2 GLYPH<s49>GLYPH<s53>GLYPH<s48> GLYPH<s49>GLYPH<s48>GLYPH<s48> GLYPH<s53>GLYPH<s48> GLYPH<s48> GLYPH<s32>GLYPH<s40>GLYPH<s110>GLYPH<s109>GLYPH<s41> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s56> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s54> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s52> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s50> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s48> GLYPH<s50> GLYPH<s48>GLYPH<s46>GLYPH<s56> GLYPH<s48>GLYPH<s46>GLYPH<s52> GLYPH<s48>GLYPH<s46>GLYPH<s48> GLYPH<s32>GLYPH<s40> GLYPH<s109>GLYPH<s41>GLYPH<s32> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s56> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s54> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s52> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s50> GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s48> GLYPH<s40>GLYPH<s84>GLYPH<s32>GLYPH<s47>GLYPH<s32>GLYPH<s84>$_{GLYPH<s99>}$GLYPH<s32>GLYPH<s41>$^{GLYPH<s50>}$ GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s41> GLYPH<s35>GLYPH<s49> GLYPH<s35>GLYPH<s50> GLYPH<s35>GLYPH<s51> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s32>GLYPH<s35>GLYPH<s49>GLYPH<s32>GLYPH<s32> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s32>GLYPH<s35>GLYPH<s50> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s32>GLYPH<s35>GLYPH<s51> GLYPH<s56>GLYPH<s48> GLYPH<s54>GLYPH<s48> GLYPH<s52>GLYPH<s48> GLYPH<s50>GLYPH<s48> GLYPH<s48> GLYPH<s40> GLYPH<s109>GLYPH<s41> GLYPH<s49>GLYPH<s54> GLYPH<s49>GLYPH<s52> GLYPH<s49>GLYPH<s50> GLYPH<s49>GLYPH<s48> GLYPH<s56> GLYPH<s54> GLYPH<s52> GLYPH<s50> GLYPH<s48> GLYPH<s54>GLYPH<s48> GLYPH<s52>GLYPH<s48> GLYPH<s50>GLYPH<s48> GLYPH<s48> GLYPH<s40>GLYPH<s110>GLYPH<s109>GLYPH<s41> GLYPH<s52> GLYPH<s51> GLYPH<s50> GLYPH<s49> GLYPH<s48> GLYPH<s84>GLYPH<s32>GLYPH<s40>GLYPH<s75>GLYPH<s41> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s35>GLYPH<s49> GLYPH<s32>GLYPH<s112>GLYPH<s111>GLYPH<s119>GLYPH<s101>GLYPH<s114>GLYPH<s45>GLYPH<s108>GLYPH<s97>GLYPH<s119>GLYPH<s32>GLYPH<s102>GLYPH<s105>GLYPH<s116> GLYPH<s32>GLYPH<s115>GLYPH<s45>GLYPH<s119>GLYPH<s97>GLYPH<s118>GLYPH<s101>GLYPH<s32>GLYPH<s66>GLYPH<s67>GLYPH<s83>GLYPH<s32>GLYPH<s102>GLYPH<s105>GLYPH<s116> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s32>GLYPH<s35>GLYPH<s49> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s32>GLYPH<s35>GLYPH<s50> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s101>GLYPH<s41>GLYPH<s32>GLYPH<s35>GLYPH<s51>GLYPH<s32>GLYPH<s32> GLYPH<s32>GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s41> GLYPH<s35>GLYPH<s49> GLYPH<s35>GLYPH<s51> GLYPH<s35>GLYPH<s50> GLYPH<s70>GLYPH<s101>GLYPH<s40>GLYPH<s84>GLYPH<s101>GLYPH<s44>GLYPH<s83>GLYPH<s41> FIG. 2: (Color online) Δ λ plotted vs. ( T/T c ) 2 for three Fe(Te,Se) crystals in the main panel and Fe(Te,S) crystal in the inset in the temperature range up to T c / 3. The curves for Fe(Te,Se)#1 and #2 are shifted vertically for clarity by 50 nm and 25 nm, respectively. FIG. 1: (Color online) Main panel: Variation of the London penetration depth, Δ λ ( T ), for three Fe(Te,Se) samples and one Fe(Te,S) sample. Inset, Δ λ ( T ) for Fe(Te,Se) #1 in the low temperature range up to T c / 3 shown along with the fitting curves assuming exponential or power-low behavior. mosphere. The sealed ampoule was heated to a soaking temperature of 430-450 $^{◦}$C for 24h, followed by a rapid heating to the growth temperature at 850 $^{◦}$C and then slowly cooled to 820 $^{◦}$C. The excess flux was removed from the crystals by decanting. The end composition of the sample used to measure the penetration depth was Fe$_{1}$$_{.