647 research outputs found
Percolation of Superconductivity
In case of superconductors whose electrons attract each other only if they
are near certain centers, the question arises 'How many such centers are needed
to make the ground state superconducting?' We shall examine it in the context
of a random U Hubbard model. In short we study the case where U_i is -|U| and 0
with probability c and 1-c respectively on a lattice whose sites are labelled i
using the Gorkov decoupling and the Coherent Potential Approximation (CPA). We
argue that for this model there is a critical concentration c_0 below which the
system is not a superconductor.Comment: 18 pages, 10 figure
Exploring dynamical magnetism with time-dependent density-functional theory: from spin fluctuations to Gilbert damping
We use time-dependent spin-density-functional theory to study dynamical
magnetic phenomena. First, we recall that the local-spin-density approximation
(LSDA) fails to account correctly for magnetic fluctuations in the paramagnetic
state of iron and other itinerant ferromagnets. Next, we construct a
gradient-dependent density functional that does not suffer from this problem of
the LSDA. This functional is then used to derive, for the first time, the
phenomenological Gilbert equation of micromagnetics directly from
time-dependent density-functional theory. Limitations and extensions of Gilbert
damping are discussed on this basis, and some comparisons with phenomenological
theories and experiments are made
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
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
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
Gap Nodes and Time Reversal Symmetry Breaking in Strontium Ruthenate
We study the superconducting state of SrRuO on the bases of a
phenomenological but orbital specific description of the electron-electron
attraction and a realistic quantitative account of the electronic structure in
the normal state. We found that a simple model which features both `in plane'
and `out of plane' coupling with strengths meV and
meV respectively reproduced the experimentally observed power law
behaviour of the low temperature specific heat , superfluid density
and thermal conductivity in quantitative detail. Moreover, it predicts
that the quasi-particle spectrum on the -sheet is fully gaped and the
corresponding order parameter breaks the time reversal symmetry. We have also
investigated the stability of this model to inclusion of further interaction
constants in particular %those which describe `proximity coupling' between
orbitals contributing to the sheet of the Fermi surface and the
and sheets. We found that the predictions of the model are
robust under such changes. Finally, we have incorporated a description of weak
disorder into the model and explored some of its consequences. For example we
demonstrated that the disorder has a more significant effect on the -wave
component of the order parameter than on the p-wave one.Comment: EPJ B submitte
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