Session V08: Superconducting Heterostructures and Quantum Dots

Towards the Kitaev model with quantum dot chains in semiconductor nanowires

We experimentally explore whether chains of quantum dots in semiconductor nanowires can be used to emulate important one-dimensional Hamiltonians such as Kitaev model that describes a topological p-wave superconductor. In our devices, quantum dots are electrostatically confined in an InSb nanowire by an array of closely spaced narrow gates. Quantum dots are coupled to NbTiN superconducting leads. We study transport properties and magneto-spectroscopy of Andreev bound states in these quantum dots with magnetic fields parallel to the nanowire axis. Up to three quantum dots and three superconducting contacts are defined in a single nanowire. We compare results from quantum dot chain experiments to those of experiments on closely related Majorana nanowires.   

Analytical Noise Spectra Calculations for High-Temperature Superconducting Nanowires

In this talk we present analytical results for the noise spectrum of a superconducting heterostructure formed by a high temperature superconductor (HTS) nanowire. The scattering matrix is calculated from first principles using the Bogoliubov-de Gennes equations with appropriate boundary conditions and without the Andreev approximation. The current-current correlation function is calculated directly using the scattering coefficients and from this the noise spectrum (and in particular the shot noise contribution) is generated. The effects of the anisotropy of the order parameter in the scattering properties of the structures are investigated. We also consider some of the consequences of the quasi-1d geometry in terms of the restrictions on allowed propagation modes through the nanowire. Recent advances in materials synthesis have brought the production of high quality HTS nanowires closer widespread use in the design and construction of superconducting devices, which makes theoretical characterization of these structures of particular importance.   

Competition between the Yu-Shiba-Rusinov and Kondo screening in double quantum dot based Cooper pair splitters

The Andreev transport properties of double quantum dot based Cooper pair splitters with one superconducting and two normal leads are studied theoretically in the Kondo regime. The influence of the Yu-Shiba-Rusinov screening resulting from superconducting pairing correlations on the local density of states, Andreev transmission coefficient, and Cooper pair splitting efficiency is thoroughly analyzed. It is shown that finite superconducting pairing potential quickly suppresses the SU(2) Kondo effect, which can however reemerge for relatively large values of coupling to superconductor. In the SU(4) Kondo regime, a crossover from the SU(4) to the SU(2) Kondo state is found as the coupling to superconductor is enhanced.   

Andreev Blockade in a Double Quantum Dot with a Superconducting Lead

A normal metal source reservoir can load two electrons onto a double quantum dot in the spin-triplet configuration. We show that if the drain lead of the dot is a spin-singlet superconductor, these electrons cannot form a Cooper pair and are blockaded on the double dot. We call this phenomenon Andreev blockade because it arises due to suppressed Andreev reflections. We identify transport characteristics unique to Andreev blockade. Most significantly, it occurs for any occupation of the dot adjacent to the superconductor, in contrast with the well-studied Pauli blockade which requires odd occupations. Andreev blockade is lifted if quasiparticles are allowed to enter the superconducting lead, but it should be observable in the hard gap superconductor-semiconductor devices. Andreev blockade should be considered in the design of topological quantum circuits, hybrid quantum bits and quantum emulators.   

Transport in superconductor/ferromagnet multilayers with Néel-type domain walls

We experimentally investigate the coupling between magnetization reversal and magneto-transport in the metallic superconductor/ferromagnet multilayers. The ferromagnet consists itself of a multilayer in which the materials’ stack (Co, Pt, Ir and Ru) is engineered in order to tailor i) the magnetic domains morphology and ii) their size. These variables have a striking effect on the magneto-transport properties below the superconducting critical temperature. In particular, in the presence of magnetic domains the mixed-state resistance is radically diminished to an extent that depends on the domains structure morphology. Contrarily, when the ferromagnet is saturated, an excess resistance is observed. Furthermore, and interestingly, those changes in the longitudinal resistance are accompanied by an anomalous transverse resistance which strongly depends on the applied magnetic field direction and magnetic history. We will discuss the mechanisms contributing to that behavior, which include Pearl vortex pinning and generation, as well as inhomogeneous superconductivity induced by the magnetic template.   

