51 research outputs found
Magnetic-Field-Induced Topological Reorganization of a P-wave Superconductor
In this work we illustrate the detrimental impact of the Cooper pair's
spin-structure on the thermodynamic and topological properties of a
spin-triplet superconductor in an applied Zeeman field. We particularly focus
on the paradigmatic one-dimensional case (Kitaev chain) for which we
self-consistently retrieve the energetically preferred Cooper pair spin-state
in terms of the corresponding spin d-vector. The latter undergoes a substantial
angular and amplitude reorganization upon the variation of the strength and the
orientation of the field and results to a modification of the bulk topological
phase diagram. Markedly, when addressing the open chain we find that the
orientation of the d-vector varies spatially near the boundary, affecting in
this manner the appearance of Majorana fermions at the edge or even altering
the properties of the bulk region. Our analysis reveals the limitations and
breakdown of the bulk-boundary correspondence in interacting topological
systems.Comment: 5 pages, 3 panels of figures; Minor corrections in the new version,
which will appear in Phys. Rev. B as a Rapid Communicatio
High orbital-moment Cooper pairs by crystalline symmetry breaking
The pairing structure of superconducting materials is regulated by the point
group symmetries of the crystal. Here, we study spin-singlet multiorbital
superconductivity in materials with unusually low crystalline symmetry content
and unveil the the appearance of even-parity (s-wave) Cooper pairs with high
orbital moment. We show that the lack of mirror and rotation symmetries makes
pairing states with quintet orbital angular momentum symmetry-allowed. A
remarkable fingerprint of this type of pairing state is provided by a
nontrivial superconducting phase texture in momentum space with -shifted
domains and walls with anomalous phase winding. The pattern of the quintet
pairing texture is shown to depend on the orientation of the orbital
polarization and the strength of the mirror and/or rotation symmetry breaking
terms. Such momentum dependent phase makes Cooper pairs with net orbital
component suited to design orbitronic Josephson effects. We discuss how an
intrinsic orbital dependent phase can set out anomalous Josephson couplings by
employing superconducting leads with nonequivalent breaking of crystalline
symmetry.Comment: 11 pages, 9 figure
Spectroscopic Signatures of Gate-Controlled Superconducting Phases
We investigate the tunneling conductance of superconductor-insulator-normal
metal (SIN) and superconductor-insulator-superconductor (SIS) heterostructures
with one superconducting side of the junction that is electrically driven and
can exhibit -pairing through a modification of the surface inversion
asymmetric couplings. In SIN tunneling we find that the variation of the
electrically driven interactions generally brings an increase of
quasi-particles in the gap due to orbitally polarized depaired states,
irrespective of the inter-band phase rearrangement. The peak of SIN conductance
at the gap edge varies with a trend that depends both on the strength of the
surface interactions as well as on the character of the gate-induced
superconducting state. While this shift can be also associated with thermal
effects in the SIN configuration, for the SIS geometry at low temperature the
electric field does not yield the characteristic matching peak at voltages
related with the difference between the gaps of the superconducting electrodes.
This observation sets out a distinctive mark for spectroscopically
distinguishing the thermal population effects from the quantum gate-driven
signatures. In SIS the electrostatic gating yields a variety of features with
asymmetric peaks and broadening of the conductance spectral weight. These
findings indicate general qualitative trends for both SIN and SIS tunneling
spectroscopy which could serve to evaluate the impact of gate-control on
superconductors and the occurrence of non-centrosymmetric orbital antiphase
pairing.Comment: 15 pages, 8 panels of figure
Synthetic Weyl Points and Chiral Anomaly in Majorana Devices with Nonstandard Andreev-Bound-State Spectra
We demonstrate how to design various nonstandard types of Andreev-bound-state
(ABS) dispersions, via a composite construction relying on Majorana bound
states (MBSs). Here, the MBSs appear at the interface of a Josephson junction
consisting of two topological superconductors (TSCs). Each TSC harbors multiple
MBSs per edge by virtue of a chiral or unitary symmetry. We find that, while
the ABS dispersions are -periodic, they still contain multiple crossings
which are protected by the conservation of fermion parity. A single junction
with four interface MBSs and all MBS couplings fully controllable, or, networks
of such coupled junctions with partial coupling tunability, open the door for
topological bandstructures with Weyl points or nodes in synthetic dimensions,
which in turn allow for fermion-parity (FP) pumping with a cycle set by the
ABS-dispersion details. In fact, in the case of nodes, the FP pumping is a
manifestation of chiral anomaly in 2D synthetic spacetime. The possible
experimental demonstration of ABS engineering in these devices, further
promises to unveil new paths for the detection of MBSs and higher-dimensional
chiral anomaly.Comment: Manuscript (8 pages, 3 panels of figures) + Supplemental Material (13
pages, 2 panels of figures); Version to appear in Physical Review Letter
Electrically Tunable Superconductivity Through Surface Orbital Polarization
We investigate the physical mechanisms for achieving an electrical control of
conventional spin-singlet superconductivity in thin films by focusing on the
role of surface orbital polarization. Assuming a multi-orbital description of
the metallic state, due to screening effects the electric field acts by
modifying the strength of the surface potential and, in turn, yields
non-trivial orbital-Rashba couplings. The resulting orbital polarization at the
surface and in its close proximity is shown to have a dramatic impact on
superconductivity. We demonstrate that, by varying the strength of the electric
field, the superconducting phase can be either suppressed, i.e. turned into
normal metal, or undergo a transition with the phase being marked
by non-trivial sign change of the superconducting order parameter between
different bands. These findings unveil a rich scenario to design
heterostructures with superconducting orbitronics effects.Comment: version ad published; 14 pages, 15 panels of figure
Zero magnetic-field orbital vortices in s-wave spin-singlet superconductors
Breaking of time-reversal and point-group spatial symmetries can have a
profound impact on superconductivity. One of the most extraordinary effects,
due to the application of a magnetic field, is represented by the Abrikosov
vortices with charged supercurrents circulating around their cores. Whether a
similar phenomenon can be obtained by exploiting spatial symmetry breaking,
e.g. through electric fields or mechanical strain, is a fundamentally relevant
but not yet fully settled problem. Here, we show that in two-dimensional
spin-singlet superconductors with unusually low degree of spatial symmetry
content, vortices with supercurrents carrying angular momentum around the core
can form and be energetically stable. The vortex has zero net magnetic flux
since it is made up of counter-propagating Cooper pairs with opposite orbital
moments. By solving self-consistently the Bogoliubov - de Gennes equations in
real space, we demonstrate that the orbital vortex is stable and we unveil the
spatial distribution of the superconducting order parameter around its core.
