Untersuchung zur Energiebänderstruktur von Ta,Nb, W und Mo mit Isochromatenmessungen

Results of first measurements with the above transition metals are presented. Assuming with some other simplifications a commond-band for all measured metals it turns out to be possible to interpret the different shapes of the isochromats with full details. To that end a theoretically estimated density of states is used and verified in that way. In addition characteristic energy losses of the electrons in the different metals have to be considered for the interpretation. It is demonstrated that measurements of isochromats favorably complete the other methods of solid state spectroscopy byX-rays.     

On the theory of the resonant interaction of electrons with a high-frequency electric field in one-dimensional two-barrier nanostructures with symmetric barriers of finite height and width

The theory of the interaction of electrons with a high-frequency electric field in one-dimensional two-barrier nanostructures with symmetric barriers of finite height and widths was developed. An exact solution to the Schrödinger equation was found for electrons in this nanostructure in the absence of high-frequency electric field. An analytical expression for the direct current I ₀ induced in this structure by an incident electron flux with energy ε differing slightly from the resonant level energy ε _r (|ε ? ε _r | << ε _r ) was derived. In the small-signal approximation, the active (field-phased) component I _c of the alternating electric current was calculated. At ε > ε _r , the current I _c is negative in the entire frequency range, which suggests the possibility of ac electric field amplification and generation in the two-barrier resonant-tunneling structure with the barriers of finite height and width. Within the applicability of the theory (?ω << ε _r ), the frequency at which amplification and generation of the ac electric field are possible reaches ω ? 10¹³ s ^?1; the power transferred by electrons to the field is ～1 W/cm².     

Experimentelle Untersuchung von Elektronen-Photonen-Kaskaden in Blei-Szintillator-Anordnungen für Primärenergien zwischen 100 und 440 MeV

Showers generated by electrons of 200 and 440 MeV energy in single lead plates of 2, 5, and 10 radiation lengths are measured in scintillator material (NE 102 A) varying between 1.24 and 10.5 g/cm². The mean energy deposited in a scintillator is derived fromNagel's Monte-Carlo calculations. The photon contributionn _γ to the total pulseheight is of the order 15% near shower maximum t_max, it dominates the electron contributionn _e in a 3 cm thick scintillator for shower depthst ≧ 4.2 · t_max. The slope of the total ionizationn _e+n _γ behind the shower maximum can be approximated byn(t) ～ exp(?0.264t). The shower absorption in scintillator depends on the thicknessx (g/cm²) as exp (?0.068x). An arrangement of counter trays, scintillators and lead plates is calibrated with electrons between 100 and 440 MeV energy. A suitably defined track-lengthS is shown to give minimum error in energy measurement and to depend linearly on electron energy. The track-length constant of 22 MeV/r.l. is compared with the results of other authors.     

Ein experimenteller Beitrag zu der durch langsame Primärelektronen ausgelösten Sekundärelektronenemission

The energy distribution of secondary electrons emitted from a highly degassed polycristalline Pt surface was investigated as a function of low energy (V _p ) primary electrons 5<V _p <150eV. The measurements were carried out in an UHV of better than 10^?10 mm Hg. The dependence of the numberN _S ^(Ev) of secondary electrons of a fixed energyE _v (3<E _v <12eV) on the energyV _p of the primaries (=isochromates) is studied. A lot of observations can be detected:

1. I.
   The exit depth of true secondary electrons is strongly dependent of their energy.     
   
   

Zur Supraleitung von Schichtpaketen aus Normal- und Supraleitern

Superimposed films of a super- and a normalconducting metal are produced on a quartz plate by evaporation. In most cases lead and occasionally also tin is used as superconductor. The normal metals are copper, manganese and chromium. For more than 50 samples of different combinations the transitiontemperatures are measured. The mean free path of the electrons for the metals can be varied by the conditions of condensation. By means of a phenomenological theory, which we owe in principle to a discussion with ProfessorFröhlich, Liverpool, we succeed in formulating expressions fort=f(D _s ,D _n ,l _s ,l _n ,ξ₀).D,l andξ ₀ are the thickness, mean free path and the coherence length of the supra- or the normal metals andt is the reduced transitiontemperature. The developed equations can be well fitted to the measured data also to those of other authors.     

