Anisotropic Coulomb Research Articles

A robot inspired by a non‐smooth point mass model of a worm

AbstractA non‐smooth point mass model which mimics the locomotion of an earthworm is presented. The planar model consists of a chain with five elements on a rough ground. The distance between each pair of neighbouring elements is restricted by unilateral constraints. The contacts between the elements and the ground are subjected to anisotropic Coulomb friction. First, the equations of motion governing the impact‐free motion are derived. Then, the impact equations are formulated which, together with the generalized Newton's impact law, describe the dynamics at instantaneous impacts. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of multiorbital and anisotropic Coulomb interactions on isotope effect coefficient in doped Fe-based superconductors

The present work describes the theoretical analysis of isotope effect coefficient as a function of transition temperature in two orbital per site model Hamiltonian in iron based superconducting system. The expression of isotope effect coefficient has been computed numerically and self-consistently by employing Green's function technique within the BCS- mean-field approximation. It is observed that the isotope effect coefficient increases with the increase of the hybridization while with the increase in Coulomb interaction it starts decreasing. On increasing the carrier density per site in two orbital per site iron pnictide system, isotope effect coefficient (α) exhibits large values (much higher than BCS limit) at lower temperatures. While in the underdoped case, isotope effect coefficient shows minimum value in superconducting states of the iron based systems. Furthermore, it has been found that the large value of the isotope effect coefficient is the indication of the fact that the contribution of phonon alone is inadequate as the origin of superconductivity in these systems. Finally, the obtained theoretical results have been compared with experimental and existing theoretical observations in iron based superconductors.

Mott transition, spin-orbit effects, and magnetism in Ca2RuO4

In this work, we study the effects of spin-orbit and Coulomb anisotropy on the electronic and magnetic properties of the Mott insulator ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$. We use the local-density approximation + dynamical mean-field approach and spin-wave theory. We show that, contrary to a recent proposal, the Mott metal-insulator transition is not induced by the spin-orbit interaction. We confirm that, instead, it is mainly driven by the change in structure from long to short $\mathbf{c}$-axis layered perovskite. We show that the magnetic ordering and the anisotropic Coulomb interactions play a small role in determining the the size of the gap. The spin-orbit interaction turns out to be essential for describing the magnetic properties. It not only results in a spin-wave gap, but it also enlarges significantly the magnon bandwidth.

Anisotropic magnetoresistance and piezoelectric effect in GaAs Hall samples

In this work, we argue that an anisotropic interaction potential may stabilize anisotropic liquid phases of electrons even in a strong magnetic field regime where normally one expects to see only isotropic quantum Hall or isotropic Fermi liquid states. We use this approach to support a theoretical framework that envisions the possibility of an anisotropic liquid crystalline state of electrons in the lowest Landau level. In particular, we argue that an anisotropic liquid state of electrons may stabilize in the lowest Landau level close to the liquid-solid transition region at filling factor $\nu=1/6$ for a given anisotropic Coulomb interaction potential. Quantum Monte Carlo simulations for a liquid crystalline state with broken rotational symmetry indicate stability of liquid crystalline order consistent with the existence of an anisotropic liquid state of electrons stabilized by anisotropy at filling factor $\nu=1/6$ of the lowest Landau level.

Anisotropic electronic states in the fractional quantum Hall regime

Recent experiments indicate the presence of new anisotropic fractional quantum Hall states at regimes not anticipated before. These experiments raise many fundamental questions regarding the inner nature of the electronic system that leads to such anisotropic states. Interplay between electron mass anisotropy and electron-electron correlation effects in a magnetic field can create a rich variety of possibilities. Several anisotropic electronic states ranging from anisotropic quantum Hall liquids to anisotropic Wigner solids may stabilize due to such effects. The electron mass anisotropy in a two-dimensional electron gas effectively leads to an anisotropic Coulomb interaction potential between electrons. An anisotropic interaction potential may strongly influence the stability of various quantum phases that are close in energy since the overall stability of an electronic system is very sensitive to local order. As a result there is a possibility that various anisotropic electronic phases may emerge even in the lowest Landau level in regimes where one would not expect them. In this work we study the state with filling factor 1/6 in the lowest Landau level, a state which is very close to the critical filling factor where the liquid-solid transition takes place. We investigate whether an anisotropic Coulomb interaction potential is able to stabilize an anisotropic electronic liquid state at this filling factor. We describe such an anisotropic state by means of a liquid crystalline wave function with broken rotational symmetry which can be adiabatically connected to the actual wave function for the corresponding isotropic phase. We perform quantum Monte Carlo simulations in a disk geometry to study the properties of the anisotropic electronic liquid state under consideration. The findings indicate stability of liquid crystalline order in presence of an anisotropic Coulomb interaction potential. The results are consistent with the existence of an anisotropic electronic liquid state in the lowest Landau level.

