Strong-coupling Model Research Articles

On the validity of the standard model for primary charge separation in the bacterial reaction center

In the standard model the primary electron donor state (P∗) is localized on the special pair, meaning that the coupling between the lowest excited state (P_) of the special pair and charge transfer states involving radical anions of accessory cofactors is weak. This model is scrutinized using hole burning data for bacterial and green plan reaction centers (RC), antenna protein complexes and ππ∗ states of probe molecules imbedded in glasses and polymers. Attention is focused on the linear electron-phonon coupling strength (optical reorganization energy) and the dynamics afforded by the zero-phonon hole of the P∗ ← P transition. The hole burning data, together with the structural differences between the special pair of bacterial and green plant RC as well as quantum chemical calculations pertaining to the contribution of internal charge transfer states (of P) to P_, indicate that the standard model is invalid. A model is proposed that has P_ strongly coupled to a quasi-degenerate P+BL− state, where BL is the necessary bacteriochlorophyll on the L branch of the RC. In the adiabatic limit this model appears to be capable of explaining the non-single exponential decay of P870∗ (Rhodobacter sphaeroides) while resolving a serious discrepancy between the hole burning and time domain results on the decay of P∗. In the strong coupling limit, the long-standing question of whether or not P+BL− serves as a real or virtual state in the primary charge separation process becomes irrelevant. The model, especially in the adiabatic limit, may well provide the basis for understanding the extraordinarily fast energy transfer dynamics from higher energy Qy-states to P∗ in the bacterial RC. Potential problems with the strong coupling model are considered and possible approaches to its testing discussed.

Weakly-coupled models for motor enzyme function

In strongly-coupled models for motor enzyme function, such as the original Huxley (1957) model for muscle, ATP binding and subsequent hydrolysis are required for the detachment and reattachment of every force-producing cross-bridge. In weakly-coupled models, cross-bridges can be 'mechanically detached' without ATP binding when they have been pushed far beyond their free energy minimum and have accumulated so much strain that the attached state is less stable than the detached state. Weakly-coupled models assume that these mechanically detached cross-bridges can rejoin the pool of detached molecules that can reattach as force-producing cross-bridges, without going through an ATP hydrolysis cycle. This paper bases this assumption on a thermodynamically rigorous model for interaction between a motor enzyme molecule and binding sites on a cytoskeletal protein filament, equivalent to other examples of ligand binding interactions. It attempts to identify more clearly the features that must be added to the idea of ligand binding equilibrium to simulate a weakly-coupled motor enzyme model. Models that assume a vectorial conformational change and a longitudinal series elastic element appear to be incompatible with the assumptions of weakly-coupled cross-bridge models. A stochastic computational method has been used to examine the properties of these models. The computations have examined the behaviour of a model containing a four-state ATPase cycle, but the model is computationally a nine-state model because a force-generating attached state is allowed to equilibrate with different detached states at negative and at positive distortions, and because three adjacent sites are considered as possible attachment sites for each of the two attached states of the ATPase cycle.

High pressure NMR study of methyl group tunnelling in dimethyl sulphide

The rotational tunnel splittings of pairs of methyl groups in dimethyl sulphide have been determined by low field proton NMR as a function of hydrostatic pressure up to 4·5 kBar. Two principal splittings are clearly observed, one being equal to 750 kHz for all pressures, the other decreasing exponentially from 100 kHz at atmospheric pressure to 40 kHz at 4·0 kBar. For the first time, tunnelling sidebands of a sideband in the NMR spectrum have been seen. The temperature dependence of the spin-lattice relaxation time has also been measured as a function of pressure and has revealed double minima at higher pressure. The results have been analysed using both weak and strong methyl coupling models.

“Identical” superdeformed band in 151Dy: further evidence for the pseudospin coupling scheme

One of the four new excited superdeformed bands found in 151Dy is characterized by transition energies that lie midway between the γ-ray energies of the yrast superdeformed band in 152Dy. This result adds significant new information on the issue of “identical” bands, as it confirms predictions based on the strong coupling model and the pseudospin coupling scheme.

Level structure and reflection asymmetry in 227Th.

