arXiv subjects

M. W. C. Dharma-wardana

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

At least 37 records · Page 2Linked to original sources

arXiv2018

Ion-ion dynamic structure factor, acoustic modes and equation of state of two-temperature warm dense aluminum

The ion-ion dynamical structure factor and the equation of state of warm dense aluminum in a two-temperature quasi-equilibrium state, with the electron temperature higher than the ion temperature, are investigated using molecular-dynamics simulations based on ion-ion pair potentials constructed from a neutral pseudoatom model. Such pair potentials based on density functional theory are parameter-free and depend directly on the electron temperature and indirectly on the ion temperature, enabling efficient computation of two-temperature properties. Comparison with ab initio simulations and with other average-atom calculations for equilibrium aluminum shows good agreement, justifying a study of quasi-equilibrium situations. Analyzing the van Hove function, we find that ion-ion correlations vanish in a time significantly smaller than the electron-ion relaxation time so that dynamical properties have a physical meaning for the quasi-equilibrium state. A significant increase in the speed of sound is predicted from the modification of the dispersion relation of the ion acoustic mode as the electron temperature is increased. The two-temperature equation of state including the free energy, internal energy and pressure is also presented.

cond-mat.stat-mech↗

arXiv2017

Isochoric, isobaric and ultrafast conductivities of aluminum, lithium and carbon in the warm dense matter (WDM) regime

We study the conductivities $σ$ of (i) the equilibrium isochoric state ($σ_{\rm is}$), (ii) the equilibrium isobaric state ($σ_{\rm ib}$), and also the (iii) non-equilibrium ultrafast matter (UFM) state ($σ_{\rm uf}$) with the ion temperature $T_i$ less than the the electron temperature $T_e$. Aluminum, lithium and carbon are considered, being increasingly complex warm dense matter (WDM) systems, with carbon having transient covalent bonds. First-principles calculations, i.e., neutral-pseudoatom (NPA) calculations and density-functional theory (DFT) with molecular-dynamics (MD) simulations, are compared where possible with experimental data to characterize $σ_{\rm ic}, σ_{\rm ib}$ and $σ_{\rm uf}$. The NPA $σ_{\rm ib}$ are closest to the available experimental data when compared to results from DFT+MD, where simulations of about 64-125 atoms are typically used. The published conductivities for Li are reviewed and the value at a temperature of 4.5 eV is examined using supporting X-ray Thomson scattering calculations. A physical picture of the variations of $σ$ with temperature and density applicable to these materials is given. The insensitivity of $σ$ to $T_e$ below 10 eV for carbon, compared to Al and Li, is clarified.

cond-mat.mtrl-sci↗

arXiv2017

A calculation of the Deuterium Hugoniot using the classical-map hypernetted-chain (CHNC) approach

The Hugoniot for Deuterium is calculated using the classical-map hyper-netted-chain (CHNC) approach using several models of the effective temperature that may be assigned to the electron-ion interaction. This effective temperature embodies the exchange-correlation and kinetic energy functional that is assigned to the electron-ion interaction. Deuterium pair distribution functions (calculated using the neutral-pseudo atom method) showing the formation of molecular pre-peaks are displayed to clarify the soft-turning over of the hugoniot in the pressure range of 0.2-0.6 Megabars. This contribution updates a previous CHNC calculation of the deuterium hugoniot given in Phys. Rev. B, 66, 014110 (2002).

physics.chem-ph↗

arXiv2017

Hot-electron relaxation in dense `two-temperature' hydrogen

Recent theories of hot-electron relaxation in dense hydrogen or deuterium are examined in the light of recent molecular-dynamics simulations as well as various theoretical developments within the two-temperature model. The theoretical work since 1998 have led to the formulation of the $f$-sum version of the Fermi Golden rule formula as the most convenient method for the calculation of the rate of cooling of hot electrons where energy is transferred to cold ions. The attempt to include relaxation via the ion-acoustic modes of the two coupled subsystems, i.e., electrons and ions has led to a coupled-mode formulation which has now been established by a variety of formal methods. However, various simplified calculational models of the system with coupled-modes, as well as sophisticated molecular dynamics simulations seem to disagree. It is expected that coupled-mode calculations which use the simple Coulomb potential $V_{ei}(r)=-|e|Z/r$ for the electron-ion interaction within RPA will greatly over-estimate the coupled-mode contribution. A weak pseudopotential $U_{ei}(r)$ would probably bring the estimated coupled-mode contribution to agree with that obtained by simulations. It is suggested that the available `reduced models' have been constructed without much attention to the satisfaction of important sum rules, Kramers-Krönig relations etc. We also deal with the question of how strongly coupled ion-ion systems can be addressed by an extension of the second-order linear response theory which is the basis of current formulations of energy relaxation in warm-dense matter systems. These are of interest in a variety of fields including hot-electron semi-conductor devices, inertial-fusion studies of hot compressed hydrogen, as well as in astrophysical applications.

