Session F49: Superconducting Proximity Effect and Josephson Junctions II

THz emission characteristics from mesa arrays of Bi2Sr2CaCu2O8+δ single crystals

To develop the high power terahertz emission from Bi₂Sr₂CaCu₂O_8+δ (Bi2212) high-T_c superconducting THz emitters (Bi2212-THz emitters), it is very important to manage self- heating of these devices. We have developed stand-alone mesa structures (SAMs) of Bi2212 single crystals sandwiched by good thermal conducting sapphire substrates to reduce self-heating effects. These thermal managed devices enable us to extend the range of the radiation frequencies up to around 2.4 THz. For applications of THz emitters, 1 mW level of output power and 1 kHz level of radiation linewidth are required. In order to obtain such high performance characteristics from our Bi2212-THz emitters, we have studied not only about thicker SAMs but also arrays of SAMs. The SAMs up to around 8 μm thickness with keeping good shape of mesa structures can be obtained by a wet etching method so far. We made arrays of those SAMs by using the thermal managed device structures. At this moment, we observed about 10 μW level of output power from two arrays of SAMs with the dimensions of 69 × 190 × 4.2~4.4 μm³. The details of the current situation of our Bi2212-THz emitters will be discussed.   

Mesa sidewall effect on coherent terahertz radiation from Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions

Coherent terahertz sources based on the stack of Bi₂Sr₂CaCu₂O_8+δ intrinsic Josephson junctions (IJJs) have been attracted in the academic and industrial fields. See Ref. [U. Welp et al., Nat. Photonics, vol. 7, p. 702 (2013)] for recent review. In this study, we demonstrate that tapered sidewalls produce a difference in the junction voltages along the stack and thus synchronization among the IJJs fails to occur with distributed oscillation frequencies. Also, our systematic experiment using shielding metal masks reveals that only the ac Josephson current flowing along the sidewall can generate the terahertz waves outside of the mesa. These results suggest that the condition of the sidewall is essentially important to control the radiation characteristics. In this talk, we will present the details of the fabrication process and experimental results and will discuss the role of the mesa sidewalls on the coherent radiation.   

Electromagnetic field analysis of coherent terahertz radiation emitted from stacks of intrinsic Josephson junctions of BSCCO with microstrip antennas

The observation of coherent terahertz radiation from a stack of intrinsic Josephson junctions (IJJs) in Bi₂Sr₂CaCu₂O_8+δ (BSCCO) highlighted the potential of using high-T_c superconductors as compact and convenient terahertz sources. So far, synchronized IJJ mesa arrays were demonstrated to produce the output power up to 0.6 mW [Benseman et al., APL (2013)], which is presently the highest recorded level among all available terahertz sources. To increase the output power further up to milliwatt-level, we need to mitigate impedance mismatch at the boundary between an emitting IJJ stack and a dielectric medium or free space. In this study, we implement the electromagnetic field analysis of the radiation emitted from IJJs stacks with microstrip antennas, which are designed for highly efficient radiation. We established a microfabrication process based on the lift-off technique to pattern the microstrip antenna. In this talk, we will discuss the characteristic radiation behavior associated with the bath and local temperatures and variation in polarization characteristics depending on the bias point.   

THz emission peak at low voltage bias from stacked intrinsic Josephson junction Bi2Sr2CaCu2O8 terahertz sources

The extremely anisotropic high-temperature superconductor Bi₂Sr₂CaCu₂O₈ contains stacked 'intrinsic' Josephson junctions with a large superconducting gap energy. Mesa-shaped devices constructed from this material therefore show promise as a source of coherent, continuous-wave radiation in the 'terahertz gap' range.
THz emission from these devices has previously been typically observed at frequencies between around 0.4 THz – 2 THz, corresponding to Josephson voltages of between 0.8 mV and 4 mV per intrinsic junction. However, we have recently observed a new type of emission which occurs at bias currents below the retrapping current of the stacked Josephson junctions. This unusual THz emission state is highly reproducible, is metastable over a timescale of hours, and shows systematic temperature dependence. The maximum emitted THz power that can be generated from this mode is comparable to that generated by the more well-established and better-understood THz emission modes in Bi₂Sr₂CaCu₂O₈ mesa sources.
We will discuss possible mechanisms for the unusual emission mode, together with future directions of related research.   

Stacked intrinsic Josephson junction Bi2Sr2CaCu2O8 terahertz sources: Design issues for achieving high power output close to Tc

The high-temperature superconductor Bi₂Sr₂CaCu₂O₈ contains stacked 'intrinsic' Josephson junctions, with unrivalled packing density and a high superconducting gap energy. Cuboid ‘mesa’ devices constructed from this material are consequently a promising technology for coherent, continuous-wave radiation in the 'terahertz gap' range, spanning from approximately 0.3 – 1.5 THz.
A key issue for practical applications of such devices is their cryocooling requirements, and it is therefore highly desirable to optimize their performance at temperatures that can be achieved by nitrogen cryogenics. Here we report generation of 0.15 milliwatts of coherent emission power at 0.5 THz, at a bath temperature of 77 Kelvin. This was achieved by exciting the (3, 0) cavity mode of a stack containing 580 junctions, and T_c of 86.5 Kelvin. In order to minimize self-heating, the THz source was mounted on a copper substrate using PbSn solder.
We will discuss the choice of mesa dimensions and cavity mode, and implications for the design of devices which are intended to operate close to the material’s superconducting critical temperature.   

