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Register a new userMeasurement of the c-axis optical reflectance of AFe$_2$As$_2$ (A=Ba, Sr) single crystals: Evidence of different mechanisms for the formation of two energy gaps
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We present the c-axis optical reflectance measurement on single crystals of BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of FeAs based superconductors. Different from the ab-plane optical response where two distinct energy gaps were observed in the SDW state, only the smaller energy gap could be seen clearly for textbf{E}$parallel$c-axis. The very pronounced energy gap structure seen at a higher energy scale for textbf{E}$parallel$ab-plane is almost invisible. We propose a novel picture for the band structure evolution across the SDW transition and suggest different driving mechanisms for the formation of the two energy gaps.
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From first-principles density functional theory calculations combined with varying temperature Raman experiments, we show that AFe$_2$As$_2$ (A=Ba, Sr), the parent compound of the FeAs based superconductors of the new structural family, undergoes a spin-Peierls-like phase transition at low temperature. The coupling between the phonons and frustrated spins is proved to be the main cause of the structural transition from the tetragonal to orthorhombic phase. These results well explain the magnetic and structural phase transitions in AFe$_2$As$_2$(A=Ba, Sr) recently observed by neutron scattering.
We report a detailed investigation on the lower critical field $H_{c1}$ of the superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (FeAs-122) single crystals. A pronounced kink is observed on the $H_{c1}(T)$ curve, which is attributed to the existence of two superconducting gaps. By fitting the data $H_{c1}(T)$ to the two-gap BCS model in full temperature region, a small gap of $Delta_a(0)=2.0pm 0.3$ meV and a large gap of $Delta_b(0)=8.9pm 0.4$ meV are obtained. The in-plane penetration depth $lambda_{ab}(0)$ is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors.
We performed optical spectroscopy measurement on single crystals of BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of FeAs based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave (SDW) state, formation of partial energy gaps was clearly observed with the presence of surprisingly two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces (FS). Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the SDW instability is more likely to be driven by the FS nesting of itinerant electrons rather than a local-exchange mechanism.
We have successfully grown high quality single crystals of SrFe$_2$As$_2$ and A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr, Ba) using flux method. The resistivity, specific heat and Hall coefficient have been measured. For parent compound SrFe$_2$As$_2$, an anisotropic resistivity with $rho_c$ / $rho_{ab}$ as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivity due to the SDW instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows 2-fold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K doped A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr. Ba), the SDW instability is suppressed and the superconductivity appears with T$_c$ above 35 K. The rather low anisotropy in upper critical field between H$parallel$ab and H$parallel$c indicates inter-plane coupling play an important role in hole doped Fe-based superconductors.
A new method is presented for measuring terms in the elastoresistivity tensor $m_{ij}$ of single crystal samples with tetragonal symmetry. The technique is applied to a representative underdoped Fe-arsenide, Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$, revealing an anomalously large and anisotropic elastoresistance in comparison to simple metals. The $m_{66}$ coefficient follows a Curie-Weiss temperature dependence, providing direct evidence that the tetragonal-to-orthorhombic structural phase transition that occurs at $T_s$ = 97.5 K in this material is not the result of a true-proper ferro-elastic transition. Rather, the material suffers a pseudo-proper transition for which the lattice strain is not the primary order parameter.
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