EuMnSb 2 is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat capacity data as well as polycrystalline 151 Eu M¨ossbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below T N Mn = 323(1) K with the ordered Mn magnetic moment µ Mn lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below T N Eu1 = 23(1) K with the ordered Eu magnetic moment µ Eu canted away from the layer normal and µ Mn retaining its higher-temperature order. µ Eu is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) Pn (cid:48) m (cid:48) a (cid:48) with the chemical and magnetic unit cells having the same dimensions. Cooling below T N Eu2 = 9(1) K reveals a third AFM phase where µ Mn remains unchanged but µ Eu develops an additional in-plane canting. This phase has MSG P 11 2 1 a (cid:48) . We additionally find evidence of short-range magnetic correlations associated with the Eu between 12 K (cid:46) T (cid:46) 30 K. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg-model. We then discuss implications of the various AFM states in EuMnSb 2 and its topological properties.

Two charge density wave transition can be detected in $\mathrm{La}\mathrm{Au}{\mathrm{Sb}}_{2}$ at $\ensuremath{\sim}110$ and $\ensuremath{\sim}90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ by careful electrical transport measurements. Whereas control of the Au site occupancy in $\mathrm{La}{\mathrm{Au}}_{x}{\mathrm{Sb}}_{2}$ (for $0.9\ensuremath{\lesssim}x\ensuremath{\lesssim}1.0$) can suppress each of these transitions by $\ensuremath{\sim}80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the application of hydrostatic pressure can completely suppress the lower transition by $\ensuremath{\sim}7.5\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$ and the upper transition by $\ensuremath{\sim}17\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$. Clear anomalies in the resistance as well as the magnetoresistance are observed to coincide with the pressures at which the charge density wave transitions are driven to zero.

We report on the crystal and magnetic structures and magnetic and transport properties of SrMnSb2 single crystals grown by the self-flux method. Magnetic susceptibility measurements reveal an antiferromagnetic (AFM) transition at T-N = 295(3) K. Above T-N, the susceptibility slightly increases and forms a broad peak at T similar to 420 K, which is a typical feature of two-dimensional magnetic systems. Neutron diffraction measurements on single crystals confirm the previously reported C-type AFM structure below T-N. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects are observed in SrMnSb2 single crystals. Analysis of the oscillatory component by a Fourier transform shows that the prominent frequencies obtained by the two different techniques are practically the same within error regardless of sample size or saturated magnetic moment. Transmission electron microscopy (TEM) reveals the existence of stacking faults in the crystals, which result from a horizontal shift of Sb atomic layers suggesting possible ordering of Sb vacancies in the crystals. Increase of temperature in susceptibility measurements leads to the formation of a strong peak at T similar to 570 K that upon cooling under magnetic field the susceptibility shows a ferromagnetic transition at T-C similar to 580 K. Neutron powder diffraction on crushed single crystals does not support a ferromagnetic phase above T-N. Furthermore, x-ray magnetic circular dichroism (XMCD) measurements of a single crystal at the L-2,(3) edge of Mn shows a signal due to induced canting of AFM moments by the applied magnetic field. All evidence strongly suggests that a chemical transformation at the surface of single crystals occurs above 500 K concurrently producing a minute amount of ferromagnetic impurity phase.

