5,793 research outputs found
In situ imaging of field emission from individual carbon nanotubes and their structural damage
©2002 American Institute of Physics. The electronic version of this article is the complete one and can be found online at: http://link.aip.org/link/?APPLAB/80/856/1DOI:10.1063/1.1446994Field emission of individual carbon nanotubes was observed by in situ
transmission electron microscopy. A fluctuation in emission current was due to a
variation in distance between the nanotube tip and the counter electrode owing
to a "head-shaking" effect of the nanotube during field emission. Strong
field-induced structural damage of a nanotube occurs in two ways: a
piece-by-piece and segment-by-segment pilling process of the graphitic layers,
and a concentrical layer-by-layer stripping process. The former is believed
owing to a strong electrostatic force, and the latter is likely due to heating
produced by emission current that flowed through the most outer graphitic
layers
Quantum simulation of exotic PT-invariant topological nodal loop bands with ultracold atoms in an optical lattice
Since the well-known PT symmetry has its fundamental significance and
implication in physics, where PT denotes the combined operation of
space-inversion P and time-reversal T, it is extremely important and intriguing
to completely classify exotic PT-invariant topological metals and to physically
realize them. Here we, for the first time, establish a rigorous classification
of topological metals that are protected by the PT symmetry using KO-theory. As
a physically realistic example, a PT-invariant nodal loop (NL) model in a 3D
Brillouin zone is constructed, whose topological stability is revealed through
its PT-symmetry-protected nontrivial Z2 topological charge. Based on these
exact results, we propose an experimental scheme to realize and to detect
tunable PT-invariant topological NL states with ultracold atoms in an optical
lattice, in which atoms with two hyperfine spin states are loaded in a
spin-dependent 3D OL and two pairs of Raman lasers are used to create
out-of-plane spin-flip hopping with site-dependent phase. Such a realistic
cold-atom setup can yield topological NL states, having a tunable ring-shaped
band-touching line with the two-fold degeneracy in the bulk spectrum and
non-trivial surface states. The states are actually protected by the combined
PT symmetry even in the absence of both P and T symmetries, and are
characterized by a Z2-type invariant (a quantized Berry phase). Remarkably, we
demonstrate with numerical simulations that (i) the characteristic NL can be
detected by measuring the atomic transfer fractions in a Bloch-Zener
oscillation; (ii) the topological invariant may be measured based on the
time-of-flight imaging; and (iii) the surface states may be probed through
Bragg spectroscopy. The present proposal for realizing topological NL states in
cold atom systems may provide a unique experimental platform for exploring
exotic PT-invariant topological physics.Comment: 11 pages, 6 figures; accepted for publication in Phys. Rev.
Hidden itinerant-spin phase in heavily-overdoped La2-xSrxCuO4 revealed by dilute Fe doping: A combined neutron scattering and angle-resolved photoemission study
We demonstrated experimentally a direct way to probe a hidden propensity to
the formation of spin density wave (SDW) in a non-magnetic metal with strong
Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an
incommensurate magnetic order below 20 K in heavily-overdoped La2-xSrxCuO4
(LSCO). Elastic neutron scattering suggested that this order cannot be ascribed
to the localized spins on Cu or doped Fe. Angle-resolved photoemission
spectroscopy (ARPES), combined with numerical calculations, revealed a strong
Fermi surface nesting inherent in the pristine LSCO that likely drives this
order. The heavily-overdoped Fe-doped LSCO thus represents the first plausible
example of the long-sought "itinerant-spin extreme" of cuprates, where the
spins of itinerant doped holes define the magnetic ordering ground state. This
finding complements the current picture of cuprate spin physics that highlights
the predominant role of localized spins at lower dopings. The demonstrated set
of methods could potentially apply to studying hidden density-wave
instabilities of other "nested" materials on the verge of density wave
ordering.Comment: Abstract and discussion revised; to appear in Phys. Rev. Let
The Quintuplet Cluster: Extended Structure and Tidal Radius
The Quintuplet star cluster is one of only three known young ( Myr)
massive (M M) clusters within pc of the Galactic
Center. In order to explore star cluster formation and evolution in this
extreme environment, we analyze the Quintuplet's dynamical structure. Using the
HST WFC3-IR instrument, we take astrometric and photometric observations of the
Quintuplet covering a field-of-view, which is times
larger than those of previous proper motion studies of the Quintuplet. We
generate a catalog of the Quintuplet region with multi-band, near-infrared
photometry, proper motions, and cluster membership probabilities for
stars. We present the radial density profile of candidate Quintuplet
cluster members with M out to pc from the cluster
center. A lower limit of pc is placed on the tidal radius,
indicating the lack of a tidal truncation within this radius range. Only weak
evidence for mass segregation is found, in contrast to the strong mass
segregation found in the Arches cluster, a second and slightly younger massive
cluster near the Galactic Center. It is possible that tidal stripping hampers a
mass segregation signature, though we find no evidence of spatial asymmetry.
Assuming that the Arches and Quintuplet formed with comparable extent, our
measurement of the Quintuplet's comparatively large core radius of
pc provides strong empirical evidence that young massive
clusters in the Galactic Center dissolve on a several Myr timescale.Comment: 25 pages (21-page main text, 4-page appendix), 18 figures, submitted
to Ap
Making topologically trivial non-Hermitian systems non-trivial via gauge fields
Non-Hermiticity significantly enriches the concepts of symmetry and topology
in physics. Particularly, non-Hermiticity gives rise to the ramified
symmetries, where the non-Hermitian Hamiltonian is transformed to
. For time-reversal () and sublattice symmetries, there are six
ramified symmetry classes leading to novel topological classifications with
various non-Hermitian skin effects. As artificial crystals are the main
experimental platforms for non-Hermitian physics, there exists the symmetry
barrier for realizing topological physics in the six ramified symmetry classes:
While artificial crystals are in spinless classes with , nontrivial
classifications dominantly appear in spinful classes with . Here, we
present a general mechanism to cross the symmetry barrier. With an internal
parity symmetry , the square of the combination can be
modified by appropriate gauge fluxes. Using the general mechanism, we
systematically construct spinless models for all non-Hermitian spinful
topological phases in one and two dimensions, which are experimentally
realizable. Our work suggests that gauge structures may significantly enrich
non-Hermitian physics at the fundamental level.Comment: 6+10 pages, 3+6 pages. Accepted for publication in Physical Review
Letter
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