11,416 research outputs found
Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias
A standard drift-diffusion model of space charge wave propagation in
semiconductors has been studied numerically and analytically under dc voltage
bias. For sufficiently long samples, appropriate contact resistivity and
applied voltage - such that the sample is biased in a regime of negative
differential resistance - we find chaos in the propagation of nonlinear fronts
(charge monopoles of alternating sign) of electric field. The chaos is always
low-dimensional, but has a complex spatial structure; this behavior can be
interpreted using a finite dimensional asymptotic model in which the front
(charge monopole) positions and the electrical current are the only dynamical
variables.Comment: 12 pages, 8 figure
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Reversible writing of high-mobility and high-carrier-density doping patterns in two-dimensional van der Waals heterostructures
A key feature of two-dimensional materials is that the sign and concentration of their carriers can be externally controlled with techniques such as electrostatic gating. However, conventional electrostatic gating has limitations, including a maximum carrier density set by the dielectric breakdown, and ionic liquid gating and direct chemical doping also suffer from drawbacks. Here, we show that an electron-beam-induced doping technique can be used to reversibly write high-resolution doping patterns in hexagonal boron nitride-encapsulated graphene and molybdenum disulfide (MoS2) van der Waals heterostructures. The doped MoS2 device exhibits an order of magnitude decrease of subthreshold swing compared with the device before doping, whereas the doped graphene devices demonstrate a previously inaccessible regime of high carrier concentration and high mobility, even at room temperature. We also show that the approach can be used to write high-quality p–n junctions and nanoscale doping patterns, illustrating that the technique can create nanoscale circuitry in van der Waals heterostructures
Observation of an unusual field dependent slow magnetic relaxation and two distinct transitions in a family of new complexes
An unusual field dependent slow magnetic relaxation and two distinct
transitions were observed in a family of new rare earth-transition metal
complexes, [Ln (bipy) (HO) M(CN)] 1.5 (bipy) 4HO (bipy = 2,2'-bipyridine; Ln = Gd,Y; M = Fe,
Co). The novel magnetic relaxation, which is quite different from those
in normal spin glasses and superparamagnets but very resembles qualitatively
those in single-molecule magnet Mn-Ac even if they possess different
structures, might be attributed to the presence of frustration that is
incrementally unveiled by the external magnetic field. The two distinct
transitions in [GdFe] were presumed from DC and AC susceptibility as well as
heat capacity measurements.Comment: Revtex, 6 figure
Chaos in resonant-tunneling superlattices
Spatio-temporal chaos is predicted to occur in n-doped semiconductor
superlattices with sequential resonant tunneling as their main charge transport
mechanism. Under dc voltage bias, undamped time-dependent oscillations of the
current (due to the motion and recycling of electric field domain walls) have
been observed in recent experiments. Chaos is the result of forcing this
natural oscillation by means of an appropriate external microwave signal.Comment: 3 pages, LaTex, RevTex, 3 uuencoded figures (1.2M) are available upon
request from [email protected], to appear in Phys.Rev.
The Radial Structure of the Cygnus Loop Supernova Remnant --- Possible evidence of a cavity explosion ---
We observed the North-East (NE) Limb toward the center region of the Cygnus
Loop with the ASCA Observatory. We found a radial variation of electron
temperature (kTe) and ionization timescale (log(\tau)) whereas no variation
could be found for the abundances of heavy elements. In this paper, we
re-analyzed the same data set and new observations with the latest calibration
files. Then we constructed the precise spatial variations of kTe, log(\tau),
and abundances of O, Ne, Mg, Si, and Fe over the field of view (FOV). We found
a spatial variation not only in kTe and in log(\tau) but also in most of heavy
elements. As described in Miyata et al. (1994), values of kTe increase and
those of log(\tau) decrease toward the inner region. We found that the
abundance of heavy elements increases toward the inner region. The radial
profiles of O, Ne, and Fe show clear jump structures at a radius of 0.9 Rs,
where Rs is the shock radius. Outside of 0.9 Rs, abundances of all elements are
constant. On the contrary, inside of 0.9 Rs, abundances of these elements are
20--30 % larger than those obtained outside of 0.9 Rs. The radial profile of
kTe also shows the jump structure at 0.9 Rs. This means that the hot and metal
rich plasma fills the volume inside of 0.9 Rs. We concluded that this jump
structure was the possible evidence for the pre-existing cavity produced by the
precursor. If the ejecta fills inside of 0.9 Rs, the total mass of the ejecta
was roughly 4\Msun. We then estimated the main-sequence mass to be roughly
15\Msun, which supports the massive star in origin of the Cygnus Loop supernova
remnant and the existence of a pre-existing cavity.Comment: 37 pages, 14 figures. Accepted for publication of Ap
Optically Thick Radio Cores of Narrow-Waist Bipolar Nebulae
We report our search for optically thick radio cores in sixteen narrow-waist
bipolar nebulae. Optically thick cores are a characteristic signature of
collimated ionized winds. Eleven northern nebulae were observed with the Very
Large Array (VLA) at 1.3 cm and 0.7 cm, and five southern nebulae were observed
with the Australia Telescope Compact Array (ATCA) at 6 cm and 3.6 cm. Two
northern objects, 19W32 and M 1-91, and three southern objects, He 2-25, He
2-84 and Mz 3, were found to exhibit a compact radio core with a rising
spectrum consistent with an ionized jet. Such jets have been seen in M 2-9 and
may be responsible for shaping bipolar structure in planetary nebulae.Comment: 29 pages, accepted for publication in Ap
Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet CsCuClBr
We report on a systematic study of the magnetic properties on single crystals
of the solid solution CsCuClBr (0 x 4), which
include the two known end-member compounds CsCuCl and CsCuBr,
classified as quasi-two-dimensional quantum antiferromagnets with different
degrees of magnetic frustration. By comparative measurements of the magnetic
susceptibility () on as many as eighteen different Br concentrations,
we found that the inplane and out-of-plane magnetic correlations, probed by the
position and height of a maximum in the magnetic susceptibility, respectively,
do not show a smooth variation with x. Instead three distinct concentration
regimes can be identified, which are separated by critical concentrations
x = 1 and x = 2. This unusual magnetic behavior can be explained
by considering the structural peculiarities of the materials, especially the
distorted Cu-halide tetrahedra, which support a site-selective replacement of
Cl- by Br- ions. Consequently, the critical concentrations x (x)
mark particularly interesting systems, where one (two) halidesublattice
positions are fully occupied.Comment: 15 pages, 4 figure
Quasiperiodic time dependent current in driven superlattices: distorted Poincare maps and strange attractors
Intriguing routes to chaos have been experimentally observed in semiconductor
superlattices driven by an ac field. In this work, a theoretical model of time
dependent transport in ac driven superlattices is numerically solved. In
agreement with experiments, distorted Poincare maps in the quasiperiodic regime
are found. They indicate the appearance of very complex attractors and routes
to chaos as the amplitude of the AC signal increases. Distorted maps are caused
by the discrete well-to-well jump motion of a domain wall during spiky
high-frequency self-sustained oscillations of the current.Comment: 10 pages, 4 figure
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