We report on the crystal and magnetic structures and magnetic and transport properties of SrMnSb2 single crystals grown by the self-flux method. Magnetic susceptibility measurements reveal an antiferromagnetic (AFM) transition at TN=295(3) K. Above TN, the susceptibility slightly increases and forms a broad peak at T∼420 K, which is a typical feature of two-dimensional magnetic systems. Neutron diffraction measurements on single crystals confirm the previously reported C-type AFM structure below TN. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects are observed in SrMnSb2 single crystals. Analysis of the oscillatory component by a Fourier transform shows that the prominent frequencies obtained by the two different techniques are practically the same within error regardless of sample size or saturated magnetic moment. Transmission electron microscopy (TEM) reveals the existence of stacking faults in the crystals, which result from a horizontal shift of Sb atomic layers suggesting possible ordering of Sb vacancies in the crystals. Increase of temperature in susceptibility measurements leads to the formation of a strong peak at T∼570 K that upon cooling under magnetic field the susceptibility shows a ferromagnetic transition at TC∼580 K. Neutron powder diffraction on crushed single crystals does not support a ferromagnetic phase above TN. Furthermore, x-ray magnetic circular dichroism (XMCD) measurements of a single crystal at the L2,3 edge of Mn shows a signal due to induced canting of AFM moments by the applied magnetic field. All evidence strongly suggests that a chemical transformation at the surface of single crystals occurs above 500 K concurrently producing a minute amount of ferromagnetic impurity phase. Disciplines Condensed Matter Physics Authors Yong Liu, Tao Ma, Warren E. Straszheim, Farhan Islam, Brandt A. Jensen, Wei Tian, Thomas Heitmann, R. A. Rosenberg, John M. Wilde, Bing Li, Andreas Kreyssig, Alan I. Goldman, Benjamin G. Ueland, Robert J. McQueeney, and David Vaknin PHYSICAL REVIEW B 99, 054435 (2019) Crystal growth, microstructure, and physical properties of SrMnSb2 Yong Liu,1,* Tao Ma,1 Lin Zhou,1 Warren E. Straszheim,1 Farhan Islam,1 Brandt A. Jensen,1 Wei Tian,2 Thomas Heitmann,3 R. A. Rosenberg,4 J. M. Wilde,1,5 Bing Li,1,5 Andreas Kreyssig,1,5 Alan I. Goldman,1,5 B. G. Ueland,1,5 Robert J. McQueeney,1,5 and David Vaknin1,5,† 1Division of Materials Sciences and Engineering, Ames Laboratory, U.S. DOE, Ames, Iowa 50011, USA 2Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 3The Missouri Research Reactor, University of Missouri, Columbia, Missouri 65211, USA 4Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 5Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA (Received 12 September 2018; published 28 February 2019) We report on the crystal and magnetic structures and magnetic and transport properties of SrMnSb2 single crystals grown by the self-flux method. Magnetic susceptibility measurements reveal an antiferromagnetic (AFM) transition at TN = 295(3) K. Above TN, the susceptibility slightly increases and forms a broad peak at T ∼ 420 K, which is a typical feature of two-dimensional magnetic systems. Neutron diffraction measurements on single crystals confirm the previously reported C-type AFM structure below TN. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects are observed in SrMnSb2 single crystals. Analysis of the oscillatory component by a Fourier transform shows that the prominent frequencies obtained by the two different techniques are practically the same within error regardless of sample size or saturated magnetic moment. Transmission electron microscopy (TEM) reveals the existence of stacking faults in the crystals, which result from a horizontal shift of Sb atomic layers suggesting possible ordering of Sb vacancies in the crystals. Increase of temperature in susceptibility measurements leads to the formation of a strong peak at T ∼ 570 K that upon cooling under magnetic field the susceptibility shows a ferromagnetic transition at TC ∼ 580 K. Neutron powder diffraction on crushed single crystals does not support a ferromagnetic phase above TN. Furthermore, x-ray magnetic circular dichroism (XMCD) measurements of a single crystal at the L2,3 edge of Mn shows a signal due to induced canting of AFM moments by the applied magnetic field. All evidence strongly suggests that a chemical transformation at the surface of single crystals occurs above 500 K concurrently producing a minute amount of ferromagnetic impurity phase. DOI: 10.1103/PhysRevB.99.054435

We have grown a series of nickel substituted single crystals of the layered ferromagnet (FM) Fe$_3$GeTe$_2$. The large single crystalline samples of (Fe$_{1-x}$Ni$_x$)$_3$GeTe$_2$ with $x = 0-0.84$ were characterized with single crystal X-ray diffraction, magnetic susceptibility, electrical resistance and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only minor structural variation. In addition, FM order is suppressed from $T_\mathrm{C}=212$~K for $x=0$ down to $T_\mathrm{C}=50$~K for $x=0.3$, which is accompanied with a strong suppression of saturated and effective moment, and Curie-Weiss temperature. Beyond $x=0.3$, the FM order is continuously smeared into a FM cluster glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe$_3$GeTe$_2$ and subsequent dilution effects from the Ni substitution.

