Stimulated emission of Cooper pairs in a high-temperature cuprate superconductor

TL;DR

This study provides experimental evidence for stimulated recombination of Cooper pairs in a high-temperature cuprate superconductor, revealing a momentum space dichotomy in pair formation and contributing to understanding supercurrent mechanisms.

Contribution

It demonstrates the first experimental observation of stimulated recombination of Cooper pairs in a cuprate superconductor using time- and angle-resolved photoemission spectroscopy.

Findings

Slower quasiparticle buildup in the superconducting state with weak pump pulses

Stimulated recombination occurs inside the Fermi arc region

Reveals momentum space dichotomy in Cooper pair formation

Abstract

The concept of stimulated emission of bosons has played an important role in modern science and technology, and constitutes the working principle for lasers. In a stimulated emission process, an incoming photon enhances the probability that an excited atomic state will transition to a lower energy state and generate a second photon of the same energy. It is expected, but not experimentally shown, that stimulated emission contributes significantly to the zero resistance current in a superconductor by enhancing the probability that scattered Cooper pairs will return to the macroscopically occupied condensate instead of entering any other state. Here, we use time- and angle-resolved photoemission spectroscopy to study the initial rise of the non-equilibrium quasiparticle population in a Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ cuprate superconductor induced by an ultrashort laser pulse. Our finding reveals significantly slower buildup of quasiparticles in the superconducting state than in the normal state. The slower buildup only occurs when the pump pulse is too weak to deplete the superconducting condensate, and for cuts inside the Fermi arc region. We propose this is a manifestation of stimulated recombination of broken Cooper pairs, and signals an important momentum space dichotomy in the formation of Cooper pairs inside and outside the Fermi arc region.