We report a new polymorph of LuRuGe, obtained in indium flux. This phase exhibits the noncentrosymmetric ZrNiAI-type structure with the space group P (6) over bar 2m as determined by single-crystal X-ray diffraction. This polymorph can convert into another centrosymmetric polymorph (TiNiSi-type structure, space group Pnma) at high temperatures. We performed electrical transport, magnetization, and specific heat measurements on this new phase. It shows metallic behavior with a Hall sign change from negative at 2 K to positive at 125 K. LuRuGe exhibits Pauli paramagnetism as the ground state with no local magnetic moments from either the Ru or Lu site. The Debye temperature circle dot = 348 K and electronic coefficient gamma(e) = 3.6 mJ K-2 mol(-1) are extracted from the low-temperature specific heat data in LuRuGe. We also carried out first-principles density functional theory calculations to map out the electronic band structure and density of states. There are several electronic bands crossing the Fermi level, supporting a multiband scenario consistent with the Hall sign change. The density of states around the Fermi level is mainly from Ru 4d and Ge 4p electrons, indicating a strong hybridization between those atomic orbitals.

We report a new compound, Y4RuGe8, with a transition metal vacancy-ordered CeNiSi2-type superstructure, which has a superconducting transition at 1.3 K. Y4RuGe8 crystals were grown by indium flux at relatively low temperatures (below 1273 K), which makes it possible to stabilize such a vacancy-ordered phase. The crystal structure of Y4RuGe8 was solved by single-crystal X-ray diffraction and confirmed by transmission electron microscopy. The as-grown Y4RuGe8 crystals are always twinned, crystallizing in the space group P (1) over bar (no. 2) with the lattice parameters a = 5.7680(1) angstrom, b = 8.2042(2) angstrom, c = 11.5093(3) angstrom, alpha = 79.696(1)degrees, beta = 88.491(1)degrees, and gamma = 79.637(2)degrees; this structure is a superstructure deriving from the higher symmetry CeNiSi2-type structure (Cmcm, no. 63) due to the ordering of Ru vacancies. The ordering of Ru sites breaks slightly distorted Ge planes in the CeNiSi2 prototype into infinite cis-trans Ge chains in Y4RuGe8. The presence of bulk superconductivity in Y4RuGe8 is well supported by zero resistance and a jump in specific heat at the critical transition temperature. The Sommerfeld coefficient (19 mJ K-2 mol(-1)) of the specific heat is greater than that (11 mJ K-2 mol(-1)) estimated using the bare density of states (4.7 states/eV/f.u.) from first-principles calculations. The ab initio calculations indicate that 4d electrons of both Y and Ru and 4p electrons of Ge are the main contributors to the total density of states at the Fermi level in Y4RuGe8.

We report a type-II intermetallic superconductor Y7Ru4InGe12 with a transition temperature (T-c) of similar to 5.8 K, which is confirmed by zero resistivity, diamagnetic magnetic susceptibility, and specific-heat jump. Single crystals of Y7Ru4InGe12 were grown from a reactive indium flux. Y7Ru4InGe12 crystallizes in the tetragonal space group P4/m and features a [Y7Ru4InGe12] polyanionic network with Y atoms located in three different channels. The upper critical fields of Y7Ru4InGe12 at 0 K are determined to be similar to 5.3 and 2.4 T along the c axis and the ab plane, respectively. The estimated coherence length along the c axis (similar to 174 angstrom) is much larger than the estimated mean free path along the c axis (similar to 29 angstrom) in Y7Ru4InGe12, placing its superconductivity in the so-called dirty regime. The compound exhibits a large superconducting specific-heat jump Delta C/gamma T-e(c )approximate to 2.4, significantly well above the weak-coupling Barden-Cooper-Schrieffer theoretical value of 1.43 and pointing to a strong-coupling scenario in Y7Ru4InGe12. Density-functional-theory calculations show that the density of states in Y7Ru4InGe12 exhibits a broad peak near the Fermi level which mainly derives from Y-4d, Ru-4d, and Ge-4p states.

