Persistent current induced by magnetic impurities

We calculate the average persistent current in a normal conducting, mesoscopic ring in the diffusive regime. In the presence of magnetic impurities, a contribution to the persistent current is identified, which is related to fluctuations in the electron spin density. Assuming a spin-flip scattering rate which is comparable to the Thouless energy E_c and low temperature, this new contribution to the persistent current is of the order $I\sim E_c^2/(kT\phi_0)$, which is considerably larger than the persistent current induced by the electron-electron interaction.Comment: 19 pages, 7 figures, accepted by Z. Phys.

Comment on "Magnetic response of Disordered Metallic Rings: Large Contributions of Far Levels"

Comment on cond-mat/0205390; PRL 90, 026805 (2003

Quantum Coherence in an Exactly Solvable One-dimensional Model with Defects

Using the Quantum Inverse Scattering Method we construct an integrable Heisenberg-XXZ-model, or equivalently a model for spinless fermions with nearest-neighbour interaction, with defects. Each defect involves three sites with a fine tuning between nearest-neighbour and next-nearest-neighbour terms. We investigate the finite size corrections to the ground state energy and its dependence on an external flux as a function of a parameter $\nu$, characterizing the strength of the defects. For intermediate values of $\nu$, both quantities become very small, although the ground state wavefunction remains extended.Comment: accepted by Europhys. Lett., latex, 7 pages. A postscript version including the figures is available at: http://www.physik.uni-augsburg.de/theo2/Publications

Persistent Currents versus Phase Breaking in Mesoscopic Metallic Samples

Persistent currents in mesoscopic normal metal rings represent, even a decade after their first experimental observation, a challenge to both, theorists and experimentalists. After giving a brief review of the existing -- experimental and theoretical -- results, we concentrate on the (proposed) relationship of the size of the persistent current to the phase breaking rate. In particular, we consider effects induced by noise, scattering at two-level systems, and magnetic impurities.Comment: accepted by JLT

Electronic transport properties of (fluorinated) metal phthalocyanine

The magnetic and transport properties of the metal phthalocyanine (MPc) and F$_{16}$MPc (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Ag) families of molecules in contact with S-Au wires are investigated by density functional theory within the local density approximation, including local electronic correlations on the central metal atom. The magnetic moments are found to be considerably modified under fluorination. In addition, they do not depend exclusively on the configuration of the outer electronic shell of the central metal atom (as in isolated MPc and F$_{16}$MPc) but also on the interaction with the leads. Good agreement between the calculated conductance and experimental results is obtained. For M = Ag, a high spin filter efficiency and conductance is observed, giving rise to a potentially high sensitivity for chemical sensor applications.Comment: 8 pages (two-column), 8 figure

Density functional theory for a model quantum dot: Beyond the local-density approximation

We study both static and transport properties of model quantum dots, employing density functional theory as well as (numerically) exact methods. For the lattice model under consideration the accuracy of the local-density approximation generally is poor. For weak interaction, however, accurate results are achieved within the optimized effective potential method, while for intermediate interaction strengths a method combining the exact diagonalization of small clusters with density functional theory is very successful. Results obtained from the latter approach yield very good agreement with density matrix renormalization group studies, where the full Hamiltonian consisting of the dot and the attached leads has to be diagonalized. Furthermore we address the question whether static density functional theory is able to predict the exact linear conductance through the dot correctly - with, in general, negative answer.Comment: 8 page

Diamagnetic orbital response of mesoscopic silver rings

We report measurements of the flux-dependent orbital magnetic susceptibility of an ensemble of 10^5 disconnected silver rings at 217 MHz. Because of the strong spin-orbit scattering rate in silver this experiment is a test of existing theories on orbital magnetism. Below 100 mK the rings exhibit a magnetic signal with a flux periodicity of h/2 e consistent with averaged persistent currents, whose amplitude is estimated to be of the order of 0.3 nA. The sign of the oscillations indicates diamagnetism in the vicinity of zero magnetic field. This sign is not consistent with theoretical predictions for average persistent currents unless considering attractive interactions in silver. We propose an alternative interpretation taking into account spin orbit scattering and finite frequency.Comment: 4 pages, 4 figures, revtex4, accepted for publication in Physical Review Letter

Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3

The electronic structure of the corundum-type transition-metal oxides V2O3 and Ti2O3 is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal 3d a_{1g} density of states, which is present in both compounds. The a_{1g} states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c-axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding a_{1g} states, which is caused by hybridization with the e_g^{pi} bands. In contrast, the antibonding a_{1g} states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a_{1g} orbitals for the metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi} hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Flux Dependence of Persistent Current in a Mesoscopic Disordered Tight Binding Ring

We reconsider the study of persistent currents in a disordered one-dimensional ring threaded by a magnetic flux, using he one-band tight-binding model for a ring of N-sites with random site energies. The secular equation for the eigenenergies expressed in terms of transfer matrices in the site representation is solved exactly to second order in a perturbation theory for weak disorder and fluxes differing from half-integer multiples of the elementary flux quantum. From the equilibrium currents associated with the one-electron eigenstates we derive closed analytic expressions for the disorder averaged persistent current for even and odd numbers, Ne, of electrons in the ground state. Explicit discussion for the half-filled band case confirms that the persistent current is flux periodic as in the absence of disorder, and that its amplitude is generally suppressed by the effect of the disorder. In comparison to previous results, based on an approximate analysis of the secular equation, the current suppression by disorder is strongly enhanced by a new flux-dependent factor.Comment: 15 pages, LaTex 2