Two-scale analysis of the Hubbard model
Prof. Ferdinando Mancini
Lanczos method I
Dr. Maciej Bak
The basics of Lanczos method; prof. Prelovsek's approach. First Part
Lanczos method II
Dr. Maciej Bak
The basics of Lanczos method; prof. Prelovsek's approach. Second Part
Energy-scale-dependent analysis of the single-impurity Anderson model
Dr. Adolfo Avella
We set up an energy scale dependent approximation that allows to resolve low-energy features embedded in a high-energy background. The dynamics of the fluctuation field is split into high- and low- energy sectors. A self-consistent set of equations is derived. On the top of a broad high-energy background a Kondo peak is derived at low enough temperatures and its parameter dependence is studied. The method reproduces exact results with very low numerical effort and it is applicable within the full range of external parameters. The Anderson model is studied and resolved as significative example.
Quasi SUSY in Physics or Is Higgs the analog of the Amplitude Mode?
Prof. V. Srinivasan
In systems with spontaneous symmetry breaking, where there is a Goldstone mode, which is a bound state of fermions, there exists another bosonic mode, making the ratios of masses of the physical fermion and boson modes as 0:1:2. This mass relation generates an approximate SUSY or a quasi SUSY.
Effects of orbital correlations on spin dynamics in ferromagetic manganites
Dr. Sergey Krivenko
We demonstrate that orbital fluctuations have strong effects on magnon dynamics in ferromagnetic manganites when these compounds ar close to a transition to an orbital ordred state. In particular we show that the scattering of the spin excitations by low-lying orbital modes with cubic symmetry causes both the magnon softening and dampig observed experimentaly.
t-J model: COM with decoupling
Dr. Maciej Bak
In the standard Composite Operator Method we choose an operator basis of the system, calculate their equations of motion, and project new, higher-order operators appearing in equations of motion, back on basis. Eigenenergies of the system are usually calculated by means of the I and m matrices, appearing in the proces of linearization, which are averages of anticommutators of basis operators and equations of motion with basis operators on different site. Averages are fixed by Pauli principle. A new approach is presented, where linearization of equations of motion is reached not by averaging their (anti)commutators with basis but by a new decoupling scheme. Hence we avoid calculation of m matrix and get eigenenergies in a more direct way. This largely simplifies calculations and allows us to increase the size of the basis; the stability of solutions should be also improved.
Effects of two-site correlations in the two-dimensional Hubbard model
Dr. Satoru Odashima
Electronic states of the two dimensional Hubbard model are investigated by use of the Composite Operator Method. We present the way to incorporate the effects of two-site composite excitations as dynamical corrections to the electronic propagator. The used approximation goes well beyond the conventional two-pole approximations.
Non-Fermi liquid behavior in the stripe phase of the extended Hubbard model
Dr. Roberta Citro
Within a strong-coupling perturbative approach, based on a Cumulant Expansion of the extended single-band Hubbard model, we show that the on-shell inverse scattering time deviates from the normal Fermi-liquid behavior near the points of the Fermi surface connected by the characteristic wave-vector of an incommensurate charge density wave. The violation of the Fermi liquid behavior is associated with a square root behavior of quasiparticle lifetime in proximity of a stripe phase. Some relevant features observed in ARPES experiments on Bi2212 are qualitatively reproduced.
Temperature dependence of optical spectral weights in quarter-filled ladder systems
Dr. Mario Cuoco
The temperature dependence of the integrated optical conductivity I(T) reflects the changes of the kinetic energy as spin and charge correlations develop. It provides a unique way to explore experimentally the kinetic properties of strongly correlated systems. We calculated I(T) in the frame of a t-J-V model at quarter-filling for ladder systems, like NaV_2O_5, and show that the measured strong T dependence of I(T) can be explained by the destruction of short range antiferromagnetic correlations.
Ergodicity constant in Heisenberg model
Dr. Maciej Bak
A review of obtained results