Narrow Results

Density functional theory is adopted for the electronic structure and phonon calculation of Ca-doped bilayer graphene. The AA and AB stacking configurations are simulated, and the atoms are relaxed so that the atomic forces working on them are close to zero ($5.0\times 10^{-3}$ eV/Å). In the final relaxation, the symmetry of $C_{8}Ca{C}_8$ is $D_{2h}$ for AA stacking and $C_S$ for AB stacking. The formation energy of AA stacking (1.72 eV) is much lower than that of AB stacking ($8.07$ eV). According to the electronic structure calculations, the Dirac point shifts down from the Fermi level, indicating that the Ca atom behaves as an n-type dopant. The calculated Fermi velocities for pristine bilayer graphene are $7.69\times 10^5$ (AA stacking) and $7.75\times 10^5$ m/s (AB stacking). Those for Ca-doped bilayer graphene are $7.29\times 10^5$ (AA stacking) and $7.22\times 10^5$ m/s (AB stacking). Phonon calculations revealed that considering the vibrational effect, the defect concentration is $1.4\times 10^{16}$ cm${}^{-3}$ in the AA stacking system. Meanwhile, concentration is deficient in the AB stacking system due to the asymmetric defect configuration.

Two-dimensional (2D) correlation analysis is applied to the temperature dependence on the Raman spectra of superconducting YNi₂B₂C single crystal. 2D correlation analysis on the temperature dependence of the B_1g superconducting gap peak shows strong correlation with the NiB_1g phonon peak near 200 cm^-1. The analysis on the B_1g phonon peak itself shows that the peak is composed of essentially two peaks at 197 and 203 cm^-1, and only the weight of the two peaks changes as the temperature changes. The result suggests that there are two distinctive environments of the Ni-B bonds in YNi₂B₂C system at the superconducting state.

We investigate the superconducting transition temperature, T_c in the presence of the magnetic field, H in CeCoIn₅. It is shown that phonon-enhanced spin fluctuations drive this superconductivity once more as suggested by us (Phys. Rev.B61, 4289). We know the magnetic dependence of our transition temperature is in good correspondence with experimental data. It is elucidated that the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting states are closely related to the temperature gradient contributed by the external magnetic field.

We investigate theoretically manganese oxides where the colossal magnetoresistance (CMR) is observed. We obtain the Curie temperature T_c using a Kondo lattice model for paramagetic to ferromagnetic transition. We also acquire the charge-ordered transition temperature T_CO applying spin-Peierls transition. We calculate the isotope exponents of Mn ion and O ion such as α_c for T_c and α_CO for T_CO. Calculated α_c of O ion is less than 0.9 and α_CO is ~-(0.1~0.9).

We propose a photon correlation theory for ferroelectrics especially focused on KDP-type where we consider a photon field from its constituent electrons and ions. We derive a Curie–Weiss type dielectric susceptibility from the microscopic Hamiltonian and we obtain the ferroelectric transition temperature, T_c. We calculate the free energy by applying the bosonic operator formalism to the Hamiltonian. A linear temperature-dependent specific heat C_v conforming with experimental data for some ferroelectric materials is obtained. We calculate the isotope exponent, α, on T_c. We also derive phonon dispersion relations in the presence of electron-phonon interactions to show soft modes at T_c.

We investigate the metallic ferromagnetism for materials with incomplete 3d-orbitals. The ferromagnetism occurs in electrons of s-orbitals by phonon-enhanced spin flippings of d-electrons via s-d exchange interactions, which was discussed by us [Phys. Rev. B61, 4289 (2000)]. We know the electron-electron interaction, U_sd, mediated by phonon-enhanced spin flippings is repulsive for metallic ferromagnetic materials but attractive for high transition temperature superconductors (HTSC). The electron-electron interaction, U_sd, is an order of magnitude stronger than that by Kondo-type bare spin-flippings. We elucidate non-occurrence of ferromagnetism in Pd even though it has very strong exchange interactions. We also show that the charge sum rule is recovered in the case of inclusion of U_sd. We calculate the resistivity in normal states.

Phonon spectra as well as thermodynamic characteristic of superlattice are analyzed using the method of two-time dependent Green's functions. Free and internal energy of the system as well as specific heat and entropy are found. The temperature behavior of superlattice specific heat is compared with specific heats of bulk structures and thin films. It was shown that at extremely low temperature, superlattice and film specific heats are practically equal and considerably lower than specific heat of bulk sample. The consequences of this fact to the thermal, conducting and superconducting properties of materials was discussed.

