Numerical Studies on the Magnetism of Fe-Ni-Mn Alloys in the Invar Region

By means of self-consistent semi-empirical LCAO calculations we study the itinerant magnetism of (Fe_{0.65}Ni_{0.35})_{1-y} Mn_y alloys for y between 0 and 0.22 at T=0 K, neglecting only the transverse spin components. We find that the magnetic behaviour is quite complicated on a local scale. In addition to ferromagnetic behaviour, also metastable spin-glass-like configurations are found. In the same approach, using a direct numerical calculation by the Kubo-Formalism without any fit parameters, we also calculate the electrical conductance in the magnetic state and find that the $y$-dependence observed in the experiments is well reproduced by our calculations, except of an overall factor of rougly 5, by which our resistivities are too large.Comment: 12 pages (Latex, to be applied 2 times) + 13 figures (eps-files

Quantum dots in magnetic fields: thermal response of broken symmetry phases

We investigate the thermal properties of circular semiconductor quantum dots in high magnetic fields using finite temperature Hartree-Fock techniques. We demonstrate that for a given magnetic field strength quantum dots undergo various shape phase transitions as a function of temperature, and we outline possible observable consequences.Comment: In Press, Phys. Rev. B (2001

Calculation of PandP_ and T_ odd effects in $"" sup 205_TIF including electron correlation

A method and codes for two-step correlation calculation of heavy-atom molecules have been developed, employing the generalized relativistic effective core potential and relativistic coupled cluster (RCC) methods at the first step, followed by nonvariational one-center restoration of proper four-component spinors in the heavy cores. Electron correlation is included for the first time in an ab initio calculation of the interaction of the permanent P,T-odd proton electric dipole moment with the internal electromagnetic field in a molecule. The calculation is performed for the ground state of TlF at the experimental equilibrium, R_e=2.0844 A, and at R=2.1 A, with spin-orbit and correlation effects included by RCC. Calculated results with single cluster amplitudes only are in good agreement (3% and 1%) with recent Dirac-Hartree-Fock (DHF) values of the magnetic parameter M; the larger differences occurring between present and DHF volume parameter (X) values, as well as between the two DHF calculations, are explained. Inclusion of electron correlation by GRECP/RCC with single and double excitations has a major effect on the P,T-odd parameters, decreasing M by 17% and X by 22%.Comment: 5 pages, REVTeX4 style Accepted for publication in Phys.Rev.Letter

Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality

We study the energy spectra of small three-dimensional (3D) and two-dimensional (2D) semiconductor quantum dots through different theoretical approaches (single-site Hubbard and Hartree-Fock hamiltonians); in the smallest dots we also compare with exact results. We find that purely 2D models often lead to an inadequate description of the Coulomb interaction existing in realistic structures, as a consequence of the overestimated carrier localization. We show that the dimensionality of the dots has a crucial impact on (i) the accuracy of the predicted addition spectra; (ii) the range of validity of approximate theoretical schemes. When applied to realistic 3D geometries, the latter are found to be much more accurate than in the corresponding 2D cases for a large class of quantum dots; the single-site Hubbard hamiltonian is shown to provide a very effective and accurate scheme to describe quantum dot spectra, leading to good agreement with experiments.Comment: LaTeX 2.09, RevTeX, 25 pages, 9 Encapsulated Postscript figures. To be published in Physical Review

Partitioning of on-demand electron pairs

We demonstrate the high fidelity splitting of electron pairs emitted on demand from a dynamic quantum dot by an electronic beam splitter. The fidelity of pair splitting is inferred from the coincidence of arrival in two detector paths probed by a measurement of the partitioning noise. The emission characteristic of the on-demand electron source is tunable from electrons being partitioned equally and independently to electron pairs being split with a fidelity of 90%. For low beam splitter transmittance we further find evidence of pair bunching violating statistical expectations for independent fermions

Quantum kinetics and thermalization in a particle bath model

We study the dynamics of relaxation and thermalization in an exactly solvable model of a particle interacting with a harmonic oscillator bath. Our goal is to understand the effects of non-Markovian processes on the relaxational dynamics and to compare the exact evolution of the distribution function with approximate Markovian and Non-Markovian quantum kinetics. There are two different cases that are studied in detail: i) a quasiparticle (resonance) when the renormalized frequency of the particle is above the frequency threshold of the bath and ii) a stable renormalized `particle' state below this threshold. The time evolution of the occupation number for the particle is evaluated exactly using different approaches that yield to complementary insights. The exact solution allows us to investigate the concept of the formation time of a quasiparticle and to study the difference between the relaxation of the distribution of bare particles and that of quasiparticles. We derive a non-Markovian quantum kinetic equation which resums the perturbative series and includes off-shell effects. A Markovian approximation that includes off-shell contributions and the usual Boltzmann equation (energy conserving) are obtained from the quantum kinetic equation in the limit of wide separation of time scales upon different coarse-graining assumptions. The relaxational dynamics predicted by the non-Markovian, Markovian and Boltzmann approximations are compared to the exact result. The Boltzmann approach is seen to fail in the case of wide resonances and when threshold and renormalization effects are important.Comment: 39 pages, RevTex, 14 figures (13 eps figures

Correlation and symmetry effects in transport through an artificial molecule

Spectral weights and current-voltage characteristics of an artificial diatomic molecule are calculated, considering cases where the dots connected in series are in general different. The spectral weights allow us to understand the effects of correlations, their connection with selection rules for transport, and the role of excited states in the experimental conductance spectra of these coupled double dot systems (DDS). An extended Hubbard Hamiltonian with varying interdot tunneling strength is used as a model, incorporating quantum confinement in the DDS, interdot tunneling as well as intra- and interdot Coulomb interactions. We find that interdot tunneling values determine to a great extent the resulting eigenstates and corresponding spectral weights. Details of the state correlations strongly suppress most of the possible conduction channels, giving rise to effective selection rules for conductance through the molecule. Most states are found to make insignificant contributions to the total current for finite biases. We find also that the symmetry of the structure is reflected in the I-V characteristics, and is in qualitative agreement with experiment.Comment: 25 figure files - REVTEX - submitted to PR