Superconducting Transition Temperatures of up to 47 K from Simultaneous Rare-Earth Element and Antimony Doping of 112-Type CaFeAs2
Kazutaka Kudo, Yutaka Kitahama, Kazunori Fujimura, Tasuku Mizukami, Hiromi Ota, Minoru Nohara
aa r X i v : . [ c ond - m a t . s up r- c on ] A ug Journal of the Physical Society of Japan
LETTERS
Superconducting Transition Temperatures of up to 47 K from SimultaneousRare-Earth Element and Antimony Doping of 112-Type CaFeAs Kazutaka Kudo , ∗ , Yutaka Kitahama , Kazunori Fujimura ,Tasuku Mizukami , Hiromi Ota , and Minoru Nohara , † Department of Physics, Okayama University, Okayama 700-8530, Japan Research Center of New Functional Materials for Energy Production, Storage and Transport, Okayama University,Okayama 700-8530, Japan Division of Instrumental Analysis, Department of Instrumental Analysis & Cryogenics, Advanced Science ResearchCenter, Okayama University, Okayama 700-8530, Japan
The e ff ects of simultaneous Sb doping on the superconductivity of 112-type Ca − x RE x FeAs ( RE = La, Ce, Pr, andNd) were studied through measurements of the magnetization and electrical resistivity. In Sb-free materials, the super-conducting transition temperature T c of the La-doped sample was 35 K, while those of the Pr- and Nd-doped sampleswere ∼
10 K; no superconductivity was observed in the Ce-doped sample. Sb doping increased the T c of all RE -dopedsamples: T c increased to 47, 43, 43, and 43 K for RE = La, Ce, Pr, and Nd, respectively. We also found that the enhancedsuperconductivity results from the increase in the lattice parameter b , which increases the As-Fe-As bond angle to becloser to the ideal tetrahedron value. These observations provide insight for further increasing the T c of the 112 phase. The discovery of iron-based superconductors has stimu-lated the development of novel superconducting materials such as RE FeAsO (1111 type), AE Fe As (122 type),
5, 6 A FeAs (111 type), and FeSe (11 type) that contain rare-earth ( RE ), alkali-earth ( AE ), and alkali ( A ) elements, as wellas the development of compounds with perovskite- and / orrocksalt-type and pyrite-type spacer layers. For thisclass of materials, the maximum superconducting transitiontemperature T c is 55 K, and new iron-based superconduct-ing materials will need to be developed to further increase T c .To this end, novel 112-type iron arsenides ofCa − x La x FeAs , Ca − x Pr x FeAs , and Ca − x RE x FeAs ( RE = Ce, Nd, Sm, Eu, and Gd) reported by Katayama etal. , Yakita et al. , and Sala et al. , respectively, havereceived considerable attention, and it has been recognizedthat the RE substitution is necessary for stabilizing the 112phase. These compounds crystallize in a monoclinicstructure with a space group of P (No. 4) or P / m (No.11)
21, 22 and consist of alternately stacked FeAs and arseniczigzag bond layers, which are a notable feature. The arseniczigzag bond layers are considered to be composed of As − ions with a 4 p configuration as found in RET As ( T = Ag, Au). Thus, the chemical formula for these compoundscan be written as (Ca + − x RE + x )(Fe + As − )As − · xe − with anexcess charge of xe − / Fe injected into the Fe + As − layers.Most 112-type iron arsenides exhibit superconductivity:La-doped compounds show bulk superconductivity at 35 K, while Pr-, Nd-, Sm-, Eu-, and Gd-doped compounds showsuperconductivity at 10–15 K with a small shielding volumefraction (VF) of 5–20%.
21, 22
Ce-doped compounds rarely ∗ [email protected] † [email protected] exhibit superconductivity. Recently, it was reported that the simultaneous doping ofisovalent P or Sb drastically improves the superconductivityin the La-doped 112 phase; T c increased to 41 and 43 K asa result of 0.5% P and 1% Sb doping, respectively. In thisLetter, we report that a large amount of Sb doping further in-creases the T c of Ca − x La x Fe(As − y Sb y ) to 47 K, which isthe second highest T c after 1111-type iron-based supercon-ductors. Moreover, we show that bulk superconductivity at43, 43, and 43 K is induced by the simultaneous Sb dopingof Ca − x RE x FeAs with RE = Ce, Pr, and Nd, respectively,and that an increase in the lattice parameter b , which modi-fies the As-Fe-As bond angle, is important for optimizing thesuperconductivity in the 112 phase.Single crystals of Ca − x RE x Fe(As − y Sb y ) ( RE = La, Ce,Pr, and Nd) were grown by heating a mixture of Ca, RE ,FeAs, As, and Sb powders with nominal compositions of x = . ≤ y ≤ .
