Strong-coupling Model Research Articles

“Dark matter” in crystals: Objects invisible in low-energy electron scattering

The possibility of existing lengthy objects that fundamentally cannot be observed in low-energy electron scattering has been demonstrated. It has been shown in the one-dimensional strong coupling model that a pair of defects can be constructed so that the electron transmission coefficient in the low-energy limit (in the framework of the applicability of the effective mass method) is equal to one. In this limit, these objects are described not as a certain reflectionless potential, but as a homogeneous medium, where the effective size of an object vanishes; i.e., its collapse occurs.

Two fluid-structure approaches for 3D simulation of St. Jude Medical bileaflet valve opening.

This paper deals with the three-dimensional (3D) numerical simulation of the opening transient for a bileaflet mechanical heart valve (St. Jude Medical Hemodynamic Plus, 27 mm characteristic size). The discussion concerns two different types of ""partitioned"" fluid-structure models, with weak and strong coupling. Both approaches are based on a moving deforming mesh method as implemented in the CFD commercial software Fluent (Fluent Inc., USA), and both yielded dynamically consistent results from a quantitative viewpoint. There is a slight improvement (peak relative difference <1%) of the numerical solution (angles vs. time) when using the strong instead of the weak coupling model. This is of utmost importance when the considerable reduction of the computational costs for the weak scheme (about 60% less) is taken into account. However, the strong coupling model provides a more consistent physical description of the interaction when looking at small-scale features like stress close to geometrical singularities. In the experiments, the differences both in terms of time-to-peak delay (15% of the opening time) and in terms of underestimation of the peak velocity (8%) seemed due to defects in the fluid inertia assessment, which depends on initial and boundary data.

Pressure effects on soft-mode dynamics in KDP-type ferroelectrics

Molecular dynamics simulation and the modified strong dipole-proton coupling (MSDPC) model are used to show how the soft mode of ferroelectric phase transition in KH 2PO 4 (KDP) changes its spectral shape at room temperature due to the decrease in the hydrogen-bond length. The effect of hydrogen-bond shortening, which simulates the effect of pressure increase in experiment, appears as the strong decrease in the soft-mode damping accompanied with the weak increase in the soft-mode frequency. These results are in accordance with the experimentally detected effect of pressure increase on the soft-mode Raman scattering spectra.

Local strong-coupling pairing ind-wave superconductors with inhomogeneous bosonic modes

Recent local tunneling data indicate strong nanoscale inhomogeneity of a superconducting gap in high temperature superconductors. Strong local nanoscale inhomogeneity in the bosonic scattering mode has also been observed in the same samples. We argue that these two inhomogeneities are directly related to each other. To address local boson scattering effects, we develop a local strong coupling model of superconducting pairing in a coarse grained superconducting state. Each patch is characterized by local coupling to the bosonic mode as well as by local mode energy. We find that local gap value on each patch grows with the local strength of electron-boson interaction. At the same time local gap value decreases with the local boson mode energy, an observation consistent with the tunneling experiments. We argue that features in the tunneling spectrum due to boson scattering are consistent with experimentally observed spectra. We also address the $^{16}\mathrm{O}$ to $^{18}\mathrm{O}$ isotope substitution. Since both coupling constant and boson energy could change upon isotope substitution, we prove that interplay between these two effects can produce results that are very different from the conventional BCS model.

Topology of the Fermi surface and the coexistence of orbital antiferromagnetism and superconductivity in cuprates

It has been shown that the strong coupling model taking into account a rise in the spin antiferromagnetic insulating state explains the doping dependence of the topology and shape of the Fermi contour of superconducting cuprates. Hole pockets with shadow bands in the second Brillouin zone form the Fermi contour with perfect ordinary and mirror nesting, which ensures the coexistence of orbital antiferromagnetism and superconductivity with a large pair momentum for T < TC. The weak pseudogap region (T* < T < T*) corresponds to the orbital antiferromagnetic ordering, which coexists with the incoherent state of superconducting pairs with large momenta in the strong pseudogap region (TC < T < T*).

