^{57}Fe Mössbauer study of stoichiometric iron based superconductor CaKFe_4As_4: a comparison to KFe_2As_2 and CaFe_2As_2
Sergey L. Bud'ko, Tai Kong, William R. Meier, Xiaoming Ma, Paul C. Canfield
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Fe M¨ossbauer study of stoichiometric iron basedsuperconductor CaKFe As : a comparison to KFe As and CaFe As ∗ , Tai Kong , William R. Meier Ames Laboratory US DOE and Department of Physics and Astronomy, Iowa StateUniversity, Ames, IA 50011, USA
Xiaoming Ma
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics,Chinese Academy of Sciences, Beijing 100190, China.Department of Physics, South University of Science and Technology of China, Shenzhen,Guangdong 518055, China
Paul C. Canfield
Ames Laboratory US DOE and Department of Physics and Astronomy, Iowa StateUniversity, Ames, IA 50011, USA
Abstract Fe M¨ossbauer spectra at different temperatures between ∼ ∼ As . Thedata indicate that CaKFe As is a well formed compound with narrow spectrallines, no traces of other, Fe - containing, secondary phases in the spectra and nostatic magnetic order. There is no discernible feature at the superconductingtransition temperature in any of the hyperfine parameters. The temperature de-pendence of the quadrupole splitting approximately follows the empirical “ T / law”.The hyperfine parameters of CaKFe As are compared with those for KFe As measured in this work, and the literature data for CaFe As , and were found tobe in between those for these two, ordered, 122 compounds, in agreement withthe gross view of CaKFe As as a structural analog of KFe As and CaFe As that has alternating Ca - and K - layers in the structure. Keywords: superconductors, M¨ossbauer spectroscopy, hyperfine parameters ∗ Corresponding author
Email address: [email protected] (Sergey L. Bud’ko) currently at Department of Chemistry, Princeton University Preprint submitted to Journal of Physics and Chemistry of Solids January 17, 2017 a r X i v : . [ c ond - m a t . s up r- c on ] J a n . Introduction The discovery of iron-based superconductors [1] was followed by an out-pouring of theoretical and experimental studies of those and related materials[2, 3, 4, 5]. Of these studies some were addressing specific details of the su-perconducting and the associated vortex state, whereas others were targetedcomprehensive characterization of the general physical properties of iron-basedsuperconductors and related materials. M¨ossbauer effect spectroscopy is widelyaccepted as one of the most sensitive techniques in terms of energy resolution.Historically, this technique has been applied to studies of superconductors fordecades,[6] however its sensitivity specifically to the superconducting state isambiguous. [7] It is quite natural that M¨ossbauer spectroscopy was widely usedfor studies of iron-based superconductors that naturally contain the commonM¨ossbauer nuclide, Fe, in the structure,[8, 9, 10, 11, 12]: no additional dop-ing with Fe (that can alter the properties of the material) is needed, anduse of partially enriched Fe was required in only few, very specific cases. Thistechnique was very successful in addressing the evolution of magnetic order, [13]structural phase transitions, [14] and phase purity [15, 16, 17] in these materials.Recently, a new family of iron-based superconductors with rather high super-conducting transition temperature, T c ∼ −
36 K, has been discovered. [18]It was found that structurally ordered CaAFe As (1144) compounds can beformed for A = K, Rb, Cs, and the key to the formation is the difference in ionicsizes between the Ca and the A ion. This family is not a (Ca − x A x )Fe As solidsolution, where the Ca and A ions randomly occupy a single crystallographicsite. [19] Along the c -axis, the Ca and A ions in this family form alternatingplanes that are separated by the Fe-As slabs (Fig. 1). In essence, the CaAFe As structure is similar to the CaFe As structure, just with layer by layer segrega-tion of the Ca and A ions. The ordering of the layers causes a change of thespace group from I /mmm to P /mmm and the Fe site in the 1144 structurehas its point symmetry lowered to orthorhombic (from the tetragonal in the 122structure). The 1144 structure was also found for SrAFe As (A = Rb, Cs) [18]and EuAFe As (A = Rb,Cs). [21, 22]We were able to grow single-crystalline, single-phase samples of CaKFe As and measure their anisotropic thermodynamic and transport properties. [23]The data indicated that CaKFe As is an ordered stoichiometric superconduc-tor with T c = 35 K and no other phase transition for 1.8 K ≤ T ≤
