temperature regions (25–80 K and 125–275 K). It has been demonstrated that the observed MD at low and high temperatures are respectively due to the spin freezing and the spin–lattice coupling. Furthermore, the very large dielectric constant and the low loss suggest that La_1.8Pr_0.2CoFeO₆ is very important from the application point of view.

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO₂, which consists of a distorted lattice of Mn³⁺ spins with single-ion anisotropy. Physical property measurements on CuMnO₂ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to ∼25 meV above T_N = 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.

We investigate the properties of a quantum dot embedded between the normal and superconducting leads which is additionally side-attached to the topological superconducting nanowire, hosting the Majorana modes. This setup enables formation of the trivial (finite-energy) bound states induced in the quantum dot through the superconducting proximity effect, coexisting/competing with the topological (zero-energy) mode transmitted from the topological superconductor. We analyze their interplay, focusing on a role played by the external magnetic field. To distinguish between these bound states we analyze the qualitative and quantitative features manifested in the subgap charge tunneling originating under nonequilibrium conditions from the Andreev (particle to hole) scattering processes.