Anisotropic Coulomb Explosion of CO Ligands in Group 6 Metal Hexacarbonyls: Cr(CO)6, Mo(CO)6, W(CO)6

Multiple ionization and subsequent Coulomb explosion have been studied for many organic molecules and their clusters; however, the metal complexes, particularly the large Coulombic interactions expected between a metal and its ligands, have not yet been explored. In this study, the angular distribution of CO(+), oxygen, and carbon ions ejected from metal hexacarbonyls (M(CO)6, M: Cr, Mo, W) having Oh symmetry by Coulomb explosion in femtosecond laser fields (>1 × 10(14) W cm(-2)) is investigated. The emissions of oxygen ions are well-explained in terms of the geometric alignment along a line inclined 45° relative to the CO-M-CO axis in a M(CO)4 plane. Unlike the explosion behavior of the oxygen ions located on the outer part of the molecule, the explosion behavior of the carbon ions was affected by the laser intensity, kinetic energy, and metal. This finding that the emission trends of carbon sandwiched between oxygen and metal atoms were the opposite of those for oxygen was explained by the obstruction by oxygen, the deformation of structure in bending coordinates, and the strong interaction with charged metal. The anisotropic Coulomb explosion of metal complexes reflecting their structural symmetry and central metal charge is a promising candidate for use in the investigation of large Coulombic interactions at the molecular level.

Anisotropic Coulomb explosion of acetylene and diacetylene derivatives

We investigated the significant differences in the angular distribution of carbon ions ejected from dimethylacetylenes (CH3CCCH3 and CH3CCCCCH3) and diiodoacetylenes (ICCI and ICCCCI) induced by a Coulomb explosion. The longest linear chain molecule ever reported was exposed to intense femtosecond laser fields (40fs, <5×1014Wcm−2) in this study. In the cases of the diiodoacetylenes, the angular distributions of the carbon ions were orthogonal with respect to the laser polarization direction and became narrower as the length of the molecules became longer. This is in sharp contrast to the dimethylacetylenes, in which the angular distributions of the carbon ions were parallel with respect to the laser polarization direction and became broader as the length of the molecules became longer. The specific angular distributions of carbon ions ejected from the diiodoacetylenes are explained in terms of the frozen molecular rotation, the blocking of carbon ion emission, and the bending of carbon chains at highly charged states due to the presence of iodines.

The novel electronic and magnetic properties in 5d transition metal oxides system

In 5d transition metal oxides, novel properties arise from the interplay of electron correlations and spin–orbit interactions. In this paper, we briefly review our theoretical progress relating to 5d compounds. We focus on describing the topological Weyl-Semimetal in pyrochlore iridates, the Axion insulator in spinel osmates, the Slater insulator in perovskite osmates. We also discuss the anisotropic unscreened Coulomb interaction in ferroelectric metal LiOsO3.