The level structure of $^{227}\mathrm{Th}$ has been studied using the electron capture decay of $^{227}\mathrm{Pa}$ and the ${\mathrm{\ensuremath{\beta}}}^{\mathrm{\ensuremath{-}}}$ decay of $^{227}\mathrm{Ac}$ to give a total of 20 levels where only four were previously known. This level structure then makes possible a more complete interpretation of the recently studied $^{231}\mathrm{U}$ alpha decay into $^{227}\mathrm{Th}$. The level structure can be interpreted using both strong coupling and intermediate coupling models in terms of K=1/${2}^{\ifmmode\pm\else\textpm\fi{}}$,K=3/${2}^{\ifmmode\pm\else\textpm\fi{}}$, and more tentative K=5/${2}^{\ifmmode\pm\else\textpm\fi{}}$ parity double bands which implies that $^{227}\mathrm{Th}$ has reflection asymmetry. Futhermore, the detailed interpretation of the level structure of $^{227}\mathrm{Th}$ solves the long standing problem of the ground state spin-parity of $^{227}\mathrm{Th}$ which is shown to be 1/${2}^{+}$.

M1 transitions between superdeformed states in 194, 195Tl: The fingerprint of the i13/2 proton intruder orbital

Recent data from the EUROGAM array have revealed dipole transitions linking signature partner superdeformed bands in 194Tl and 195Tl nuclei. Measurements of the decay branching ratios, taken together with the average quadrupole moment of the neighboring superdeformed nuclei, enable the absolute M1 strengths to be determined. From these data and using the strong coupling model, we find that two SD bands in 195Tl are due to the 81st proton being in the [642]5/2+ orbital and four from the six SD bands in 194Tl correspond to a configuration where the intrinsic spins of the single proton and single neutron are aligned.

Specific heat of 2H-NbSe 2 in high magnetic fields

Recently, the unusual behaviour of the specific heat of the superconductors YBa 2Cu 3O 7, Y 2Ba 4Cu 7O 15, and YBa 2Cu 4O 8 at high field and low temperature was attributed to Zeeman splitting of localized states in the vortex cores [D. Sanchez et al., Physica C 200 (1992) 1]. In an attempt to detect similar effects in classic high-κ superconductors, we have measured the mixed-state specific heat of NbSe 2 single crystals in magnetic fields up to H c2(0). The specific heat in the superconducting state and zero field is found to follow accurately a strong-coupling model that takes into account the anisotropy of the gap. We analyse then the mixed-state specific heat using the London model. A small Schottky-like excess specific heat remains unexplained. In qualitative agreement with the observations made in YBaCuO, this contribution is suggested to originate from the localized vortex core levels observed by STM techniques.

A strong coupled model for analysing dynamic behaviours of non-linear electromechanical systems

A strong coupled model solving simultaneously electric circuit, magnetic field and mechanical motion equations is presented. The magnetic field is solved by a formulation in terms of magnetic vector potential. The working variable in the mechanical equation can be either the displacement or the velocity. The magnetic force is calculated by the virtual work method associated with the edge element. The non-linear system is solved by the Newton-Raphson method. The final matrix equation is symmetrized. A numerical example shows that, compared with a conventional sequential coupling model, the convergence of the present method is less dependent on time step values. >

Elementary-spin-excitation spectrum of undoped and doped single-band Hubbard models.

We have calculated the elementary-spin-excitation spectrum of undoped and doped Hubbard models in one and two dimensions. We have obtained the gap of the spectrum at Q=(\ensuremath{\pi},\ensuremath{\pi}) in the doped strong-coupling Hubbard model by a Lanczos diagonalization study of the \ensuremath{\surd}10 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}10 site cluster. The result is qualitatively in agreement with the experimental observation of the ``spin gap'' by Rossat-Mignod et al. We conclude that the ``spin gap'' can be accounted for with the doped strong-coupling Hubbard model without any symmetry reductions and is one of the intrinsic properties of the Hubbard model.

Magnetic Phases and Generalized t-J Models in Doped Mott-Hubbard Insulators

We review some recent results obtained for the dynamics of a single hole and for the ground states at finite hole doping in t—J model. Next, we address the role of orbital degeneracy in doped Mott—Hubbard insulators and show examples of effective strong coupling models which include the orbital degrees of freedom. These new- t-J models have interesting phase diagrams, with the new magnetic phases stabilized by a competition between magnetic energy and excitonic excitations. It is argued that the doped holes always bind to the excitons and that the new phases identified on the mean-field level give rise to local distortions of the lattice. We conclude that realistic t—J models derived from the electronic structure of particular compounds may be successfully applied for understanding both the observed magnetic ground states, and the results of photoemission experiments, as we have demonstrated recently for NiO.

