A Tunable Monolithic SQUID in Twisted Bilayer Graphene

Magic-angle twisted bilayer graphene (MATBG) hosts a number of correlated states of matter that can be tuned by electrostatic doping. Superconductivity has drawn considerable attention and the mechanism behind it is a topic of active discussion. MATBG has been experimentally characterized by numerous transport and scanning-probe experiments. The material has also emerged as a versatile platform for superconducting electronics, as proven by the realization of monolithic Josephson junctions. However, even though phase-coherent phenomena have been measured, no control of the superconducting phase has been demonstrated so far. Here, we present a Superconducting Quantum Interference Device (SQUID) in MATBG, where the superconducting phase difference is controlled through the magnetic field. We observe magneto-oscillations of the critical current, demonstrating long-range coherence agreeing with an effective charge of 2e for the superconducting charge carriers. We tune to both asymmetric and symmetric SQUID configurations by electrostatically controlling the critical currents through the junctions. With this tunability, we study the inductances in the device, finding values of up to 2{\mu}H. Furthermore, we directly observe the current-phase relation of one of the Josephson junctions of the device. Our results show that superconducting devices in MATBG can be scaled up and used to reveal properties of the material. We expect this to foster a more systematic realization of devices of this type, increasing the accuracy with which microscopic characteristics of the material are extracted. We also envision more complex devices to emerge, such as phase-slip junctions or high kinetic inductance detectors.Comment: Supplementary Information is included in the .pd

Tunable quantum interferometer for correlated moir\'e electrons

Magic-angle twisted bilayer graphene (MATBG) can host an intriguing variety of gate-tunable correlated states, including superconducting and correlated insulator states. Junction-based superconducting devices, such as Josephson junctions and SQUIDs, have been introduced recently and enable the exploration of the charge, spin, and orbital nature of superconductivity and the coherence of moir\'e electrons in MATBG. However, complementary fundamental coherence effects - in particular, the Little-Parks effect in a superconducting and the Aharonov-Bohm effect in a normal conducting ring - remained to be observed. Here, we report the observation of both these phenomena in a single gate-defined ring device where we can embed a superconducting or normal conducting ring in a correlated or band insulator. We directly observe the Little-Parks effect in the superconducting phase diagram as a function of density and magnetic field, confirming the effective charge of $2e$. By measuring the Aharonov-Bohm effect, we find that in our device, the coherence length of normal conducting moir\'e electrons exceeds a few microns at 50 mK. Surprisingly, we also identify a regime characterized by $h/e$-periodic oscillations but with superconductor-like nonlinear transport. Taken together, these experiments establish a novel device platform in MATBG, and more generally in tunable 2D materials, to unravel the nature of superconductivity and other correlated quantum states in these materials

High mobility transport in isotopically-enriched 12^{12}C and 13^{13}C exfoliated graphene

Graphene quantum dots are promising candidates for qubits due to weak spin-orbit and hyperfine interactions. The hyperfine interaction, controllable via isotopic purification, could be the key to further improving the coherence. Here, we use isotopically enriched graphite crystals of both $^{12}$C and $^{13}$C grown by high-pressure-high-temperature method to exfoliate graphene layers. We fabricated Hall bar devices and performed quantum transport measurements, revealing mobilities exceeding $10^{5}$$\textrm{cm}^{2}/Vs$ and a long mean free path of microns, which are as high as natural graphene. Shubnikov-de Haas oscillations, quantum Hall effect up to the filling factor of one, and Brown-Zak oscillations due to the alignment of hBN and graphene are observed thanks to the high mobility. These results constitute a material platform for physics and engineering of isotopically-enriched graphene qubits.Comment: 6 pages, 2 figure

Characterization of Ion Cyclotron Wall Conditioning Using Material Probes in LHD

The ion cyclotron wall conditioning (ICWC) is one of the conditioning methods to reduce impurities and to remove tritium from the plasma facing components. Among the advantages of ICWC are the possible operation under strong magnetic field for fully torus area based on the charge exchange damage observed in thin SS samples arranged on a hexahxedron block holder with three different facings, the areas influenced by ICWC is estimated. On the plasma facing area of the material holder, high density of helium bubbles is observed by transmission electron microscope (TEM). But the other areas show no observable damage. The fact that the bubble were observed only in a sample facing the plasma implies that the effective particles, most probably charge exchange neutrals come to the wall straightly Thus, cleaning of the surfaces un-exposed to plasma directly and those in shadow area is difficult by ICWC

The X-ray Polarization Probe mission concept

The X-ray Polarization Probe (XPP) is a second generation X-ray polarimeter following up on the Imaging X-ray Polarimetry Explorer (IXPE). The XPP will offer true broadband polarimetery over the wide 0.2-60 keV bandpass in addition to imaging polarimetry from 2-8 keV. The extended energy bandpass and improvements in sensitivity will enable the simultaneous measurement of the polarization of several emission components. These measurements will give qualitatively new information about how compact objects work, and will probe fundamental physics, i.e. strong-field quantum electrodynamics and strong gravity.Comment: submitted to Astrophysics Decadal Survey as a State of the Profession white pape