10 (grown quantities of the112 phase drastically decrease for y ≥ . ◦ C for 3 h, heated to 1100 ◦ Cat a rate of 46 ◦ C / h, and then cooled to 1050 ◦ C at a rateof 1.25 ◦ C / h before furnace cooling. The obtained sampleswere characterized by powder X-ray di ff raction (XRD) us-ing a Rigaku RINT-TTR III X-ray di ff ractometer with Cu K α radiation and by single-crystal XRD using a Rigaku Sin-gle Crystal X-ray Structural Analyzer (Varimax with Saturn).Ca − x RE x Fe(As − y Sb y ) samples were obtained together witha powder mixture of RE As, FeAs, FeAs , and CaFe As . Thesingle crystals of Ca − x RE x Fe(As − y Sb y ) separated from the
1. Phys. Soc. Jpn.
LETTERS -1.0-0.50.0 π M / H ( e m u / c m ) x La x Fe(As y Sb y ) x = 0.16, y = 0.00 x = 0.15, y = 0.01 x = 0.12, y = 0.10
30 Oe H // ab ρ ab ( m Ω c m ) x La x Fe(As y Sb y ) x = 0.16, y = 0.00 x = 0.15, y = 0.01 x = 0.12, y = 0.10 Fig. 1. (Color online) (a) Temperature dependence of the magnetization M of Ca − x La x Fe(As − y Sb y ) measured at a magnetic field H of 30 Oeparallel to the ab plane under zero-field-cooling and field-cooling con-ditions. (b) Temperature dependence of the electrical resistivity ρ ab ofCa − x La x Fe(As − y Sb y ) parallel to the ab plane. mixture were plate-like with typical dimensions of (0.3–0.7) × (0.3–0.7) × .The RE content x was analyzed by energy-dispersive X-ray spectrometry (EDS), but the Sb content y could not bedetermined by EDS because the Sb peak positions in the EDSspectra overlapped those of Ca. We thus assumed a nomi-nal y . Although the nominal x was fixed at 0.1, the x valuesmeasured by EDS varied with the nominal y . For example,Ca − x La x Fe(As − y Sb y ) had compositions of x = y = y . Incontrast, for Ca − x RE x FeAs ( RE = Ce, Pr, and Nd), we foundcompositions of x = y = x and y .The magnetization M in a magnetic field of 30 Oe parallelto the ab plane was measured using a Quantum Design mag-netic property measurement system (MPMS). The T c values V o l u m e ( Å ) x (b)706050403020100 T c ( K ) Ca x La x Fe(As y Sb y ) y = 0.100.010.00 (a) 949290 β ( ° ) x (e)10.510.410.310.2 c ( Å ) (d)4.003.953.903.85 a , b ( Å ) ab (c) Fig. 2. (Color online) Dependence of the (a) T c , (b) cell volume, (c) a and b parameters, (d) c parameter, and (e) β angle of Ca − x La x Fe(As − y Sb y ) on x . were determined from the onset of the diamagnetism, whichis characteristic of the superconducting transition. Electricalresistivity ρ ab parallel to the ab plane was also measured us-ing a standard DC four-terminal method in a Quantum Designphysical property measurement system (PPMS).The enhancement in the superconductivity of the Sb-dopedCa − x La x FeAs can be observed in the temperature depen-dence of M shown in Fig. 1(a). Previous studies
20, 24 havereported that La-doped samples with y = T c =
34 K, and T c increased to 43K for y = The values of VF at the lowest tempera-ture were previously estimated to be 66% and 78% for y = and 0.01, respectively, indicating bulk superconduc-tivity. We found that a large amount of Sb doping leads to afurther increase in T c to 47 K. As shown in Fig. 1(a), the La-doped sample with y = T c =
47 K, and the VF at 2 K was estimated to beapproximately 100%, which supports the emergence of bulksuperconductivity. Further evidence of the enhanced super-conductivity was obtained from the temperature dependenceof ρ ab , as shown in Fig. 1(b). We found that ρ ab for the La-doped y = y = and 0.01 samples.The increased T c of Ca − x La x Fe(As − y Sb y ) can be ex-plained by two e ff ects originating from the simultaneous Sbdoping: a decrease in the La content and an increase in thecell volume. In Ca − x La x FeAs , it is known that T c increaseswith decreasing x and exhibits a maximum value of 35 K atthe lowest x of 0.15, as shown in Fig. 2(a); a sample with alower x could potentially have a higher T c . Simultaneous Sb
