arXiv subjects

M. W. C. Dharma-wardana

Publications and source records attributed to M. W. C. Dharma-wardana.

At least 55 records · Page 3Linked to original sources

arXiv2007

Pair-distribution functions of two-temperature two-mass systems: Comparison of MD, HNC, CHNC, QMC and Kohn-Sham calculations for dense hydrogen

Two-temperature, two-mass quasi-equilibrium plasmas may occur in electron-ion plasmas,nuclear-matter, as well as in electron-hole condensed-matter systems. Dense two-temperature hydrogen plasmas straddle the difficult partially - degenerate regime of electron densities and temperatures which are important in astrophysics, in inertial-confinement fusion research, and other areas of warm dense matter physics. Results from Kohn-Sham calculations and QMC are used to benchmark the procedures used in classical molecular-dynamics simulations, HNC and CHNC methods to derive electron-electron and electron-proton pair - distribution functions. Then, nonequilibrium molecular dynamics for two -temperature, two-mass plasmas are used to obtain the pair distribution. Using these results, the correct HNC and CHNC procedures for the evaluation of pair-distribution functions in two-temperature two-mass two-component charged fluids are established. Results for a mass ratio of 1:5, typical of electron-hole fluids, as well as for compressed hydrogen are presented. PACS Numbers: 52.25.Kn, 52.25Gj, 71.10.-w, 52.27.Gr, 26.30.+k

cond-mat.stat-mech↗

arXiv2007

Electronic structure and the minimum conductance of a graphene layer on SiO2 from density-functional methods.

The effect of the SiO$_2$ substrate on a graphene film is investigated using realistic but computationally convenient energy-optimized models of the substrate supporting a layer of graphene. The electronic bands are calculated using density-functional methods for several model substrates. This provides an estimate of the substrate-charge effects on the behaviour of the bands near $E_F$, as well as a variation of the equilibrium distance of the graphene sheet. A model of a wavy graphene layer is examined as a possible candidate for understanding the nature of the minimally conducting states in graphene.

cond-mat.mes-hall↗

arXiv2007

The ground state of clean and defected graphene: Coulomb interactions of massless Dirac fermions, pair-distribution functions and spin-polarized phases

First-principles density functional calculations for graphene and defected graphene are used to examine when the quasi-2D electrons near the Fermi energy in graphene could be represented by massless fermions obeying a Dirac-Weyl (DW) equation. The DW model is found to be inapplicable to defected graphene containing even $\sim$3% vacancies or N substitution. However, the DW model holds in the presence of weakly adsorbed molecular layers. The possibility of spin-polarized phases (SPP) of DW-massless fermions in pure graphene is considered. The exchange energy is evaluated from the analytic pair-distribution functions as well as in $k$-space. The kinetic energy enhancement of the sipn-polarized phase nearly cancels the exchange enhancement, and the correlation energy plays a dominant residual role. The correlation energies are estimated via a model four-component 2D electron fluid whose Coulomb coupling matches that of graphene. While SPPs appear with exchange only, the inclusion of correlations suppresses them in ideal graphene.

cond-mat.str-el↗

arXiv2006

Coulomb interactions of massless Dirac fermions in graphene; pair-distribution functions and exchange-driven spin-polarized phases

The quasi-2D electrons in graphene behave as massless fermions obeying a Dirac-Weyl equation in the low-energy regime near the two Fermi points. The stability of spin-polarized phases (SPP) in graphene is considered. The exchange energy is evaluated from the analytic pair-distribution functions, and the correlation energies are estimated via a closely similar four-component 2D electron fluid which has been investigated previously. SPPs appear for sufficiently high doping, when the exchange energy alone is considered. However, the inclusion of correlations is found to {\it suppress} the spin-phase transition in ideal graphene.

