By the first-principles electronic structure calculations, we find that energetically the most favorable cleaved $A$Fe$_2$As$_2$(001) surface ($A$=Ba, Sr, or Ca) is $A$-terminated with a $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ or $(1\times 2)$ order. The $(1\times 2)$ ordered structure yields a $(1\times 2)$ dimerized STM image, in agreement with the experimental observation. The $A$ atoms are found to diffuse on the surface with a small energy barrier so that the cleaving process may destroy the $A$ atoms ordering. At the very low temperatures this may result in an As-terminated surface with the $A$ atoms in randomly assembling. The As-terminated BaFe$_2$As$_2$ surface in orthorhombic phase is $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ buckled, giving rise to a switchable $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ STM pattern upon varying the applied bias. No any reconstruction is found for the other As-terminated surfaces. There are surface states crossing or nearby the Fermi energy in the As-terminated and $(1\times 2)$ $A$-terminated surfaces. A unified physical picture is thus established to help understand the cleaved $A$Fe$_2$As$_2$(001) surfaces.
Here we report the first-principle FLAPW-GGA calculations of the elastic properties of two related phases, namely, the ternary arsenide SrFe2As2 and the quaternary oxyarsenide LaOFeAs as the basic phases for the newly discovered "1111" and "122" 38-52K superconductors. The independent elastic constants (Cij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. Additionally, numerical estimates of the elastic parameters of the polycrystalline LaOFeAs and SrFe2As2 ceramics are performed for the first time.
We report the first-principle FLAPW-GGA calculations of the elastic properties of two related layered phases, namely, the ternary arsenide SrFe2As2 and the quaternary oxyarsenide LaFeAsO - basic phases for the newly discovered "122" and "1111" 38-55K superconductors. The independent elastic constants (Cij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. The numerical estimates of the elastic parameters of the polycrystalline SrFe2As2 and LaFeAsO ceramics are performed for the first time. Additionally, the peculiarities of chemical bonding in these phases are discus…
We have studied the electronic and magnetic structures of the ternary iron arsenides AFe$_2$As$_2$ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As $4p$ orbitals. The correction from the spin-orbit interaction to the band structure is small. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based…
Using a combination of linear response theory and constrained orbital hybridization approach, we study the mechanism of magnetic exchange interaction of iron-based superconductor. We reproduce the observed highly anisotropic exchange interaction, and our constrain-orbital calculation unambiguously identifies that the anisotropic feature of exchange interaction is not sensitive to the unequal d$_{xz}$/d$_{yz}$ orbital population.
Superconductivity was achieved in Ti-doped iron-arsenide compound Sr4Cr0.8Ti1.2O6Fe2As2 (abbreviated as Cr-FeAs-42622). The x-ray diffraction measurement shows that this material has a layered structure with the space group of \emph{P4/nmm}, and with the lattice constants a = b = 3.9003 A and c = 15.8376 A. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2…
To examine the role of magnetism in superconductivity of iron-based superconductors, we first present first-principles optical calculations on three representative parent compounds: LaFeAsO, BaFe2As2 and LiFeAs. Both nonmagnetic (NM) and spin-density wave (SDW) with striped antiferromagnetic order are considered. Due to their layer structure, these compounds in NM states show anomalously large optical anisotropy with a conductivity ratio $σ_{zz}/σ{xx}$ as large as 313% in LaFeAsO and 530% in LiFeAs. Interestingly the giant optical anisotropy is significantly suppressed by SDW. On the other han…
We report the synthesis and crystal structures of three new superconducting iron-platinum arsenides (CaFe1-xPtxAs)10Pt4-yAs8 (x = 0-0.15, y = 0-0.4). The structures are stacking variants of FeAs- and slightly puckered Pt4-yAs8-layers with square coordinated platinum separated by calcium-layers, respectively. Arsenic atoms in the Pt4-yAs8-layers form (As2)4- dumbbells according to Zintl's concept, providing charge balance in (Ca2+Fe2+As3-)10(Pt2+)3[(As2)4-]4. Superconductivity was observed at 13-35 K. We suggest that the highest Tc above 30 K occurs in the 1048 phase with clean FeAs-layers that…
Very recently (November, 2010, PRB, 82, 180520R), the first ThCr2Si2-type ternary superconductor K0.8Fe2Se2 with enhanced TC ~ 31 K has been discovered. This finding has stimulated much activity in search for related materials and triggered intense studies of their properties. Indeed, very soon superconductivity (TC ~ 28-32 K) was also found in the series of related ternary systems (so-called 122 phases) such as CsxFe2-ySe2, RbxFe2-ySe2, (TlK)xFeySe2, (TlRb)xFeySe2 etc., which formed a new group of superconducting iron-based materials without toxic As. In this paper the recent progress in synt…
Recent observations of selective emergence (suppression) of superconductivity in the uncollapsed (collapsed) tetragonal phase of LaFe$_2$As$_2$ has rekindled interest in understanding what features of the band structure control the superconducting T$_c$. We show that the proximity of the narrow Fe-d$_{xy}$ state to the Fermi energy emerges as the primary factor. In the uncollapsed phase this state is at the Fermi energy, and is most strongly correlated and source of enhanced scattering in both single and two particle channels. The resulting intense and broad low energy spin fluctuations suppre…
By the first-principles electronic structure calculations, we find that the ground state of ternary iron selenides AFe$_2$Se$_2$ (A=K, Cs, or Tl) is in a bi-collinear antiferromagnetic order, in which the Fe local moments ($\sim2.8μ_B$) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe-Fe square lattice. This bi-collinear antiferromagnetic order results from the interplay among the nearest, the next nearest, and the next next nearest neighbor superexchange interactions, mediated by Se $4p$-orbitals.
