Upper limit on magnetic field expulsion in optically driven K 3 C 60

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 YBa2Cu3O6.48. 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 YBa2Cu3O6.48, 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 (χv>-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 0.8Tc, for which χv>-0.1. A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for K3C60 with the current resolution.