cond-mat.str-el↗

arXiv2005

Ground-state energy and Wigner crystallization in thick 2D-electron systems

The ground state energy of the 2-D Wigner crystal is determined as a function of the thickness of the electron layer and the crystal structure. The method of evaluating the exchange-correlation energy is tested using known results for the infinitely-thin 2D system. Two methods, one based on the local-density approximation(LDA), and another based on the constant-density approximation (CDA) are established by comparing with quantum Monte-Carlo (QMC) results. The LDA and CDA estimates for the Wigner transition of the perfect 2D fluid are at $r_s=38$ and 32 respectively, compared with $r_s=35\pm5$ from QMC. For thick-2D layers as found in Hetero-junction-insulated-gate field-effect transistors, the LDA and CDA predictions of the Wigner transition are at $r_s=20.5$ and 15.5 respectively. Impurity effects are not considered here.

cond-mat.str-el↗

arXiv2005

Spin and temperature dependent study of exchange and correlation in thick two-dimensional electron layers

The exchange and correlation $E_{xc}$ of strongly correlated electrons in 2D layers of finite width are studied as a function of the density parameter $r_s$, spin-polarization $ζ$ and the temperature $T$. We explicitly treat strong-correlation effects via pair-distribution functions, and introduce an equivalent constant-density approximation (CDA) applicable to all the inhomogeneous densities encountered here. The width $w$ defined via the CDA provides a length scale defining the $z$-extension of the quasi-2D layer resident in the $x$-$y$ plane. The correlation energy $E_c$ of the quasi-2D system is presented as an interpolation between a 1D gas of electron-rods (for $w/r_s>1$) coupled via a log(r) interaction, and a 3D Coulomb fluid closely approximated from the known {\it three-dimensional} correlation energy when $w/r_s$ is small. Results for the $E_{xc}(r_s,ζ,T)$, the transition to a spin-polarized phase, the effective mass $m^*$, the Landé $g$-factor etc., are reported here.

cond-mat.str-el↗

arXiv2005

Idealized Slab Plasma approach for the study of Warm Dense Matter

Recently, warm dense matter (WDM) has emerged as an interdisciplinary field that draws increasing interest in plasma physics, condensed matter physics, high pressure science, astrophysics, inertial confinement fusion, as well as materials science under extreme conditions. To allow the study of well-defined WDM states, we have introduced the concept of idealized-slab plasmas that can be realized in the laboratory via (i) the isochoric heating of a solid and (ii) the propagation of a shock wave in a solid. The application of this concept provides new means for probing the dynamic conductivity, equation of state, ionization and opacity. These approaches are presented here using results derived from first-principles (density-functional type) theory, Thomas-Fermi type theory, and numerical simulations.

physics.plasm-ph↗

arXiv2005

The static and dynamic conductivity of warm dense Aluminum and Gold calculated within a density functional approach

The static resistivity of dense Al and Au plsmas are calculated where all the needed inputs are obtained from density functional theory (DFT). This is used as input for a study of the dynamic conductivity. These calculations involve a self-consistent determination of (i) the equation of state (EOS) and the ionization balance, (ii) evaluation of the ion-ion, and ion-electron pair-distribution functions, (iii) Determination of the scattering amplitudes, and finally the conductivity. We present data for the static resistivity of Al for compressions 0.1-2.0, and in the temperature range T= 0.1 - 10 eV. Results for Au in the same temperature range and for compressions 0.1-1.0 is also given. In determining the dynamic conductivity for a range of frequencies consistent with standard laser probes, a knowledge of the electronic eigenstates and occupancies of Al- or Au plasma becomes necessary. They are calculated using a neutral-pseudoatom model. We examine a number of first-principles approaches to the optical conductivity, including many-body perturbation theory, molecular-dynamics evaluations, and simplified time-dependent DFT. The modification to the Drude conductivity that arises from the presence of shallow bound states in typical Al-plasmas is examined and numerical results are given at the level of the Fermi Golden rule and an approximate form of time-dependent DFT.

cond-mat.mtrl-sci↗

arXiv2005

Layer-thickness effects in quai-two dimensional electron liquids

We use a mapping of the quasi-2D electron liquid to a classical fluid and use the hypernetted-chain equation inclusive of bridge corrections, i.e., CHNC, to calculate the electron pair-distribution functions and exchange-correlation energies as a function of the density, layer width, spin-polarization and temperature. The theory is free of adjustable parameters and is in good accord with recent effective-mass and spin-susceptibility results for HIGFET 2-D layers.

