Dimer Coupling Research Articles

Interplay of coulomb and exciton-phonon coupling controls singlet fission dynamics in two pentacene polymorphs.

Pentacene is an important model organic semiconductor in both the singlet exciton fission (SF) and organic electronics communities. We have investigated the effect of changing crystal structure on the SF process, generating multiple triplet excitons from an initial singlet exciton, and subsequent triplet recombination. Unlike for similar organic semiconductors that have strong SF sensitive to polymorphism, we find almost no quantitative difference between the kinetics of triplet pair (TT) formation in the two dominant polymorphs of pentacene. Both pairwise dimer coupling and momentum-space crystal models predict much faster TT formation from the bright singlet excited state of the Bulk vs ThinFilm polymorph, contrasting with the experiment. GW and Bethe-Salpeter equation calculations, including exciton-phonon coupling, reveal that ultrafast phonon-driven transitions in the ThinFilm polymorph compensate the intrinsically slower purely Coulomb-mediated TT formation channel, rationalizing the similarity in observed rates. Taking into account the influence of subtle structural distinctions on both the direct and phonon-mediated SF pathways reveals a predictive capability to these methods, expected to be applicable to a wide variety of molecular crystals.

Dimer Coupling Energies of the Si(001) Surface.

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ξ_{∥}^{+}/ξ_{⊥}^{+}=5.2 of the fluctuating c(4×2) domains above the critical temperature T_{c}=(190.6±10) K. We obtain effective couplings J_{∥}=(-24.9±1.3) meV along the dimer rows and J_{⊥}=(-0.8±0.1) meV across the dimer rows, i.e., antiferromagneticlike coupling of the dimers with c(4×2) symmetry.

Enhanced phonon-plasmon interaction in film-coupled dimer nanoridges mediated by surface acoustic waves

The interaction between phonons and localized plasmons in coupled nanoparticles can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we demonstrate by numerical analysis an enhanced optomechanical interaction in a film-coupled gold dimer nanoparticles mediated by surface acoustic waves. Two gold nanoridges are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide substrate. Numerical simulations of the optical properties reveal the existence of three surface localized plasmons in the infrared range with enhanced scattering and narrower linewidths than with a single nanoridge. The physical origin of such modes as well as their tunability as function of key geometrical parameters are successfully captured with a simple model based on effective Metal-Insulator-Metal (MIM)-like plasmonic cavity. We calculate the optomechanic coupling rates between the GHz localized mechanical modes and plasmonic modes of the dimer, finding that the strongest coupling is observed for the in-phase compressional mode followed by the out-of-phase flexural mode. Both such modes can be excited by launching a surface acoustic wave (Sezawa wave) at the inlet in front of the dimer structure. It is also found that the flexural mode which is inactive optomechanically in case of a monomer becomes active due to dimer coupling, with a significant phonon-plasmon coupling rate. The findings in this work may facilitate design of new optomechanical components monitored with fast coherent acoustics, leading to new generation of light acousto-optic modulators where strong optomechanical interactions are required.

Co and Pt Dual-Single-Atoms with Oxygen-Coordinated Co-O-Pt Dimer Sites for Ultrahigh Photocatalytic Hydrogen Evolution Efficiency.

The platinum single-atom-catalyst is verified as a very successful route to approach the size limit of Pt catalysts, while how to further improve the catalytic efficiency of Pt is a fundamental scientific question and is challenging because the size issue of Pt is approached at the ultimate ceiling as single atoms. Here, a new route for further improving Pt catalytic efficiency by cobalt (Co) and Pt dual-single-atoms on titanium dioxide (TiO2 ) surfaces, which contains a fraction of nonbonding oxygen-coordinated Co-O-Pt dimers, is reported. These Co-Pt dimer sites originate from loading high-density Pt single-atoms and Co single-atoms, with them anchoring randomly on the TiO2 substrate. This dual-single-atom catalyst yields 13.4% dimer sites and exhibits an ultrahigh and stable photocatalytic activity with a rate of 43.467mmol g-1 h-1 and external quantum efficiency of ≈83.4% at 365nm. This activity far exceeds those of equal amounts of Pt single-atom and typical Pt clustered catalysts by 1.92 and 1.64 times, respectively. The enhancement mechanism relies on the oxygen-coordinated Co-O-Pt dimer coupling, which can mutually optimize the electronic states of both Pt and Co sites to weaken H* binding. Namely, the "mute" Co single-atom is activated by Pt single-atom and the activity of the Pt atom is further enhanced through the dimer interaction. This strategy of nonbonding interactive dimer sites and the oxygen-mediated catalytic mechanisms provide emerging rich opportunities for greatly improving the catalytic efficiency and developing novel catalysts with creating new electronic states.

