Superconducting—normal phase boundaries of (La,Th)Ce systems

The dependence of the superconducting transition temperature T_c on Ce impurity concentration n is reported for various ($\text{La, Th}$)Ce systems. These results document for the first time the evolution of the superconducting— normal phase boundary (T_c vs n curve) as an impurity undergoes magnetic- nonmagnetic transition which for Ce impurities in ($\text{La, Th}$)Ce systems proceeds with increasing Th concentration. Observed for only the second time, re-entrant T_c vs n curves are reported for two La-rich La, Th host compositions.     

The impurity resistivity and resistivity minima in AlCr and AlMn alloys

The impurity resistivity of $\text{Al}$Cr between 1.5 and 50°K was determined with a characteristic temperature for the T² variation θ₁=960±40°K. The behaviour of the resistivity minimum both in $\text{Al}$Cr and $\text{Al}$Mn alloys with impurity concentration provides evidence that a T³ phonon resistivity is found also in aluminium with anomalous impurity resistivity.     

Effect of pressure on the Curie temperature of PdFe,PdCo, PtFe and PtCo alloys

The Curie temperatures of $\text{Pd}$Fe, $\text{Pd}$Co, $\text{Pt}$Fe and $\text{Pt}$Co alloys with Fe or Co concentrations between 1 and 10 at. % have been measured by thermal scanning of the ⁵⁷Fe Mössbauer resonance under pressures up to 170 kbar. Systematically, the largest (positive) values of the logarithmic derivative, dlnT_e/dlnV, are found in the most dilute alloys. The data are compared with models, which relate the variation of T_c under pressure to changes in the impurity spin-conduction electron coupling parameter, J_i, and to changes of the pure host metal susceptibility, χ_h. The discrepancies between the experimental data and these models are discussed.     

Resistivity of dilute AlMg and AlMn between 0° and 100°C a generalized approach

We have measured the resistivity of $\text{Al}$Mg and $\text{Al}$Mn up to 0.5 at. % impurity concentration between 0° and 100°C. The results for the resistivity for both systems can be analysed within one generalized model in which spinfluctuations as well as changes in the electron-phonon interaction are considered. For $\text{Al}$Mn a calculation of the impurity contribution to d?/dT is attempted. For $\text{Al}$Mg there are indications that the superconducting T_c will increase with Mg concentration.     

Specific heat of superconducting Th,Sc and Th,Y alloys with Ce impurities

Measurements of the specific heat jump ΔC at the superconducting critical temperature T_c on ($\text{Th, Sc}$)Ce and ($\text{Th, Y}$)Ce Ce impurity, solid solution alloy systems indicate that the former systems obey the BCS law of corresponding states (LCS) characteristic of superconductors with non-magnetic impurities while the latter systems present deviations from the LCS linear relation between reduced parameters which are attributed to the development of localized moments at the Ce ions as the Y concentration increases.     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Ce impurities

The dependence of the superconducting transition temperature T_c on Ce impurity concentration and the specific heat jump at T_c as a function of T_c are reported for the system ($\text{La}$Ce)Sn₃. The experimental results are analyzed using a theory due to Kaiser concerning the effect on superconductivity of nonmagnetic localized resonant impurity states This analysis yields values for the intraatomic Coulomb repulsion parameter U_eff and the Ce local density of states at the Fermi level N_? (E_F). The results of low temperature normal state heat capacity and magnetic susceptibility measurements which give independent estimates of N_? (E_F) are also reported. A large pressure dependence of the T_c of ($\text{La}$Ce)Sn₃ alloys was observed for pressures up to 20 kbar. This behavior is similar to that previously observed in several other superconducting matrix-Ce impurity systems in which the Ce solute 4f electron shell undergoes a continuous-pressure induced demagnetization.     

