Session M49: Superconductivity: Devices and Applications

Development of NbTiN constriction junctions and superconducting quantum interference devices

Superconducting Quantum Interference Devices (SQUIDs) have become a leading tool for magnetometry with a wide range of applications. In this work, we investigate the properties of NbTiN SQUIDs created with a patterned constriction junction. The NbTiN film was grown through reactive sputtering, and then patterned using e-beam lithography and ion milling. SQUIDs were fabricated with constrictions as narrow as 50nm across, and loop areas of approximately 1μm². At 250mK, field dependent voltage oscillations were observed up to .3T, with an effective area consistent with the physical dimensions of the device.   

Reducing 1/f flux noise in superconducting devices in situ with UV light.

Magnetic flux noise with a 1/f power spectrum is pervasive in superconducting devices, and presents a fundamental limit to both the low-frequency energy resolution of dc SQUIDs and the high-frequency coherence of flux-sensitive superconducting qubits. Studies of the scaling of the magnitude of the flux noise with SQUID geometry are consistent with generation of the noise by fluctuating magnetic dipoles on the superconducting surface. Kumar et al. showed that the magnetic dipoles are adsorbed molecular oxygen, and reduced the noise with processes in a room-temperature hermetic sample enclosure. We demonstrate a factor of about three reduction of the 1/f flux noise power spectrum in Nb SQUIDs by applying 240-nm UV light from a Light Emitting Diode at cryogenic temperatures. Subsequent exposure of the SQUIDs to oxygen at atmospheric temperature and pressure restored the flux noise to its original value. Re-exposure to UV at cryogenic temperatures again reduced the flux noise. These results are consistent with molecular oxygen being the source of the noise. Our work demonstrates a practical technique to reduce the flux noise of superconducting devices in situ.   

Integration of two-dimensional transition-metal dichalcogenide superconductors into superconducting circuits operated at DC and GHz frequencies

Two-dimensional (2D) transition-metal dichalcogenide (TMD) superconductors have unique and desirable properties for integration with conventional superconducting circuits, including the ability to form atomically-flat and clean interfaces with stable tunnel barriers, increased kinetic inductance due to the atomically-thin geometry, and resilience to large in-plane magnetic fields. We created 2D-3D Josephson junction contacts with R=0 and critical currents between 0.15uA-128uA. We study the flux response and observe a Fraunhofer pattern with a frequency proportional to a large fraction of the area of the 2D superconductor. This experimental result is confirmed by our numerical modeling, using the Ginzburg-Landau equation to describe screening currents induced in the flake by the magnetic field. We attribute the large effective area and small distortions of the Fraunhofer pattern to the almost uniform penetration of the TMD by the magnetic field and the distribution of screening currents. We have also embedded these 2D-3D contacts in an RF tank circuit (Q>4000) to measure the kinetic inductance. Our work lays the foundation for the analysis of TMD nano-devices in superconducting circuits.   

A quantum heat switch based on a single driven qubit

With the growing interest to nano and quantum technologies, a lot of attention has been paid to the challenge of controlling the heat currents at the nanoscale. Here we study the photonic heat transfer through a coherently driven qubit coupled to a hot and cold baths and show that such setup behaves as a quantum heat switch. More precisely, the heat flow from the hot bath can be activated or completely suppressed varying the parameters of the drive, its intensity or its frequency. We show the complete suppression of the heat flow is a quantum effect occuring for specific drive parameters and analyze the role of the coherences in the free qubit energy eigenbasis. Such scheme can be tested in state-of-the-art circuit QED setups, e.g. involving a transmon qubit coupled to thermal resistances.   

Tunable photonic thermal rectification via superconducting qubit

The intense studies into superconducting circuit quantum electrodynamics (QED) together with the progress in ultrasensitive nanoscale bolometry provide a unique platform for studying heat transport in the quantum limit. On chip integration of superconducting qubits coupled with superconducting resonators together with the tools of ultra-sensitive microwave bolometry have been considered as promising systems to realize such quantum devices as the recently demonstrated heat valve [1] and rectifier [2]. We will present our recent results of the flux-tunable photonic heat transport between thermal reservoirs coupled via a superconducting artificial atom. To study the possibility of manipulating heat currents and the directionality of photonic heat transport, we couple the Xmon qubit to two asymmetric resonators, and implement a flux-tunable wireless quantum thermal rectifier. We will present spectroscopy results of the resonator-qubit-resonator assembly [3].
[1] A. Ronzani et al, Nature Physics 14, p.991 (2018)
[2] J. Senior et al, arXiv:1908.05574 (2019)
[3] A. Gubaydullin et al, in preparation (2019)   

