Long-range Coulomb Interaction Research Articles

Effect of Coulomb interaction on transport gap in ideal graphene nanoribbons

Quantum-mechanical calculations of electron transport in ideal graphene nanoribbons show that the transport gap that is predicted by noninteracting theories vanishes if the long-range Coulomb interaction between electrons is taken into account. This is a result of charge screening with the lowest subband edge being pinned to the chemical potential. However, the transport gap reappears if a ribbon is connected to wider leads, which is typically realized in an experimental setup that is based on lithographically patterned graphene ribbons. The gap is determined by scattering at the lead-to-ribbon interface, which can already be captured by the noninteracting theory.

LAMMPS - a flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales

Since the classical molecular dynamics simulator LAMMPS was released as an open source code in 2004, it has become a widely-used tool for particle-based modeling of materials at length scales ranging from atomic to mesoscale to continuum. Reasons for its popularity are that it provides a wide variety of particle interaction models for different materials, that it runs on any platform from a single CPU core to the largest supercomputers with accelerators, and that it gives users control over simulation details, either via the input script or by adding code for new interatomic potentials, constraints, diagnostics, or other features needed for their models. As a result, hundreds of people have contributed new capabilities to LAMMPS and it has grown from fifty thousand lines of code in 2004 to a million lines today. In this paper several of the fundamental algorithms used in LAMMPS are described along with the design strategies which have made it flexible for both users and developers. We also highlight some capabilities recently added to the code which were enabled by this flexibility, including dynamic load balancing, on-the-fly visualization, magnetic spin dynamics models, and quantum-accuracy machine learning interatomic potentials. Program SummaryProgram Title: Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)CPC Library link to program files:https://doi.org/10.17632/cxbxs9btsv.1Developer's repository link:https://github.com/lammps/lammpsLicensing provisions: GPLv2Programming language: C++, Python, C, FortranSupplementary material:https://www.lammps.orgNature of problem: Many science applications in physics, chemistry, materials science, and related fields require parallel, scalable, and efficient generation of long, stable classical particle dynamics trajectories. Within this common problem definition, there lies a great diversity of use cases, distinguished by different particle interaction models, external constraints, as well as timescales and lengthscales ranging from atomic to mesoscale to macroscopic.Solution method: The LAMMPS code uses parallel spatial decomposition, distributed neighbor lists, and parallel FFTs for long-range Coulombic interactions [1]. The time integration algorithm is based on the Størmer-Verlet symplectic integrator [2], which provides better stability than higher-order non-symplectic methods. In addition, LAMMPS supports a wide range of interatomic potentials, constraints, diagnostics, software interfaces, and pre- and post-processing features.Additional comments including restrictions and unusual features: This paper serves as the definitive reference for the LAMMPS code.

Structure and shock properties of amorphous silica predicted by a metal-organic framework force field

The laser damage induced by nano-absorbing centers generally results in a local high temperature and pressure environment, leading to denser phases and complex hydrodynamic processes. Here we parameterize the metal-organic framework force field to overcome the notorious unphysical agglomeration at small atomic distance in a Buckingham term. The structure and shock properties of amorphous silica are predicted well by the parameterized force field. By avoiding the Ewald summation of long-range coulomb interaction, the periodic boundary condition is not in such demand that the computational efficiency is greatly improved. The parameterized force field implicates a prospect for the atomic investigation of laser-induced hydrodynamic processes around the free surface or interface.

Fragile versus stable two-dimensional fermionic quasiparticles

We provide a comprehensive theoretical investigation of the Fermi liquid quasiparticle description in two-dimensional electron gas interacting via the long-range Coulomb interaction by calculating the electron self-energy within the leading-order approximation, which is exact in the high-density limit. We find that the quasiparticle energy is larger than the imaginary part of the self-energy up to very high energies, implying that the basic Landau quasiparticle picture is robust up to far above the Fermi energy. We find, however, that the quasiparticle picture becomes fragile in a small discrete region around a critical wave vector where the quasiparticle spectral function strongly deviates from the expected quasiparticle Lorentzian line shape with a vanishing renormalization factor. We show that such a non-Fermi liquid behavior arises due to the coupling of quasiparticles with the collective plasmon mode. This situation is somewhat intermediate between the one-dimensional interacting electron gas (i.e., Luttinger liquid), where the Landau Fermi liquid theory completely breaks down since only bosonic collective excitations exist, and three-dimensional electron gas, where quasiparticles are well-defined and more stable against interactions than in one and two dimensions. We use a number of complementary definitions for a quasiparticle to examine the interacting spectral function, contrasting two-dimensional and three-dimensional situations critically.

