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Register a new userUpper critical field, anisotropy, and superconducting properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals
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The temperature dependent resistivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (x = 0.23, 0.25, 0.28 and 0.4) single crystals and the angle dependent resistivity of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals were measured in magnetic fields up to 9 T. The measurements of temperature dependent resistivity for samples with different doping levels revealed very high upper critical fields which increase with the transition temperature monotonously, and a very low superconducting anisotropy ratio $Gamma=H_{c2}^{ab}/H_{c2}^c approx$ 2. By scaling the resistivity in the frame of the anisotropic Ginzburg-Landau theory, the angle dependent resistivity of the Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy $Gamma$ was determined alternatively for each temperature and was found to be between two and three.
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Single crystals of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$, $x<0.37$, have been grown and characterized by structural, magnetic and transport measurements. These measurements show that the structural/magnetic phase transition found in pure BaFe$_2$As$_2$ at 134 K is suppressed monotonically by Ru doping, but, unlike doping with TM=Co, Ni, Cu, Rh or Pd, the coupled transition seen in the parent compound does not detectably split into two separate ones. Superconductivity is stabilized at low temperatures for $x>0.2$ and continues through the highest doping levels we report. The superconducting region is dome like, with maximum T$_c$ ($sim16.5$ K) found around $xsim 0.29$. A phase diagram of temperature versus doping, based on electrical transport and magnetization measurements, has been constructed and compared to those of the Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM=Co, Ni, Rh, Pd) series as well as to the temperature-pressure phase diagram for pure BaFe$_2$As$_2$. Suppression of the structural/magnetic phase transition as well as the appearance of superconductivity is much more gradual in Ru doping, as compared to Co, Ni, Rh and Pd doping, and appears to have more in common with BaFe$_2$As$_2$ tuned with pressure; by plotting $T_S/T_m$ and $T_c$ as a function of changes in unit cell dimensions, we find that changed in the $c/a$ ratio, rather than changes in $c$, $a$ or V, unify the $T(p)$ and $T(x)$ phase diagrams for BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ respectively.
The effect of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature ($T_c$) of K$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard $T^2$ Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi liquid like behavior of the resistivity for K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ and some KFe$_2$As$_2$ samples can be explained by disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of $T_c$ for stoichiometric KFe$_2$As$_2$. The analysis of $T_c$ in the K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ at pressures below 1.5 GPa showed, that the reduction of $T_c$ with Na substitution follows the Abrikosov-Gorkov law with the critical temperature $T_{c0}$ of the clean system (without pair-breaking) which linearly depends on the pressure. Our observations, also, suggest that $T_c$ of K$_{1-x}$Na$_x$Fe$_2$As$_2$ is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently in de Haas-van Alphen experiments [T. Terashima et al., Phys.Rev.B89, 134520(2014)] measured pressure dependencies of the Fermi-surface cross-sections has been found. These calculations, also, indicate that the observed minimum around 2~GPa in the pressure dependence of $T_c$ may occur without a change of the pairing symmetry.
Resistivity, Hall effect and magnetoresistance have been investigated systematically on single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ranging from undoped to optimally doped regions. A systematic evolution of the quasiparticle scattering has been observed. It is found that the resistivity in the normal state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ is insensitive to the potassium doping concentration, which is very different from the electron doped counterpart Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, where the resistivity at 300 K reduces to half value of the undoped one when the system is optimally doped. In stark contrast, the Hall coefficient R$_H$ changes suddenly from a negative value in the undoped sample to a positive one with slight K-doping, and it keeps lowering with further doping. We interpret this dichotomy due to the asymmetric scattering rate in the hole and the electron pockets with much higher mobility of the latter. The magnetoresistivity shows also a non-monotonic doping dependence indicating an anomalous feature at about 80 K to 100 K, even in the optimally doped sample, which is associated with a possible pseudogap feature. In the low temperature region, it seems that the resistivity has the similar values when superconductivity sets in disregarding the different T$_c$ values, which indicates a novel mechanism of the superconductivity. A linear feature of resistivity $rho_{ab}$ vs. $T$ was observed just above $T_c$ for the optimally doped sample, suggesting a quantum criticality.
Single crystals of BaFe$_2$As$_2$ and (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated and can have dimensions as large as 3 x 3 x 0.2 mm$^3$. The BaFe$_2$As$_2$ single crystals manifest a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat and the anisotropic magnetic susceptibility. In the (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near $T_c$ is small, but resolvable, giving $Delta C_p/gamma T_c approx 1$. The application of a 140 kOe magnetic field suppresses $T_c$ by only $sim 4$ K when applied along the c-axis and by $sim 2$ K when applied perpendicular to the c-axis. The ratio of the anisotropic upper critical fields, $gamma = H_{c2}^{perp c} / H_{c2}^{| c}$, varies between 2.5 and 3.5 for temperatures down to $sim 2$ K below $T_c$.
In-plane resistivity measurements as a function of temperature and magnetic field up to 35~T with precise orientation within the crystallographic $ac-$plane were used to study the upper critical field, $H_{c2}$, of the hole-doped iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Compositions of the samples studied were spanning from underdoped $x=$0.17 ($T_c$=12~K) and $x$=0.22 ($T_c$=20~K), both in the coexistence range of stripe magnetism and superconductivity, though optimal doping $x$=0.39 ($T_c$=38.4~K), $x$=0.47 ($T_c$=37.2~K), to overdoped $x$=0.65 ($T_c$=22~K), $x$=0.83 ($T_c$=10~K). We find notable doping asymmetry of the shapes of the anisotropic $H_{c2}(T)$ suggesting important role of paramagnetic limiting effects in $H parallel a$ configuration in overdoped compositions and multi-band effects in underdoped compositions.
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