Multi-terminal superconducting quantum interference device

External flux control of the superconducting phase difference is predicted to be a powerful tool to generate and manipulate Majorana bound states in a topological multi-terminal Josephson Junction (JJ). We study multi-terminal superconducting quantum interference devices (SQUID) fabricated from a semiconductor/superconductor (InAs/Al) heterostructure. Such a device consists of superconducting loops connecting some of terminals of a multi-terminal JJ. Due to non-local superconducting coherence, we observe for the first time an effect of “dragging” of the phase difference between one pair of terminals produced by the flux applied through the loop connecting two other terminals. Our ground state measurement implies the existence of novel multi-terminal Andreev bound states (ABS) which depend on more than two superconducting phase differences. Progress in tunneling spectroscopy of such states will also be discussed.   

Synthesis of Quantum-Voltage Waveforms with Pulse-Driven High-Tc Josephson Junctions

Josephson junctions (JJs) driven by a single-frequency microwave source have been established as quantum-accurate dc voltage standards [1]. Similar JJ devices are biased by patterns of fast pulses to create precise time-varying voltages [2], used for ac voltage standards and pseudo-random voltage noise references integrated into noise thermometry systems. Voltage standards typically use niobium-based JJs, but superconductors with higher-transition temperature, T_c, can be an attractive alternative. These high-T_c JJ circuits can have critical currents near 1 milliamp at temperatures between 60 K and 80 K and thus can use compact low-power cryocoolers.

Investigation of high-temperature JJs in quantum voltage devices at NIST has focused primarily on grain-boundary junctions in YBa₂Cu₃O_7-x. By driving these high-T_c junctions with pulse codes clocked at microwave frequencies we have demonstrated synthesis of programmable quantum-based voltage waveforms in a miniaturized cryocooler system. We’ll also discuss relevant uncertainties, sensitivity of the output to bias currents, and applicability of this waveform-synthesis system to Johnson-noise thermometry.

[1] C.A. Hamilton, Rev. Sci. Inst., 71, 3611, 2000.
[2] S.P. Benz, and C.A. Hamilton, Appl. Phys. Lett., 68, 3171, 1996.   

Demonstration of a circuit-QED maser based on a nanowire Josephson junction

On-chip coherent microwave generation can be achieved by strongly coupling a dc biased Josephson junction to a high quality factor superconducting cavity [1]. The incorporation of a gate-tunable semiconducting nanowire Josephson junction in such a device enables fast control of the junction-cavity coupling and laser emission, paving the way for efficient scalable on-chip microwave control of superconducting qubits.
In these devices, the efficient down-conversion at higher order harmonics stimulates the creation of multiple coherent photons with a single Cooper pair tunneling event, increasing the phase coherence. However, performance is currently limited by incoherent dissipative processes such as quasiparticle poisoning (QPP) that suppress the laser coherence.
Here we report on the quenching of the coherence above the 4th order harmonic. We track the emission dynamics of the 5th order harmonic in time and observe emission blinking due to QPP events with characteristic timescales T_poisoning = 200 ± 10 μs and T_unpoisoning = 17 ± 2 μs.

Reference:
[1] M.C. Cassidy et al., Science 355, 939 (2017)   

Epitaxial Josephson junctions for superconducting qubits made by wafer-bonding

State-of-the-art transmons most often utilize Al/AlO x /Al Josephson junctions (JJs) even though the
amorphous AlO x is known to be defective and lossy. A scalable process to realize epitaxial JJs with low
loss barrier dielectrics could improve qubit performance beyond state-of-the-art; however, challenges in
materials integration have not yet been overcome. In this work, a wafer-bonding technique is used to
realize epitaxial JJ structures utilizing conventional semiconductors for the JJ dielectric. Semiconductors
and epitaxial superconductors are grown by molecular beam epitaxy on III-V semiconductor substrates
and bonded to low-loss substrates. Selective etching is used to remove the III-V substrate followed by
surface cleaning and superconductor regrowth, resulting in epitaxial Al/Semiconductor/Al tri-layers on
low loss substrates.   

Confluence of complex surface impedance and critical current studies of model defects in niobium films near critical temperature

Studying and manipulation of vortex matter is key to understanding fundamental physics of superconducting materials, improving the properties of superconducting devices, and developing new concepts for superconductor applications. Two major approaches to experimentally characterize vortex dynamics are to measure (i) the critical current density or (ii) surface impedance as a function of applied field. In this work we combine both approaches to characterize thin 200 nm Nb films used in superconducting electronic circuits. We measured the vortex pinning constant (2 kN/m^2), viscous drag (~1e-8 N-s/m^2), and depinning frequency (~ 10 GHz) near the critical temperature (~0.85Tc) using a Parallel Plate Resonator technique. In addition, we measured the depairing critical currents (~ 0.75 MA/cm^2 at 0.9Tc) and the pinning force density (~1 MN/m^3 at 0.989Tc) as a function of temperature near Tc. The test results agree well with vortex wiggling experiments using a Scanning SQUID Microscope, and our numerical simulations of vortex potentials and vortex attempt frequencies based on the Ginzburg-Landau model and Kramers theory of escape rate of a Brownian particle from a potential well, respectively.   

