Past Research Projects from 2013 to 2022:

Superconducting Qubits and Microwave Components

In the 8 months at LLNL, I moved to the field of quantum computing and worked on three projects:

• Fabricated superconducting qubits with photolithography
• Simulated and optimized optical to microwave transduers with COMSOL Multiphysics.
• Invented a new architecture for quantum anomalous Hall insulator (QAHI)-based circulators that can be easily integrated to microwave monolithic integrated circuits, and designed the CAD and PCB for the experiments.

Josephson Effects in Quantum Materials

1. Time-Reversal-Symmetry Breaking Superconductivity in SnTe Nanowires:

In a typical Josephson junction, retrapping current is always equal to or smaller than the switching current. However, we observed an unusual Josephson effect in SnTe nanowires where the retrapping current is larger than the switching current due to an unusual symmetry breaking. This unusual intrinsic asymmetry in the DC Josephson effect, along with the prominent half-integer Shapiro steps and anomalous magnetic diffraction patterns with a local minimum of \(I_C\) at \(B=0\), can be attributed to the two-band superconducting proximity effect with time reversal symmetry broken by ferroelectric distortion in SnTe, supported by the cryo-TEM characterization. The non-reciprocal transport property observed in this overdamped SnTe Josephson junction may be used as a Josephson diode that can be operated at high frequencies and applicable as a building block of low-dissipative superconducting electronics. My contribution includes noticing this unusual source-drain asymmetry in the DC Josephson effect, conducting low-temperature transport measurements, data analysis and simulating Shapiro steps and the DC Josephson effect by resistively shunted junction (RSJ) model with a symmetry breaking two-channel current-phase relation.

npj Quantum Materials 6, 61 (2021)

Ballistic Graphene Josephson Junctions

1. Ballistic Transport Characteristics near Zero Field:

We studied the ballistic transport in graphene Josephson junctions. The Josephson junctions are made by encapsulating monolayer graphene with hexagonal boron nitride (hBN) flakes with one-dimensional molybdenum rhenium (MoRe) contacts. We systematically studied the the temperature activation in several devices covering both long and short junction regimes. We also observed a peculiar periodicity doubling in the magnetic diffraction pattern near the Dirac point. The side-gated and top-gated devices (see below) were also measured to investigate the possible origins of this double periodicity. Recently, we also observed overdamped phase diffusion in a long ballistic graphene Josephson junction, which contradicts with most of graphene-based junctions. My contribution includes fabricating and characterizing the devices, and participating in discussing and analyzing the data.

• Phys. Rev. Lett. 117, 237002 (2016); arXiv:1604.07320
• Phys. Rev. Res. 1, 033084 (2019) (open access)
• Phys. Rev. Res. 4, 023203 (2022) (open access)



2. Driven Phase Dynamics:

Graphene Josephson junctions are typically strongly underdamped, resulting the hysteresis in critical currents. By applying a radio-frequency radiation, we first observed that graphene Josephson junctions also show peculiar Shapiro steps that are different from the conventional resistively and capacitively shunted junction (RCSJ) model. We confirmed by simulation that this is due to the large shunting capacitance of the bonding pads. More interestingly, the zeroth steps manifest strong hysteresis, especially near the Dirac point. In another junction, we also observed a bistable transition between steps +1 and -1 at the first elongated node. Graphene plays a unique role in this chaotic behavior, which is rarely seen in other underdamped systems. The switching time between the bistable states (in the order of a few seconds to a few mintues) is caused by its unique current-phase relation (CPR). The gate-tunability of graphene makes it an ideal platform to explore nonlinear dynamics in driven Josephson junctions. My contribution in this project was fabricating and characterizing the graphene Josephson junctions.

• Nano Letters 20, 6998 (2020); arXiv:2003.08369
• Phys. Rev. Res. 2, 23093 (2020) (open access)



3. Multiterminal Josephson Junctions:

Multiterminal Josephson junctions have been proposed to emulate an effective topological band structure, where the superconducting phases play the role of quasimomenta in such artificial topological matter. We demonstrated the first four-terminal ballistic graphene Josephson junction, showing the coexistence of superconducting and dissipative currents across the device. My contribution was designing and fabricating the device.

Nano Lett. 19, 1039 (2019); arXiv:1810.11632.