Author(s): Frandsen, BA; Taddei, KM; Bugaris, DE; Stadel, R; Yi, M; Acharya, A; Osborn, R; Rosenkranz, S; Chmaissem, O; Birgeneau, RJ | Abstract: © 2018 American Physical Society. We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr1-xNaxFe2As2. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels (x=0.48, near optimal superconductivity) and high temperatures (above room temperature for x=0, decreasing to 150 K for x=0.48), with a typical length scale of 1-3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels ($x \gtrsim 0.48$, near optimal superconductivity) and high temperatures (above room temperature for $x = 0$, decreasing to 150~K for $x = 0.48$), with a typical length scale of 1--3~nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

Author(s): Frandsen, BA; Taddei, KM; Bugaris, DE; Stadel, R; Yi, M; Acharya, A; Osborn, R; Rosenkranz, S; Chmaissem, O; Birgeneau, RJ | Abstract: We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels ($x gtrsim 0.48$, near optimal superconductivity) and high temperatures (above room temperature for $x = 0$, decreasing to 150~K for $x = 0.48$), with a typical length scale of 1--3~nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

A new polymorph of the RE2Ru3Ge5 (RE = Pr, Sm, Dy) compounds has been grown as single crystals via an indium flux. These compounds crystallize in tetragonal space group P4/mnc with the Sc2Fe3Si5-type structure, having lattice parameters a = 11.020(2) Å and c = 5.853(1) Å for RE = Pr, a = 10.982(2) Å and c = 5.777(1) Å for RE = Sm, and a = 10.927(2) Å and c = 5.697(1) Å for RE = Dy. These materials exhibit a structural transition at low temperature, which is attributed to an apparent charge density wave (CDW). Both the high-temperature average crystal structure and the low-temperature incommensurately modulated crystal structure (for Sm2Ru3Ge5 as a representative) have been solved. The charge density wave order is manifested by periodic distortions of the one-dimensional zigzag Ge chains. From X-ray diffraction, charge transport (electrical resistivity, Hall effect, magnetoresistance), magnetic measurements, and heat capacity, the ordering temperatures (TCDW) observed in the Pr and Sm analogues are ∼200 and ∼175 K, respectively. The charge transport measurement results indicate an electronic state transition happening simultaneously with the CDW transition. X-ray absorption near-edge spectroscopy (XANES) and electronic band structure results are also reported.

We report on temperature-dependent pair distribution function measurements of Sr_{1-x}Na_{x}Fe_{2}As_{2}, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic C_{4} phase. Quantitative refinements indicate that the instantaneous local structure in the C_{4} phase comprises fluctuating orthorhombic regions with a length scale of ∼2  nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic C_{4} phase and the neighboring C_{2} and superconducting phases.

Previously synthesized only as powders, single crystals of the RE2Ru3Ge5 (RE = La, Ce, Nd, Gd, Tb) series of compounds have now been obtained from molten In. These materials crystallize with the U2Co3Si5-type structure in orthorhombic space group Ibam with lattice parameters a ≈ 10.00-9.77 Å (La-Tb), b ≈ 12.51-12.35 Å, and c ≈ 5.92-5.72 Å. The structure is a three-dimensional framework consisting of RuGe5 and RuGe6 units, as well as Ge-Ge zigzag chains. This structure type and those of the other five (Sc2Fe3Si5, Lu2Co3Si5, Y2Rh3Sn5, Yb2Ir3Ge5, and Yb2Pt3Sn5) to compose the RE2T3X5 phase space are discussed in depth. For the three compounds with RE = Nd, Gd, Tb, multiple magnetic transitions and metamagnetic behavior are observed. Electronic band structure calculations performed on La2Ru3Ge5 indicate that these materials have a negative band gap and are semimetallic in nature.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Since its discovery in 2014, the magnetic tetragonal C-4 phase has been identified in a growing number of hole-doped 122 Fe-based superconducting compounds. Exhibiting a unique double-Q magnetic structure and a strong competition with both superconducting and magnetic order parameters, the C-4 phase and the conditions of its formation are of significant interest to understanding the fundamental mechanisms in these materials. Particularly, separating the importance of direct changes to the relative size of hole and electron pockets at the Fermi surface (achieved via charge doping) from the role of structural changes due to differences of ionic radii of dopants is useful to determine the underlying parameter which causes the C-4 instability. Here, we report the discovery of the C-4 phase in a fourth member of the hole-doped 122 materials Ca1-xNaxFe2As2 (0.20 <= x <= 0.50) as determined from neutron and x-ray powder diffraction studies. The maximum of the C-4 dome is observed at x = 0.44 with a reentrant temperature T-r = 52 K and an extent of Delta x similar to 0.07 in composition. It is observed that for a range of compositions within the C-4 dome (0.40 <= x <= 0.42), there is a second reentrance (Tr-2 < Tr) where the antiferromagnetic C-2 phase is recovered-a feature previously only seen in Ba1-xKxFe2As2. A phase diagram is presented for Ca1-xNaxFe2As2 and compared to the other Na-doped 122' s-A(1-x)Na(x)Fe(2)As(2) with A = Ba, Sr, and Ca. The structural parameters for these three systems are compared and the importance of the "chemical pressure" due to changing the A-site ion (A = Ba, Sr, Ca) is discussed.