cond-mat.stat-mech↗

arXiv2017

Theory of complex fluids in the warm-dense-matter regime, and application to an unusual phase-transitions in liquid carbon

Data from recent laser-shock experiments, density-functional theory (DFT) with molecular-dynamics (MD), and path-integral Monte Carlo (PIMC) simulations on carbon are compared with predictions from the neutral-pseudo-atom (NPA)+ hyper-netted-chain (HNC) approach for carbon, a complex liquid in the warm-dense matter regime. The NPA results are in good agreement, not only with high-density regimes that have been studies via PIMC, but even at low densities and low temperatures where transient covalent bonding dominates ionic correlations. Thus the `pre-peak' due to the C-C bond at $\sim$1.4-1.6 Å$\,$ and other features found in the pair-distribution function from DFT+MD simulations at 0.86 eV and 3.7 g/cm$^3$ etc., are recovered accurately in the NPA+HNC calculations. Such C-C bonding peaks have not been captured via average-atom ion-sphere (IS) models. Evidence for an unusual liquid $\to$ vapor and metal$\to$ semi-metal transition occurring simultaneously is presented. Here a strongly correlated metallic-liquid with transient C-C bonds, i.e., carbon at density $\sim$ 1.0 g/cm$^3$ and mean ionization $Z=4$ transits abruptly to a disordered mono-atomic vapour at 7 eV, with $Z\simeq$ 3. Other cases where $Z$ drops abruptly are also noted. The nature of $Z$, its discontinuities, and the role of exchange correlation, are reviewed. The limitations of IS models in capturing the physics of transient covalent bonding in warm dense matter are discussed.

cond-mat.stat-mech↗

arXiv2017

Chronic Kidney Disease of Unknown aetiology (CKDu) and multiple-ion interactions in drinking water

Recent experimental work on the nephrotoxicity of contaminants in drinking water using laboratory mice, motivated by the need to understand the origin of chronic kidney disease of unknown aetiology is examined within our understanding of the hydration of ions and proteins. Qualitative considerations based on Hofmeister-type action of these ions, as well as quantitative electrochemical models for the Gibbs free-energy change for ion-pair formation are used to explain why Cd$^{2+}$ in the presence of F$^-$ and water hardness due to Mg$^{2+}$ ions (but not Ca$^{2+}$) can be expected to be more nephrotoxic, while AsO$_3^{3-}$ in the presence of F$^-$ and hardness may be expected to be less nephrotoxic. The analysis is applied to a variety of ionic species typically found in water to predict their likely combined electro-chemical action. These results clarify the origins of chronic kidney disease in the north-central province of Sri Lanka. The conclusion is further strengthened by a study of the dietary load of Cd and As, where the dietary loads are found to be safe, especially when the mitigating effects of micronutrient ionic forms of Zn and Se, as well as corrections for bio-availability are taken in to account. The resulting aetiological picture supports the views that F$^-$, Cd$^{2+}$ (to a lesser extent), and Mg$^{2+}$ ions found in stagnant household well water act together with enhanced toxicity, becoming the most likely causative factor of the disease. Similar incidence of CKDu found in other tropical climates may have similar geological origins.

q-bio.TO↗

arXiv2016

A critical assessment of models of pair-interactions and screening used in analyzing recent warm-dense matter (WDM) experiments

Ultra-fast laser experiments yield increasingly reliable data on warm-dense matter (WDM), but rely on entrenched simplistic theoretical models. We re-analyze two topical experiments, avoiding (i) {\it ad hoc} core-repulsion models, (ii) "Yukawa screening" models and (iii) electron-ion equilibrium assumptions. An accurate, rapid density-functional neutral-pseudoatom model coupled to a hyper-netted-chain (HNC) equation with a bridge term is used to compute structure factors, X-Ray scattering, compressibility, phonons and resistivity. Electronic-structure codes are used to confirm the calculations. The Yukawa and core-repulsion models are shown to be misleading.

