Strong Coulomb Interaction Research Articles

Influence of the nuclear dynamical deformation on production cross sections of superheavy nuclei

The superheavy nuclear production mechanism can be described by fusion reactions with the dinuclear system concept, in which the deformations of the two nuclei are assumed to stay at their ground state with nucleons transferring between them. Actually the two nuclei have to deform due to very strong nuclear and Coulomb interactions between them. These deformations can be analytically described by a Fokker-Planck equation, and by combining them with a master equation that describes the nucleon transfer between nuclei the superheavy nuclear production cross sections can be investigated systematically. The calculated results are in good agreement with available data, and the evaporation residue cross sections for synthesizing the superheavy nuclei $Z=119$ and 120 are predicted.

First-principles calculation for hydrogen-doped hematite

Hexagonal α-Fe2O3 is one of the most common functional material used as magnetic semiconductor, and plays an important part in various applications, such as electronic devices etc. Based on the density functional theory, the lattice parameters, density of states and Bader charge analysis of α-Fe2O3 have been calculated using the first-principles calculation with GGA+U method. As Fe is a transition metal element, the value of U can be more accurate by considering the influence of the strong on-site Coulomb interaction between 3d electrons. First, the crystal equilibrium volume, the magnetic moment of Fe atom, and the band gap value of α-Fe2O3 are synthetically researched and compared with those with different U. Results indicate that the calculation model of α-Fe2O3 are in good agreement with the experimental model when the value of U is 6 eV. These parameters can also be adapted to the following doping calculaton. The α-Fe2O3 unit cell has both tetrahedral and octahedral interstitial sites. The calculation of doping formation energy shows that the α-Fe2O3 system is most stable when the doped hydrogen atom is in the tetrahedral interstitial site. The density of states show that the valence band and conduction band compositions are similar for the bulk and hydrogen-doped α-Fe2O3. That is, the valence bands are dominated mainly by both O 2p and Fe 3d orbitals with the O 2p orbitals playing a leading role, while the conduction band is dominated by Fe 3d orbitals. The band gap of α-Fe2O3 decreases from 2.2 to 1.63 eV after hydrogen doping. Also, a strong hybrid peak occurs near the Fermi level after hydrogen doping, which is chiefly composed of Fe 3d orbital, and the O 2p orbital also has a small contribution. The H 1s orbital is mainly in the lower level below the top valence band. Results of the Bader charge analysis and the density of states calculation for partial correlated atoms suggest that the new hybrid peak is chiefly caused by Fe atom which is closest to the hydrogen atom in the crystal cell. In this process, H atom loses electrons, and the nearest neighbors of H atom, i.e. O and Fe atoms, almost obtain all the electrons H atom loses, so H and O atoms are bonded together strongly, causing the hybrid peak, to expand the width of the top valence band and shift down the bottom of the conduction band, so that the band gap decreases and the electrical conductivity increases. Hydrogen doping is suggested to be an effective method to modify the band.

Soft landing of cell-sized vesicles on solid surfaces for robust vehicle capture/release.

Based on a concept of a smooth and steady landing of fragile objects without destruction via a soft cushion, we have developed a model for the soft landing of deformable lipid giant unilamellar vesicles (GUVs) on solid surfaces. The foundation for a successful soft landing is a solid substrate with a two-layer coating, including a bottom layer of positively charged lysozymes and an upper lipid membrane layer. We came to a clear conclusion that anionic GUVs when sedimented on a surface, the vesicle rupture occurs upon the direct contact with the positively charged lysozyme layer due to the strong coulombic interactions. In contrast, certain separation distances was achieved by the insertion of a soft lipid membrane cushion between the charged GUVs and the lysozyme layer, which attenuated the coulombic force and created a mild buffer zone, ensuring the robust capture of GUVs on the substrate without their rupture. The non-covalent bonding facilitated a fully reversible stimuli-responsive capture/release of GUVs from the biomimetic solid surface, which has never been demonstrated before due to the extreme fragility of GUVs. Moreover, the controllable capture/release of cells has been proven to be of vital importance in biotechnology, and similarity the present approach to capture/release cells is expected to open the previously inaccessible avenues of research.