We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of ( B a 1 − x K x ) F e 2 A s 2 ( 0.18 ≤ x ≤ 1 ). The critical current density J c reaches maximum in the underdoped sample x = 0.26 and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier U 0 sharply decreases in the overdoped sample x = 0.70 . These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples x = 0.38 , 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in T c become small in the samples x = 0.43 and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the ( B a 1 − x K x ) F e 2 A s 2 crystals: (i) strong δl pinning, which results from the fluctuations in the mean free path l and δ T c pinning from the spatial variations in T c in the underdoped regime, and (ii) weak δ T c pinning in the optimally doped and overdoped regime. Disciplines Condensed Matter Physics | Materials Science and Engineering Authors Yong Liu, Lin Zhou, Kewei Sun, Warren E. Straszheim, Makariy A. Tanatar, Ruslan Prozorov, and Thomas A. Lograsso This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/112 PHYSICAL REVIEW B 97, 054511 (2018) Doping evolution of the second magnetization peak and magnetic relaxation in (Ba1−xKx)Fe2As2 single crystals Yong Liu,1,* Lin Zhou,1 Kewei Sun,1 Warren E. Straszheim,1 Makariy A. Tanatar,1,2 Ruslan Prozorov,1,2 and Thomas A. Lograsso1,3 1Division of Materials Sciences and Engineering, Ames Laboratory, U.S. DOE, Ames, Iowa 50011, USA 2Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 3Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA (Received 11 June 2017; revised manuscript received 26 November 2017; published 16 February 2018) We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of (Ba1−xKx)Fe2As2 (0.18 x 1). The critical current density Jc reaches maximum in the underdoped sample x = 0.26 and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier U0 sharply decreases in the overdoped sample x = 0.70. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples x = 0.38, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in Tc become small in the samples x = 0.43 and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the (Ba1−xKx)Fe2As2 crystals: (i) strong δl pinning, which results from the fluctuations in the mean free path l and δTc pinning from the spatial variations in Tc in the underdoped regime, and (ii) weak δTc pinning in the optimally doped and overdoped regime. DOI: 10.1103/PhysRevB.97.054511

M. A. Tanatar, ∗ E. C. Blomberg, 2 Hyunsoo Kim, 2 Kyuil Cho, 2 W. E. Straszheim, Bing Shen, Hai-Hu Wen, 4, 5 and R. Prozorov Ames Laboratory, Ames, Iowa 50011, USA Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, P. R. China Canadian Institute for Advanced Research, Toronto, Ontario, Canada (Dated: November 10, 2018)

We report on our attempt to hole-dope the antiferromagnetic semiconductor LaMnAsO by substitution of the La3+ site by Ca2+. We use neutron and x-ray diffraction, magnetic susceptibility, and transport techniques to characterize polycrystalline (La1−xCax)MnAsO samples prepared by solid-state reaction and find that the parent compound is highly resistant to substitution with an upper limit x≤0.01. Magnetic susceptibility of the parent and the x=0.002(xnom=0.04) compounds indicate a negligible presence of magnetic impurities (i.e., MnO or MnAs). Rietveld analysis of neutron and x-ray diffraction data shows the preservation of both the tetragonal (P4/nmm) structure upon doping and the antiferromagnetic ordering temperature, TN=355±5 K. Disciplines Engineering Physics | Materials Science and Engineering Authors Yong Liu, Warren E. Straszheim, Pinaki Das, Farhan Islam, Thomas W. Heitmann, Robert McQueeney, and David Vaknin This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/179 PHYSICAL REVIEW MATERIALS 2, 054410 (2018) Synthesis and characterization of Ca-doped LaMnAsO Yong Liu,1,* Warren E. Straszheim,1 Pinaki Das,2 Farhan Islam,2 Thomas W. Heitmann,3 Robert J. McQueeney,2 and David Vaknin2 1Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA 2Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 3The Missouri Research Reactor, University of Missouri, Columbia, Missouri 65211, USA (Received 27 November 2017; published 18 May 2018) We report on our attempt to hole-dope the antiferromagnetic semiconductor LaMnAsO by substitution of the La3+ site by Ca2+. We use neutron and x-ray diffraction, magnetic susceptibility, and transport techniques to characterize polycrystalline (La1−xCax)MnAsO samples prepared by solid-state reaction and find that the parent compound is highly resistant to substitution with an upper limit x 0.01. Magnetic susceptibility of the parent and the x = 0.002 (xnom = 0.04) compounds indicate a negligible presence of magnetic impurities (i.e., MnO or MnAs). Rietveld analysis of neutron and x-ray diffraction data shows the preservation of both the tetragonal (P 4/nmm) structure upon doping and the antiferromagnetic ordering temperature, TN = 355 ± 5 K. DOI: 10.1103/PhysRevMaterials.2.054410