cond-mat.mtrl-sci↗

arXiv2016

Current issues in finite-$T$ density-functional theory and Warm-Correlated Matter

Finite-temperature DFT has become of topical interest, partly due to the increasing ability to create novel states of warm-correlated matter (WCM). Subclasses of WCM are Warm-dense matter (WDM), ultra-fast matter (UFM), and high-energy density matter (HEDM), containing electyrons (e) and ions (i). Strong e-e, i-i and e-i correlation effects and partial degeneracies are found in these systems where the electron temperature $T_e $ is comparable to the electron Fermi energy. The ion subsystem may be solid, liquid or plasma, with many states of ionization with ionic charge $Z_j$. Quasi-equilibria with the ion temperature $T_i\ne T_e$ are common. The ion subsystem in WCM can no longer be treated as a passive "external potential", as is customary in $T=0$ density functional theory (DFT) dominated by solid-state theory or quantum chemistry. Hohenberg-Kohn-Mermin theory can be used for WCMs if finite-$T$ exchange-correlation (XC) functionals are available. They are functionals of both the one-body electron density $n_e$ and the one-body ion densities $ρ_j$. A method of approximately but accurately mapping the quantum electrons to a classical Coulomb gas enables one to treat electron-ion systems entirely classically at any temperature and arbitrary spin polarization, using exchange-correlation effects calculated {\it in situ}, directly from the pair-distribution functions. This eliminates the need for any XC-functionals, or the use of a Born-Oppenheimer approximation. This classical map has been used to calculate the equation of state of WDM systems, and construct a finite-$T$ XC functional that is found to be in close agreement with recent quantum path-integral simulation data. In this review current developments and concerns in finite-$T$ DFT, especially in the context of non-relativistic warm-dense matter and ultra-fast matter will be presented.

physics.chem-ph↗

arXiv2016

The dynamic conductivity and the plasmon profile of Aluminum in the ultra-fast-matter regime; an analysis of recent X-ray scattering data from the LCLS

We use an explicitly isochoric two-temperature theory where the ion temperature is not assumed to be equal to the electron temperature, to analyze recent X-ray laser scattering data for Aluminum in the ultra-fast-matter (UFM) regime up to 6 eV, obtained at the Standford Linac Coherent Light Source (LCLS). The observed surprisingly low conductivities are explained by including strong electron-ion scattering effects using the phase shifts calculated via the density-functional neutral-pseudo-atom model (NPA). The applicability of the Mermin model to UFM is questioned. The static and dynamic conductivity, complex collision frequency and the plasmon line-shape are evaluated within a Born approximation and without the use of the Mermin response. This approach, where all needed quantities are obtained from the NPA model, provides good agreement with the reported experimental results.

cond-mat.stat-mech↗

arXiv2014

Strongly-Coupled Coulomb Systems using finite-$T$ Density Functional Theory: A review of studies on Strongly-Coupled Coulomb Systems since the rise of DFT and SCCS-1977

The conferences on "Strongly Coupled Coulomb Systems" (SCCS) arose from the "Strongly Coupled Plasmas" meetings, inaugurated in 1977. The progress in SCCS theory is reviewed in an `author-centered' frame to limit its scope. Our efforts, i.e., with François Perrot, sought to apply density functional theory (DFT) to SCCS calculations. DFT was then poised to become the major computational scheme for condensed matter physics. The ion-sphere models of Salpeter and others evolved into useful average-atom models for finite-$T$ Coulomb systems, as in Lieberman's Inferno code. We replaced these by correlation-sphere models that exploit the description of matter via density functionals linked to pair-distributions. These methods provided practical computational means for studying strongly interacting electron-ion Coulomb systems like warm-dense matter (WDM). The staples of SCCS are wide-ranged, viz., equation of state, plasma spectroscopy, opacity (absorption, emission), scattering, level shifts, transport properties, e.g., electrical and heat conductivity, laser- and shock- created plasmas, their energy relaxation and transient properties etc. These calculations need pseudopotentials and exchange-correlation functionals applicable to finite-$T$ Coulomb systems that may be used in ab initio codes, molecular dynamics, etc. The search for simpler computational schemes has proceeded via proposals for orbital-free DFT, statistical potentials, classical maps of quantum systems using classical schemes like HNC to include strong coupling effects (CHNC). Laughlin's classical plasma map for the fractional quantum Hall effect (FQHE) is a seminal example where we report new results for graphene.

