Superconductivity in Ca_1-xLa_xFeAs₂: A Novel 112-Type Iron Pnictide with Arsenic Zigzag Bonds

Abstract

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 ...

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…the space group P 2 1 or P 2 1 /m rather than the originally assumed tetragonal structure [13,14]. Second harmonic generation experiments confirmed the space group P 2 1 for La doped compounds [15].…”

“…(The former structure is reported, for example, for the La doped compound in Ref. [14], and the latter is reported, for example, for the Pr doped compound in Ref. [13].…”

“…We use the Coulomb interaction U = 5.0 eV and the Hund's coupling J = 0.8 eV, which were shown to describe Ca 1−x La x FeAs 2 compounds [19,36]. The temperature is set to T = 116 K. The experimental crystal structure determined by X-ray diffraction [14] is used. The virtual crystal approximation is adopted to investigate the electronic structure of the La doped compounds.…”

“…Attempts to synthesize iron pnictide compounds in this structure were not originally successful, but new Mn based materials in this structure were found [7,8] and it was observed theoretically [7] and experimentally [8][9][10][11] that the spacer layers exhibit Dirac cones [12]. Recently, Fe superconductors in the 112 structure were synthesized, (Ca,Pr)FeAs 2 [13], and Ca 1−x La x FeAs 2 [14].…”

Dependences on RE of superconducting properties of transition metal co-doped (Ca,RE)FeAs2 with RE= La–Gd

“…Searches for new iron-based superconductors have been still continuing, resulted in a recent discovery of new series of iron-based superconductors (Ca,RE)FeAs 2 ((Ca,RE)112) [5][6][7][8][9] . (Ca,RE)112 has several unique properties, such as monoclinic structure with space group of P2 1 or P2 1 /m, and co-existence of As -1 and As -3 in the crystal lattice [5,6] . Moreover, anisotropy of (Ca,La)112 was reported to be 2.0-4.2 [10] , which is smaller than that of 1111 type superconductors, while interlayer distance between Fe layer (d Fe-Fe ) is longer in (Ca,La)112.…”