Anisotropic purity of entangled photons from Cooper pairs in heterostructures

It has been theoretically proposed that a forward-biased p-n junction with a superconducting layer (P-N-S) would produce pure, polarization-entangled photons due to inherent spin-singlet pairing of electrons. However, any heterostructure interface generically induces Rashba spin-orbit coupling, which in turn generates a mixed singlet-triplet superconducting order parameter. Here we study the effect of triplet pairing on the purity of photons produced through Cooper pair recombination. A unique directional dependence of the state purity is found: in a triplet superconductor with fixed d-vector, pure, entangled photons are produced when the photon polarization axis is parallel to d, with diminished purity for other directions. Induced triplet pairing in a singlet superconductor is shown to degrade the state purity, while induced singlet pairing in a triplet superconductor is shown to enhance the production of entangled pairs. These considerations may aid the design of functional devices to produce entangled photons.   

Subgap states and Dynes parameter in hard-gap tunnel junctions

Normal metal – insulator - superconductor (NIS) tunnel junctions are the basic building blocks for Josephson junctions, superconducting qubits and many other applications. Ideally, the single particle current is zero within the superconducting gap; while photon assisted tunneling (PAT) can lead to sub-gap currents (Dynes parameter). Here, we examine hard gap NIS tunnel junctions in a well filtered and shielded environment where PAT is negligible. We observe discrete, sharp current steps in the sub-gap regime which can be used as a primary thermometer, in excellent agreement with regular NIS thermometry [1], but cooling as low as 4 mK on a nuclear refrigerator [2]. In an in-plane magnetic field, the steps exhibit Zeeman splitting with g = 2 and display diamagnetic shifts. We model the steps as geometric resonances within the weakly disordered normal metal giving enhanced Andreev reflection due to multiple reflections. The model shows that disorder is a possible microscopic origin for Dynes-type linear leakage current.
[1] A. V. Feshchenko at al., Phys. Rev. Appl. 4, 034001 (2015).
[2] M. Palma, C. P. Scheller, at al., Appl. Phys. Lett. 11, 253101 (2017).   

Induced superconducting gap vs. barrier thickness in hybrid Al/Al0.15In0.85As/InAs heterostructures

The two dimensional electron gas (2DEG) in InAs proximitized by aluminum (Al) is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al_0.15In_0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG will largely depend on the barrier thickness. We present a systematic study of the strength of the induced gap in hybrid Al/Al_0.15In_0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness. We estimate the induced gap by analyzing multiple Andreev reflections in SNS junctions and differential conductance measurement in S-QPC-N junctions in the tunneling regime. We compare our results with self-consistent calculations of induced gap for our particular geometry.   

Interaction of Skyrmions and Pearl Vortices in Superconductor-Chiral Ferromagnet Heterostructures

We investigate a hybrid heterostructure with magnetic skyrmions (Sk) inside a chiral ferromagnet
interfaced by a thin superconducting lm via an insulating barrier. The barrier prevents the electronic
transport between the superconductor and the chiral magnet, such that the coupling can only
occur through the magnetic elds generated by these materials. We nd that Pearl vortices (PV)
are generated spontaneously in the superconductor within the skyrmion radius, while anti-Pearl
vortices (PV) compensating the magnetic moment of the Pearl vortices are generated outside of
the Sk radius, forming an energetically stable topological hybrid structure. Finally, we analyze the
interplay of skyrmion and vortex lattices and their mutual feedback on each other. In particular, we
argue that the size of the skyrmions will be greatly a ected by the presence of the vortices o ering
another prospect of manipulating the skyrmionic size by the proximity to a superconductor.   

Tunneling Anomalous Hall Effect (TAHE) in Ferromagnet/Superconductor Junctions

The competition of two antagonistic interactions, spin-singlet superconducting pairing and ferromagnetic exchange, in one heterojunction leads to extraordinary phenomena. Owing to the additionally broken inversion symmetry, not only the interplay of superconductivtity and ferromagnetism, but also the induced strong interfacial spin-orbit fields (SOFs) offer interesting subjects for experimental and theoretical investigations; several studies unraveled an intriguing impact of interfacial SOFs on transport already in normal-conducting systems, e.g., TAMR¹ and TAHE² effects. Our theoretical work focuses on ferromagnet/superconductor junctions, demonstrating the existence of a superconducting TAHE effect. While the effect's fundamental characteristics are comparable to what has been found in the normal-conducting analog², our numerical simulations predict a much larger tunability of the TAHE conductance in the superconducting scenario, mostly due to the presence of Andreev reflection. Together with a simultaneously generated transverse supercurrent response in the superconductor, these findings might offer an interesting future perspective for experimentalists.