Strong coupling effects in the polarized IR spectra of the chain hydrogen bond systems in imidazole crystals: H/D isotopic ‘self-organization’ effects in the IR spectra of isotopically diluted imidazole single crystals

This paper presents the investigation results of the polarized IR spectra of H1245 imidazole crystals and of D1H245, D1245 and H1D245 imidazole deuterium derivative crystals. The spectra were measured using polarized light at the room temperature and at 77 K by a transmission method, for two different crystalline faces. Theoretical analysis of the results concerned linear dichroic effects, H/D isotopic and temperature effects, observed in the spectra of the hydrogen and of the deuterium bonds in imidazole crystals, at the frequency ranges of ν N–H and ν N–D bands. The basic crystal spectral properties can be satisfactorily interpreted in a quantitative way for a hydrogen bond linear dimer model. Such a model explains not only a two-branch structure of the ν N–H and ν N–D bands in crystalline spectra, but also some essential linear dichroic effects in the band frequency ranges, for isotopically diluted crystals. Model calculations, performed within the limits of the strong-coupling model, allowed for quantitative interpretation and for understanding of the basic properties of the hydrogen bond IR spectra of imidazole crystals, H/D isotopic, temperature and dichroic effects included. The results allowed verification of theoretical models proposed recently for the imidazole crystal spectra generation mechanisms. In the scope of our studies, the mechanism of H/D isotopic self-organization processes, taking place in the crystal hydrogen bond lattices, was also recognized. It was proved that for isotopically diluted crystalline samples of imidazole, a non-random distribution of protons and deuterons exclusively occurs in some restricted fragments (domains) of open chains of the hydrogen-bonded molecules. Nevertheless, these co-operative interactions between the hydrogen bonds do not concern adjacent fragments of neighboring hydrogen bond chains in the lattice. Analysis of the isotopic self-organization effects in the spectra of imidazole crystals delivered crucial arguments for understanding of the nature of the hydrogen bond spectra generation mechanisms.

Polarization IR spectra of the hydrogen bond in 1-naphthylacetic and 2-naphthylacetic acid crystals: H/D isotopic effects. Temperature and polarization effects

This paper deals with experimental studies and with the quantitative interpretation of the polarized IR crystalline spectra of 1-naphthylacetic and 2-naphthylacetic acid. The spectra were measured in the ν O–H and in the ν O–D band frequency ranges, at the temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the strong-coupling model, when assuming that the isolated (COOH) 2 and (COOD) 2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. A vibronic mechanism, promoting the symmetry forbidden transition in the IR, for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers, was found to be of a considerably minor importance, when compared with analogous spectral properties of arylcarboxylic acid crystals. Also no Fermi resonance impact on the ν O–H band shape was identified in the 1-naphthylacetic and 2-naphthylacetic acid crystal spectra. These effects were ascribed to weakening of electronic couplings between the hydrogen bonds and the phenyl rings, due to the separation of these molecular fragments by methylene groups in 1-naphthylacetic and 2-naphthylacetic acid molecules.

Correlation between hydrogen bond geometry and phase transition temperature in KDP-type ferroelectrics

Molecular dynamics simulation of the modified strong dipole-proton coupling model is used to show that ferroelectric phase transition temperature, T c, in KH 2PO 4 (KDP) and KD 2PO 4 (DKDP) linearly depends on hydrogen bond length, R O−O, and separation between peaks in proton (deuteron) thermal distribution in the paraelectric phase, δ. Separation between lines for KDP and DKDP is larger in the T c- R O−O dependence than in the T c- δ dependence indicating equal effect of different R O−O and different proton and deuteron masses on the whole change of T c between KDP and DKDP and establishing δ as a more important determinant of T c in these crystals.

Development and Application of the Modified Strong Dipole-Proton Coupling Model for KDP-Type Crystals

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KDP-type ferroelectrics, was used to investigate local properties of these crystals. The results showed both order-disorder and displacive characteristics of pure KDP and DKDP, as in many other experimental studies (NMR, neutron scattering) of local properties. Paramagnetic center SeO4 3− introduced in KDP and DKDP lattice as a local probe in EPR technique was treated within MSDPC model as a “soft” impurity. Simulated dipole reorientational dynamics of the impurity showed characteristics similar to the slow local dynamics of the probe detected in an EPR experiment. Additionally, the MSDPC model was adapted to describe dielectric properties of KDP-type antiferroelectrics. Antiferroelectric phase transition of the first order and isotopic shift in Tc were reproduced.

