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Register a new userAnharmonic properties from a generalized third order ab~initio approach: theory and applications to graphite and graphene
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We have implemented a generic method, based on the 2n+1 theorem within density functional perturbation theory, to calculate the anharmonic scattering coefficients among three phonons with arbitrary wavevectors. The method is used to study the phonon broadening in graphite and graphene mono- and bi-layer. The broadening of the high-energy optical branches is highly nonuniform and presents a series of sudden steps and spikes. At finite temperature, the two linearly dispersive acoustic branches TA and LA of graphene have nonzero broadening for small wavevectors. The broadening in graphite and bi-layer graphene is, overall, very similar to the graphene one, the most remarkable feature being the broadening of the quasi acoustical ZO branch. Finally, we study the intrinsic anharmonic contribution to the thermal conductivity of the three systems, within the single mode relaxation time approximation. We find the conductance to be in good agreement with experimental data for the out-of-plane direction but to underestimate it by a factor 2 in-plane.
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We develop a theoretical and computational framework to study polarons in semiconductors and insulators from first principles. Our approach provides the formation energy, excitation energy, and wavefunction of both electron and hole polarons, and takes into account the coupling of the electron or hole to all phonons. An important feature of the present method is that it does not require supercell calculations, and relies exclusively on electron band structures, phonon dispersions, and electron-phonon matrix elements obtained from calculations in the crystal unit cell. Starting from the Kohn-Sham (KS) equations of density-functional theory, we formulate the polaron problem as a variational minimization, and we obtain a nonlinear eigenvalue problem in the basis of KS states and phonon eigenmodes. In our formalism the electronic component of the polaron is expressed as a coherent superposition of KS states, in close analogy with the solution of the Bethe-Salpeter equation for the calculation of excitons. We demonstrate the power of the methodology by studying polarons in LiF and Li2O2. We show that our method describes both small and large polarons, and seamlessly captures Frohlich-type polar electron-phonon coupling and non-Frohlich coupling to acoustic and optical phonons. To analyze in quantitative terms the electron-phonon coupling mechanisms leading to the formation of polarons, we introduce spectral decompositions similar to the Eliashberg spectral function. We validate our theory using both analytical results and direct calculations on large supercells. This study constitutes a first step toward complete ab initio many-body calculations of polarons in real materials.
There has been a lot of excitement around the observation of superconductivity in twisted bilayer graphene, associated to flat bands close to the Fermi level. Such correlated electronic states also occur in multilayer rhombohedral stacked graphene (RG), which has been receiving increasing attention in the last years. In both natural and artificial samples however, multilayer stacked Bernal graphene (BG) occurs more frequently, making it desirable to determine what is their relative stability and under which conditions RG might be favored. Here, we study the energetics of BG and RG in bulk and also multilayer stacked graphene using first-principles calculations. It is shown that the electronic temperature, not accounted for in previous studies, plays a crucial role in determining which phase is preferred. We also show that the low energy states at room temperature consist of BG, RG and mixed BG-RG systems with a particular type of interface. Energies of all stacking sequences (SSs) are calculated for N = 12 layers, and an Ising model is used to fit them, which can be used for larger N as well. In this way, the ordering of low energy SSs can be determined and analyzed in terms of a few parameters. Our work clarifies inconsistent results in the literature, and sets the basis to studying the effect of external factors on the stability of multilayer graphene systems in first principles calculations.
We present an textit{ab initio} study based on density-functional theory of first- and second-order Raman spectra of graphene-based materials with different stacking arrangements and numbers of layers. Going from monolayer and bilayer graphene to periodic graphitic structures, we investigate the behavior of the first-order G-band and of the second-order 2D-band excited by the same set of photon energies. The former turns out to be very similar in all considered graphene-based materials, while in the latter we find the signatures of individual structures. With a systematic analysis of the second-order Raman spectra at varying frenquency of the incident radiation, we monitor the Raman signal and identify the contributions from different phonon modes that are characteristic of each specific arrangement. Supported by good agreement with experimental findings and with previous theoretical studies based on alternative approaches, our results propose an effective tool to probe and analyze the fingerprints of graphene-based and other low-dimensional materials.
A procedure is presented that combines density functional theory computations of bulk semiconductor alloys with the semiconductor Bloch equations, in order to achieve an ab initio based prediction of the optical properties of semiconductor alloy heterostructures. The parameters of an eight-band kp-Hamiltonian are fitted to the effective band structure of an appropriate alloy. The envelope function approach is applied to model the quantum well using the kp-wave functions and eigenvalues as starting point for calculating the optical properties of the heterostructure. It is shown that Luttinger parameters derived from band structures computed with the TB09 density functional reproduce extrapolated values. The procedure is illustrated by computing the absorption spectra for a (AlGa)As/Ga(AsP)/(AlGa)As quantum well system with varying phosphide content in the active layer.
We have performed ab-initio molecular dynamics simulations to elucidate the mechanism of the phase transition at high pressure from hexagonal graphite (HG) to hexagonal diamond (HD) or to cubic diamond (CD). The transition from HG to HD is found to occur swiftly in very small time of 0.2 ps, with large cooperative displacements of all the atoms. We observe that alternate layers of atoms in HG slide in opposite directions by (1/3, 1/6, 0) and (-1/3, -1/6, 0), respectively, which is about 0.7 {AA} along the pm[2, 1, 0] direction, while simultaneously puckering by about pm0.25 {AA} perpendicular to the a-b plane. The transition from HG to CD occurred with more complex cooperative displacements. In this case, six successive HG layers slide in pairs by 1/3 along [0, 1, 0], [-1, -1, 0] and [1, 0, 0], respectively along with the puckering as above. We have also performed calculations of the phonon spectrum in HG at high pressure, which reveal soft phonon modes that may facilitate the phase transition involving the sliding and puckering of the HG layers. The zero-point vibrational energy and the vibrational entropy are found to have important role in stabilizing HG up to higher pressures (>10 GPa) and temperatures than that estimated (<6 GPa) from previous enthalpy calculations.
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