H. M. Tütüncü

Structural, elastic, electronic, and phonon properties of zinc-blende and wurtzite BeO

The results of ab initio calculations of the structural, elastic, electronic, and phonon properties of BeO in both zinc-blende and wurtzite structures are presented. Our calculations are based on the application of plane-wave basis, pseudopotentials, and the generalized gradient approximation of the density functional scheme. Our total energy calculations indicate that the wurtzite phase has lower energy (around 5.8 meV) than zinc-blende phase. It is found that for the two crystal phases the values of the equilibrium atomic volume and of the bulk modulus and its pressure derivative are almost identical. The zinc-blende and wurtzite phases are predicted to be characterized with indirect (and smaller) and direct band gaps, respectively. The maximum longitudinal optical and transverse optical phonon frequencies in the zinc-blende phase compare well to the average of the corresponding E1 and A1 modes in the wurtzite phase.

Electronic structure and zone-center phonon modes in multiferroic bulk BiFeO3

We present ab initio investigations of the electronic structure and zone-center phonon modes in bulk BiFeO3 with rhombohedral R3c symmetry. The electronic structure has been studied using the plane wave pseudopotential method, with electron-electron interaction treated within the local spin density approximation (LSDA), a spin-polarized generalized gradient approximation (σGGA), and the LSDA+U and σGGA+U schemes. It is found that with a reasonable choice of a rotationally invariant value of U both the LSDA+U and σGGA+U schemes produce electronic band gaps in the range of values derived from optical spectroscopy. The computed electronic structure results are utilized to determine the zone-center phonon modes by applying a linear response technique. We provide a clearer analysis of the energy values and polarization characteristics of the Raman active modes than is presently available from theoretical and experimental studies.

Lattice Dynamics of the Zinc-Blende and Wurtzite Phases of Nitrides

Abstract An adiabatic bond charge model has been applied to calculate the phonon dispersion and the density of states of the nitride materials InN, GaN, AlN and BN in the zinc-blende and wurtzite (WZ) phases. The results for the two phases are compared, the angular dependence of the zone-centre mode in the WZ phase is examined, and a discussion is provided on the anti-crossing behaviour of the A 1 acoustic and E 2 optical phonon modes in the WZ phase.

Electronic, elastic and phonon properties of the rock-salt LaSb and YSb

The electronic structures and elastic constants of LaSb and YSb are calculated using an ab initio pseudopotential scheme within the generalized gradient approximation. In agreement with experimental studies, it has been found that the upper valence bands in LaSb are characterized by Sb-5p and La-5d states. Our calculated elastic constants for LaSb are compared with available experimental data. A linear-response approach to density functional theory is used to derive phonon dispersion curves and the density of states for LaSb and YSb. Our phonon results for LaSb are in good agreement with experimental data. Frequency differences between phonon modes in these materials are discussed in terms of differences involving total mass, reduced mass and the lattice constant.

Theory of localized phonons on III-V(110) surfaces

We have calculated localized phonons on the (110) surfaces of GaAs, GaP, InAs and InP using the adiabatic bond-charge model, within a repeated slab scheme. Our results compare well with available high-resolution electron-energy-loss spectroscopy measurements and recent first principles theoretical results. Trends in frequencies and polarisation of surface phonon modes are examined. We find that the displacement patterns of the zone centre modes are similar for all the considered surfaces and that the difference in their energies can be explained in terms of the reduced mass difference. We also present a discussion on the trend in the location and dispersion of zone boundary Rayleigh wave as well as the highest surface optical frequency Fuchs-Kliewer phonon mode.

Electronic structure, phonons and electron?phonon interaction in MgXNi3 (X = B, C and N)

Ab initio calculations have been performed to study the electronic structure, phonon dispersion relations, and the electron–phonon interaction for cubic superconductors MgXNi3 with X = B, C and N. The electronic calculations are based upon the application of a plane wave basis, ultrasoft pseudopotentials, and the local density approximation of the density functional scheme. The electronic structure results are used, within the implementation of a linear response technique, for calculations of phonon states. Our calculations predict anomalous dispersion of the transverse acoustic branch along [111] in MgCNi3, and of all the three acoustic branches in a larger region of the Brillouin zone for MgNNi3. An explanation for the differences in the phonon dispersion and electron–phonon coupling parameter between these three materials has been put forward.

Phonon dispersion on a GaAs(110) surface studied using the adiabatic bond charge model

We have used the adiabatic bond charge model within a repeated slab scheme to study lattice dynamics on the GaAs(110) surface. The results provide a more detailed analysis of the surface phonon modes than is available from existing theoretical and experimental techniques.

Electrons, phonons and superconductivity in rocksalt and tungsten–carbide phases of CrC

We present results of ab initio theoretical investigations of the electronic structure, phonon dispersion relations, electron-phonon interaction and superconductivity in the rocksalt and tungsten-carbide phases of CrC. It is found that, compared to the stable tungsten-carbide phase, the metastable rocksalt phase is characterized by a much larger electronic density of states at the Fermi level. The phonon spectra of the rocksalt phase exhibit anomalies in the dispersion curves of both the transverse and longitudinal acoustic branches along the main symmetry directions. A combination of these characteristic electronic and phonon properties leads to an order of magnitude larger value of the electron-phonon coupling constant (λ = 2.66) for the rocksalt phase compared to that for the tungsten-carbide phase (λ = 0.24). Our calculations suggest that superconducting transition temperature values of 0.01 K and 25-35 K may be expected for the tungsten-carbide and rocksalt phases, respectively.

Phonons and superconductivity in the cubic perovskite Cr3RhN

We have performed an ab initio study of structural, electronic, vibrational, and superconducting properties of Cr3RhN. The calculations have been carried out by employing the density functional theory, a linear-response formalism, and the plane-wave pseudopotential method. A striking feature of the phonon spectra in this cubic perovskite is the existence of a number of soft modes at the zone centre as well as at Brillouin zone boundaries. Calculations of the Eliashberg spectral function α2F(ω) and the electron-phonon prefactor α2(ω) reveal that the low frequency soft phonon branches couple more strongly with electrons. Using the calculated electron-phonon coupling parameter λ=1.03 and the logarithmically averaged frequency ωln=255 K, we suggest that this material is a superconductor with the transition temperature close to 17 K.

Ab initio calculation of phonons in bulk HfC and the HfC(001)(1 × 1) surface

Structural, elastic and dynamical properties of the rock-salt HfC have been studied by employing an ab initio pseudopotential method and a linear response scheme, within the generalised gradient approximation. In addition to achieving good agreement with experimental measurements of the phonon dispersion results along the symmetry direction Γ-X, we have presented accurate dispersion results along several other directions in the Brillouin zone. Sharp features in the phonon density of states are ascertained and their origin discussed. This is followed by investigations of the structural, electronic and phonon properties of the HfC(001) surface. The agreement between our surface phonon spectrum and experimental measurements is very good. By analysing the atomic displacement patterns of the zone-centre and zone-edge phonon modes we have determined the energy locations and polarisation characteristics of Fuchs–Kliewer, Wallis, Lucas and Rayleigh modes.