Demonstrating phase control in spin-triplet ferromagnetic Josephson junctions

Ferromagnetic Josephson junctions show promise for application in 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) as the central layer. These junctions have been shown to exhibit phase control [2] but have a low critical current when compared to singlet junctions. We have shown that removing the SAF while maintaining the PMA increases the critical current by a significant amount. We demonstrate phase-control by fabricating two of these junctions in a SQUID loop, and measuring SQUID oscillations for combinations of the parallel and anti-parallel magnetic states in the two junctions.

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

Ferromagnetic Josephson junctions with perpendicular magnetic anisotropy

Ferromagnetic Josephson junctions are a strong candidate to become a dissipationless cryogenic memory alternative to dissipative CMOS technologies. In order to build such a memory, the ferromagnetic Josephson junctions must be optimised for both the switching of the ferromagnetic layer and the critical current of the junction. The critical current of the junction decays exponentially with the ferromagnet thickness, making a thin ferromagnet preferable, however typically for in-plane ferromagnets the switching properties worsen as the ferromagnet is thinned. In addition, stray fields from an in-plane ferromagnet add a vector potential in the Josephson junction. To solve these issues, here we explore the use of ferromagnets with perpendicular magnetic anisotropy. We find that, unlike for in-plane ferromagnets, the switching properties of Co in a Nb-Pt-Co(d_Co)-Pt-Nb wedge are optimised for ultra-thin (≤0.6 nm) films. We will also report our progress characterising Josephson junctions with perpendicular Co and CoB ferromagnetic layers.   

High Field Superconductivity and Magnetic Moment Enhancement in Proximity Exchange Coupled GdN/NbN Nano-bridges

Proximity effects in ferromagnetic insulator/superconductor (FI/SC) interface have been highly intriguing in terms of fundamental exploration as well as technological relevance [1,2]. We perform detailed investigations on GdN (FI) and NbN (SC) interface via magneto-transport properties of GdN/NbN-based nano-bridges and thin film heterostructures. We observe a clear enhancement of magnetic moment and a drastic increase of coercive field, below the superconducting transition temperature of NbN, which may be attributed to exchange coupled effect between GdN and NbN at the interface. Most interestingly, our nano-bridges exhibit a large magnetic field dependent superconductivity enhancement for fields > 5T similar to the Jaccarino effect [3]. The observed effect is temperature dependent implying a free energy dependence at the interface. Finally, we also discuss the exchange coupled interface effects and their implications in GdN/NbN system.

Ref.:
[1] J. S. Moodera, T. S. Santos and T. Nagahama, J. Phys.: Condens. Matter 19 165202 (2007);
[2] Y. Zhu et al., Nat. Mat. 16, 195 (2017);
[3] V. Jaccarino and M. Peter, Phys. Rev. Lett. 9, 290 (1962).   

Optimizing supercurrent transmission and magnetic behavior in ferromagnetic Josephson junctions

Josephson junctions with ferromagnetic layers where the ground-state phase difference can be reliably controlled are a potential candidate for applications in cryogenic memory devices, which can greatly reduce the ever-growing energy requirements for large-scale computing. Phase control has been successfully demonstrated with junctions containing a Ni fixed layer and a NiFe free layer[1,2]. However, there are still a number of improvements that can be made to increase the efficiency and reliability of these junctions. We present work on trying to improve the efficiency by using thin layers of Ni “dusting” around the NiFe free layer to further increase the transmission of supercurrent through these junctions. We also present work on improving the switching reliability by replacing the fixed Ni layer with unbalanced Ni SAFs which may have more desirable magnetic properties.

[1] E. C. Gingrich, B. M. Niedzielski, J. A. Glick, Y. Wang, D. L. Miller, R. Loloee, W. P. Pratt, and N. O. Birge, Nat. Phys. 12, 564 (2016).
[2] I. Dayton, T. Sage, E. Gingrich, M. Loving, T. Ambrose, N. Siwak, S. Keebaugh, C. Kirby, D. Miller, A. Herr, Q. Herr, and O. Naaman, IEEE Magn. Lett. 9, 3301905 (2018).   