2. Phys. Soc. Jpn.
LETTERS -0.50.0 π M / H ( e m u / c m ) (b) Ca x Pr x Fe(As y Sb y ) x = 0.20, y = 0.00 x = 0.18, y = 0.01 x = 0.24, y = 0.05 -0.50.0 π M / H ( e m u / c m ) x Nd x Fe(As y Sb y ) x = 0.19, y = 0.00 x = 0.19, y = 0.01 x = 0.16, y = 0.05 -0.50.0 π M / H ( e m u / c m ) x = 0.19, y = 0.00 x = 0.16, y = 0.01 x = 0.24, y = 0.10 (a) Ca x Ce x Fe(As y Sb y )
30 Oe H // ab ρ ab ( m Ω c m ) x Nd x Fe(As y Sb y ) x = 0.19, y = 0.00 x = 0.19, y = 0.01 x = 0.16, y = 0.05 ρ ab ( m Ω c m ) x = 0.20, y = 0.00 x = 0.18, y = 0.01 x = 0.24, y = 0.05 (e) Ca x Pr x Fe(As y Sb y ) ρ ab ( m Ω c m ) (d) Ca x Ce x Fe(As y Sb y ) x = 0.19, y = 0.00 x = 0.16, y = 0.01 x = 0.25, y = 0.05 Fig. 3. (Color online) (a)(b)(c) Temperature dependence of the magneti-zation M of Ca − x RE x Fe(As − y Sb y ) ( RE = Ce, Pr, and Nd) measured at amagnetic field H of 30 Oe parallel to the ab plane under zero-field-coolingand field-cooling conditions. (d)(e)(f) Temperature dependence of the electri-cal resistivity ρ ab of Ca − x RE x Fe(As − y Sb y ) ( RE = Ce, Pr, and Nd) parallelto the ab plane. doping allows the La content to be reduced to x = T c increased to 47 K. In general, chemical sub-stitution modifies the number of charge carriers and induces achemical pressure. The primal role of La doping is charge car-rier modification because the cell volume [Fig. 2(b)] and lat-tice parameters [Figs. 2(c), (d), and (e)] of Ca − x La x FeAs exhibit no significant changes upon La doping owing to thesimilar ionic radii of Ca + and La + . Thus, a decrease in theLa content corresponds to a reduction in the number of chargecarriers. Secondly, simultaneous Sb doping applies a negativechemical pressure that increases the cell volume, as shownin Fig. 2(b), because the ionic radius of Sb − (Sb − ) is largerthan that of As − (As − ). The increased cell volume is a re-sult of an increase in the in-plane lattice parameters a and b , as shown in Fig. 2(c); in contrast, the c parameter hardlychanges, as shown in Fig. 2(d). In iron-based superconduc-tors, the expansion can e ff ectively optimize the superconduc-tivity, which is sensitive to modifications in the crystal struc-ture. Note that the enhancement of T c by 0.5% P dopingin the La-doped system, which was reported in our previousarticle, should be attributed to a di ff erent mechanism be-cause the small amount of P doping neither reduced the Lacontent nor changed the lattice parameters. However, the ex-act mechanism is still unclear.A similar enhancement in the superconductivity was alsoobserved in Sb-doped Ca − x RE x FeAs ( RE = Ce, Pr, and Nd).As shown in Figs. 3(a), (b), and (c), in the Sb-free samples, a , b ( Å ) ab (a)949290 β ( ° ) RE La Ce Pr Nd(c)10.510.410.310.2 c ( Å ) (b) Fig. 4. (Color online) Dependences of the (a) a and b parameters, (b) c pa-rameter, and (c) β angle of Ca − x RE x Fe(As − y Sb y ) ( RE = La, Ce, Pr, andNd) on RE . The open and closed symbols indicate the lattice parameters ofSb-free samples and Sb-doped samples, respectively. The dotted and brokenlines are, respectively, guides to the eye for the lattice parameters of Sb-freesamples and Sb-doped samples with the highest T c for each RE -doped sys-tems. the Ce-doped system shows no bulk superconductivity downto 2 K, while the Pr- and Nd-doped systems exhibit super-conductivity at 10 and 11 K, respectively, with a small VFof 5%. These results are consistent with previous reports.
