Surface condition and electron emission from cold cathodes in vacuum and in noble gas glow discharge

An optogalvanic method is used to measure photoemission coefficient γ_ph in a gas discharge exposed to the resonant radiation of helium atoms. The range of working current j/P _He ² (j is the current density, and P _He is the gas pressure) extends from 2 to 1000 μA/(cm² Torr), and field strength E/N at the cathode varies from 0.45 to 13 kTd. Up to j/P _He ² = 10 μA/(cm²Torr²), photoemission coefficient γ_ph grows and then tends toward saturation at a level of γ_ph = 0.30 ± 0.01. Under the no-discharge conditions, γ_ph = 0.35 ± 0.05. It is concluded that the emissivity of cold cathodes in a gas discharge is governed by adsorption of the working gas on the cathode surface and its implantation into the cathode. With allowance for this factor, the contribution of photoemission to the discharge current is reconsidered. It is shown that, for cathodes with diameter d _c ? l _c (l _c is the length of the cathode layer), a normal or weakly abnormal glow discharge in noble gases is largely of a photoelectron character. In light noble gases, the photoelectron character of the discharge persists even for a strongly abnormal discharge. The energy dependences of coefficients γ of kinetic and potential emission in helium are calculated with allowance for implantation of helium atoms into the cathode and compared with published data. The influence of particle implantation on γ in a vacuum is estimated.     

Two-photon absorption method for the study of collisional relaxation of atoms in a perturber bath

A two-photon absorption study was performed with sodium vapour at about 300°C perturbed by noble gases at densities between 0.5 x 10¹⁹ and 6 x 10¹⁹ atoms/cm³. The collisional relaxation of the 3P_{$\text{3}\text{2}$} and 3P_{$\text{1}\text{2}$} states has been measured for several noble gases. The origin of relaxation is interpreted as the result of a reactive collision leading to an excited molecular state. The recombination rate coefficients of the three-body reaction are calculated and their values are compared to those given by Scheps and Gallagher.     

Supraleitung und Temperverhalten abschreckend kondensierter Germanium-Edelmetall-Legierungen

Thin films of Ge-Cu, Ge-Ag and Ge-Au alloys have been condensed onto substrates held at 4 K. An amorphous, metallic phase has been obtained. The superconductivity of this phase is believed to be due to Ge which is forced into a liquidlike structure with a higher coordination number than that of the semiconducting diamond structure. The maximum transition temperatures of the Ge-Cu and Ge-Au films are 3.3 and 3.6 K, respectively, whereas Ge-Ag films show a maximumT _c of 1.2 K. The difference inT _c is explained by the band structure of the noble metals.     

The zero-distribution of the great partition function in the complex fugacity plane of gases of point particles with logarithmic interaction

The equation of state for gases of point particles with logarithmic interaction is derived. The system exhibits a phase transition at a critical temperature T_c. The critical temperature is a function of the dimension of the system. A hard core must be added below T_c to prevent the system from collapsing. The specific heat diverges on both sides as |T_c - T|^-2 in any dimension.For T≤T_c there are no zeros of the grand partition function in the complex fugacity plane, for T >T_c all zeros occupy the whole negative real axis. The density of zeros will be calculated.     

Positronium: Precision determination of the ground-state fine-structure interval Δv,and measurement of density shifts in the noble gases

The positronium n = 1 fine-structure interval, Δv, has been measured in nitrogen yielding Δv = 203.3849(12) GHz. Comparison with theory shows the need to calculate higher-order theoretical terms. The positronium density shifts in the noble gases were also measured.     

Superconductivity in palladium-noble metal-hydrogen systems

The apparently puzzling observations in the superconducting behavior of palladium—noble metal—hydrogen (deuterium) systems, viz., the occurrence of higherT _c and a monotonic increase inT _c for Au, Ag and Cu systems; the absence of the H(D) induced opposite isotope effect; the existence of an optimum noble metal content, different for different alloy systems, beyond which superconductivity ceases; and the existence of an optimum hydrogen content, different for different alloys, giving the highestT _c , have been accounted for by taking due cognition of the electronic and the crystallographic behavior of these alloys and their role in superconductivity, the participation of H(D)-optic phonons, in conjunction with anharmonicity, in BCS-type pairing interaction and the manifestations of the metallurgical and thermodynamical properties of these alloys in superconductivity.     

