Bound-pair excitations in superconductors

Ichiro Takahashi

Martin Greiter

The spinon and holon excitations of two-leg Heisenberg or lightly doped $t\ensuremath{-}J$ ladders are shown to be bound in pairs by string confinement forces given approximately by the antiferromagnetic exchange energy across the rungs, ${F=J}_{\ensuremath{\perp}}〈{\mathbf{S}}_{i}{\mathbf{S}}_{j}{〉}_{\ensuremath{\perp}}/b.$ These forces originate from fictitious flux tubes associated with the half-Fermi statistics of the excitations. It is conjectured that similar confinement forces, determined by the antiferromagnetic exchange energy across the layers, are partially responsible for the spin gap and the pairing of charge carriers in multilayer CuO superconductors.

É. R. Velibekov

Munehiro Nishida, Toshiyuki Fujii, Noriyuki Hatakenaka

We propose a new type of mobile qubit that utilizes a bound pair of half fluxons in a long superconductor-ferromagnet-superconductor (SFS) Josephson junction. The qubit states are composed of the lowest two levels of the quantized nonlinear internal oscillation of the bound pair. The energy levels are estimated by the numerical quantization based on a collective coordinate method. The qubit operation scheme is discussed, showing an estimate of the interaction strength between a bound pair and a microcircuit.

R. Prabhu

Victor Gurarie

We study small oscillations of the order parameter in weakly and strongly paired superconductors driven slightly out of equilibrium, in the collisionless approximation. While it was known for quite some time that the amplitude of the oscillations in a weakly paired superconductor decays as t(-1/2), we show that in a superconductor sufficiently strongly paired so that its fermions form bound states usually referred to as molecules, these oscillations decay as t(-3/2). The transition between these two regimes happens when the chemical potential of the superconductor vanishes; thus, the behavior of the oscillations can be used to distinguish weakly and strongly paired superconductors. Finally, we interpret the result in the strongly paired superconductor as the probability of the molecular decay as a function of time.