Magnetism of dimer ensemble with random exchange energy

We present the study of magnetism of dimer ensemble deposited on a non-magnetic conducting substrate. The dimers consist of ions with spin 1/2 and have a random size. The magnetic ions are coupled by RKKY-interaction with random energy value. We consider two interaction models. The first assumes that dimer coupling energy has a Gaussian distribution, and the second model is based on a log-normal distribution of dimer size. The magnetization and susceptibility are evaluated from the exact solution for dimer energy and eigenstates. We show that the average exchange value and its dispersion have a strong effect on the system properties. The magnetic saturation exists under strong magnetic fields. High-temperature leads to the Curie’s law for susceptibility. The saturation magnetization of system with ferromagnetic coupling lowers, while the antiferromagnetic dimer ensemble can have non-zero magnetization under the strong magnetic field. It is determined by the dispersion of exchange energy.

Energy transfer under natural incoherent light: Effects of asymmetry on efficiency.

The non-equilibrium stationary coherences that form in donor-acceptor systems are investigated to determine their relationship to the efficiency of energy transfer to a neighboring reaction center. It is found that the effects of asymmetry in the dimer are generally detrimental to the transfer of energy. Four types of systems are examined, arising from combinations of localized trapping, delocalized (Forster) trapping, eigenstate dephasing, and site basis dephasing. In the cases of site basis dephasing, the interplay between the energy gap of the excited dimer states and the environment is shown to give rise to a turnover effect in the efficiency under weak dimer coupling conditions. Furthermore, the nature of the coherences and associated flux is interpreted in terms of pathway interference effects. In addition, regardless of the cases considered, the ratio of the real part and the imaginary part of the coherences in the energy-eigenbasis tends to a constant value in the steady state limit.

Characterization of a Mixed-Valence Ru(II)/Ru(III) Ion-Pair Complex. Unexpected High-Frequency Electron Paramagnetic Resonance Evidence for Ru(III)–Ru(III) Dimer Coupling

In this contribution, we report the synthesis and full characterization of the first mixed-valence Ru(II)/Ru(III) ion-pair complex, [RuII(bipy)2(pic)]+[cis-RuIIICl2(pic)2]-, in the solid state and in aqueous solution, where bipy = 2,2'-bipyridine and pic- = picolinate. In addition, unexpected high-frequency electron paramagnetic resonance evidence for interactions between two neighboring Ru(III) ions, resulting in a triplet state, S = 1, was found.

Quantum transfer energy in the framework of time-dependent dipole-dipole interaction

Abstract In this work, we examine the process of the quantum transfer of energy considering time-dependent dipole-dipole interaction in a dimer system characterized by two-level atom systems. By taking into account the effect of the acceleration and speed of the atoms in the dimer coupling, we demonstrate that the improvement of the probability for a single-excitation transfer energy extremely benefits from the incorporation of atomic motion effectiveness and the energy detuning. We explore the relevance between the population and entanglement during the time-evolution and show that this kind of nonlocal correlation may be generated during the process of the transfer of energy. Our work may provide optimal conditions to implement realistic experimental scenario in the transfer of the quantum energy.

How Markovian is exciton dynamics in purple bacteria?

We investigate the extent to which the dynamics of excitons in the light-harvesting complex LH2 of purple bacteria can be described using a Markovian approximation. To analyse the degree of non-Markovianity in these systems, we introduce a measure based on fitting Lindblad dynamics, as well as employing a recently introduced trace-distance measure. We apply these measures to a chromophore-dimer model of exciton dynamics and use the hierarchical equation-of-motion method to take into account the broad, low-frequency phonon bath. With a smooth phonon bath, small amounts of non-Markovianity are present according to the trace-distance measure, but the dynamics is poorly described by a Lindblad master equation unless the excitonic dimer coupling strength is modified. Inclusion of underdamped, high-frequency modes leads to significant deviations from Markovian evolution in both measures. In particular, we find that modes that are nearly resonant with gaps in the excitonic spectrum produce dynamics that deviate most strongly from the Lindblad approximation, despite the trace distance measuring larger amounts of non-Markovianity for higher frequency modes. Overall we find that the detailed structure in the high-frequency region of the spectral density has a significant impact on the nature of the dynamics of excitons.