cond-mat.str-el↗

arXiv2004

Metallic behaviour of carrier-polarized C$_{60}$ molecular layers: Experiment and Theory

Although C$_{60}$ is a molecular crystal with a bandgap E$_g$ of ~2.5 eV, we show that E$_g$ is strongly affected by injected charge. In sharp contrast to the Coulomb blockade typical of quantum dots, E$_g$ is {\it reduced} by the Coulomb effects. The conductance of a thin C$_{60}$ layer sandwiched between metal (Al, Ag, Au, Mg and Pt) contacts is investigated. Excellent Ohmic conductance is observed for Al electrodes protected with ultra-thin LiF layers. First-principles calculations, Hubbard models etc., show that the energy gap of C$_{60}$ is dramatically reduced when electrons hop from C$_{60}^-$ to C$_{60}$.

cond-mat.mtrl-sci↗

arXiv2004

Local environment of Nitrogen in GaN{y}As{1-y} epilayers on GaAs (001) studied using X-ray absorption near edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is used to study the N environment in bulk GaN and in GaN{y}As{1-y} epilayers on GaAs (001), for y \~5%. Density-functional optimized structures were used to predict XANES via multiple-scattering theory. We obtain striking agreement for pure GaN. An alloy model with nitrogen pairs on Ga accurately predicts the threshold energy, the width of the XANES ``white line'', and features above threshold, for the given X-ray polarization. The presence of N-pairs may point to a role for molecular N_2 in epitaxial growth kinetics.

cond-mat.mtrl-sci↗

arXiv2004

The effective mass and g-factor of the strongly correlated 2-D electron fluid. Evidence for a coupled-valley condensate in the Si system

The effective mass m*, and the Lande g-factor of the uniform 2-D electron fluid (2DEF) are calculated as a function of the spin polarization zeta, and the density parameter r_s, using a non-perturbative analytic approach. Our theory is in good accord with the m*g* data of Zhu et al. for zeta=0 for the GaAs-2DEF, and striking agreement with the data of Shashkin et al for the Si-2DEF. While g* is enhanced in GaAs, m* is enhanced in Si. The latter arises from singlet-pair excitations in the two valleys forming a coupled-valley state occurring at the critical density of ~1.10^{11}$ e/cm^2.

cond-mat.str-el↗

arXiv2004

Molecular electronics exploiting sharp structure in the electrode density-of-states. Negative differential resistance and Resonant Tunneling in a poled molecular layer on Al/LiF electrodes

Density-functional calculations are used to clarify the role of an ultrathin LiF layer on Al electrodes used in molecular electronics. The LiF layer creates a sharp density of states (DOS), as in a scanning-tunneling microscope (STM) tip. The sharp DOS, coupled with the DOS of the molecule leads to negative differential resistance (NDR). Electron transfer between oriented molecules occurs via resonant tunneling. The I-V characteristic for a thin-film of tris (8-hydroxyquinoline)- aluminum (AlQ) molecules, oriented using electric-field poling, and sandwiched between two Al/LiF electrodes is in excellent agreement with theory. This molecular device presents a new paradigm for a convenient, robust, inexpensive alternative to STM or mechanical break-junction structures.

cond-mat.mtrl-sci↗

arXiv2004

Spin and Valley dependent analysis of the two-dimensional low-density electron system in Si-MOSFETS

The 2-D electron system (2DES) in Si metal-oxide field-effect transistors (MOSFETS) consists of two distinct electron fluids interacting with each other. We calculate the total energy as a function of the density $n$, and the spin polarization $ζ$ in the strongly-correlated low-density regime, using a classical mapping to a hypernetted-chain (CHNC) equation inclusive of bridge terms. Here the ten distribution functions, arising from spin and valley indices, are self-consistently calculated to obtain the total free energy, the chemical potential, the compressibility and the spin susceptibility. The T=0 results are compared with the 2-valley Quantum Monte Carlo (QMC) data of Conti et al. (at T=0, $ζ=0$) and found to be in excellent agreement. However, unlike in the one-valley 2DES, it is shown that {\it the unpolarized phase is always the stable phase in the 2-valley system}, right up to Wigner Crystallization at $r_s=42$. This leads to the insensitivity of $g^*$ to the spin polarization and to the density. The compressibility and the spin-susceptibility enhancement calculated from the free energy confirm the validity of a simple approach to the two-valley response based on coupled-mode formation. The three methods, QMC, CHNC, and Coupled-mode theory agree closely. Our results contain no {\it ad hoc} fit parameters. They agree with experiments and do not invoke impurity effects or metal-insulator transition phenomenology.

