ALBERT

Notes: Argon, hydrogen, and germane plasmas were investigated in a variably configured radio frequency (rf) diode glow-discharge reactor system with a range of rf powers. Plasma parameters such as electron temperature, plasma density, plasma potential, and floating potential were determined and the internal distribution of voltages within the glow discharge was considered. An equivalent circuit for the discharge is presented and fundamental dependence between the voltage ratio and the electrode area ratio of squared power law compared with the usual fourth power law is measured and described. The main goal of this work was to obtain more information about the energy of ion bombardment on the growing film surface.

Notes: The ion-induced secondary electron emission coefficient γ from the a-Si:H film deposited from a SiH4 glow discharge is measured in situ using an electrostatic grid analyzer. At low SiH4 pressure and discharge power density, γ≈0.033±0.004 and the overall electrical power dissipation does not vary with the gas and wall temperature between 25 and 250 °C, when the SiH4 molecular density is kept constant. However, the a-Si:H film deposition rate does depend on the temperature, which reveals the effects of thermally activated gas phase of H and SiH2 radicals arising from SiH4 dissociation.

Notes: Analysis of the leading nonlinear term of the susceptibility χ3(T) of the singlet-ground-state heavy-fermion superconductor UPd2Al3 shows evidence for a jump at TN and a "third-order'' Curie–Weiss type behavior at higher temperatures, inferring quadrupolar or magnetoelastic interactions. These effects are closely related to quadrupolar correlations observed in measurements of the nonlinear susceptibility and calculated in a crystal-electric-field approach for the singlet-ground-state systems URu2Si2 and PrNi5 .

Notes: We have performed a series of calculations on small models (number of atoms ranging from 10 to 34) of graphite and lithium intercalated graphite (LIG) at the UHF level with a minimal basis set for the valence electrons and an effective core potential for the core electrons (CEP-4G), where the basis and the CEP is optimal for free atoms. We have shown that small model hosts, such as C10 (Bernal i.e., AB) and C12 (primitive hexagonal, i.e., AA), enable us to make several predictions regarding LIG. Firstly, lithium looses its valence electron upon entering either type of host lattice and eventually falls into a body-centered position in an AA host lattice. Secondly, lithium strongly destabilizes the AB lattice, while it strongly stabilizes the AA lattice. Thirdly, the barrier for site hopping in the limit of infinite dilution (Ea) can be estimated along with a related quantity which we call the hilltop energy (see text). Further, we have shown that by building up to host models no larger than C32 (AA) we can make a better estimate of Ea (0.72 eV), determine that the dynamics of Li (within any two-dimensional solvated sheet) is largely determined by ionic interactions with screening from adjacent carbon layers effecting an approximately 20% reduction of naked two-dimensional Li Coulombic forces, and give a reasonable indication of how much energy is liberated as Li is moved from infinity to a vacant site in unsaturated LIG (1.1±0.7 eV).

Notes: Material properties of low-dimensional organic donor–acceptor (DA) metals are comparatively discussed. It is shown that the low-temperature superconductivity in organic solids and the nonconventional temperature dependence of the dc electrical conductivity σ have the same microscopic precursor, softening of lattice modes caused by a divergence of the generalized susceptibility on effectively flat Fermi surfaces (FS) (T dependence of σ), as well as the resulting enhancement of the electron–phonon coupling (superconductivity). Prerequisite for this behavior are sizeable electronic charge fluctuations 〈(Δn2i)〉 in the strongly correlated organic DA salts at any temperature. Nonvanishing fluctuations, which are maintained also in the limit of almost perfect interatomic correlations, are possible as a result of nonintegral effective electron densities n¯i≠1.0. The electronic charge fluctuations at T=0 K and finite temperature are studied by an analytic many-body model. The T dependence of the fluctuations is discussed as a function of the accessible electron density n¯i.Charge densities n¯i≠1.0 as realized in the highly conducting organic metals attenuate a possible temperature control on the fluctuations. For the integral 1:1 charge transfer salts of D+A− stoichiometry, negligible fluctuations (insulating Mott configurations) for temperatures 〈500 K are predicted. In addition to the T dependence of the electronic fluctuations, the mean-squared atomic fluctuations are studied as a function of temperature. The fluctuation amplitude diverges with increasing mode softening. Experimental consequences of this behavior are touched. A Kubo–Mori projector technique in a single-particle approximation is adopted to quantify the strong interrelation between the T variation of σ and FS properties (mode softening). Boundary expressions following a T−2 and T−1 law of σ(T) are formulated. The first relation is valid for systems with sizeable frequency renormalization; the second one for conventional metals with vanishing mode softening. The theoretical results suggest a new subclassification of the DA salts into two classes; FS properties serve as underlying criterion. The validity of the transport theory is demonstrated for the two-dimensional (BEDT-TTF)2ClO4 system. A two-mode model in the strong-coupling approximation is used to calculate superconducting transition temperatures TC of organic superconductors in the presence of Kohn mode softening. A remarkable influence of the frequency renormalization on the magnitude of TC is evaluated. An additional enhancement of TC is possible due to the divergence of the mean-squared atomic (lattice) displacement. Calculated TC numbers of (BEDT-TTF)2A superconductors are in fair agreement with experimental results.

