10 research outputs found
Transport Through Andreev Bound States in a Graphene Quantum Dot
Andreev reflection-where an electron in a normal metal backscatters off a
superconductor into a hole-forms the basis of low energy transport through
superconducting junctions. Andreev reflection in confined regions gives rise to
discrete Andreev bound states (ABS), which can carry a supercurrent and have
recently been proposed as the basis of qubits [1-3]. Although signatures of
Andreev reflection and bound states in conductance have been widely reported
[4], it has been difficult to directly probe individual ABS. Here, we report
transport measurements of sharp, gate-tunable ABS formed in a
superconductor-quantum dot (QD)-normal system, which incorporates graphene. The
QD exists in the graphene under the superconducting contact, due to a
work-function mismatch [5, 6]. The ABS form when the discrete QD levels are
proximity coupled to the superconducting contact. Due to the low density of
states of graphene and the sensitivity of the QD levels to an applied gate
voltage, the ABS spectra are narrow, can be tuned to zero energy via gate
voltage, and show a striking pattern in transport measurements.Comment: 25 Pages, included SO
Electrical control of the superconducting-to-insulating transition in graphene–metal hybrids
Unconventional superconductivity in magic-angle graphene superlattices
The understanding of strongly-correlated materials, and in particular
unconventional superconductors, has puzzled physicists for decades. Such
difficulties have stimulated new research paradigms, such as ultra-cold atom
lattices for simulating quantum materials. Here we report on the realization of
intrinsic unconventional superconductivity in a 2D superlattice created by
stacking two graphene sheets with a small twist angle. For angles near
, the first `magic' angle, twisted bilayer graphene (TBG) exhibits
ultra-flat bands near charge neutrality, which lead to correlated insulating
states at half-filling. Upon electrostatic doping away from these correlated
insulating states, we observe tunable zero-resistance states with a critical
temperature up to 1.7 K. The temperature-density phase diagram shows
similarities with that of the cuprates, including superconducting domes.
Moreover, quantum oscillations indicate small Fermi surfaces near the
correlated insulating phase, in analogy with under-doped cuprates. The relative
high , given such small Fermi surface (corresponding to a record-low 2D
carrier density of , renders TBG among the strongest
coupling superconductors, in a regime close to the BCS-BEC crossover. These
novel results establish TBG as the first purely carbon-based 2D superconductor
and as a highly tunable platform to investigate strongly-correlated phenomena,
which could lead to insights into the physics of high- superconductors and
quantum spin liquids.Comment: 18 pages, 9 figures (with Methods). A few typos correcte