Polarization IR spectra of hydrogen bonded 1-naphthoic acid and 2-naphthoic acid crystals: electronic effects in the spectra

This article deals with measurements and a theoretical interpretation of the polarization IR spectra of the hydrogen bond in h 8-1-naphthoic and h 8-2-naphthoic acid crystals and crystals of their deuterium isotopomers: d 7-1-naphthoic acid and d 7-2-naphthoic acid. Polarization spectra were measured at the room temperature and at 77 K. Similarly as for other carboxylic acid crystal cases, the intensity distribution in the bands may be discussed on the basis of the strong-coupling model, when assuming that the isolated (COOH) 2 and (COOD) 2 cycles determine basic spectral properties of hydrogen bonds in the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. Another band shaping mechanism, i.e. a vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations, in the centrosymmetric dimers of hydrogen bonds, was also considered. It was shown that for 1-naphthoic acid crystal spectra the promotion mechanism is ca. two times weaker than in the case of 2-naphthoic acid crystal. This relation was found independent of the hydrogen isotopes linked with the aromatic rings. A new kind of the long-range isotopic effects H/D in the spectra was indicated, depending on the influence of the aromatic ring hydrogen atoms in the ν O–H band fine structure patterns. The role of the aromatic ring electronic properties was also discussed, in order to explain reasons of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra.

Calculation of strongly-coupled rotational bands in terms of the tilted axis cranking model

Recently observed strongly-coupled rotational bands associated with the ν[505] 11 2 − quasiparticle state are studied by means of a microscopic tilted axis cranking (TAC) model. The results of calculation for the routhians and the B(M1)/ B(E2) ratios are investigated in the light of other existing models, i.e., the strong-coupling model and the conventional cranking model. It is demonstrated that only the TAC model can successfully reproduce these two observables at the same time. The reason of the success is clarified by making connections between these models.

Molecular dynamics simulation of the soft mode for hydrogen-bonded ferroelectrics

Numerical simulations of dipole dynamics have been studied by employing a recently proposed modified strong dipole proton coupling (MSDPC) model of the phase transition in the hydrogen-bonded type of ferroelectrics. The obtained results in the form of the spectral density of the polarization fluctuation along the polar axis and along the lateral direction are simulated and correlated with known experimental data obtained primarily by Raman and infrared spectroscopy on KDP and DKDP lattices. A central peak (CP) appears in the calculated spectral density of the longitudinal polarization fluctuation in the ferroelectric phase of KDP and DKDP. The simulated CP indicates an order-disorder mechanism of excitation in the paraelectric phase near T c in both crystals. By increasing the temperature to 460 K, the crossover to the displacive behavior for KDP lattice is predicted. The model also predicts a low-frequency flat anomaly for KDP and a CP for DKDP in the spectral density of transversal polarization fluctuations, as was earlier detected by infrared and Raman spectroscopy. The superiority of the MSDPC model is additionally tested in the description of the experimentally obtained pressure-induced change of the CP line shape for KDP.

Thermodynamic potentials for the infinite range Ising model with strong coupling

The specific Gibbs free energy has been calculated for the infinite range Ising model with fixed and finite interaction strength. The model shows a temperature driven first-order phase transition that differs from the infinite ranged Ising model with weak coupling. In the temperature-field phase diagram the strong coupling model shows a line of first-order phase transitions that does not end in a critical point.

Model of a quantum dot coherently interacting with an ultrashort pulse of electromagnetic radiation

Based on a model of strong coupling for two electrons that are localized inside a quantum dot characterized by only two levels of size quantization, a set of equations is derived, which allows one to describe coherent transient processes in an ensemble of isolated dots. This set of equations is a generalization of the Bloch equations, which are used in studying the processes occurring under the action of ultrashort pulses of electromagnetic interaction. The model obtained is reduced to the model of two-level atoms only if radiation is circularly polarized.