We have studied the pressure effect on electronic structures and magnetic orders of ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$ by the first-principles electronic structure calculations. At low pressure, the compound is in the blocked checkerboard antiferromagnetic (AFM) semiconducting phase, as observed by the neutron scatting measurements. Applying pressure induces two phase transitions, first from the blocked checkerboard AFM semiconducting phase to a collinear AFM metallic phase around 12 GPa, and then to a non-magnetic metallic phase around 25 GPa, respectively. Our results help to cl…
Platelet-like single crystals of the Ca(Fe1-xCox)2As2 series having lateral dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant variation. Pristine CaFe2As2 shows a combined spin-density-wave and structural transition near T = 166 K which gradually shifts to lower temperatures and splits with increasing Co-doping. Both transitions terminate abruptly at a critical Co-concentration of xc = 0.075. For x \geq 0.05…
By the first-principles electronic structure calculations, we find that the ground state of the Fe-vacancies ordered TlFe$_{1.5}$Se$_2$ is a quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy gap of 94 meV, in agreement with experimental measurements. This antiferromagnetic order is driven by the Se-bridged antiferromagnetic superexchange interactions between Fe moments. Similarly, we find that crystals AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs) are also antiferromagnetic semiconductors but with a zero-gap semiconducting state or semimetallic state nearly degenerated with t…
The results of the ab initio FLAPW-GGA calculations of the band structure of the newly synthesized tetragonal (space group P4/nmm) layered iron phosphide-oxide: 17K superconductor Sr4Sc2Fe2P2O6 are presented. For Sr4Sc2Fe2P2O6 the optimized structural data, the energy bands, total and partial densities of states, Fermi surface topology, low-temperature electron specific heat and molar Pauli paramagnetic susceptibility have been determined and discussed in comparison with hypothetical isostructural iron arsenide-oxide phase Sr4Sc2Fe2As2O6 and related layered FeAs and FeP superconductors.
Discovering new materials with ultrahigh thermal conductivity has been a critical research frontier and driven by many important technological applications ranging from thermal management to energy science. Here we have rigorously investigated the fundamental lattice vibrational spectra in ternary compounds and determined the thermal conductivity using a predictive ab initio approach. Phonon transport in B-X-C (X = N, P, As) groups is systematically quantified with different crystal structures and high-order anharmonicity involving a four-phonon process. Our calculation found an ultrahigh room…
We have studied electronic and magnetic structures of K$_{0.8+x}$Fe$_{1.6}$Se$_2$ by performing the first-principles electronic structure calculations. The ground state of the Fe-vacancies ordered K$_{0.8}$Fe$_{1.6}$Se$_2$ is found to be a quasi-two-dimensional blocked checkerboard antiferromagnetic (AFM) semiconductor with an energy gap of 594 meV and a large ordering magnetic moment of 3.37 $μ_B$ for each Fe atom, in excellent agreement with the neutron scattering measurement. The underlying mechanism is the chemical-bonding-driven tetramer lattice distortion. K$_{0.8+x}$Fe$_{1.6}$Se$_2$ wit…
Despite the fact that 1111-type iron arsenides hold the record transition temperature of iron-based superconductors, their electronic structures have not been studied much because of the lack of high-quality single crystals. In this study, we completely determine the Fermi surface in the antiferromagnetic state of CaFeAsF, a 1111 iron-arsenide parent compound, by performing quantum oscillation measurements and band-structure calculations. The determined Fermi surface consists of a symmetry-related pair of Dirac electron cylinders and a normal hole cylinder. From analyses of quantum-oscillation…
Two new ternary borides $TM$$_7$Fe$_3$B$_8$ ($TM$ = Nb, Ta) were synthesized by high-temperature thermal treatment of samples obtained by arc-melting. This new type of structure with space group $P$6/$mmm$, comprises $TM$ slabs containing isolated planar hexagonal [B$_6$] rings and iron centered $TM$ columns in a Kagome type of arrangement. Chemical bonding analysis in Nb$_7$Fe$_3$B$_8$ by means of the electron localizability approach reveals two-center interactions forming the Kagome net of Fe and embedded B, while weaker multicenter bonding present between this net and Nb atoms. Magnetic sus…
Composition-dependent Thin-film Synthesis of Layered Ternary Iron Nitrides FeMN2 (M = W, Mo)
National Laboratory of the Rockies（落基山国家实验室）；Korea Advanced Institute of Science and Technology (KAIST)（韩国科学技术院）；Stanford University（斯坦福大学）；Colorado School of Mines（科罗拉多矿业学院）；Idaho National Laboratory（爱达荷国家实验室）
The iron arsenide RbFe_2As_2 with the ThCr_2Si_2-type structure is found to be a bulk superconductor with T_c=2.6 K. The onset of diamagnetism was used to estimate the upper critical field H_c2(T), resulting in dH_c2/dT=-1.4 T/K and an extrapolated H_c2(0)=2.5 T. As a new representative of iron pnictide superconductors, superconducting RbFe_2As_2 contrasts with BaFe_2As_2, where the Fermi level is higher and a magnetic instability is observed. Thus, the solid solution series (Rb,Ba)Fe_2As_2 is a promising system to study the crossover from superconductivity to magnetism.
