arXiv subjects
Publications and source records attributed to M. Crisan.
We show that in the Anderson model for a two-dimensional non-Fermi liquid a magnetic instability can lead to the itinerant electron ferromagnetism. The critical temperature and the susceptibility of the paramagnetic phase have been analytically calculated. The usual Fermi behaviour is re-obtained taking the anomalous exponent to be zero.
Using the field-theoretical methods we studied the evolution from BCS description of a non-Fermi superconductor to that of Bose-Einstein condensation (BEC) in one loop approximation. We showed that the repulsive interaction between composite bosons is determined by the exponent $α$ of the Anderson propagator in a two dimensional model. For $\a\neq 0$ the crossover is also continous and for $\a=0$ we obtain the case of the Fermi liquid.
We showed that an electronic system with weak disorder considered in the finite-charge infinite U Hubbard model can present a non-Fermi behavior. The imaginary pert of the self-energy has been calculated and a linear temperature dependence was obtained. This result is in agreement with the non-Fermi behavior observed at the insulator-metal crossover in La_{2-x}Sr_xCuO_4.
Using the renormalization approach proposed by Millis for the itinerant electron systems we calculated the specific heat coefficient $γ(T)$ for the magnetic fluctuations with susceptibility $χ^{-1}\sim |δ+ω|^α+f(q)$ near the Lifshitz point. The constant value obtained for $α=4/5$ and the logarithmic temperature dependence, specific for the non-Fermi behavior, have been obtained in agreement with the experimental dat.
We investigate the fluctuation contribution to the specific heat of a two-dimensional superconductor with a non-Fermi normal state described by a Anderson Green's function $G(k,iω)=ω_c^{-α}/(iω-ε_k)^{1-α}$. The specific heat corrections contain a term proportional to $(T^{2α-T_c^{2α}})^{-1}$ and another logarithmic one. We defined a coherence length as function of the non-Fermi parameter $α$, which showed that a crossover study between BCS and Bose-Einstein condensation is possible by varying $α$ in an interval $0 ÷α_{cr}$. By comparing our theoretical results with the experimental data for HTSC materials, we reobtained the value for $α$, corresponding to such systems, of the order $0.3 ÷0.45$. We also reobtained the critical temperature for such a superconductor using the Thouless criterion.
We studied the influence of the amplitude fluctuations of a non-Fermi superconductor on the energy spectrum of the 2D Anderson non-Fermi system. The classical fluctuations give a temperature dependence in the pseudogap induced in the fermionic excitations.
The flow equation method has been used to calculate the energy of single impurity in a superconductor for the Anderson model with $U \neq 0$. We showed that the energy of the impurity depends only of the $Δ^{2}_{R}$ (renormalized order parameter) which depends on the renormalized Hubbard repulsion$U^R$. For a strong Hubbard repulsion $U^R = U$ and $Δ^R = Δ^I$ the effect of the $s-d$ interactions are nonrelevant, a result which is expected for this model.
Using the flow-equations method we showed analytically the occurence of dispersion for the local bosons in a model of hybridized of local bosons and fermions.