12 research outputs found
Metastable electron-electron states in double-layer graphene structures
The prototypical exciton model of two interacting Dirac particles in graphene
was analyzed in [1] and it was found that in one of the electron-hole
scattering channels the total kinetic energy vanishes, resulting in a singular
behaviour. We show that this singularity can be removed by extending the
quasiparticle dispersion, thus breaking the symmetry between upper and lower
Dirac cones. The dynamics of an electron-electron pair are then mapped onto
that of a single particle with negative mass and anisotropic dispersion. We
show that the interplay between dispersion and repulsive interaction can result
in the formation of bound, Cooper-pair-like, metastable states in
double-layered hybrid structures.Comment: 8 pages, 3 figure
Fluidity Onset in Graphene
Viscous electron fluids have emerged recently as a new paradigm of
strongly-correlated electron transport in solids. Here we report on a direct
observation of the transition to this long-sought-for state of matter in a
high-mobility electron system in graphene. Unexpectedly, the electron flow is
found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a
wide temperature range, showing signatures of viscous flows only at relatively
high temperatures. The transition between the two regimes is characterized by a
sharp maximum of negative resistance, probed in proximity to the current
injector. The resistance decreases as the system goes deeper into the
hydrodynamic regime. In a perfect darkness-before-daybreak manner, the
interaction-dominated negative response is strongest at the transition to the
quasiballistic regime. Our work provides the first demonstration of how the
viscous fluid behavior emerges in an interacting electron system.Comment: 8pgs, 4fg
Pulse calibration and non-adiabatic control of solid-state artificial atoms
Transitions in an artificial atom, driven non-adiabatically through an
energy-level avoided crossing, can be controlled by carefully engineering the
driving protocol. We have driven a superconducting persistent-current qubit
with a large-amplitude, radio-frequency field. By applying a bi-harmonic
waveform generated by a digital source, we demonstrate a mapping between the
amplitude and phase of the harmonics produced at the source and those received
by the device. This allows us to image the actual waveform at the device. This
information is used to engineer a desired time dependence, as confirmed by
detailed comparison with simulation.Comment: 4.1 pages, 3 figure
Mimicking graphene physics with a plane hexagonal wire mesh
This is the final version of the article. Available from AIP Publishing via the DOI in this record.A hexagonal metallic-wire mesh is fabricated and experimentally characterized to demonstrate graphene-physics in an electromagnetic analogue. In contrast to previous studies, our structure has a smaller ratio of out-of-plane to in-plane dimensions, more akin to real graphene. This allows for the development of a simple analytical treatment using equivalent electric circuit theory, and we demonstrate that the predicted dispersion curves of the supported eigenmodes agree well with those obtained from experimental measurements.The authors wish to acknowledge the financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1) and from the Higher Education Funding Council for England (HEFCE)
Mapping Dirac quasiparticles near a single Coulomb impurity on graphene
The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have not been tested. Graphene, a two-dimensional material in which electrons behave like massless Dirac fermions, provides a unique opportunity to test such predictions. Graphene’s response to a Coulomb potential also offers insight into important material characteristics, including graphene’s intrinsic dielectric constant, which is the primary factor determining the strength of electron–electron interactions in graphene. Here we present a direct measurement of the nanoscale response of Dirac fermions to a single Coulomb potential placed on a gated graphene device. Scanning tunnelling microscopy was used to fabricate tunable charge impurities on graphene, and to image electronic screening around them for a Q = +1|e| charge state. Electron-like and hole-like Dirac fermions were observed to respond differently to a Coulomb potential. Comparing the observed electron–hole asymmetry to theoretical simulations has allowed us to test predictions for how Dirac fermions behave near a Coulomb potential, as well as extract graphene’s intrinsic dielectric constant: ε[subscript g] = 3.0±1.0. This small value of ε[subscript g] indicates that electron–electron interactions can contribute significantly to graphene properties.United States. Office of Naval Research. Multidisciplinary University Research Initiative (Award N00014-09-1-1066)United States. Dept. of Energy. Office of Science (Contract DE-AC02-05CH11231)National Science Foundation (U.S.) (Award DMR-0906539
Ultrafast time-evolution of chiral N\'eel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering
Non-collinear spin textures in ferromagnetic ultrathin films are attracting a
renewed interest fueled by possible fine engineering of several magnetic
interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This
allows the stabilization of complex chiral spin textures such as chiral
magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report
here on the ultrafast behavior of chiral DWs after optical pumping in
perpendicularly magnetized asymmetric multilayers, probed using time-resolved
circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe
a picosecond transient reduction of the CD-XRMS, which is attributed to the
spin current-induced coherent and incoherent torques within the continuously
dependent spin texture of the DWs. We argue that a specific demagnetization of
the inner structure of the DW induces a flow of hot spins from the interior of
the neighboring magnetic domains. We identify this time-varying change of the
DW textures shortly after the laser pulse as a distortion of the homochiral
N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a
direction transverse to the DW. Our study highlights how time-resolved CD-XRMS
can be a unique tool for studying the time evolution in other systems showing a
non-collinear electric/magnetic ordering such as skyrmion lattices,
conical/helical phases, as well as the recently observed antiskyrmion lattices,
in metallic or insulating materials