Session D22: Focus Session: Fe-based Superconductors - Crystal Growth, Structure, and Properties of K_xFe_1-ySe Phases

Superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$

Single crystal alloys KxFe$_{2-y}$Se$_{2-z}$S$_{z}$ offer valuable insight into the strength of electronic correlations in the normal state and structural characteristics associated with superconductivity. I will discuss the evolution of the superconducting and magnetic ground states as a function of sulfur concentration $z$ and some noticeable changes in the average and local crystal structure associated with this [1-4]. Conductivity and magnetic properties coincide with stoichiometry changes and with particular local environment of Fe atoms on the two Fe sites in the crystal structure. The ratio of superconducting T$_{c}$ and Fermi temperature T$_{F}$ is also suppressed by sulfur doping, indicating the suppression of electronic correlations. The superconductivity persists with relatively high T$_{c}$ even when electronic correlations in the normal state are greatly reduced. The results for z = 0 will be compared with other experimental techniques that probe nanoscale phase separation and degree of vacancy order [5-6]. It will be shown that local structure and population of particular Fe sites is rather important for obtaining the bulk superconducting phase. Superconducting volume fraction and homogeneity of superconducting phase is in direct competition with Fe vacancy order [7].\\[4pt] [1] Hechang Lei et al., Phys. Rev. Lett. 107, 137002 (2011)\\[0pt] [2] Hechang Lei et al., Phys. Rev. B 83, 180503 (2011)\\[0pt] [3] Kefeng Wang et al., Phys. Rev. B 174503 (2011)\\[0pt] [4] Kefeng Wang et al., Phys. Rev. B 84, 054526 (2011)\\[0pt] [5] Z. Wang et al., Phys. Rev. B 83, 140505 (2011)\\[0pt] [6] Y. J. Yan et al., arXiv:1104.4941 (2011)\\[0pt] [7] Hyejin Ryu et al., arXiv:1111.2597.   

Crystal growth and detailed structural characterization of superconducting and non-superconducting phases in the K$_{1-x}$Fe$_{2-y}$Se$_2$ system

Amid the flurry of activity on K$_{1-x}$Fe$_{2-y}$Se$_2$ superconductors, it remains established that the stoichiometric compound K$_2$Fe$_4$Se$_5$ is an antiferromagnetic semiconductor. This raises the question of whether subtle Fe$^{1+/3+}$ doping causes K$_{1-x}$Fe$_{2-y}$Se$_2$ to become a bulk superconductor, and if so, is there a structural distinction between superconducting and non-superconducting phases? We have grown K$_{1-x}$Fe$_{2-y}$Se$_2$ samples that show superconductivity with $T_C$ = 31 K, even when growth conditions are starkly different from those reported in the literature. Here we present high-resolution synchrotron X-ray diffraction measurements, alongside single-crystal x-ray and electron diffraction, to elucidate the phase space in this system. Combined with magnetometry, heat capacity, and transport measurements, our structure-property relations help prescribe how chemical composition and heat treatment induce superconductivity and vacancy ordering in the K$_{1-x}$Fe$_{2-y}$Se$_2$ system.   

Suppression of superconductivity and spin-glass behavior in Cu-doped K0.8Fe2Se2

Single crystals with nominal compositions of K0.8Fe2-xCuxSe2 were grown and studied with low temperature electrical transport and magnetic susceptibility measurements. We show that the superconductivity present in undoped K0.8Fe2Se2 crystals with transition temperature of 31 K is very quickly suppressed with Cu doping into the Fe site, and the system very quickly becomes insulating. We discuss anomalous behavior at higher doping, including spin-glass like behavior with further Cu doping.   

What is the true parent state in alkali-doped Iron Selenides

By performing first-principles electronic structure calculations and analyzing effective magnetic model of alkali-doped iron selenides, we show that the materials without iron vacancies should approach a novel checkerboard phase in which each four Fe sites group together in tetragonal structure. The checkerboard phase is the ground state with a block antiferromagnetic (AFM) order and a small charge density wave order in the absence of superconductivity. Both of them can also coexist with superconductivity. The results explain mysterious 2 by 2 ordered patterns and hidden orders observed in various different experiments, clarify the missing link between AFM and superconducting phases, suggest that the block-AFM state is the parent state, and unify the understanding of various observed phases in alkali-doped iron selenides. (Reference: Wei Li, et al, arxiv:1110.0372 (2011)   