21, 22
Sb doping resulted in higher T c values of 21 and 43 K in Ce-doped systems of y = y = y = T c of higher than 40 K irrespective of RE . More im-portantly, Sb substitution resulted in a substantial increase inthe VF, indicating the emergence of bulk superconductivity.Evidence of the enhanced superconductivity was also foundin the temperature dependence of ρ ab . As shown in Figs. 3(d),(e), and (f), ρ ab of Ca − x RE x Fe(As − y Sb y ) ( RE = Ce, Pr, andNd) with y = − x Ce x FeAs sample is attributed tofilamentary superconductivity because there is no visible dia-magnetic signal at T c . In Ca − x RE x Fe(As − y Sb y ) ( RE = Ce,
3. Phys. Soc. Jpn.
LETTERS
Pr, and Nd), while we have observed the importance of thevolume e ff ect [Figs. 4(a) and (b)] in the enhanced supercon-ductivity in the same manner as discussed for the La-dopedsystem, the precise x and y dependences of the T c are stillunclear.On considering the well-known relation between T c and lo-cal structure in the iron-based superconductors, the mostimportant factor that determines the volume e ff ect, which en-hances superconductivity, is the increase in the b parameter.In general, iron-based superconductors with high T c , suchas 1111-type superconductors with T c >
50 K, satisfy thefollowing structural conditions: the Fe and As atoms forman ideal tetrahedral structure (i.e., the As-Fe-As bond angle α = ◦ )
25, 26 and the As height with respect to the Feplane, h Pn , is approximately 1.38 Å.
27, 28
Because of the mon-oclinic structure, the 112 phase possesses two bond angles: α a and α b , which correspond to the α -angle along the a and b axes, respectively. Our preliminary structural analysis ofCa − x La x Fe(As − y Sb y ) with T c =
47 K showed that α b isdrastically improved by Sb doping: α a = ◦ and α b = ◦ in the Sb-free sample, while α a = ◦ and α b = ◦ in the Sb-doped sample. The result suggeststhat the increase in b parameter [Fig. 4(a)], which increases α b , is a key factor for enhancing the superconductivity in the112 phase. On the other hand, Sb doping does not modify h Pn as much, because the c parameter is almost insensitive toSb doping [Fig. 4(b)]. In both Sb-free and Sb-doped samples,the h Pn values are slightly larger than the ideal value: h Pn = and Sb-doped samples,respectively. We expect that the T c of the 112 phase can beincreased above 50 K, if the b parameter can be increased toapproximately equal a and, simultaneously, the c parametercan be slightly decreased.In summary, the e ff ects of Sb doping on the T c of 112-type Ca − x RE x FeAs ( RE = La, Ce, Pr, and Nd) were studiedthrough measurements of the magnetization and electrical re-sistivity. Sb-doping results in an increase in T c such that the T c values of Sb-doped Ca − x RE x FeAs with RE = La, Ce, Pr,and Nd were 47, 43, 43, and 43 K, respectively. We found thatan increase in the b parameter, which improves the As-Fe-Asbond angle, is important for enhancing the superconductivityin the 112 phase. Acknowledgments
This work was partially supported by Grants-in-Aidfor Scientific Research (B) (26287082) and (C) (25400372) from the JapanSociety for the Promotion of Science (JSPS) and the Funding Program forWorld-Leading Innovative R&D on Science and Technology (FIRST Pro-gram) from JSPS.1) K. Ishida, Y. Nakai, and H. Hosono, J. Phys. Soc. Jpn. , 062001(2009).2) J. Paglione and R. L. Greene, Nat. Phys. , 645 (2010).3) D. C. Johnston, Adv. Phys. , 803 (2010).4) Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem.Soc. , 3296 (2008).5) M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. , 107006 (2008).6) K. Kudo, K. Iba, M. Takasuga, Y. Kitahama, J. Matsumura, M. Danura,Y. Nogami, and M. Nohara, Sci. Rep. , 1478 (2013).7) J. H. Tapp, Z. Tang, B. Lv, K. Sasmal, B. Lorenz, P. C. W. Chu, and A.M. Guloy, Phys. Rev. B , 060505(R) (2008).8) F.