Gap-anisotropic model for the narrow-gap Kondo insulators

A theory is presented which accounts for the dynamical generation of a hybridization gap with nodes in the Kondo insulating materials $CeNiSn$ and $CeRhSb$. We show that Hunds interactions acting on virtual $4f^2$ configurations of the cerium ion can act to dynamically select the shape of the cerium ion by generating a Weiss field which couples to the shape of the ion. In low symmetry crystals where the external crystal fields are negligible, this process selects a nodal Kondo semimetal state as the lowest energy configuration.Comment: Substantially Revised Versio

Antiferromagnetic gap in the Hubbard model

We compute the temperature dependence of the antiferromagnetic order parameter and the gap in the two dimensional Hubbard model at and close to half filling. Our approach is based on truncations of an exact functional renormalization group equation. The explicit use of composite bosonic degrees of freedom permits a direct investigation of the ordered low temperature phase. We show that the Mermin--Wagner theorem is not practically applicable for the spontaneous breaking of the continuous spin symmetry in the antiferromagnetic state. The critical behavior is dominated by the fluctuations of composite Goldstone bosons.Comment: new discussion of critical behavior 4 pages,2 figures, LaTe

Two-gap model for underdoped cuprate superconductors

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).Comment: 5 pages, 1 enclosed figure. For further information see http://htcs.or

Random gap model for graphene and graphene bilayers

The effect of a randomly fluctuating gap, created by a random staggered potential, is studied in a monolayer and a bilayer of graphene. The density of states, the one-particle scattering rate and transport properties (diffusion coefficient and conductivity) are calculated at the neutrality point. All these quantities vanish at a critical value of the average staggered potential, signaling a continuous transition to an insulating behavior. The calculations are based on the self-consistent Born approximation for the one-particle scattering rate and a massless mode of the two-particle Green's function which is created by spontaneous symmetry breaking. Transport quantities are directly linked to the one-particle scattering rate. Moreover, the effect of disorder is very weak in the case of a monolayer but much stronger in bilayer graphene.Comment: 5 pages, 1 figur

Superconducting gap within a modified interlayer tunneling model

A modified version of the interlayer tunneling model, including interlayer single particle hopping (ISPH), is considered as a phenomenological model to describe cuprate superconductors. The effective ISPH (t_\perp^{eff}) is taken along with a probability factor P, that involves the normal state pseudogap (E_g). This makes t_\perp^{eff} to mimic experimental observations that, ISPH is small in the underdoped regime and increases towards overdoping. Within the modified model, we establish the absence of bilayer splitting as observed in case of layered cuprates. Transition temperature (T_c) and the superconducting gap are calculated. A match, to the T-dependent superconducting gap data from experiment, is obtained and high values of the ratio of the superconducting gap to T_c are recovered. Depending on the values of E_g, T_c as a function of interlayer coupling shows mixed behaviour. This is a prediction and can be checked further.Comment: Latex file, 14 pages, 4 figures (postscript files) include

Gap and out-gap breathers in a binary modulated discrete nonlinear Schr\"odinger model

We consider a modulated discrete nonlinear Schr\"odinger (DNLS) model with alternating on-site potential, having a linear spectrum with two branches separated by a 'forbidden' gap. Nonlinear localized time-periodic solutions with frequencies in the gap and near the gap -- discrete gap and out-gap breathers (DGBs and DOGBs) -- are investigated. Their linear stability is studied varying the system parameters from the continuous to the anti-continuous limit, and different types of oscillatory and real instabilities are revealed. It is shown, that generally DGBs in infinite modulated DNLS chains with hard (soft) nonlinearity do not possess any oscillatory instabilities for breather frequencies in the lower (upper) half of the gap. Regimes of 'exchange of stability' between symmetric and antisymmetric DGBs are observed, where an increased breather mobility is expected. The transformation from DGBs to DOGBs when the breather frequency enters the linear spectrum is studied, and the general bifurcation picture for DOGBs with tails of different wave numbers is described. Close to the anti-continuous limit, the localized linear eigenmodes and their corresponding eigenfrequencies are calculated analytically for several gap/out-gap breather configurations, yielding explicit proof of their linear stability or instability close to this limit.Comment: 17 pages, 12 figures, submitted to Eur. Phys. J.

Range of the t--J model parameters for CuO2_{2} plane: experimental data constraints

The t-J model effective hopping integral is determined from the three-band Hubbard model for the charge carriers in CuO$_{2}$ plane. For this purpose the values of the superexchange constant $J$ and the charge-transfer gap $E_{gap}$ are calculated in the framework of the three-band model. Fitting values of $J$ and $E_{gap}$ to the experimental data allows to narrow the uncertainty region of the three-band model parameters. As a result, the $t/J$ ratio of the t-J model is fixed in the range $2.4 \div 2.7$ for holes and $2.5 \div 3.0$ for electrons. Formation of the Frenkel exciton is justified and the main features of the charge-transfer spectrum are correctly described in the framework of this approach.Comment: 20pp., REVTEX 3.0, (11 figures), report 66