Department of Theoretical Physics (F-1)
Department of Theoretical Physics (F-1)The group for theoretical physics of nuclei, elementary particles and fields investigated static and dynamic properties of hadrons, renormalizability of Wilson loops in quantum chromodynamics, unified theories of particles and fields, and atomic and nuclear three-body problems.
It has been shown that the problem of excessive pionic contribution to the electric polarizability of the nucleon in the linear sigma model cannot be solved by assuming a weak pionic field. Using projected coherent states for pions in the nucleon and increasing the chromomagnetic nucleon-delta splitting, the correct magnetic polarizability can be obtained, with the electric polarizability only twice as large compared to the experiment. The process gamma + gamma = rho0 + rho0 is a stringent test of mesonic and quark models. Two effective mesonic models involving rho and pion exchanges have been eliminated since they do not reproduce simultaneously all characteristic "resonance" features of the process. A similar calculation using the Glashow-Weinberg-Salam model for the gamma + gamma = Z + Z process has shown that the detection of a hypothetical fourth generation of fermions below the threshold via fermion-loop effects will be obscured by boson loops unless longitudinal Z's are selected.
Regarding quark confinement in the nonrelativistic quark model, it has been proved that the linear or quadratic potential is indeed nonnegative definite. It has also been shown that the covalent bond and the Van der Waals force contribute a small but noticeable amount to nuclear forces. An analogy with the atomic case shows that, in the hadronic case, both contributions are weakened due to colour effects. In the framework of perturbative quantum chromodynamics, the renormalization properties of the Wilson line integral along contours with cusps have been studied. It has been shown that, in the temporal gauge, a cusp renormalization constant arises non-locally.
Research on the unified theories of particles and fields showed that particles which live in at least five dimensional ordinary space and in the same dimensional Grassmann space of anticommuting coordinates manifest themselves in the four dimensional ordinary subspace as spinors, vectors or scalars. In the case of gravitational field, vierbeins have as their super partners spin connections with spin three half. A relativistic theory of point particles and extended objects without constraints was formulated. It was shown that the additional degrees of freedom are related to the variable tension within the object. The general theory of moving relativistic membranes with electric charge was also studied, and mass of a spherically symmetric radially oscillating mebrane was calculated. Covariant linear transport equations were studied, which in the strong scattering limit became equivalent to the Klein-Gordon, Dirac, Maxwell and Proca equations. Such transport equations were used to explain quantum-mechanical faster-than-light effects.
In the field of atomic physics, calculations of the mu-dt ion were finished, using the CFHH (Correlation-Function Hyperspherical-Harmonic) Method. Different parametrizations of the nonlinear correlation function yielded more stable values of fusion rate and other quantities than the differences in literature using other methods.
The group for theoretical condensed matter physics studied phase transitions in glassy systems, in liquid crystals and on solid surfaces, and investigated electronic models of high temperature superconductors. In collaboration with the J. Nehru University, New Delhi, the relaxation dynamics of quantum spin and dipolar glasses was investigated. A new model for the coupling between the proton pseudospin coordinates and thermal vibrations in proton glasses was suggested, in which proton tunneling is treated as coherent motion. It was shown that the line shape of the NMR resonance contains a sharp central peak, which is due to coherent proton tunneling and is thus a typical quantum effect. Together with the laboratory for dielectric spectroscopy at IJS it was shown that the static freezing temperature of deuteron glasses can be determined from the measured dynamic dielectric constant, since it corresponds to the point of divergence of the longest relaxation time. A theory of the nonlinear susceptibility of orientational glasses has also been formulated. In contrast to static approaches, time dependent fluctuations of the order parameter were considered, and the quasistatic nonlinear susceptibility thus obtained was shown to diverge at the static freezing temperature.
By means of an extended phenomenological Landau model, the phase transition from the modulated to the homogeneous ferroelectric liquid-crystalline phase under the influence of an external magnetic field was studied. The effects of finite dimensions on the static and dynamic properties were considered, and an explanation of the dielectric response of ferroelectric liquid crystals to a bias electric field was given, which agrees with the measured dielectric response. Static and dynamic properties of simply modulated phases were investigated within the framework of the phenomenological model of Orihara and Ishibashi, and the structure of a new incommensurate ferrielectric phase was determined. In collaboration with the laboratory of magnetic resonances at IJS and Kent State University, Ohio, the surface molecular anchoring in microconfined nematic liquid crystals near the transition to the smectic-A phase was investigated. The molecular surface anchoring profile was monitored, the density of point defects determined, and the surface anchoring strength estimated.
In collaboration with the Forschungszentrum Juelich, the transitions between reconstructed and unreconstructed noble metal surfaces in an electrolytic environment were investigated. The phase diagram was calculated, and it was shown that, by varying the electrode potential, it is possible to tune the surface structure between the reconstructed and unreconstructed phases. The nature of the transition between the double-stepped and single-stepped structures on vicinal Si(001) surfaces was investigated. It was shown that the transition is gradual, implying that double steps gradually unbind into pairs of single steps as the temperature is increased or the miscut angle decreased.
The pairing mechanism in high temperature superconductors is still not understood. Some of the normal state properties of copper oxides represent an open problem as well. In this context, two prototype models for strongly correlated electrons were investigated (the t-J model and the Hubbard model). It was shown that, in a one-dimensional system with staggered field, the hole paired state is stabilized and the superconducting correlations become pronounced. The exact solution of a bound pair in the anisotropic t-J model on the Cayley tree was found. This solution appears to be the only known exact solution of this model in the infinite system. Various symmetries of the superconducting correlations were investigated for the odd symmetry of the gap function. The results indicate that, at some range of the model parameters, odd-frequency correlations become camparable with BCS pairing corelations. The problem of one hole in the antiferromagnetic background was treated within the spin wave formalism. Various correlation functions were determined, e.g. spin backflow. The results agree well with the existing small cluster calculations. The influence of electron-phonon coupling in the heavy fermion systems was also studied. Spectral functions of the degenerate Anderson model with one impurity were calculated using the quantum Monte Carlo technique. The results agree with those obtained with other methods.
The group for theoretical biophysics studied polymer mediated inter and intra-membrane forces. The variational approach was used to obtain the configurational statistics of charged polymer chains. The force between two charged spherical aggregates mediated by the oppositely charged polymer chain was determined by applying the Monte Carlo method. In studies of shapes of phospholipid vesicles it was found that the bilayer couple model for some values of system parameters predicts nonaxisymmetric vesicle shapes. Here the equilibrium vesicle shapes were determined by minimization of the membrane bending energy where the vesicle volume and the areas of the two phospholipid monolayers composing the bilayer were kept constant. The generalized bilayer couple model was then introduced. In this model, the vesicle shapes were obtained by minimization of the membrane elastic energy which includes the local and non-local bending energies. A mathematical model was developed for the transmembrane transport of erythrocyte lipids. The results of the corresponding analysis indicated that the asymmetric distribution of the erythocyte lipids arises as a consequence of the balance between the passive and active fluxes of these molecules.