Observation of Edge States in Multilayer WTe2

Three-dimensional (3D) topological semimetals possess unconventional surface and edge states, which play central roles in exotic topological phases. WTe₂, as a type-II Weyl semimetal, has 2D Fermi arcs on the (001) surface in the bulk and 1D helical edge states in its monolayer. However, in the intermediate regime between the bulk and monolayer, the edge states have not been resolved owing to its closed band gap which makes the bulk states dominant. Here, we report the signatures of the edge states by superconducting quantum interference measurements in multilayer WTe₂ Josephson junctions and we directly map the localized supercurrent. In thick WTe₂ (~60 nm), the supercurrent is uniformly distributed on the (001) surface. In thin WTe₂ (10 nm), however, the supercurrent becomes confined to the 1D edge modes and its width reaches up to 1.4 um. Furthermore, an asymmetric Josephson effect , predicted as a unique characteristic of inversion-symmetry-breaking topological systems, is observed in thin WTe₂ whereas it is absent in the thick one. The ability to combine superconductivity with these edge states establishes WTe₂ as a promising topological system with exotic quantum phases and a rich physics of extra dimensionality and tunability.   

An Alternate Scheme to Mode-Lock Quantum Phase Slips.

It has been proposed that locking the voltage oscillations of a Quantum Phase Slip Junction (QPSJ) to a microwave source could achieve the transfer of an integer number of elementary charges per microwave cycle. Such a synchronized transfer of charge would correspond to current steps in the current-voltage characteristic of the QPSJ, which could be used to realize a current standard. These proposals focus on realizing a circuit dual to the Josephson voltage standard and therefore require placing a QPSJ close to a resistor whose value is larger than the superconducting quantum of resistance. The observation of current steps for such a circuit is however extremely challenging experimentally, mostly due to the large Joule heating occurring in the resistor. To address this problem, we consider alternative designs that relax the constraint on the resistance and instead rely on the feedback provided by a high-impedance resonator. Through numerical simulations, we demonstrate that mode-locking indeed occurs in such systems and that synchronized charge transfer can be achieved in a resonant environment, without the need for a large resistor. Such a circuit may offer an alternative path toward the realization of a current standard.   

Exploring parity transitions of Majorana bound states in S-TI-S lateral Josephson junctions

It is predicted that S-TI-S lateral Josephson junctions, fabricated by depositing electrodes of a conventional superconductor (S) on the surface of a topological insulator (TI), exhibit Majorana bound states (MBS) at locations in the junction where the phase difference is an odd-multiple of π, i.e. at the cores of the Josephson vortices. There is evidence for this from the lifting of odd nodes in the critical current diffraction patterns and the observation of supercurrent features at magnetic field where the MBS enter the junction. In this talk, we report progress on experiments designed to probe the parity states of MBS pairs and the transitions between them via measurements of the critical current distribution and supercurrent fluctuations. These experiments give information about parity lifetimes and open the door to understand the factors that limit them.   

Topological Josephson Junctions in Corbino geometry

One-dimensional hybrid semiconductor/superconductor wires and, more recently, long Josephson junctions are the most versatile synthetic topological superconductors where non-abelian excitations can be realized and thoroughly investigated. In both realizations quasiparticle states appear at the physical boundaries, either at the ends of the nanowires or at the edges of the Josephson junction. Signatures of these states have been observed, studied and investigated by several groups with respect to various parameters such as magnetic field, chemical potential and the superconducting phase difference. In this work we report experimental investigation of Josephson junctions in the Corbino geometry fabricated from InAs/Al heterostructures. These devices are in a long junction regime with periodic boundary conditions. Transport studies of these junctions in a normal regime (zero in-plane magnetic field) and in the regime where some regions of the junction are in a topologically non-trivial regime (non-zero in-plane magnetic field) will be presented.   

Engineering the spin-orbit interaction in InAs-based quantum wells for superconductor-semiconductor hybrid structures

Superconductor (SC)-semiconductor hybrid structures open an access to exotic superconductivity, enabling the creation of Majorana zero modes in appropriate devices. The strength of the spin-orbit interaction (SOI) in the structure represents an important parameter, driving for example the size of the energy gap induced into the SC. InAs-based epitaxial hybrid heterostructures have emerged as a promising candidate for such systems, exploiting the rather large intrinsic SOI of InAs and the epitaxial matching with SCs such as aluminum (Al).
In this work, we present a systematic study of two-dimensional electron gases (2DEGs) realized in strongly asymmetric InAs/InGaAs/InAlAs quantum well layouts. We show that the SOI in such 2DEGs is substantially enhanced by engineering the asymmetry through the position of the InAs inset and the doping layer. We characterize the strength of the SOI in the 2DEGs by analyzing SOI-induced features in low temperature magneto transport, such as zero-field spin splitting-induced beating patterns and weak antilocalization, as well as by exploring the energy gap induced into epitaxial superconducting Al.   

Anomalously large second Josephson harmonic in quantum well-based junctions

The current-phase relation (CPR) of a Josephson junction (JJ) contains information about the microscopic mechanisms behind supercurrent. The sinusoidal CPR can successfully describe most JJs made with different materials and synthesis technologies. However, CPR can also deviate from simply sinusoidal form, in particular it can feature higher order sinusoidal terms. We investigate InAs quantum well JJs with epitaxial Al contacts. The distance between Al electrodes is of order 100 nm shorter than the mean free path in the quantum well. We perform diffraction pattern measurements, SQUID measurements and Shapiro step measurements all pointing at a strong intrinsic second order harmonic.