Discovery of superconductivity in AlB2-type hexagonal YGa2

A crystalline sample YGa₂with the AlB₂-type hexagonal structure has been synthesized using the self-flux method. We measured electrical resistivity and specific heat down to 0.4 K and in fields up to 200 mT. The obtained data reveal type-II superconductivity. Unusual behaviour of the temperature dependence of upper critical fieldH_c2(T_c) was attributed to the crystal twinning of two phases possessing close lattice parameters. We determined thermodynamic parameters in the superconducting and normal states of YGa₂. DFT calculations indicated a large change in the Fermi velocity of carriers nearby cylinder-like structured Fermi surface along the Γ -Aline.

Physical properties of the layered oxypnictide Sr2ScFeAsO3: a Mössbauer study down to 1.7 K

A polycrystalline sample of Sr2ScFeAsO3 was studied by 57Fe Mössbauer spectroscopy down to 1.7 K. In contrast to the earlier Mössbauer data, the obtained in this work results indicate that Sr2ScFeAsO3 is in paramagnetic state down to 10 K, while the spectra recorded at 4.6 K and 1.7 K show a weak magnetic order of Fe moments in the Fe2As2 layers. Temperature dependences of isomer shift and quadrupole splitting/shift are compared with specific heat and electrical resistivity data from earlier investigations revealing different local Debye temperatures for the Fe2As2 and perovskite-related Sr2ScO3 layers. Finally, a fast decrease of the carrier density was observed below 80 K and this effect seems to be responsible for the absence of superconductivity in the studied compound.

Specific heat and Hall effect of the ferromagnetic Kondo lattice UCu0.9Sb2

We have investigated the electrical resistivity ρ, specific heat C _p and Hall coefficient R _H on a single crystal of a ferromagnetic Kondo lattice UCu_0.9Sb₂. The experimental [Formula: see text], C _p (T) and [Formula: see text] data evidence a bulk magnetic phase transition at [Formula: see text] K, and additionally exhibit an unexpected bump located in the temperature range T _C/10-T _C/3. UCu_0.9Sb₂ has an enhanced electronic specific heat coefficient [Formula: see text] mJ molK^-2, corresponding to Kondo temperature [Formula: see text] K. An analysis of the Hall effect data for j//(a, b)-plane and H// c-axis reveals that the low-temperature ordinary Hall coefficient R ₀ is positive, suggesting that p-type electrical conductivity is dominant. The density of the carriers at 2 K is about 0.6 holes f.u.^-1, which may categorize the studied compound into class of low carrier density compounds. Combined γ and R ₀ data divulge an effective mass of charge carriers [Formula: see text] 27 m _e . This finding together with quite low Hall mobility [Formula: see text] cm² Vs^-1 and Kadowaki-Woods ratio [Formula: see text] [Formula: see text] cm (mol K² mJ^-1)², manifest the development of heavy-fermion state in the ferromagnetic UCu_0.9Sb₂ compound at low temperatures.

Superconductivity Emerging from an Electronic Phase Separation in the Charge Ordered Phase of RbFe_{2}As_{2}

^{75}As, ^{87}Rb, and ^{85}Rb nuclear quadrupole resonance (NQR) and ^{87}Rb nuclear magnetic resonance measurements in a RbFe_{2}As_{2} iron-based superconductor are presented. We observe a marked broadening of the ^{75}As NQR spectrum below T_{0}≃140  K which is associated with the onset of a charge order in the FeAs planes. Below T_{0} we observe a power-law decrease in the ^{75}As nuclear spin-lattice relaxation rate down to T^{*}≃20  K. Below T^{*} the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in light of an orbital selective behavior expected for the electronic correlations.

Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe₂As₂, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe₂As₂ crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.

Direct evidence for a pressure-induced nodal superconducting gap in the Ba0.65Rb0.35Fe2As2 superconductor

The superconducting gap structure in iron-based high-temperature superconductors (Fe-HTSs) is non-universal. In contrast to other unconventional superconductors, in the Fe-HTSs both d-wave and extended s-wave pairing symmetries are close in energy. Probing the proximity between these very different superconducting states and identifying experimental parameters that can tune them is of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth in the optimally doped nodeless s-wave Fe-HTS Ba0.65Rb0.35Fe2As2. Upon pressure, a strong decrease of the penetration depth in the zero-temperature limit is observed, while the superconducting transition temperature remains nearly constant. More importantly, the low-temperature behaviour of the inverse-squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linear-in-temperature behaviour at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the superconducting gap.

Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft-X-ray angle-resolved photoemission spectroscopy

We present a soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichiometric pnictide superconductor LaRu(2)P(2). The observed electronic structure is in good agreement with density functional theory (DFT) calculations. However, it is significantly different from its counterpart in high-temperature superconducting Fe pnictides. In particular, the bandwidth renormalization present in the Fe pnictides (~2-3) is negligible in LaRu(2)P(2) even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu(2) P(2) has a different origin with respect to the iron pnictides. Finally, we demonstrate that the increased probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is essential in determining the bulk electronic structure in the experiment.

