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Register a new userExceptional Suppression of Flux-Flow Resistivity in FeSe$_{0.4}$Te$_{0.6}$ by Back-Flow from Excess Fe Atoms and Se/Te Substitutions
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We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single crystals with- and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as $rho_f(Bll B_{rm c2})/rho_n=alpha B/B_{rm c2}$. The observed $alpha$ value of $approx0.66$ was considerably smaller than that of other Fe-based materials ($alphageq1$) and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is the first-time observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems.
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We report on the anisotropy of the vortex motion surface impedance of a fst thin film grown on a CaF$_2$ substrate. The dependence on the magnetic field intensity up to 1.2 T and direction, both parallel and perpendicular to the sample $c$-axis, was explored at fixed temperature at two distinct frequencies, $sim16;$GHz and $sim27;$GHz, by means of bitonal dielectric resonator. The free flux flow resistivity $rho_{ff}$ was obtained by exploiting standard models for the high frequency dynamics, whereas the angle dependence was studied in the framework of the well known and widely used Blatter-Geshkenbein-Larkin (BGL) scaling theory for anistropic superconductors. Excellent agreement with the scaling law prescription by the fluxon flux flow resistivity was obtained. From the scaling analysis, a low-field mass anisotropy $sim1.8$ was obtained, well within the value ranges reported in literature. The angular dependence of the pinning constant suggests that pinning is dominated by random, isotropic point pins, consistently with critical current density measurements.
It has been clarified that bulk superconductivity in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ can be induced by annealing in an appropriate atmosphere to remove the harmful effects of excess iron. In order to clarify the details of the annealing process, we studied the changes in the physical properties and reaction products of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ annealed in pnictogen (P, As, Sb) atmospheres. Crystals annealed in a pnictogen atmosphere show bulk superconductivity and the values of $T_{c}$ and $J_{c}$ are about $14~$K and 2-4 $times 10{^5}~$A/cm$^2$ ($2~$K, self-field), respectively. It is also found that the reaction rate increases with the increase in the saturated vapor pressure of the pnictogen. Unexpectedly, the reaction products of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ after annealing in a P atmosphere mainly consist of FeTe$_2$. In addition, the amount of P required to obtain the optimal $T_{c}$ is much smaller than the amount of excess iron, which is similar to the case of oxygen annealing. P, oxygen, and to some extent As could serve as catalysts to form FeTe$_2$ to remove excess iron.
We study Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ multi-band superconductor with $T_c=14$K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 45cm$^{-1}$ ($2Delta=5.6$meV) in the $XY$($B_{2g}$) scattering geometry, consistent with twice of the superconducting gap energy (3 meV) revealed by ARPES on the hole-like Fermi pocket with $d_{xz}/d_{yz}$ character. We analyze the superconductivity induced phonon self-energy effects for the $B_{1g}$(Fe) phonon and estimate the electron-phonon coupling constant $lambda^Gamma approx 0.026$, which is insufficient to explain superconductivity with $T_c=14$K.
Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a systematical study of O$_2$-annealing dynamics in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ by controlling the amount of O$_2$, annealing temperature, and time. Bulk superconductivity can be gradually induced by increasing the amount of O$_2$ and annealing time at suitable temperatures. The optimally annealed crystals can be easily obtained by annealing with ~ 1.5% molar ratio of oxygen at 400 $^{circ}$C for more than 1 hour. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe.
We report a systematic study of the superconducting (SC) and normal-state anisotropy of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystals with controlled amounts of excess Fe ($y$ = 0, 0.07, and 0.14). The SC state anisotropy $gamma_{H}$ was obtained by measuring the upper critical fields under high magnetic fields over 50 T for both $Hparallel ab$ and $Hparallel c$. On the other hand, the normal state anisotropy $gamma_{rho}$ was obtained by measuring the resistivity with current flowing in the $ab$ plane ($rho_{ab}$) and along the $c$ axis ($rho_c$). To precisely measure $rho_{ab}$ and $rho_c$ in the same part of a specimen avoiding the variation dependent on pieces or parts, we adopt a new method using a micro-fabricated bridge with an additional neck part along $c$ axis. The $gamma_{H}$ decreases from a value dependent on the amount of excess Fe at $T_{rm{c}}$ to a common value $sim$ 1 at 2 K. The different $gamma_{H}$ at $T_{rm{c}}$ ($sim$1.5 for $y$ = 0, and 2.5 for $y$ = 0.14) suggests that the anisotropy of effective mass $m_c^*/m_{ab}^*$ increases from $sim$ 2.25 ($y$ = 0) to 6.25 ($y$ = 0.14) with the excess Fe. The almost isotropic $gamma_{H}$ at low temperatures is due to the strong spin paramagnetic effect at $Hparallel ab$. By contrast, the $gamma_{rho}$ shows a much larger value of $sim$ 17 ($y$ = 0) to $sim$ 50 ($y$ = 0.14) at the temperature just above $T_{rm{c}}$. Combined the results of $gamma_{H}$ and $gamma_{rho}$ near $T_{rm{c}}$, we found out that the discrepant anisotropies between the SC and normal states originates from a large anisotropy of scattering time $tau_{ab}$/$tau_c$ $sim$ 7.8. The $tau_{ab}$/$tau_c$ is found to be independent of the excess Fe.
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