697 research outputs found
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:
(hopping between nearest neighbours), (Coulomb repulsion (attraction))
supplemented by the intersite Coulomb interactions (parameters:
(parallel spins), (antiparellel spins)) and the hopping of
the intrasite Cooper pairs (parameter: ). As a first step we find the
eigenvalues and eigenvectors of the dimer and we
represent each partial Hamiltonian
() 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 interactions ascribed to two different
energy levels which do not vanish even for and their
coupling constants are equal to 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 and . 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 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
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