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Colossal permittivity with ultralow dielectric loss in In + Ta co-doped rutile TiO2

Colossal permittivity (CP) materials have many important applications in electronics but their development has generally been hindered due to the difficulty in achieving a relatively low dielectric loss. In this work, we report an In + Ta co-doped TiO2 material system that manifests high dielectric permittivity and low dielectric loss based on the electron-pinned defect-dipole design. The dielectric loss can be reduced down to e.g. 0.002 at 1 kHz, giving high performance, low temperature dependent dielectric properties i.e. er > 104 with tanδ < 0.02 in a broad temperature range of 50–400 K. Density functional theory calculations coupled with the defect analysis uncover that electron-pinned defect dipoles (EPDDs), in the form of highly stable triangle-diamond and/or triangle-linear dopant defect clusters with well-defined relative positions for Ti reduction, are also present in the host material for the CP observed. Such a high-performance dielectric material would thus help for practical applications and points to further discovery of promising new materials of this type.

Anharmonic effects on thermodynamic properties of a graphene monolayer

We extend the unsymmetrized self-consistent-field method (USF) for anharmonic crystals to layered non-Bravais crystals to investigate structural, dynamical and thermodynamic properties of a free-standing graphene monolayer. In this theory, the main anharmonicity of the crystal lattice has been included and the quantum corrections are taken into account in an -expansion for the one-particle density matrix. The obtained result for the thermal expansion coefficient (TEC) of graphene shows a strong temperature dependence and agrees with experimental results by Bao et al. (Nat. Nanotechnol., 4 (2009) 562). The obtained value of TEC at room temperature (300 K) is and it becomes positive for . We find that quantum effects are significant for . The interatomic distance, effective amplitudes of the graphene lattice vibrations, adiabatic and isothermal bulk moduli, isobaric and isochoric heat capacities are also calculated and their temperature dependences are determined.

Single and Paired Point Defects in a 2D Wigner Crystal

Using the path-integral Monte Carlo method, we calculate the energy to form single and pair vacancies and interstitials in a two-dimensional Wigner crystal of electrons. We confirm that the lowest energy point defects of a 2D electron Wigner crystal are interstitials, with a creation energy roughly 2/3 that of a vacancy. The formation energy of the defects goes to zero at melting, suggesting that point defects may be the melting mechanism and that the melting could be a continuous transition. In addition, we find that the interaction between defects is strongly attractive, so that most defects will exist as bound pairs.

Formation energy and interaction of point defects in two-dimensional colloidal crystals

The manipulation of individual colloidal particles using optical tweezers has allowed vacancies to be created in two-dimensional (2D) colloidal crystals, with unprecedented possibility of real-time monitoring the dynamics of such defects [Nature (London) 413, 147 (2001)]. In this paper, we employ molecular dynamics simulations to calculate the formation energy of single defects and the binding energy between pairs of defects in a 2D colloidal crystal. In the light of our results, experimental observations of vacancies could be explained and then compared to simulation results for the interstitial defects. We see a remarkable similarity between our results for a 2D colloidal crystal and the 2D Wigner crystal [Phys. Rev. Lett. 86, 492 (2001)]. The results show that the formation energy to create a single interstitial is 12%\char21{}28% lower than that of the vacancy. Because the pair binding energies of the defects are strongly attractive for short distances, the ground state should correspond to bound pairs with the interstitial bound pairs being the most probable.

A quantum Monte Carlo study on electron correlation in all-metal aromatic clusters MAl4− (M = Li, Na, K, Rb, Cu, Ag and Au)

Using fixed-node diffusion quantum Monte Carlo (FN-DMC) simulation we investigate the electron correlation in all-metal aromatic clusters MAl4(-) (with M = Li, Na, K, Rb, Cu, Ag and Au). The electron detachment energies and electron affinities of the clusters are obtained. The vertical electron detachment energies obtained from the FN-DMC calculations are in very good agreement with the available experimental results. Calculations are also performed within the Hartree-Fock approximation, density-functional theory (DFT), and the couple-cluster (CCSD(T)) method. From the obtained results, we analyse the impact of the electron correlation effects in these bimetallic clusters and find that the correlation of the valence electrons contributes significantly to the detachment energies and electron affinities, varying between 20% and 50% of their total values. Furthermore, we discuss the electron correlation effects on the stability of the clusters as well as the accuracy of the DFT and CCSD(T) calculations in the present systems.

