Renormalized quasiparticles in antiferromagnetic states of the Hubbard model

We analyze the properties of the quasiparticle excitations of metallic antiferromagnetic states in a strongly correlated electron system. The study is based on dynamical mean field theory (DMFT) for the infinite dimensional Hubbard model with antiferromagnetic symmetry breaking. Self-consistent solutions of the DMFT equations are calculated using the numerical renormalization group (NRG). The low energy behavior in these results is then analyzed in terms of renormalized quasiparticles. The parameters for these quasiparticles are calculated directly from the NRG derived self-energy, and also from the low energy fixed point of the effective impurity model. From these the quasiparticle weight and the effective mass are deduced. We show that the main low energy features of the k-resolved spectral density can be understood in terms of the quasiparticle picture. We also find that Luttinger's theorem is satisfied for the total electron number in the doped antiferromagnetic state.     

Local field correction effect on inelastic Coulomb scattering lifetime of two-dimensional quasiparticles at low temperatures

We have investigated the effect of local field correction on the inelastic Coulomb scattering lifetime of high mobility quasiparticles in a quantum layer at low temperatures. By replacing temperature-dependent dynamic dielectric function for the zero-temperature one in calculations, we have considered our improved zero-temperature STLS local field correction for low temperatures in lifetime calculations and compared the results with those obtained from the RPA and Hubbard approximation. It has been found that the quasiparticle lifetime decreases by including the STLS local field factor in all temperatures, energies and layer thicknesses.     

Ferromagnetism in the two-dimensional Hubbard model with long-range hopping

The combination of small-cluster exact-diagonalization calculations and the quantum Monte Carlo method is used to examine ferromagnetism in the two-dimensional Hubbard model with a generalized type of hopping. It is found that the long-range hopping with exponentially decaying hopping amplitudes t _ij ～ ? q ^Ri?Rj stabilizes the ferromagnetic state for a wide range of electron interactions U and electron concentrations n > 1. The critical value of the hopping parameter q _c above which the ferromagnetic state becomes stable is calculated numerically and the ground-state phase diagram of the model is discussed for physically the most interesting cases.     

Generalized diffusion and quasi-elastic scattering widths in two-dimensional systems

The paper presents approximate calculations of dimensionality and lattice symmetry effects in the wave vector and frequency dependent diffusional response of disordered lattices. Numerical results are given and comparatively discussed for the tracer diffusion correlation function and the quasi-elastic incoherent scattering width in triangular, simple square and simple cubic lattices, only the restriction of double occupancy avoidance being taken into account. The quasi-elastic coherent scattering width is estimated for a triangular system possessing liquid-like structural disorder by means of the simple inclusion of a spread in jump lengths around a preferred set of jumps. Qualitative contact is made with recent neutron scattering experiments on alkali-metal graphite intercalation compounds.     

D-wave pairing in the two-dimensional Hubbard model with low filling

It is shown that the two-dimensional electron system of low-density, when described by the Hubbard model, is unstable towards superconductive transition in thed-wave pairing state.     

Anomalous magnetoresistance of two-dimensional systems in the presence of spin-orbit scattering

A theory of weak localization in two-dimensional semiconductor structures and metal films is developed for spin relaxation by the Elliott-Yafet mechanism. The theory is valid in the entire range of classically weak magnetic fields. It is shown that effects due to spin-orbit interaction substantially modify magnetoresistance in both diffusive and ballistic regimes.     

Nature of quasiparticles in itinerant correlated electron systems with geometrical frustration

Using the exact diagonalization technique, we study the effect of geometrical frustration on single-particle, spin and charge excitations in the Hubbard model in a metallic state close to half-filling. As the frustration increases, the magnetic order in the system is suppressed and the peak in the single-particle spectrum becomes sharper, indicating enhanced quasiparticle formation. Careful examination of spin and charge excitations shows that increasing frustration also leads to the merge of spin and charge excitation energies to that of the single-particle excitation. This is consistent with a Fermi liquid having well-defined quasiparticles with both spin and charge characteristics. The calculated results show that geometrical frustration plays an important role in defining the nature of quasiparticles in itinerant correlated electron systems.     

