23 research outputs found
Fractional Chern insulator edges and layer-resolved lattice contacts
Fractional Chern insulators (FCIs) realized in fractional quantum Hall
systems subject to a periodic potential are topological phases of matter for
which space group symmetries play an important role. In particular, lattice
dislocations in an FCI can host topology-altering non-Abelian topological
defects, known as genons. Genons are of particular interest for their potential
application to topological quantum computing. In this work, we study FCI edges
and how they can be used to detect genons. We find that translation symmetry
can impose a quantized momentum difference between the edge electrons of a
partially-filled Chern band. We propose {\it layer-resolved lattice contacts},
which utilize this momentum difference to selectively contact a particular FCI
edge electron. The relative current between FCI edge electrons can then be used
to detect the presence of genons in the bulk FCI. Recent experiments have
demonstrated graphene is a viable platform to study FCI physics. We describe
how the lattice contacts proposed here could be implemented in graphene subject
to an artificial lattice, thereby outlining a path forward for experimental
dectection of non-Abelian topological defects.Comment: 5+7 pages, 10 figures, v2: modified figure
Current-phase relations of few-mode InAs nanowire Josephson junctions
Gate-tunable semiconductor nanowires with superconducting leads have great
potential for quantum computation and as model systems for mesoscopic Josephson
junctions. The supercurrent, , versus the phase, , across the junction
is called the current-phase relation (CPR). It can reveal not only the
amplitude of the critical current, but also the number of modes and their
transmission. We measured the CPR of many individual InAs nanowire Josephson
junctions, one junction at a time. Both the amplitude and shape of the CPR
varied between junctions, with small critical currents and skewed CPRs
indicating few-mode junctions with high transmissions. In a gate-tunable
junction, we found that the CPR varied with gate voltage: Near the onset of
supercurrent, we observed behavior consistent with resonant tunneling through a
single, highly transmitting mode. The gate dependence is consistent with
modeled subband structure that includes an effective tunneling barrier due to
an abrupt change in the Fermi level at the boundary of the gate-tuned region.
These measurements of skewed, tunable, few-mode CPRs are promising both for
applications that require anharmonic junctions and for Majorana readout
proposals
Images of Edge Current inï¾ InAs/GaSbï¾ Quantum Wells
Quantum spin Hall devices with edges much longer than several microns do not display ballistic transport; that is, their measured conductances are much less thanï¾ e2/hï¾ per edge. We imaged edge currents inï¾ InAs/GaSbï¾ quantum wells with long edges and determined an effective edge resistance. Surprisingly, although the effective edge resistance is much greater thanï¾ h/e2, it is independent of temperature up to 30ï¾ K within experimental resolution. Known candidate scattering mechanisms do not explain our observation of an effective edge resistance that is large yet temperature independent
Emergent Dirac gullies and gully-symmetry breaking quantum Hall states in ABA trilayer graphene
We report on quantum capacitance measurements of high quality, graphite- and
hexagonal boron nitride encapsulated Bernal stacked trilayer graphene devices.
At zero applied magnetic field, we observe a number of electron density- and
electrical displacement-tuned features in the electronic compressibility
associated with changes in Fermi surface topology. At high displacement field
and low density, strong trigonal warping gives rise to emergent Dirac gullies
centered near the corners of the hexagonal Brillouin and related by three fold
rotation symmetry. At low magnetic fields of ~T, the gullies manifest
as a change in the degeneracy of the Landau levels from two to three. Weak
incompressible states are also observed at integer filling within these
triplets Landau levels, which a Hartree-Fock analysis indicates are associated
with Coulomb-driven nematic phases that spontaneously break rotation symmetry.Comment: Main text: 5 pages, 3 Figures. Supplements: 8 pages, 5 figure
Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO<sub>3</sub>
Strontium titanate is a low-temperature, non-Bardeen-Cooper-Schrieffer
superconductor that superconducts to carrier concentrations lower than in any
other system and exhibits avoided ferroelectricity at low temperatures. Neither
the mechanism of superconductivity in strontium titanate nor the importance of
the structure and dielectric properties for the superconductivity are well
understood. We studied the effects of twin structure on superconductivity in a
5.5-nm-thick layer of niobium-doped SrTiO embedded in undoped
SrTiO. We used a scanning superconducting quantum interference device
susceptometer to image the local diamagnetic response of the sample as a
function of temperature. We observed regions that exhibited a superconducting
transition temperature 10% higher than the temperature at
which the sample was fully superconducting. The pattern of these regions varied
spatially in a manner characteristic of structural twin domains. Our results
emphasize that the anisotropic dielectric properties of SrTiO are
important for its superconductivity, and need to be considered in any theory of
the mechanism of the superconductivity.Comment: 14 pages, 11 figures, Supplemental Information available at
http://stanford.edu/group/moler/papers/Noad_STOsuperconductivity_SI.pd