cond-mat.str-el↗

arXiv2003

Structure of the Local-field factor of the 2-D electron fluid. Possible evidence for correlated scattering of electron pairs

The static local-field factor (LFF) of the 2-D electron fluid is calculated {\it nonperturbatively} using a mapping to a classical Coulomb fluid $\lbrack$Phys. Rev. Lett., {\bf 87}, 206$\rbrack$. The LFF for the paramagnetic fluid {\it differs markedly} from perturbation theory where a maximum near 2$k_F$ is expected. Our LFF has a quasi-linear small-k region leading to a maximum close to 3$k_F$, in agreent with currently available quantum Monte Carlo data. The structure in the LFF and its dependence on the density and temperature are interpretted as a signature of correlated scattering of electron pairs of opposite spin.The lack of structure at $2k_F$ implies weakened Friedel oscillations, Kohn anomalies etc.

cond-mat.str-el↗

arXiv2002

The static response and the local-field factor of the 2-D electron fluid

The static local-field factor (LFF) of the interacting 2-D fluid is calculated nonperturbatively using a mapping of the quantum fluid to a classical Coulomb fluid [Phys. Rev. Let., {\bf 87}, 206 (2002)]. The LFF for the paramagnetic fluid DIFFERS MARKEDLY from expectations from standard perturbation theory. Our result for the LFF has a maximum close to 3$k_F$, while perturbation methods yield a maximum near 2$k_F$. The available quantum Monte Carlo data seem to agree with our results.These findings imply that many effects, e.g., Friedel oscillations, Kohn anomalies, effective electron-electron interactions, etc., are affected by the strong correlations among anti-parallel spins in 2D.

cond-mat.str-el↗

arXiv2002

Spin-polarized stable phases of the 2-D electron fluid at finite temperatures

The Helmholtz free energy F of the interacting 2-D electron fluid is calculated nonperturbatively using a mapping of the quantum fluid to a classical Coulomb fluid [Phys. Rev. Letters, vol. 87, 206404 (2001)]. For density parameters rs such that rs<~25, the fluid is unpolarized at all temperatures t=T/EF where EF is the Fermi energy. For lower densities, the system becomes fully spin polarized for t<~0.35, and partially polarized for 0.35<t< 2, depending on the density. At rs ~25-30, and t ~0.35, an ''ambispin'' phase where F is almost independent of the spin polarization is found. These results support recent claims, based on quantum Monte Carlo results, for a stable, fully spin-polarized fluid phase at T = 0 for rs larger than about 25-26.

cond-mat.str-el↗

arXiv2002

Designing molecules to bypass the singlet-triplet bottleneck in the electroluminescence of organic light-emitting-diode materials

Electroluminescence in organic light emitting diode (OLED) materials occurs via the recombination of excitonic electrons-hole pairs Only the singlet excitons of commonly used OLED materials, e.g., Aluminum trihydroxyquinoline (AlQ$_3$), decay radiatively, limiting the external quantum efficiency to a maximum 25%. Thus 75% of the energy is lost due to the triplet bottleneck for radiative recombination. We consider molecules derived from AlQ$_3$ which bypass the triplet bottleneck by designing structures which contain strong spin-orbit coupling. As a first stage of this work, groundstate energies and vertical excitation energies of Al-arsenoquinolines and Al-boroarsenoquinolines are calculated. It is found that the substitution of N by As leads to very favourable results, while the boron substitution leads to no advantage.

cond-mat.mtrl-sci↗