Pair Breaking In Superconductors Research Articles

Pair breaking in superconductors with strong spin-orbit coupling

We study the influence of symmetry-breaking perturbations on superconductivity in multiorbital materials, with a particular focus on an external magnetic field. We introduce the field-fitness function which characterizes the pair-breaking effects of the perturbation on a given superconducting state. For even-parity superconductors we find that this field-fitness function for an external magnetic field is one, implying that the paramagnetic response is controlled only by a generalized effective $g$ factor. For odd-parity superconductors, the interplay of the effective $g$ factor and the field-fitness function can lead to counterintuitive results. We demonstrate this for $p$-wave pairing in the effective $j=\frac{3}{2}$ electronic states of the Luttinger-Kohn model.

Photons and magnetic fields: The use of synchrotron sources to study pairbreaking in superconductors

Pair-breaking effects in metallic superconductors have been studied by both linear and nonlinear spectroscopy using infrared synchrotron radiation. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory, in magnetic fields up to 10 T. The optical conductivity of thin-film superconductors in applied magnetic fields has been estimated from the results of far-infrared transmission and reflection measurements. The combined measurements have been analyzed to give the real and imaginary parts of the conductivity. In turn, these quantities allow the magnetic-field dependence of the superconducting energy gap, pairbreaking parameter, and superfluid density to be estimated. Photons also may break Cooper pairs. Pump-probe studies of excess quasiparticle relaxation in the these superconducting films show a relaxation rate proportional to the excess quasiparticle number density, as expected for bimolecular recombination driven by a large excess quasiparticle population. Application of a magnetic field parallel to the sample surface is found to slow significantly the quasiparticle recombination process.

A note on dephasing and pair-breaking

It is shown that the dephasing which suppresses the weak localization correction to the conductivity has the same physical origin than pair-breaking in superconductors and thus-following de Gennes-may be expressed in terms of the correlation function of the operator for time reversal.

Thermal pair-breaking in superconductors with strong electron-phonon interaction

The influence of inelastic scattering in superconductors with a strong electron- phonon interaction was considered. The pair-breaking scattering leads to smearing of singularities in the electron density of states and to the appearance of states inside a “gap”. Normal electrons in such superconductors appear due to the filling of the states instead of the excitations across the gap. This leads to the temperature dependencies of some thermodynamic and kinetic characterics to be more close to the phenomenological Gorter-Casimir two-fluid model rather then BCS theory.

Pairbreaking in superconductors near and below antiferromagnetic transitions

The Abrikosov-Gorkov theory for pairbreaking in superconductors due to a dilute concentration of magnetic impurities is generalized to the case when the magnetic ions are present in a large concentration or on a regular lattice. The case in which the magnetic ions undergo a transition to an antiferromagnetic phase is considered in detail. The physical considerations for increased or decreased pairbreaking over the Abrikosov-Gorkov value due to the increased range of correlations near the antiferromagnetic transition and due to inelastic scattering of electrons off spin waves in the antiferromagnetic phase are discussed in detail. Recent experimental results are considered in terms of the new theory.

Phonon absorption and pair breaking in superconductors at 35 GHz

Measurements of the absorption of longitudinal sound in aluminium at a microwave frequency of 35 GHz are reported. The phase problems associated with the very short sound wavelength have been partly overcome by using a semiconductor avalanche bolometer which detects scattered phonons rather than unscattered coherent ultrasound. A simple theory is developed to relate the bolometer signal to the ultrasonic attenuation coefficient and to its variation with temperature. The results for the pair breaking region of the superconducting state would appear to be in agreement with the BCS Bobetic theory, and the broadening of the transition to the region where quasi-particle scattering dominates is interpreted in terms of energy gap anisotropy.