Notes: The material properties of low-dimensional 2:1 donor (D)–acceptor (A) salts with integral mutual charge transfer (CT) are exceptional in the class of the synthetic metals. The electronic charge fluctuations in the majority component of D+2A− and A−2D+ compounds are of extremal character in the presence of strong electronic correlations. In the minority component the fluctuations are extensively suppressed. The corresponding solids are one-chain conductors. The charge fluctuations are investigated by a simple analytic model formulated on the basis of the local approach for the many-particle problem and the bond-orbital approximation for the independent-particle wave function. The Fermi surfaces of the D+2A− and A+2D− CT salts are effectively flat also for smaller anisotropy ratios. Structural phase transitions of the Peierls type leading to one-dimensional as well as two- and/or three-dimensional ordering are analyzed in a tight-binding framework. There exists a large range, where the one-dimensional dimerization is suppressed, but where n-dimensional (n=2, 3) ordering can still exist. Fermi-surface nesting in materials with smaller anisotropy ratios allows for the conservation of metastable solid-state configurations, i.e., configurations where the electron–lattice interaction is enhanced, but where a metal–insulator transition is suppressed. For the quasi-one-dimensional metals with D+2A− and A−2D+ stoichiometry it is shown that electronic correlations lead to a remarkable enhancement of the metal–insulator transition temperature TP. Experimental data available for the respective 2:1 CT salts can be rationalized by simple analytic models. A possible violation of particle–hole symmetry between D+2A− and A−2D+ compounds in their ability to stabilize a low-temperature superconducting ground state is suggested.

Notes: A four-grid electrostatic energy analyzer for measurements of the ion velocity distribution and the emission of secondary electrons on the electrodes of low-pressure radio frequency glow-discharge systems has been conceived. Problems arising from poor analyzer design are discussed and the performance of the presented analyzer is shown for measurements of the ion velocity distribution in pure hydrogen, helium, and argon discharges. Moreover, the secondary electron yields on aluminium, stainless steel, copper, and amorphous silicon exposed to radio frequency argon, helium, and hydrogen plasmas are determined in situ, for the first time to our knowledge. In parallel-plate radio frequency discharges secondary electron emission involves the contributions of ions, fast neutrals, metastables, and photons impinging on the electrode surfaces. Therefore, secondary electron emission must be considered as a global phenomenon. Global secondary electron emission can be up to ten times larger than only ion-induced secondary electron emission. Typically, the global emission coefficient is of the order of 0.1 (secondary electrons per ion) but can exceed 1 when the contribution of metastables is important. This is a noteworthy result since secondary electron emission is often neglected or underestimated in modeling of electrical discharges.

Notes: We demonstrate the first GaAs/AlGaAs multiquantum well infrared detector device with double wavelength selectivity and high wavelength resolution. By applying an efficient light coupling mechanism, which is based upon the excitation and emission of surface plasmons, we have achieved a responsivity R of 2.20 A/W and a detectivity D* of 2.2×1011 cm (square root of)Hz/W for λ=7.38 μm at a temperature T=5 K for a 300 K background irradiance and a 90° field of view, which are one of the highest values reported to date.