In this paper, the comparison between power-law long-range interaction and Kac–Baker long-range interaction in the DNA molecule is investigated. This is done by employing an extended version of spin-like model of the DNA molecule with long-range interaction between intra-strand nucleotides and helicoidal coupling between inter-strand nucleotides when an RNA-polymerase binds to the DNA at biological temperature. Results show that LRIs have an undeniable effect on the DNA dynamics and that one is free to use either PLLRI or KBLRI to study DNA behaviors.

Bismuth telluride (Bi₂Te₃) is one of the most intricate materials with its semiconducting, insulating and pressure-induced superconducting properties. Although different theoretical works have been carried out to understand the confusing properties of Bi₂Te₃, information about the high pressure structural, elastic, mechanical and phonon properties of this significant material is still rare. Unlike earlier theoretical approaches, two-body interatomic potentials in the Morse potential form have been employed for the first time to predict the density, phase transition pressure, elastic constants, bulk, shear and Young moduli and elastic wave velocities of Bi₂Te₃ under pressures up to 12 GPa. $\alpha \to \beta$ phase transition pressure of Bi₂Te₃ was found to be 10 GPa. The results of above elastic quantities agree well with experiments and are better than some of the published theoretical data. In addition, the effect of pressure on the phonon dispersion and density of states (DOS) were also evaluated with the same potential and their results are satisfactory, especially for the low-frequency acoustic portions of phonons.

In this paper, interatomic interactions, zone-center phonon frequencies, mechanical properties, sound velocities and Debye temperature of indium thiospinels MIn₂S₄ (M = Cd, Zn and Mg) have been calculated using rigid-ion model. We found that the first neighbor interaction is stronger than the second neighbor interaction. We have compared our calculated results with the available experimental and theoretical data and find good agreement with the experimental results.

The ab initio computations have been performed to examine the structural, elastic, electronic and phonon properties of cubic $\mathrm{\text{La}}X$$(X=\mathrm{\text{Cd}}$, $\mathrm{\text{Hg}}$ and $\mathrm{\text{Zn}})$ compounds in the $B2$ phase. The optimized lattice constants, bulk modulus, and its pressure derivative and elastic constants are evaluated and compared with available data. Electronic band structures and total and partial densities of states (DOS) have been derived for $\mathrm{\text{La}}X$$(X=\mathrm{\text{Cd}}$, $\mathrm{\text{Hg}}$ and $\mathrm{\text{Zn}})$ compounds. The electronic band structures show metallic character; the conductivity is mostly governed by $\mathrm{\text{La}}$-$5d$ states for three compounds. Phonon-dispersion curves have been obtained using the first-principle linear-response approach of the density-functional perturbation theory. The specific heat capacity at a constant volume $C_V$ of $\mathrm{\text{La}}X$$(X=\mathrm{\text{Cd}}$, $\mathrm{\text{Hg}}$ and $\mathrm{\text{Zn}})$ compounds are calculated and discussed.

In present paper, it is focused on theoretical study of TiO₂ with particular emphasis on vibrational phonon properties by means of van der Waals three-body force shell model (VTBFSM). Present model includes van der Waals interactions (VWI) and three-body interactions (TBI) in the framework of both ion polarizable rigid shell model (RSM) with short-range interactions effective up to the second neighbor. The Coulombic ion–ion interaction contribution to the dynamical matrix is calculated by the Kellerman.¹ Therefore, it may be inferred that the present model is the most realistic model for complete harmonic dynamical behavior of the crystals. The lattice dynamical study of phonon properties of transition metal titanium dioxide (TiO₂) has been presented by means of VTBFS model. Titanium is an extremely light semiconducting material, exhibiting stable bcc structure in a certain temperature range 0–300 K normal pressures Debye temperature variation and combined density of states (CDS) of TiO₂ has been studied in this work. At high temperature, strength and oxidation resistance of Ti alloys has improved. The model predictions are found to be in reasonable agreement with experimental data.²

The structure of ZrB₂ under high-pressure was predicted by Particle Swarm Optimization method (CALYPSO). We investigated the structure stability, phonon dispersion curve, elasticity, electronic structure and thermodynamic properties of ZrB₂ under high-pressure and high-temperature via first-principles calculations. It maintained the hexagonal structure when the pressure lowers below 600 GPa at 0 K, which is confirmed by the calculated phonon dispersion curve. Studies indicate that the elastic modulus and Poisson’s ratio increase monotonically with pressure, as supported by some theoretical and experimental evidences. Calculated anisotropic factors demonstrate that compression and shear isotropy of ZrB₂ weakens as the pressure increases. Using the quasi-harmonic approximation Debye model, the Debye temperature, sound velocity, expansion coefficient, thermal capacity under the high-temperature and pressure were also predicted.