300 K. Itappeared to have properties very close to what is referred to as an optimally-doped, on a generalized phase diagram, iron-based superconductor. Being anordered stoichiometric compound with a high value of T c and a single crystal-lographic Fe site, CaKFe As offers an exceptional opportunity to determinewhether any of the hyperfine parameters exhibit an anomaly at superconduct-ing transition. Additionally, this time with a local probe, we can evaluate thephase purity (in terms of possible Fe-containing phases) of the samples and thepresence of static magnetic moment on the iron site. Furthermore, we can com-pare the temperature dependencies of the hyperfine parameters with those ofthe closely related compounds, CaFe As and KFe As .2n this work we will present results of the Fe M¨ossbauer spectroscopymeasurements between ∼ ∼
300 K on a mosaic of the oriented sin-gle crystals of CaKFe As and will compare the results with similar sets ofdata for CaFe As and KFe As . Whereas there are available literature datafor CaFe As , [14, 24, 25] the published M¨ossbauer data for KFe As appar-ently is limited to three temperature points [13] so we have chosen to collect acomprehensive set of data for KFe As as a part of this work.
2. Experimental
CaKFe As single crystals were grown by high temperature solution growthout of excess FeAs. The growth and basic physical properties are described indetail in Ref. [23]. The crystals were screened as described in Ref. [23] toavoid possible contaminations by CaFe As and KFe As minority phases. Thesuperconducting transition in the CaKFe As crystals used for the M¨ossbauerstudy was sharp with T c ∼
35 K (Fig. 2).KFe As single crystals were also grown using a high-temperature solutiongrowth technique. Starting elements were packed in an alumina frit-disc crucibleset [26] with a molar ratio of K:Fe:As = 8:2:10. The crucible set together withthe material were then welded in a Ta tube and sealed in a silica ampoule undera partial Ar atmosphere. A detailed drawing of such an assembly can be foundin Ref [27]. The ampoule was slowly heated up to 920 ◦ C over ∼
40 hours, heldat 920 ◦ C for 10 hours, quickly cooled to 850 ◦ C over 3 hours and then slowlycooled to 700 ◦ C over 3 days. At 700 ◦ C, the silica ampoule was inverted anddecanted in a centrifuge. Remaining solution (primarily K-As) on the singlecrystals was rinsed off using ethanol. The resulting crystals had high residualresistivity ratio ( ρ (300K) /ρ (5K) ∼ T c values consistent with other highquality KFe As crystals. [28, 29]M¨ossbauer spectroscopy measurements were performed using a SEE Co.conventional, constant acceleration type spectrometer in transmission geome-try with a Co(Rh) source kept at room temperature. Both for CaKFe As and KFe As the absorber was prepared as a mosaic of single crystals held ona paper disk by a small amount of Apiezon N grease. The gaps between theindividual crystals were kept as small as possible. The mosaic had the c axisperpendicular to the disks and arbitrary in-plane orientation for each of thecrystals. The c axis of the crystals in the mosaic was parallel to the M¨ossbauer γ beam. The absorber was cooled to a desired temperature using a Janis modelSHI-850-5 closed cycle refrigerator (with vibration damping). The driver ve-locity was calibrated using an α -Fe foil, and all isomer shifts (IS) are quotedrelative to the α -Fe foil at room temperature. The M¨ossbauer spectra were fit-ted using either the commercial software package MossWinn, [30] or the MossApackage [31] with both analyses giving very similar results.3 . Results and Discussion As A subset of M¨ossbauer spectra for CaKFe As , taken at different temper-atures, is shown in Fig. 3. The absorption lines are asymmetric, suggestingthat each spectrum is a quadrupole split doublet with rather small value of thequadrupole splitting, QS. There are no extra features observed, confirming thatthe samples are single phase. The results of fits to these data are shown in Fig.4. The linewidth of the spectra (Fig. 4d - FWHM) varies between ∼ . − . T c = 35 K that rises