Anisotropic plasmon-coupling dimerization of a pair of spherical electron gases

We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled non-concentric spherical two-dimensional electron gases (S2DEGs). Our results show that the plasmon excitations for such pairs depend on the orientation with respect to the external electromagnetic probe field. The origin of this anisotropy of the inter-sphere Coulomb interaction is due to the directional asymmetry of the electrostatic coupling of electrons in excited states which depend on both the angular momentum quantum number L and its projection M on the axis of quantization taken as the probe E-field direction. We demonstrate the anisotropic inter-sphere Coulomb coupling in space and present semi-analytic results in the random-phase approximation both perpendicular and parallel to the axis of quantization. For the incidence of light with a finite orbital or spin angular momentum, the magnetic field generated from an induced oscillating electric dipole on one sphere can couple to an induced magnetic dipole on another sphere in a way that is dependent on whether the direction is parallel or perpendicular to the probe E field. Such an effect from the plasmon spatial correlation is expected to be experimentally observable by employing circularly polarized light or a helical light beam for incidence. The S2DEG serves as a simple model for fullerenes as well as metallic dimers, when the energy bands are far apart.

Importance of anisotropic Coulomb interactions and exchange to the band gap and antiferromagnetism ofβ-MnO2from DFT+U

First principles density functional theory (DFT) is used to investigate the electronic structure of \beta-MnO2. From collinear spin polarized calculations we find that DFT+U_Eff predicts a gapless ferromagnet in contrast with experiment which indicates an insulating antiferromagnet. The inclusion of anisotropic Coulomb and exchange interactions in the DFT+U approach, defining U and J explicitly, corrects these errors and leads to an antiferromagnetic ground state with a fundamental gap of 0.8 eV consistent with low temperature experiments. To our knowledge, this work on \beta-MnO2 represents the first demonstration of a case in which the application of fully anisotropic interactions in DFT+U determines the magnetic order and consequent band gap, while the more commonly used effective U approach fails. Such effects are argued to be of importance in many insulating materials. The mechanism leading to an increase in band gap due to anisotropic interactions is highlighted by analytical calculation of DFT+U d-orbital eigenvalues obtained within a Kanamori-type model. Magnetic coupling constants obtained by the fitting of a Heisenberg spin Hamiltonian to the energies of a range of magnetic states assist in rationalizing the finding that anisotropic interactions enhance the stability of the experimentally observed helical antiferromagnetic order. The plane wave PAW method yields poorer results for the exchange couplings than full-potential LAPW calculations. Finally, we compare the DFT+U results with exchange couplings obtained from hybrid functionals. It is argued that anisotropic interactions should be included in DFT+U if the results are to be properly compared with those from hybrid functionals.

Fractional quantum Hall states in two-dimensional electron systems with anisotropic interactions

We study the anisotropic effect of the Coulomb interaction on a 1/3-filling fractional quantum Hall system by using an exact diagonalization method on small systems in torus geometry. For weak anisotropy the system remains to be an incompressible quantum liquid, although anisotropy manifests itself in density correlation functions and excitation spectra. When the strength of anisotropy increases, we find the system develops a Hall-smectic-like phase with a one-dimensional charge density wave order and is unstable towards the one-dimensional crystal in the strong anisotropy limit. In all three phases of the Laughlin liquid, Hall-smectic-like, and crystal phases the ground state of the anisotropic Coulomb system can be well described by a family of model wave functions generated by an anisotropic projection Hamiltonian. We discuss the relevance of the results to the geometrical description of fractional quantum Hall states proposed by Haldane [ Phys. Rev. Lett. 107 116801 (2011)].

Generation of a quasi-monoergetic proton beam from laser-irradiated sub-micron droplets

Proton bursts with a narrow spectrum at an energy of (2.8 ± 0.3 MeV) are accelerated from sub-micron water spray droplets irradiated by high-intensity (∼5 × 1019 W/cm2), high-contrast (∼1010), ultra-short (40 fs) laser pulses. The acceleration is preferentially in the laser propagation direction. The explosion dynamics is governed by a residual ps-scale laser pulse pedestal which “mildly” preheats the droplet and changes its density profile before the arrival of the high intensity laser pulse peak. As a result, the energetic electrons extracted from the modified target by the high-intensity part of the laser pulse establish an anisotropic electrostatic field which results in anisotropic Coulomb explosion and proton acceleration predominantly in the forward direction. Hydrodynamic simulations of the target pre-expansion and 3D particle-in-cell simulations of the measured energy and anisotropy of the proton emission have confirmed the proposed acceleration scenario.

