London penetration depth and superfluid density of single-crystalline Fe_1+y ( Te_1-x Se_x ) and Fe_1+y ( Te_1-x S_x )

The in-plane London penetration depth, λ (T), was measured in single crystals of the iron-chalcogenide superconductors Fe_1.03 (Te _0.63 Se_0.37) and Fe_1.06 (Te_0.88 S_0.14) 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 λ (T) at low temperatures. The absolute value of the penetration depth in the T→0 limit was determined for Fe_1.03 (Te_0.63 Se _0.37) by using an Al coating technique, giving λ (0) 560±20 nm. The superfluid density ρs (T) = λ² (0) / λ² (T) was fitted with a self-consistent two-gap γ model. While two different gaps are needed to describe the full-range temperature variation in ρs (T), a nonexponential low-temperature behavior requires pair-breaking scattering, and therefore an unconventional (e.g., s± or nodal) order parameter.