cond-mat.stat-mech↗

arXiv2013

Density-Functional theory, finite-temperature classical maps, and their implications for foundational studies of quantum systems

The advent of the Hohenberg-Kohn theorem in 1964, its extension to finite-T, Kohn-Sham theory, and relativistic extensions provide the well-established formalism of density-functional theory (DFT). This theory enables the calculation of all static properties of quantum systems {\it without} the need for an n-body wavefunction ψ. DFT uses the one-body density distribution instead of ψ. The more recent time-dependent formulations of DFT attempt to describe the time evolution of quantum systems without using the time-dependent wavefunction. Although DFT has become the standard tool of condensed-matter computational quantum mechanics, its foundational implications have remained largely unexplored. While all systems require quantum mechanics (QM) at T=0, the pair-distribution functions (PDFs) of such quantum systems have been accurately mapped into classical models at effective finite-T, and using suitable non-local quantum potentials (e.g., to mimic Pauli exclusion effects). These approaches shed light on the quantum \to hybrid \to classical models, and provide a new way of looking at the existence of non- local correlations without appealing to Bell's theorem. They also provide insights regarding Bohmian mechanics. Furthermore, macroscopic systems even at 1 Kelvin have de Broglie wavelengths in the micro-femtometer range, thereby eliminating macroscopic cat states, and avoiding the need for {\it ad hoc} decoherence models.

quant-ph↗

arXiv2012

Electron-ion and ion-ion potentials for modeling warm-dense-matter: applications to laser-heated or shock-compressed Al and Si

The pair-interactions U_{ij}(r) determine the thermodynamics and linear transport properties of matter via the pair-distribution functions (PDFs), i.e., g_{ij}(r). Great simplicity is achieved if U_{ij}(r) could be directly used to predict material properties via classical simulations, avoiding many-body wavefunctions. Warm dense matter (WDM) is encountered in quasi-equilibria where the electron temperature $T_e$ differs from the ion temperature T_i, as in laser-heated or in shock-compressed matter. The electron PDFs g_{ee}(r) as perturbed by the ions are used to evaluate fully non-local exchange-correlation corrections to the free energy, using Hydrogen as an example. Electron-ion potentials for ions with a bound core are discussed with Al and Si as examples, for WDM with T_e \ne T_i, and valid for times shorter than the electron-ion relaxation time. In some cases the potentials develop attractive regions, and then become repulsive and `Yukawa-like' for higher $T_e$. These results clarify the origin of initial phonon-hardening and rapid release. Pair-potentials for shock-heated WDM show that phonon hardening would not occur in most such systems. Defining meaningful quasi-equilibrium static transport coefficients consistent with the dynamic values is addressed. There seems to be no meaningful `static conductivity' obtainable by extrapolating experimental or theoretical σ(ω, T_i, T_e) to ω\to 0, unless T_i \to T_e as well. Illustrative calculations of quasi-static resistivities R(T_i,T_e) of laser-heated as well as shock-heated Aluminum and Silicon are presented using our pseudopotentials, pair-potentials and classical integral equations. The quasi-static resistivities display clear differences in their temperature evolutions, but are not the strict ω\to 0 limits of the dynamic values.

physics.plasm-ph↗

arXiv2011

The electron effective mass in the strongly correlated 2D-uniform electron fluid from finite-temperature calculations

The very-low temperature thermal effective mass m* of paramagnetic and ferromagnetic electrons in a uniform electron fluid in two dimensions is studied. Analytical and numerical evaluations are used to meaningfully define an m*, even in the the Hartree-Fock approximation. The Hartree-Fock m* decreases linearly with the electron-disk radius r_s. Correlation effects lead to strong cancellations between exchange and correlation. Thus the effective mass is enhanced with increasing r_s for the unpolarized fluid, while m* decreases with the r_s of the polarized fluid. The effective mass is calculated from the coefficient of the quadratic temperature dependence of exchange-correlation free energy F_{xc}. This is calculated in a physically transparent manner using a new formula for the effective mass. This uses the T=0 pair-distribution functions of Gori-Giorgi et al., and the temperature derivative of a quantum analogue of the potential of mean-force well known in the statistical mechanics of classical fluids. The results are compared with recent quantum Monte-Carlo simulations at T=0, as well as with other available experimental and theoretical data for the effective mass.