Phase string effect and mutual Chern-Simons theory of Hubbard model

The fermion sign plays a dominant role in Fermi liquid theory. However, in Mott insulators, the strong Coulomb interaction suppresses the charge fluctuations and eliminates the fermion signs due to electron permutation. In this article, we first review the phase string theory of the Hubbard model for a bipartite lattice, which unifies the Fermi liquid at weak coupling and the antiferromagnetic Mott insulator at strong coupling. We first derive the exact sign structure of the Hubbard model for an arbitrary Coulomb interaction U. In small U limit, the conventional fermion sign is restored, while at large U limit, it leads to the phase string sign structure of the t-J model. For half filling, we construct an electron fractionalization representation, in which chargons and spinons are coupled to each other via emergent mutual Chern-Simons gauge fields. The corresponding ground state ansatz and low energy effective theory capture the ground state phase diagram of the Hubbard model qualitatively. For weak coupling regime, the Fermi liquid quasiparticle is formed by the bound state of a chargon and a spinon, and the long range phase coherence is determined by the background spin correlation. The Mott transition can be realized either by forming the chargon gap or by condensing the background spinons.

Ultrafast dynamics of defect-assisted electron-hole recombination in monolayer MoS2.

In this Letter, we present nondegenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼ 2 ps and a slow time scale that lasts longer than ∼ 100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two-dimensional atomic materials, together with strong electron and hole correlations in two-dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two-dimensional atomic materials.

Recent experimental progress of fractional quantum Hall effect: 5/2 filling state and graphene

Abstract The phenomenon of fractional quantum Hall effect (FQHE) was first experimentally observed 33 years ago. FQHE involves strong Coulomb interactions and correlations among the electrons, which leads to quasiparticles with fractional elementary charge. Three decades later, the field of FQHE is still active with new discoveries and new technical developments. A significant portion of attention in FQHE has been dedicated to filling factor 5/2 state, for its unusual even denominator and possible application in topological quantum computation. Traditionally, FQHE has been observed in high-mobility GaAs heterostructure, but new materials such as graphene also open up a new area for FQHE. This review focuses on recent progress of FQHE at 5/2 state and FQHE in graphene.

Nonrandom point defect configurations and driving force transitions for grain boundary segregation in trivalent cation doped ZrO₂.

The energetically favorable spatial configuration of M(3+) ions and oxide-ion vacancies near a symmetrical grain boundary (GB) in cubic zirconia is determined for various trivalent species M(3+) (M = Al, Sc, Y, Gd, La), and the driving force for grain boundary segregation (GBS) quantitatively examined using atomistic Monte Carlo simulations in conjunction with static lattice calculations. For a high concentration of ∼10 mol %, it is found that point defects near a GB plane preferentially occupy specific sites to minimize total lattice energy, rather than being randomly distributed. Systematic analysis shows that energetically stable configurations of segregants vary depending on their ionic radii. Analysis of the driving force for GBS as a function of dopant concentration reveals that three important factors govern GBS. First, occupation of specific sites by point defects is necessary to minimize the total lattice energy; enrichment of point defects near the GB plane with random configuration does not decrease the total lattice energy significantly because of strong Coulombic interactions. Second, the factors governing GBS change with increasing dopant concentration. At dilute concentrations, relief of bond strain is the dominant factor, while at high concentrations Coulombic interactions, which depend strongly on the specific arrangement of defects, become another dominant factor. Third, the stabilization of matrix cations, Zr(4+) ions, is the dominant factor to lower the driving force for GBS at all concentrations. In contrast, the stabilization of M(3+) ions does not necessarily contribute to GBS of point defects at high concentrations. These findings suggest practical ways to control GBS to enhance materials' properties or minimize detrimental effects.

Structural phase analysis, band gap tuning and fluorescence properties of Co doped TiO2 nanoparticles

This paper reports on structural and optical properties of Co (0, 3, 5 & 7mol%) doped TiO2 (titania) nanoparticles (NPs) synthesized by employing acid modified sol–gel method. The crystalline phase of the pure and doped NPs was observed with X-ray diffraction (XRD) followed by Raman scattering technique. Field emission scanning electron microscope and transmission electron microscopy give the morphological details. Fourier transform infrared spectra indicate the bonding interactions of Co ions with the titania lattice framework. Optical studies were attained with UV–visible absorption and fluorescence emission spectroscopy. XRD analysis reveals that all prepared samples have pure anatase phase with tetragonal symmetry devoid of any other secondary phase. The average crystallite size of all samples was calculated using Scherrer’s formula and was found to vary from 8 to 10nm with doping concentration of Co. The Raman spectroscopy further confirmed the formation of TiO2 in anatase structure in both pure and Co doped TiO2 NPs. The most intense Raman active Eg peak of TiO2 NPs shifted to higher energy on doping. Both UV–visible and fluorescence spectra show a blue shift in their absorption and band edge emission subsequently on increasing with Co percentage in titania host matrix, wherever there is an indication of quantum confinement effect with widening of band gap on decreasing in NPs size. There is also a possibility of strong Coulomb interaction effect on the optical processes involving the Co ions. However, the intensities of different emission spectra are not the same but decrease profoundly for doping samples due to concentration quenching effect.