The resulting amplitude has a characteristic pattern with a pronounced angular
anisotropy that deviates from the profile of conventional magnetic vortices.
These hallmarks guide predictions and proposals for the experimental detection.Comment: (main: 6 pages, 3 panels of figures; supplemental material: 5 pages 5
panels of figures
Colossal orbital-Edelstein effect in non-centrosymmetric superconductors
In superconductors that lack inversion symmetry, the flow of supercurrent can
induce a non-vanishing magnetization, a phenomenon which is at the heart of
non-dissipative magneto-electric effects, also known as Edelstein effects. For
electrons carrying spin and orbital moments a question of fundamental relevance
deals with the orbital nature of magneto-electric effects in conventional
spin-singlet superconductors with Rashba coupling. Remarkably, we find that the
supercurrent-induced orbital magnetization is more than one order of magnitude
greater than that due to the spin, giving rise to a colossal magneto-electric
effect. The induced orbital magnetization is shown to be sign tunable, with the
sign change occurring for the Fermi level lying in proximity of avoiding
crossing points in the Brillouin zone and in the presence of superconducting
phase inhomogeneities, yielding domains with opposite orbital moment
orientation. The orbital-dominated magneto-electric phenomena, hence, have
clear-cut marks for detection both in the bulk and at the edge of the system
and are expected to be a general feature of multi-orbital superconductors
without inversion symmetry breaking.Comment: 7 pages, 5 figure
Sign reversal diode effect in superconducting Dayem nanobridges
Supercurrent diodes are nonreciprocal electronic elements whose switching
current depends on their flow direction. Recently, a variety of composite
systems combining different materials and engineered asymmetric superconducting
devices have been proposed. Yet, ease of fabrication and tunable sign of
supercurrent rectification joined to large efficiency have not been assessed in
a single platform so far. Here, we demonstrate that all-metallic
superconducting Dayem nanobridges naturally exhibit nonreciprocal supercurrents
in the presence of an external magnetic field, with a rectification efficiency
up to . Our niobium nanostructures are tailored so that the diode
polarity can be tuned by varying the amplitude of an out-of-plane magnetic
field or the temperature in a regime without magnetic screening. We show that
sign reversal of the diode effect may arise from the high-harmonic content of
the current phase relation of the nanoconstriction in combination with vortex
phase windings present in the bridge or an anomalous phase shift compatible
with anisotropic spin-orbit interactions
Back-action supercurrent diodes
Back-action refers to a response that retro-acts on a system to tailor its
properties with respect to an external stimulus. This self-induced effect
generally belongs to both the natural and technological realm, ranging from
neural networks to optics and electronic circuitry. In electronics, back-action
mechanisms are at the heart of many classes of devices such as amplifiers,
oscillators, and sensors. Here, we demonstrate that back-action can be
successfully exploited to achieve transport in
superconducting circuits. Our device realizes a supercurrent diode, since the
dissipationless current flows in one direction whereas dissipative transport
occurs in the opposite direction. Supercurrent diodes presented so far rely on
magnetic elements or vortices to mediate charge transport or external magnetic
fields to break time-reversal symmetry. In our implementation, back-action
solely turns a conventional reciprocal superconducting weak link with no
asymmetry between the current bias directions into a diode, where the critical
current amplitude depends on the bias sign. The self-interaction of the
supercurrent with the device stems from the gate tunability of the critical
current, which uniquely promotes up to 88% of magnetic field-free signal
rectification and diode functionality with selectable polarity. The concept we
introduce is very general and can be applied directly to a large variety of
devices, thereby opening novel functionalities in superconducting electronics
Local field theory for disordered itinerant quantum ferromagnets
An effective field theory is derived that describes the quantum critical
behavior of itinerant ferromagnets in the presence of quenched disorder. In
contrast to previous approaches, all soft modes are kept explicitly. The
resulting effective theory is local and allows for an explicit perturbative
treatment. It is shown that previous suggestions for the critical fixed point
and the critical behavior are recovered under certain assumptions. The validity
of these assumptions is discussed in the light of the existence of two
different time scales. It is shown that, in contrast to previous suggestions,
the correct fixed point action is not Gaussian, and that the previously
proposed critical behavior was correct only up to logarithmic corrections. The
connection with other theories of disordered interacting electrons, and in
particular with the resolution of the runaway flow problem encountered in these
theories, is also discussed.Comment: 17pp., REVTeX, 5 eps figs, final version as publishe
- …