Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations

We present a novel approach that allows to calculate the dielectric response of periodic systems in the quantum Monte Carlo formalism. We employ a many-body generalization for the electric enthalpy functional, where the coupling with the field is expressed via the Berry-phase formulation for the macroscopic polarization. A self-consistent local Hamiltonian then determines the ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo calculations where the polarization's fixed point is estimated from the average on an iterative sequence, sampled via forward-walking. This approach has been validated for the case of an isolated hydrogen atom, and then applied to a periodic system, to calculate the dielectric susceptibility of molecular-hydrogen chains. The results found are in excellent agreement with the best estimates obtained from the extrapolation of quantum-chemistry calculations.Comment: 5 page 2figure

GW quasi-particle spectra from occupied states only

We introduce a method that allows for the calculation of quasi-particle spectra in the GW approximation, yet avoiding any explicit reference to empty one-electron states. This is achieved by expressing the irreducible polarizability operator and the self-energy operator through a set of linear response equations, which are solved using a Lanczos-chain algorithm. We first validate our approach by calculating the vertical ionization energies of the benzene molecule and then show its potential by addressing the spectrum of a large molecule such as free-base tetraphenylporphyrin.Comment: 4 pages, 3 figure

Motivasidanhasilstudisiswamiskinsmpnegeri3kecamatan Pujud

Motivation and study result of SMPN 3 PUJUD's poor students: Budi Pramaharja, 1205188179, 2014. Study program: guidance and counseling; science department, Education and science faculty: Riau University. Counselor I: Prof. Dr. Zulfan Saam, M. Si; Counselor II: Dra. Tri Umari, M. Si.This research aims to know the illustration of poor student' study motivation in SMPN 3 PUJUD that back grounded by some reason that seen in school such as : the problem about low motivation and the study result of poor student in SMPN 3 PUJUD.The method that be used in this research is Descriptive method in percentage, the location of research is in SMPN 3 PUJUD. This research subject is VII, VIII, IX students' class. Giving the study motivation is taken by sample technique or saturate sample. Because the sum of population in this research is small. Data and the instrument of data collection is questionnaire form about motivation and study of result poor student in SMPN 3 PUJUD that done by researcher and be guided at lattice work with the alternative of multiple choices: frequent, often, sometimes, seldom and never. Data analysis technique uses the percentage formula (Anas Sudijono, 2004:170)Based on the research and data processing that had been done, can be resumed : (1) based on the result of data processing can be know the general descriptive of study motivation: VII, VIII, IX students' class of SMPN 3 PUJUD, PUJUD district, school year 2013-2014, that is by paying attention the intensity of giving the poor students' study motivation, with the result that low level category. The poor students' study motivation is be categorized low level, be market by : a) have not the lesson book, b) less the parent's attention, c) seldom visit the library, d) less the media in giving the assist to the development of potential students in studying, so that become the adult human and responsible. (2) the description of poor students' study result in SMPN 3 PUJUD can be know that is the students of VII, VIII, IX class, school year 2013-2014, low level dominantly . the low subject is under KKM on Pendidikan Pancasila dan Kewarganegaraan (citizen) lesson by rating 47,5 before remedial, and on IPA (science) lesson by rating 45 before remedial. (3) constraints that be faced by the poor students like as helping parent, long distance from house to school, this case result the low study motivation and study result for students

Pengaruh Konseling Kelompok terhadap Pengurangan Stres Akademik Siswa Kelas X Sman 2 Siakhulu T.p 2014/2015