}$$_{06}$(Te$_{0}$$_{.}$$_{88}$S$_{0}$$_{.}$$_{14}$)[10]. fairly sharp superconducting transition with Δ T$_{c}$ < 1 . 5 K reflecting the high quality of the single crystals, the Fe(Te,S) crystal shows a broader transition. The ”maxi- mum slope”, T slope c , determined by taking the maximum of the derivative d Δ λ ( T ) /dT gives T slope c ≈ 12 . 0 K for Fe(Te,Se) and T slope c ≈ 7 . 5 K for Fe(Te,S). As for the onset values, T onset c ≈ 13 K for Fe(Te,Se) and T onset c ≈ 8 K for Fe(Te,S). The low-temperature variation of λ ( T ) is examined in the inset in Fig. 1. The dashed line rep- resents the best fit to a standard s -wave BCS function, Δ λ ( T ) = λ (0) √ π Δ$_{0}$ / 2 T exp( − Δ$_{0}$ /T ), with λ (0) and Δ$_{0}$ being free fitting parameters. The experimental data do not show any indication of saturation down to 0 . 04 T$_{c}$ and the fit is not adequate. Also obtained from the fit is Δ$_{0}$ = 0 . 5 T$_{c}$ , which is impossible in a single-gap scenario, hence ruling out conventional s -wave BCS superconduc- tivity. We will come back to a multi-gap s -wave fitting later in the discussion of the superfluid density. On the other hand, fitting with the power-law, Δ λ ( T ) ∝ AT $^{n}$, n = 2 . 1 ± 0 . 01, produces excellent agreement with the data. The in-plane London penetration depth, λ ( T ), was measured by using a self-oscillating tunnel-diode res- onator (TDR) [16, 18–20]. The sample was mounted on a sapphire rod which was inserted into a tank-circuit induc- tor. The weak ac magnetic field H$_{ac}$ ∼ 20 mOe produced by the coil is much smaller than the lower critical field H$_{c}$$_{1}$ < 100 Oe, so the sample was in the Meissner state and its magnetic response was determined by the London penetration depth. To probe the ab − plane supercurrent response, the sample was placed with its crystallographic c − axis along H$_{ac}$ . The shift of the resonance frequency, Δ f ≡ f ( T ) − f$_{0}$ , is measured to obtain the total magnetic susceptibility χ ( T ) via Δ f = − G 4 πχ ( T ). Here f$_{0}$ ∼ 14 MHz is the resonance frequency of an empty resonator, G = f$_{0}$V$_{s}$/ 2 V$_{c}$ (1 − N ) is the calibration factor that de- pends on the demagnetization factor N , sample volume V$_{s}$ and coil volume V$_{c}$ . The calibration factor is deter- mined for each sample by measuring the full frequency change resulting from physically pulling the sample out of the coil at the lowest temperature. In the Meissner state the magnetic susceptibility, 4 πχ , can be written in terms of λ and the effective sample dimension R as: 4 πχ = ( λ/R ) tanh( R/λ ) − 1, from which λ can be ob- tained [19]. In order to examine how close the overall power-law variation is to quadratic, we plot Δ λ versus ( T/T$_{c}$ ) 2 for Fe(Te,Se) in the main panel of Fig. 2 and for Fe(Te,S) in the inset. All samples follow the Δ λ ( T ) ∝ T 2 behavior rather well. To probe how robust the power n is, we performed a data fit over a floating temperature range, from T =0 to T$_{up}$ , using a functional form of Δ λ ( T ) = a$_{0}$ + AT $^{n}$. The difference between the a$_{0}$ term determined from an extrapolation from the T 2 plot in Fig. 2 and the power-law fit turned out to be negligible, 1.5 ± 0.5 nm, and had no significant effect on the fit. The dependence of the other fitting parameters, n and A , on T$_{up}$ (selected in the range from T$_{c}$/ 6 to T$_{c}$/ 3) is summarized in Fig. 3. The main panel in Fig. 1 shows the full-temperature range penetration depth for Fe(Te,Se) and Fe(Te,S) su- perconductors. While the Fe(Te,Se) samples show a