108 research outputs found
Triplet superconductivity and proximity effect induced by Bloch and N\'{e}el domain walls
Noncollinear magnetic interfaces introduced in superconductor
(SC)/ferromagnet/SC heterostructures allow for spin-flipping processes and are
able to generate equal-spin spin-triplet pairing correlations within the
ferromagnetic region. This leads to the occurrence of the so-called long-range
proximity effect. Particular examples of noncollinear magnetic interfaces
include Bloch and N\'{e}el domain walls. Here, we present results for
heterostructures containing Bloch and N\'{e}el domain walls based on
self-consistent solutions of the spin-dependent Bogoliubovde Gennes
equations in the clean limit. In particular, we investigate the thickness
dependence of Bloch and N\'{e}el domain walls on induced spin-triplet pairing
correlations and compare with other experimental and theoretical results,
including conical magnetic layers as noncollinear magnetic interfaces. It is
shown that both, Bloch and N\'{e}el domain walls lead to the generation of
unequal-spin spin-triplet pairing correlations of similar strength as for
conical magnetic layers. However, for the particular heterostructure geometries
investigated, only Bloch domain walls lead to the generation of equal-spin
spin-triplet pairing correlations. They are stronger than those generated by an
equivalent thickness of conical magnetic layers. In order for N\'{e}el domain
walls to induce equal-spin spin-triplet pairing correlations, they have to be
oriented such that the noncollinearity appears within the plane parallel to the
interface region.Comment: 11 pages, 4 figure
Spin-flipping with Holmium: Case study of proximity effect in superconductor/ferromagnet/superconductor heterostructures
Superconductor/ferromagnet/superconductor heterostructures exhibit a
so-called long-range proximity effect provided some layers of conical magnet
Holmium are included in the respective interface regions. The Ho layers lead to
a spin-flip process at the interface generating equal-spin spin-triplet pairing
correlations in the ferromagnet. These equal-spin spin-triplet pairing
correlations penetrate much further into the heterostructure compared to the
spin-singlet and unequal-spin spin-triplet correlations which occur in the
absence of Ho. Here we present calculations of this effect based on the
spin-dependent microscopic Bogoliubov-de Gennes equations solved within a
tight-binding model in the clean limit. The influence of the ferromagnet and
conical magnet layer thickness on the induced equal-spin spin-triplet pairing
correlations is obtained and compared to available experimental data. It is
shown that, in agreement with experiment, a critical minimum thickness of
conical magnet layers has to be present in order to observe a sizeable amount
of equal-spin spin-triplet pairing correlations.Comment: 8 pages, 6 figure
Proximity effect in superconductor/conical magnet heterostructures
The presence of a spin-flip potential at the interface between a
superconductor and a ferromagnetic metal allows for the generation of
equal-spin spin-triplet Cooper pairs. These Cooper pairs are compatible with
the exchange interaction within the ferromagnetic region and hence allow for
the long-range proximity effect through a ferromagnet or half-metal. One
suitable spin-flip potential is provided by incorporating the conical magnet
Holmium (Ho) into the interface. The conical magnetic structure is
characterised by an opening angle with respect to the crystal -axis
and a turning (or pitch) angle measuring the rotation of magnetisation
with respect to the adjacent layers. Here, we present results showing the
influence of conical magnet interface layers with varying and
on the efficiency of the generation of equal-spin spin-triplet pairing. The
results are obtained by self-consistent solutions of the microscopic
Bogoliubovde Gennes equations in the clean limit within a tight-binding
model of the heterostructure. In particular, the dependence of unequal-spin and
equal-spin spin-triplet pairing correlations on the conical magnetic angles
and are discussed in detail.Comment: 12 pages, 6 figure
Proximity effect in superconductor/conical magnet/ferromagnet heterostructures
At the interface between a superconductor and a ferromagnetic metal
spin-singlet Cooper pairs can penetrate into the ferromagnetic part of the
heterostructure with an oscillating and decaying spin-singlet Cooper pair
density. However, if the interface allows for a spin-mixing effect, equal-spin
spin-triplet Cooper pairs can be generated that can penetrate much further into
the ferromagnetic part of the heterostructure, known as the long-range
proximity effect. Here, we present results of spin-mixing based on
self-consistent solutions of the microscopic Bogoliubov-de Gennes equations
incorporating a tight-binding model. In particular, we include a conical magnet
into our model heterostructure to generate the spin-triplet Cooper pairs and
analyse the influence of conical and ferromagnetic layer thickness on the
unequal-spin and equal-spin spin-triplet pairing correlations. It will be show
that, in agreement with experimental observations, a minimum thickness of the
conical magnet is necessary to generate a sufficient amount of equal-spin
spin-triplet Cooper pairs allowing for the long-range proximity effect.Comment: 14 pages, 7 figures, 1 tabl
The Berry curvature of the Bogoliubov quasiparticle Bloch states in the unconventional superconductor SrRuO
We will extend the concept of electron band Berry curvatures to
superconducting materials. We show that this can be defined for the
Bogoliubov-de Gennes equation describing the superconducting state in a
periodic crystal. In addition, the concept is exploited to understand the
driving mechanism for the optical Kerr effect in time reversal symmetry
breaking superconductors. Finally, we establish a sum rule analogue to the
normal state Hall sum rule making quantitative contact between the imaginary
part of the optical conductivity and the Berry curvature. The general theory
will be applied and tested against the drosophila of the p-wave paired
materials SrRuO
The electronic Hamiltonian for cuprates
A realistic many-body Hamiltonian for the cuprate superconductors should include both copper d and oxygen p states, hopping matrix elements between them, and Coulomb energies, both on-site and inter-site. We have developed a novel computational scheme for deriving the relevant parameters ab initio from a constrained occupation local density functional. The scheme includes numerical calculation of appropriate Wannier functions for the copper and oxygen states. Explicit parameter values are given for La2CuO4. These parameters are generally consistent with other estimates and with the observed superexchange energy. Secondly, we address whether this complicated multi-band Hamiltonian can be reduced to a simpler one with fewer basis states per unit cell. We propose a mapping onto a new two-band effective Hamiltonian with one copper d and one oxygen p derived state per unit cell. This mapping takes into account the large oxygen-oxygen hopping given by the ab initio calculations
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