Light-induced enhancement of superconductivity in iron-based superconductor FeSe0.5Te0.5

Abstract

Illumination of light on matter normally causes heating and destroys the ordered ground states. Despite this common understanding, recent advances in ultrafast light sources have enabled the non-thermal control of quantum phases. Here, we report the light-induced enhancement of superconductivity in a thin film of an iron chalcogenide FeSe0.5Te0.5, which exhibits multiple quantum condensates associated with the multi-orbital character. Upon the photoexcitation, we observed a transient increase of the superfluid density as indicated by the optical conductivity in the frequency range of superconducting gaps. The light-induced enhancement of superconductivity is further corroborated by the photoinduced enhancement of terahertz third harmonic generation, which is accounted for by the Higgs mode response. The ultrafast dynamics of two superfluid components revealed by frequency- and time-resolved terahertz measurements indicate the interplay between the condensates through the interband Cooper pairings while suggesting the potential tunability of the pairing interaction by light in the ultrafast timescale.

Contribution of Collective Excitations to Third Harmonic Generation in Multiband Superconductors: the case of MgB$_2$

“…In the former case, the authors observed marked oscillations of the transmitted signal as a function of the pump-probe delay t pp at a frequency corresponding to twice the smaller gap. This result [34,35] implies a marked resonance of the non-linear kernel at the lowest optical gap, as also suggested by THG experiments in FeSe 0.5 Te 0.5 [38]. However, to really establish the predominance of the Higgs over the BCS response one would need again to estimate the role of disorder for each band, that was not explicitly provided in Ref.…”

“…In the present manuscript we provide a detailed analysis of the non-linear response of multiband superconductors, with the twofold aim from one side to clarify the nature of the collective fluctuations, in particular the Higgs mode, and from the other side to establish the experimental conditions for their detection via non-linear optics, that has been recently applied both to MgB 2 [24,27] and iron-based superconductors [37][38][39]. For what concerns the amplitude fluctuations, we will show that the redistribution of spectral weight between the two peaks at 2∆ α in the spectral functions depends crucially on the structure of the coupling matrix.…”

Properties and challenges of hot-phonon physics in metals: MgB$_2$ and other compounds

“…Alternatively, coherent modes (phonon, collective excitations, etc) can be also triggered in [7][8][9][10][11][12][13][14][15][16]. A later probe (optical, electronic, transport, diffraction) is thus used to assess the physical properties at a time delay t. Within this context, pump-probe experiments have proven to be able to induce a variety of physical processes otherwise not allowed (or very slow) under steady conditions, as for instance insulator-metal transitions [17][18][19][20][21][22][23], light-induced superconductivity [24][25][26][27][28][29], chemical reactions [30][31][32][33][34], structural phase transitions [17,35], local heat-transport, non-linear optics [14,36,37], exciton dynamics of relevance for light harvesting [38][39][40], detecting the effects of surface plasmons and/or other collective modes [41,42], manipulation of magnetic degrees of freedom [43][44][45][46] etc. In accordance with the different physical processes under examination, a wide variety of time-resolved techniques have been developed, encompassing an...…”

Surface resistance and amplitude mode under uniform and static external field in conventional superconductors

“…1) The excitation and relaxation of quasiparticles and collective modes have been observed by pump-probe spectroscopy. 2,3) As a major development, the amplitude mode, which was difficult to observe because it did not appear in the linear response, has been discussed in the nonlinear response with experimental [4][5][6][7] and theoretical methods. [8][9][10][11][12] On the other hand, the generation of nonequilibrium states is possible not only by optical excitation but also by stationary fields (magnetic field and current), which has been studied for a long time.…”