-C. Hsu, J.-Y. Luo, K.-W. Yeh, T.-K. Chen, T.-W. Huang, P. M. Wu, Y.-C. Lee, Y.-L. Huang, Y.-Y. Chu, D.-C. Yan, and M.-K. Wu, Proc. Natl.Acad. Sci. U.S.A. , 14262 (2008).9) N. Kawaguchi, H. Ogino, Y. Shimizu, K. Kishio, and J. Shimoyama,Appl. Phys. Express , 063102 (2010).10) X. Zhu, F. Han, G. Mu, P. Cheng, B. Shen, B. Zeng, and H.-H. Wen,Phys. Rev. B , 220512(R) (2009).11) H. Ogino, K. Machida, A. Yamamoto, K. Kishio, J. Shimoyama, T. To-hei, and Y. Ikuhara, Supercond. Sci. Technol. , 115005 (2010).12) P. M. Shirage, K. Kihou, C.-H. Lee, H. Kito, H. Eisaki, and A. Iyo, Appl.Phys. Lett. , 172506 (2010).13) S. Kakiya, K. Kudo, Y. Nishikubo, K. Oku, E. Nishibori, H. Sawa, T.Yamamoto, T. Nozaka, and M. Nohara, J. Phys. Soc. Jpn. , 093704(2011).14) M. Nohara, S. Kakiya, K. Kudo, Y. Oshiro, S. Araki, T. C. Kobayashi, K.Oku, E. Nishibori, and H. Sawa, Solid State Commun. , 635 (2012).15) N. Ni, J. M. Allred, B. C. Chan, and R. J. Cava, Proc. Natl. Acad. Sci. , E1019 (2011).16) C. L¨ohnert, T. St¨urzer, M. Tegel, R. Frankovsky, G. Friederichs, and D.Johrendt, Angew. Chem. Int. Ed. , 9195 (2011).17) C. Hieke, J. Lippmann, T. St¨urzer, G. Friederichs, F. Nitsche, F. Winter,R. P¨ottgen, and D. Johrendt, Phil. Mag. , 3680 (2013).18) K. Kudo, D. Mitsuoka, M. Takasuga, Y. Sugiyama, K. Sugawara, N.Katayama, H. Sawa, H. S. Kubo, K. Takamori, M. Ichioka, T. Fujii, T.Mizokawa, and M. Nohara, Sci. Rep. , 3101 (2013).19) Z.-A. Ren, W. Lu, J. Yang, W. Yi, X.-L. Shen, Z.-C. Li, G.-C. Che, X.-L.Dong, L.-L. Sun, F. Zhou, and Z.-X. Zhao, Chin. Phys. Lett. , 2215(2008).20) N. Katayama, K. Kudo, S. Onari, T. Mizukami, K. Sugawara, Y.Sugiyama, Y. Kitahama, K. Iba, K. Fujimura, N. Nishimoto, M. Nohara,and H. Sawa, J. Phys. Soc. Jpn. , 123702 (2013).21) H. Yakita, H. Ogino, T. Okada, A. Yamamoto, K. Kishio, T. Tohei,Y. Ikuhara, Y. Gotoh, H. Fujihisa, K. Kataoka, H. Eisaki, and J. Shi-moyama, J. Am. Chem. Soc. , 846 (2014).22) A. Sala, H. Yakita, H. Ogino, T. Okada, A. Yamamoto, K. Kishio, S.Ishida, A. Iyo, H. Eisaki, M. Fujioka, Y. Takano, M. Putti, and J. Shi-moyama, Appl. Phys. Express , 073102 (2014).23) D. Rutzinger, C. Bartsch, M. Doerr, H. Rosner, V. Neu, Th. Doert, andM. Ruck, J. Solid State Chem. , 510 (2010).24) K. Kudo, T. Mizukami, Y. Kitahama, D. Mitsuoka, K. Iba, K. Fujimura,N. Nishimoto, Y. Hiraoka, and M. Nohara, J. Phys. Soc. Jpn. , 025001(2014).25) C.-H. Lee, A. Iyo, H. Eisaki, H. Kito, M. T. Fernandez-Diaz, T. Ito, K.Kihou, H. Matsuhata, M. Braden, and K. Yamada, J. Phys. Soc. Jpn ,083704 (2008).26) H. Usui and K. Kuroki, Phys. Rev. B , 024505 (2011).27) K. Kuroki, H. Usui, S. Onari, R. Arita, and H. Aoki, Phys. Rev. B ,224511 (2009).28) Y. Mizuguchi, Y. Hara, K. Deguchi, S. Tsuda, T. Yamaguchi, K. Takeda,H. Kotegawa, H. Tou, and Y. Takano: Supercond. Sci. Technol. ,054013 (2010).29) By doping P in a range of more than 1%, the La doped 112 phase wasnot obtained.30) The values of α and h P n were determined on the basis of the crystalstructure with the space group P proposed by Katayama et al . Wehave checked that they are nearly unchanged even when using the spacegroup P / m proposed by Yakita et al .
31) The increased a parameter [Fig. 4(a)] results in an increased α a , butthe nearly ideal condition of α aa