Optoelectronic excitations and photovoltaic effect in strongly correlated materials

Solar cells based on conventional semiconductors have low efficiency in converting solar energy into electricity because the excess energy beyond the gap of an incident solar photon is converted into heat by phonons. Here we show by ab initio methods that the presence of strong Coulomb interactions in strongly correlated insulators (SCI) causes the highly photo-excited electron-hole pair to decay fast into multiple electron-hole pairs via impact ionization (II). We show that the II rate in the insulating $M_1$ phase of vanadium dioxide (chosen for this study as it is considered a prototypical SCI) is two orders of magnitude higher than in Si and much higher than the rate of hot electron/hole decay due to phonons. Our results indicate that a rather broad class of materials may be harnessed for an efficient solar-to-electrical energy conversion that has been not considered before.

An equation of motion analysis of the two stage Kondo effect in T-shaped double-quantum-dot systems

We use the equation of motion method in connection with a generalized Anderson Hamiltonian to evaluate the electronic transmission in a T-shaped double quantum dot system. We consider the strong Coulomb interaction regime (U1→∞, U2→∞) using a decoupling procedure that includes terms beyond the standard Hartree–Fock approximation. The typical manifestation of the two stage Kondo effect is reflected in system's electronic transmission coefficient that presents both a large Lorentzian and a sharp dip around ω=0. We provide an analytical result for the first stage Kondo temperature that includes corrections due to the presence of an additional quantum dot in the system. For the second stage Kondo effect, we identify the logarithmic correction that is responsible for the additional dip in the system's electronic transmission coefficient.

Magnon qubit and quantum computing on magnon Bose-Einstein condensates

Recently, great progress has been made in the creation of a solid-state quantum computer using superconducting qubits on Cooper pairs of charged electrons. However, this approach has met limitations due to decoherence effects caused by the strong Coulomb interaction of the superconducting qubit with the environment. Here, we propose the solution of this problem by switching to another Bose-Einstein condensate (BEC), uncharged long-lived magnons, wherein the magnon BEC qubit can be realized due to the magnon blockade isolating a pair of the magnon condensate energy levels in the mesoscopic and nanoscopic ferromagnetic dielectric sample. We demonstrate the single-qubit gates by operating quantum transition between these states in the external microwave field. We also consider implementation of the two-qubit gates by using the interaction between such magnon BEC qubits due to exchange by virtual photons in a microwave cavity. Finally, we discuss the condition for long-lived magnon BEC qubits, a scalable architecture, and promising advantages of the multiqubit quantum computer based on the magnon qubit.

The influence of f-electron hopping on ground states and valence transitions in the extended Falicov-Kimball model

The ground-state phase diagram of the extended Falicov-Kimball model with f-f electron hopping is studied numerically in the one-dimensional case. To identify the nature of ground states three complementary numerical methods are used, and namely, (i) the small-cluster exact-diagonalization method, (ii) the density-matrix-renormalization-group method (DMRG) and (iii) an approximate, but very accurate, numerical method based on the reduction of the Hilbert space. It is found that the physics of the Falicov-Kimball model found for the zero value of the f-electron hopping integral tf (including the existence of the devil’s staircase structure) persists also at finite values of tf. The critical values of tcf below which the physics of the Falicov-Kimball model dominates are calculated numerically and it is shown that they depend very strongly on the f-electron concentration nf and only very weakly on the Coulomb interaction. In particular, we have found that for strong Coulomb interactions the value of tcf rapidly increases from tcf ~ 0.003 found for nf = 1 / 4 up to relatively large tcf ~ 0.4 found for nf near the half-filled band case nf = 1 / 2. In addition, the complete picture of valence transitions is presented for non-zero tf and strong Coulomb interactions.

Orbital Ferromagnetism in Interacting Few-Electron Dots with Strong Spin-Orbit Coupling

Investigations of quantum dots yield information about the properties of semiconductors on the smallest scales. Researchers show that quantum dots can exhibit an orbital ferromagnet phase due to the interplay of strong Rashba spin-orbit coupling and Coulomb interactions.

