Nonlinear nonequilibrium quasiparticle relaxation in Josephson junctions

Nonequilibrium electrons in superconductors relax and eventually recombine into Cooper pairs. Relaxation is facilitated by electron-boson interaction and is accompanied by emission of nonequilibrium bosons. Here I solve numerically a full set of nonlinear kinetic balance equations for stacked Josephson junctions, which allows analysis of strongly nonequilibrium phenomena. It is shown that nonlinearity becomes significant already at very small disequilibrium. The following new, essentially nonlinear effects are obtained: (i) At even-gap voltages $V=2n \Delta/e$ $(n=2,3,...)$ nonequilibrium bosonic bands overlap. This leads to enhanced emission of $\Omega = 2 \Delta$ bosons and to appearance of dips in tunnel conductance. (ii) A new type of radiative solution is found at strong disequilibrium. It is characterized by the fast stimulated relaxation of nonequilibrium quasiparticles. A stack in this state behaves as a light emitting diode and directly converts electric power to boson emission, without utilization of the ac-Josephson effect. This leads to very high radiation efficacy and to significant radiative cooling of the stack. The phenomenon can be used for realization of a new type of superconducting cascade laser in the THz frequency range.Comment: 4 pages, 4 figures and a supplementar

Comment on Counterintuitive consequence of heating in strongly driven intrinsic junctions of Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} mesas

In a recent paper [Phys.Rev.B 81, 224518 (2010)], C. Kurter et al, analyzed the effect of strong self-heating in large-area Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) mesa structures. They conclude that $dI/dV$ conductance peaks in their mesas occur when mesas are heated to the superconducting critical temperature $T_c$. Further on they extrapolate this statement for all mesas, including much smaller and much better thermally anchored mesas used in Intrinsic Tunnelling Spectroscopy (ITS). Here I show that their conclusion does not hold neither for previously reported data, nor even for their own mesas; the very remote extrapolation to ITS is invalid. I also point out a number of inconsistencies and misleading references.Comment: 2 page