¹ PRL 99, 056601 (2007)
² PRL 115, 056602 (2015)   

Optimizing spin-triplet supercurrent though a ferromagnetic Josephson junction

Ferromagnetic Josephson junctions show promise for application to energy efficient cryogenic memory [1]. Both spin-singlet and spin-triplet supercurrents are being studied by our group for this purpose. Engineering adjacent F layers in a three-layer system to have perpendicular magnetizations allows singlet pairs to convert to spin-aligned triplet pairs. Recent work in our group exploited a synthetic antiferromagnet (SAF) with perpendicular magnetic anisotropy (PMA) for the middle F layer [2]. These junctions exhibit phase control [2], but have a low critical current when compared to singlet junctions. To make the triplet junction a viable option for memory we show that removing the SAF while maintaining the PMA increases the critical current. To estimate the fraction of the supercurrent carried by spin-triplet pairs in our junctions, we also fabricate comparison junctions with the order of the F layers shuffled to suppress generation of spin-triplet pairs.

[1] I. Dayton, et al., IEEE Magn. Lett. 9, pp 1-5, (2018).
[2] J. A. Glick, et al., Science Advances 4, eaat9457 (2018).   

Geometry and pairing-symmetry effects in superconductor/half-metallic ferromagnet vertical junctions

The interactions between superconductors (S) and ferromagnets (F) are widely studied nowadays due to the possibility of obtaining equal-spin triplet pairing, which is immune to the exchange field and therefore can propagate over long distances into the F. This effect has fundamental interest as well as a potential for spintronic applications. In this context, we study vertical S/F/S junctions made of a half-metallic F (La_0.7Sr_0.3MnO₃ or La_0.7Ca_0.3MnO₃) with either a d-wave S (YBa₂Cu₃O₇) or a s-wave one (Mo₈₀Si₂₀), aiming to understand to role of the pairing symmetry on the proximity effect and, particularly, on the generation of equal-spin triplet correlations. We will compare low-temperature magneto-transport measurements made in vertical micro-junctions having different geometries, namely three-probe vs. a novel four-probe junction scheme, which is designed to separate conductance features linked to the top electrode’s contact resistance from effects due to transport across the entire S/F/S junction.   

Transport in ferromagnetic/superconducting heterostructures

We consider the transport theory in nano-scale ferromagnetic/superconducting heterostructures with multiple ferromagnetic layers. We do so using fully self consistent, numerical methods in the clean limit. Many properties unique to these systems arise due to the proximity effects between ferromagnets and superconductors. We determine the conductance in each spin channel and analyze its dependence on the magnetic misalignment angle between the ferromagnets, as well as on other physical parameters pertinent to multi-layered systems. We quantitatively describe some of these properties and we discuss our results in connection to proposed experiments.   

Thermoelectric effects in superconductor-ferromagnetic hybrids

The conflicting spin order in superconductor-ferromagnetic systems has lead to a host of facinating phenomena. However, most experimental studies have focused on electrical and spin transport measurements. While there have been numerous theortical predictions of exciting thermal phenomena, thermal properties of these systems remains largley unexplored. In particular, thermoelectric effects which are usually negligible in superconductors have been predicted to increase dramatically in superconductor hybrid structures when in the presence of a spin splitting exchange field. This presentation will focus of the experimental design and measurement of the Seebeck coeffient in superconductor-ferromagnetic hybrids.   

High-critical-field superconducting heterostructures using anodic oxidation

In-situ growth of Al on top of shallow InAs 2DEG heterostructures gives close to perfect proximity effect [1-2]. The transparent super/semi interface combined with customizable 2DEG lithography hold promise to many interesting applications, e.g. topological quantum computation [3-4].
When the aluminum is chemically etched the underlying InAs is degraded by surface impurities. [2]
Instead of etching the aluminum, here we show that controllably oxidizing the Al through anodic oxidation (AO) gives up to a factor 2 increase in InAs mobility and Quantum Hall effect emerges before 3 Tesla.
We will also show how AO can be used to controllably thin down Al, thus increasing its superconducting properties [2,5-6], obtaining an in-plane critical field > 6 T and a perpendicular critical field > 3T on a mesoscopic structure.
Besides enhancing superconducting properties of established devices, this technique paves the way to new research topics, eg. Quantum Hall edge states proximitized by the surface Al with close to unity transparency.

[1] M. Kjærgaard et al. Nature commun. 12841
[2] J. Shabani et al. Phys. Rev. B 93, 159908
[3] F. Nichele et al. Phys. Rev. Lett. 119, 136803
[4] A. Fornieri et al. arXiv:1809.03037
[5] Y. Ivry et al. Phys. Rev. B 90, 214515
[6] P. Tedrow et al. Phys. Rev. B 25, 171