The title of this research is ‘' Effect of Group Counseling towards the Reduction of Academic Stress of First – Year Students of Academic Year 2014/2015 at SMAN 2 Siak Hulu''. The purposes of this research were 1) to know the description of students' academic stress before given group counseling service, 2) to know the dynamic description of the group counseling implementation in order to reduce students' academic stress, 3) to know the description of students' academic stress reduction after given group counseling service, 4) to know how much the difference of students' academic stress reduction before and after given group counseling service is, 5) to know how much the influence of group counseling towards the reduction of students' academic stress is. The instrument used in this research was academic stress scale. The subjects of this research were 101 first – year students of academic year 2014/2015 at SMAN 2 Siak Hulu. The sample of this research was 30 students and they were selected by simple random sampling. The description of academic stress before given group counseling was in high category (50%), medium category (46,67%), and low category (3,33%). The description of students' academic stress reduction after given group counseling was in high category (10%), medium category (53,33%), and low category (36,67%). Based on the result of the calculation of the correlation coefficient between X1 and X2 was 0,505 and the determination coefficient (r2) was 0,255. This means that the contribution of group counseling towards the reduction of academic stress was 25,5%. Therefore, it can be known that ttest was bigger than ttable, (7,93 > 2,000) so that Ha was accepted . So, it means that there was influence of group counseling towards the reduction of academic stress of first – year students of academic year 2014/2015 at SMAN 2 Siak Hulu

Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study

We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were generated by a combination of classical and ab initio molecular-dynamics simulations and their dynamical matrices were computed within the framework of the density functional theory. The Raman scattering intensities were determined using the bond-polarizability model and a good agreement with experimental spectra was found. We confirm that the modes associated to the fourfold and threefold rings produce most of the Raman intensity of the D1 and D2 peaks, respectively, in the VV Raman spectra. Modifications of the Raman spectra upon compression are found to be in agreement with experimental data. We show that the modes associated to the fourfold rings still exist upon compression but do not produce a strong Raman intensity, whereas the ones associated to the threefold rings do. This result strongly suggests that the area under the D1 and D2 peaks is not directly proportional to the concentration of small rings in amorphous SiO2.Comment: 21 pages, 8 figures. Phys. Rev. B, in pres

Photoelasticity of crystalline and amorphous silica from first principles

Based on density-functional perturbation theory we have computed from first principles the photoelastic tensor of few crystalline phases of silica at normal conditions and high pressure (quartz, $\alpha$-cristobalite, $\beta$-cristobalite) and of models of amorphous silica (containig up to 162 atoms), obtained by quenching from the melt in combined classical and Car-Parrinello molecular dynamics simulations. The computational framework has also been checked on the photoelastic tensor of crystalline silicon and MgO as prototypes of covalent and ionic systems. The agreement with available experimental data is good. A phenomenological model suitable to describe the photoelastic properties of different silica polymorphs is devised by fitting on the ab-initio data.Comment: ten figure

Resonant Lifetime of Core-Excited Organic Adsorbates from First Principles

We investigate by first-principles simulations the resonant electron-transfer lifetime from the excited state of an organic adsorbate to a semiconductor surface, namely isonicotinic acid on rutile TiO$_2$(110). The molecule-substrate interaction is described using density functional theory, while the effect of a truly semi-infinite substrate is taken into account by Green's function techniques. Excitonic effects due to the presence of core-excited atoms in the molecule are shown to be instrumental to understand the electron-transfer times measured using the so-called core-hole-clock technique. In particular, for the isonicotinic acid on TiO$_2$(110), we find that the charge injection from the LUMO is quenched since this state lies within the substrate band gap. We compute the resonant charge-transfer times from LUMO+1 and LUMO+2, and systematically investigate the dependence of the elastic lifetimes of these states on the alignment among adsorbate and substrate states.Comment: 24 pages, 6 figures, to appear in Journal of Physical Chemistry

Understanding how excess lead iodide precursor improves halide perovskite solar cell performance

The presence of excess lead iodide in halide perovskites has been key for surpassing 20% photon-to-power conversion efficiency. To achieve even higher power conversion efficiencies, it is important to understand the role of remnant lead iodide in these perovskites. To that end, we explored the mechanism facilitating this effect by identifying the impact of excess lead iodide within the perovskite film on charge diffusion length, using electron-beam-induced current measurements, and on film formation properties, from grazing-incidence wide-angle X-ray scattering and high-resolution transmission electron microscopy. Based on our results, we propose that excess lead iodide in the perovskite precursors can reduce the halide vacancy concentration and lead to formation of azimuthal angle-oriented cubic alpha-perovskite crystals in-between 0 degrees and 90 degrees. We further identify a higher perovskite carrier concentration inside the nanostructured titanium dioxide layer than in the capping layer. These effects are consistent with enhanced lead iodide-rich perovskite solar cell performance and illustrate the role of lead iodide