Phase separation in the two-dimensional bosonic Hubbard model with ring exchange

We show that soft-core bosons in two dimensions with a ring exchange term exhibit a tendency for phase separation. This observation suggests that the thermodynamic stability of normal Bose liquid phases driven by ring exchange should be carefully examined.     

Mechanism of superfluid-insulator transition in two-dimensional Bose Hubbard model

To understand the mechanism of Mott transitions in case of no magnetic influence, superfluid-insulator (Mott) transitions are studied for the S = 0 Bose Hubbard model on the square lattice, using a variational Monte Carlo approach. In trial many-body wave functions, we introduce various types of attractive correlation factors between a doubly-occupied site (doublon, D) and an empty site (holon, H), which play a central role for the transition. We propose an improved picture of D–H binding; a Mott transition occurs when the D–H pair length becomes equivalent to the minimum D–D distance, which lengths are appropriately estimated. We confirm this picture is valid for all the wave functions with attractive D–H factors we consider, and point out it can be universal for nonmagnetic Mott transitions.     

Roles of zeros of the Green function in Fermi arc and non-Fermi liquid in the two-dimensional Hubbard model

We clarify effects of zeros of the Green function on a Fermi arc and on a non-Fermi liquid behavior in the two-dimensional Hubbard model by means of the cellular dynamical mean-field theory (CDMFT). We study in detail the state with a hole-pocket Fermi surface and zeros of the Green function, which was found for a slightly doped Mott insulator in an earlier CDMFT calculation [T.D. Stanescu, G. Kotliar, Phys. Rev. B 74 (2006) 125110; T.D. Stanescu, M. Civelli, K. Haule, G. Kotliar, Ann. Phys. (N.Y.) 321 (2006) 1682]. As thermal or other extrinsic scatterings of electrons broaden the zeros, regions around the zero surface gain an imaginary part of the self-energy, which strongly suppresses the spectral intensity, especially on the closer side of the hole pocket to the zero surface. Then the rest emerges as a Fermi arc. Quasiparticle weight becomes ill defined on the closer side of the Fermi pocket while it is well defined on the opposite side, which means that a differentiation of electrons occurs in the momentum space, indicating an emergence of a non-Fermi liquid phase.     

Unusually flat hole dispersion relation in the two-dimensional Hubbard model and restoration of coherence by addition of pair-hopping processes

The dispersion relation of a doped hole in the half-filled 2D Hubbard model is shown to follow a law around the and points in the Brillouin zone. Upon addition of pair-hopping processes this dispersion relation is unstable towards a law. The above follows from T=0 Quantum Monte-Carlo calculations of the single particle spectral function on lattices. We discuss finite dopings and argue that the added term restores coherence to charge dynamics and drives the system towards a d _{x2 - y2} superconductor. Received 22 March 1999     

Positron-lithium scattering with the inclusion of positronium formation

Detailed studies have been made of elastic scattering and positronium formation in low energy collisions of positrons with lithium atoms for the two partial wavesl=0,1. For this system, as for all alkali atoms, the positronium formation channel is open even at zero positron energy. A two-channel version of the Kohn variational method is used with trial functions containing many variational parameters, and reasonably well converged results are obtained. The s-wave positronium formation cross section is infinite at zero positron energy but it then falls rapidly to become several orders of magnitude smaller than the elastic scattering cross section which has a maximum value of approximately 100 ₀ ² at a positron energy of 0.5 eV. For p-wave scattering the positronium formation cross section rises to a value of approximately 10 ₀ ² at an energy of 0.1 eV, with the elastic scattering cross section rising to a maximum of approximately 60 ₀ ² just below the first excitation threshold at 1.84 eV.     

Absorption of ultrasound waves during dynamic processes in disperse systems

Measurements of ultrasound wave absorption are conducted at a frequency of 3 MHz in 3% suspensions of starch, gelatin, and lactose. It is shown that the dynamics of the additional ultrasound wave absorption coefficient in the suspensions carries information on the processes of swelling, dissolution, and the phase and structural periods occurring in the interaction of the disperse and dispersoid phases; it also reflects the influence of the temperature field on these processes.