Rapid differential decay of metastable populations in a copper halide laser

The time dependences of the populations ofthe copper metastable levels (²D_{$\text{5}\text{2}$}, ²D_{$\text{3}\text{2}$} have been measured in a longitudinally-excited CuBr laser of tube diameter 12 mm at a dissociation energy density of 15 mJ cm^-3. The metastable populations are created within 3 μs of the dissociation discharge pulse and thereafter decay to the ground level by electron collisions. The decay of the ²D_{$\text{3}\text{2}$} population is faster than that of the ²D_{$\text{5}\text{2}$} population and, in the case of CuBr, both decay rates are increased significantly by the decomposition products which arise from the presence of cupric oxybromide as an impurity. Decay times for the ²D_{$\text{5}\text{2}$}, ²D_{$\text{3}\text{2}$} populations as short as 4, 1.8 μs respectively have been observed under optimum laser conditions. The increase in the relative intensity of the 578.2 nm line at the shortest interpulse delays (ca 10 μs) is due to the more rapid decay of the ²D_{$\text{3}\text{2}$} population. At times up to the several microseconds after the dissociation current pulse the gas in the laser tube remains highly conducting. The rise-time of the pumping pulse is then determined by the external circuit rather than by the transit time of an ionization wave. The controlled effect of the cupric oxyhalide impurity in both CuBr and CuCl lasers offers the prospect of more efficient operation of copper halide lasers at repetition rates in excess of 20 kHz.     

Spin-flip and resistivity scattering of conduction electrons due to metallic impurities dissolved in lithium and sodium

We analyse the impurity induced scattering cross sections with (σ_sf) or without (σ_r) spin-flip, for conduction electrons in lithium and sodium. We show that the experimental values of $\text{σ}{}_{\mathrm{sf}}\text{σ}{}_{\mathrm{r}}$, are approximately equal to the Elliott parameter $\text{(}\text{λ}\text{Δ}\text{)}{}^2$ (λ is the spin-orbit splitting of an impurity state (A) close to the impurity conduction band (C), and Δ is the energy separation between (C) and (A). We conclude that the Elliott calculation is valid not only for a spin-independent scattering potential, but also for the perturbation due to heavy impurities (strong spin-orbit coupling) dissolved in light metals (weak spin-orbit coupling).     

Impurity conduction and negative magnetoresistance in compensated n type indium phosphide,at low temperature

Electrical resistivity, mobility, magnetoresistance in compensated n type InP with a carrier density n = 7,2 10¹⁶/cm³ have been measured in the temperature range 1,7 K – 100 K, in magnetic fields up to 0,9 Tesla. Under a temperature T_C, which corresponds to the formation of an impurity band in which the electron conduction is metallic, the magnetoresistance becomes negative. Longitudinal and transverse magnetoresistances have similar behaviours. Longitudinal magnetoresistance which is treated here, has the behaviour of a system of paramagnetic spins for which the Curie point is determined. The variation of the negative magnetoresistance versus magnetic field and temperature is quite well approximated by a Langevin function L(u) with $\text{u}\text{=}\text{μ}{}^1\text{B}\text{kT}$; $\text{μ}{}^1\text{=}\text{n}\text{μ}{}_{\mathrm{<mtext>B</mtext>}}$ is the effective magnetic moment where μ_B is the Bohr magneton and n = 5. The value of the effective magnetic moment and the average separation of donors, allow the discussion of the negative magnetoresistance on the basis of the Toyozawa model. Other models are discussed and it seems that in the range of impurity concentrations concerned, the picture of localised moments is more consistant that a “mobility edge” model.     

Multiphoton excitation of visible emission in Y2O2S: Er,Yb phosphors by 810 nm excitation

During investigations of anti-Stokes effects in erbium—ytterbium doped yttrium oxysulphide phosphors we have found an excitation of the erbium green emission due to 810 nm radiation (⁴I$\text{15}\text{2}$ → ⁴I$\text{9}\text{2}$ transition), the effect being of a two-photon type. There is also a two-photon excited blue emission (²H$\text{9}\text{2}$ → ⁴I$\text{15}\text{2}$ transition) due to the same radiation. Introduction of ytterbium impurity has a deleterious effect on the above 810 nm excitation, in contrast to its sensitization of the 1000 nm anti-Stokes excitation. To distinguish between various excitation schemes a two-beam excitation technique has been used. Results show that ytterbium inclusions lead to loss of energy from the 810 nm excitation by increasing the probability of downward transitions from the ⁴F$\text{9}\text{2}$ level to ⁴I$\text{11}\text{2}$ and ⁴I$\text{13}\text{2}$ states, reached after onephoton absorption. This effect is indicated by an increase in 1500 nm emission (⁴I_{$\text{13}\text{2}$} → ⁴I_{$\text{15}\text{2}$} with ytterbium concentration.     