We report the occurrence of reentrantmetallic behavior in theWeyl semimetal NbP. When the appliedmagnetic field H is above a critical value H-c, a reentrance appears as a peak in the temperature-dependent resistivity rho(xx) (T) at T = T-p, similar to that observed in graphite where it was attributed to local superconductivity. The Tp(H) relationship follows a power-law dependence T-p similar to (H-H-c)(1/nu) where. can be derived from the temperature dependence of the zero-field resistivity rho(0)(T) similar to T-nu. From concurrent measurements of the transverse rho(xx) (T) and Hall rho(xy)(T) magnetoresistivities, we reveal a clear correlation between the rapidly increasing rho(xy) (T) and the occurrence of a peak in the rho(xx) (T) curve. Quantitative analysis indicates that the reentrantmetallic behavior arises from the competition of the magnetoconductivity sigma(xx) (T) with an additional component Delta sigma(xx) (T) = kappa(H)sigma(xx)(T) where kappa(H) = [rho(xy)(T)/rho(xx)(T)](2) is the Hall factor. We find that the Hall factor (kappa(H) approximate to 0.4) at peak temperature T-p is nearly field independent, leading to the observed T-p (H) relationship. Furthermore, the reentrant metallic behavior in rho(xx) (T) also is reflected in the behavior of rho(xx) (H) that ranges from nonsaturating at T > 70K to saturation at liquid-helium temperatures. The latter can be explained with the magnetic field dependence of the Hall factor kappa(H) (H). Our paper demonstrates that a semiclassical theory can account for the "anomalies" in the magnetotransport phenomena of NbP without invoking an exotic mechanism.

Large single crystals of SrIr4In2Ge4 were synthesized using the In flux method. This compound is a hybridization gap semiconductor with an experimental optical band gap of Eg = 0.25(3) eV. It crystallizes in the tetragonal EuIr4In2Ge4 structure type with space group I4̅2m and unit cell parameters a = 6.9004(5) Å and c = 8.7120(9) Å. The electronic structure is very similar to both EuIr4In2Ge4 and the parent structure Ca3Ir4Ge4, suggesting that these compounds comprise a new family of hybridization gap materials that exhibit indirect gap, semiconducting behavior at a valence electron count of 60 per formula unit, similar to the Heusler alloys.

The new ternary copper selenide NaCu4Se3 crystallizes in the RbCd4As3 structure type with the trigonal space group R3̅m and lattice constants a = 4.0316(4) Å and c = 31.438(8) Å. Its structure is built from two-dimensional slabs of (2)/∞[Cu4Se3] separated by Na(+) cations. The compound is formally mixed-valent with Se(2-)/Se(-) atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of ∼300 S cm(-1) at room temperature and a thermopower of ∼10 μV K(-1). Hall effect measurements indicate holes as the dominant carrier with a concentration of ∼6.12(1) × 10(21) cm(-3) at 300 K. Density functional theory electronic structure calculations indicate p-type metallic behavior for the (2)/∞[Cu4Se3] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism.

La2Re3B7 and La3Re2B5 have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 °C in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La2Re3B7 (space group Pcca) having lattice parameters a = 7.657(2) Å, b = 6.755(1) Å, and c = 11.617(2) Å, and La3Re2B5 (space group Pmma) having lattice parameters a = 10.809(2) Å, b = 5.287(1) Å, and c = 5.747(1) Å. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron-boron bonding. La3Re2B5 features fairly common B2 dumbbells, whereas La2Re3B7 has unique one-dimensional subunits composed of alternating triangular B3 and trans-B4 zigzag chain fragments. Also observed in La3Re2B5 is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La2Re3B7 is a semimetal, which is observed in the electrical resistivity data as measured on single crystals, with behavior obeying the Bloch-Grüneisen model and a room-temperature resistivity ρ300 K of ∼375 μΩ cm. The electronic band structure calculations also suggest that La3Re2B5 is a regular metal.

Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high-temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of certain hole-doped compounds, revealing the existence of a new magnetic phase at compositions close to the onset of superconductivity. Here, we present Mössbauer data that show that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. Instead, this state is naturally explained as the interference between two commensurate spin-density waves, a rare example of collinear double-Q magnetic order. Our results demonstrate the itinerant character of the magnetism of the iron pnictides, and the primary role played by magnetic degrees of freedom in determining their phase diagram. A combination of neutron scattering, X-ray scattering and Mössbauer spectroscopy experiments reveal the existence of a collinear double-Q magnetic ordering in an iron arsenide superconductor.