Single Temperature for Monte Carlo Optimization on Complex Landscapes

We propose a new strategy for Monte Carlo (MC) optimization on rugged multidimensional landscapes. The strategy is based on querying the statistical properties of the landscape in order to find the temperature at which the mean first passage time across the current region of the landscape is minimized. Thus, in contrast to other algorithms such as simulated annealing, we explicitly match the temperature schedule to the statistics of landscape irregularities. In cases where these statistics are approximately the same over the entire landscape or where nonlocal moves couple distant parts of the landscape, a single-temperature MC scheme outperforms any other MC algorithm with the same move set. We also find that in strongly anisotropic Coulomb spin glass and traveling salesman problems, the only relevant statistics (which we use to assign a single MC temperature) are those of irregularities in low-energy funnels. Our results may explain why protein folding is efficient at constant temperature.

Proton Acceleration due to Anisotropic Coulomb Explosion of a Double-Layer Target Irradiated by an Intense Laser Pulse

The ion acceleration by a hundred TW laser pulse irradiating a double-layer target is examined using three-dimensional particle-in-cell simulations. For a sufficiently high ion charge-to-mass ratio in the first layer of the target, a strongly inhomogeneous expansion of the first layer occurs due to its Coulomb explosion and the onset of the radiation pressure dominant acceleration regime. The time-varying electric potential of the inhomogeneously expanding ion cloud efficiently accelerates protons. Using the optimum material for the first layer and the optimum laser pulse incidence angle one can obtain a high-energy quasimonoenergetic proton beam.

Dynamics of a mobile system with an internal acceleration-controlled mass in a resistive medium

The dynamics of a mobile system with a movable internal mass is investigated in this paper. Due to the periodic motion of the internal acceleration-controlled mass as well as the anisotropy of the external resistance, the system can move in a resistive medium. Major attention is given to the steady-state motion and stick–slip effect of the system as a whole. For anisotropic Coulomb's dry friction, in light of the non-smooth factors in both the internal control mode and external resistance, method of averaging is adopted to obtain an approximate expression of the average steady-state velocity when the stick–slip motion is absent. Optimizing the parameters of the internal controlled mass enables one to realize a maximal average steady-state velocity of the system. In view of the stick–slip effect, the steady-state motion of the system is classified into eight types, and the characteristics of each type are analyzed. Stick–slip motion is of our interest and receives extra attention. Two strategies of control are put forward based on the characteristics of stick–slip motion. Making use of these two control strategies, directed motions of the system are possible and the direction of the motion can be simply controlled by modifying the values of internal accelerations. To achieve an always forward motion with higher average steady-state velocity, further optimization is carried out. For anisotropic linear resistance, the approximate expression of average steady-state velocity can be also obtained by the method of averaging. No stick–slip motion may occur in this instance. All the analytical results are numerically simulated in order to verify their correctness.

Stepwise Neutral−Ionic Phase Transitions in a Covalently Bonded Donor/Acceptor Chain Compound

Neutral (N)-ionic (I) transitions in organic donor (D)/acceptor (A) charge-transfer complexes are intriguing because a 'reservoir of functions' is available. For systematically controlling N-I transitions, tuning the ionization potential of D and the electron affinity of A is extremely important. However, the effect of Coulomb interactions, which likely causes a number of charge-gap states at once in a system bringing about stepwise transitions, is a long-standing mystery. Here, we show definite evidence for stepwise N-I transitions caused by contributions from anisotropic interchain Coulomb interactions in a metal-complex-based covalently bonded DA chain compound, [Ru(2)(2,3,5,6-F(4)PhCO(2))(4)(DMDCNQI)]·2(p-xylene) (1; 2,3,5,6-F(4)PhCO(2)(-) = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N'-dicyanoquinonediimine), where the [Ru(2)(II,II)(2,3,5,6-F(4)PhCO(2))(4)] moiety has a paddlewheel diruthenium(II,II) motif with a Ru-Ru bond. An intermediate-temperature phase involving self-organized N and I chains was observed in the temperature range between 210 K (= T(2)) and 270 K (= T(1)) with N phase at T > T(1) and I phase at T < T(2). Accompanying the charge transitions, the spin-ground states as well as the ferrimagnetic ordering in the I phase vary. The stepwise feature of the N-I transition with a highly sensitive magnetic response should bring about new dynamical functionalities associated with charge, spin, and lattice.