above the scatteringof the data or the error bars.Fig. 4a presents measured isomer shift (IS) which increases upon cooling.The isomer shift includes contributions from both the chemical shift and thesecond-order Doppler shift. The latter is known to increase convexly upon de-creasing temperature, due to gradual depopulation of the excited phonon states,but should be constant at low temperature, because of the quantum mechanicalzero-point motion. The chemical shift should not depend on temperature. Themain contribution to the temperature dependence of the isomer shift then isconsidered to be from the second-order Doppler shift, and is usually describedby the Debye model: [32] IS ( T ) = IS (0) − k B TM c (cid:18) T Θ D (cid:19) (cid:90) Θ D /T x dxe x − , (1)where c is the velocity of light, M is the mass of the Fe nucleus, and IS (0)is the temperature-independent part. For the isomer shift data in Fig.4a Debyefit yields Θ D = 370 ± T / law”[33], QS ( T ) = QS (1 − βT / ), where QS ( T ) is temperature dependent quadrupolesplitting, QS is its value at T = 0 K, β is a parameter that was found [33] tovary between 1 × − and 7 × − K − / . In our case the value of β ≈ . × − K − / falls within the expected range.Whereas the physics behind the “ T / law” is not fully understood (it isconsidered that that it originates from thermal vibrations of the lattice [34]), thisrelation describes reasonably well the temperature dependencies of QS observedin non-cubic metals. [33, 35, 36, 37]The spectral area under the doublet increases on cooling (Fig. 4c). Thetemperature dependence of the spectral area can also be fitted with the Debyemodel [32]: f = exp (cid:40) − E γ k B Θ D M c (cid:34)
14 + (cid:18) T Θ D (cid:19) (cid:90) Θ D /T xdxe x − (cid:35)(cid:41) , (2)4here f is the recoilless fraction, which is proportional to the area for a thinsample and E γ is the γ -ray energy. The estimate of the Debye temperaturefrom the fit gives Θ D = 247 ± IS . Although part of thisdiscrepancy could be due to deviations from the thin absorber conditions of themeasurements, it should be mentioned that similar differences were found earlierin studies of Lu Fe Si , FeSe . Te . and Fe doped YBa Cu O . compounds.[7, 38, 39] This discrepancy may be explained by the fact the area reflects theaverage mean-square displacements, whereas IS is related to the mean-squarevelocity of the M¨ossbauer atom. Both quantities may respond in a different wayto lattice anharmonicities.The temperature dependent linewidth of the spectra is shown in Fig. 4d.Overall the linewidth increases by a few percent on cooling from room temper-ature to the base temperature. The observed spectral lines for CaKFe As aresomewhat narrower than those in CaFe As [14, 24] and KFe As (see below)single crystal measurements, and measurably sharper than the M¨ossbauer spec-tra lines in the substituted Ca(Fe . Co . ) As [40] that vary in the rangeof 0.28 - 0.35 mm/s between room temperature and 5 K. This suggests thatCaKFe As crystals used in this work are well ordered.In the AFe As (A = Ba, Sr, Ca, Cs, Rb, K) compounds the point symmetry( − m
2) and the location of the Fe site in the crystal structure constrains theprincipal axis of the local electric field gradient tensor to the c -crystalline axis;as a result, a doublet lines intensity ratio of 3:1 is expected for the mosaic withthe c - axis parallel to the γ - beam. Per contra, in the CaKFe As the pointsymmetry (2 mm ) of the Fe site formally does not impose such constrain [18] andsome deviation from the 3 : 1 ratio is expected. This said, the Fe - As1 and Fe- As2 bond lengths as well as As1 - Fe -As1 and As2 - Fe - As2 bonds angles arevery similar and we would not expect significant difference from the AFe As case. The experimentally observed room temperature ratio is ∼ . ∼ . As single crystals[14, 25, 41] and several possible reasons for the doublet lines intensity ratiobeing different from 3:1 were discussed, e.g. a thick absorber conditions of themeasurements and some misorientation of the crystals that form the absorbermosaic. The same arguments, in addition to the different point group symmetryfor Fe, are probably appropriate when considering the CaKFe As results. As Fig. 5 shows a subset of M¨ossbauer spectra of KFe As taken at differenttemperatures. The asymmetry is even less pronounced than