Entangled electron current through finite size normal-superconductor tunneling structures

We investigate theoretically the simultaneous tunneling of two electrons from a superconductor into a normal metal at low temperatures and voltages. Such an emission process is shown to be equivalent to the Andreev reflection of an incident hole. We obtain a local tunneling Hamiltonian that permits to investigate transport through interfaces of arbitrary geometry and potential barrier shapes. We prove that the bilinear momentum dependence of the low-energy tunneling matrix element translates into a real space Hamiltonian involving the normal derivatives of the electron fields in each electrode. The angular distribution of the electron current as it is emitted into the normal metal is analyzed for various experimental setups. We show that, in a full three-dimensional problem, the neglect of the momentum dependence of tunneling causes a violation of unitarity and leads to the wrong thermodynamic (broad interface) limit. More importantly for current research on quantum information devices, in the case of an interface made of two narrow tunneling contacts separated by a distance r, the assumption of momentum-independent hopping yields a nonlocally entangled electron current that decays with a prefactor proportional to r ^-2 instead of the correct r ^-4.Received: 14 June 2004, Published online: 24 September 2004PACS: 74.45. + c Proximity effects; Andreev effect; SN and SNS junctions - 74.50. + r Tunneling phenomena; point contacts, weak links, Josephson effects     

Resonant electron tunneling in (111) GaAs/AlAs structures

Pseudopotential and scattering matrix methods are used to study the spectral details of the transmission of electrons as a function of the angle of incidence on a heterojunction in various structures based on (111) GaAs/AlAs. A simplified two-valley (Γ-L) model for describing the electronic states in such structures has been proposed and its parameters have been determined. The existence of quasilocalized “interfacial” states has been established. A formula has been found which well approximates the behavior of the scattering matrix elements near these resonances, and their decay times are estimated. Academician V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika No. 9, pp. 89–99, September, 1998.     

Doublet resonances of an electron in symmetric three-barrier resonant tunneling structures

The evolution and collapse of electron resonances and their spectral parameters in a symmetric three-barrier resonant tunneling structure (TBRTS) are studied theoretically. The resonance energy and width of the quasi-stationary states of an electron are analyzed. The quasi-stationary states are calculated by the transmission coefficient method, the method of the probability distribution function (the probability of finding an electron in a TBRTS), and the scattering cross section method.     

Valence band formation in small silver clusters

UV photoemission spectra of valence electrons in small silver clusters have been compared with spectra from bulk silver samples using synchrotron radiation, 16 <hν< 27 eV. Spectra for single silver atoms supported on carbon are indicative of a completely filled 4d¹⁰ initial state configuration. With increased cluster size, both density of states and valence band modulations with respect to photon energy resemble the bulk metal more closely. Spectral modulation, characteristic of conservation of crystal momentum, appears to require a cluster consisting of ～150 atoms.     

Sm富勒烯的价带光电子能谱

用紫外光电子能谱和同步辐射光电子能谱研究了Sm掺杂C₆₀薄膜的价带电子结构.Sm的价电子大部分转移给C₆₀，化学键以离子性为主.对于任何化学配比都没有观察到费米边，所以Sm富勒烯超导相在室温下为半导体性质.获得了很接近单相Sm_2.75C₆₀的样品在费米能级附近的电子态密度分布.固溶相的光电子发射与Sm_2.75C₆₀有明显区别.Sm_xC_{60关键词： 60}的Sm填隙化合物')" href="#">C₆₀的Sm填隙化合物 价带光电子能谱 电子结构     

Valence electron shake-up in photoelectron spectra

Valence electron shake-up phenomena are important in ultraviolet photoelectron spectroscopy because extra bands are formed which may complicate the interpretation, and because the primary bands are displaced in energy which may invalidate comparisons with calculated ionization energies. It would be of value during experimental work to have access to simple methods for an approximate estimation of these effects. Such methods are presented here together with applications to the photoelectron spectra of ethylene and benzene.     

Inelastic electron tunneling

Inelastic Electron Tunneling Spectroscopy (IETS) is a new technique for measuring the vibrational spectrum of minute quantities of organic compounds. Sensitivity is its key advantage over the conventional techniques of infrared and Raman spectroscopy. This article will first discuss the technique itself: its theoretical basis, selection rules, sensitivity, vibrational mode shifts due to surface interactions and superconductivity, and sample preparation. Then it will discuss applications of the technique to problems in biology, radiation physics, surface physics, and catalysis.     