Phenomenological Analysis and Microscopic Structure of "Identical" Superdeformed Bands

A reliable phenomenological analysis of superdeformed(SD)bands shows that the so-called identical SD bands in general may have different bandhead moments of inertia ( δJ o/ J o≥10-2). Because the dynamic moment of inertia J varies with ω much faster than the kinematic moment of inertia J, and the ω variation of moments of inertia may be quite different for various SD bands, under certain conditions a near equality of J (hence E_γ) of two identical SD bands may occur in certain frequency range (| δEγ / Eγ |=| δJ / J |～10-3), and the angular momentum alignments may appear to be approximately quantized. But the situation turns out to be different in other frequency regions. The present phenomenological analysis seems to be consistent with the configuration assignments made by the available microscopic theory in the framework of strong-coupling model. No pseudospin symmetry is involved in the present analysis.

Generalized commutators and deformation of strong coupling superconductivity

We consider quantum deformation of a strong-coupling superconductivity model based on creation/annihilation operators which satisfy generalized commutator relations. It is found that the nature of the superconducting phase transition can be changed from the usual second-order to a first-order transition, if the deformation parameter exceeds a certain critical value. Metastable normal and superconducting states can exist when the transition is of the first order.

First measurement of magnetic properties in a superdeformed nucleus: 193Hg.

Dipole transitions linking signature partner superdeformed bands in 193Hg have been observed. Measurements of the photon decay branching ratios, taken together with the average superdeformed quadrupole moment measured in neighboring nuclei, enable the absolute M1 strengths to be determined. From these data, using the strong coupling model, we find (gK-gR)K/Q0=-0.14±0.01 (e b)−1, gK=-0.65±0.14 with Ω=2.8±0.8. These data are consistent with the superdeformed neutron orbital being [512]5/2− as predicted by cranked Woods-Saxon calculations.

Microscopic description of tunneling systems in a structural model glass.

We present a quantitative method which systematically finds tunneling systems in glasses and hence allows a microscopic check of the standard tunneling model. We apply this method to a two-component model amorphous alloy. The major assumptions of the standard tunneling model are qualitatively verified. Small quantitative differences in the distribution of the tunneling matrix elements explain why the experimental temperature dependence of the specific heat is superlinear and the thermal conductivity is subquadratic. Connections to the soft-potential model and recent strong-coupling models are discussed.

On the nature of the circadian clock in mammals

The evidence for the essential role of the suprachiasmatic nucleus (SCN) for the generation and maintenance of circadian rhythms in mammals is briefly reviewed. The pharmacology of the phase-response curve is considered and a new circadian measure, the phase-dose-response surface (PDRS), is introduced. The role of neurotransmission, ion fluxes, and non-neuronal cellular elements in the generation and maintenance of circadian rhythmicity is considered. Cell culture of the SCN is proposed as a tool for the functional analysis of clock mechanism. The critical contribution of coupling and synchronization of clock elements is reviewed in the context of the explicit predictions generated by a strong coupling model of the circadian clock. Finally, the nature of the circadian output signal is analyzed from a phylogenetic viewpoint.

Hubbard model in the strong-coupling approximation.

We report numerical results for the two-dimensional Hubbard model in the strong-coupling approximation, including the three-site terms omitted in the t-J model, for a 4\ifmmode\times\else\texttimes\fi{}4 system with periodic boundary conditions. We study the one- and two-hole band structures, the root-mean-square hole-hole separation, and the hole-hole binding energy for both the t-J and the strong-coupling models. In the physical parameter region (8U/t40) believed appropriate for the high-temperature superconductors, the three-site terms enhance the hole mobility, increasing the one- and two-hole bandwidths and decreasing the binding energy, but changing the hole-hole separation only slightly; quantitative differences between the models range up to 40%. The transition (at large U/t) to the ferromagnetic Nagaoka state is sharper in the strong-coupling model; our results do not support a transition through all intermediate S values in the bulk.