Electronic, mechanic and lattice dynamic properties of yttrium-based compounds, X₃Y, where X = Pd, Pt and Rh were investigated using the density functional theory. The electronic band calculations demonstrated that X₃Y compounds are metallic at the cubic crystal structures. The calculated elastic constants using the energy-strain method indicate that the three materials are mechanically stable. The calculated bulk modulus and Young’s modulus values suggest that the Pt₃Y is stiffer than that of the other two. The type of bonding and ductility in the X₃Y compounds were also evaluated based on their B/G ratios, Cauchy pressures (C${}_{12}$–C${}_{44}$) and band structure calculations. These compounds were found to be ductile in nature. The density functional perturbation theory was used to derive full phonon frequencies and total and projected phonon density of states. The computed full phonon spectra for X₃Y compounds show that these compounds in the L1₂ phase are dynamically stable. Debye temperature and specific heat of these compounds were also calculated and evaluated using quasi harmonic approximation.

This work performs a computational prediction on SnSe₂ and Te-doped bilayer material. Doping the Te element onto SnSe₂ can lead to a decrease in bandgap $E_g$ energy (from 1.95 eV to 0.94 eV) and cause a drop in both conduction band (CB) and valence band (VB), apart from the valence band maximum (VBM). This may lead to an increase in electrical conductivity. The decrease in $g_2$ parameter may result in an impact on electronic effective mass and transport properties when the SnSe₂ bilayer is doped with Te. A redshift observed on both the optical absorption and the dielectric constant plots may suggest that a tensile force could be possibly induced when we dope the SnSe₂ compound. The identical elastic constants of C${}_{14}$ and C${}_{65}$ evaluated could describe the nature of the close-packed hexagonal bilayer. Finally, the ductility slightly increases when we mix the Te element onto the SnSe₂ alloy. It is also worth noting that both SnSe₂ and Te-doped are dynamically stable, according to the phonon dispersion.

We investigate theoretically manganese oxides where the colossal magnetoresistance (CMR) is observed. Recent studies show that, if there are both an electron and a hole in the e_g-band of Mn, the Jahn–Teller distortions split the band into a conduction band and a localized band . We find a Kondo lattice model with hole conduction in manganese oxides when the electron in the localized e_g-band plays the role of the Kondo impurity. The Curie temperature (T_c) and the resistivity are calculated. We also calculate the antiferromagnetic temperature by a spin-Peierls transition.

We investigated theoretically ferromagnetic semiconductors. We obtained the Curie temperature T_c for carriers of the one-band and two-band types using a modified Kondo lattice model for paramagnetic to ferromagnetic transition. We also acquired the electrical resistivity and optical conductivity from the basic principles. We calculated the magnetic field dependence of the resistivity in the presence of the external magnetic field, H. A relatively lower value of T_c in the case of ferromagnetic semiconductors in comparison with CMR manganites is elucidated.

The method to obtain phonon dispersion of achiral single-wall carbon nanotubes (SWNTs) from 6×6 matrix proposed by Mahan and Jeon⁷ has been extended to chiral SWNTs. The number of calculated phonon modes of a chiral SWNT (10, 1) is much larger than that of a zigzag one (10, 0) because the number of atoms in the translational unit cell of chiral SWNT is larger than that of an achiral one even though they have relative similar radius. The possible application of our approach to other models with more phonon potential terms beyond Mahan and Jeon's model is discussed.

A simple and explicit formula for specific heat of icoschedral Al-Mn-Pd quasicrystals is obtained, theoretical predictions of which are in good agreement with that given by experiment.

Based on the elasto-/hydro-dynamic model the dynamic properties of the five-fold symmetry quasicrystals with point groups 5, are investigated, by using the finite difference method. The problems including dynamic initiation of crack growth and fast crack propagation of this material are studied. The results show that the phonon–phason coupling effect plays an important role to the dynamic properties of the quasicrystals.