Influence of noble gases on electron cyclotron heated hydrogen plasma

A quartz-chamber 2.45-GHz electron cyclotron resonance ion source (ECRIS) is designed at Peking University for the diagnostic purpose. Experimental results show that the added noble gases in hydrogen could cause a decrease of electron temperature and an increase of electron density. In this work, a numerical model is upgraded for ECR plasma generated by mixed gases. On this basis, the model is applied for the composed hydrogen plasma with additional noble gases. And dependences of neutral gas density and electron density on electron temperature are presented for individual gas (He, Ar, and H₂) and gas mixture, respectively. The calculated results are basically in accordance with the diagnostic results of ECRIS.     

Experimental studies of two-photon absorption of sodium perturbed by collisions with noble gas atoms

This study was performed with sodium vapor at about 300°C perturbed by noble gases at densities between 0.5 x 10¹⁹ and 6 x 10¹⁹ atoms/cm³. We have measured the rate of two-photon absorption for step-wise processes, which occur if the exciting frequency is resonant with one of the transition frequencies ω_ri or ω_fr with i, r, and f referring, respectively, to the initial, the relay and the final state. By varying the exciting frequency, we obtain collision profiles which may be difficult to obtain by other methods. The shifts and widths for the transitions 3S-3P_{$\text{3}\text{2}$}, 3S-3P_{$\text{1}\text{2}$}, and 3P_{$\text{3}\text{2}$}-5S have been measured for several noble gases. The observed variation of the profiles with perturber gas pressure suggests possible quenching of the relay state by noble gas molecules present in the mixture.     

Electron impact ionization and excitation rate coefficients in the negative glow region of a glow discharge

Some easy to use reasonable approximations for electron impact rate coefficients have been considered. The most important rate coefficients for electron collisions in noble gases are electron-neutral ionization and electron impact excitation. Electron-neutral ionization besides electron impact excitation of some states of the argon and helium atom in direct current (dc) glow discharge plasma has been calculated. The plasma parameters of electron are significant factors for computing the rate coefficients. We present first results of probe diagnostic that includes the double probe measurements of the plasma parameters, namely, electron temperature (T_e) and electron density (n_e). Electron properties obtained from the double probe characteristic curves including T_e and n_e as well as the calculated rate coefficients (ionization and excitation) were studied as a function of the axial distance from the cathode while the discharge operating parameters of voltage and pressure were varied. Two regions of the glow discharge were investigated: cathode fall region and negative glow. Particular emphasis was placed on the negative glow region.     

Pseudogap phenomena in ultracold atomic Fermi gases

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tc superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum sinmlator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tc superconductivity. One obstacle to the ultimate understand- ing of high Tc superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics comnnmity, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fuctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tc superconductivity.     

Study of weakly interacting trapped Bose gas

We have obtained expressions for single particle density and two particle density ofweakly interacting trapped quantum gases. These are valid for all temperature and in anydimension. These expressions have been simplified and expressed in terms ofnon-interacting single particle density. The ground fluctuations for T<T_cin grand canonical ensemble has been treated with care using the method of Kocharovskyet al. [Phys. Rev. A 61, 053606 (2000)]. Some numerical results are presentedin one and three dimension for isotropic harmonically trapped Bose gas with contactinteractions. It is seen that boson density decreases with increasing repulsiveinteractions. The expression for critical temperature is also shown to agree with earlierresult and is in accordance with experiments.     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

Role of magnetism in the formation of a short-range order in an Fe-Ga alloy

The formation of a short-range order in an Fe-Ga bcc alloy has been studied by Monte Carlo simulation with the use of effective interaction potentials calculated within the density functional theory for the ferromagnetic and paramagnetic states. It has been found that the pronounced short-range order of the D0₃ type is formed at Ga concentrations close to the boundary of the two-phase region at T < T _c, whereas no short-range order is observed at T < T _c. The results obtained are in agreement with the experimental X-ray diffraction analysis. The relation of the features of the short-range order in the Fe-Ga alloy to the magnetostriction value has been discussed.     

Specific heat of A-15 Nb3Si produced by explosive compression

The specific heat from 1.2 to 23 K has been measured on a new high T_c superconductor, A-15 Nb₃Si. The sample was prepared by explosive compression and has an onset of bulk superconductivity at 18.0 K, with a transition width of 0.7 K. The density of states for pure A-15 Nb₃Si implied from the specific heat data is 0.94 ± 0.20 states/eV-atom, ΔC/γ T_c is 2.0 ± 0.2.     