1/B resistance oscillations within the superconducting regime of heterostructure with disordered superconductor

In-situ growth of Al on top of shallow InAs 2DEG heterostructures gives close to perfect superconducting proximity effect [1]. In recent work, we used anodic oxidation to thin down the Al by oxidizing from the top down, allowing us to create an ultra-thin and disordered Al film with a large perpendicular critical field B_c >3 T on a mesoscopic structure. [2]
For B>B_c, the sample resistance rises to >1 kΩ whereas for B<B_c, we observe reproducible resistance oscillations of <30 Ω. The oscillations have a 1/B periodicity from which standard SdH analysis yields a density matching closely with the carrier density of the underlying 2DEG. This indicates strong electronic contact between the disordered Al layer and the high mobility InAs 2DEG, creating a novel material system to study disordered 2D superconductivity.
Furthermore, fully oxidizing the Al layer gives rise to quantum Hall effect (ρ_xx=0) emerging at B~2.5 T [2] making this material system a candidate for studying proximitized quantum Hall edge states with close to unity transparency to the superconducting Al, contacting the 2DEG from the top.

[1] M. Kjærgaard et al. Nature commun. 12841 (2016)
[2] A. C. C. Drachmann et al. work in progress (2019)   

Spectroscopy of barrier defects coupled to superconductors in Van der Waals tunneling devices

Superconductor-Quantum Dot (SC-QD) coupling is a subject of an intense research effort. We report on SC-QD devices realized in Van der Waals tunneling heterostructures consisting of transition metal dichalcogenide (TMD) semiconducting barrier and the 2-band superconductor NbSe₂. The QDs in our study are naturally occurring defects in the TMD barrier. As reported earlier by Dvir et. al. [1], Andreev Bound states may form due to proximitized defects in an NbSe₂-WSe₂ system. Here we focus on two device structures. In the first, tunneling through an MoS₂ barrier into a graphene-NbSe₂ stack reveals zero-energy Kondo features which arise due to the presence of carriers associated with the NbSe₂ low-energy band turning normal. In the 2^nd device, defect-dot energies are tunable by use of a graphene tunneling electrode which permits the penetration of electric field from an electrostatic gate. This allows the QD to be used as spectroscopic probe to study resonant tunnelling into the superconductor, allowing low-energy spectroscopy of the inner band of NbSe₂.

References
T. Dvir, M. Aprili, C. H. L. Quay, H. Steinberg, arxiv:1906.01215   

Local measurement of temperature dependent London Penetration Depth in Superconductors using Nitrogen Vacancy Centers in Diamond

Local optical magneto-sensing based on nitrogen-vacancy (NV) centers in diamond [1] was used to measure temperature-dependent London penetration depth, λ (T), in two different unconventional superconductors, Ba(Fe_1-xCo_x)₂As₂ (BaCo122) and YBa₂Cu₃O_7-δ (YBCO). The results are compared with the sensitive bulk measurements of the same samples using tunnel-diode resonator (TDR). Each sample was measured in two orientation to separate the effects of platelet geometry and surface morphology, imaged using SEM. Not only this study compares the absolute numbers from two different techniques, it also allows us to discuss a more realistic picture of the magnetic field penetration into superconducting samples.   

A propellant-free superconducting solenoid thruster driven by geomagnetic field

We computed the force exerted on a solenoid in a magnetic field, and compared with the experimental results to verify the effect of magnetic moment gradient on exerted force. Based on the concept, a superconducting solenoid thruster driven by geomagnetic field is devised, which has advantages of low energy consumption and propellant-free. To further enhance the force exerted on the solenoid, we replaced an aluminum core with an iron core. Moreover, we placed the solenoid in the geomagnetic field and measured the forces corresponding to different gradients due to variation of cross-sectional area and electric current magnitude. The results demonstrated a proportionality between the force and these two factors, as predicted by the theory. By measuring and calculating the force exerted on a superconducting solenoid at low temperature, we can simulate the condition when such a thruster is used in space. The results showed that the force exerted on a superconducting solenoid at low temperature matched the predicted force relation versus the gradient of varying cross-sectional area and electric current magnitude at room temperature. Therefore, the relation can provide a reference for design of an alternative thruster for space exploration.