Origin of Spontaneous Currents in a Superconductor-Ferromagnet Proximity System

We have previously shown that a ferromagnet-superconductor heterostructure may possess a spontaneous current circulation parallel to the interface. This current is caused by Andreev bound states in the thin ferromagnetic layer, and can be fully spin-polarized. Here we investigate the total energy of the system in cases where the current either does or does not flow. We show that the current is a true quantum ground state effect, and examine the effect of the current on the different contributions to the total energy.Comment: 6 pages, 4 figures. Vortex 2005 conference, accepted for publication Physica

Electronic Correlations within Fermionic Lattice Models

We investigate two-site electronic correlations within generalized Hubbard model, which incorporates the conventional Hubbard model (parameters: $t$ (hopping between nearest neighbours), $U$ (Coulomb repulsion (attraction)) supplemented by the intersite Coulomb interactions (parameters: $J^{(1)}$(parallel spins), $J^{(2)}$ (antiparellel spins)) and the hopping of the intrasite Cooper pairs (parameter: $V$). As a first step we find the eigenvalues $E_{\alpha}$ and eigenvectors $|E_{\alpha}>$ of the dimer and we represent each partial Hamiltonian $E_{\alpha} |E_{\alpha} > < E_{\alpha} |$ ($\alpha =1,2,..,16$) in the second quantization with the use of the Hubbard and spin operators. Each dimer energy level possesses its own Hamiltonian describing different two-site interactions which can be active only in the case when the level will be occupied by the electrons. A typical feature is the appearence of two generalized $t-J$ interactions ascribed to two different energy levels which do not vanish even for $$ and their coupling constants are equal to $\pm t$ in this case. The competition between ferromagnetism, antiferromagnetism and superconductivity (intrasite and intersite pairings) is also a typical feature of the model because it persists in the case $U=J^{(1)}=J^{(2)}=V=0$ and $t\neq 0$. The same types of the electronic, competitive interactions are scattered between different energy levels and therefore their thermodynamical activities are dependent on the occupation of these levels. It qualitatively explains the origin of the phase diagram of the model. We consider also a real lattice as a set of interacting dimers to show that the competition between magnetism and superconductivity seems to be universal for fermonic lattice models.Comment: 12 page

Double non-equivalent chain structure on vicinal Si(557)-Au surface

We study electronic and topographic properties of the vicinal Si(557)-Au surface using scanning tunneling microscopy and reflection of high energy electron diffraction technique. STM data reveal double wire structures along terraces. Moreover behavior of the voltage dependent STM tip - surface distance is different in different chains. While the one chain shows oscillations of the distance which are sensitive to the sign of the voltage bias, the oscillations in the other chain remain unchanged with respect to the positive/negative biases. This suggests that one wire has metallic character while the other one - semiconducting. The experimental results are supplemented by theoretical calculations within tight binding model suggesting that the observed chains are made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.

Spin Polarized Current in the Ground State of Superconductor - Ferromagnet - Insulator Trilayers

We study the ground state properties of a superconductor - ferromagnet - insulator trilayer on the basis of a Hubbard Model featuring exchange splitting in the ferromagnet and electron - electron attraction in the superconductor. We solve the spin - polarized Hartree - Fock - Gorkov equations together with the Maxwell's equation (Ampere's law) fully self-consistently. For certain values of the exchange splitting we find that a spontaneous spin polarized current is generated in the ground state and is intimately related to Andreev bound states at the Fermi level. Moreover, the polarization of the current strongly depends on the band filling.Comment: 13 pages, 14 figure

Fulde-Ferrell-Larkin-Ovchinnikov-like state in Ferromagnet-Superconductor Proximity System

We discuss some properties of the ferromagnet-superconductor proximity system. In particular, the emphasis is put on the physics of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state. In addition to Andreev reflections it features a number of unusual thermodynamic and transport properties, like: oscillatory behavior of the pairing amplitude, density of states and superconducting transition temperature as a function of the ferromagnet thickness. Surprisingly, under certain conditions spontaneous spin polarized current is generated in the ground state of such a system. We provide some informations regarding experimental observations of this exotic state.Comment: Talk given at Advanced Research Workshop on ''Physics of Spin in Solids: Materials, Methods & Applications'', Baku (October 2003

High resolution scanning tunneling spectroscopy of ultrathin Pb on Si(111)-(6x6) substrate

The electronic structure of Si(111)-(6x6)Au surface covered with submonolayer amount of Pb is investigated using scanning tunneling spectroscopy. Already in small islands of Pb with thickness of 1 ML Pb$_{(111)}$ and with the diameter of only about 2 nm we detected the quantized electronic state with energy 0.55 eV below the Fermi level. Similarly, the I(V) characteristics made for the Si(111)-(6x6)Au surface reveal a localized energy state 0.3 eV below the Fermi level. These energies result from fitting of the theoretical curves to the experimental data. The calculations are based on tight binding Hubbard model. The theoretical calculations clearly show prominent modification of the I(V) curve due to variation of electronic and topographic properties of the STM tip apex.Comment: 7 pages, 7 figures, accepted for publication in Surface Scienc

Spontaneous currents in a ferromagnet - normal metal - superconductor trilayer

We discuss the ground state properties of the system composed of a normal metal sandwiched between ferromagnet and superconductor within a tight binding Hubbard model. We have solved the spin-polarized Hartree-Fock-Gorkov equations together with the Maxwell's equation (Ampere's law) and found a proximity induced Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in this system. Here we show that the inclusion of the normal metal layer in between those subsystems does not necessarily lead to the suppression of the FFLO phase. Moreover, we have found that depending on the thickness of the normal metal slab the system can be switched periodically between the state with the spontaneous current flowing to that one with no current. All these effects can be explained in terms of the Andreev bound states formed in such structures.Comment: 6 pages, 4 figure

Narratives of Diversity in the Corporate Boardroom: What Corporate Insiders Say About Why Diversity Matters

Over the last generation, the concept of diversity has become commonplace and taken-for-granted in discourses ranging from law to education to business. In higher education, for example, it is hard to imagine a faculty job search or a student admissions discussion that was not heavily laden with talk of diversity, in the sense of the representative inclusion of women and racial and ethnic minorities in a group or organization. In this paper we present the results of an interview-based study of the discourse of diversity in a particular business setting: the corporate boardroom. Our principal observation is that—thirty-one years after the Supreme Court’s Bakke decision introduced the term into public discourse--corporate insiders appear not to have arrived at a master narrative to explain the pursuit of diversity on boards of directors. Instead, their accounts stress a variety of factors and feature few concrete examples

Dangerous Categories: Narratives of Corporate Board Diversity

In this article, we report the results of a series of interviews with corporate directors about racial, ethnic, and gender diversity on corporate boards. On the one hand, our respondents were clear and nearly uniform in their statements that board diversity was an important goal worth pursuing. Yet when asked to provide examples or anecdotes illustrating why board diversity matters, many subjects acknowledged difficulty in illustrating theory with reference to practice. This expressed reluctance to come to specific terms with general claims about the value of director diversity inspired our title phrase: dangerous categories. That is, while diversity evokes universal acclaim in the abstract, our respondents’ narratives demonstrate that it is an elusive and even dangerous subject to talk about concretely. So we are left with narratives that simultaneously extol difference and express embarrassment with it.This expressed reluctance to come to specific terms with general claims about the value of director diversity inspired our title phrase: dangerous categories. That is, while diversity evokes universal acclaim in the abstract, our respondents’ narratives demonstrate that it is an elusive and even dangerous subject to talk about concretely. So we are left with narratives that simultaneously extol difference and express embarrassment with it