Bayrammurad Saparov

Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe2As2.

We use multiscale techniques to determine the extent of local inhomogeneity and superconductivity in Ca0.86Pr0.14Fe2As2 single crystal. The inhomogeneity is manifested as a spatial variation of the praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-Tc superconductivity emerges from cloverlike defects associated with Pr dopants. The highest Tc is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of nonuniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

Properties of binary transition-metal arsenides (TAs)

We present thermodynamic and transport properties of transition-metal (T) arsenides, TAs, with Txa0=xa0Sc to Ni (3d), Zr, Nb, Ru (4d), Hf and Ta (5d). Characterization of these binaries is carried out with powder x-ray diffraction, temperature-xa0and field-dependent magnetization and resistivity, temperature-dependent heat capacity, Seebeck coefficient, and thermal conductivity. All binaries show metallic behavior except TaAs and RuAs. TaAs, NbAs, ScAs and ZrAs are diamagnetic, while CoAs, VAs, TiAs, NiAs and RuAs show approximately Pauli paramagnetic behavior. FeAs and CrAs undergo antiferromagnetic ordering below TNxa0≈xa071xa0K and TNxa0≈xa0260xa0K, respectively. MnAs is a ferromagnet below TCxa0≈xa0317xa0K and undergoes hexagonal–orthorhombic–hexagonal transitions at TSxa0≈xa0317xa0K and 384xa0K, respectively. For TAs, Seebeck coefficients vary between xa0+xa040 and xa0−xa040xa0μV xa0K−1 in the 2–300xa0K range, whereas thermal conductivity values stay below 18xa0Wxa0m−1xa0K−1. The Sommerfeld coefficients γ are less than 10xa0mJxa0K−2xa0mol−1. At room temperature with application of 8xa0T magnetic field, large positive magnetoresistance is found for TaAs (∼25%), MnAs (∼90%) and NbAs (∼75%).

Fermi-surface reconstruction and complex phase equilibria in CaFe2As2.

Fermi-surface topology governs the relationship between magnetism and superconductivity in iron-based materials. Using low-temperature transport, angle-resolved photoemission, and x-ray diffraction, we show unambiguous evidence of large Fermi-surface reconstruction in CaFe2As2 at magnetic spin-density-wave and nonmagnetic collapsed-tetragonal (cT) transitions. For the cT transition, the change in the Fermi-surface topology has a different character with no contribution from the hole part of the Fermi surface. In addition, the results suggest that the pressure effect in CaFe2As2 is mainly leading to a rigid-band-like change of the valence electronic structure. We discuss these results and their implications for magnetism and superconductivity in this material.

Complex structures of different CaFe2As2 samples

The interplay between magnetism and crystal structures in three CaFe2As2 samples is studied. For the nonmagnetic quenched crystals, different crystalline domains with varying lattice parameters are found, and three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist between TS = 95u2005K and 45u2005K. Annealing of the quenched crystals at 350°C leads to a strain relief through a large (~1.3%) expansion of the c-parameter and a small (~0.2%) contraction of the a-parameter, and to local ~0.2u2005Å displacements at the atomic-level. This annealing procedure results in the most homogeneous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at TN/TS = 168(1) K. In the 700°C-annealed crystal, an intermediate strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120u2005K. The origin of such strong shifts in the transition temperatures are tied to structural parameters. Importantly, with annealing, an increase in the Fe-As length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter, which enhances the Fermi surface nesting in CaFe2As2, are also discussed.

