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

QED effects in molecules: test on rotational quantum states of H2_2

Quantum electrodynamic effects have been systematically tested in the progression of rotational quantum states in the $X ^{1}\Sigma_{g}^{+}, v=0$ vibronic ground state of molecular hydrogen. High-precision Doppler-free spectroscopy of the $EF ^{1}\Sigma_{g}^{+} - X ^{1}\Sigma_{g}^{+}$ (0,0) band was performed with 0.005 cm$^{-1}$ accuracy on rotationally-hot H$_2$ (with rotational quantum states J up to 16). QED and relativistic contributions to rotational level energies as high as 0.13 cm$^{-1}$ are extracted, and are in perfect agreement with recent calculations of QED and high-order relativistic effects for the H$_2$ ground state.Comment: 4 pages, 3 figures, to be published in Physical Review Letter