cond-mat.mes-hall↗

arXiv2011

The Classical-Map Hyper-Netted-Chain (CHNC) method and associated novel density-functional methods for Warm Dense Matter

The advent of short-pulse lasers, nanotechnology, as well as shock-wave techniques have created new states of matter (e.g., warm dense matter) that call for new theoretical tools. Ion correlations, electron correlations as well as bound states, continuum states, partial degeneracies and quasi-equilibrium systems need to be addressed. Bogoliubov's ideas of timescales can be used to discuss the quasi-thermodynamics of non-equilibrium systems. A rigorous approach to the associated many-body problem turns out to be the computation of the underlying pair-distribution functions g_ee, g_ei and g_ii, that directly yield non-local exchange-correlation potentials, free energies etc., valid within the timescales of each evolving system. An accurate classical map of the strongly-quantum uniform electron-gas problem given by Dharma-wardana and Perrot is reviewed. This replaces the quantum electrons at T=0 by an equivalent classical fluid at a finite temperature T_q, and having the same correlation energy. It has been shown, but not proven, that the classical fluid g_ij are excellent approximations to the quantum g_ij. The classical map is used with classical molecular dynamics (CMMD) or hyper-netted-chain integral equations (CHNC) to determine the pair-distribution functions (PDFs), and hence their thermodynamic and linear transport properties. The CHNC is very efficient for calculating the PDFs of uniform systems, while CMMD is more adapted to non-uniform systems. Applications to 2D and 3D quantum fluids, Si metal-oxide-field-effect transistors, Al plasmas, shock-compressed deuterium, two-temperature plasmas, pseudopotentials, as well as calculations for parabolic quantum dots are reviewed.

cond-mat.stat-mech↗

arXiv2010

The Coulomb bridge function and the Pair-distribution functions of the 2-dimensional electron liquid in the quantum regime

The electron-electron pair distribution functions (PDF) of the 2-D electron fluid (2DEF) in the quantum regime (at T=0) are calculated using a classical-map-hyper-netted-chain (CHNC) scheme and compared with currently available Quantum Monte-Carlo (QMC) simulations in the coupling range r_s=1 to 50. We iteratively extract the bridge function of the "equivalent" classical 2-D liquid in the quantum regime. These bridge functions B(r) are relatively insensitive to spin-polarization effects. The structure of the bridge functions changes significantly for r_s>6, suggesting the onset of strongly correlated clusters. The new B(r), appropriate for the long-range Coulomb potential, can be used to replace the hard-sphere B(r) previously used in these calculations. They provide accurate classical representations of the QMC-PDFs even at very strong coupling, and probably at finite-T near T=0.

cond-mat.mtrl-sci↗

arXiv2009

The Classical-Map Hyper-Netted-Chain (CHNC) technique for inhomogeneous electron systems. Application to quantum dots

The Classical-map Hyper-Netted-Chain (CHNC) technique is a simple method of calculating quantum pair-distribution functions, spin-dependent energies, etc., of strongly-interacting {\it uniform} systems. We present CHNC calculations of charge densities and energies of {\it non-uniform} systems, viz., quantum dots, and compare with quantum Monte Carlo and density -functional results. Results for up to 210 electrons are reported.

cond-mat.str-el↗

arXiv2008

Magnetism and structure at a vacancy in graphene

The electronic structure, bonding and magnetism in graphene containing vacancies are studied using density-functional methods. The single-vacancy graphene ground state is spin polarized and structurally flat. The unpolarized state is non planar only for finite segments. Systems containing periodic arrays of vacancies displays magnetic transitions and metal-insulator transitions.

cond-mat.mes-hall↗

arXiv2007

Temperature Relaxation in Hot Dense Hydrogen

Temperature equilibration of hydrogen is studied for conditions relevant to inertial confinement fusion. New molecular-dynamics simulations and results from quantum many-body theory are compared with Landau-Spitzer (LS) predictions for temperatures T from 50 eV to 5000 eV, and densities with Wigner-Seitz radii r_s = 1.0 and 0.5. The relaxation is slower than the LS result, even for temperatures in the keV range, but converges to agreement in the high-T limit.

cond-mat.str-el↗