A quantum Monte Carlo study on electron correlation effects in small aluminum hydride clusters

Using the fixed-node diffusion quantum Monte Carlo (DMC) method, we investigate the electron correlation in several small relaxed and unrelaxed neutral, cationic, and anionic aluminum hydride clusters. We calculate the clusters total energies and use them to obtain the binding energies. Our results are in very good agreement with the available ab initio calculations and anion-photoelectron spectroscopy experiments. The calculations have also been performed in the Hartree–Fock (HF) approximation in order to analyse the impact of electron correlation. For the total atomic binding energy, i.e. the energy necessary to separate all the atoms, this impact varies from 20% up to about 50%, whereas for the electron binding energy, i.e. the energy required to detach or attach an electron to the cluster, it ranges from 1% up to 73%. The decomposition of the electron binding energies clearly shows that both charge redistribution and electron correlation are important for determining the detachment energies, whereas electrostatic and exchange interactions are responsible for the ionization potential.

Mechanism of point-defect diffusion in a two-dimensional colloidal crystal

The dynamics and mechanism of migration of a vacancy point defect in a two-dimensional (2D) colloidal crystal are studied using numerical simulations. We find that the migration of a vacancy is always realized by topology switching between its different configurations. From the temperature dependence of the topology switch frequencies, we obtain the activation energies for possible topology transitions associated with the vacancy diffusion in the 2D crystal.

Quantum monte carlo study of the energetics of small hydrogenated and fluoride lithium clusters.

An investigation of the energetics of small lithium clusters doped either with a hydrogen or with a fluorine atom as a function of the number of lithium atoms using fixed‐node diffusion quantum Monte Carlo (DMC) simulation is reported. It is found that the binding energy (BE) for the doped clusters increases in absolute values leading to a more stable system than for the pure ones in excellent agreement with available experimental measurements. The BE increases for pure, remains almost constant for hydrogenated, and decreases rapidly toward the bulk lithium for the fluoride as a function of the number of lithium atoms in the clusters. The BE, dissociation energy as well as the second difference in energy display a pronounced odd–even oscillation with the number of lithium atoms. The electron correlation inverts the odd–even oscillation pattern for the doped in comparison with the pure clusters and has an impact of 29%–83% to the BE being higher in the pure cluster followed by the hydrogenated and then by the fluoride. The dissociation energy and the second difference in energy indicate that the doped cluster Li3H is the most stable whereas among the pure ones the more stable are Li2, Li4, and Li6. The electron correlation energy is crucial for the stabilization of Li3H.

A quantum Monte Carlo study of the structural and electronic properties of small cationic and neutral lithium clusters

Using the fixed-node diffusion quantum Monte Carlo method, we calculate the total energy of small cationic and neutral lithium clusters. We estimate the ionization potential, atomic binding energy, dissociation energy, and the second difference in energy. We present a critical analysis of the structural and electronic properties of the clusters. The bond lengths and binding and dissociation energies obtained from the calculations are in excellent agreement with the available experimental results. A comparative analysis of the dissociation energy and the second difference in energy indicates that the cationic clusters Li3+, Li5+, and Li7+ are the most stable ones. We have also studied the electron correlation effects in the lithium clusters. The cationic clusters of odd-number size are relatively more favored in terms of correlation energy than their neighbors of even-number size. In the range of cluster sizes under investigation, we find that the contribution of electron correlation to ionization potential is not larger than 28% of its total values, whereas it enhances significantly the dissociation energy of the clusters reaching up to 70% of its total values for the most stable ones.

Electron Correlation Effects in All-Metal Aromatic Clusters: A Quantum Monte Carlo Study.

The electron correlation effects on the atomic and electronic structure of a few multiaromatic clusters XAl3– (X = Si, Ge, and Sn) are investigated using the diffusion quantum Monte Carlo method (DMC). We found that the vertical detachment energies are in very good agreement with available photoelectron spectroscopy data. The total binding energy of the clusters is dominated by the electron correlation energy contribution with about 55% of its total values. However, the binding energy gained in adding the dopant X into the Al3– unit to form the cluster XAl3– has been almost equally distributed among the contributions from Hartree–Fock (HF) and correlation energies. The resonance energy is found to be about 100 kcal/mol, which is roughly five times that of the organic aromatic compounds such as benzene. On the basis of some thermodynamical extremum principles we found that the order of decreasing stability of the clusters is SiAl3– ≈ GeAl3– > SnAl3–, the electron correlation impacting more the most stable ...