Quantenstatistik eines Gases mit verschiedener Bahn- und Spintemperatur

A system of particles with spin in a magnetic field may possess an orbital temperatureT _o different from the spin temperatureT _s (?0), if it is possible to neglect the energetic interaction between the orbital and the spin system. The calculation of the quantum statistical most probable distribution of identical independent particles on the orbital and spin energy levels yields the introduction of three Lagrange multipliers—according to the fact that the orbital and the spin energy and the number of particles are fixed—representing the orbital and spin temperature and a generalizedPlanck's “characteristic function”. Apart from the Boltzmann-approximation being valid in the case of small spin values forT _o ?T _e (T _e =customary degeneration temperature) and arbitraryT _s ?0, the distributions and the orbital and the spin energy depend onboth the temperaturesT _o andT _s coming from the principle of exclusion forFermi resp.Bose particles. The equations of state are discussed. There are four heat capacities, which possess characteristic peaks. In stead of the well-known temperature independence of the paramagnetism of degenerated conducting electrons one obtains χ～T _o /T _s . The behaviour of the Einstein-condensation of aBose gas is considered.     

Wirkungsquerschnitte für elastische und unelastische Elektronenstreuung an amorphen Kohlenstoff- und Germaniumschichten

Absolute intensities of electrons scattered by amorphous carbon and germanium foils were measured for various energies (20–60 keV) and foil thicknesses (200–800 Å). Electrons scattered elastically were separated from those scattered inelastically by means of a retarding field. Thus total cross sectionsσ _e for elastic scattering andσ _u for inelastic scattering were obtained. Agreement of observed and theoretical values is satisfactory, especially regarding the dependence on electron energy.     

Electron–phonon heat exchange in quasi-two-dimensional nanolayers

We study the heat power P transferred between electrons and phonons in thin metallic films deposited on free-standing dielectric membranes. The temperature range is typically below 1 K, such that the wavelengths of the excited phonon modes in the system is large enough so that the picture of a quasi-two-dimensional phonon gas is applicable. Moreover, due to the quantization of the components of the electron wavevectors perpendicular to the metal film’s surface, the electrons spectrum forms also quasi two-dimensional sub-bands, as in a quantum well (QW). We describe in detail the contribution to the electron–phonon energy exchange of different electron scattering channels, as well as of different types of phonon modes. We find that heat flux oscillates strongly with thickness of the film d while having a much smoother variation with temperature (T _e for the electrons temperature and T _ph for the phonons temperature), so that one obtains a ridge-like landscape in the two coordinates, (d, T _e ) or (d, T _ph ), with crests and valleys aligned roughly parallel to the temperature axis. For the valley regions we find P ∝ T _e ^3.5 – T _ph ^3.5 . From valley to crest, P increases by more than one order of magnitude and on the crests P cannot be represented by a simple power law. The strong dependence of P on d is indicative of the formation of the QW state and can be useful in controlling the heat transfer between electrons and crystal lattice in nano-electronic devices. Nevertheless, due to the small value of the Fermi wavelength in metals, the surface imperfections of the metallic films can reduce the magnitude of the oscillations of P vs. d, so this effect might be easier to observe experimentally in doped semiconductors.     

Das magnetische Verhalten von FeGe2-Einkristallen,auch für Abweichungen von der stöchiometrischen Zusammensetzung

Tetragonal stoichiometric pure FeGe₂ single crystals are para- or antiferro-magnetic with an isotropic mass susceptibilityε ₃₀₀=6.5·10^?6 cgs _m and Δ(1/χ)/ΔT=150 between 90 and 435 °K. Deviations from the stoichiometry give on the germaniumside a corresponding decrease ofχ. Excess iron risesχ and aboveΔFe=+2% a drastic field- and temperature-dependence ofχ sets in. Crystals with nonstoichiometric overall composition (by chemical analysis) are made up of two phases: In a matrix of pure FeGe₂ precipitates lie parallel to thec-axis, as revealed by polishing and etching. This behaviour explains that Mössbauer experiments give no variation of the Néel point with composition. The precipitates make the iron rich crystals magnetically anisotropic.     

