We report superconductivity in the novel 112-type iron-based compound Ca 1Àx La x FeAs 2 . Single-crystal X-ray diffraction analysis revealed that the compound crystallizes in a monoclinic structure (space group P2 1 ), in which Fe 2 As 2 layers alternate with Ca 2 As 2 spacer layers such that monovalent arsenic forms zigzag chains. Superconductivity with a transition temperature (T c ) of 34 K was observed for the x ¼ 0:1 sample, while the x ¼ 0:21 sample exhibited trace superconductivity at 45 K. First-principles band calculations demonstrated the presence of almost cylindrical Fermi surfaces, favorable for the high T c in La-doped CaFeAs 2 .KEYWORDS: iron-based superconductors, Ca-La-Fe-As, 112-type, CaFeAs 2Since the discovery of superconductivity with a transition temperature (T c ) of 26 K in LaFeAsO 1Àx F x , 1) there has been tremendous effort towards synthesizing novel iron pnictide superconductors. [2][3][4][5][6][7][8][9][10][11][12][13][14] All of the iron pnictide superconductors identified so far consist of a common structural motif, i.e., Fe 2 As 2 layers that are alternately stacked with various kinds of spacer layers. Therefore, the central goal for realizing a higher T c has been finding a novel spacer layer that can suitably tune the electronic states of Fe 2 As 2 layers.Recently, superconductivity has been discovered in Ca 10 (Pt n As 8 )(Fe 2Àx Pt x As 2 ) 5 , which consists of As-As dimers with a formal electron count of As 2À in the spacer layer. [15][16][17][18] Because of the 4p 3 electron configuration of elemental arsenic, arsenic can form various bonding structures: (i) Isolated arsenic with a formal electron count of As 3À . Examples include A 3 As (A ¼ Li, Na, and K) and iron-based superconductors. (ii) Dimerized As-As with a single bond. Its formal electron count is As 2À . Sr 2 As 2 and Ca 10 (Pt n As 8 )(Fe 2Àx Pt x As 2 ) 5 with As-As dimer bonds in the spacer layer can be categorized here. (iii) A one-dimensional chain connected by arsenic single bonds with a formal electron count of As À . This category includes KAs as an example. Realizing novel iron-based superconductors with spacer layers composed of complex bonding networks of arsenic such as (iii) has been a longstanding challenge: Shim et al. have theoretically proposed the hypothetical compound BaFeAs 2 (112-type) with spacer layers of the arsenic square network, and suggested that such compounds can be used to examine the role of charge and polarization fluctuations as well as the importance of two-dimensionality in the mechanism of superconductivity. 19) Although the 112-type iron pnictides AEFeAs 2 (AE ¼ Ca, Sr, Ba) have not yet been synthesized, the isostructural compounds RET As 2 (RE = rare-earth elements; T ¼ Cu, Ag, Au) have been studied intensively. 20,21) In this letter, we present a report on the novel 112-type iron-based superconductor Ca 1Àx La x FeAs 2 . Although pure CaFeAs 2 was not obtained, we found that the substitution of a small amount of La for Ca stabilizes the 112 phase. Thus, Ca 1Àx La x FeAs 2 ...

A number of iron-based superconductors have been discovered, 1-3 which include LaFeAsO (1111-type structure), 4 BaFe 2 As 2 (122-type), 5 LiFeAs (111-type), 6 and FeSe (11-type), 7 as well as compounds with complex oxide spacer layers [8][9][10][11] and arsenide spacer layers such as Ca 10 (Pt 4 As 8 )(Fe 2 As 2 ) 5 . [12][13][14][15][16][17] The maximum superconducting transition temperature T c is 55 K of the 1111-type structure. 18 In order to further increase T c , an exploration of novel structure types should be performed.Very recently, Katayama et al. 19 and Yakita et al. 20 have reported superconductivity in Ca 1−x La x FeAs 2 and Ca 1−x Pr x FeAs 2 , respectively, with a novel 112-type structure. Ca 1−x La x FeAs 2 crystalizes in a monoclinic structure with the space group P2 1 (No. 4) and consists of alternately stacked Fe 2 As 2 and arsenic zigzag bond layers. 19 Although pure CaFeAs 2 was not obtained, Katayama et al. found that the substitution of a small amount of La for Ca stabilizes the 112 phase and induces superconductivity at T c = 34 K for x = 0.16. Interestingly, Katayama et al. 19 suggested that the trace superconductivity of Ca 1−x La x FeAs 2 could exhibit T c = 45 K.In this paper, we report that a large increase in T c occurs with the phosphorus or antimony doping of Ca 1−x La x FeAs 2 . P-doped Ca 0.84 La 0.16 FeAs 2 and Sb-doped Ca 0.85 La 0.15 FeAs 2 exhibited T c values of 41 and 43 K, respectively, while P/Sbfree Ca 0.85 La 0.15 FeAs 2 exhibited T c = 35 K.Single crystals of Ca 1−x La x Fe(As 1−y Pn y ) 2 (Pn = P and Sb) were grown by heating a mixture of Ca, La, FeAs, As, P, and Sb powders. A stoichiometric amount of the mixture was placed in an aluminum crucible and sealed in an evacuated quartz tube. The preparation was carried out in a glove box filled with argon gas. Ampules were heated at 700 • C for 3 h, heated to 1100 • C at a rate of 46 • C/h, and cooled to 1050 • C at a rate of 1.25 • C/h, followed by furnace cooling. The obtained samples were characterized by powder Xray diffraction (XRD) analysis, performed using a Rigaku RINT-TTR III X-ray diffractometer with CuK α radiation. The Ca 1−x La x Fe(As 1−y Pn y ) 2 was obtained together with a powder mixture of LaAs, FeAs, FeAs 2 , and CaFe 2 As 2 . We separated platelike single crystals of the present system with typ-

