We report the achieving of depairing current limit along c-axis in Fe1+yTe1−xSex single crystals. A series of crystals with Tc ranging from 8.6 K to 13.7 K (different amount of excess Fe, y) were fabricated into c-axis bridges with a square-micrometer cross-section. The critical current density, Jc, was directly estimated from the transport current-voltage measurements. The transport Jc reaches a very large value, which is about one order of magnitude larger than the depinning Jc, but comparable to the calculated depairing Jc ∼ 2 × 10 6 A/cm 2 at 0 K, based on the Ginzburg-Landau (GL) theory. The temperature dependence of the depairing Jc follows the GL-theory (∝ (1-T /Tc) 3/2 ) down to ∼ 0.83 Tc, then increases with a reduced slope at low temperatures, which can be qualitatively described by the Kupriyanov-Lukichev theory. Our study provides a new route to understand the behavior of depairing Jc in iron-based superconductors in a wide temperature range. * sunyue@phys.aoyama.ac.jp † hkitano@phys.aoyama.ac.jp

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The application of an electrochemical method to the iron-based chalcogenide superconductors has great potentials in enhancing their properties such as the superconducting transition temperature. Unfortunately, this method has been limited to polycrystalline powders or thin film samples with a large surface area. Here, we demonstrate that the electrochemical method can be usefully applied to single-crystal devices of FeSe1−xTex superconductors by combining it with the focused ion beam (FIB) microfabrication techniques. Our results open a new route to developing the high-quality superconducting devices fabricated using layered iron-based chalcogenides, whose properties are electrochemically controlled.

We present a study of the first and second switches (SWs) with nearly identical switching current I SW in Bi 2 Sr 2 Ca 0.85 Y 0.15 Cu 2 O y intrinsic Josephson junctions (IJJs). In contrast to early studies where a large difference between I 1st SW and I 2nd SW was observed, a crossover temperature T cr to the quantum phase escapes for the 2nd SW is strongly reduced (∼2.8 K) and agrees with that for the 1st SW. This confirms that the anomalous enhancement of T 2nd cr , commonly observed in the IJJs with I 1st SW < I 2nd SW , originates from the phase running state after the 1st SW. Detailed analysis of the microwave-induced escapes strongly suggests that nonlinear bifurcated phenomena possibly occurs in IJJs.Macroscopic quantum tunneling (MQT) phenomena discovered in the intrinsic Josephson junctions (IJJs) of Bi 2 Sr 2 CaCu 2 O y (Bi2212) superconductors 1) have attracted much interest for more than a decade because of the unusual properties on the MQT rate. [2][3][4][5] The most striking feature is that a crossover temperature, T cr , to the MQT state in the phase switch from the first to the second resistive branch (2nd SW) is largely enhanced compared to that from the zero voltage to the first resistive branch (1st SW) in the multi-branched current-voltage (I-V) characteristics of IJJs. 6-15) An early study 6) suggested that such an enhancement was due to the self-heating effects in the first resistive state, based on the consideration that a series array of uniform Josephson junctions should show the same value of T cr as long as the current flowing in each junction agrees with the externally-applied bias current.However, the anomalous increase of T 2nd cr was found to be independent of the device structure of IJJs with different heat-transfer environment 7) and the order of phase switches, 10) throwing doubt on the self-heating effects after the 1st SW. Detailed analyses of the switching *

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