Nonperturbative Dyson-Schwinger equation approach to strongly interacting Dirac fermion systems

Studying the strong correlation effects in interacting Dirac fermion systems is one of the most challenging problems in modern condensed matter physics. The long-range Coulomb interaction and the fermion-phonon interaction can lead to a variety of intriguing properties. In the strong-coupling regime, weak-coupling perturbation theory breaks down. The validity of $1/N$ expansion with $N$ being the fermion flavor is also in doubt since $N$ equals to 2 or 4 in realistic systems. Here, we investigate the interaction between $(1+2)$- and $(1+3)$-dimensional massless Dirac fermions and a generic scalar boson, and develop an efficient nonperturbative approach to access the strong-coupling regime. We first derive a number of self-consistently coupled Ward-Takahashi identities based on a careful symmetry analysis and then use these identities to show that the full fermion-boson vertex function is solely determined by the full fermion propagator. Making use of this result, we rigorously prove that the full fermion propagator satisfies an exact and self-closed Dyson-Schwinger integral equation, which can be solved by employing numerical methods. A major advantage of our nonperturbative approach is that there is no need to employ any small expansion parameter. Our approach provides a unified theoretical framework for studying strong Coulomb and fermion-phonon interactions. It may also be used to approximately handle the Yukawa coupling between fermions and order-parameter fluctuations around continuous quantum critical points. Our approach is applied to treat the Coulomb interaction in undoped graphene. We find that the renormalized fermion velocity exhibits a logarithmic momentum dependence but is nearly energy independent, and that no excitonic gap is generated by the Coulomb interaction. These theoretical results are consistent with experiments in graphene.

Superconducting state of Sr2RuO4 in the presence of longer-range Coulomb interactions

The symmetry of the superconducting condensate in Sr$_2$RuO$_4$ remains controversial after nuclear magnetic resonance (NMR) experiments recently overturned the dominant chiral $p$-wave paradigm. Several theoretical proposals have been put forward to account for existing experiments, including a $d+ig$-wave admixture, conjectured to be stabilized by longer-range Coulomb interactions. We perform a material-specific microscopic theoretical study of pairing by spin- and charge-fluctuations in Sr$_2$RuO$_4$, including the effects of spin-orbit coupling, and both local and longer-range Coulomb repulsion. The latter has important consequences for Sr$_2$RuO$_4$ due to the near-degeneracy of symmetry-distinct pairing states in this material. We find that both the $g$- and $d_{x^2-y^2}$-wave channels remain noncompetitive compared to leading nodal $s'$, $d_{xy}$, and helical ($p$) solutions. This suggests nodal time-reversal symmetry broken $s'+id_{xy}$ or $s'+ip$ phases, promoted by longer-range Coulomb repulsion, as the most favorable candidates for Sr$_2$RuO$_4$. We analyse the properties of these states, and show that the $s'+id_{xy}$ solution agrees with the bulk of available experimental data, including recent discoveries from NMR, muon spin relaxation ($\mu$SR), and ultrasound measurements.

Free-Carrier-Induced Ferroelectricity in Layered Perovskites.

Doping ferroelectrics with carriers is often detrimental to polarization. This makes the design and discovery of metals that undergo a ferroelectriclike transition challenging. In this Letter, we show from first principles that the oxygen octahedral rotations in perovskites are often enhanced by electron doping, and this can be used as a means to strengthen the structural polarization in certain hybrid-improper ferroelectrics-compounds in which the polarization is not stabilized by the long-range Coulomb interactions but is instead induced by a trilinear coupling to octahedral rotations. We use this design strategy to predict a cation ordered Ruddlesden-Popper compound that can be driven into a metallic ferroelectriclike phase via electrolyte gating.