Verification of an anisotropic Coulomb adhesion‐friction law for contact surfaces with periodic structure

AbstractSliding friction forces and so‐called adhesion forces are the main mechanical characteristics to describe contact interaction. Both together are representing 2D surface constitutive laws in analogy to e.g. elasto‐plasticity for 3D continua. The classical model to generalize the Coulomb friction law into anisotropic domains is to introduce an anisotropic friction tensor. Michalowski and Mroz in [1] proposed the structure of the friction tensor considering the sliding of a rigid block on an inclined surface. Zmitrowicz in [2] developed the theoretical basis for the structure of the friction tensor on symmetry groups for the tensor. The current contribution is aimed at verification of this modeling process based on a homogenization procedure for a very fine discretization representing the exact structure of the surface. The validation issue with realistic experiments given in [4] is discussed as well. (© 2010 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A local moment approach to the degenerate Anderson impurity model

The local moment approach is extended to the orbitally degenerate (SU(2N)) Anderson impurity model (AIM). Single-particle dynamics are obtained over the fullrange of energy scales, focusing on particle–hole symmetry in the strongly-correlated regimewhere the onsite Coulomb interaction leads to many-body Kondo physics with entangledspin and orbital degrees of freedom. The approach captures many-body broadening of theHubbard satellites and recovers the correct exponential vanishing of the Kondo scale for allN, and its universal scaling spectra are found to be in very good agreement withnumerical renormalization group (NRG) results. In particular the high-frequencylogarithmic decays of the scaling spectra, obtained here in closed form for arbitraryN, coincide essentially perfectly with available numerics from the NRG. A particular case ofan anisotropic Coulomb interaction, in which the model represents a system ofN ‘capacitivelycoupled’ SU(2) AIMs, is also discussed. Here the model is generally characterized by two low-energy scales,the crossover between which is seen directly in its dynamics.

Regulation of Thermal Conductivity in Hot Galaxy Clusters by MHD Turbulence

The role of thermal conduction in regulating the thermal behavior of cooling flows in galaxy clusters is reexamined. Recent investigations have shown that the anisotropic Coulomb heat flux caused by a magnetic field in a dilute plasma drives a dynamical instability. A long standing problem of cooling flow theory has been to understand how thermal conduction can offset radiative core losses without completely preventing them. In this Letter we propose that magnetohydrodynamic turbulence driven by the heat flux instability regulates field-line insulation and drives a reverse convective thermal flux, both of which may mediate the stabilization of the cooling cores of hot clusters. This model suggests that turbulent mixing should accompany strong thermal gradients in cooling flows. This prediction seems to be supported by the spatial distribution of metals in the central galaxies of clusters, which shows a much stronger correlation with the ambient hot gas temperature gradient than with the parent stellar population.

Electric field of a pointlike charge in a strong magnetic field and ground state of a hydrogenlike atom

In an external constant magnetic field, so strong that the electron Larmour length is much shorter than its Compton length, we consider the modification of the Coulomb potential of a point charge owing to the vacuum polarization. We establish a short-range component of the static interaction in the Larmour scale, expressed as a Yukawa-like law, and reveal the corresponding ``photon mass'' parameter. The electrostatic force regains its long-range character in the Compton scale: the tail of the potential follows an anisotropic Coulomb law, decreasing away from the charge slower along the magnetic field and faster across. In the infinite-magnetic-field limit the potential is confined to an infinitely thin string passing though the charge parallel to the external field. This is the first evidence for dimensional reduction in the photon sector of quantum electrodynamics. The one-dimensional form of the potential on the string is derived that includes a $\ensuremath{\delta}$ function centered in the charge. The nonrelativistic ground-state energy of a hydrogenlike atom is found with its use and shown not to be infinite in the infinite-field limit, contrary to what was commonly accepted before, when the vacuum polarization had been ignored. These results may be useful for studying properties of matter at the surface of extremely magnetized neutron stars.