44 research outputs found
Tunable hybridization of Majorana bound states at the quantum spin Hall edge
Confinement at the helical edge of a topological insulator is possible in the
presence of proximity-induced magnetic (F) or superconducting (S) order. The
interplay of both phenomena leads to the formation of localized Majorana bound
states (MBS) or likewise (under certain resonance conditions) the formation of
ordinary Andreev bound states (ABS). We investigate the properties of bound
states in junctions composed of alternating regions of F or S barriers.
Interestingly, the direction of magnetization in F regions and the relative
superconducting phase between S regions can be exploited to hybridize MBS or
ABS at will. We show that the local properties of MBS translate into a
particular nonlocal superconducting pairing amplitude. Remarkably, the symmetry
of the pairing amplitude contains information about the nature of the bound
state that it stems from. Hence, this symmetry can in principle be used to
distinguish MBS from ABS, owing to the strong connection between local density
of states and nonlocal pairing in our setup.Comment: 10 pages, 6 figure
Creation of spin-triplet Cooper pairs in the absence of magnetic ordering
In superconducting spintronics, it is essential to generate spin-triplet
Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid
structures in which a superconductor (SC) is combined with a magnetically
ordered material (or an external magnetic field). We, instead, identify a novel
way to create and isolate spin-triplet Cooper pairs in the absence of any
magnetic ordering. This achievement is only possible because we drive a system
with strong spin-orbit interaction--the Dirac surface states of a strong
topological insulator (TI)--out of equilibrium. In particular, we consider a
bipolar TI-SC-TI junction, where the electrochemical potentials in the outer
leads differ in their overall sign. As a result, we find that nonlocal singlet
pairing across the junction is completely suppressed for any excitation energy.
Hence, this junction acts as a perfect spin triplet filter across the SC
generating equal-spin Cooper pairs via crossed Andreev reflection.Comment: 12 pages, 8 figure
Entanglement detection from conductance measurements in carbon nanotube Cooper pair splitters
Spin-orbit interaction provides a spin filtering effect in carbon nanotube
based Cooper pair splitters that allows us to determine spin correlators
directly from current measurements. The spin filtering axes are tunable by a
global external magnetic field. By a bending of the nanotube the filtering axes
on both sides of the Cooper pair splitter become sufficiently different that a
test of entanglement of the injected Cooper pairs through the Bell inequality
can be implemented. This implementation does not require noise measurements,
supports imperfect splitting efficiency and disorder, and does not demand a
full knowledge of the spin-orbit strength. Using a microscopic calculation we
demonstrate that entanglement detection by violation of the Bell inequality is
within the reach of current experimental setups.Comment: 8 pages, 5 figure
Confinement-induced zero-bias peaks in conventional superconductor hybrids
Majorana bound states in topological superconductors have been predicted to appear in the form of zero-bias conductance peaks of height 2e2/h, which represents one of the most studied signatures so far. Here, we show that quasi-zero-energy states, similar to Majorana bound states, can naturally form in any superconducting hybrid junction due to confinement effects, in the absence of spin fields and, thus, without relation to topology. Remarkably, these topologically trivial quasi-zero-energy states produce zero-bias conductance peaks, that could be similar to Majorana signatures, but develop a different peak height ((4e2/h) and are less stable under gating or depletion of the confined region. Our results put forward confinement as an alternative mechanism to explain the ubiquitous presence of trivial zero-bias peaks and quasi-zero-energy states in superconductor hybrid
Phase-tunable multiple Andreev reflections in a quantum spin Hall strip
A quantum spin Hall strip where different edges are contacted by -wave
superconductors with a phase difference supports Majorana Kramers pairs
(MKPs). We study the transport properties of this setup in a four-terminal
normal metal (N)/insulator (I)/superconductor (S) and S/I/S junction. The
tunneling spectroscopy for the N/I/S junction reveals that the signature of
MKPs is that the conductance is quantized by at zero bias and
suppressed at the gap edges. In the S/I/S junction, the subharmonic gap
structure displays a phase-tuned even-odd effect, where all odd spikes
disappear in the presence of MKPs and the remaining even spikes split when
superconductors forming the junction have different gaps. We explain these
features by showing that midgap bound states enhance the transmission of the
even order multiple Andreev reflections, while the reduced density of states at
the gap edges suppresses the odd order ones.Comment: 9 pages, 5 figure
Topological superconductivity in a magnetic-texture coupled Josephson junction
Topological superconductors are appealing building blocks for robust and
reliable quantum information processing. Most platforms for engineering
topological superconductivity rely on a combination of superconductors,
materials with intrinsic strong spin-orbit coupling, and external magnetic
fields, detrimental for superconductivity. We propose a setup where a
conventional Josephson junction is linked via a magnetic-textured barrier.
Antiferromagnetic and ferromagnetic insulators with periodically arranged
domains are compatible with our proposal which does not require intrinsic
spin-orbit or external magnetic fields. We find that the topological phase
depends on the magnitude and period of the barrier magnetization. The
superconducting phase controls the topological transition, which could be
detected as a sharp suppression of the supercurrent across the junction.Comment: 18 pages, 10 figure