75 research outputs found
General solution of 2D and 3D superconducting quasiclassical systems: coalescing vortices and nanoisland geometries
An extension of quasiclassical Keldysh-Usadel theory to higher spatial
dimensions than one is crucial in order to describe physical phenomena like
charge/spin Hall effects and topological excitations like vortices and
skyrmions, none of which are captured in one-dimensional models. We here
present a numerical finite element method which solves the non-linearized 2D
and 3D quasiclassical Usadel equation relevant for the diffusive regime. We
show the application of this on three model systems with non-trivial
geometries: (i) a bottlenecked Josephson junction with external flux, (ii) a
nanodisk ferromagnet deposited on top of a superconductor and (iii)
superconducting islands in contact with a ferromagnet. In case (i), we
demonstrate that one may control externally not only the geometrical array in
which superconducting vortices arrange themselves, but also to cause
coalescence and tune the number of vortices. In case (iii), we show that the
supercurrent path can be tailored by incorporating magnetic elements in planar
Josephson junctions which also lead to a strong modulation of the density of
states. The finite element method presented herein paves the way for gaining
insight in physical phenomena which have remained largely unexplored due to the
complexity of solving the full quasiclassical equations in higher dimensions.Comment: 16 pages, 8 figures. Added several new result
Field-free nucleation of antivortices and giant vortices in non-superconducting materials
Giant vortices with higher phase-winding than are usually
energetically unfavorable, but geometric symmetry constraints on a
superconductor in a magnetic field are known to stabilize such objects. Here,
we show via microscopic calculations that giant vortices can appear in
intrinsically non-superconducting materials, even without any applied magnetic
field. The enabling mechanism is the proximity effect to a host superconductor
where a current flows, and we also demonstrate that antivortices can appear in
this setup. Our results open the possibility to study electrically controllable
topological defects in unusual environments, which do not have to be exposed to
magnetic fields or intrinsically superconducting, but instead display other
types of order.Comment: Revised version; 4 pages manuscript, 4 pages supplemental, 6 figure
Spin accumulation induced by a singlet supercurrent
We show that a supercurrent carried by spinless singlet Cooper pairs can
induce a spin accumulation in the normal metal interlayer of a Josephson
junction. This phenomenon occurs when a nonequilibrium spin-energy mode is
excited in the normal metal, for instance by an applied temperature gradient
between ferromagnetic electrodes. Without supercurrent, the spin accumulation
vanishes in the Josephson junction. With supercurrent, a spatially
antisymmetric spin accumulation is generated that can be measured by tunneling
to a polarized detector electrode. We explain the physical origin of the
induced spin accumulation by the combined effect of a Doppler shift induced by
a flow of singlet Cooper pairs, and the spin-energy mode excited in the normal
metal. This effect shows that spin control is possible even with singlet Cooper
pairs in conventional superconductors, a finding which could open new
perspectives in superconducting spintronics.Comment: 4 pages, 4 figure
Quasiclassical boundary conditions for spin-orbit coupled interfaces with spin-charge conversion
The quasiclassical theory of superconductivity provides a methodology to
study emergent phenomena in hybrid structures comprised of superconductors
interfaced with other materials. A key component in this theory is the boundary
condition that the Green functions describing the materials must satisfy.
Recently, progress has been made toward formulating such a boundary condition
for interfaces with spin-orbit coupling, the latter playing an important role
for several phenomena in spintronics. Here, we derive a boundary condition for
spin-orbit coupled interfaces that includes gradient terms which enables the
description of spin-Hall like effects with superconductors due to such
interfaces. As an example, we show that the boundary conditions predict that a
supercurrent flowing through a superconductor that is coupled to a normal metal
via a spin-orbit interface can induce a non-local magnetization in the normal
metal.Comment: 9 pages, 2 figures. Corrected here a few minor typos present in
published versio
Grain-boundary topological superconductor
We show that a grain boundary (GB) defect, constituted by a one-dimensional
dislocation superlattice, can host a topological superconductor (SC) with a
pair of co-habitating Majorana zero modes (MZMs) at its end when immersed in a
parent two-dimensional topological SC. For the realization of our proposal, it
is essential that the single-dislocation defects feature MZMs, implying that
the parent topological SC should be gapped with the Fermi surface away from the
point. As we demonstrate, both numerically and analytically, the GB
topological SC with two localized MZMs then emerge in a finite range of both
the angle and the magnitude of the applied exchange magnetic field. Finally, we
argue that the proposed GB SC can be realized in various superconducting
materials where the GB defects can be readily manipulated.Comment: 6 pages, 4 figure
Superconducting phase diagram and spin diode effect via spin accumulation
Spin-split superconductors offer new functionality compared to conventional
superconductors such as diode-effects and efficient thermoelectricity. The
superconducting state can nevertheless only withstand a small amount of
spin-splitting . Here, we self-consistently determine the spin transport
properties and the phase diagram of a spin-split superconductor in the presence
of an injected spin accumulation. Energy and spin relaxation are accounted for
in the relaxation time approximation via a single effective inelastic
scattering parameter. We find that the spin-splitting field in the
superconductor enables a spin diode effect in the presence of such spin
relaxation. Moreover, we consider the superconducting phase diagram of such a
system when it is in contact with a spin accumulation, and find that the
inclusion of energy and spin relaxation alters the phase diagram qualitatively.
