Above-T c anomalies of the infrared-active phonons in RBa2Cu4O8 (R=Dy,Ho) and Y2Ba4Cu7O15−δ superconductors

We report a far-infrared reflectivity study of DyBa₂Cu₄O₈, HoBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– superconductors in the temperature range 10–300 K. The assignment of thez-polarized infrared-acitve vibrations of the rare-earth-ion (R) was verified by the substitution ofR. Since the structure of the Y₂Ba₄Cu₇O_15– compound inz-direction is an ordered sequence of YBa₂Cu₃O_7– and YBa₂Cu₄O₈ blocks, the phonon spectrum of Y₂Ba₄Cu₇O_15– is a superposition of their respective phonon modes. The temperature dependence of the infrared-active phonons in RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was analyzed. A narrowing of the plane-oxygen phonon upon sample cooling was found to occur atT _c , as it was reported for single-chain YBa₂Cu₃O₇ materials; however a pronounced frequency softening of this mode for both RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was found to start well aboveT _c , suggesting the interaction of infrared-active phonons with an excitation gap similar to that observed in spin-dependent measurements.     

Dynamic short-range order observed in the (Zn)[Fe2]O4 spinel by neutron diffraction, μSR,and Mössbauer experiments

Using neutron diffraction (ND), muon-spin rotation/relaxation (SR), and⁵⁷Fe-Mössbauer spectroscopy (MS) we have investigated magnetic properties of the normal spinel (Zn)[Fe₂]O₄. In compounds which are slowly cooled from 1200°C to room temperature inversion is below detection limits. AtT _N = 10.5 K the spinel exhibits long-range antiferromagnetic order (LRO). The transition as seen in thermal-scan spectra by MS is very sharp. However, ND andSR experiments show that already at temperatures of 10T _N a short-range antiferromagnetic ordering (SRO) develops which extends through 70% of the sample volume just aboveT _N . BelowT _N SRO and LRO coexist. At 4.2 K still 25% of the sample is short-range ordered. The regions over which the SRO extends have a size of 3 nm. Their fluctuation rates are in the GHz range. Modern ab initio cluster calculations successfully describe the magnetic hyperfine fields as well as the electric field gradient (EFG) tensor at the Fe sites. Covalency of the Fe-O and Zn-O bonds is important. The physical origin of the regions exhibiting SRO, however, remains unresolved at this point.     

Structure and Dynamics of a Lattice of Tetragonal Germanates R2Ge2O7 (R = Tb–Lu,Y): Ab Initio Calculation

Physics of the Solid State - The crystal structure, phonon spectrum, and elastic constants of a series of rare-earth germanates (including yttrium germanate R2Ge2O7 (R = Tb–Lu, Y)) with a...     

Phase Composition,Microstructure, and Electroconductivity of HfO2–R2O3 Solid Electrolytes (R = Sc,Y, Ho,Er, Tm,Yb, and Lu)

Physics of the Solid State - The effect of the addition of 11 mol % R2O3 (R = Sc, Y, Ho, Er, Tm, Yb, Lu) on the phase and elemental composition, microstructure, and electroconductivity of hafnium...     

Young’s modulus and fracture during Knoop indentation of potassium, rubidium, cesium, and ammonium acid phthalate single crystals on the (010) plane

The correlation of the anisotropy of the Young??s modulus of organic single crystals of potassium, rubidium, cesium, and ammonium acid phthalates with strain and fracture patterns during Knoop indentation on the (010) cleavage plane in the [001] and [100] directions has been studied. The data on the maximum anisotropy of the strain and fracture patterns of the ammonium acid phthalate single crystal have been discussed in view of the published data on the structure, mechanical, elastic, and X-ray spectral properties of these crystals.     

Observation of magnetic order of Cu-moment aroundx=0.12 in La2−x Ba x CuO4 and La2−x sr x CuO4 by μ+SR

Zero field positive muon spin rotation method ( ⁺SR) is applied on La_2–x Sr _x CuO₄ around forx=0.12 at which the high-T _c superconductivity (SC) is suppressed. The magnetically ordered state of Cu-moments, which is not a spin glass state but an antiferromagnetic like state, appears below 15 K for 0.105x0.120. The magnetic phase boundary is very similar to the one of the La_2–x Ba _x CuO₄ in which the structural transition from the low temperature orthorhombic (LTO) phase to the low temperature tetragonal (LTT) phase is observed aroundx=0.12. The present study suggests that there is no big difference of the electronic state of the CuO₂ plane between the La-Ba system and the La-Sr system and that the magnetic ordering of Cu-moments plays an important role for the suppression of the high-T _c SC aroundx=0.12 in both of the systems, although the LTO-LTT structural transition has not been observed yet in the La-Sr system.     

Crystal structure and ionic conductivity of solid electrolytes M5RESi4O12, where M=Na,Ag, RE=Sm→Lu

Conductivity measurements of the crystals of the N₅RESi₄O₁₂ family, where RE=Sm→Lu, have been made. A reversible phase transition R32

R$\text{3}$c has been established. The conductive channels have been determined using a high temperature X-ray structural analysis of the Na₅RESi₄O₁₂ crystals, as well as the ion exchanged Ag phase.     