We have grown a series of nickel-substituted single crystals of the layered ferromagnet (FM) Fe3GeTe2. The large single-crystalline samples of (Fe1−xNix)3GeTe2 with x=0–0.84 were characterized with single-crystal xray diffraction, magnetic susceptibility, electrical resistance, and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only a minor structural variation. In addition, FM order is suppressed from TC=212K for x=0 down to TC=50Kfor x=0.3, which is accompanied by a strong suppression of saturated and effective moments, and Curie-Weiss temperature. Beyond x=0.3, the FM order is continuously smeared into a FM cluster-glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe3GeTe2 and subsequent dilution effects from the Ni substitution. Disciplines Condensed Matter Physics Authors Gil Drachuck, Zaher Salman, Morgan W. Masters, Valentin Taufour, Tej Lamichhane, Qisheng Lin, Warren E. Straszheim, Sergey L. Bud’ko, and Paul C. Canfield This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/333 PHYSICAL REVIEW B 98, 144434 (2018) Effect of nickel substitution on magnetism in the layered van der Waals ferromagnet Fe3GeTe2 Gil Drachuck,1,2 Zaher Salman,3 Morgan W. Masters,1 Valentin Taufour,1,4 Tej N. Lamichhane,1,2 Qisheng Lin,2,5 Warren E. Straszheim,2 Sergey L. Bud’ko,1,2 and Paul C. Canfield1,2 1Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 2Ames Laboratory, U.S. DOE, Iowa State University, Ames, Iowa 50011, USA 3Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 4Department of Physics, University of California Davis, Davis, California 95616, USA 5Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA (Received 10 September 2018; revised manuscript received 5 October 2018; published 24 October 2018) We have grown a series of nickel-substituted single crystals of the layered ferromagnet (FM) Fe3GeTe2. The large single-crystalline samples of (Fe1−xNix )3GeTe2 with x = 0–0.84 were characterized with single-crystal x-ray diffraction, magnetic susceptibility, electrical resistance, and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only a minor structural variation. In addition, FM order is suppressed from TC = 212 K for x = 0 down to TC = 50 K for x = 0.3, which is accompanied by a strong suppression of saturated and effective moments, and Curie-Weiss temperature. Beyond x = 0.3, the FM order is continuously smeared into a FM cluster-glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe3GeTe2 and subsequent dilution effects from the Ni substitution. DOI: 10.1103/PhysRevB.98.144434

The interplay of magnetism and superconductivity (SC) has been a focus of interest in condensed matter physics for decades. EuFe2As2 has been identified as a potential platform to investigate interactions between structural, magnetic, electronic effects as well as coexistence of magnetism and SC with similar transition temperatures. However, there are obvious inconsistencies in the reported phase diagrams of Eu(Fe1-xCox)2As2 crystals grown by different methods. For transition metal arsenide (TMA)-flux-grown crystals, even the existence of SC is open for dispute. Here we re-examine the phase diagram of single-crystalline Eu(Fe1-xCox)2As2 grown by TMA flux. We found that the lattice parameter c shrinks linearly with Co doping, almost twice as fast as that of the tin-flux-grown crystals. With Co doping, the spin-density-wave (SDW) order of Fe sublattice is quickly suppressed, being detected only up to x = 0.08. The magnetic ordering temperature of the Eu2+ sublattice (TEu) shows a systematic evolution with Co doping, first going down and reaching a minimum at x = 0.08, then increasing continuously up to x = 0.24. Over the whole composition range investigated, no signature of SC is observed.

We report on our attempt to hole-dope the antiferromagnetic semiconductor LaMnAsO by substitution of the ${\mathrm{La}}^{3+}$ site by ${\mathrm{Ca}}^{2+}$. We use neutron and x-ray diffraction, magnetic susceptibility, and transport techniques to characterize polycrystalline $({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x})\mathrm{MnAsO}$ samples prepared by solid-state reaction and find that the parent compound is highly resistant to substitution with an upper limit $x\ensuremath{\le}0.01$. Magnetic susceptibility of the parent and the $x=0.002\phantom{\rule{4pt}{0ex}}({x}_{\mathrm{nom}}=0.04)$ compounds indicate a negligible presence of magnetic impurities (i.e., MnO or MnAs). Rietveld analysis of neutron and x-ray diffraction data shows the preservation of both the tetragonal $(P4/nmm)$ structure upon doping and the antiferromagnetic ordering temperature, ${T}_{\mathrm{N}}=355\ifmmode\pm\else\textpm\fi{}5$ K.