Electron self-energy from quantum charge fluctuations in the layered t−J model with long-range Coulomb interaction

Employing a large-N scheme of the layered t-J model with the long-range Coulomb interaction, which captures fine details of the charge excitation spectra recently observed in cuprate superconductors, we explore the role of the charge fluctuations on the electron self-energy. We fix temperature at zero and focus on quantum charge fluctuations. We find a pronounced asymmetry of the imaginary part of the self-energy Im$\Sigma({\bf k}, \omega)$ with respect to $\omega = 0$, which is driven by strong electron correlation effects. The quasiparticle weight is reduced dramatically, which occurs almost isotropically along the Fermi surface. Concomitantly an incoherent band and a sharp side band are newly generated and acquire sizable spectral weight. All these features are driven by usual on-site charge fluctuations, which are realized in a rather high-energy region and yield plasmon excitations. On the other hand, the low-energy region with the scale of the superexchange interaction J is dominated by bond-charge fluctuations. Surprisingly, compared with the effect of the on-site charge fluctuations, their effect on the electron self-energy is much weaker even if the system approaches close to bond-charge instabilities. Furthermore, quantum charge dynamics does not produce a clear kink nor a pseudogap in the electron dispersion.

Unified theory of spin and charge excitations in high- Tc cuprate superconductors: A quantitative comparison with experiment and interpretation

We provide a unified interpretation of both paramagnon and plasmon modes in high-$T_c$ copper-oxides, and verify it quantitatively against available resonant inelastic $x$-ray scattering (RIXS) data across the hole-doped phase diagram. Three-dimensional extended Hubbard model, with included long-range Coulomb interactions and doping-independent microscopic parameters for both classes of quantum fluctuations, is used. Collective modes are studied using VWF+$1/\mathcal{N}_f$ approach which extends variational wave function (VWF) scheme by means of an expansion in inverse number of fermionic flavors ($1/\mathcal{N}_f$). We show that intense paramagnons persist along the anti-nodal line from the underdoped to overdoped regime and undergo rapid overdamping in the nodal direction. Plasmons exhibit a three-dimensional character, with minimal energy corresponding to anti-phase oscillations on neighboring $\mathrm{CuO_2}$ planes. The theoretical spin- and charge excitation energies reproduce semi-quantitatively RIXS data for $\mathrm{(Bi, Pb)_2 (Sr, La)_2 CuO_{6+\delta}}$. The present VWF+$1/\mathcal{N}_f$ analysis of dynamics and former VWF results for static quantities combine into a consistent description of the principal properties of hole-doped high-$T_c$ cuprates as strongly correlated systems.

Flow of polar and nonpolar liquids through nanotubes: A computational study

We perform ab initio density functional calculations to study the flow of water, methanol, and dimethyl ether through nanotubes of carbon and boron nitride with different diameters and chiralities. The liquids we choose are important solvents, with water and methanol being polar and dimethyl ether being nonpolar. In terms of activation barriers for liquid transport, we find the molecular-level drag to decrease with decreasing nanotube diameter but to be rather independent of the chiral index. We also find molecules with higher polarity to generally experience higher drag during flow. Counterintuitively, we find the drag for water in boron nitride nanotubes not to exceed that in carbon nanotubes due to frustration in competing long-range Coulomb interactions.

Connected Coulomb columns: analysis and numerics

We consider a version of Gamow’s liquid drop model with a short range attractive perimeter-penalizing potential and a long-range Coulomb interaction of a uniformly charged mass in . Here we constrain ourselves to minimizing among the class of shapes that are columnar, i.e., constant in one spatial direction. Using the standard perimeter in the energy would lead to non-existence for any prescribed cross-sectional area due to the infinite mass in the constant spatial direction. In order to overcome this issue we use a connected perimeter instead. We prove existence of minimizers for this connected isoperimetric problem with long-range interaction and study the shapes of minimizers in the small and large cross section regimes. For an intermediate regime we use an Ohta–Kawasaki phase field model with connectedness constraint to study the shapes of minimizers numerically.

DNA Framework-Engineered Long-Range Electrostatic Interactions for DNA Hybridization Reactions.

Long-range electrostatic interactions beyond biomolecular interaction interfaces have not been extensively studied due to the limitation in engineering electric double layers in physiological fluids. Here we find that long-range electrostatic interactions play an essential role in kinetic modulation of DNA hybridizations. Protein and gold nanoparticles with different charges are encapsulated in tetrahedral frameworks to exert diverse electrostatic effects on site-specifically tethered single DNA strands. Using this strategy, we have successfully modulated the hybridization kinetics in both bulk solution and single molecule level. Experimental and theoretical studies reveal that long-range Coulomb interactions are the key factor for hybridization rates. This work validates the important role of long-range electrostatic forces in nucleic acid-biomacromolecule complexes, which may encourage new strategies of gene regulation, antisense therapy, and nucleic acid detection.