The effects of simultaneous Sb doping on the superconductivity of 112-type Ca 1¹x RE x FeAs 2 (RE = La, Ce, Pr, and Nd) were studied through measurements of the magnetization and electrical resistivity. In Sb-free materials, the superconducting transition temperature T c of the La-doped sample was 35 K, while those of the Pr-and Nd-doped samples were ³10 K; no superconductivity was observed in the Ce-doped sample. Sb doping increased the T c of all REdoped samples: T c increased to 47, 43, 43, and 43 K for RE = La, Ce, Pr, and Nd, respectively. We also found that the enhanced superconductivity results from the increase in the lattice parameter b, which increases the As-Fe-As bond angle to be closer to the ideal tetrahedron value. These observations provide insight for further increasing the T c of the 112 phase.

Co-doping of lanthanum and phosphorus in CaFe2As2 induces superconductivity at 45 K. This superconducting transition temperature is higher than the 38 K transition in Ba1−xKxFe2As2, which is the maximum found thus far among the 122 phases. Superconductivity with a substantial shielding volume fraction was observed at 0.12 ≤ x ≤ 0.18 and y = 0.06 in Ca1−xLaxFe2(As1−yPy)2. The superconducting phase of the present system seems to be not adjacent to an antiferromagnetic phase.

The specific heat of two polymorphs of BaPd 2 As 2 was measured. The ThCr 2 Si 2 -type polymorph (space group I4/mmm, D 17 4h , No. 139) is a previously reported superconductor with a transition temperature T c ≃ 3.5 K, while the CeMg 2 Si 2 -type polymorph (P4/mmm, D 1 4h , No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr 2 Si 2 -type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg 2 Si 2 -type. Moreover, a large specific-heat jump at T c indicated that the superconductivity of ThCr 2 Si 2 -type is a strong-coupling type, which is likely derived from soft phonons.Energetically low-lying phonons often result in strongcoupling superconductivity with an enhanced superconducting transition temperature T c .1-10) Thus, engineering materials to produce low-lying phonons has become an important issue in superconductivity. A promising route to producing low-lying phonons is a local anharmonic vibration of the ion that is weakly bound in a cage-like structure. Such a vibration is called a rattling phonon and characterized by a lowfrequency Einstein mode. A remarkable example is the β-pyrochlore oxide KOs 2 O 6 with T c = 9.6 K, [1][2][3][4] as well as Ba 3 Ir 4 Ge 16 with T c ≃ 6 K. 5,6) Another route to producing low-lying phonons is structural instability that becomes evident from the occurrence of a structural phase transition due to applied pressure or chemical doping. Prominent examples of such superconductors include the 122-type pnictides BaNi 2 (As 1−x P x ) 2 with T c = 3.3 K, 7) BaNi 2 (Ge 1−x P x ) 2 with T c = 2.9 K, 8) and Ba(Ni 1−x Cu x ) 2 As 2 with T c = 3.2 K, 9) all of which exhibit strong-coupling superconductivity because of a structural phase transition and the subsequent phonon softening characterized by an anomalously low Debye frequency. The MnP-type compound IrGe with T c = 4.7 K is a rare example that exhibits strong-coupling superconductivity with lowlying phonons but does not exhibit a structural phase transition.10) Such superconductors suggest how low-lying phonons may be produced for enhancing superconductivity.The 12) The ThCr 2 Si 2 -type structure consists of PdAs 4 tetrahedra, while the CeMg 2 Si 2 -type consists of PdAs 4 planar squares, as shown in Figs. 1(a) and 1(b), respectively. The intergrowth structure, shown in Fig. 1(c), consists of Pd 4 As tetrahedra, which are characteristic of the CaBe 2 Ge 2 -type structure such as BaPt 2 As 2 , 13, 14) * E-mail: kudo@science.okayama-u.ac.jp † E-mail: nohara@science.okayama-u.ac.jp as well as PdAs 4 planar squares, which are characteristic of the CeMg 2 Si 2 -type structure. Guo et al. reported superconductivity at T c = 3.85 K for the ThCr 2 Si 2 -type structure. 15)This T c of BaPd 2 As 2 is anomalously high compared with T c = 1.27 and 0.92 K for CaPd 2 As 2 and SrPd 2 As 2 , respectively, 16) which are isoelectronic and isostructural with BaPd 2 As 2 of the ThCr 2 Si 2 -type structure. On the other hand, no bulk superconductiv...