Session Y37: Focus Session: Fe-based Superconductors: Vortices and Critical Fields

Slow Abrikosov- to fast moving Josephson-vortex transition in iron-pnictide superconductors

We have observed a novel type of transition of vortex matter from well-pinned Abrikosov to highly mobile Josephson vortices in the iron pnictide high-$T_c$ superconductor SmFeAs(O,F) ($T_c \sim 50K$). This A-to-J transition between the two regimes upon cooling through the temperature $T^*$ is hallmarked by an extraordinary jump of vortex mobility and a pronounced peak in the critical current density. The dissipation below $T^*$ reaches significant fractions of the normal state resistance at all temperatures and fields, far below $H_{c2}\|ab$, estimated well above $100T$ at low temperatures. We show the temperature $T^*$ to coincide with the temperature at which the interlayer coherence length $\xi_c(T)$ equals the SmO layer thickness, hence leading to Josephson-like vortices below and Abrikosov-like vortices above $T^*$. This transition is surprising, as the material is an only moderately anisotropic superconductor ($\gamma \sim 5 - 7$), unlike strongly anisotropic, clearly two-dimensional cuprates. The observation of this A-to-J transition highlights the significance of structural layeredness and gives microscopic information about the order parameter in SmFeAs(O,F). This profound change in the nature of the vortex matter in these compounds has eluded discovery until now, as its detection poses two main experimental challenges: The Josephson nature of the vortex matter may only be observed (1) for fields precisely aligned with the FeAs layers($<0.1$deg). Even slightest field misalignments away from the FeAs planes ($>0.1$deg) restore dissipation free current transport and very high critical current densities ($\sim 10^6 A/cm^2$) at low temperatures. Secondly (2), currents flowing perpendicular to the layers are essential for the observation, forcing the vortices to slide between the layers. To this end, thin ($<10\mu m$) high quality single crystals of SmFeAs(O,F) were microstructured and contacted using a Focused Ion Beam.   

Superconductivity amid phase inhomogeneity: the case of K$_{x}$Fe$_{2-y}$Se$_{2}$

The recently discovered Fe-based superconductors, K$_{x}$Fe$_{2-y}$Se$_{2}$, is studied using neutron diffraction and the pair density function analysis to investigate the nature of the atomic disorder induced by the K and Fe site vacancies. In this system, both superconductivity and magnetic ordering can coexist, while superconductivity is observed in a narrow range of potassium concentration, between 0.6 $<$ x $<$ 0.8. While no crystal transition occurs across with x, the Fe site vacancies are ordered in the $\sqrt{5} \times \sqrt{5}$ structure. At high temperatures, the Fe vacancies are not ordered. Why does superconductivity appear in the vicinity of the 0.8 composition? To provide a clue towards the answer, instead of probing the periodic structure, we probed the local atomic structure that provides information regarding the short-range correlations in real space. The results suggest a strong dependence of the Fe-Fe bond lengths to the K concentration. What is unique to this system is that a double-well bond distribution of short and long Fe - Fe bonds exists, originating from the fully occupied Fe site. As the K concentration increases to x=1, the distribution shifts weight from the short to the long while in the superconducting case, it is equal between the two.   

Angle and frequency dependent low field microwave absorption in electronically doped Ca 122 pnictides: Comparison of high Tc = 42 K phase in Pr, Nd, Ce and La doping

The motivation of this study is to investigate the properties of a unique interfacial superconducting phase in electron-doped Ca 122 pnictides by the Low Field Microwave Absorption (LFMA) technique. Samples are exposed to microwave radiation with frequency $\nu_{MW}$ between 1 and 24 GHz, and also to a low strength magnetic field which modulates at $\nu_{mag} = 100$ kHz. Due to their single crystalline nature, the pnictides can be oriented relative to the MW polarization and magnetic field direction. Studying this orientation dependence reveals filament-like micro-interfaces between highly doped regions and poorly doped regions. These interfaces may be responsible for a high $T_c \sim 42$ K superconducting phase. We demonstrate that this higher $T_c$ can be clearly distinguished from the low temperature bulk SC phase by the angle-dependent LFMA method. Additionally, variation of the MW frequency yields changes in the spectra which are in agreement with theoretical predictions.   

High, magnetic field independent critical currents in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ with composite defects

We investigate the enhancement of vortex pinning by compound defects that are composed of correlated and point defects in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ crystals with T$_{c}$ 37.5. Initial irradiation by high-energy heavy ions to a dose matching field of $B=$\textit{21}T increases vortex pinning via columnar defects with no degradation of the superconducting transition temperature. Subsequent proton irradiations further enhance the critical current \textit{Jc(H) }by suppressing the motion of vortex kinks between the columnar defects. At a temperature of 5K, we find a critical current density of 5.8 MA/cm$^{2}$ that is essentially magnetic field independent in fields up to 7 T. This work supported by the Center for Emergent~Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The operation of the ATLAS facility was supported by the U.S. D.O.E., Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. The work in China was supported by the NSF of China, the MOST of China (2011CBA00102 and 2012CB821403) and PAPD.   