Transition temperature and specific heat discontinuity for superconducting alloys containing Kondo impurities

The dependence of the transition temperature and the specific heat discontinuity of a superconductor on the magnetic impurity concentration is calculated selfconsistently using the spin-boson approach for the Kondo problem. The results agree well with experimental dat for the (La, $\text{Ce}$) Al₂ alloy.     

A note on the impurity spin magnetisation of dilute alloys

Double time temperature dependent anticommutator Green's function has been used to obtain magnetisation of the impurity spin in s-band host metals. The function φ_Ji which is related to the impurity spin magnetisation shows a break in the magnetisation vs temperature curve when $\text{|(}\text{H}{}_0\text{J}\text{) × (}\text{g}{}_{\mathrm{i}}\text{S}{}_{\mathrm{e}}\text{(S}{}_{\mathrm{e}}\text{+ 1))|}$ lies within 1.50. The effect of crystal field of the host lattice on the impurity spin magnetisation has also been considered.     

Electronic structure of thallous centres and Tl+ - Vk recombination in KCl crystal

Parameters of impurity Tl ion required for the calculation of different thallous centres in alkali halides are obtained for the semiempirical method of the intermediate neglect of differential overlap (INDO). The electronic structure of Tl⁺, Tl²⁺ centres in KCl is calculated. The potential energy curves for the recombination of nearest Tl⁺, V_$\text{k}$ centres against the breathing vibrational mode of the V_$\text{k}$ centre are calculated. This recombination (hole trapping) is found to be nonradiative tunneling with small activation energy rather than radiative one. A number of experimental data is also discussed in the light of the present calculations.     

The oxidation of Ca,Sr, and Ba studied by appearance potential spectroscopy

The reaction of oxygen with evaporated films of Ca, Sr and Ba has been studied with soft X-ray appearance potential spectroscopy (SXAPS). Both O 1s and Ca 2p_{$\text{1}\text{2}\text{,}\text{3}\text{2}$}, Sr 2p_{$\text{3}\text{2}$} and Ba 3d_{$\text{5}\text{2}$} spectra were recorded. For Ca and Sr two distinct reactivity regimes were distinguishable: (i) a low exposure regime characterized by a single broad peak in the O 1s spectra and unchanged metal spectra, (ii) a high exposure regime or oxide regime characterized by multipeaked O1s spectra and appreciable changes in the metal spectra. The exposure at which the nucleation of oxide starts is determined. The bonding in the metal-oxygen complex is discussed.     

Magnetic moments of the 12+ isomeric states in 194, 196, 198Pb

The g-factor of the 12⁺ isomeric state in lead isotopes with A = 194, 196, 198 was measured using the time-differential perturbed angular distribution method (TDPAD). The values obtained are respectively g(194) = ?0.158(6); g(196) = ?0.157(7); g(198) = ?0.144(11). A more precise determination of the 12⁺ level half-life is also made. The g-factprs of these nuclear states, which are described with v(i_{$\text{13}\text{2}$})^?2 as the main configuration, are surprisingly constant over a large mass range (between A = 206 and 194). A core polarization analysis explains this trend: the polarization induced on neutrons in i_{$\text{13}\text{2}$} orbit decreases with the mass number A (blocking effect), but a compensation is provided by the other spin-orbit partners f_{$\text{7}\text{2}$}-f_{$\text{5}\text{2}$} and P_{$\text{3}\text{2}$}-P_{$\text{1}\text{2}$}.     

Impurity density dependence of cyclotron-resonance line width

It is shown that the by now commonly accepted expression for the cyclotron-resonance line width leads to a n_s${}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$-dependence for InSb, where n_s is the impurity concentration. The order of magnitude is in agreement with experiment.     