in the CaKFe As spectra above. Still, good fit of the data can be obtained by using a doubletwith small quadrupole splitting. For KFe As , similarly to the CaKFe As ,the principal axis of the local electric field gradient tensor should be parallelto the c -crystalline axis and the doublet lines intensity ratio of 3:1 is expected.If the fits are performed with this ratio left as a free parameter, within the5rror bars the expected A /A A /A IS , from room temperature to the basetemperatures are very similar.The linewidth and its temperature dependence (Fig. 6d) are similar to thoseobserved for CaKFe As . The temperature dependent isomer shift and spectralarea are well fit using the Debye model, as described above (Fig. 6a,c). Thesefits yield the values of Θ D of 474 ±
20 K (from IS ( T )) and 325 ± As , theDebye temperatures are higher in KFe As , suggesting that the lattice is stiffer.The values of quadrupole splitting (Fig. 6b) for KFe As are significantlysmaller than those for CaKFe As , moreover QS decreases with decrease oftemperature, as opposed to increase following the “ T / law” in CaKFe As .Although the theoretical foundations of the empirical “ T / law” are not wellunderstood and different (constant, vs T / ) QS ( T ) behavior has been observedfor related (Ce . FeCo Sb vs Ce . Fe Sb ) materials,[36] this observationin iron-arsenides calls for further studies. As , KFe As , and CaFe As Since, naively speaking, the CaKFe As structure can be viewed as beingcostructed from the alternating slabs of CaFe As and KFe As structures, itwould be of use to compare the Fe hyperfine parameters of these three com-pounds. Whereas CaKFe As and KFe As do not exhibit static magnetic or-der or a structural transition below room temperature, CaFe As is known tobe more complex. CaFe As grown out of Sn flux [42] exhibits concomitantstructural (high temperature tetragonal to low temperature orthorhombic) andmagnetic (paramagnetic to low temperature antiferromagnetic) transitions at ≈
173 K. [42, 43]. In the following we will refer to this sample as CaFe As -AFM and use the hyperfine parameters from the single crystal work, Ref. [24].In the CaFe As sample grown out of FeAs flux, by judicious choice of annealing/ quenching conditions [41] we can stabilize low temperature ambient pressurecollapsed tetragonal (cT) phase with the structural transition at ≈
90 K. Thissample will be referred in the following as CaFe As - cT, and the hyperfineparameters from the ref. [14] will be used.The hyperfine parameters for the CaKFe As , KFe As , and CaFe As com-pounds at room temperature and the base temperature are summarized in theTable 1. The temperature dependencies are presented in the plots below. Thespectral linewidths of these compounds are very similar (Fig. 7). The smallestone is observed for CaKFe As , possibly pointing out to very well formed crys-tals. The isomer shift in CaKFe As (Fig. 8). has values in between those forKFe As and CaFe As . Note that the IS ( T ) for CaFe As - cT has a smallbut distinct feature associated with the cT transition. Similarly, the quadrupolesplitting of CaKFe As (Fig. 9) has values in between the values for two other6ompounds. Both, CaFe As - cT and CaFe As - AFM have clear featuresassociated with the cT and the structural / AFM transitions, respectively. Itis curious (although it might be a mere coincidence) that QS ( T ) of CaKFe As and the absolute values of QS ( T ) of CaFe As - AFM below the structural /AFM transition are laying basically on top of each other. As for the overalltemperature behavior, it appears that only for KFe As QS ( T ) decreases withdecrease of temperature. It would be of interest to see if different temperaturedependences are observed in quadrupole frequencies when measured in thesematerials by As nuclear magnetic resonance. As for normalized (at the re-spective base temperatures) spectral areas (Fig. ?? ), again, CaFe As - cTand CaFe As - AFM have anomalies at the corresponding transitions. Thedata points for CaKFe As , CaFe As - cT and CaFe As - AFM (below thestructural / AFM transition) are, grossly speaking, following the same tempera-ture dependence. The data for KFe As are somewhat distinct, pointing eitherto stiffer phonon spectrum or some additional contribution to the temperaturedependence of the spectral area in this compound.
