Transiently enhanced interlayer tunneling in optically driven high- Tc superconductors

Abstract

Recent pump-probe experiments reported an enhancement of superconducting transport along the c axis of underdoped YBa2Cu3O 6+δ (YBCO), induced by a midinfrared optical pump pulse tuned to a specific lattice vibration. To understand this transient nonequilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in the presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature Tc. This model reproduces the essential features of the enhancement measured below Tc. To reproduce the experimental results above Tc, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.

Theory of parametrically amplified electron-phonon superconductivity

“…We note that photo-induced enhancement of superconductivity has also been observed in high-Tc cuprates [64] along with several theoretical proposals for explaining these experiments [20,[65][66][67][68][69]. Cuprate superconductors are considerably more complicated than conventional electron-phonon superconductors that we considered in this paper.…”

Photoinduced Nonequilibrium Response in Underdoped YBa2Cu3O6+xZhang1, Wang2, Xiang3 et al. 2020Phys. Rev. X12080View full textAdd to dashboardCite

“…The resonant pumping of specific phonon mode is believed to be the key to light-induced superconductivity. The observations also motivated theoretical studies on transient superconductivity along the direction of the resonant phonon pumping [7][8][9][10][11][12][13] .…”

Parametric resonance of Josephson plasma waves: A theory for optically amplified interlayer superconductivity in YBa2Cu3O6+x

“…The appearance of this additional blueshifted lower plasmon edge has been interpreted as indication of the photoinduced enhancement of the interlayer Josephson coupling. Earlier theoretical work aimed at understanding photoinduced superconductivity [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] has not been able to explain the dramatic enhancement of JP edges following excitation of the phonons. Another striking phenomenon that has been recently observed in YBa 2 Cu 3 O 6+x is the coherent excitation of Josephson plasma wave oscillations by the photoexcited phonons [1].…”