Deutung der optischen Absorptionsspektren der Übergangselemente der Eisenreihe,insbesondere der des Rubins

Crystal field, ligand field and molecular orbital theories have been used to explain the optical spectra of transition elements. It is shown an additional possibility to explain the absorption bands of the transition elements, especially those of ruby, as transitions not between the nonbondingt _2g and the antibondinge _g ^* but between the bondinga _1g and the nonbondingt _2g.     

Study of photon electroproduction on the nucleon at high and low energy by Virtual Compton Scattering

Virtual Compton Scattering (VCS) and Deeply Virtual Compton Scattering (DVCS) on the nucleon are two processes accessed via the photon electroproduction reaction (eN → eγN). In the first part of this paper we are interested by the DVCS on the neutron. We measured the (D(e, eγ)X-H(e, e'γ)X) unpolarized cross section and we extracted, for the first time, a non-zero contribution of (neutron-DVCS + coherent- deuteron-DVCS) at Q ² = 1.75 GeV² and x _B = 36 from Jefferson Lab experiment E08-025. VCS on the proton has been studied at Mainz Microtron MAMI at the four-momentum transfer squared Q ² = 0.5 GeV², below the pion production threshold. In the second part of this paper we present our preliminary results of the structure functions (P _LL ? (P _TT/ε)) and P _LT, and the electric and magnetic generalized polarizabilities α _E (Q ²) and β _M (Q ²) extracted from this experiment.     

Coherent electron-hole BCS state: Study of dynamics

The first experimental study of the evolution of a coherent electron-hole (e-h) BCS-like state in bulk GaAs at room temperature is presented. We explicitly demonstrate that the total spontaneous emission from e-h pairs located within the conduction and valence bands approaches zero when the radiative recombination of the e-h BCS state occurs. This confirms that a vast majority of electrons and holes available are condensed at the very bottoms of the bands and form the BCS state. The average lifetime of this state is measured to be around 300 fs. We also show that the coherence of electrons and holes of the BCS state is preserved for a much longer time compared to the intraband relaxation time T₂.     

Magnetische Suszeptibilität der flüssigen B-Elemente

From experimental data of the magnetic susceptibilityχ for various B-elements (Tab. 2 and 3) the contributionχ _e from electrons of the outer shell has been derived. Theχ _e-values of theliquid B-elements are found to be distributed systematically in the periodic table (Tab. 4). Most of these values can readily be interpreted by simplified theories (Larmor-Langevin-term for non-metallic, Pauli-Landau-term offree electrons for metallic melts). Some complicated cases (e.g. liquid Te) can be explained by considering the chemical bond. Besidesχ _e, the temperature dependency of the susceptibility and its change by melting are discussed in detail (Tab. 6 and 7).     

Converting heat to electricity by a graphene stripe with heavy chiral fermions

A conversion of thermal energy into electricity is considered in the electricallypolarized graphene stripes with zigzag edges where the heavy chiral fermion (HCF) statesare formed. The stripes are characterized by a high electric conductance G _e and by a significantSeebeck coefficient S. The electric current in the stripes is induced due toa non-equilibrium thermal injection of “hot” electrons. This thermoelectric generationprocess might be utilized for building of thermoelectric generators with an exceptionallyhigh figure of merit ZδT ?1 and with an appreciable electric power densities ~1 MW/cm².     