4. Summary
The measurements of Fe M¨ossbauer spectra on oriented mosaics of sin-gle crystals of CaKFe As and KFe As were performed and the results werecompared with the literature data for CaFe As .CaKFe As can be characterized as a well formed compound with narrowspectral lines and no traces of other, Fe - containing, secondary phases in thespectra. There is no feature in hyperfine parameters at T c and no indica-tion of static magnetic order. The values of the Fe hyperfine parameters ofCaKFe As are in between those for KFe As and CaFe As , in agreement withthe gross view of CaKFe As as a structural analog of KFe As and CaFe As with alternating Ca - and K - layers in the structure. The QS ( T ) generallyfollows the empirical “ T / law”. Debye fits of the temperature dependenciesof the isomer shift and the spectral area yield the Debye temperatures of ∼ ∼
247 K respectively.KFe As has smaller quadrupole splitting and isomer shift in comparisonwith CaKFe As and CaFe As . Its QS decreases slightly on cooling that dif-fers from the generic behavior observed in many non-cubic metals. The Debyetemperatures evaluated from the temperature dependent IS and spectral areaare ∼
474 K and ∼
325 K respectively, these values being ∼
100 K higher thanthose for CaKFe As . Acknowledgments
This work was supported by the U.S. Department of Energy, Office of BasicEnergy Science, Division of Materials Sciences and Engineering. The researchwas performed at the Ames Laboratory. Ames Laboratory is operated for the7.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. In addition, W. R. M. was supported by the Gordon andBetty Moore Foundations EPiQS Initiative through Grant GBMF4411.
ReferencesReferences [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130(2008) 3296.[2] P. C. Canfield and S. L. Bud’ko, Annu. Rev. Condens. Matter Phys. 1 (2010)27.[3] D. C. Johnston, Adv. Phys. 59 (2010) 803.[4] G. R. Stewart, Rev. Mod. Phys. 83 (2011) 1589.[5] N.-L. Wang, H. Hosono and P.-C. Dai (eds.), Iron-based Superconductors.Materials, Properties and Mechanisms (Pan Stanford Publishing , Boca Ra-ton, FL , 2012) .[6] P. P. Craig, R. D. Taylor, and D. E. Nagle, Nuovo Cim. 22 (1961) 402.[7] Xiaoming Ma, Sheng Ran, Hua Pang, Fashen Li, Paul C. Canfield, andSergey L. Bud’ko, J. Phys. Chem. Solids 83 (2015) 58.[8] Israel Nowik, Israel Felner, Physica C 469 (2009) 485.[9] M. G. Kozin, I. L. Romashkina, Izv. Ros. Akad. Nauk, Ser. Fiz. 74 (2010)360 [Bull. Rus. Acad. Sci.: Physics 74 (2010) 330].[10] A. B(cid:32)lachowski, K. Ruebenbauer, and J. ˙Zukrowski, The Annales UMCS,Sectio AAA - Physica 66 (2011) 125.