Current-voltage characteristics of a tunnel junction with resonant centers

We calculate the I-V characteristics of a tunnel junction containing impurities in the barrier. We consider the indirect resonant tunneling involving the impurities. The Coulomb repulsion energy $E_c$ between two electrons with opposite spins simultaneously residing on the impurity is introduced by an Anderson Hamiltonian. At low temperatures $T \ll E_c$ the I-V characteristic is linear in $V$ both for $VE_c$ and changes slope at $V=E_c$. This behavior reflects the energy spectrum of the impurity electrons - the finite value of the charging energy $E_c$. At $T \sim E_c$ the junction reveals an ohmic-like behavior as a result of the smearing out of the charging effects by the thermal fluctuations.Comment: 9 pages, REVTEX 3.0, 2 .ps figures from [email protected], NUB-308

Isospin breaking in pion-nucleon scattering at threshold by radiative processes

We investigate the dispersive contribution by radiative processes such as (pi- proton to neutron gamma) and (pi- proton to Delta gamma) to the pion-nucleon scattering lengths of charged pions in the heavy baryon limit. They give a large isospin violating contribution in the corresponding isoscalar scattering length, but only a small violation in the isovector one. These terms contribute 6.3(3)% to the 1s level shift of pionic hydrogen and give a chiral constant F_pi^2f_1=-25.8(8) MeV.Comment: 9 pages with 1 figur

A model of gravitation with global U(1)-symmetry

It is shown that an embedding of the general relativity $4-$space into a flat $12-$space gives a model of gravitation with the global $U(1)-$symmetry and the discrete $D_{1}-$one. The last one may be transformed into the $SU(2)-$symmetry of the unified model, and the demand of independence of $U(1)-$ and $SU(2)-$transformations leads to the estimate $\sin^{2}\theta_{min}=0,20$ where $\theta_{min}$ is an analog of the Weinberg angle of the standard model.Comment: 7 page