Gloria M. Japiassu

On the k‐dependence of the hybridization in two‐band superconductors

We study the influence of a k‐dependent hybridization on superconductivity of a two‐band model, within an extended BCS picture. Considering intraband d‐d pairing and using homothetic relations for the bands and for the hybridization we calculate the energies and the gap of the quasiparticles, the order parameter and the critical temperature Tc. We verify that Tc vanishes at a critical hybridization which depends on the ratio of the effective band masses. Numerical results and possible applications are also presented.

Two‐band model for Kondo insulators: Thermodynamic and scaling properties

Recently a new family of heavy fermions (e.g., Ce3Bi4Pt3) has been discovered which is characterized by an insulating ground state. Similar behavior has been found in some transition metal compounds like FeSi. We introduce a two‐band model to describe the physical properties of these systems. Coulomb correlations in the narrow band are treated beyond the mean field approach. We find that a critical value of the hybridization Vc is required to open a gap that produces an insulating ground state. We discuss the scaling properties of these systems close to Vc. The temperature‐dependent magnetic susceptibility calculated in our model is in good agreement with experimental results.

Superconductivity in two-band systems: application to transition metals and high Tc materials

Abstract We study the effect of hybridization on superconductivity within a two band model, in the framework of an extended BCS theory. We describe interband effects through a one-body mixing term differently from the standard many body approach. Using homothetic relations for tje bands and the hybridization we obtain the energies of the quasiparticles, the critical temperature and the order parameter. We discuss the application of our results for transition metals and high Tc superconductors.

Hybridization effect on superconductivity in transition metals

Abstract The effect of hybridization on superconductivity in transition metals is studied within a two-band model in the framework of an extended BCS theory. It is described by an one-body Hamiltonian differently from the usual two-particle scheme for the interband interaction. We use the equation of motion method to obtain a self-consistent equation for the critical temperature. The one-body interaction term causes a renormalization of the density of states and the narrowing of the d-band generates an enhancement of the superconducting transition temperature. Numerical results are presented and compared with previous works.

Specific heat jump in two-band superconductors

Abstract We discuss the effect of k-dependent hybridization in the specific heat jump ΔC at the superconducting critical temperature Tc, considering an exact analytical expression obtained from the extended BCS gap equation. Our model intends to discuss a superconducting material described by a two-band model, within an extended BCS picture. We consider a broad s–p conducting band, hybridized with a narrow d-band, assuming that: (i) electron–electron repulsion Ur, formulated via the Hubbard model and (ii) electron–electron attraction due to electron–“fonon” interaction Ua, treated in a BCS framework, both occur at the narrow d-band. One observes that there is a critical value Vc for the hybridization (depending on the ratio α of the effective masses of the narrow and broad bands), above which there is no ΔC jump. Moreover, one observes that: (i) the ratio α of the effective band masses in a homothetic band picture plays an important role in the jump, and (ii) the Hubbard like repulsive interaction is very small and therefore plays no significant role in the range of temperatures where superconductivity occurs.

Scaling approach to heavy fermions: Pressure effects in CeAl3

We discuss experimental results on CeAl3 under external pressure using a recently proposed scaling theory of heavy fermion systems. Specific heat, resistivity, and electron spin resonance (ESR) data at different pressures are examined to obtain relations between the critical exponents characterizing the quantum critical behavior of this system. We observe that the critical exponents governing the behavior of CeAl3 belong to the same class of universality of those obtained for CeRu2Si2.

Role of intraband and interband pairings in two-band superconductors: deviation from BCS results

Abstract We study the stability of the intraband ( r =dd) and interband ( r =sd) couplings in the presence of the k -dependent hybridization in a two-band system. We calculate the critical temperature T c, r associated to both type of couplings, and we verified that there is a critical value of the hybridization V c , for which T c, r vanishes. Then we obtain the value of e r =2Δ ∗ r (0)/k B T c ,r and the isotope coefficients γ r showing that under some circumstances, one can obtain values for e r and γ r which differ considerably from standard BCS results for both Cooper pairings.

Change of universality class of metal–insulator transition due to magnetic ordering

Using a two-band model we report a theory to describe the metal–insulator (MI) transition as a function of an external applied magnetic field in Kondo insulators. To deal with electronic correlations we use a functional integral approach in the static approximation. We show the existence of a critical value of the Coulomb correlation Uc, such that for U Uc, the transition is to a ferromagnetic metal and it is described by different critical exponents.

Temperature-driven metal-non-metal transition in Kondo insulators

Abstract We present a theory to describe the metal-insulator transition as a function of pressure and temperature in Kondo insulators. We use a two-band model and treat the electronic correlations via a functional integral approach in the static approximation. The effects of pressure and temperature on the electronic structure of the system and the phase diagram are discussed. We obtain the critical line and the exponents characterizing the vanishing of the gap close to this line and compare our results with those obtained using a periodic Anderson lattice model (PAM).

Metal‐insulator transition in the presence of excitonic correlation

We study the possibility of an excitonic transition in a two‐band model and show that a true phase transition does not occur in the presence of hybridization since the one‐body mixing term acts as a conjugate field to the order parameter of the excitonic phase. We suggest an alternative interpretation for recent experiments on rare earth semiconductors based on a metal‐insulator transition associated with the opening or closing of a hybridization gap at a critical pressure.