2S12 state ions in inorganic luminescent materials

Selected examples of impurity ions with one unpaired electron in an unfilled shell are Pb³⁺ ions in II–IV compounds (ZnO, ZnSe, ZnTe), Pb³⁺ in CaWO₄,F centres in alkali halides and Ag⁰ and Au⁰ atoms in radiophotoluminescent (RPL) glasses. Comparative studies of EPR, luminescence, thermoluminescence and related phenomena were performed on systems with ²S_{$\text{1}\text{2}$} state ions as central ions and systematically varying ligand ions in the temperature range from 1.6°K to 450°K. Luminescence bands attributed to internal transitions ²P_{$\text{3}\text{2}$,$\text{1}\text{2}$}→²S_{$\text{1}\text{2}$} ground state are reported especially with respect to the corresponding EPR data. All data are summarized in a molecular orbital model for the luminescence complex.     

The crystal structure of SnYb3Rh4Sn12, a new ternary superconducting stannide

The crystal structure of superconducting SnYb₃Rh₄Sn₁₂ has been determined from single-crystal X-ray diffraction data. This compound is cubic, space group Pm3n, $\text{a}{}_{\mathrm{o}}\text{= 9.676}\text{A}\text{?}$ (1) and has two formulae per unit cell. The structure was solved from Patterson and subsequent Fourier synthesis. The least squares refinement was based on 375 independent reflections. The final R and wR factors were 0.015 and 0.014, respectively. The two Sn(1) atoms occupy the 2a (000) positions, the six Yb atoms the 6d ($\text{1}\text{4}$$\text{1}\text{2}$ 0) positions, the eight Rh atoms the 8e ($\text{1}\text{4}$$\text{1}\text{4}$$\text{1}\text{4}$) positions and the twenty-four Sn(2) atoms the 24k (Oyz) positions (y ～ 0.31, z ～ 0.15). The Sn(2) atoms form a tridimensional array of corner-sharing trigonal prisms whose centers are occupied by the rhodium atoms. The Sn(1) and the Yb atoms occupy the icosahedral and cuboctahedral holes of this array, respectively. They form a sublattice which has the arrangement found in the structure of the A15 compounds. The structure of SnYb₃Rh₄Sn₁₂ can be described as containing two interpenetrated structures, namely Yb₃Sn and RhSn₃, or as having an A15 arrangement of clusters of atoms such as (SnSn₁₂) and (YbSn₁₂). These clusters are bound together by face-sharing among them; and by the rhodium atoms. An analogy is drawn between SnYb₃Rh₄Sn₁₂ and the perovskite-like ternary oxides A′A″₃B₄O₁₂.     

Determination of the deformation potential of shallow donors in silicon

The stress dependence of the energy of the ground state of group V impurities (P, Sb) in silicon was investigated by measurement of the Hall effect in a wide range of pressures. A conclusion was reached that the deformation potential of the lowest impurity state of shallow donors (Ξ¹_u) in silicon differs from the deformation potential of the conduction band (Ξ_u), the value of this difference being dependent on the type of the impurity. According to our data, the most probable values for ($\text{Ξ}{}_{\mathrm{u}}\text{ Ξ}{}^1{}_{\mathrm{u}}$) are 0.12 eV for phosphorous-doped silicon and 0.06 eV for antimony-doped silicon.     

Monopoles,duality, triality

The existence of magnetic charges could be a raison d'être not only for the quantization of electricity in units $\text{1}\text{3}\text{e}$ but also for the confinement of the quarks at the end of “observable”, “electric” Dirac strings (quarks have no magnetic charge in this scheme). We first review the Dirac quantization condition, using a “sum over histories” approach, and get a more general result: A string attached to the dyon (e₁, g₂) is observable by a dyon (e₂, g₂) unless (1?x) e₂g₁ ? xe₁g₂ = nh, where x is an arbitrary parameter which reflects an ambiguity in the action principle. The Dirac and Schwinger-Zwanziger quantization rules are special cases, with $\text{x = 0}\text{and}\text{1}\text{2}$, respectively. Then, we look for the values of x and of the magnetic charges to “explain” that (i) the electric charge is quantized in units $\text{1}\text{3}\text{e}$, (ii) the string attached to an electron is unobservable, (iii) the string attached to a quark is observable. We find a denumerable set of solutions. In most cases, the magnetic charges also are connected with observable strings.