5,740 research outputs found
Topological Superconductivity and Majorana Fermions in Metallic Surface-States
Heavy metals, such as Au, Ag, and Pb, often have sharp surface states that
are split by strong Rashba spin-orbit coupling. The strong spin-orbit coupling
and two-dimensional nature of these surface states make them ideal platforms
for realizing topological superconductivity and Majorana fermions. In this
paper, we further develop a proposal to realize Majorana fermions at the ends
of quasi-one-dimensional metallic wires. We show how superconductivity can be
induced on the metallic surface states by a combination of proximity effect,
disorder, and interactions. Applying a magnetic field along the wire can drive
the wire into a topologically non-trivial state with Majorana end-states.
Unlike the case of a perpendicular field, where the chemical potential must be
fined tuned near the Rashba-band crossing, the parallel field allows one to
realize Majoranas for arbitrarily large chemical potential. We then show that,
despite the presence of a large carrier density from the bulk metal, it is
still possible to effectively control the chemical potential of the surface
states by gating. The simplest version of our proposal, which involves only an
Au(111) film deposited on a conventional superconductor, should be readily
realizable.Comment: 9 Pages, 6 Figure
Engineering a p+ip Superconductor: Comparison of Topological Insulator and Rashba Spin-Orbit Coupled Materials
We compare topological insulator materials and Rashba coupled surfaces as
candidates for engineering p+ip superconductivity. Specifically, in each type
of material we examine 1) the limitations to inducing superconductivity by
proximity to an ordinary s-wave superconductor, and 2) the robustness of the
resulting superconductivity against disorder. We find that topological
insulators have strong advantages in both regards: there are no fundamental
barriers to inducing superconductivity, and the induced superconductivity is
immune to disorder. In contrast, for Rashba coupled quantum wires or surface
states, the the achievable gap from induced superconductivity is limited unless
the Rashba coupling is large. Furthermore, for small Rashba coupling the
induced superconductivity is strongly susceptible to disorder. These features
pose serious difficulties for realizing p+ip superconductors in semiconductor
materials due to their weak spin-orbit coupling, and suggest the need to seek
alternatives. Some candidate materials are discussed.Comment: 10 pages, 4 Figures; Changes for v2: References added, Includes an
expanded discussion of surface vs bulk disorder (see Sec. IVc. and Appendix
A
Multichannel Generalization of Kitaev's Majorana End States and a Practical Route to Realize Them in Thin Films
The ends of one-dimensional p+ip superconductors have long been predicted to
possess localized Majorana fermion modes. We show that Majorana end states
survive beyond the strict 1D single-channel limit so long as the sample width
does not exceed the superconducting coherence length, and exist when an odd
number of transverse quantization channels are occupied. Consequently we find
that the system undergoes a sequence of topological phase transitions driven by
changing the chemical potential. These observations make it feasible to
implement quasi-1D p+ip superconductors in metallic thin-film microstructures,
which offer 3-4 orders of magnitude larger energy scales than
semiconductor-based schemes. Some promising candidate materials are described.Comment: 5 pages, 5 figures, final published version, appendix on samples with
random edge geometries adde
Majorana End-States in Multi-band Microstructures with Rashba Spin-Orbit Coupling
A recent work [1] demonstrated, for an ideal spinless p+ip superconductor,
that Majorana end-states can be realized outside the strict one-dimensional
limit, so long as: 1) the sample width does not greatly exceed the
superconducting coherence length and 2) an odd number of transverse sub-bands
are occupied. Here we extend this analysis to the case of an effective p+ip
superconductor engineered from Rashba spin-orbit coupled surface with induced
magnetization and superconductivity, and find a number of new features.
Specifically, we find that finite size quantization allows Majorana end-states
even when the chemical potential is outside of the induced Zeeman gap where the
bulk material would not be topological. This is relevant to proposals utilizing
semiconducting quantum wires, however, we also find that the bulk energy gap is
substantially reduced if the induced magnetization is too large. We next
consider a slightly different geometry, and show that Majorana end-states can
be created at the ends of ferromagnetic domains. Finally, we consider the case
of meandering edges and find, surprisingly, that the existence of well-defined
transverse sub-bands is not necessary for the formation of robust Majorana
end-states.Comment: 9 pages, 9 figure
Superconductivity and Ferromagnetism in Oxide Interface Structures: Possibility of Finite Momentum Pairing
We introduce a model to explain the observed ferromagnetism and
superconductivity in LAO/STO oxide interface structures. Due to the polar
catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface
layer. We argue that this charge localizes and orders ferromagnetically via
exchange with the conduction electrons. Ordinarily this ferromagnetism would
destroy superconductivity, but due to strong spin-orbit coupling near the
interface, the magnetism and superconductivity can coexist by forming an
FFLO-type condensate of Cooper pairs at finite momentum, which is surprisingly
robust in the presence of strong disorder.Comment: 6 pages of Supplementary materials added containing details of
calculation and further discussion of the FFLO state with disorder,
references added, final version as publishe
Zero-bias peaks in spin-orbit coupled superconducting wires with and without Majorana end-states
One of the simplest proposed experimental probes of a Majorana bound-state is
a quantized (2e^2/h) value of zero-bias tunneling conductance. When temperature
is somewhat larger than the intrinsic width of the Majorana peak, conductance
is no longer quantized, but a zero-bias peak can remain. Such a non-quantized
zero-bias peak has been recently reported for semiconducting nanowires with
proximity induced superconductivity. In this paper we analyze the relation of
the zero-bias peak to the presence of Majorana end-states, by simulating the
tunneling conductance for multi-band wires with realistic amounts of disorder.
We show that this system generically exhibits a (non-quantized) zero-bias peak
even when the wire is topologically trivial and does not possess Majorana
end-states. We make comparisons to recent experiments, and discuss the
necessary requirements for confirming the existence of a Majorana state.Comment: 5 pages, 4 Figure
Edge-Ferromagnetism from Majorana Flat-Bands: Application to Split Tunneling-Conductance Peaks in the High-Tc Cuprates
In mean-field descriptions of nodal d-wave superconductors, generic edges
exhibit dispersionless Majorana fermion bands at zero-energy. These states give
rise to an extensive ground-state degeneracy, and are protected by
time-reversal (TR) symmetry. We argue that the infinite density of states of
these flat-bands make them inherently unstable to interactions, and show that
repulsive interactions lead to edge FM which splits the flat bands. This edge
FM offers an explanation for the observation of splitting of zero-bias peaks in
edge tunneling in High-Tc cuprate superconductors. We argue that this mechanism
for splitting is more likely than previously proposed scenarios, and describe
its experimental consequences.Comment: 10 pages, 5 figure
Convolutional Framework for Accelerated Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) is a noninvasive imaging technique that
provides exquisite soft-tissue contrast without using ionizing radiation. The
clinical application of MRI may be limited by long data acquisition times;
therefore, MR image reconstruction from highly undersampled k-space data has
been an active area of research. Many works exploit rank deficiency in a Hankel
data matrix to recover unobserved k-space samples; the resulting problem is
non-convex, so the choice of numerical algorithm can significantly affect
performance, computation, and memory. We present a simple, scalable approach
called Convolutional Framework (CF). We demonstrate the feasibility and
versatility of CF using measured data from 2D, 3D, and dynamic applications.Comment: IEEE ISBI 2020, International Symposium on Biomedical Imagin
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