Supercurrents in the quantum Hall regime

1. Initial Quest of Quantum Hall Supercurrents Only Flowing along the edges:

Since Finkelstein's group at Duke University discovered supercurrent in the quantum Hall regime in hBN-encapsulated graphene in 2016, we started designing different experiments to investigate the transport mechanisms of quantum Hall supercurrent. The quantum Hall supercurrent has a SQUID-like magnetic interference between two branches of supercurrents near the vacuum edges. It can happen in two different scenarios:

1. The supercurrent is carried by a round trip of Andreev bound state across the entire junction mediated by chiral quantum Hall edge states and Andreev reflections near the superconductors, and therefore relying on both edges.
2. The supercurrent occurs at two "independent" channels of trivial states near the edges.

We have compared several devices with different widths and lengths, and also designed a device with one elongated L-shaped vacuum edge to see whether killing one edge can suppress the supercurrent. However, the doping discrepancy between the elongated vacuum edge and the inner part of the junction created by the contacts limits us from completely turning off the supercurrent in the N-doped regime. Indeed, devices with higher tunability is needed to show the dominant scenario (see below). My contribution includes fabricating and designing the samples, performing transport measurements of some devices, and analyzing the data.

• MRS Adv. 3, 2855 (2018); arXiv:1807.11923
• J. Low Temp. Phys. 191, 288 (2018); arXiv:1801.01447



2. Manipulation of Quantum Hall Supercurrents with Gated Vacuum Edges:
• We reported a superconducting interference device in the quantum Hall regime, where the vaccum edges are side-gated by graphene but separated by etched trenches. The individual quantum Hall edge channel in the junction can be locally tuned into different filling factors by each side gate. The supercurrents at 1 to 2 Tesla, carried by quantum Hall edge states, can be induced by the side gates, and the edge-mediated supercurrents can be controlled independently by the corresponding gates. I helped fabricating and characterizing the sample and explaining the cause of the usual plateau heights.

  

  Sci. Adv. 5, eaaw8693 (2019) (open access); mentioned on Phys.org
• Prior to the side gated junction above, I fabricated a similar device with top-gated vacuum edges instead. The primary goal was to tune on/off the coherent edge current flows with the top gates. However, the top gates were not effective to shut off the supercurrent due to the doping dicrepancy between the top gated and contact regions. More details about this unpublished result can be found in Chapter 6 of my dissertation.

  

3. Chiral quasiparticle tunneling between quantum Hall edges proximitized by superconductors:

We studied a graphene Josephson junction with a T-shaped contact, forming a short and narror constriction less than 100 nm. A supercurrent is observed up to about 2.5T, higher than any previously reported field in the quantum Hall regime. However, it does not have a SQUID-like magnetic interference pattern, suggesting that the supercurrent occurs at the short channel, instead of the edge channels. At higher fields, tunneling at this short channel also results in additional conductance besides the quantum Hall conductance over a wide range of magnetic field. The graphite backgate of this device also allows us to clearly observe v=1 state above 4T. My contribution includes designing the experiment, lithographic fabrication of the junction, performing quantum transport measurement, analyzing the data, and noticing the subtle enhanced conductance at high fields.

Phys. Rev. B 100, 121403(R) (2019); arXiv:1904.11689.



4. Manipulation of Quantum Hall Supercurrents by Injecting a Normal Current:

I fabricated a three-terminal graphene Josephson junction with one of the edge connected to a third gold contact. We studied how the normal injection current from the gold contact can alter the quantum Hall supercurrents. The effectiveness of the current injection depends on the chirality, which depends on the sign of the charge carriers and the direction of the magnetic field of the carriers. It also shows non-reciprocal transport in the quantum Hall regime that is similar to a circulator. In particular, a supercurrent observed between \(\nu\) = 0 and 2 plateaus manifests a periodicity of \(h/e\), and is robust against the normal current. This \(h/e\)-periodicity is different from the \(h/2e\)-periodicity of previously observed quantum Hall supercurrents. The double-periodicity may indicate that this supercurrent is primarily carried by the single-particle chiral edge states, as theoretically predicted. My contribution of this project includes fabricating the device, performing transport measurement, and analyzing the data. See Chapter 7 of my dissertation for more details about this unpublished work.