1,741 research outputs found
On the Stiefel-Whitney classes of GKM manifolds
We show that under standard assumptions on the isotropy groups of an integer
GKM manifold, the equivariant Stiefel-Whitney classes of the action are
determined by the GKM graph. This is achieved via a GKM-style description of
the equivariant cohomology with coefficients in a finite field
even though in this setting the restriction map to the fixed point set is not
necessarily injective. This closes a gap in our argument why the GKM graph of a
-dimensional integer GKM manifold determines its nonequivariant
diffeomorphism type. We introduce combinatorial Stiefel-Whitney classes of GKM
graphs and use them to derive a nontrivial obstruction to realizability of GKM
graphs in dimension and higher.Comment: 19 pages, comments are welcome
GKM manifolds are not rigid
We construct effective GKM -actions with connected stabilizers on the
total spaces of the two -bundles over with identical GKM graphs.
This shows that the GKM graph of a simply-connected integer GKM manifold with
connected stabilizers does not determine its homotopy type. We complement this
by a discussion of the minimality of this example: the homotopy type of integer
GKM manifolds with connected stabilizers is indeed encoded in the GKM graph for
smaller dimensions, lower complexity, or lower number of fixed points.
Regarding geometric structures on the new example, we find an almost complex
structure which is invariant under the action of a subtorus. In addition to the
minimal example, we provide an analogous example where the torus actions are
Hamiltonian, which disproves symplectic cohomological rigidity for Hamiltonian
integer GKM manifolds.Comment: v2: simplified construction of the minimal example. Comments are
welcome
Electronic Band Dispersion of Graphene Nanoribbons via Fourier-Transformed Scanning Tunneling Spectroscopy
Atomically precise armchair graphene nanoribbons of width (7-AGNRs) are
investigated by scanning tunneling spectroscopy (STS) on Au(111). The analysis
of energy-dependent standing wave patterns of finite length ribbons allows, by
Fourier transformation, the direct extraction of the dispersion relation of
frontier electronic states. Aided by density functional theory calculations, we
assign the states to the valence band, the conduction band and the next empty
band of 7-AGNRs, determine effective masses of , and , respectively, and a band gap of eV.Comment: 20 pages, 7 figure
Hinter den Kulissen: Über die Beziehung von Illusion und Realität
Nicht nur der Protagonist des Spielilms The Truman Show ist mit Täuschungen konfrontiert. Vieles spricht dafür, dass auch wir in einer Scheinwelt leben. Ob multiple Identitäten im Internet oder ästhetische Normen im Fernsehen: Wir leben mit Illusionen und haben uns an sie gewöhnt. Wie sich diese problematisieren lassen, zeigt der Film von Peter Weir – auch wenn hierbei eher deutlich wird, wie ein Blick ,hinter die Kulissen‘ nicht aussehen sollte
Microfluidic cell transport with piezoelectric micro diaphragm pumps
The automated transport of cells can enable far-reaching cell culture research. However, to date, such automated transport has been achieved with large pump systems that often come with long fluidic connections and a large power consumption. Improvement is possible with space- and energy-efficient piezoelectric micro diaphragm pumps, though a precondition for a successful use is to enable transport with little to no mechanical stress on the cell suspension. This study evaluates the impact of the microfluidic transport of cells with the piezoelectric micro diaphragm pump developed by our group. It includes the investigation of different actuation signals. Therewith, we aim to achieve optimal fluidic performance while maximizing the cell viability. The investigation of fluidic properties proves a similar performance with a hybrid actuation signal that is a rectangular waveform with sinusoidal flanks, compared to the fluidically optimal rectangular actuation. The comparison of the cell transport with three actuation signals, sinusoidal, rectangular, and hybrid actuation shows that the hybrid actuation causes less damage than the rectangular actuation. With a 5% reduction of the cell viability it causes similar strain to the transport with sinusoidal actuation. Piezoelectric micro diaphragm pumps with the fluidically efficient hybrid signal actuation are therefore an interesting option for integrable microfluidic workflows
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