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8

We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (∼40 meV), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high-temperature superconductors, indicating they have a common origin.

Evidence for a spin pseudogap in the normal state of superconducting Mo(3)Sb(7)

Using muon spin relaxation (μSR) and inelastic neutron scattering (INS) we have investigated the normal state of the superconductor Mo(3)Sb(7) and the reference compound Ru(3)Sn(7). The μSR experiments on Ru(3)Sn(7) reveal static and relatively slow dynamic relaxations, which are ascribed to a random static nuclear dipole field and thermally activated muon motion, respectively. INS experiments on Ru(3)Sn(7), on the other hand, reveal three phononic excitations at 11, 18 and 23 meV, substantiating the assertion of Einstein and Debye oscillations derived from the specific heat and electrical resistivity data. The distinct difference in the μSR as well as INS spectra between Ru(3)Sn(7) and Mo(3)Sb(7) provides strong evidence for a magnetic/electronic nature of the phase transition at T(*) = 50 K in the Mo-based compound. On the basis of the μSR and INS data, the energy spin pseudogap of 150(10) K was estimated. The observed weak magnetism in the dynamic susceptibility χ('')(Q,ω) and residual longitudinal field relaxation at 5 K imply a static ordering or quantum fluctuations.

Simulation of coagulation, flocculation, and sedimentation

The computer program ZB2 was used to study simulated coagulation, flocculation, and sedimentation. The effect of the initial velocity of soil particles on the values of reaction-rate constants was investigated. In this respect, the results obtained with the program ZB2 corresponded to the theory and realities of coagulation carried out under practical conditions. The effect of coagulant excess/deficiency on the formation of the first floc and on the rate of coagulation of 50% of a soil was also estimated. An increase in simulated coagulant excess caused a decrease in the simulated rate of soil coagulation. In this respect, the results obtained with the program ZB2 did not correspond to the realities of coagulation carried out under practical conditions. An attempt was made to explain this inconsistency referring to the coagulation-flocculation theory. The simulation program ZB2 may provide a basis for developing "local" programs simulating coagulation-flocculation, which can be successfully applied at wastewater treatment plants.

LaBaNiO(4): a Fermi glass

Polycrystalline samples of LaSr(1-x)Ba(x)NiO(4) show a crossover from a state with metallic transport properties for x = 0 to an insulating state as [Formula: see text]. The end member LaBaNiO(4) with a nominal nickel Ni 3d(7) configuration might therefore be regarded as a candidate for an antiferromagnetic insulator. However, we do not observe any magnetic ordering in LaBaNiO(4) down to 1.5 K, and despite its insulating transport properties several other physical properties of LaBaNiO(4) resemble those of metallic LaSrNiO(4). Based on an analysis of electrical and thermal-conductivity data as well as magnetic-susceptibility and low-temperature specific-heat measurements, we suggest that LaBaNiO(4) is a Fermi glass with a finite electron density of states at the Fermi level but these states are localized.

Scanning tunneling spectroscopy in the superconducting state and vortex cores of the beta-pyrochlore KOs2O6

We performed the first scanning tunneling spectroscopy measurements on the pyrochlore superconductor KOs2O6 (T(c)=9.6 K) in both zero magnetic field and the vortex state at several temperatures above 1.95 K. This material presents atomically flat surfaces, yielding spatially homogeneous spectra which reveal fully gapped superconductivity with a gap anisotropy of 30%. Measurements performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line lattice. From the shape of the vortex cores, we extract a coherence length of 31-40 A, in agreement with the value derived from the upper critical field H(c2). We observe a reduction in size of the vortex cores (and hence the coherence length) with increasing field which is consistent with the unexpectedly high and unsaturated upper critical field reported.

Enhanced susceptibility in LNiO3 perovskites ( L = La,Pr,Nd,Nd0.5Sm0. 5)

The temperature dependence of the resistivity rho(T) and of the dc magnetic susceptibility chi(T) were measured on high-quality LNiO3 (L = La,Pr,Nd,Nd0.5Sm0.5) samples synthesized under high oxygen pressure. Subtraction of the rare-earth contribution to chi(T) allows the presentation of the evolution of the susceptibility of the NiO3 array from Pauli to Curie-Weiss paramagnetism with decreasing bandwidth. A metal-insulator transition occurring at a temperature T(t) = T(N) is first order for L = Pr and Nd; it becomes second order and produces no anomaly in chi(-1)(T) at a T(t)>T(N) for L = Nd0.5Sm0.5. In the antiferromagnetic domain T<T(N), the chi(T) curve for the NiO3 array resembles that of a paramagnet.

Electron microscope study of superconductor YBa2Cu3O6+δ

Electron microscopic observations of small single-crystal grains of YBa2Cu3O6+δ confirmed the orthorhombic symmetry and revealed very fine domain structure with twin boundaries parallel to (110) crystallographic planes.