μSR studies in the heavy fermion systems UNi2Al3, UPd2Al3 and U2PtC2

We have performed Muon Spin Relaxation (SR) measurements of several heavy fermion systems: UNi₂Al₃ (single crystal), UPd₂Al₃ (single crystal) and U₂PtC₂ (polycrystal). ZF -SR measurements of UNi₂Al₃ show magnetic order below the Néel temperatureT _N=4.6K, with the muon precession frequency(T 0)=2.5MHz at low temperatures. The local field distribution obtained at low temperatures in UNi₂Al₃ resembles that of the incommensurate spin density wave (SDW) system (TMTSF)₂PF₆, suggesting an incommensurate spin arrangement in UNi₂Al₃. TF -SR measurements give the penetration depth(T 0)=6100Å for UPd₂Al₃ and(T 0)=7000Å for U₂PtC₂ respectively. Since UPd₂Al₃ and U₂PtC₂ are clean superconductors (/l-0), we can combine n _s/m^* with the Sommerfeld constant n _s ^1/3 m^* to calculate the Fermi temperatureT _F ^3/4–1/4, which represents the characteristic energy scale of the superconducting carriers. We obtainT _C/T_F 0.01 for both UPd₂Al₃ and U₂PtC₂, comparable to other exotic superconductors, but quite different from simple metal BCS superconductors, whereT _C/T_F 0.01.     

Gamow-Teller β transition from the 2− ground state of88Rb to the 3− excited state of88Sr

The Gamow-Tellerβ-transition from the ground state 2^? of⁸⁸Rb to the 3^? level at 2.734 MeV of⁸⁸Sr is studied. The nuclear matrix element 〈σ〉 and the lofft value are calculated using complete nuclear wave functions for the initial and final states. It is shown that, contrary to the normal assumption, the component ¦π2p ₃ ^2/?1 π1g _9/2〉 of the final state does give a very important contribution to 〈σ〉 due to the presence of strong cancellation effects. Although our calculations favour a wave function for the 3^? level⁸⁸Sr where neutron 1h—1p configurations are not included, there are still some facts which make that our results cannot be taken as conclusive. Nuclear Structure⁸⁸Rb,⁸⁸Sr, calculated the Gamow-Teller nuclear matrix element 〈σ〉 and the logft value using a shell model.     

The effect of LaPO4 in the LixLa2/3Ti1−xPxO3+x system

Lithium ion conductors of the overall composition Li_xLa_2/3Ti_1−xP_xO_3+x (hereafter referred to as LTP) based on La_2/3TiO₃ were prepared by solid state reaction at high temperature (1300 °C). AC impedance measurements indicate that the total conductivities are of the order of 10⁻⁴ S·cm⁻¹ when x=0.28 − 0.35 at room temperature and have an activation energy of 18 kJ·mol⁻¹ in the temperature range from room temperature to 400 °C. X-ray powder diffraction patterns showed that the LTP system has a complex composition, which contains the solid solution perovskite Li_3xLa_2/3−xTiO₃ and LaPO₄.     

Symmetry of superconducting compositions in the BaPb1−xBixO3 system

Reexamination of the symmetry of superconducting phases in the BaPb_1−xBi_xO₃ system was carried out by x-ray diffraction using a synchrotron source which offers exceptional resolution. These studies confirm our original finding of tetragonal symmetry. We believe that confusion on the symmetry has resulted because some compositions in this system are two phase mixtures.     

The electronic structure of the mixed 5f system UC x N1−x

We present results of electronic structure calculations for UC _x N_1–x obtained with the relativistic Korringa-Kohn-Rostocker Greens function (RKKR-GF) method. While on the anion sites the disorder is treated within the Coherent Potential Approximation (CPA) the small change in the cation potential upon alloying is accounted for in the averaget-matrix approximation (ATA). The presence of strong local spin fluctuations forx0.6 restricts our treatment to the carbon rich phase, and even there we find that the observed linear specific heat coefficient is much enhanced over the bandstructure value. The computed X-ray photoemission spectra vary smoothly with composition the main change consisting in the melting away of the side peak at –3 eV binding energy seen in pure UC with the adjunction of nitrogen.     