We report on the results of a high-energy x-ray diffraction study of Al-Pd-Mn to investigate the solidification products obtained during free-cooling using an electrostatic levitation furnace. The primary solidification product from the melt is i-Al-Pd-Mn which coexists with a significant remaining liquid component. As the sample cools further, we find that the solidification pathway is consistent with the liquidus projection and pseudo-binary cut through the ternary phase diagram reported previously. At ambient temperature we have identified the major phase to be the xi'-phase orthorhombic approximant, along with minor phases identified as Al and, most likely, the R-phase orthorhombic approximant. We have also observed a distinct prepeak in the liquid at high temperature, signifying the presence of extended atomic order. Interestingly, this prepeak was not observed in previous neutron diffraction measurements on the Al-Pd-Mn system. No undercooling was observed preceding the solidification of the i-Al-Pd-Mn phase from the melt which may signal the close similarity of the short-range order in the solid and liquid. However, this can not be clearly determined because of the potential for heterogenous nucleation associated with the presence of an Al2O3 impurity at the surface of the sample.

GeTe, a self -doping semiconductor, is a well-known base compound for thermoelectric and phase change materials. It is known, that replacement of Ge in Ag6.5Sb6.5Ge37Te50 (TAGS -85) material by rare earth Dy significantly enhances both the power factor and thermoelectric figure of merit. Here we demonstrate how replacement of Ge in GeTe by rare earths with different atomic size and localized magnetic moments affect XRD patterns, magnetization, Te-125 NMR spectra and spin -lattice relaxation, and the Seebeck coefficient of the alloys with a nominal composition of Ge49Te50R1 (R = La, Pr, Gd, Dy, and Yb). SEM, EDS and WDS data show that rare earth atoms in the matrix are present at smaller extent compared to a nominal composition, whereas rare earth also is present in inclusions. Rare earths affect the Seebeck coefficient, which is a result of interplay between the reduction due to higher carrier concentration and enhancement due to magnetic contribution. The effect of replacement of Ge in GeTe by Dy on the Seebeck coefficient is smaller than that observed in Ag6.5Sb6.5Ge36 Te50Dy1. This can be explained by larger amount of rare earth, which can be embedded into the lattice of materials containing [Ag+ Sb] atomic pairs and possible effect from these pairs.(C) 2017 Elsevier B.V. All rights reserved.

(Left) Scanning electron microscopy micrographs and corresponding contact angle (CA) images for (top) thermally annealed inkjet printed graphene (IPG) and (bottom) laser annealed (IPG). (Right) Molecular dynamic simulation results correlating CA and changes in graphene petal orientation due to laser treated IPG.

The study of the iron-based superconductor FeSe has blossomed with the availability of high-quality single crystals, obtained through flux/vapor-transport growth techniques below the structural transformation temperature of its tetragonal phase, T approximate to 450 degrees C. Here, we report on the variation of sample morphology and properties due to small modifications in the growth conditions. A considerable variation of the superconducting transition temperature T-c, from 8.8 K to 3 K, which cannot be correlated with the sample composition, is observed. Instead, we point out a clear correlation between T-c and disorder, as measured by the residual resistivity ratio. Notably, the tetragonal-to-orthorhombic structural transition is also found to be quite strongly disorder dependent (T-s approximate to 72-90 K) and linearly correlated with T-c.

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba1-x K x )Fe2As2 has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping (x ≈ 0.4) to becoming nodal at x > 0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, λ(T), across the full phase diagram for different concentrations of point-like defects introduced by 2.5-MeV electron irradiation. Fitting the low-temperature variation with the power law, Δλ ~ Tn , we find that the exponent n is the highest and the Tc suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around x ≃ 0.8, indicating the onset of nodal behavior. Our analysis using a self-consistent t-matrix approach suggests the ubiquitous and robust nature of s± pairing in IBSs and argues against a previously suggested transition to a d-wave state near x = 1 in this system.

We report how the superconducting phase forms in pseudo-single-crystal KxFe2-ySe2. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an "imperfect" order-disorder transition. It should be emphasized that the phase separation in pseudo-single-crystal KxFe2-ySe2 is caused by the iron-vacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Z. Wang et al., Phys. Rev. B 91, 064513 (2015)]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction ormelt growth. By focused ion beam scanning electronmicroscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase.

The nature of the phase separation in KxFe2-ySe2 single crystals is not well understood. Here, temperature dependent SEM images provide compelling evidence that phase separation corresponds to a nucleation and growth process rather than a spinodal decomposition. The superconducting phase is the remnant of high temperature phase after iron vacancy order-disorder transition. Three-dimension (3D) spatial distribution of superconducting phase in the iron vacancy ordered matrix was revealed by using focused ion beam scanning electron microscopy (FIB-SEM). Our results clearly show that superconducting phase forms a hierarchical structure. Understanding the formation of this hierarchical structure not only can guide the synthesis of bulk superconductors in the future, but also greatly enrich our knowledge about the interplay between phase separation and solid-state phase transformations.