[11] Amar Nath and Airat Khasanov, in: M¨ossbauer Spectroscopy: Applica-tions in Chemistry, Biology, and Nanotechnology, edited by Virender K.Sharma, Gostar Klingelhofer, and Tetsuaki Nishida (John Wiley & Sons,Inc., Hoboken, NJ, 2013), p. 535.[12] A. K. Jasek, K. Kom¸edera, A. B(cid:32)lachowski, K. Ruebenbauer, J. ˙Zukrowski,Z. Bukowski, and J. Karpinski, Philos. Mag. 95 (2015) 493.[13] Marianne Rotter, Marcus Tegel, Inga Schellenberg, Falko M. Schappacher,Rainer P¨ottgen, Joachim Deisenhofer, Axel G¨unther, Florian Schrettle, AloisLoidl, and Dirk Johrendt, New J. Phys. 11 (2009) 125014.[14] Sergey L. Bud’ko, Xiaoming Ma, Milan Tomi´c, Sheng Ran, Roser Valent´ı,and Paul C. Canfield, Phys. Rev. B 93 (2016) 024516.815] Israel Felner, Israel Nowik, Bing Lv, Joshua H. Tapp, Zhongjia Tang, andArnold M. Guloy, Hyperfine Int. 191 (2009) 61.[16] D. H. Ryan, W. N. Rowan-Weetaluktuk, J. M. Cadogan, R. Hu, W. E.Straszheim, S. L. Bud’ko, and P. C. Canfield, Phys. Rev. B 83 (2011) 104526.[17] Vadim Ksenofontov, Gerhard Wortmann, Sergey A. Medvedev, VladimirTsurkan, Joachim Deisenhofer, Alois Loidl, and Claudia Felser, Phys. Rev.B 84 (2011) 180508.[18] Akira Iyo, Kenji Kawashima, Tatsuya Kinjo, Taichiro Nishio, ShigeyukiIshida, Hiroshi Fujihisa, Yoshito Gotoh, Kunihiro Kihou, Hiroshi Eisaki,and Yoshiyuki Yoshida, J. Amer. Chem. Soc. 138 (2016) 3410.[19] D. M. Wang, X. C. Shangguan, J. B. He, L. X. Zhao, Y. J. Long, P. P.Wang, and L. Wang, J. Supercond. Nov. Magn. 26 (2013) 2121.[20] K. Momma and F. Izumi, J. Appl. Crystallogr., 44 (2011) 1272.[21] Yi Liu, Ya-Bin Liu, Zhang-Tu Tang, Hao Jiang, Zhi-Cheng Wang, AbduweliAblimit, Wen-He Jiao, Qian Tao, Chun-Mu Feng, Zhu-An Xu, and Guang-Han Cao, Phys. Rev. B 93 (2016) 214503.[22] Yi Liu, Ya-Bin Liu, Qian Chen, Zhang-Tu Tang, Wen-He Jiao, Qian Tao,Zhu-An Xu, Guang-Han Cao, Science Bulletin 61 (2016) 1213.[23] W. R. Meier, T. Kong, U. S. Kaluarachchi, V. Taufour, N. H. Jo, G.Drachuck, A. E. B¨ohmer, S. M. Saunders, A. Sapkota, A. Kreyssig, M. A.Tanatar, R. Prozorov, A. I. Goldman, Fedor F. Balakirev, Alex Gurevich,S. L. Bud’ko, and P. C. Canfield, Phys. Rev. B 94 (2016) 064501.[24] Xiaoming Ma, Sheng Ran, Paul C. Canfield, Sergey L. Bud’ko, J. AlloysCompd 657 (2016) 379.[25] M. Alzamora, J. Munevar, E. Baggio-Saitovitch, S. L. Bud’ko, Ni Ni, P. C.Canfield, and D. R. S´anchez, J. Phys.: Cond. Mat. 23 (2011) 145701.[26] Paul C. Canfield, Tai Kong, Udhara S. Kaluarachchi, and Na Hyun Jo,Philos. Mag. 96 (2016) 84.[27] Tai Kong, Sergey L. Bud’ko, and Paul C. Canfield, Phys. Rev. B 91 (2015)020507.[28] Kunihiro Kihou, Taku Saito, Shigeyuki Ishida, Masamichi Nakajima, Ya-suhide Tomioka, Hideto Fukazawa, Yoh Kohori, Toshimitsu Ito, Shin-ichiUchida, Akira Iyo, Chul-Ho Lee, and Hiroshi Eisaki, J. Phys. Soc. Jpn. 79(2010) 124713.[29] Yong Liu, M. A. Tanatar, V. G. Kogan, Hyunsoo Kim, T. A. Lograsso, andR. Prozorov, Phys. Rev. B 87 (2013) 134513.930] Z. Klencz´ar, MossWinn 4.0 Manual (2016).[31] C. Prescher, C. McCammon and L. Dubrovinsky, J. Appl. Cryst. 45 (2012)329.[32] Philipp G¨utlich, Eckhard Bill, and Alfred X. Trautwein,
