London penetration depth and superfluid density of single-crystalline Fe1+y(Te1-xSex)and Fe1+y(Te1-xSx)

The in-plane London penetration depth, lambda(T), was measured in single crystals of the iron-chalcogenide superconductors Fe-1.03(Te0.63Se0.37) and Fe-1.06(Te0.88S0.14) by using a radio-frequency tunnel diode resonator. Similar to the iron-arsenides and in stark contrast to the iron-phosphides, iron-chalcogenides exhibit a nearly quadratic temperature variation of lambda(T) at low temperatures. The absolute value of the penetration depth in the T -> 0 limit was determined for Fe-1.03(Te0.63Se0.37) by using an Al coating technique, giving lambda(0) approximate to 560 +/- 20 nm. The superfluid density rho(s)(T)=lambda(2)(0)/lambda(2)(T) was fitted with a self-consistent two-gap gamma model. While two different gaps are needed to describe the full-range temperature variation in rho(s)(T), a nonexponential low-temperature behavior requires pair-breaking scattering, and therefore an unconventional (e.g., s(+/-) or nodal) order parameter.