Low-temperature anharmonic vibrations in the cuprates: a bipolaron model

Il Nuovo Cimento D - Starting from the recent experiments that have revealed large-amplitude anharmonic vibrations in the cuprates, the possibility of formation of bipolarons has been investigated....     

An equations-of-motion method for anharmonic vibrations: Application to a degenerate shell model

An equations-of-motion method is proposed for calculating anharmonic effects in doubly even spherical nuclei. The method can be viewed as an extension of the QRPA theory for spherical nuclei, and its applicability is demonstrated for the case of a degenerate shell model of configuration ($\text{21}\text{2}$)⁶ by comparison with the results of a previous exact diagonalization.     

Localized vibrations in perfect anharmonic lattices: Trapping on phonons

A theory is developed, which allows one to calculate intrinsic local modes (ILMs) in infinite lattices and to find the effect of an ILM on phonons. One prediction is the appearance of linear local modes in the lattice nearby an ILM. To check these results, high-precision MD calculations of the vibrations of diatomic chains have been carried through. The results fully confirm the predictions.     

Entanglement correlation in mixed states for anharmonic vibrations in ozone

Entanglement dynamics of anharmonic vibrations in molecule O₃ is studied in terms of the linear entropy and negativity with various initial states that are, respectively, taken to be the mixed density matrices of coherent states on each normal mode. It is shown that with a suitable parameter in initial states, the entropy in one mode can be positively correlated or anti-correlated with negativity. The behavior of correlation between two entropies for two modes, negativity, and mutual entropy is discussed as well.     

Local anharmonic vibrations strong correlations and superconductivity: A quantum simulation study

We investigate the importance of local anharmonic vibrations of the bridging oxygen in the copper oxide high-T _c materials in the context of superconductivity. For the numerical simulation we employ the projector quantum Monte Carlo method to study the ground state properties of the coupled electron-phonon system. The quantum Monte Carlo simulation allows an accurate treatment of electronic interactions which investigates the influence of strong correlations on superconductivity mediated by additional quantum degrees of freedom. As a generic model for such a system, we study the two-dimensional single band Hubbard model coupled to local pseudo spins (bridging oxygen), which mediate an effective attractive electron-electron interaction leading to superconductivity. The results are compared to those of an effective negativeU model.     

Magnetic domains and stripes in a spin-fermion model for cuprates

Monte Carlo simulations applied to a model of interacting fermions and classical spins show the existence of antiferromagnetic spin domains and charge stripes upon hole doping. The stripes have a filling of approximately 1/2 hole per site, and they separate spin domains with a pi phase shift among them. The observed stripes run either along the x or y axes. No particular boundary conditions or external fields are needed to stabilize these structures. When magnetic incommensurate peaks are observed at momentum pi(1,1-delta), charge incommensurate peaks appear at (0,2delta). The charge fluctuations responsible for the stripe formation also induce a pseudogap in the density of states.     

Modulational instability in the anharmonic Peyrard-Bishop model of DNA

We report on the presence of modulational instability and the generation of soliton-like excitations in DNA nucleotides. Taking the Peyrard-Bishop-Dauxois model of DNA dynamics as an example, we show that the original difference equation for the DNA dynamics can be reduced to the Salerno equation. We derive the MI criterion in this case. The effect of the anharmonic stacking term on the domain of instability/stability is pointed out. Numerical simulations show the validity of the analytical approach with the generation of wave packets provided that the wave numbers fall in the instability domain. The impact of the anharmonic stacking interactions is investigated and these are found to give rise to a spectrum of behaviours which corroborates experimental facts.     

Lattice dynamics of an anharmonic crystal: evidence for interactions between atomic vibrations at high temperatures

In studying the lattice dynamics of a strongly anharmonic crystal, we solve a set of nonlinear Langevin equations for interacting oscillators while a multiwell potential is calculated in the displacive limit from the first principles. The model applied to the peculiar vibrations of beta-Zr along [111] allows us to analyze all contributions to the spectral density and their influence on each other. We predict the effect of induced anharmonicity for quick vibrations due to their interaction with intrinsically anharmonic slow vibrations. This effect results in the broadband distribution in energy of inelastic neutron scattering known as the symmetry-forbidden phonon-branch splitting.     

Low-temperature solution of the Sherrington-Kirkpatrick model

I propose a simple scaling ansatz for the full replica symmetry breaking solution of the Sherrington-Kirkpatrick model in the low energy sector. This solution is shown to become exact in the limit x --> 0, Bx --> infinity of the Parisi replica symmetry breaking scheme parameter . The distribution function of the frozen fields has been known to develop a linear gap at zero temperature. The scaling equations are integrated to find an exact numerical value for the slope of the gap thetaP(x,y)/delta|(y --> 0) = 0.301 046.... I also use the scaling solution to devise an inexpensive numerical procedure for computing finite time scale (x =1) quantities. The entropy, the zero field cooled susceptibility, and the local field distribution function are computed in the low-temperature limit with high precision, barely achievable by currently available methods.     

Temperature dependence of the properties of strong-coupling bipolaron in a quantum dot

Temperature dependence of the properties of strong-coupling bipolaron in a quantum dot (QD) is studied based on the Lee-Low-Pines-Huybrechts variational method and quantum statistical theory. Results of the numerical calculation show that the vibration frequency as well as the absolute value of the induced potential and the effective potential all increase with increasing coupling strength and temperature, respectively, and they also increase with decreasing relative distance of electrons. The bipolarons are closer and more stable when the temperature is higher and coupling strength is larger. The influence of radius of QD and dielectric constant ratio on the effective potential is little.     

A Cud-d excitation model for the pairing in the high-T c cuprates

A new model for the pairing mechanism in the ceramic superconductors is presented. Like the magnetic models, we assume the limit of large correlation energies for the Cud electrons. We postulate that the pairing of the Op conduction holes occurs viad–d orbital excitations within thee _g manifold of thed hole of Cu⁺⁺, which is split because of tetragonal or lower symmetry at the Cu sites. This valence conserving charge degree of freedom has been ignored in the magnetic pairing models. Thed–d excitation model may provide a simple qualitative understanding of many experimental results.