72 research outputs found

### Domain walls and their experimental signatures in s+is superconductors

Arguments were recently advanced that hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$
exhibits $s+is$ state at certain doping. Spontaneous breaking of time reversal
symmetry in $s+is$ state, dictates that it possess domain wall excitations.
Here, we discuss what are the experimentally detectable signatures of domain
walls in $s+is$ state. We find that in this state the domain walls can have
dipole-like magnetic signature (in contrast to the uniform magnetic signature
of domain walls $p+ip$ superconductors). We propose experiments where
quench-induced domain walls can be stabilized by geometric barriers and be
observed via their magnetic signature or their influence on the magnetization
process, thereby providing an experimental tool to confirm $s+is$ state.Comment: Replaced with a version in print in Physical Review Letters; Minor
changes; 8 pages, 9 figure

### Topological defects in mixtures of superconducting condensates with different charges

We investigate the topological defects in phenomenological models describing
mixtures of charged condensates with commensurate electric charges. Such
situations are expected to appear for example in liquid metallic deuterium.
This is modeled by a multicomponent Ginzburg-Landau theory where the
condensates are coupled to the same gauge field by different coupling constants
whose ratio is a rational number. We also briefly discuss the case where
electric charges are incommensurate. Flux quantization and finiteness of the
energy per unit length dictate that the different condensates have different
winding and thus different number of (fractional) vortices. Competing
attractive and repulsive interactions lead to molecule-like bound state between
fractional vortices. Such bound states have finite energy and carry integer
flux quanta. These can be characterized by $\mathbb{C}P^1$ topological
invariant that motivates their denomination as skyrmions.Comment: Replaced with a version in print in Phys. Rev. B; Improved and
extended as compared to the first version; 14 pages, 8 figure

### Skyrmionic state and stable half-quantum vortices in chiral p-wave superconductors

Observability of half-quantum vortices and skyrmions in p-wave
superconductors is an outstanding open question. Under the most common
conditions, fractional flux vortices are not thermodynamically stable in bulk
samples. Here we show that in chiral p-wave superconductors, there is a regime
where, in contrast lattices of integer flux vortices are not thermodynamically
stable. Instead skyrmions made of spatially separated half-quantum vortices are
the topological defects produced by an applied external field.Comment: Replaced with a version in print in Physical Review B, Rapid
Communications; References added; 8 pages, 9 figure

### Microscopic prediction of skyrmion lattice state in clean interface superconductors

When an in-plane field is applied to a clean interface superconductor, a
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like phase is stabilized. This phase
has a $\mathrm{U}(1)\times\mathrm{U}(1)$ symmetry and, in principle, this
symmetry allows for flux carrying topological excitations different from
Abrikosov vortices (which are the simplest defects associated with $S^1 \to
S^1$ maps). However, in practice, largely due to electromagnetic and other
intercomponent interactions, such topological excitations are very rare in
superconducting systems. Here we demonstrate that a realistic microscopic
theory for interface superconductors, such as SrTiO$_3$/LaAlO$_3$, predicts an
unconventional magnetic response where the flux-carrying objects are skyrmions,
characterized by homotopy invariants of $S^2 \to S^2$ maps. Additionally, we
show that this microscopic theory predicts that stable fractional vortices form
near the boundary of these superconductors. It also predicts the appearance of
type-1.5 superconductivity for some range of parameters. Central to these
results is the assumption that the Rashba spin orbit coupling is much larger
than the superconducting gap.Comment: Replaced with a version in print in Phys. Rev. B; Improved and
extended as compared to the first version; 10 pages, 6 figure

### Chiral CP^2 skyrmions in three-band superconductors

It is shown that under certain conditions, three-component superconductors
(and in particular three-band systems) allow stable topological defects
different from vortices. We demonstrate the existence of these excitations,
characterized by a $CP^2$ topological invariant, in models for three-component
superconductors with broken time reversal symmetry. We term these topological
defects "chiral $GL^{(3)}$ skyrmions", where "chiral" refers to the fact that
due to broken time reversal symmetry, these defects come in inequivalent left-
and right-handed versions. In certain cases these objects are energetically
cheaper than vortices and should be induced by an applied magnetic field. In
other situations these skyrmions are metastable states, which can be produced
by a quench. Observation of these defects can signal broken time reversal
symmetry in three-band superconductors or in Josephson-coupled bilayers of
$s_\pm$ and s-wave superconductors.Comment: minor presentation changes; replaced journal version; 30 pages, 21
figure

- β¦