In particular, these mechanisms turn out to induce a superconducting state in
large parts of the phase diagram where a normal state would otherwise be the
ground-state. We identify an FFLO-like state even in the presence of impurity
scattering which can be controllably tuned on and off via the electrically
induced spin accumulation. We explain the underlying physics from how the
superconducting order parameter depends on the non-equilibrium modes in the
system as well as the behavior of these modes in the presence of energy and
spin relaxation when a spin-splitting field is present.Comment: 8 pages, 5 figure
Improving the Correctness of Automated Program Repair
Developers spend much of their time fixing bugs in software programs. Automated program repair (APR) techniques aim to alleviate the burden of bug fixing from developers by generating patches at the source-code level. Recently, Generate-and-Validate (G&V) APR techniques show great potential to repair general bugs in real-world applications. Recent evaluations show that G&V techniques repair 8–17.7% of the collected bugs from mature Java or C open-source projects. Despite the promising results, G&V techniques may generate many incorrect patches and are not able to repair every single bug.
This thesis makes contributions to improve the correctness of APR by improving the quality assurance of the automatically-generated patches and generating more correct patches by leveraging human knowledge. First, this thesis investigates whether improving the test-suite-based validation can precisely identify incorrect patches that are generated by G&V, and whether it can help G&V generate more correct patches. The result of this investigation, Opad, which combines new fuzz-generated test cases and additional oracles (i.e., memory oracles), is proposed to identify incorrect patches and help G&V repair more bugs correctly. The evaluation of Opad shows that the improved test-suite-based validation identifies 75.2% incorrect patches from G&V techniques. With the integration of Opad, SPR, one of the most promising G&V techniques, repairs one additional bug.
Second, this thesis proposes novel APR techniques to repair more bugs correctly, by leveraging human knowledge. Thus, APR techniques can repair new types of bugs that are not currently targeted by G&V APR techniques. Human knowledge in bug-fixing activities is noted in the forms such as commits of bug fixes, developers’ expertise, and documentation pages. Two techniques (APARE and Priv) are proposed to target two types of defects respectively: project-specific recurring bugs and vulnerability warnings by static analysis.
APARE automatically learns fix patterns from historical bug fixes (i.e., originally crafted by developers), utilizes spectrum-based fault-localization technique to identify highly-likely faulty methods, and applies the learned fix patterns to generate patches for developers to review. The key innovation of APARE is to utilize a percentage semantic-aware matching algorithm between fix patterns and faulty locations. For the 20 recurring bugs, APARE generates 34 method fixes, 24 of which (70.6%) are correct; 83.3% (20 out of 24) are identical to the fixes generated by developers. In addition, APARE complements current repair systems by generating 20 high-quality method fixes that RSRepair and PAR cannot generate.
Priv is a multi-stage remediation system specifically designed for static-analysis security-testing (SAST) techniques. The prototype is built and evaluated on a commercial SAST product. The first stage of Priv is to prioritize workloads of fixing vulnerability warnings based on shared fix locations. The likely fix locations are suggested based on a set of rules. The rules are concluded and developed through the collaboration with two security experts. The second stage of Priv provides additional essential information for improving the efficiency of diagnosis and fixing. Priv offers two types of additional information: identifying true database/attribute-related warnings, and providing customized fix suggestions per warning. The evaluation shows that Priv suggests identical fix locations to the ones suggested by developers for 50–100% of the evaluated vulnerability findings. Priv identifies up to 2170 actionable vulnerability findings for the evaluated six projects. The manual examination confirms that Priv can generate patches of high-quality for many of the evaluated vulnerability warnings
Inverse proximity effect in -wave and -wave superconductors coupled to topological insulators
We study the inverse proximity effect in a bilayer consisting of a thin -
or -wave superconductor (S) and a topological insulator (TI). Integrating
out the topological fermions of the TI, we find that spin-orbit coupling is
induced in the S, which leads to spin-triplet -wave (-wave) correlations
in the anomalous Green's function for an -wave (-wave) superconductor.
Solving the self-consistency equation for the superconducting order parameter,
we find that the inverse proximity effect can be strong for parameters for
which the Fermi momenta of the S and TI coincide. The suppression of the gap is
approximately proportional to , where is the
dimensionless superconducting coupling constant. This is consistent with the
fact that a higher gives a more robust superconducting state. For an
-wave S, the interval of TI chemical potentials for which the suppression of
the gap is strong is centered at , and
increases quadratically with the hopping parameter . Since the S chemical
potential typically is high for conventional superconductors, the inverse
proximity effect is negligible except for above a critical value. For
sufficiently low , however, the inverse proximity effect is negligible, in
agreement with what has thus far been assumed in most works studying the
proximity effect in S-TI structures. In superconductors with low Fermi
energies, such as high- cuprates with -wave symmetry, we again find a
suppression of the order parameter. However, since is much smaller in
this case, a strong inverse proximity effect can occur at for much
lower values of . Moreover, the onset of a strong inverse proximity effect
is preceded by an increase in the order parameter, allowing the gap to be tuned
by several orders of magnitude by small variations in .Comment: 11 pages, 4 figures, updated versio
High magnetic field superconductivity in a two-band superconductor
When applying an external magnetic field to a superconductor, orbital and
Pauli paramagnetic pairbreaking effects govern the limit of the upper critical
magnetic field that can be supported before superconductivity breaks down.
Experimental studies have shown that many multiband superconductors exhibit
values of the upper critical magnetic field that violate the theoretically
predicted limit, giving rise to many studies treating the underlying mechanisms
that allow this. In this work we consider spin-splitting induced by an external
magnetic field in a superconductor with two relevant bands close to the Fermi
level, and show that the presence of interband superconducting pairing produces
high-field reentrant superconductivity violating the
Pauli-Chandrasekhar-Clogston limit for the value of the upper critical magnetic
field
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