Elastic electron scattering from Ar@C60 z+: Dirac partial-wave analysis

Elastic electron scattering from Ar@C60 z+ (z = 1, 6) is investigated by employing Dirac partial-wave analysis within the static-exchange model. The C60 cage is approximated using compact annular square well (ASW) and diffuse Gaussian annular square well (GASW) confinement potentials. In order to investigate the roles of Ar z+and C60 z+ targets in the e–Ar@C60 z+ collision, the scattering dynamics of individual targets is also presented. A quantitative analysis of the total cross-section (TCS) and the differential cross-section (DCS) is presented, highlighting the effects of the charge state andpolarizability of the target. The ramifications of compact versus diffuse confinement potentials in modeling electron scattering with endohedral targets are also presented. This work concludes that the long-range Coulomb interaction dominates the elastic scattering process, compared to the short-range contributions, which makes the TCS independent of the target details, and only sensitive to the charge state.

Correlated Magnetic Weyl Semimetal State in Strained Pr2 Ir2 O7.

Correlated topological phases (CTPs) with interplay between topology and electronic correlations have attracted tremendous interest in condensed matter physics. Therein, correlated Weyl semimetals (WSMs) are rare in nature and, thus, have so far been less investigated experimentally. In particular, the experimental realization of the interacting WSM state with logarithmic Fermi velocity renormalization has not been achieved yet. Here, experimental evidence of a correlated magnetic WSM state with logarithmic renormalization in strained pyrochlore iridate Pr2 Ir2 O7 (PIO) which is a paramagnetic Luttinger semimetal in bulk, is reported. Benefitting from epitaxial strain, "bulk-absent" all-in-all-out antiferromagnetic ordering can be stabilized in PIO film, which breaks time-reversal symmetry and leads to a magnetic WSM state. With further analysis of the experimental data and renormalization group calculations, an interacting Weyl liquid state with logarithmically renormalized Fermi velocity, similar to that in graphene, is found, dressed by long-range Coulomb interactions. This work highlights the interplay of strain, magnetism, and topology with electronic correlations, and paves the way for strain-engineering of CTPs in pyrochlore iridates.

Pseudogap metal induced by long-range Coulomb interactions

In correlated electron systems the metallic character of a material can be strongly suppressed near an integer concentration of conduction electrons as Coulomb interactions forbid the double occupancy of local atomic orbitals. While the Mott-Hubbard physics arising from such on-site interactions has been largely studied, several unexplained phenomena observed in correlated materials challenge this description and call for the development of new ideas. Here we explore a general route for obtaining correlated behavior that is decidedly different from the spin-related Mott-Hubbard mechanism and instead relies on the presence of unscreened, long-range Coulomb interactions. We find a pseudogap metal phase characterized by a divergent quasiparticle mass and the opening of a Coulomb pseudogap in the electronic spectrum. The destruction of the Fermi-liquid state occurs because the electrons move in a nearly frozen, disordered charge background, as collective charge rearrangements are drastically slowed down by the frustrating nature of long-range potentials on discrete lattices. The present pseudogap metal realizes an early conjecture by Efros, that a soft Coulomb gap should appear for quantum lattice electrons with strong unscreened interactions due to self-generated randomness.

Electronic structures, charge transfer, and charge order in twisted transition metal dichalcogenide bilayers

Moir\'e superlattices of transition metal dichalcogenide (TMD) bilayers have been shown to host correlated electronic states, which arise from the interplay of long wavelength moir\'e potential and long-range Coulomb interaction. Here, we theoretically investigate structural relaxation and single-particle electronic structure of twisted TMD homobilayer. From the large-scale density functional theory calculation and continuum model with layer degrees of freedom, we find that the out-of-plane gating field creates a tunable charge transfer gap at the Dirac point between the first and second moir\'e valence bands. We further study the charge orders at the fractional band fillings. In the flat band limit, we find from Monte Carlo simulations a series of charge-ordered insulating states at various fillings $n=\frac{1}{4},\frac{1}{3},\frac{1}{2},\frac{2}{3},1$. We predict that gating field induces a phase transition between different electron crystals at fixed filling $n=\frac{1}{2}$ or $\frac{2}{3}$. At half-filling $n=1$, the ground state is a Mott insulator with electronically driven ferroelectricity. This work demonstrates that TMD homobilayer provides a powerful platform for the investigation of tunable charge transfer insulator and charge orders.