Normal state transport properties in single crystals of Ba1−xKxFe2As2 andNdFeAsO1−xFx

Hall effect and magnetoresistance Δρ/ρ(0) (MR) in the normal state have been measured on single crystals of Ba_1?xK_xFe₂As₂ and NdFeAsO_1?xF_x. Detailed analysis reveal the following conclusions: (1) For the parent phases of Ba_1?xK_x Fe₂As₂ and NdFeAsO_1?xF_x, large Hall effect and MR with strong temperature dependence were observed below a characteristic temperature corresponding to the antiferromagnetic/ structural transition. The field dependence of the Hall resistivity ρ_xy exhibits a non-linear behavior, which is accompanied by the violation of the B-square feature of the longitudinal magnetoresistivity Δρ_xx(B)/ρ_xx(0). A closer inspection further indicates that they are well related to each other and could be attributed to the multi-band effect or spin-related scattering. (2) The superconducting samples show much smaller Hall coefficient and MR in the normal state. The Hall coefficient shows a weaker temperature dependence compared to the parent phase, while the mean scattering rate 1/τ_H has a power-law like temperature dependence as 1/τ_H ∝ Tⁿ (n = 2–3). (3) For a Ba_1?xK_xFe₂ As₂ sample with T_c = 36 K, the field dependence of MR is complicated and the feature varies in different temperature regions. A drastic change of Δρ/ρ(0) was found between 80 K and 100 K, which corresponds very well to the maximum of the temperature derivative of the resistivity. This may be attributed to the spin-related scattering of electrons. (4) A comparison between the parent phase and the superconducting sample with T_c = 50 K in NdFeAsO_1?xF_x suggests that the electronic transport properties in the normal state cannot be easily understood with the simple multi-band model, while a picture concerning a suppression to the quasiparticle density of states at the Fermi energy is more reasonable.     

Mössbauer effect study of Ni1−xCuxMnyFe2−yO4 system

The Mössbauer effect technique has been employed for the study of magnetic properties of spinel series Ni_1?xCu_xMn_yFe_2?yO₄ with 0.0≤x≤1.0, and y=0.6. The substitution of Mn³⁺ and Cu²⁺ ions results in a slight decrease of the hyperfine field at B‐ as well as A‐sites. The area ratio of Fe³⁺ ions at the A‐ and B‐site at 77 K indicates that Cu²⁺, Ni²⁺ and Mn³⁺ ions occupy the octahedral sites in an evidence for complete inverse spinel in this system. The temperature dependence of the hyperfine parameters has been studied for composition with x=0.5 where Nèel point T_N and Debye temperature θ_D are found to be 650 and 679 K, respectively. The temperature dependence of the sublattice magnetization σ(T) obeys a one‐third‐power law in the range 0.5N<0.99.     

Variational study of the ground state energy of theE−b 1,b 2 Jahn-Teller system

In this paper a variational method for the ground state energy approximation of theE−b ₁,b ₂ Jahn-Teller system is presented. This method is based on the choice of a suitable variational ground state wave function. This trial wave function — a correlated squeezed state — is used to account for the correlation and anharmonicity of the interaction between the two vibrational modes; the anharmonicity of both modes is taken into account by the squeeze effects of these modes. The ground state of mode 1 in this trial wave function is considered as a linear combination of the two displaced harmonic oscillators. The ground state energies for the linearE - e Jahn-Teller system calculated by this method are not only in good agreement with the exact diagonalization results, but they are also better than those from the previous analytical studies. Another conclusion which results from the presented model is the following one: the squeezing effect of mode 1 for the linearE - e Jahn-Teller system is substantially smaller, in contrast with the results which are presented in the previous analytical studies.     

A computational study of the electron detachment energies of Al2As2 − and Al3As3 −

Ab initio and density functional theory calculations are reported for the low-lying electronic states of Al₂As₂ ^?, Al₂As₂, Al₃As₃ ^?, and Al₃As₃. The ²B_2g ground electronic state of Al₂As₂ ^? has a rhombic structure with the Al atoms occupying the shorter diagonal. In contrast, the As atoms occupy the shorter diagonal of the ground state rhombic structure of Al₂As₂. Electron detachment energies computed for Al₂As₂ ^? are presented and discussed. The adiabatic electron affinity of Al₂As₂ ^? is calculated to be 2.1 eV at the CCSD(T) level, using B3LYP and MP2 optimized geometries. The ground states of both Al₃As₃ ^? (²A₁′) and Al₃As₃ (¹A₁′) have planar hexagonal D_3h geometry. Electron detachment energies computed for the anion are reported. At the CCSD(T)//B3LYP level, the electron affinity of Al₃As₃ is calculated to be 2.47 eV.     

Neutral fermion excitations in the Moore-Read state at filling factor ν = 5/2

We present evidence supporting the weakly paired Moore-Read phase in the half-filled second Landau level, focusing on some of the qualitative features of its excitations. Based on numerical studies, we show that systems with odd particle number at the flux N(?)=2N-3 can be interpreted as a neutral fermion mode of one unpaired fermion, which is gapped. The mode is found to have two distinct minima, providing a signature that could be observed by photoluminescence. In the presence of two quasiparticles the same neutral fermion excitation is shown to be gapless, confirming expectations for non-abelian statistics of the Ising model with degenerate fusion channels 1 and ψ.     