Energetic and conformational features linked to the monomeric and dimeric states of bovine BLG

Bovine β-lactoglobulin (BLG) belong to the lipocalin family. This is a group of proteins involved in the binding and transporting of hydrophobic molecules. Experimental and theoretical reports have stated its complex structural behavior in solution, with coupled effects between homodimerization and ligand recognition. Nonetheless, structural evidence at the atomic level about the cause of this coupled effect has not been reported to date. To address this issue microsecond molecular dynamics (MD) simulations were combined with the molecular mechanics generalized Born surface area (MM/GBSA) approach, clustering analysis and principal component analysis (PCA), to explore the conformational complexity of BLG protein-protein self-association and palmitic acid (PLM) or dodecyl sulfate (SDS) ligand recognition in the monomeric and dimeric state. MD simulations, coupled to the MM/GBSA method, revealed that dimerization exerts contrasting effects on the ligand-binding capacity of BLG. Protein dimerization decreases PLM affinity, promoting dimer association. For SDS the dimeric state increases affinity, enhancing dimer dissociation. MD simulations based on PCA revealed that while few differences in the conformational subspace are observed between the free and bound monomer and dimer coupling for PLM, substantial changes are observed between the free and bound monomer and dimer coupling for SDS.

Excitonic Coupling and Femtosecond Relaxation of Zinc Porphyrin Oligomers Linked with Triazole Bridge: Dynamics and Modeling.

The synthesis of new zinc porphyrin oligomers linked by a triazole bridge was carried out via "click" reaction. A split in the porphyrin oligomer B-band was observed. It was considered as evidence of exciton-excitonic coupling. The relaxation of excited states in Q-band porphyrin oligomers was studied by the femtosecond laser spectroscopy technique with a 20 fs pump pulse. The transient oscillations of two B-band excitonic peaks have a π-radian shift. For explanation of the coherent oscillation, a theoretical model was developed. The model considered the combination of the exciton-excitonic coupling between porphyrin rings in dimer and weak exciton-vibronic coupling in one porphyrin ring. By varying the values of the structural parameters of porphyrins (the strength values of this couplings and measure of symmetry breaking), we obtained correspondence between the experimental data (phase shift and amplitudes of the spectrum oscillations) and the predictions of the model developed here.

Ferromagnetic resonance of a magnetic dimer with dipolar coupling

We develop a general formalism for analyzing the ferromagnetic resonance characteristics of a magnetic dimer consisting of two magnetic elements (in a horizontal or vertical configuration) coupled by dipolar interaction, taking account of their finite-size and aspect ratio. We study the effect on the resonance frequency and resonance field of the applied magnetic field (in amplitude and direction), the inter-element coupling, and the (uniaxial) anisotropy in various configurations. We obtain analytical expressions for the resonance frequency in various regimes of the interlayer coupling. We (numerically) investigate the behavior of the resonance field in the corresponding regimes. The critical value of the applied magnetic field at which the resonance frequency vanishes may be an increasing or a decreasing function of the dimer's coupling, depending on the anisotropy configuration. It is also a function of the nanomagnets aspect ratio in the case of in-plane anisotropy. This and several other results of this work, when compared with experiments using the standard ferromagnetic resonance with fixed frequency, or the network analyzer with varying frequency and applied magnetic field, provide a useful means for characterizing the effective anisotropy and coupling within systems of stacked or assembled nanomagnets. Comparing with the experimental data for the frequency splitting of coupled FeV nano disks, we find that our theory provides the same order of magnitude for the dipolar coupling.

Effect of Triclosan on Self‐Assembly of Alkyl Ammonium Surfactants Adsorbed within Montmorillonite Galleries in Silicone Elastomer Composites

SummaryPoly(propylene) homopolymer/fly ash (50% wt) composites have been prepared from two samples of fly ash that differ in terms of the geographic source of coal feedstock, which left one of the samples with a higher group I and II metal content. A screening study indicated that Lubrizol Solplus® C800 (an unsaturated carboxylic acid dimer coupling agent), initiated with dicumyl peroxide, conferred the best strength–toughness balance to the composites. In some cases, however this was related to detachment of the interfacial region due to possible extraction of PP‐C800 adduct‐group I and II ion reaction products. Interfacial adhesion was arguably too high in the case of the fly ash sample bearing a lower level of group I and II ions. These deductions were also supported by DSC data.