Effect of molybdenum 4d hole substitution in BaFe2As2

We investigate the thermodynamic and transport properties of molybdenum-doped BaFe2As2 (122) crystals, the first report of hole doping using a 4d element. The chemical substitution of Mo in place of Fe is possible up to {approx} 7%. For Ba(Fe1-xMox)2As2, the suppression rate of the magnetic transition temperature with x is the same as in 3d Cr-doped 122 and is independent of the unit cell changes. This illustrates that the temperature-composition phase diagram for hole-doped 122 can be simply parameterized by x, similar to the electron-doped 122 systems found in the literature. Compared to 122 with a coupled antiferromagnetic order (TN) and orthorhombic structural transition (T0) at {approx}132 K, 1.3% Mo-doped 122 (x=0.013) gives TN=T0=125(1) K according to neutron diffraction results and features in specific heat, magnetic susceptibility, and electrical resistivity. The cell volume expands by {approx}1% with maximum Mo doping and TN is reduced to {approx}90 K. There is a T* feature that is identified for lightly Cr- or Mo-doped 122 crystals, which is x dependent. This low-temperature transition may be a trace of superconductivity.

Spin glass and semiconducting behavior in one-dimensional BaFe2 Se3 ( 0.2) crystals

We investigate the physical properties and electronic structure of BaFe2-{delta}Se3 crystals, which were grown out of tellurium flux. The crystal structure of the compound, an iron-deficient derivative of the ThCr2Si2-type, is built upon edge-shared FeSe4 tetrahedra fused into double chains. The semiconducting BaFe2-{delta}Se3 with {delta} approx 0.2 ({rho}295K = 0.18 {Omega}cdotcm and Eg = 0.30 eV) does not order magnetically, however there is evidence for short-range magnetic correlations of spin glass type (Tf approx 50 K) in magnetization, heat capacity and neutron diffraction results. A one-third substitution of selenium with sulfur leads to a slightly higher electrical conductivity ({rho}295K = 0.11 {Omega}cdotcm and Eg = 0.22 eV) and a lower spin glass freezing temperature (Tf approx 15 K), corroborating with higher electrical conductivity reported for BaFe2S3. According to the electronic structure calculations, BaFe2Se3 can be considered as a one-dimensional ladder structure with a weak interchain coupling.

Crystal, magnetic, and electronic structures, and properties of new BaMnPnF (Pn = As, Sb, Bi)

New BaMnPnF (Pn = As, Sb, Bi) are synthesized by stoichiometric reaction of elements with BaF2. They crystallize in the P4/nmm space group, with the ZrCuSiAs-type structure, as indicated by X-ray crystallography. Electrical resistivity results indicate that Pn = As, Sb, and Bi are semiconductors with band gaps of 0.73u2005eV, 0.48u2005eV and 0.003u2005eV (extrinsic value), respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below TN = 338(1) K for Pn = As, and below TN = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above TN. The ordered magnetic moments are 3.65(5)u2005μB/Mn for Pn = As, and 3.66(3)u2005μB/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.

Magnetic structure and spin excitations in BaMn2Bi2

We present a single crystal neutron scattering study of BaMn2Bi2, a recently synthesized material with the same ThCr2Si2type structure found in several Fe-based unconventional superconducting materials. We show long range magnetic order, in the form of a G-type antiferromagnetic structure, to exist up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering we observe a spin-gap of 16 meV, with spin-waves extending up to 55 meV. We find these magnetic excitations are well fit to a J1-J2-Jc Heisenberg model and present values for the exchange interactions. The spin wave spectrum appears to be unchanged by the 100 K structural phase transition.

Room‐Temperature Ba(Fe1−xCox)2As2 is not Tetragonal: Direct Observation of Magnetoelastic Interactions in Pnictide Superconductors

Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration.

Pressure-Induced Structural Phase Transition in CeNi: X-ray and Neutron Scattering Studies and First-Principles Calculations

The pressure-induced structural phase transition in the intermediate-valence compound CeNi has been investigated by x-ray and neutron powder diffraction techniques. It is shown that the structure of the pressure-induced CeNi phase (phases) can be described in terms of the Pnma space group. Equations of state for CeNi on both sides of the phase transition are derived and an approximate P-T phase diagram is suggested for P<8 GPa and T<300 K. The observed Cmcm→Pnma structural transition is then analyzed using density functional theory calculations, which successfully reproduce the ground state volume, the phase transition pressure, and the volume collapse associated with the phase transition.