High-resolution spectroscopy of the a4Σ−3/2 − X12Π3/2 system of gold monosulphide in the near infrared

ABSTRACT

We have recorded the a⁴Σ^? _3/2 ? X₁ ²Π_3/2 (0,0), (1,0), and (2,0) bands of gaseous gold monosulphide (AuS) at sub-Doppler resolution in the near-infrared region. The molecules were made in a hollow cathode discharge source by the reaction of sputtered gold with carbonyl sulphide. The high resolution of the laser excitation spectrum enabled the determination of molecular constants describing the rotational and ¹⁹⁷Au hyperfine structure in both states, as well as the spin–rotation interaction in the a⁴Σ^? _3/2 state. The natures of the two electronic states are discussed in the context of the observed hyperfine structure.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

Novel magnetic behaviour of the system ZnxCa1−xFe2O4

New magnetic system Zn_xCa_1−xFe₂O₄ (x = 0.4 to 0.6) is reported having extremely unusual properties like small ordered moments ≈ 0.13 ∼ 0.23 μ_B, large ordering temperatures T_c ≈ 700–750 K and surprising lack of hyperfine field (HF) at Fe for temperatures greater than T¹(≈ 0.25 T_c) in spite of bulk magnetization in the system. The HF appears below T¹ implying evolution of local moments far below T_c. Sharp susceptibility maxima resembling those of spin-glasses are observed for x = 0.4 and 0.5, which may be attributed to the single domain-superparamagnetic (SD-SP) transition and the overall behaviour shows a cluster spin-glass type of ordering.     

Magnetic properties of Fe−Mn−Al alloy system in the FCC disordered phase

In this work we report the experimental studies of Fe?Mn?Al alloys in the FCC disordered phase at room temperature by Mössbauer spectroscopy and X-ray diffraction. In this phase the alloys are antiferromagnetic with a constant mean hyperfine field ( \(\bar H\) ) near 26 kOe in the composition range from 0 to 7.5 at.% Al and 50 to 65 at.% Fe. When the Al or Fe concentration increases, the \(\bar H\) value gradually decreases to zero and the alloy becomes paramagnetic. In the same way when the Al concentration increases the lattice parameter increases linearly but when the Fe concentration increases the lattice parameter remains nearly constant for alloys with 5 at.% Al and decreases for alloys with 10 at.% Al.     

Superconductivity in the Ba1−x LaxPbO3 system

An analysis of the relationship between the local crystal and electronic structure of the Ba_1?x K_xBiO₃ and BaPb_1?x Bi_xO₃ perovskite systems, which was made in terms of an empirical model based on EXAFS spectroscopy studies of the above compounds, led to the conclusion that superconductivity is possible in the Ba_1?x La_xPbO₃ system. The Ba_1?x La_xPbO₃ multiphase compound synthesized at a pressure of 6.7 GPa was found to contain a superconducting phase with a critical temperature T _c ?11 K.     

Formation of CS− 2 in the γ radiolysis of CS2−3 MP glassy system at 77°K

Abstract

Tris-acetate buffer (0.14 M, pH7), alone or containing Shigella flexneri in suspension, was irradiated with 250 kVp X-rays or 7 MeV electrons in closed vessels or in vessels bubbled with gas containing oxygen. The removal of dissolved oxygen by the radiation was followed with a polarographic electrode.

The rate of removal of oxygen at tensions below 5 mm Hg was 2.0 mm Hg/K rad with both electron and X-irradiation of buffer solution. It was about 10% greater with Shigella in suspension. The rate of removal was lower at higher oxygen tensions, and the mean G_-o₂ for oxygen removal between air tension and zero was 2.8, compaied with 3.4 at all tensions below 5 mm Hg down to less than 0.2 mrn Hg.

Bubbling of the solution during inadiation did not alter the G-value significantly, since the fall in oxygen tension could be correlated well with the measured equilibration time constant of the solution and the G-value obtained in closed vessels.     

Magnetic moment of the 2140.2 keV 5− state in136Ba

We have determined theg-factor of the 2140.2 keV 5^– state of¹³⁶Ba using the timeintegral perturbed angular correlation (IPAC) method at an external magnetic field of 17.0 kG;g=–0.38±0.04. The half-lives of the 2140.2 keV 5^– and 2207.1 keV 6⁺ states were redetermined to be 1.5±0.1 ns and 3.1±0.1 ns, respectively. The radioactive source of¹³⁶Cs was prepared by the¹³⁸Ba(, pn) reaction.