De Haas-van Alphen effect and the Fermi surface in κ-(BEDT-TTF)2I3

The de-Haas-van Alphen effect and the Fermi surface have been studied in the κ-typ organic superconductor (BEDT-TTF)2I3. The originally proposed Fermi surface using the tight-binding band calculation is different from the ones reported afterwards for the other κ-type salts. The Fermi surface has simple 2-dimensional cylindrical shape and no band gap exists at the zone boundary due to the crystal symmetry. The absence of band gap at the boundary is known as the band sticking effect. We confirmed the band sticking effect experimentally using the magnetization measurement. The observed quantum oscillation frequency corresponds to the 100% area of the first Brillouin zone. The Fourier analysis has revealed another frequency corresponding to the 20% area of the first zone other than the fundamental one. This frequency corresponds to the one reported for the other κ-type salts (BEDT-TTF)2Ag(CN)2H2O and (BEDT-TTF)2Cu(NCS)2. The effective mass of the fundamental frequency and the Dingle temperature are 2.7m0 and 0.9 K respectively. The many body renormalization effect has benn discussed considering mass enhancement and strong coupling model for the superconductor.

Fluid-structure coupling by the entrained fluid in submerged concentric double-shell vibration.

Vibrational characteristics of a point-driven “double shell” (two concentric submerged cylindrical shells coupled by the entrained fluid) are investigated. Strong-and weak-coupling theories are proposed to model the interaction of how the inner and outer shells are coupled by the entrained fluid. Each theory is based on Flugge's infinite-shell equations, the Helmholtz wave equation, and boundary conditions at the fluid-struc tureinterfaces which depend on the coupling situation. Strong coupling forms a stand ingwavefield in the entrained fluid; weak coupling, a free wavefield. Experiments are carried out where generalized near-field acoustical holography (GENAH) is employed to provide the experimental vibration characteristics in wavenumber-frequency space of the finite double-shell. It is confirmed theoretically and experimentally that the outer shell of the double shell exhibits two separate dispersion curves: A higher-frequency curve bears in-phase vibrations with respect to the inner shell and suggests a forced response of the inner shell; the other lower-frequency curve, out-of-phase vibrations, and an induced response of the outer shell. Fluid-structure coupling condition de finitelyaffects the lower-frequency curve, but barely the higher-frequency one. Com parisonbetween theoretical and experimental results demonstrates the validity of the strong-coupling model.

Eta -pairing superconductivity in the spin-polarized strong-coupling negative-U Hubbard model.

We have calculated the effect of a single unpaired electron on superconductivity in the strong-coupling negative-U Hubbard model employing a diagram technique with generalized Matsubara frequencies. Using a generalization of the Hamiltonian to n orbitals per site and applying a geometry-controlled approximation in the limit of high coordination numbers z, we perform a loop expansion of the model. This allows us to incorporate fluctuation corrections to the mean-field solution, which emerges as result in lowest order of the loop expansion. We show that in the limit \ensuremath{\Vert}U\ensuremath{\Vert}\ensuremath{\rightarrow}\ensuremath{\infty} the mean-field solution exhibits a phase transition to a state that is characterized by a ``staggered'' local superconducting order parameter with its sign alternating from site to site. This superconducting state (referred to as the ``\ensuremath{\eta}-pairing'' state by a number of authors) for the attractive case is related to the Nagaoka ground state of the repulsive Hubbard model by a partial particle-hole transformation. We have found that the model in the slightly spin-polarized case (i.e., when a single unpaired electron is present) for highly coordinated lattices and in lowest loop order is equivalent to a pair-hopping model with temperature-dependent repulsive coupling. In this paper we report the mean-field results for the transition temperature, the order parameter, the chemical potential, the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ and excitation spectra both above and below ${\mathit{T}}_{\mathit{c}}$ and demonstrate that the system exhibits a complete Meissner effect.

On the superconductivity in 2D systems with nesting

Superconducting properties of La 2CuO 4 are studied within the model of weak coupling for the Emery hamiltonian. The weak coupling model for the 2D CuO 2 lattice is obtained by means of the canonical transformation from the strong coupling model. The elementary excitations within the Emery model are the Zhang-Rice magnetic polarons whose motion depends essentially on the spin state of the Cu-sublattice. The effective spectrum of magnetic polarons for the RVB ground state is shown to satisfy the nesting condition near the Fermi surface in some narrow range of doping level. Several mechanisms are found to contribute to effective interaction, i.e. the usual Hubbard repulsion and sign changing momentum-dependent correlation contributions. Superconducting d-pairing emerges provided repulsion interaction dominates, and s-pairing can be realized if the sign-changing interaction gives the main contribution. In both cases the superconductivity arises owing to nesting which is responsible for the strong anisotropy of the pairing interaction. Nesting conditions are not satisfied for the Neel state and hence superconductivity does not arise in this case.