Weakening of Solvation-Induced Ordering by Composition Fluctuation in Salt-Doped Block Polymers.

The spontaneous ordering of block polymers doped with ions is affected by both selective solvation and long-range Coulombic interaction. The mean-field treatment was recently shown to overestimate the solvation-induced ordering, requiring a large solvation radius to fit experimental phase diagrams, which may be relieved by including composition fluctuations. Treating the composition fluctuations in such systems is challenging because of the need of resolving heterogeneous dielectric profile that couples with the ordering itself. Starting from a minimal model, we develop a Landau-Brazovskiĭ expansion for the free energy of salt-doped block polymer near the ordering transition. It is found that the wavelength for typical composition fluctuations first decreases with salt doping, due to Coulombic interaction, then increases due to ionic solvation. Two mechanisms that weaken the solvation-enhanced ordering are identified: the Brazovskiĭ-type composition fluctuation that stabilizes disordered phase, and the coupling between mismatch in dispersion interaction and the dielectric permittivity through monomeric polarizability.

Prediction of anomalous LA-TA splitting in electrides

Electrides are an emerging class of materials with excess electrons localized in interstices and acting as anionic interstitial quasi-atoms (ISQs). The spatial ion–electron separation means that electrides can be treated physically as ionic crystals, and this unusual behavior leads to extraordinary physical and chemical phenomena. Here, a completely different effect in electrides is predicted. By recognizing the long-range Coulomb interactions between matrix atoms and ISQs that are unique in electrides, a nonanalytic correction to the forces exerted on the matrix atoms is proposed. This correction gives rise to a longitudinal acoustic-transverse acoustic splitting in the acoustic branch of lattice phonons near the zone center, similar to the well-known longitudinal optical–transverse optical splitting in the phonon spectra of ionic compounds. The factors that govern this splitting are investigated, with isotropic fcc-Li and anisotropic hP4-Na as the typical examples. It is found that not all electrides can induce a detectable splitting, and criteria are given for this type of splitting. The present prediction unveils the rich phenomena in electrides and could lead to unprecedented applications.

Hydrodynamic sound and plasmons in three dimensions

In a recent paper by Lucas and Das Sarma [Physical Review B 97, 115449 (2018)], a solvable model of collective modes in 2D metals was considered in the hydrodynamic regime. In the current work, we generalize the hydrodynamic theory to 3D metals for which the calculation of sound modes in a strongly-coupled quantum Coulomb plasma can be made explicit. The specific theoretical question of interest is how the usual linearly dispersing hydrodynamic sound mode relates to the well-known gapped 3D plasmon collective mode in the presence of long-range Coulomb interaction. We show analytically that both the zero sound in the collisionless regime and the first sound in the hydrodynamic region become massive in 3D, acquiring a finite gap because of the long-range Coulomb interaction, while their damping rates become quadratic in momentum. We also discuss other types of long-range potential, where the dispersion of sound modes is modified accordingly. The general result is that the leading order hydrodynamic sound mode is always given by the leading-order plasmon frequency in the presence of long-range Coulomb interaction, but the next-to-leading-order dispersion corrections differ in hydrodynamic and colllisionless regimes.

Mean-field simulation of hopping resistivity in a model with Coulomb interactions and current correlations

Simulations of hopping resistivity by mean-field methods rely on randomly generated site energies. Thus such effects as the presence of the Coulomb gap and electron current correlations have been simulated using predominantly Monte Carlo methods. This paper presents a method of mean-field calculation of hopping resistivity in a wide range of temperatures and electric fields. This method considers the long-range Coulomb interactions and at least partially accounts for the electron current correlations. Resulting curves reproduce the Mott law in the form predicted by Efros and Shklovskii and the experimentally observed electric field dependencies. Based on the results, a decrease of hopping energy is predicted with increasing temperature in the nearest-neighbor hopping regime. The report analyses the differences between the mean-field and the Monte Carlo approaches.