Specific heat study of magnetic excitations in a one-dimensional S=1 Heisenberg magnet with strong planar anisotropy

The results of experimental studies of the specific heat of the magnetic chain compounds Ni(C2H8N2)2Ni(CN)4, Ni(C11H10N2O)2Ni(CN)4, and Ni(C10H8N2)2Ni(CN)4⋅H2O are reported. All compounds are identified as S=1 planar Heisenberg magnetic chains with large planar anisotropy and different values of the in-plane anisotropy constant. The low-temperature specific heat data are interpreted assuming the existence of noninteracting excitons and antiexcitons as elementary excitations from the singlet-ground state. The extended strong-coupling model is used for analysis of the data at higher temperatures. The applicability of the models used with respect to the value of the in-plane anisotropy is discussed.

Morphology and growth kinetics of 3He crystals below 1 mK

The shapes as well as the growth and melting properties of bcc-3He single crystals have been investigated with a low temperature Fabry-Perot interferometer. Eleven types of facets were clearly identified during slow crystal growth at the temperature of 0.55 mK, where the solid is in the antiferromagnetically ordered u2d2 phase. The growth rates of the individual facets have been measured and the results indicate significant growth anisotropy. The observed linear dependence of the growth velocity on the driving force shows that facets grow due to the presence of screw dislocations, while the step velocity is limited by the spin wave velocity due to the strong interaction of the moving step with magnons in the solid. The measured growth rates of the facets and the assumed growth mechanism gave us the unique opportunity to obtain the step free energies for ten different types of facets observed during a single growth sequence. The dependence of the free energy of the step on the step height is compared with predictions of the weak- and strong-coupling models. Our results suggest that 3He crystals have rather strong coupling of the liquid/solid interface to the crystal lattice and that the step-step interactions are of elastic origin.

Electronic properties ofCeIn3under high pressure near the quantum critical point

We present a detailed study of the phase diagram of the antiferromagnetic Kondo-lattice compound ${\mathrm{CeIn}}_{3}$ under pressure up to 100 kbar by resistivity measurements. Antiferromagnetic order vanishes at a critical pressure ${P}_{c}=26.5\mathrm{kbar}.$ At this quantum critical point a complete superconducting transition is found in the pressure range from 24 to 27 kbar. Normal state properties near the critical point show strong deviations from Fermi-liquid behavior. In magnetic fields just above the upper critical field Fermi-liquid behavior is restored. For $Pg{P}_{c},$ $\ensuremath{\rho}(T)$ shows a clean ${T}^{2}$ dependence up to a crossover temperature ${T}_{I}$ which increases linearly with pressure. The temperature dependence of the upper critical field ${H}_{c2}$ can be described with a strong coupling model in the clean limit.

Polarization IR spectra of the hydrogen bond in phenylacetic acid crystals: H/D isotopic effects: Temperature and polarization effects

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of phenylacetic acid and of its deuterium isotopomers d 2 and d 7. The spectra were measured in the ν O–H and in the ν O–D band frequency ranges at temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the strong-coupling model, when assuming that the isolated (COOH) 2 and (COOD) 2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. A vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers, was found to be of a considerably minor importance, when compared with analogous properties of arylcarboxylic acid crystals. The spectra of phenylacetic acid crystals, unlike the spectra of arylacetic acid crystals do not exhibit the so-called H/D long-range isotopic effects, depending on an influence of the aromatic ring hydrogen atoms on the ν O–H band fine structure patterns. Also no Fermi resonance impact on the ν O–H band shape was identified in the phenylacetic acid crystal spectra. These effects were ascribed to weakening of electronic couplings between the hydrogen bonds and the phenyl rings, due to the separation of these groups in phenylacetic acid molecules by methylene groups.

Flow equation analysis of the anisotropic Kondo model

We use the new method of infinitesimal unitary transformations to calculate zero temperature correlation functions in the strong-coupling phase of the anisotropic Kondo model. We find the dynamics on all energy scales including the crossover behaviour from weak to strong coupling. The integrable structure of the Hamiltonian is not used in our approach. Our method should also be useful in other strong-coupling models since few other analytical methods allow the evaluation of their correlation functions on all energy scales.

Relativistic effects in PtF6 and isoelectronic octahedral groups: Magnetic susceptibility, 19F NMR, and ab initio calculations

The temperature-independent paramagnetism and high O h symmetry of PtF6 molecules in solids (at 77 K) and of the octahedral molecular complexes [IrF6]− in KIrF6 and [OsF6]2− in K2OsF6 are shown to be consistent with the model of strong spin-orbit coupling of 5d4 electrons.