M¨ossbauer Spec-troscopy and Transition Metal Chemistry. Fundamentals and Applications ,Springer-Verlag, Berlin, Heidelberg, (2011).[33] R. Vianden, Hyperfine Int. 15/16 (1983) 189.[34] K. Nishiyama, F. Dimmling, Th. Kornrumpf, and D. Riegel, Phys. Rev.Lett. 37 (1976) 357.[35] H. C. Verma and G. N. Rao, Hyperfine Int. 15/16 (1983) 207.[36] Gary J. Long, Dimitri Hutot, Fernande Grandjean, Donald T. Morelli, andGregory P. Meisner, Phys. Rev. B 60 (1999) 7410.[37] Ichiro Tamura, Tsuyoshi Ikeno, Toshio Mizushima, and Yosikazu Isikawa,J. Phys. Soc. Jpn. 81 (2012) 074703.[38] J. Lind´en, J.-P. Lib¨ack, M. Karppinen, E.-L. Rautama, H. Yamauchi, SolidState Commun. 151 (2011), 130.[39] V.M. Cherepanov, M.A. Chuev, E. Yu. Tsymbal, Ch. Sauer, W. Zinn, S.A.Ivanov, V.V. Zhurov, Solid State Commun. 93 (1995) 921.[40] A. E. B¨ohmer, A. Sapkota, A. Kreyssig, S. L. Bud’ko, G. Drachuck, S. M.Saunders, A. I. Goldman, P. C. Canfield, preprint, arXiv:1612.07341 (2016),Phys. Rev. Lett. - in press.[41] S. Ran, S. L. Bud’ko, D. K. Pratt, A. Kreyssig, M. G. Kim, M. J. Kramer,D. H. Ryan, W. N. Rowan-Weetaluktuk, Y. Furukawa, B. Roy, A. I. Gold-man, and P. C. Canfield, Phys. Rev. B 83 (2011) 144517.[42] N. Ni, S. Nandi, A. Kreyssig, A. I. Goldman, E. D. Mun, S. L. Bud’ko, andP. C. Canfield, Phys. Rev. B 78 (2008) 014523.[43] A. I. Goldman, D. N. Argyriou, B. Ouladdiaf, T. Chatterji, A. Kreyssig,S. Nandi, N. Ni, S. L. Bud’ko, P. C. Canfield, and R. J. McQueeney, Phys.Rev. B 78 (2008) 100506. 10 able 1: Hyperfine parameters of CaKFe As , KFe As , and CaFe As at room temperatureand base temperature sample T (K) IS (mm/s) QS (mm/s) FWHM (mm/s) Ref.CaKFe As
297 0.372(2) 0.140(3) 0.232(3) this work4.8 0.511(2) 0.149(2) 0.251(3) this workKFe As
296 0.311(1) 0.113(4) 0.244(3) this work4.7 0.431(3) 0.05(1) 0.265(4) this workCaFe As - AFM 296 0.448(2) 0.202(2) 0.254(3) [24]4.6 0.5708(7) -0.159(1) 0.280(2) [24]CaFe As - cT 293 0.430(2) 0.225(3) 0.269(3) [14]4.6 0.5902(5) 0.2758(7) 0.274(1) [14]11 igure 1: (Color online) Crystal structure of CaKFe As sketched using VESTA [20]. M/H (emu/mol)
T ( K )
C a K F e A s H | | a b , H = 5 0 O eF CZ F C
Figure 2: Low field, low temperature zero-field-cooled (ZFC) and field cooled (FC) DC sus-ceptibility of one of the CaKFe As crystals used in the mosaic for M¨ossbauer measurements. absorbtion C a K F e A s v ( m m / s ) Figure 3: (Color online) Fe Mossbauer spectra of CaKFe As at selected temperatures.Symbols-data, lines-fits. . 40 . 5 IS (mm/s)