Analysis of traditional Tibetan pills

Traditional Tibetan medicine starts to be a very popular complementary medicine in USA and Europe. These pills contain many elements essential for the human body. However, they might also contain heavy metals such as mercury, iron, arsenic, etc. This paper focuses on elemental composition of two Tibetan pills and investigation of forms of iron in them. X-ray fluorescence spectroscopy and neutron activation analysis identified the presence of several heavy metals such as mercury, iron and copper. M?ssbauer spectroscopy revealed the possible presence of α ? F e ₂ O ₃(hematite) and α ? F e O O H(goethite) in both of the investigated samples.     

Emission und Schwingungsverteilung der 1. neg. Gruppe und der 2. pos. Gruppe in einem Stickstoffplasma

The intensity of the first negative system ofN ₂ ⁺ (B ² Σ _u ⁺ -X ² Σ _g ⁺ and of the second positive system ofN ₂(C ³ Π _u -B ³ Π _g ) was observed in the discharge and in the afterglow as function of discharge current. An a. c. discharge in pure nitrogen was used at pressures of about 5 torr. The intensity of the first negative system ofN ₂ ⁺ — in the discharge and in the afterglow — rises to a maximum and decreases with further increase of the discharge current. The afterglow intensity of the second positive system ofN ₂ shows a maximum too. In the discharge, however, the intensity of the second positive system ofN ₂ increases with increasing discharge current. The relative population of the vibrational levelsN _v′ =i/N _v′ =0 (i=1,2,3,4) of theB ² Σ _u ⁺ state ofN ₂ ⁺ , in the discharge and in the afterglow, increases with increasing discharge current, while the relative population of the vibrational levelsN _v′ =i/N _v′ =0 (i=1, 2, 3, 4) of theC ³ Π _u state ofN ₂ reaches a maximum in the discharge. There seems to be evidence that the first negative system ofN ₂ ⁺ is not excited by electron impact withN ₂ molecules in ground state under the discharge conditions in question.     

Single <Emphasis Type="Italic">Z</Emphasis>′ production at compact linear collider based on <Emphasis Type="Italic">e</Emphasis>-γ collisions

We analyze the potential of the compact linear collider (CLIC) based on e-γ collisions to search for the new Z′ gauge boson. Single Z′ production on e-γ colliders in two SU(3)_C?SU(3)_L ? U(1)_N models, the minimal model and the model with right-handed neutrinos is studied in detail. The results show that new Z′ gauge bosons can be observed on the CLIC and that the cross sections in the model with right-handed neutrinos are bigger than those in the minimal one.     

Ab Initio Study of the Structural,Magnetic, Electronic,and Thermodynamic Properties of Pd<Subscript>2</Subscript>MnZ (Z = Ga,Ge, As) Heusler Alloys

The results of examination of the structural, magnetic, electronic, and thermodynamic properties of Pd₂MnZ (Z = Ga, Ge, As) Heusler alloys obtained in ab initio and Monte Carlo modeling are presented. It is demonstrated that a stable martensitic state is possible for Pd₂MnGa and Pd₂MnAs alloys. The equilibrium lattice parameter increases in the considered series of alloys with the number of valence electrons per atom (e/a). The Curie temperature of Pd₂MnZ (Z = Ga, Ge, As) alloys is determined using the calculated parameters of exchange interaction and total magnetic moments.     

Investigation of the e<Subscript>0</Subscript>-electron yield from the surface of thin films under bombardment by α and β particles from radioactive sources

The yields of near-zero (e₀) electrons from the surface of targets with different Z under their bombardment by α particles from ²²⁶Ra decay were measured by the method of e_α coincidences. The ratios of the e₀-electron yields for α particles with different energies E _α are described well by the dependence Y _e (E _α) ～ υ _α ^?1 , where υ _α is the α-particle velocity. The e₀-electron yield from the surface of thin films under their bombardment by β particles from radioactive sources with ¹⁵²Eu, ¹⁵⁴Eu, and ²²⁶Ra has been investigated by the method of eγ coincidences. It is established that ionization rate of target atoms in this case is inversely proportional to the velocity of incident β particles (υ _β ^?1 ), i.e., is proportional to the time of atomic electron perturbation induced by a β particle passing near an atom.