Spin-flip induction of Fano resonance upon electron tunneling through atomic-scale spin structures

The inclusion of inelastic spin-dependent electron scatterings by the potential profiles of a single magnetic impurity and a spin dimer is shown to induce resonance features due to the Fano effect in the transport characteristics of such atomic-scale spin structures. The spin-flip processes leading to a configuration interaction of the system’s states play a fundamental role for the realization of Fano resonance and antiresonance. It has been established that applying an external magnetic field and a gate electric field allows the conductive properties of spin structures to be changed radically through the Fano resonance mechanism.     

Inelastic electron tunneling spectroscopy on MOS structures with very thin oxide films

Inelastic electron tunneling spectroscopy at 4.2 K was used to investigate the defect structure of MOS capacitors with very thin SiO₂ films. Samples were degeneratelyP-andB-doped Si substrates, oxidized in O₂ at 600°C and provided with evaporated Pb, Au, In, Al or Mg electrodes. The observed peaks in the second derivative of theI-U characteristic were assigned to the excitation of phonons and of vibrational modes of the dopants and impurities. The results were found to correlate with infrared data. In addition, a distinct effect of Si/SiO₂ interface states on the characteristic was found.     

Valence band photoemission on metallic Ce systems

The valence band photoemission spectroscopy of metallic Ce systems is inspected with regard to the different models employed for the interpretation of the data. It is concluded, that contrary to general believe, in most systems higher lyingf-excitations within the 4f manifold are observed, and the observation of the Kondo resonance is difficult.     

Resonant tunneling,eigenvalue and energy band calculation for potential and periodic potential structures

Recursion formulae for the reflection and the transmission probability amplitudes and the eigenvalue equation for multistep potential structures are derived. Using the recursion relations, a dispersion equation for periodic potential structures is presented. Some numerical results for the transmission probability of a double barrier structure with scattering centers, the lifetime of the quasi-bound state in a single quantum well with an applied field, and the miniband of a periodic potential structure are presented.     

Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

Resonant tunneling through double-barrier structures on grapheneResonant tunneling through double-barrier structures on grapheneResonant tunneling through double-barrier structures on grapheneResonant tunneling through double-barrier structures on graphene

Quantum resonant tunneling behaviors of double-barrier structures on graphene are investigated under the tight- binding approximation. The Klein tunneling and resonant tunneling are demonstrated for the quasiparticles with energy close to the Dirac points. The Klein tunneling vanishes by increasing the height of the potential barriers to more than 300 meV. The Dirac transport properties continuously change to the Schr6dinger ones. It is found that the peaks of resonant tunneling approximate to the eigen-levels of graphene nanoribbons under appropriate boundary conditions. A comparison between the zigzag- and armchair-edge barriers is given.     

Photon-assisted tunneling in electron pumps

We measure photon-assisted tunneling in 4- and 6-junction electron pumps at photon frequencies up to 60 GHz. We determine the microwave voltage at the pumps using noise thermometry. The standard theory of leakage in the electron pump, modified to include photon-assisted tunneling, describes our experiments well. From this test of theory we argue that, in the absence of external microwaves, photon-assisted tunneling driven by 1/f noise is an important error mechanism in electron pumps.     

Effects of level broadening and electron overheating in tunneling structures based on metal clusters

The effect of level broadening and electronic subsystem overheating of disk-shaped and spherical gold clusters/islands on the current-voltage characteristic of the three-electrode structure has been studied. A scheme for calculating the electronic level broadening in the one-dimensional case of rectangular barriers has been proposed. In the two-temperature electron-ion model of a metal cluster, taking into account the size dependence of the Debye frequency, the kinetic electron temperature has been estimated as a function of the bias voltage. At low ion temperatures, the broadening and electron overheating effects result in disappearance of steps of quantum and Coulomb staircases, i.e., a strong smoothness of the current-voltage characteristic even in structures on clusters consisting of a denumerable number of atoms, which is actually observed experimentally.     

Yb2.75C60价带光电子能谱

用角积分紫外光电子能谱技术测量了Yb2 75C6 0 薄膜的价带电子态密度分布 .相纯Yb2 75C6 0 样品通过C1s芯态x射线电子谱峰的位移表征 .结果表明Yb2 75C6 0 是半导体 ,在费米能级处几乎没有电子态分布 .Yb 6s电子态和C6 0 LU MO能带的杂化效应不可忽略 ,有部分Yb 6s电子分布在Yb C6 0 杂化能带上 .     

Valence band x-ray emission spectra of compressed germanium

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic beta-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic beta-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation.