Session A49: Superconducting Proximity Effect and Josephson Junctions

Gate-Dependent Transport in Multi-Terminal Josephson Junctions

Josephson coupling of three or more superconducting leads through a material with few conducting modes has been predicted to give rise to topological effects. Such behavior, of relevance for topologically-protected quantum bits, would lead to specific transport features measured between terminals, with topological phase transitions occurring as a function of relative phase and voltage biases. Here we study the effects on transport of several top-gating arrangements on multi-terminal Josephson junctions with many conducting modes based on an InAs 2DEG proximitized with an epitaxial aluminum layer and many conducting modes. The superconducting features can be accurately simulated by a network of RCSJ junctions. [1]

1. G. V. Graziano, J. S. Lee, M. Pendharkar, C. J. Palmstrom and V. S. Pribiag. arXiv:1905.11730 (2019)   

Gate tunable multi-terminal Josephson effect

Junctions of more than two superconducting terminals are required for implementing braiding operations on Majorana fermions. Moreover, such multi-terminal Josephson Junctions (JJ) were predicted to support topological state and host zero-energy quasiparticles. Unlike conventional two-terminal JJs where the value of critical current is a number, the multi-terminal JJs exhibit a novel feature – the critical current contour (CCC) [1]. We report the measurement of non-trivial CCC shapes as a function of gate voltage and magnetic field in hybrid semiconductor/superconductor (InAs/Al) multi-terminal JJs. Multi-terminal junctions can host two different regimes: a strong neighbor-coupling regime and a multi-terminal regime, depending on the gate voltage. The geometry of a junction is also an important factor defining the operating regime. The effect of an out-of-plane magnetic field indicates an observation of the Fraunhofer interference pattern in multi-terminal JJs.
[1] N. Pankratova, H. Lee et al., “The multi-terminal Josephson effect”, arXiv:1812.06017
  

Unconventional Superconductivity in monolayer Transition Metal Dichalcogenides via proximity

Single layer Transition Metal Dichalcogenides (TMDCs) provide a unique platform to study the interplay of spin-orbit coupling, topology and electron-electron interactions. In addition, the ultrathin 2D geometry allows for proximal coupling to other materials particularly magnets and superconductors. In this talk we report on the nature of the induced superconducting state when coupled to a conventional superconductor. Going beyond the tunneling model we solve a self-consistent Bogoliubov-De Gennes equations for the heterostructure to establish the nature of the paired state. Introducing a third device component, a ferromagnet such as Chromium tri-Iodide, breaks time reversal allowing access to the topologically nontrivial electronic states of the TMDC. Modifications of the induced phase and conditions for realizing topological superconductivity will be discussed.
  

Induced superconductivity and multiple Andreev reflections in multilayer WTe2 Josephson junctions

Introducing superconductivity into topological materials has become a focus of attention in condensed matter physics as an effective way to realize Majorana modes. It has been predicted that Majorana bound states could arise from the proximity effect between an s-wave superconductor and the surface states of a strong topological insulator (TI). As one of the transition metal dichalcogenides, the topological properties of WTe₂ have been verified in both bulk (type-II Weyl semi-metal) and monolayer materials (topological insulator). Here we fabricated SNS junctions based on multilayer WTe₂ flakes. We report the observations of the proximity effect induced superconductivity revealed by magnetoresistance (MR) and I-V measurements. Distinct zero-bias conductance peaks in differential conductance measurements, might be as a sign of Majorana state, were also observed. In addition, the multi-dips of differential resistance at low temperature and magnetic field marks multiple Andreev reflections which might result from the intrinsic multi-gap superconducting states of WTe₂.   

Proximity induced Superconductivity in low carrier density BixSb2-xTe3

To investigate superconductivity in topological insulators, we have fabricated and measured arrays of islands of diameter about 160nm on topological insulators grown by molecular beam epitaxy. Typical island spacing is about 40 nm. The superconductivity is induced via the proximity effect which is strongly dependent on the electronic properties of the underlying TI film. Earlier work with n-type Bi₂Se₃ where the carrier density was 3E13 carriers per square cm showed the emergence of superconductivity with a 2D critical current density of more than 1A/m at 0.7 mK. In these samples, both bulk and 2D topological surface states were made superconducting. To study proximity effect coupling into only the topological surface states we used thin films of the alloy Bi_xSb_2-xTe₃. In these materials, the total carrier density can be tuned such that the Fermi surface is completely topological and no bulk states are occupied. In samples with carrier density of 7E12 carriers per square cm, we observed the superconducting proximity effect coupling through the surface states above 0.8 K. Nonlinear transport and magnetoresistance results will be presented and discussed. This study reveals how surface and bulk carriers participate in superconductivity in TIs.   

Superconducting Proximity Effect in Magnetically-Doped Topological Insulators using Bulk Single Crystals

Superconducting proximity effect on magnetic topological insulators (TIs) is expected to induce unconventional superconductivity, some of which can host the Majorana fermion. Recently, we observed some unusual behaviors on Nb/Fe-doped TI/ Nb Josephson junctions: e.g., a unique three-peak structure and a 4π-periodic Josephson current. However, those origins are still under debate due to possible contributions from the bulk and the edge modes to the proximity effect.
In this study, we prepared another magnetic TI with a high bulk resistivity and fabricated its Josephson junctions without edge contacts. As a result, a similar three-peak structure was also observed, suggesting that the observed unique behaviors found in the former junction also came from the surface contribution. Furthermore, we observed a unique magnetic response possibly related to bulk magnetic properties. We believe that those results promote our understanding of the proximity effect on topological materials.   

Compensated CoGd Ferrimagnets in Magnetic Josephson Junctions

The rare earth-transition metal ferrimagnet Co_1-xGd_x has been studied for application in superconducting electronics. At a carefully chosen Gd concentration, the Co and Gd moments perfectly cancel leading to a zero-moment compensation point. Point-contact Andreev reflection measurements [1] have shown that at the compensation point there is still a net spin polarization of the transport current. This raises the possibility of application in electronics requiring a magnetic material with little stray field, such as cryogenic magnetic memory. As the compensation point can vary significantly as a function of temperature and thickness, we present SQUID VSM data on thin films taken at cryogenic temperatures. We also present the results of transport measurements on superconductor-ferrimagnet-superconductor Josephson junctions over a range of thicknesses for CoGd alloys near their compensation point.
[1] Naylor et al., Phys. Rev. B 85, 064410 (2012)   

Superconducting pairing symmetry and spin-orbit coupling in proximitized graphene

Graphene has been shown to exhibit unusual topological phases upon proximity to different dichalcogenide substrates [1]. However, the effect of these perturbations in the presence of superconducting correlations has not been explored. We study the effect of different superconducting symmetry pairings over a range of chemical potential. We analyze the symmetry and identify the topological characteristics of the resulting quasiparticle spectrum at the mean field level.
Interestingly, the system with nearest neighbor spin-singlet superconductivity order parameter may exhibit inverted bands in the quasiparticle band structure, reminiscent of the electronic bands [1]. As the system parameters change, the corresponding edge states in such phase appear after a transition to a regime that closes the bulk bandgap. As we will show, however, both gapped phases belong to a topologically trivial regime, as the results of their invariant number Z2 indicate.
Our results also show that the phases in a time-reversal-invariant 2D-superconductor are not dependent on the magnitude of the singlet and triplet order parameter, and/or the chemical potential, as the topological invariant number remains unchanged.
[1] A. M. Alsharari et al., Phys. Rev. B 98, 195129 (2018).
  

Chaos and Chimera in Hysteretic RF SQUID Metamaterials

RF SQUIDs have been established as viable building blocks for microwave frequency metamaterials [1,2]. The RF SQUID resonance is tunable under applied DC flux, with upper-frequency range scaling as √(1+β_rf). Our previous design restricted the parameter β_rf below unity to avoid hysteresis, thus limited the resonance range. We have built new arrays of RF SQUID meta-atoms in the hysteretic regime to explore their interesting properties with the ultimate goal of extending the resonance frequency tunability. In particular, a strong and positive out-of-plane coupling among the SQUIDs is achieved through an alternating-overlapping-loop geometry, potentially mitigating the hysteresis from high-β_rf SQUID meta-atoms. Much theoretical work has predicted chaotic dynamics and chimera states in such systems. Observations of the above nonlinear phenomena in microwave transmission measurements and laser scanning microscopy [3] will be reported.

[1] Phys. Rev. X, 3, 041029 (2013) https://doi.org/10.1103/PhysRevX.3.041029
[2] Phys. Rev. X, 5, 041045 (2015) https://doi.org/10.1103/PhysRevX.5.041045
[3] Appl. Phys. Lett. 114, 082601 (2019) https://doi.org/10.1063/1.5064658   

Josephson current mediated by odd-frequency equal-spin triplet pairing on the surface of Weyl nodal loop semimetals

We explore proximity-induced pairing on the surface of a Weyl nodal loop semimetal (WNLS) sandwiched between two conventional spin-singlet s-wave superconductors in a Josephson junction set-up. The fully spin-polarizated drumheadlike surface states (DSSs) and the intrinsic spin-orbit interation of the WNLS cause odd-frequency (odd-ω) equal-spin triplet pairing to be induced on the surface of the WNLS, whereas, the spin-singlet pairing decays inside the WNLS very fast. We show finite Josephson current in the junction contributed by the equal-spin triplet odd-ω pairing. Odd-ω mixed-spin triplet pairing can be generated by placing an additional ferromagnet in the junction if the direction of the magnetization of the ferromagnet opposes the spin-polarization direction of the DSSs. The pairing and the current are not affected if the magnetization direction is orthogonal to the DSS spin polarization, which further confirms the equal-spin structure of the odd-ω pairing.   

Critical Current Decay in Josephson Junctions Containing Antiferromagnetic Ni41Mn59

We report on the fabrication and measurement of antiferromagnetic (AF) S/N/AF/N/S Josephson junctions where Ni₄₁Mn₅₉ is used as the AF layer, Nb as the S layer, and Cu as the N layer. From measurement of critical current in samples with NiMn thicknesses in the range from 1.2 nm to 6.0 nm, we measure the characteristic decay length within NiMn to be 2.1 nm. One potential application of AF layers inside Josephson junctions is to pin the magnetization of an adjacent ferromagnetic (F) layer by exchange bias. We have characterized NiMn/NiFe and NiMn/Ni bilayer sheet films magnetically to confirm the pinning behavior of NiMn. Such a bilayer AF/F system could potentially be used as a pinned hard layer in a proposed cryogenic memory cell whose design takes the form of an S/F/F’/S spin valve where the two ferromagnetic layers [F, F’] have different switching fields [1]. This design requires a robust magnetization of the hard layer which would be strengthened by the exchange bias effect.

[1] Dayton I.M., et. al., IEEE Magnetics Letters, 9, 3301905 (2018)   

Magneto-transport of electrons in near surface InAs quantum wells in contact with NbTiN

Indium Arsenide (InAs) near surface quantum wells have become the focus of recent interest for their use in heterostructures with superconductors. An interface between a superconductor and quantum Hall edges is predicted to exhibit excitations with non-abelian statistics. NbTiN-InAs is a promising candidate as NbTiN can sustain strong magnetic fields where InAs can host integer quantum Hall states. In this work, we study the proximity effect by monitoring the current flow along the superconductor-semiconductor interface. The data suggest the enhanced conductance is due to Andreev reflection when edge states are formed in the presence of a strong magnetic field.   

Superconductor-Semiconductor Devices with 2D Holes in Germanium

The coupling of superconductors to semiconductors receives widespread interest due to their potential for realizing topological superconductivity, interferometers and Andreev Bound State devices [1, 2]. The quality of the induced gap, heavily influenced by the transparency of the superconductor-semiconductor interface, presents a crucial challenge for realizing such devices.
Here, we study proximity induced superconductivity in germanium quantum wells coupled to aluminium. The presence of strong spin-orbit interaction, tunable g-factor and high mobility make germanium an attractive semiconducting platform. Combined with the ease of ohmic contact formation due to Fermi level pinning, Ge-Al is a promising material system for hybrid semiconductor-superconductor devices. We achieved transparencies of 65% and I_CR_N products of up to 50µV, about three times higher than previously reported [3, 4].
[1] R. M. Lutchyn et al., Phys. Rev. Lett. 105, 077001 (2010).
[2] J.-D. Pillet et al., Nature Physics 6, 965 (2010).
[3] N. W. Hendrickx et al., Phys. Rev. B 99, 075435 (2019)
[4] F. Vigneau et al., Nano Lett. 19, 1023-1027 (2019)   

Transport properties of ultra-scaled Ge/Si core/shell nanowires with highly transparent Al contacts.

Superconducting-semiconducting hybrid systems provide a rich domain for the investigation of electronic and quantum transport. Further, promising developments in their application in high performance nanoelectronics and quantum devices has motivated significant research and development. Nanowire heterostructures are of particular interest due to their quantum confinement properties allowing one to investigate transport in quasi one-dimensional systems. However, critical to their success is the fabrication of high quality and reproducible semiconductor-superconductor interfaces. Using a novel annealing technique, we have realised nanowire heterostructures consisting of crystalline-Al/Si core/shell leads contacting tuneable Ge/Si core/shell segments with atomically precise interfaces. We will present results of temperature dependent DC transport measurements on ultra-scaled devices. Reporting on the gate tuneable transport properties of these highly transparent devices including quantized conductance, tuneable Josephson current and multiple Andreev reflections and their applications as quantum devices.   

Gate-tunable proximity effect in epitaxial Al-InAs Josephson junctions using hBN gate dielectric

The transparent interface of epitaxial Al-InAs heterostructures provides an excellent platform for potential advances in topological superconductivity and superconducting quantum computation. Josephson junctions built on these heterostructures can be gated, allowing for tuning of the supercurrent. We report the fabrication and measurement of gate-tunable Al-InAs Josephson junctions in which the gate dielectric in contact with the InAs was mechanically exfoliated hexagonal boron nitride (hBN). We discuss fabrication processes that enable compatibility between layered material transfer and Al-InAs heterostructures. By comparing these devices with others using a conventional AlOx gate dielectric, we show that hBN is a suitable gate dielectric for the Al-InAs material platform. Specifically, the product of normal resistance and critical current, is comparable for both types of devices, but higher R_N for the hBN-based devices suggests the possibility of less unintentional doping of the InAs by the gate dielectric.