Structural and Superconducting Properties of K$_{0.8}$Fe$_{1.6+x}$Se$_{2 }$ single crystals

We report structural and superconducting properties of K0.8Fe1.6+xSe2 single crystals. Superconducting properties were studied by using transport, specific heat and magnetization measurements, and compared with that of FeSe$_{1-x}$Te$_{x}$. We found the $c$-axis resistive transition and specific heat behavior are distinctively different in these two classes of materials. Structural properties were studied via transmission electron microscopy and energy-dispersive X-ray spectroscopy. We found large scale structural and chemical disorder in the K0.8Fe1.6+xSe2 samples. The relationship between structural and superconducting properties in K0.8Fe1.6+xSe2 will be discussed.   

Imaging the coexistence of superconductivity and a charge density modulation in K$_{0.73}$Fe$_{1.67}$Se$_{2}$ superconductor

We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K$_{0.73}$Fe$_{1.67}$Se$_{2}$ with $T_{C}$ = 32K. On the atomically resolved FeSe surface, we observe well-defined superconducting gap and the microscopic coexistence of a charge density modulation with $\sqrt 2 \times \sqrt 2 $ periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides.   

Local Atomic and Electronic Structure of K$_{0.8}$Fe$_{1.6+x}$Se$_{2}$: Structural Order and Disorder

The local structure of superconducting single crystals of K$_{0.8}$Fe$_{1.6+x}$Se$_{2}$ was studied by x-ray absorption spectroscopy. Near-edge spectra reveal that the average valence of Fe is 2+. The structure about the Se and Fe sites shows a high degree of order in the nearest neighbor Fe-Se bonds. On the other hand, the combined Se and K local structure measurements reveal a very high level of structural disorder in the K layers. The temperature dependence of the Fe-Se atomic correlation follows that of the Fe-As correlation in LaFeAsO$_{0.89}$F$_{0.11. }$ In K$_{0.8}$Fe$_{1.6+x}$Se$_{2}$, the nearest neighbor Fe-Fe bonds has a lower Einstein temperature and higher structural disorder than in LaFeAsO$_{0.89}$F$_{0.11}$. For higher shells, an enhancement of the second nearest neighbor Fe-Fe interaction is found just below Tc and suggests that correlations between Fe magnetic ion pairs beyond the first neighbor are important in models of magnetic order and superconductivity in these materials. This research is supported by DOE BES Grant DE-FG02-07ER46402 (NJIT) for T.A.T and T.Y. and DOE BES Contract No. DE-AC0298CH10886. (BNL), for S.J.H, G.G, I.K.D, and Q. L.   

Interplay between structure and properties in TlFe$_{1.6}$Se$_2$

The degree to which Fe vacancies order greatly influences the properties of intercalated FeSe compounds. In this talk, we will consider the relationship between vacancy ordering and the properties of TlFe$_{1.6}$Se$_2$. Unlike the alkali-metal based compounds, such as K$_{0.8}$Fe$_{1.6}$Se$_2$, TlFe$_{1.6}$Se$_2$ does not become superconducting and always forms with the Tl-sites fully occupied. This results in reduced complexity and disorder, allowing the role of Fe vacancies to be probed directly. Common to all of these materials is $\sqrt{5}a \times \sqrt{5}a$ superstructure associated with ordering of the Fe vacancies, which appears coupled to a block-checkerboard antiferromagnetic order. Through subtle changes in composition and processing, the vacancies can order more or less completely, and this results in substantial variations in the magnetic properties of TlFe$_{1.6}$Se$_2$. The electronic behavior is relatively insensitive to these changes, though, as semiconducting behavior is observed in all cases. Finally, high resolution TEM reveals complex local structures, which are markedly different from those observed in K$_{0.8}$Fe$_{1.6}$Se$_2$.   

Defect Structures and nonstoichiometry in the K$_{x}$Fe$_{2-y}$Se$_{2}$ by X-ray diffraction.