Effect of different element doping on the structure and magnetic properties of composite crystal Sr14(Cu1−xMx)24O41 (M=Zn, Ni, Co)

The Zn2+, Ni2+ and Co3+ doped Sr14(Cu1−xMx)24O41 composite crystals were synthesized by a standard solid state reaction method. The temperature dependence of magnetic properties was measured for every sample. The Curie coefficient, Weiss temperature, number of dimers in CuO2 chain per formula unit (f.u.) and dimer coupling constant are obtained by fitting the temperature dependence of the magnetic susceptibility. Meantime, selected area electron diffraction patterns (EDPs) show that the diffraction spots corresponding to the CuO2 chain substructure are extended to streaks along a⁎ and b⁎ directions for all the samples, while the diffraction spots produced merely by the Cu2O3 ladder substructure are still very sharp. This means that the periodicities of chains in a⁎ and b⁎ directions are partially destroyed upon doping of Zn, Ni and Co due to that the initial phase of each chain becomes a random variable. Considering the effect of different ions substitution for Cu on the magnetic susceptibility, it is found that the degree of initial phase disorder is related to the order degree of magnetic sequence in CuO2 chain. For the un-doped sample, the decoupling of dimers is weak, the magnetic sequence is slightly destroyed, and the streaks in EDP are also very weak, which implies the degree of initial phase disorder in CuO2 chain is very low. When Zn2+ and Ni2+ ions are doped, the number of dimers per f.u. decreases, and the intensity of diffraction streaks increases. Furthermore, when the high spin magnetic ions Co3+ are doped, the number of holes in Sr14(Cu1−xCox)24O41 decreases, the magnetic sequence is destroyed very seriously, and the spots in EDP are extended to streaks almost completely. The phenomenon that the diffraction spots of CuO2 chain extend to streaks in EDP appears as evidence that the magnetic sequence in the CuO2 chain is destroyed by doping of Zn, Ni, Co.

Microscopic evidence of destroying the order of magnetic sequence in CuO2 chain by Zn, Ni, Co Doping in Sr14Cu24O41 compound

The Zn2+, Ni2+ and Co3+ doped Sr14Cu24O41 compounds were synthesized by standard solid state method. X-ray diffraction results show that the changes in lattice parameters are very small. Selected area electron diffraction patterns (EDPs) show that the diffraction spots corresponding to the CuO2 chain substructure are extended to streaks along a* and b* directions for all the samples, while the diffraction spots produced merely by the Cu2O3 ladder substructure are still very sharp. This means that the periodicities of chains in a* and b* directions are partially destroyed upon doping of Zn, Ni and Co due to that the initial phase of each chain becomes a random variable. The temperature dependence of magnetic properties was measured for every sample. And the number of dimers in CuO2 chain per formula unit (f.u.) and dimer coupling constant are obtained by fitting the temperature dependence of the magnetic susceptibility. It is found that the degree of initial phase disorder is related to the order degree of magnetic sequence in CuO2 chain. For the un-doped sample, the decoupling of dimers is weak, the magnetic sequence is slightly destroyed, and the streaks in EDP are also very weak, which implies the degree of initial phase disorder in CuO2 chain is very low. When Zn2+ and Ni2+ ions are doped, the number of dimers per f.u. decreases, and the intensity of diffraction streaks increases in comparison with the corresponding spots. Furthermore, when the high spin magnetic ions Co3+ are doped, the number of holes in Sr14Cu24O41 decreases, the magnetic sequence is destroyed very seriously, and the spots in EDP are extended to streaks almost completely. The phenomenon that the diffraction spots of CuO2 chain extend to streaks in EDP appears as evidence that the magnetic sequence in the CuO2 chain is destroyed by doping of Zn, Ni, Co.

