Photo-excitation of a light-harvesting supra-molecular triad: a Time-Dependent DFT study

We present the first time-dependent density-functional theory (TDDFT) calculation on a light harvesting triad carotenoid-diaryl-porphyrin-C60. Besides the numerical challenge that the ab initio study of the electronic structure of such a large system presents, we show that TDDFT is able to provide an accurate description of the excited state properties of the system. In particular we calculate the photo-absorption spectrum of the supra-molecular assembly, and we provide an interpretation of the photo-excitation mechanism in terms of the properties of the component moieties. The spectrum is in good agreement with experimental data, and provides useful insight on the photo-induced charge transfer mechanism which characterizes the system.Comment: Accepted for publication on JPC, March 09th 200

Spin Susceptibility of an Ultra-Low Density Two Dimensional Electron System

We determine the spin susceptibility in a two dimensional electron system in GaAs/AlGaAs over a wide range of low densities from 2$\times10^{9}$cm$^{-2}$ to 4$\times10^{10}$cm$^{-2}$. Our data can be fitted to an equation that describes the density dependence as well as the polarization dependence of the spin susceptibility. It can account for the anomalous g-factors reported recently in GaAs electron and hole systems. The paramagnetic spin susceptibility increases with decreasing density as expected from theoretical calculations.Comment: 5 pages, 2 eps figures, to appear in PR

Spin-Polarization transition in the two dimensional electron gas

We present a numerical study of magnetic phases of the 2D electron gas near freezing. The calculations are performed by diffusion Monte Carlo in the fixed node approximation. At variance with the 3D case we find no evidence for the stability of a partially polarized phase. With plane wave nodes in the trial function, the polarization transition takes place at Rs=20, whereas the best available estimates locate Wigner crystallization around Rs=35. Using an improved nodal structure, featuring optimized backflow correlations, we confirm the existence of a stability range for the polarized phase, although somewhat shrunk, at densities achievable nowadays in 2 dimensional hole gases in semiconductor heterostructures . The spin susceptibility of the unpolarized phase at the magnetic transition is approximately 30 times the Pauli susceptibility.Comment: 7 pages, 4 figure

The 2-D electron gas at arbitrary spin polarizations and arbitrary coupling strengths: Exchange-correlation energies, distribution functions and spin-polarized phases

We use a recent approach [Phys. Rev. Letters, {\bf 84}, 959 (2000)] for including Coulomb interactions in quantum systems via a classical mapping of the pair-distribution functions (PDFs) for a study of the 2-D electron gas. As in the 3-D case, the ``quantum temperature'' T_q of a classical 2-D Coulomb fluid which has the same correlation energy as the quantum fluid is determined as a function of the density parameter r_s. Spin-dependent exchange-correlation energies are reported. Comparisons of the spin-dependent pair-distributions and other calculated properties with any available 2-D quantum Monte Carlo (QMC) results show excellent agreement, strongly favouring more recent QMC data. The interesting novel physics brought to light by this study are: (a) the independently determined quantum-temperatures for 3-D and 2-D are found to be approximately the same, (i.e, universal) function of the classical coupling constant Gamma. (b) the coupling constant Gamma increases rapidly with r_s in 2-D, making it comparatively more coupled than in 3-D; the stronger coupling in 2-D requires bridge corrections to the hyper- netted-chain method which is adequate in 3-D; (c) the Helmholtz free energy of spin-polarized and unpolarized phases have been calculated. The existence of a spin-polarized 2-D liquid near r_s = 30, is found to be a marginal possibility. These results pertain to clean uniform 2-D electron systems.Comment: This paper replaces the cond-mat/0109228 submision; the new version include s more accurate numerical evaluation of the Helmholtz energies of the para- and ferromagentic 2D fluides at finite temperatures. (Paper accepted for publication in Phys. Rev. Lett.