The mechanisms of charge carrier density control in the iron pnictide and chalcogenide superconductors are important as small changes in composition produce metal-insulator transitions and generate superconductivity at temperatures of up to 37K in chalcogenides and 55K in pnictides. All of the reported materials are based on a square FeX (X = Se, As) layer built from edge-sharing of FeX$_{4}$ tetrahedra. Insertion of alkali metal cations between FeSe layers affords superconductivity in A$_{x}$Fe$_{2-y}$Se$_{2}$ (A = K, Rb, Cs, Tl: 0.7 $<$ x $<$ 1, 0 $<$ y $<$ 0.5) materials highlites the defect chemistry as the iron charge states close to +2 found in the other Fe-based superconductors require the creation of considerable defect concentrations on either or both iron and alkali metal sites. Ordering of the tetrahedral site vacancies in two crystals of refined compositions K$_{0.93(1)}$Fe$_{1.52(1)}$Se$_{2}$ and K$_{0.862(3)}$Fe$_{1.563(4)}$Se$_{2}$ produces a fivefold expansion of the parent ThCr$_{2}$Si$_{2}$ unit cell in the ab plane which can accommodate 20{\%} vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, doping mechanisms, which can operate at both average and local structure levels will be discussed. These structures will also be considered in terms of their local ordering and with relation to other defect chalcogenide layer structures and possible phase segregation.   

Local structural disorder and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$

We report significantly enhanced magnetic moment on K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals after post-annealing and quenching process. In K$_{x}$Fe$_{2-y}$Se$_{2}$ unit cell, there are two Fe sites, Fe1 which has higher symmetry with longer average Fe-Se bond length, and Fe2 which has lower symmetry with shorter average Fe-Se bond length. Temperature dependent X-ray absorption fine structure (XAFS) analysis results on quenched and as grown K$_{x}$Fe$_{2-y}$Se$_{2}$ materials show that quenched K$_{x}$Fe$_{2-y}$Se$_{2}$ materials have increased average Fe-Se bond length and decreased static disorder. This result indicates that occupancy of Fe1 sites increased after post-annealing and quenching process. This result provides clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.   

Novel synthesis method of K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystal

The discovery of superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$ with $\sim $T$_{c}$ 31 K has triggered a great interest in the field of iron-based superconductors [1]. K$_{x}$Fe$_{2-y}$Se$_{2}$ superconductor has several practical advantages of relatively high $T_{c}$, high upper critical field (H$_{c2})$ and less toxicity compared to FeAs-based superconductors. However, the procedure for producing K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystal is complicated and time-consuming: At first, the FeSe precursor was prepared, and then the single crystals of K$_{x}$Fe$_{2-y}$Se$_{2}$ were grown by the self-flux method. The simplification of the synthesis is really important for applications. We present a novel synthesis method of K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystal, which is very simple and quick. A superconducting transition of this sample appeared at T $\sim $31.6 K. After quenching the sample, the calcurated volume fraction from dc magnetic susceptibility was dramatically increased, consistent with the previous report [2]. We will also report the details of the synthesis, transport properties and microstructures of the samples. 1)J.Guo, \textit{et al}, Phys. Rev. B \textbf{82}, 180520 (2010). 2)H. Lei, \textit{at el}, arXiv: 1109.0534.   

Ordering driven superconductivity in an alkaline iron selenide

Combining the resistivity, synchrotron x-ray diffraction, and neutron diffraction measurements, we investigated the evolution of the structural, magnetic, and superconducting properties with pressure up to 37 GPa of superconducting Rb$_{0.347}$Tl$_{0.347}$Fe$_{1.752}$Se$_{2}$. Extended phase diagram of temperatures and pressure were established for this material. The results show the superconductivity is in reality driven by ordering.   

Pressure-tuned superconductivity of iron chalcogenides

In this talk, we present our recent progress in effect of pressure on superconductivity of newly discovered iron chalcogenide superconductors. We show that the either positive or negative pressure can tune superconductivity of this new kind of superconductors. Superconductivity with higher superconducting transition temperature \textit{Tc }can reemerge after elimination of the initial superconducting phase upon compression. We find that the maximum $T$c of the reemerging superconducting phase is as high as 48.7 K for K$_{0.8}$Fe$_{1.70}$Se$_{2}$ and 48 K for Tl$_{0.6}$Rb$_{0.4}$Fe$_{1.67}$Se$_{2}$, setting a new $T$c record for chalcogenide superconductors. The presence of the second superconducting phase is proposed to be related to pressure-induced quantum criticality. Our findings open up the potential route for the exploration of high-$T$c superconductivity in iron-based and other superconductors.