Nonmagnetic spin-singlet dimer formation and coupling to the lattice in the 6H perovskite Ba3CaRu2O9

Non-conservation of magnetic neutron scattering from the six-layer hexagonal perovskite Ba3CaRu2O9 on cooling reveals that (Ru5+)2 dimers form nonmagnetic spin-singlet ground states. This is in contrast to Ba3BRu2O9 (B=Co or Ni) analogues which display long range spin ordered ground states containing antiferromagnetic (Ru5+)2 dimers (Lightfoot and Battle 1990 J. Solid State Chem. 89 174). Dimer formation is weakly coupled to the lattice, resulting in an excess [101] microstrain broadening of diffraction peaks at low temperatures. Non-conservation of magnetic neutron scattering is also observed around the charge ordering transition in Ba3NaRu2O9 showing that the ground state structure is coupled to spin-singlet formation in charge ordered Ru2O9 dimers.

THE MAGNETOINDUCTIVE DIMER

In this paper, we examine a nonlinear magnetoinductive dimer and compute its linear and nonlinear symmetric, antisymmetric and asymmetric modes in closed-form, in the rotating-wave approximation. A linear stability analysis of these modes reveals that the asymmetric mode is always stable, for any allowed value of the coupling parameter and for both, hard and soft nonlinearity. An exact numerical computation of the dimer dynamics reveals a magnetic energy self-trapping whose threshold increases for increasing dimer coupling, decreases for increasing nonlinearity response and is robust against asymmetrical nonlinear responses and resonant frequencies mismatch.

Integrin-mediated Membrane Blebbing Is Dependent on Sodium-Proton Exchanger 1 and Sodium-Calcium Exchanger 1 Activity

Integrin signaling and membrane blebbing modulate cell adhesion, spreading, and migration. However, the relationship between integrin signaling and membrane blebbing is unclear. Here, we show that an integrin-ligand interaction induces both membrane blebbing and changes in membrane permeability. Sodium-proton exchanger 1 (NHE1) and sodium-calcium exchanger 1 (NCX1) are membrane proteins located on the bleb membrane. Inhibition of NHE1 disrupts membrane blebbing and decreases changes in membrane permeability. However, inhibition of NCX1 enhances cell blebbing; cells become swollen because of NHE1 induced intracellular sodium accumulation. Our study found that NHE1 induced sodium influx is a driving force for membrane bleb growth, while sodium efflux (and calcium influx) induced by NCX1 in a reverse mode results in membrane bleb retraction. Together, these findings reveal a novel function for NHE1 and NCX1 in membrane blebbing and permeability, and establish a link between membrane blebbing and integrin signaling.

RNA synthesis via dimer and trimer phosphoramidite block coupling

The solid-phase synthesis of oligoribonucleotides using dimer and trimer phosphoramidite blocks is described. This method significantly reduces the total number of steps required in the synthesis of a target RNA sequence, provides more material, and simplifies separation of the product from shorter failure sequences. The procedure is illustrated by the synthesis of UpU, ApA, and UpUpU phosphoramidite blocks and their use in the rapid synthesis of oligoribonucleotides on a solid support. Dimer and trimer amidite blocks will likely find use in the large scale solution (or solid)-phase synthesis of siRNA drugs.

Magnetic excitations of the gapped quantum spin dimer antiferromagnetSr3Cr2O8

${\text{Sr}}_{3}{\text{Cr}}_{2}{\text{O}}_{8}$ consist of a lattice of spin-1/2 ${\text{Cr}}^{5+}$ ions, which form hexagonal bilayers and which are paired into dimers by the dominant antiferromagnetic intrabilayer coupling. The dimers are coupled three dimensionally by frustrated interdimer interactions. A structural distortion from hexagonal to monoclinic leads to orbital order and lifts the frustration giving rise to spatially anisotropic exchange interactions. We have grown large single crystals of ${\text{Sr}}_{3}{\text{Cr}}_{2}{\text{O}}_{8}$ and have performed DC susceptibility, high-field magnetization and inelastic neutron scattering measurements. The neutron scattering experiments reveal three gapped and dispersive singlet to triplet modes arising from the three twinned domains that form below the transition thus confirming the picture of orbital ordering. The exchange interactions are extracted by comparing the data to a random phase approximation model and the dimer coupling is found to be ${J}_{0}=5.551(9)\text{ }\text{meV}$, while the ratio of interdimer to intradimer exchange constants is ${J}^{\ensuremath{'}}/{J}_{0}=0.64(2)$. The results are compared to those for other gapped magnets.