Distance distributions of photogenerated charge pairs in organic photovoltaic cells.

Strong Coulomb interactions in organic photovoltaic cells dictate that charges must separate over relatively long distances in order to circumvent geminate recombination and produce photocurrent. In this article, we measure the distance distributions of thermalized charge pairs by accessing a regime at low temperature where charge pairs are frozen out following the primary charge separation step and recombine monomolecularly via tunneling. The exponential attenuation of tunneling rate with distance provides a sensitive probe of the distance distribution of primary charge pairs, reminiscent of electron transfer studies in proteins. By fitting recombination dynamics to distributions of recombination rates, we identified populations of charge-transfer states and well-separated charge pairs. For the wide range of materials we studied, the yield of separated charges in the tunneling regime is strongly correlated with the yield of free charges measured via their intensity-dependent bimolecular recombination dynamics at room temperature. We therefore conclude that populations of free charges are established via long-range charge separation within the thermalization time scale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation constant, we estimate critical charge separation distances of ∼3-4 nm in all materials. In some blends, large fullerene crystals can enhance charge separation yields; however, the important role of the polymers is also highlighted in blends that achieved significant charge separation with minimal fullerene concentration. We expect that our approach of isolating the intrinsic properties of primary charge pairs will be of considerable value in guiding new material development and testing the validity of proposed mechanisms for long-range charge separation.

Precession Electron Diffraction

The strong Coulombic interaction between a high energy electron and a thin crystal film gives rise to electron diffraction patterns encoded with information that is remarkably sensitive to the crystal potential. That exquisite sensitivity can be advantageous, for example in the determination of local symmetry and bonding, but can also be problematic in that in general the dynamical scattering inherent in electron diffraction prohibits the use of conventional crystallographic methods to recover structure factor phase information and solve unknown structures. One way to reduce this problem is to use precession electron diffraction (PED), introduced 20 years ago [1] as the electron analogue of Buerger's X-ray technique, in which the electron beam is first rocked in a hollow cone above the sample and then de-rocked below, the net effect of which is equivalent to precessing the sample about a stationary electron beam. PED is now used almost routinely as a starting point to solve crystal structures that cannot be solved for a variety of reasons using x-ray or neutron methods. In this keynote lecture we explore why the PED technique has been successful for structure determination, focussing on the PED geometry, the variation of intensities with precession angle and specimen thickness, and how this `mimics' kinematic behaviour, and the use of unconventional structure solution and refinement approaches [2]. New acquisition geometries will be discussed that rely on tilt series of PED patterns to yield a more complete 3D data set. The lecture will focus on how PED has been used also as a method for nanoscale orientation mapping [3], providing more information than conventional electron diffraction and a robust method with which to determine local crystallographic orientation. By scanning the beam, accurate orientation images can be derived from series of PED patterns and, by combining with tomographic methods, sub-volume orientation information is also available.

Electron and X-ray diffraction – two worlds united

Small crystals structure solution usually done with X-ray powder diffraction (XRPD) provides bulk information and is powerful for in-situ investigations. Peak overlap in the one-dimensional data causes problems e.g. for polyphasic or impure samples and large cell parameters thus peak indexing and intensity extraction are the main issues where x-ray powder data may be supported by extra information. Electrons sample smaller volumes but strong coulombic interaction cause multiple scattering effects changing intensities often so strong that a structure solution is becoming impossible. Nevertheless, oriented electron diffraction patterns may provide sufficient information to support indexing or the assignment of impurity peaks in the case of low quality x-ray powder pattern. Reciprocal space tomography [1] uses a series of non-oriented diffraction patterns for which dynamical effects are significantly reduced and an enhanced amount of independent reflections sampled allows "ab-initio" crystal structure solution using established X-ray structure solution packages. Although structure refinement based on kinematical intensities is stable, achievable R values of 10-30% are high and final refinement may be performed based on X-ray powder data. Scanning transmission electron microscopy (STEM) for crystal tracking and nano electron diffraction (NED) is suitable for beam sensitive material, agglomerated particles, twins or intergrown phases on crystals down to 30nm size [2, 3]. Interesting properties of nanocrystalline materials are driven mainly by twinning, defects, disorder in one or two dimensions down to the amorphous state. Here low data completeness or uncertain intensity determination causes problems in structure solution. Here a mean structure may be determinable serving as a basis for disorder description and being used as a starting model being refined onto X-ray powder data maybe supported by a combination of the diffraction methods or by adding extra information.

Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide

In this work, the interactions between a representative pyridinium-based ionic liquid (IL) with cyano-functionalized anion ([C4Py][SCN]) and dimethyl sulfoxide (DMSO) were investigated in detail using attenuated total reflection infrared spectroscopy (ATR-FTIR), hydrogen nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations. It was found that H-bonds are universally involved and play important role for the miscibility of DMSO with [C4Py][SCN] IL and maintain the stability of this system. ATR-FTIR and excess spectroscopy analysis indicated that the H-bonds involving the alkyl C–Hs and C≡N are strengthened with the addition of DMSO, while the H-bonds involving pyridinium ring C–Hs as well as the H-bonds formed between the [C4Py]+ and [SCN]− are weakened. The addition of [C4Py][SCN] IL led to the H-bonds involving the C–Hs in DMSO weakened comparing with the associated H-bonds in the pure DMSO system. The results of DFT calculations indicated that the DMSO molecules cannot disrupt the strong Coulombic interaction between the [C4Py]+ and [SCN]−. Natural bond orbital (NBO) analysis further confirmed that the interaction mechanisms of DMSO molecule with the anion and cation are different in nature. These studies will shed light on exploring the applications of ILs as reaction or separation media.

Effect of plasmon-enhancement on photophysics in upconverting nanoparticles.

Surface plasmon polaritons (SPP) waves have been shown to significantly affect the near-field photophysical phenomenon. In particular, strong Coulombic interactions can enhance nearby non-linear optics and energy transfer process, while SPP waves also affect other photophysical processes like quenching observed in fluorescent and excitonic systems. Here, using different plasmonic substrates, we show the effect of plasmon-enhancement on quenching, phonon-assisted non-radiative decay, weak Purcell effect or electromagnetic field enhancement, and energy transfer rates of upconverting doped-lanthanide nanoparticles. While the resonant plasmons enhance the local electromagnetic field and the rate of energy transfer leading to enhanced upconversion photoluminescence of infrared radiation to visible light, it can also increase the quenching and non-radiative decay rates of photoexcited electron-hole pairs leading to losses and lower efficiency. These results can guide the design of optimized substrate geometry for using surface plasmons to modulate the photophysics in other applications too.

Influence of Coulomb interaction on the anisotropic Dirac cone in graphene

Anisotropic Dirac cones can appear in a number of correlated electron systems, such as cuprate superconductors and deformed graphene. We study the influence of long-range Coulomb interaction on the physical properties of an anisotropic graphene by using the renormalization group method and 1/N expansion, where N is the flavor of Dirac fermions. Our explicit calculations reveal that the anisotropic fermion velocities flow monotonously to an isotropic fixed point in the lowest energy limit in clean graphene. We then incorporate three sorts of disorders, including random chemical potential, random gauge potential, and random mass, and show that the interplay of Coulomb interaction and disorders can lead to rich and unusual behaviors. In the presence of strong Coulomb interaction and a random chemical potential, the fermion velocities are driven to vanish at low energies and the system turns out to be an exotic anisotropic insulator. In the presence of Coulomb interaction and other two types of disorders, the system flows to an isotropic low-energy fixed point more rapidly than the clean case, and exhibits non-Fermi liquid behaviors. We also investigate the nonperturbative effects of Coulomb interaction, focusing on how the dynamical gap is affected by the velocity anisotropy. It is found that the dynamical gap is enhanced (suppressed) as the fermion velocities decrease (increase), but is suppressed as the velocity anisotropy increases.

Even-denominator fractional quantum Hall effect at a Landau level crossing

The fractional quantum Hall effect (FQHE), observed in two-dimensional (2D) charged particles at high magnetic fields, is one of the most fascinating, macroscopic manifestations of a many-body state stabilized by the strong Coulomb interaction. It occurs when the filling factor ($\nu$) of the quantized Landau levels (LLs) is a fraction which, with very few exceptions, has an odd denominator. In 2D systems with additional degrees of freedom it is possible to cause a crossing of the LLs at the Fermi level. At and near these crossings, the FQHE states are often weakened or destroyed. Here we report the observation of an unusual crossing of the two \emph{lowest-energy} LLs in high-mobility GaAs 2D $hole$ systems which brings to life a new \emph{even-denominator} FQHE at $\nu=1/2$.