title: Stimulated emission of Cooper pairs in a high-temperature cuprate superconductor.
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 Bi2Sr2CaCu2O8+δ 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.
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.
authors.forename: Wentao
authors.initials: W
authors.lastname: Zhang
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Tristan
authors.initials: T
authors.lastname: Miller
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Smallwood
authors.affiliation: Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8568, Japan.
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Yoshida
authors.affiliation: Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8568, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Eisaki
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Kaindl
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Dung-Hai
authors.initials: DH
authors.lastname: Lee
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.|Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Lanzara