2,286,764 research outputs found
Radiation induced zero-resistance states: a dressed electronic structure effect
Recent results on magnetoresistance in a two dimensional electron gas under
crossed magnetic and microwave fields show a new class of oscillations,
suggesting a new kind of zero-resistance states. A complete understanding of
the effect is still lacking. We consider the problem from the point of view of
the electronic structure dressed by photons due to a in plane linearly
polarized ac field. The dramatic changes in the dressed electronic structure
lead to a interpretation of the new magnetoresistance oscillations as a
persistent-current like effect, induced by the radiation field.Comment: 5 pages, 5 figures, revtex4, changes in introduction and added
reference
Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment
Accurate alignment of the radio and optical celestial reference frames
requires detailed understanding of physical factors that may cause offsets
between the positions of the same object measured in different spectral bands.
Opacity in compact extragalactic jets (due to synchrotron self-absorption and
external free-free absorption) is one of the key physical phenomena producing
such an offset, and this effect is well-known in radio astronomy ("core
shift"). We have measured the core shifts in a sample of 29 bright compact
extragalactic radio sources observed using very long baseline interferometry
(VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and
estimate that the average shift between radio and optical positions of distant
quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional
accuracy of GAIA and SIM. We suggest two possible approaches to carefully
investigate and correct for this effect in order to align accurately the radio
and optical positions. Both approaches involve determining a Primary Reference
Sample of objects to be used for tying the radio and optical reference frames
together.Comment: 4 pages, 1 figure; to appear in IAU Symposium 248 Proceedings, "A
Giant Step: from Milli- to Micro-arcsecond Astrometry", eds. W.-J. Jin, I.
Platais, M. Perryma
Graphene field effect transistors with ferroelectric gating
Recent experiments on ferroelectric gating have introduced a novel
functionality, i.e. nonvolatility, in graphene field effect transistors. A
comprehensive understanding in the non-linear, hysteretic ferroelectric gating
and an effective way to control it are still absent. In this letter, we
quantitatively characterize the hysteretic ferroelectric gating using the
reference of an independent background doping (nBG) provided by normal
dielectric gating. More importantly, we prove that nBG can be used to control
the ferroelectric gating by unidirectionally shifting the hysteretic
ferroelectric doping in graphene. Utilizing this electrostatic effect, we
demonstrate symmetrical bit writing in graphene-ferroelectric FETs with
resistance change over 500% and reproducible no-volatile switching over 10^5
cycles.Comment: 5 Pages; 4 figures; two column forma
Transitions in spatial networks
Networks embedded in space can display all sorts of transitions when their
structure is modified. The nature of these transitions (and in some cases
crossovers) can differ from the usual appearance of a giant component as
observed for the Erdos-Renyi graph, and spatial networks display a large
variety of behaviors. We will discuss here some (mostly recent) results about
topological transitions, `localization' transitions seen in the shortest paths
pattern, and also about the effect of congestion and fluctuations on the
structure of optimal networks. The importance of spatial networks in real-world
applications makes these transitions very relevant and this review is meant as
a step towards a deeper understanding of the effect of space on network
structures.Comment: Corrected version and updated list of reference
Jeans criterion and nonextensive velocity distribution function in kinetic theory
The effect of nonextensivity of self-gravitating systems on the Jeans
criterion for gravitational instability is studied in the framework of Tsallis
statistics. The nonextensivity is introduced in the Jeans problem by a
generalized q-nonextensive velocity distribution function through the equation
of state of ideal gas in nonextensive kinetic theory. A new Jeans criterion is
deduced with a factor that, however, differs from that one in Ref.[21] and new
results of gravitational instability are analyzed for the nonextensive
parameter q. An understanding of physical meaning of q and a possible seismic
observation to find astronomical evidence for a value of q different from unity
are also discussed.Comment: 10 pages, 27 reference
On the general relativistic framework of the Sagnac effect
The Sagnac effect is usually considered as being a relativistic effect
produced in an interferometer when the device is rotating. General relativistic
explanations are known and already widely explained in many papers. Such
general relativistic approaches are founded on Einstein's equivalence principle
(EEP), which states the equivalence between the gravitational "force" and the
pseudo-force experienced by an observer in a non-inertial frame of reference,
included a rotating observer. Typically, the authors consider the so-called
Langevin-Landau-Lifschitz metric and the path of light is determined by null
geodesics. This approach partially hides the physical meaning of the effect. It
seems indeed that the light speed varies by c\pm\omega r in one or the other
direction around the disk. In this paper, a slightly different general
relativistic approach will be used. The different "gravitational field" acting
on the beam splitter and on the two rays of light is analyzed. This different
approach permits a better understanding of the physical meaning of the Sagnac
effect.Comment: 9 pages, to appear in the European Physical Journal
Hadronic Light-by-Light Scattering in the Muonium Hyperfine Splitting
We consider an impact of hadronic light-by-light scattering on the muonium
hyperfine structure. A shift of the hyperfine interval is calculated with the light-by-light scattering approximated
by exchange of pseudoscalar and pseudovector mesons. Constraints from the
operator product expansion in QCD are used to fix parameters of the model
similar to the one used earlier for the hadronic light-by-light scattering in
calculations of the muon anomalous magnetic moment. The pseudovector exchange
is dominant in the resulting shift, . Although the effect is tiny it is useful in understanding
the level of hadronic uncertainties.Comment: 16 pages, 7 figures, a reference adde
D6-branes and torsion
The D6-brane spectrum of type IIA vacua based on twisted tori and RR
background fluxes is analyzed. In particular, we compute the torsion factors of
the (co)homology groups H_n and describe the effect that they have on D6-brane
physics. For instance, the fact that H_3 contains Z_N subgroups explains why RR
tadpole conditions are affected by geometric fluxes. In addition, the presence
of torsional (co)homology shows why some D6-brane moduli are lifted, and it
suggests how the D-brane discretum appears in type IIA flux compactifications.
Finally, we give a clear, geometrical understanding of the Freed-Witten anomaly
in the present type IIA setup, and discuss its consequences for the
construction of semi-realistic flux vacua.Comment: 35 pages, 1 figure. One reference adde
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