New publication in PR:

Upper limit on magnetic field expulsion in optically driven K_3⁢C₆₀

G. De Vecchi, M. Buzzi, G. Jotzu, S. Fava, T. Gebert, G. Magnani, D. Pontiroli, M. Riccò, A. Cavalleri
Physical Review Research, 7, 043270 (2025)

Abstract

Photoexcited K₃C₆₀ displays several properties reminiscent of equilibrium superconductivity, including transient optical spectra, pressure dependence, and I–V characteristics. However, these observations do not decisively establish nonequilibrium superconductivity, which may be further evidenced by transient magnetic field expulsion measurements, as shown recently in driven YBa₂Cu₃O_6.48. Here, we search for transient magnetic field expulsion in K₃C₆₀ by measuring Faraday rotation in a magneto-optic material placed in its vicinity. Unlike in the case of YBa₂Cu₃O_6.48, inhomogeneous, metallic K₃C₆₀ powders reduce the size of the effect. With the ∼50 nT magnetic field resolution achieved in our experiments, we provide an upper limit for the photoinduced diamagnetic volume susceptibility (χ_v > −0.1). On this basis, we conclude that the photoinduced phase has weaker magnetic susceptibility than superconducting K₃C₆₀ at zero temperature. Yet, from recent nonlinear transport measurements in this granular material, we expect a light-induced state similar to the equilibrium superconductor near 0.8 T_c, for which χ_v > −0.1. A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for K₃C₆₀ with the current resolution.