Surface-field induced interband tunneling in InAs

Metal/insulator/semiconductor junctions are prepared on degeneratep-type InAs substrates with hole concentrations ranging from 2.3×10¹⁷ cm^–3 to 2.7×10¹⁸ cm^–3. The low work function of the top metal Yb, Al, or Au and charged interface states influence a two-dimensional (2D) electron inversion layer at the InAs surface. The insulator barrier that is formed by thermal oxidation is designed sufficiently thin, so that the bias voltage applied at the metal electrode mainly drops across the depletion layer separating the electron channel from the bulk. The current-voltage (I–V) characteristics exhibit strong negative differential conductance due to interband, tunneling from the 2D subband into the 3D valence band with peak-to-valley current ratios up to 3.1, 18, and 32 at 300 K, 77 K, and 4.2 K, respectively. In agreement with a theoretical model based on coherentelastic tunneling, the form of the I–V curves resembles those of double-barrier resonant tunnel devices rather than those of 3D Esaki diodes. The series resistance is obtained from the saturation of the differential conductance dI/dV at high forward bias and from the shift of structures in d² I/dV ² arising from phonon assisted tunneling.Dedicated to G. Lautz on the occasion of his 65th birthday     

Contractive multifunctions, fixed point inclusions and iterated multifunction systems

We study the properties of multifunction operators that are contractive in the Covitz-Nadler sense. In this situation, such operators T possess fixed points satisfying the relation x∈Tx. We introduce an iterative method involving projections that guarantees convergence from any starting point x₀∈X to a point x∈XT, the set of all fixed points of a multifunction operator T. We also prove a continuity result for fixed point sets XT as well as a “generalized collage theorem” for contractive multifunctions. These results can then be used to solve inverse problems involving contractive multifunctions. Two applications of contractive multifunctions are introduced: (i) integral inclusions and (ii) iterated multifunction systems.     

Exchange of stability and finite-dimensional dynamics in a bifurcation problem with marginally stable continuous spectrum

We consider solutions bifurcating from a spatially homogeneous equilibrium under the assumption that the associated linearization possesses continuous spectrum up to the imaginary axis, for all values of the bifurcation parameter, and that a real eigenvalue crosses the imaginary axis. For a model we investigate the nonlinear stability of the trivial solution with respect to spatially localized perturbations, prove the occurrence of a pitchfork bifurcation of equilibria and the nonlinear stability of the bifurcating equilibria, again with respect to spatially localized perturbations.     

Remote-doping scattering and the local field corrections in the 2D electron system in a modulation-doped Si/SiGe quantum well

The small, about 30% magnetoresistance at the onset of full spin polarization in the 2D electron system in a modulation-doped Si/SiGe quantum well gives evidence that it is the remote doping that determines the transport scattering time. Measurements of the mobility in this strongly-interacting electron system with remote-doping scattering allow us to arrive at a conclusion that the Hubbard form underestimates the local field corrections by about a factor of 2.     

Ions and atoms in superfluid helium (4He)

We review the previous conclusion [J.Y. Ryu, Y.C. Chung and S.D. Choi, Phys. Rev. B 32, 7769 (1984)] that the trace property Tr(ABC) = Tr(CAB) leads to two different cyclotron transition absorption formulae in the electron-phonon systems in the lowest order approximation. The pictorial expression and the calculated linewidths in Ge and Si show that the socalled EWC scheme is more seeming than the socalled MWC scheme. The difference is expected to disappear if we take into account all the higher order perturbation terms or start with the many body formalism in the complete scheme.     

Phosphinsubstituierte Chelatliganden. XVIII. Penta- und Tetracarbonylmetall-Komplexe des Chroms,Molybdäns und Wolframs mit sekundären und tertiären Phosphinothioformamid-Liganden

Phosphine Substituted Chelate Ligands. XVIII. Penta- and Tetracarbonylmetal Complexes of Chromium, Molybdenum, and Tungsten with Secondary and Tertiary Phosphinothioformamide Ligands Mono- and bidentately coordinated phosphinothioformamide complexes are obtained by photochemical substitution of the metal hexacarbonyls M(CO)₆ (M ? Cr ( a ), Mo ( b ), W ( c )). The M(CO)₅ · THF adducts react with secondary thioamides under exclusion of light to give the P-coordinate pentacarbonyl complexes [(CO)₅MPPh₂C(S)NHR¹] (R¹ ? Ph ( 1a – c ), Me ( 2a )). The photoreaction of M(CO)₅ · THF with secondary and tertiary thioamides at low temperatures leads to the formation of the P, S-chelate complexes . The corresponding N-silylated complexes 6a – c (R¹ ? Me₃Si, R² ? Ph) are obtained by direct photosubstitution of M(CO)₆ in cyclohexane solution. The labile bis(thioformamide) complexes [(CO)₄M(PPh₂C(S)NHMe)₂] ( 7a – c , cis-trans isomers) are synthesized in low yields according to the same procedure. The attempted alkylation of the chelate complexes 3a – c remains unsuccessful, whereas the secondary thioformamides react with n-BuLi/CH₂Br₂ to give the methylene bis(thioformirnidoesters) [Ph₂PC(NR¹)S]₂CH₂ (R¹ ? Ph (8), Me ( 9 )) in quantitative yields.     

Die freie Oberflächenenergie binärer Systeme als Funktion der zwischen den Molekeln wirkenden Kräfte

Zusammenfassung Die freien molaren Oberflächenenergien binärer Systemea ^M werden durch Anteilwerte der Mischungskomponenten ausgedrückt. Diese Anteilwerte setzen sich proportional den Molenbrüchen aus dena-Werten der reinen Komponenten und einem Wechselwirkungsglieda ^GL zusammen. Die Formel füra ^M, bzw. ^M enthält daher außer den Molenbrüchen unda-Werten der reinen Komponenten nur die Größea ^GL.Diese Gleichung ermöglicht es daher, mit Hilfe einereinzigen Messung bei beliebiger Konzentration den Gesamtverlauf der freien Oberflächenenergie bei allen Konzentrationen zu berechnen. Die Gleichung ist auch bei Dipolstoffen, welche in Lösung assoziiert sind, anwendbar. Die Dipolassoziation wirkt sich nur dann aus, wenn die betreffende Molekel klein oder das Moment besonders groß ist. In diesem Falle kann unter Annahme einer empirisch zu ermittelnden wirksamen Assoziationszahl eine Korrektur der Molenbrüche vorgenommen werden.     

Thin-layer chromatography combined with diode laser desorption/atmospheric pressure chemical ionization mass spectrometry

The desorption of an analyte by a continuous wave diode laser from a porous surface of a thin-layer plate covered with a graphite suspension is presented. The thermally desorbed analyte molecules are ionized in the gas phase by a corona discharge at atmospheric pressure. Therefore, both essential processes--the desorption and the ionization of analyte molecules, which are often performed in one step--are separated. The target preparation is easy and fast since no additional extraction process is required. The mass spectrometric background signal was mostly limited to the low mass range showing no interference with typical compounds of interest. In this study, the calmative and antihypertensive drug reserpine was chosen as model analyte, which is often used for specification of mass spectrometers. No fragmentation was observed because of efficient collisional cooling under atmospheric pressure. The influence of diode laser power and the composition of the graphite suspension were investigated, and a primary optimization was performed.