C a K F e A s ( a ) QS (mm/s) ( b ) area (mm/s) ( c )
FWHM (mm/s) ( d )
A1/A2
T ( K )( e )
Figure 4: (Color online) Temperature dependencies of the hyperfine parameters obtained fromfits of Fe Mossbauer spectra of CaKFe As at different temperatures: (a) isomer shift, (b)quadrupole splitting, (c) area normalyzed to the baseline, (d) linewidth (full width at halfmaximum), and (e) intensity ratio of the doublet lines. Symbols: data, lines (a), (c) Debyefits (see text), (b)“ T / law” (see text), (d), (e) linear fits that serve as guide to the eye. absorbtion K F e A s v ( m m / s ) Figure 5: (Color online) Fe Mossbauer spectra of KFe As at selected temperatures.Symbols-data, lines-fits. . 30 . 4 IS (mm/s)
K F e A s ( a ) QS (mm/s) ( b ) area (mm/s) ( c )
T ( K )
FWHM (mm/s) ( d )
Figure 6: (Color online) Temperature dependencies of the hyperfine parameters obtainedfrom fits of Fe Mossbauer spectra of KFe As at different temperatures: (a) isomer shift,(b) quadrupole splitting, (c) area normalyzed to the baseline, (d) linewidth (full width at halfmaximum). Symbols: data, lines (a), (c) Debye fits (see text), (b) and (d) linear fits thatserve as guide to the eye. C a K F e A s K F e A s C a F e A s - A F M C a F e A s - c T FWHM (mm/s)
T ( K )
Figure 7: (Color online) Temperature dependencies of the linewidth (full width at half maxi-mum) obtained from fits of Fe Mossbauer spectra of CaKFe As , KFe As (this work) andCaFe As [14, 24] at different temperatures. C a K F e A s K F e A s C a F e A s - A F M C a F e A s - c T IS (mm/s)
T ( K )
Figure 8: (Color online) Temperature dependencies of the isomer shift obtained from fitsof Fe Mossbauer spectra of CaKFe As , KFe As (this work) and CaFe As [14, 24] atdifferent temperatures. C a K F e A s K F e A s C a F e A s - A F M C a F e A s - c T QS (mm/s)
T ( K )
Figure 9: (Color online) Temperature dependencies of the quadrupole splitting obtained fromfits of Fe Mossbauer spectra of CaKFe As , KFe As (this work) and CaFe As [14, 24] atdifferent temperatures. Note that for CaFe As - AFM the absolute values of the quadrupolesplitting are plotted in the magnetically ordered state. C a K F e A s K F e A s C a F e A s - A F M C a F e A s - c T normalized spectral area T ( K )
Figure 10: (Color online) Temperature dependencies of the spectral area normalyzed to thecorresponding value at the base temperature (4.6 K - 4.8 K) obtained from fits of Fe Moss-bauer spectra of CaKFe As , KFe As (this work) and CaFe As [14, 24] at different tem-peratures.[14, 24] at different tem-peratures.