62 research outputs found
Quantum oscillations of the critical current and high-field superconducting proximity in ballistic graphene
Graphene-based Josephson junctions provide a novel platform for studying the
proximity effect due to graphene's unique electronic spectrum and the
possibility to tune junction properties by gate voltage. Here we describe
graphene junctions with a mean free path of several micrometres, low contact
resistance and large supercurrents. Such devices exhibit pronounced
Fabry-P\'erot oscillations not only in the normal-state resistance but also in
the critical current. The proximity effect is mostly suppressed in magnetic
fields below 10mT, showing the conventional Fraunhofer pattern. Unexpectedly,
some proximity survives even in fields higher than 1 T. Superconducting states
randomly appear and disappear as a function of field and carrier concentration,
and each of them exhibits a supercurrent carrying capacity close to the
universal quantum limit. We attribute the high-field Josephson effect to
mesoscopic Andreev states that persist near graphene edges. Our work reveals
new proximity regimes that can be controlled by quantum confinement and
cyclotron motion
On Switch-Reference Phenomena in Kolyma Yukaghir
「環太平洋の言語」成果報告書A2-002ELPR publication series A2-00
Nodal quasiparticle in pseudogapped colossal magnetoresistive manganites
A characteristic feature of the copper oxide high-temperature superconductors
is the dichotomy between the electronic excitations along the nodal (diagonal)
and antinodal (parallel to the Cu-O bonds) directions in momentum space,
generally assumed to be linked to the "d-wave" symmetry of the superconducting
state. Angle-resolved photoemission measurements in the superconducting state
have revealed a quasiparticle spectrum with a d-wave gap structure that
exhibits a maximum along the antinodal direction and vanishes along the nodal
direction. Subsequent measurements have shown that, at low doping levels, this
gap structure persists even in the high-temperature metallic state, although
the nodal points of the superconducting state spread out in finite "Fermi
arcs". This is the so-called pseudogap phase, and it has been assumed that it
is closely linked to the superconducting state, either by assigning it to
fluctuating superconductivity or by invoking orders which are natural
competitors of d-wave superconductors. Here we report experimental evidence
that a very similar pseudogap state with a nodal-antinodal dichotomous
character exists in a system that is markedly different from a superconductor:
the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer
manganite La1.2Sr1.8Mn2O7. Our findings therefore cast doubt on the assumption
that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy
are hallmarks of the superconductivity state.Comment: To appear in Natur
Excess resistivity in graphene superlattices caused by umklapp electron–electron scattering
In electronic transport, umklapp processes play a fundamental role as the only intrinsic mechanism that allows electrons to transfer momentum to the crystal lattice and, therefore, provide a finite electrical resistance in pure metals1,2. However, umklapp scattering is difficult to demonstrate in experiment, as it is easily obscured by other dissipation mechanisms1–6. Here we show that electron–electron umklapp scattering dominates the transport properties of graphene-on-boron-nitride superlattices over a wide range of temperature and carrier density. The umklapp processes cause giant excess resistivity that rapidly increases with increasing superlattice period and are responsible for deterioration of the room-temperature mobility by more than an order of magnitude as compared to standard, non-superlattice graphene devices. The umklapp scattering exhibits a quadratic temperature dependence accompanied by a pronounced electron–hole asymmetry with the effect being much stronger for holes than electrons. In addition to being of fundamental interest, our results have direct implications for design of possible electronic devices based on heterostructures featuring superlattices. © 2018, The Author(s), under exclusive licence to Springer Nature Limited
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