Superconductivity in the amorphous Pd−Si−H alloys with noble-metal substitutes

Hydrogen (or deuterium) was implanted into amorphous Pd-noble metal-Si alloys at low temperatures. In all cases superconductivity resulted. Silicon concentration was varied between 17.1 and 20.1 at.%. The transition temperature,T _c , was primarily influenced by the silicon concentration with small decreases upon addition of noble metals. All the deuteriated alloys show lowerT _c 's compared to the corresponding hydrogenated ones, manifesting normal isotope effect.     

Enhancement of Jc by Hf-doping in the superconductor MgB2: A hyperfine interaction study

Measurements of the critical current density (J_c) by magnetisation and the upper critical field (H_c2) by magnetoresistance have been performed for hafnium-doped MgB₂. There has been a remarkable enhancement of J_c as compared to that by ion irradiation without any appreciable decrease in T_c, which is beneficial from the point of view of applications. The irreversibility line extracted from J_c shows an upward shift. In addition, there has been an increase in the upper critical field which indicates that Hf partially substitutes for Mg. Hyperfine interaction parameters obtained from time differential perturbed angular correlation (TDPAC) measurements revealed the formation of HfB and HfB₂ phases along with the substitution of Hf. A possible explanation is given for the role of these species in the enhancement of J_c in MgB₂ superconductor.     

Comparative performance of closed cycle gas turbine engine with heat recovery using different gases

The unique features of the closed cycle gas turbine engine have been recently contributing towards its adoption in a multiplicity of applications such as power plants, space, and marine power supplies. Therefore, research is reactivated to serve its future developments.In this work, a performance analysis is carried out using different gases such as air, combustion gases, CO₂ and helium. Operating variables are turbine inlet temperature T₀₃, compressor pressure ratio R_c and inlet temperature T₀₁. A computer program is taioored to calculate specific work W_s and overall efficiency η_o over a wide range of operating variables. Results show that helium gives relatively higher W_s, but η_o starts to drop early after a low optimum R_c. Air and combustion gases offer nearly equal performance.     

Korrelationseffekte in den Auger-Spektren von Edelgasen

The possibility of studying correlation effects through Auger electron spectrometry has been shown recently by Krause, Carlson and Moddeman in the case of theK Auger spectrum of neon. As a further example we have measured theM _4,5 Auger spectrum of krypton with high resolution. Correlation effects have been found via the strong deviations of relative intensities of Auger diagram lines (e.g.M _4,5 N ₁ N _2,3(¹ p ₁)) from theoretical values and via the occurence of double Auger transitions, where one electron is emitted and another is excited. A critical examination of high resolution Auger spectra of noble gases, which has been measured so far, has shown that several non diagram lines can be assigned to double Auger transitions of the above kind.     

Effect of structural water on the critical characteristics of highly textured YBa2Cu3O6.9

The effect of low-temperature treatment (200°C) in a humid argon atmosphere and subsequent annealing (930°C) on the critical parameters of a highly textured YBa₂Cu₃O_6.9 has been studied. During annealing at T = 200°C, the absorbed water is incorporated into the structure of the compound, which is accompanied by the deterioration of its superconducting properties. However, after the recovery annealing at T = 930°C and subsequent oxidation, the superconducting characteristics (j _c , B _1c , and F _p ) are improved. This is explained by the formation of 124-type planar defects, which are effective pinning centers, especially in high fields applied perpendicular to the c axis (⊥ c). The optimum conditions of double annealing substantially increasing the critical current density (j _c ≥ 10⁴ A/cm²) in an external magnetic field up to 10 T and also the first critical fields have been found. In fields up to ～3 T, the critical current density j _c is isotropic despite the conservation of high texture in the samples.     

Self-focusing of electromagnetic waves in a magnetoplasma

The steady state self-focusing of a Gaussian electromagnetic beam in a magneto-plasma has been studied. On a short time scale, a non-linearity in the dielectric constant of a plasma appears due to the ponderomotive force. This force in the case of the extraordinary mode has opposite signs forω>ω _c andω<ω _c, whereω _c is the electron cyclotron frequency. The self-focusing due to this effect is predicted at frequencies except forω _c /2<ω<ω _c . The focusing of the ordinary mode is adversely affected by the magnetic field. On a larger time scale, the non-uniform heating of electrons by the beam and the resulting redistribution of the electron density is a source of non-linearity. This non-local non-linearity is several orders of magnitude higher than the ponderomotive non-linearity. We predict self-focusing of the extraordinary mode only above the gyroresonance (ω>ω _c ), while the ordinary mode can be focused at all frequencies.