Magnetic Field Induced Spin Polarization of AlAs Two-dimensional Electrons

Two-dimensional (2D) electrons in an in-plane magnetic field become fully spin polarized above a field B_P, which we can determine from the in-plane magnetoresistance. We perform such measurements in modulation-doped AlAs electron systems, and find that the field B_P increases approximately linearly with 2D electron density. These results imply that the product |g*|m*, where g* is the effective g-factor and m* the effective mass, is a constant essentially independent of density. While the deduced |g*|m* is enhanced relative to its band value by a factor of ~ 4, we see no indication of its divergence as 2D density approaches zero. These observations are at odds with results obtained in Si-MOSFETs, but qualitatively confirm spin polarization studies of 2D GaAs carriers.Comment: 4 pages, 5 figure

Correlation energy and spin polarization in the 2D electron gas

The ground state energy of the two--dimensional uniform electron gas has been calculated with fixed--node diffusion Monte Carlo, including backflow correlations, for a wide range of electron densities as a function of spin polarization. We give a simple analytic representation of the correlation energy which fits the density and polarization dependence of the simulation data and includes several known high- and low-density limits. This parametrization provides a reliable local spin density energy functional for two-dimensional systems and an estimate for the spin susceptibility. Within the proposed model for the correlation energy, a weakly first--order polarization transition occurs shortly before Wigner crystallization as the density is lowered.Comment: Minor typos corrected, see erratum: Phys. Rev. Lett. 91, 109902(E) (2003

Self-consistent Overhauser model for the pair distribution function of an electron gas in dimensionalities D=3 and D=2

We present self-consistent calculations of the spin-averaged pair distribution function $g(r)$ for a homogeneous electron gas in the paramagnetic state in both three and two dimensions, based on an extension of a model that was originally proposed by A. W. Overhauser [Can. J. Phys. {\bf 73}, 683 (1995)] and further evaluated by P. Gori-Giorgi and J. P. Perdew [Phys. Rev. B {\bf 64}, 155102 (2001)]. The model involves the solution of a two-electron scattering problem via an effective Coulombic potential, that we determine within a self-consistent Hartree approximation. We find numerical results for $g(r)$ that are in excellent agreement with Quantum Monte Carlo data at low and intermediate coupling strength $r_s$, extending up to $r_s\approx 10$ in dimensionality D=3. However, the Hartree approximation does not properly account for the emergence of a first-neighbor peak at stronger coupling, such as at $r_s=5$ in D=2, and has limited accuracy in regard to the spin-resolved components $g_{\uparrow\uparrow}(r)$ and $g_{\uparrow\downarrow}(r)$. We also report calculations of the electron-electron s-wave scattering length, to test an analytical expression proposed by Overhauser in D=3 and to present new results in D=2 at moderate coupling strength. Finally, we indicate how this approach can be extended to evaluate the pair distribution functions in inhomogeneous electron systems and hence to obtain improved exchange-correlation energy functionals.Comment: 14 pages, 7 figuers, to apear in Physical Review

Excitonic condensation in a symmetric electron-hole bilayer

Using Diffusion Monte Carlo simulations we have investigated the ground state of a symmetric electron-hole bilayer and determined its phase diagram at T=0. We find clear evidence of an excitonic condensate, whose stability however is affected by in-layer electronic correlation. This stabilizes the electron-hole plasma at large values of the density or inter-layer distance, and the Wigner crystal at low density and large distance. We have also estimated pair correlation functions and low order density matrices, to give a microscopic characterization of correlations, as well as to try and estimate the condensate fraction.Comment: 4 pages, 3 figures, 2 table

Many-body perturbation theory calculations using the yambo code

International audienceyambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as quantum-espresso and abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools

Tight-binding parameters for charge transfer along DNA

We systematically examine all the tight-binding parameters pertinent to charge transfer along DNA. The $\pi$ molecular structure of the four DNA bases (adenine, thymine, cytosine, and guanine) is investigated by using the linear combination of atomic orbitals method with a recently introduced parametrization. The HOMO and LUMO wavefunctions and energies of DNA bases are discussed and then used for calculating the corresponding wavefunctions of the two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO and LUMO energies of the bases are in good agreement with available experimental values. Our results are then used for estimating the complete set of charge transfer parameters between neighboring bases and also between successive base-pairs, considering all possible combinations between them, for both electrons and holes. The calculated microscopic quantities can be used in mesoscopic theoretical models of electron or hole transfer along the DNA double helix, as they provide the necessary parameters for a tight-binding phenomenological description based on the $\pi$ molecular overlap. We find that usually the hopping parameters for holes are higher in magnitude compared to the ones for electrons, which probably indicates that hole transport along DNA is more favorable than electron transport. Our findings are also compared with existing calculations from first principles.Comment: 15 pages, 3 figures, 7 table