Angle - dependent upper critical field of overdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

In-plane resistivity measurements were used to study the upper critical field, $H_{c2}$, of single crystals of iron-based superconductor Ba(Fe$_{1-x}$Ni$_{x}$)$_{2}$As$_{2}$ ($x=0.054$ and $x=0.072$). An applied magnetic field (up to 35 T) was precisely aligned (with the accuracy better than 0.1$^{o}$) parallel to the Fe-As layers and the measurements were taken for $H\parallel ab-$ plane and $H\parallel c-$axis as function of temperature. The determined $H_{c2}(T)$ clearly differs for the two principal directions. The dependence of the upper critical field on the angle $\Theta$ between the field direction and the $ab-$plane was measured in isothermal conditions at temperatures close to $T_{c0}$ and at low temperatures $T\ll T_{c}(H=0)$. In both temperature regimes $H_{c2}(\Theta)$ clearly deviates from sinusoidal function, expected for orbital $H_{c2}$ [1]. We discuss the origin of this behavior as possible reflection of the angular modulation of the superconducting gap magnitude and the complex warping of the Fermi surface along the $c$-axis. Work in Ames was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.\\[4pt] [1] V. G. Kogan and R. Prozorov, Rep. Prog. Phys. \textbf{75}, 114502 (2012).   

Multiband, paramagnetic effects and vortices in KFe$_2$As$_2$

We study the normal- and superconducting-state properties of the iron pnictide superconductor KFe$_{2}$As$_{2}$ using heat-capacity, thermal-expansion and magnetization measurements. In the normal state, our data show strong evidence of the existence of strong local fluctuations and of the coherence-incoherence crossover predicted by theory. In zero field, for T \textless\ T$_{\mathrm{c}}$, the temperature dependence of the heat capacity provides evidence for the existence of extremely small energy gaps. The (H,T) phase diagram is also determined down to 80 mK using calorimetric measurements. We reveal the existence of strong paramagnetic effects for field parallel to the Fe$_{\mathrm{2}}$As$_{2}$ planes. We discuss the symmetry of the order parameter and the interplay between multiband, paramagnetic and orbital effects.   

Field dependence of the thermal conductivity in the iron-based superconductor KFe$_2$As$_2$

The behavior of the thermal conductivity in the iron-arsenide KFe$_2$As$_2$ at low temperature provides compelling evidence of d-wave superconductivity [1]. Here we report a detailed study of the thermal conductivity in KFe$_2$As$_2$ as a function of magnetic field, for two field orientations: perpendicular and parallel to the FeAs planes. The data are in excellent quantitative agreement with theoretical calculations for a d-wave superconductor [2]. Our study also highlights the power of thermal conductivity as a technique to directly measure the upper critical field H$_{c2}$ in a clean type-II superconductor. \\[4pt] [1] J.-Ph. Reid et al., Phys. Rev. Lett. 109, 087001 (2012).\\[0pt] [2] A. B. Vorontsov and I. Vekhter, Phys. Rev. B 75, 224502 (2007).   

Microwave Surface Impedance Measurements on Fe(Se,Te) Single Crystals under Finite Magnetic Fields

We measured the microwave surface impedances of Fe(Se,Te) single crystals under magnetic fields up to 8 Tesla and extracted the flux flow resistivity, $\rho_f$, to investigate the quasiparticle dynamics inside the vortex core. Previously performed $\rho_f$ measurements on several iron-based superconductors have revealed that the quasiparticle dynamics inside the vortex core can be described as that in the so-called ``moderately clean'' regime, in which the mean free path is comparable to the coherence length [1,2]. The mean free path in Fe(Se,Te) in the normal state is smaller than those in other superconductors. In addition, London penetration depth shows quadratic temperature dependence because of the strong pair-breaking [3]. Therefore, it is interesting to investigate the $\rho_f$ to clarify whether the strong quasiparticle scattering affects the quasiparticle dynamics inside the vortex core. We also discuss the surface impedances of Fe(Se,Te) thin films which have a higher $T_c$ than the bulk crystals. \\[4pt] [1] T. Okada $et$ $al.$, Phys. Rev. B {\bfseries 86} (2012) 064516.\\[0pt] [2] H. Takahashi $et$ $al.$, Phys. Rev. B {\bfseries 86} (2012) 144525.\\[0pt] [3] H. Takahashi $et$ $al.$, Phys. Rev. B {\bfseries 84} (2011) 132503.\\   

Flux flow of iron based superconductors

Flux flow measured by the microwave technique is the only one possible technique to pick up the information on the quasiparticles in the vortex core. Theoretically, novel features have been suggested to show up in the flux flow of Fe-based superconductors (SCs) as multiple gapped SCs with possible sign changes. We investigated the flux flow resistivity of various different types of Fe based SCs, such as 111, 122, and 11 systems [1]. It is found that (1) the sign change is not important for the flux flow, (2) we can discuss the gap structure based on the flux flow data, even without performing angle dependent measurement, (3) vortices of Fe-based SC dissipate more energy than expected from the properties in the Meissner state. Together with the flux flow result in many other superconductors such as cuprates, Y$_{2}$C$_{3}$, and boron carbides, our result suggests the existence of a universal mechanism of dissipation for quasiparticles in the vortex core, probably related with the Andreev reflection at the core boundary. We also discuss the flux flow of cuprate superconductors, in terms of superconductivity fluctuation investigated by ac conductivity and diamagnetisms.\\[4pt] [1] K. Okada \textit{et al}.: Phys. Rev. B86 (2012) 064516, H. Takahashi \textit{et al}.: Phys. Rev. B86 (2012) 144525.