Comments on "Translation-invariant bipolarons and the problem of high-temperature superconductivity"

We comment on the recent results of Refs. [1, 2] on the bipolaron problem derived using an approximation of Gross - Tulub. It is proved that, contrary to the claim made in Refs. [1, 2], the bipolaron ground state energy calculated there in the strong-coupling approximation has not been shown to constitute a variational upper bound.Comment: 3 pages, 2 figures, submitted to Solid State Communication

Variational Approach to Hydrogen Atom in Uniform Magnetic Field of Arbitrary Strength

Extending the Feynman-Kleinert variational approach, we calculate the temperature-dependent effective classical potential governing the quantum statistics of a hydrogen atom in a uniform magnetic at all temperatures. The zero-temperature limit yields the binding energy of the electron which is quite accurate for all magnetic field strengths and exhibits, in particular, the correct logarithmic growth at large fields.Comment: Author Information under this http://www.physik.fu-berlin.de/~kleinert/institution.html Latest update of paper also at this http://www.physik.fu-berlin.de/~kleinert/30

Surface-induced magnetic anisotropy for impurity spins in granular AuFe films

The theory of the surface-induced anisotropy is extended to the case of granular films of dilute magnetic alloys. Since the surface-induced blocking of a magnetic-impurity spin appears to be very sensitive to the specific polycrystalline structure, we speculate that the apparent discrepancy between the experimental results of different groups for the size dependence of the Kondo resistivity can be linked to different microstructure of the samples. We apply our model to calculate the magnetization of impurity spins in small AuFe grains and to interpret the experimental data on the anomalous Hall effect in thin Fe doped Au films.Comment: 4 pages, 5 figures, E-mail addresses: [email protected], [email protected], [email protected]

Shape of nanosize superconducting grains: Does it influence pairing characteristics?

The Richardson exact solution for the reduced BCS Hamiltonian is applied to examine how sensitive are the pairing characteristics (condensation energy, spectroscopic gap, parity gap) to a specific configuration of single-electron energy levels in nanosize metallic grains. Using single-electron energy spectra in parallelepiped-shaped potential boxes with various volumes and aspect ratios as a model of energy levels in grains, we show that this sensitivity is extremely high. Just due to such an extreme sensitivity, the detailed shape of grains cannot be detected through the pairing characteristics, averaged over an ensemble of grains, even in the case of relatively small size dispersion within this ensemble. We analyse the effect of the pairing interaction on the excited-level spacings in superconducting grains and comment on the influence of shape-dependent fluctuations in single-electron energy spectra on the possibility to reveal this effect through tunnelling measurements.Comment: 13 pages, 5 figures, to be published in Solid State Communications, E-mail addresses: [email protected], [email protected], [email protected]

Coupled Ripplon-Plasmon Modes in a Multielectron Bubble

In multielectron bubbles, the electrons form an effectively two-dimensional layer at the inner surface of the bubble in helium. The modes of oscillation of the bubble surface (the ripplons) are influenced by the charge redistribution of the electrons along the surface. The dispersion relation for these charge redistribution modes (`longitudinal plasmons') is derived and the coupling of these modes to the ripplons is analysed. We find that the ripplon-plasmon coupling in a multielectron bubble differs markedly from that of electrons a flat helium surface. An equation is presented relating the spherical harmonic components of the charge redistribution to those of the shape deformation of the bubble.Comment: 8 pages, 1 figure, E-mail addresses: [email protected], [email protected], [email protected], [email protected]

Electronic structure of a Si δ-doped layer in a GaAs/AlxGa1-xAs/GaAs quantum barrier

We present a theoretical study of the electronic structure of a heavily Si d-doped layer in a GaAs/ AlxGa1-xAs/GaAs quantum barrier. In this class of structures the effect of DX centers on the electronic properties can be tuned by changing the AlxGa1-xAs barrier width and/or the Al concentration, which leads to a lowering of the DX level with respect to the Fermi energy without disturbing the wave functions much. A self-consistent approach is developed in which the effective confinement potential and the Fermi energy of the system, the energies, the wave functions, and the electron densities of the discrete subbands have been obtained as a function of both the material parameters of the samples and the experimental conditions. The effect of DX centers on such structures at nonzero temperature and under an external pressure is investigated for three different models: (1) the DXnc0 model with no correlation effects, (2) the d+/DX0 model, and (3) the d+/DX- model with inclusion of correlation effects. In the actual calculation, influences of the background acceptors, the discontinuity of the effective mass of the electrons at the interfaces of the different materials, band nonparabolicity, and the exchange-correlation energy of the electrons have been taken into account. We have found that (1) introducing a quantum barrier into d-doped GaAs makes it possible to control the energy gaps between different electronic subbands; (2) the electron wave functions are more spread out when the repellent effect of the barriers is increased as compared to those in d-doped GaAs; (3) increasing the quantum-barrier height and/or the application of hydrostatic pressure are helpful to experimentally observe the effect of the DX centers through a decrease of the total free-electron density; and (4) the correlation effects of the charged impurities are important for the systems under study

Reply to "On the cutoff parameter in the translation-invariant theory of the strong coupling polaron"

The present work is a reply to the paper [1]. It is proven that the argumentation of Ref. [1] is inconsistent. The variational functional for the polaron ground state energy considered in Ref. [1] contains an incomplete recoil energy. Since the variational functional of Ref. [1] is incomplete, it is not proven to provide a variational upper bound for the polaron ground-state energy. The same conclusion follows also for the bipolaron ground-state energy.Comment: 2 pages, 1 figure, submitted to Solid State Communication

Influence of the Characteristics of the STM-tip on the Electroluminescence Spectra

We analyze the influence of the characteristics of the STM-tip (applied voltage, tip radius) on the electroluminescence spectra from an STM-tip-induced quantum dot taking into account the many-body effects. We find that positions of electroluminescence peaks, attributed to the electron-hole recombination in the quantum dot, are very sensitive to the shape and size of the confinement potential as determined by the tip radius and the applied voltage. A critical value of the tip radius is found, at which the luminescence peak positions as a function of the tip radius manifest a transition from decreasing behavior for smaller radii to increasing behavior for larger radii. We find that this critical value of the tip radius is related to the confinement in the lateral and normal direction.Comment: 15 pages, 5 figure