Competition of magnetic order and superconductivity investigated by positive muon spin rotation in La2−x M x CuO4 (M = Ba,Sr) aroundx=0.12

Muon spin rotation (SR) is applied to La_2–x Ba _x CuO₄ aroundx=0.12 where the superconductivity (SC) is suppressed remarkably. The magnetic ordering of Cu-moments appears below 35 K in the narrow range ofx where a lattice instability from the low temperature orthorhombic (LTO) to the low-temperature tetragonal (LTT) structure exists. The present study suggests strongly that the magnetic ordering of Cu-moments is an important factor in the suppression of high-T _c superconductivity aroundx=0.12 in La_2–x Ba _x CuO₄. Similar results are obtained for the La_2–x Sr _x CuO₄ and La_2–x–y Sr _x Nd _y CuO₄ systems.     

Study of the effect of the substitution of O by F,S, and Cl on the superconductivity of Y1Ba2Cu3O7−δ

We have studied the superconducting transitions of Y₁Ba₂Cu₃D _x O_7–x– samples where O has been replaced by D=F, S, or Cl andx=0.2-2. No single phase compounds were obtained forx>0.2. No dramatic increase inT _c was registered. Generally the results can be interpreted as a dilution of the highT _c 1 2 3 phase by other phases as the dopant concentration increases, ultimately suppressingT _c completely. Possibly there is an increase ofT _c for an intermediate dopant range,x, in the Cl series.     

Evolution of the atomic order and valence state of rare-earth atoms and uranium in a new carbon-metal composite—diphthalocyanine pyrolysate C64H32N16 Me (Me = Y,La, Ce,Eu, and U)

The structure of a metal-carbon composite formed by the pyrolysis of diphthalocyanine of some rare-earth elements (Y, La, Ce, Eu) and uranium in the temperature range T _ann = 800–1700°C has been investigated for the first time by the methods of X-ray diffraction analysis and X-ray line shift. It has been shown that, in the general case, the studied pyrolysates consist of three phases. One phase corresponds to the structure of graphite. The second phase corresponds to nitrides, carbides, and oxides of basic metal elements with a crystallite size ranging from 5 to 100 nm. The third phase is amorphous or consisting of crystallites with a size of ～1 nm. It has been found that all the basic elements (Y, La, Ce, Eu, U) and incorporated iodine atoms in the third phase are in a chemically bound state. The previously unobserved electronic configurations have been revealed for europium. The possibility of including not only atoms of elements forming diphthalocyanine but also other elements (for example, iodine) in the composite structure is of interest, in particular, for the creation of a thermally, chemically, and radiation resistant metal-carbon matrix for the radioactive waste storage.     

Anomaly in the upper critical magnetic field common to YBa2Cu3O7−δ , HoBa2Cu3O7−δ , and Nd1.85Ce0.15CuO4−δ irradiated by helium ions

The high-temperature superconductors YBa₂Cu₃O_7−δ , HoBa₂Cu₃O_7−δ , and Nd_1.85Ce_0.15CuO_4−δ are found to possess a common anomaly in the variation of the upper critical magnetic field owing to irradiation by helium ions. While the resistivity ρ increases by many times, the increase in H _c2 from scattering by radiation defects typical of ordinary superconductors does not occur. In terms of the ordinary mechanism, the anomaly may be explained by a significant reduction in the conduction electron density resulting from a loss of oxygen, which causes a significant rise in ρ with a small change in the scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 1372–1376 (August 1999)     

Search for supersymmetry using final states with one lepton, jets, and missing transverse momentum with the ATLAS detector in √s=7 TeV pp collisions

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √s=7 TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35 pb(-1). No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for A(0)=0 GeV, tanβ=3, and μ>0 and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level.     

Effect of high pressure and chemical substitution on the crystal structure and magnetic state of R2Fe17 ? xSix (R = Lu or Y; x = 0 or 1.7)

The effect of high pressure on the crystal structure of the intermetallic compounds R₂Fe_{17 − x} Si_x (R = Lu or Y; x = 0 or 1.7) was studied using neutron diffraction. A correlation between changes in structural parameters and magnetic properties under the action of high pressure, as well as under chemical substitution of Si for Fe atoms, was analyzed in terms of localized moment and spin fluctuation models. The spin fluctuation model was found to describe more adequately the experimentally observed increase in the Curie temperature upon chemical substitution and the decrease in this temperature under the action of high pressure. Possible reasons for the suppression of a collinear ferromagnetic state and the occurrence of a noncollinear antiferromagnetic state in R₂Fe_17−x Si_x under pressure are discussed based on estimated differences between the total energy minima of these states.     

Superlattice structures in solid solution of high-Tc YBa2Cu3O7−δ and (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7−δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7−δ has been found with general stoichiometry (Pb_0.75xCu_1−0.75x)(Sr_2xBa_2−2x)(Ca_0.5xY_1−0.5x)Cu₂O_7−δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship with the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.     

Rapid ultrasound-assisted magnetic microextraction of gallic acid from urine,plasma and water samples by HKUST-1-MOF-Fe3O4-GA-MIP-NPs: UV–vis detection and optimization study

Magnetite (Fe₃O₄ nanoparticles (NPs)) HKUST-1 metal organic framework (MOF) composite as a support was used for surface imprinting of gallic acid imprinted polymer (HKUST-1-MOF-Fe₃O₄-GA-MIP) using vinyltrimethoxysilane (VTMOS) as the cross-linker. Subsequently, HKUST-1-MOF-Fe₃O₄-NPs-GA-MIP characterized by FT-IR, XRD and FE-SEM analysis and applied for fast and selective and sensitive ultrasound assisted dispersive magnetic solid phase microextraction of gallic acid (GA) by UV–Vis (UA-DMSPME-UV-Vis) detection method. Plackett–Burman design (PBD) and central composite design (CCD) according to desirability function (DF) indicate the significant variables among the extraction factors vortex (mixing) time (min), sonication time (min), temperature (°C), eluent volume (L), pH and HKUST-1-MOF-Fe₃O₄-NPs-GA-MIP mass (mg) and their contribution on the response. Optimum conditions and values correspond to pH, HKUST-1-MOF-Fe₃O₄-NPs-GA-MIP mass, sonication time and the eluent volume were set as follow 3.0, 1.6 mg, 4.0 min and 180 μL, respectively. The average recovery (ER%) of GA was 98.13% with desirability of 0.997, while the present method has best operational performance like wide linear range 8–6000 ng mL⁻¹ with a Limit of detection (LOD) of 1.377 ng mL⁻¹, limit of quantification (LOQ) 4.591 ng mL⁻¹ and precision (<3.50% RSD). The recovery of GA in urine, human plasma and water samples within the range of 92.3–100.6% that strongly support high applicability of present method for real samples analysis, which candidate this method as promise for further application.     

Crystal field effects on the magnetic and hyperfine properties of rare earth ions in RE2 (SO4)3 · 8H2O (RE = Pr,Dy and Er)

An analysis of the observed magnetic properties of Pr-sulfate, Dy-sulfate and Er-sulfate has been undertaken on the basis of the interaction of the rare earth ion with the lattice. An effective crystal field conforming to tetragonal symmetry, which is the approximate site symmetry of the RE³⁺ ion, gives an excellent simulation of the observed behavior. The g-factors and Schottky anomalies in the heat capacity are determined theoretically. Various hyperfine properties including the thermal variation of Mössbauer spectra are predicted in light of the proposed crystal field.     

Elastic moduli and step-like elastic anomalies in Ce-substituted Tl0.9Bi0.1Sr2−xCexCa0.9Y0.1Cu1.99Fe0.01O7−δ (x = 0–0.20) superconductors

Ultrasonic velocity measurements have been performed on polycrystalline Tl_0.9Bi_0.1Sr_2?xCe_xCa_0.9Y_0.1Cu_1.99Fe_0.01O_7?δ superconductor to study the influence of Ce substitution on elastic properties of the samples. Ce was observed to influence elastic moduli at 80 K which showed the largest value obtained at x = 0.10 where, coincidentally, the highest superconducting temperature T_c among samples was also shown. A longitudinal velocity anomaly was observed at around 260 K for the unsubstituted sample (x = 0). Ce substitution caused the temperature of the elastic anomaly to shift to 250 K (x = 0.1) and 262 K (x = 0.2). The existence of the step-like elastic anomaly was suggested to be due to oxygen ordering taking place in Tl–O planes. The analysis of elastic behavior in the vicinity of the elastic anomalies using the Landau free-energy model suggests that the anomaly is due to a phase transition which involves oxygen ordering.     

Structural and magnetic properties of Fe<Subscript>m</Subscript>Y<Subscript>n</Subscript> (m + n = 7, Y = Ru,Rh, Pd,and Pt) nanoalloys

The structural and magnetic properties of the small binary clusters Fe _m Y _n (with m + n = 7, Y = Ru, Rh, Pd, Pt) were studied through extensive ab initio calculations, by means of the fully unconstrained version of the density-functional method, as implemented in the SIESTA code, within the generalized gradient approximation. The lowest energy state geometries, the chemical ordering, and the electronic and the magnetic structures were calculated. We found that the lowest energy geometrical structures for the pure Ru, Rh, Pd, Pt, and Fe heptamers, are a cube without an apex, a triangular prism capped on a square face, a decahedron, a side capped double square, and a decahedron, respectively. Starting from these geometries of the pure element heptamers, we followed the changes in the geometric structure as a function of the chemical composition. We analyzed all the different chemical arrangements, which depend on the particular geometry, and magnetic moment orientations, in the whole range of compositions. In general, there are important modifications to the magnetic moment of the Y atoms as soon as one of them is substituted by an Fe atom in the cluster. In contrast, under the same circumstances, the Fe magnetic moment takes values larger than 3 μ _B and keeps almost this value, insensitive to the structure, composition and chemical order of the system.