1,970 research outputs found
Advanced memory effects in the aging of a polymer glass
A new kind of memory effect on low frequency dielectric measurements on
plexiglass (PMMA) is described. These measurements show that cooling and
heating the sample at constant rate give an hysteretic dependence on
temperature of the dielectric constant . A temporary stop of cooling
produces a downward relaxation of . Two main features are observed i)
when cooling is resumed goes back to the values obtained without the
cooling stop (i.e. the low temperature state is independent of the cooling
history) ii) upon reheating keeps the memory of all the cooling
stops({\it Advanced memory}).
The dependence of this effect on frequency and on the cooling rate is
analyzed. The memory deletion is studied too. Finally the results are compared
with those of similar experiments done in spin glasses and with the famous
experiments of Kovacs.Comment: to be published in the European Physical Journa
Point/Counterpoint: The Gettysburg Battlefield Marathon
Jeff: On November 6, the small town of Gettysburg will be swarmed by runners during the first ever Gettysburg Battlefield Marathon. The event has provoked heated discussion from many in the Civil War community, bringing up many questions regarding the use of our most hallowed grounds for recreational use. In this post, Matt and I will engage in a back and forth conversation about the concerns and advantages of the race. I’d like to begin by noting that the views that we each express in this piece may not necessarily be our own and that we may merely be bringing them up to contribute to the conversation surrounding the marathon.
[excerpt
Scattering Mechanism in Modulation-Doped Shallow Two-Dimensional Electron Gases
We report on a systematic investigation of the dominant scattering mechanism
in shallow two-dimensional electron gases (2DEGs) formed in modulation-doped
GaAs/Al_{x}Ga_{1-x}As heterostructures. The power-law exponent of the electron
mobility versus density, mu \propto n^{alpha}, is extracted as a function of
the 2DEG's depth. When shallower than 130 nm from the surface, the power-law
exponent of the 2DEG, as well as the mobility, drops from alpha \simeq 1.65
(130 nm deep) to alpha \simeq 1.3 (60 nm deep). Our results for shallow 2DEGs
are consistent with theoretical expectations for scattering by remote dopants,
in contrast to the mobility-limiting background charged impurities of deeper
heterostructures.Comment: 4 pages, 3 figures, modified version as accepted in AP
The role of the picoeukaryote Aureococcus anophagefferens in cycling of marine high molecular weight dissolved organic nitrogen
Environmental evidence suggests that Aureococcus anophagefferens (Pelagophyceae), a eukaryotic picoplankton that blooms in coastal seawaters, can outcompete other organisms because of its ability to use abundant dissolved organic nitrogen (DON). To test this hypothesis, we isolated A. anophagefferens in axenic culture and monitored its growth on high-molecular weight (HMW) DON collected from sediment pore waters, a putative source for DON in bays where blooms occur. HMW DON originating from pore water had a substantially higher protein content than surface seawater DON. We found that A. anophagefferens could deplete 25-36% of the available nitrogen in cultures with HMW DON as the sole source of nitrogen and that this corresponded well with the protein fraction in pore-water HMW DON. High rates of cell surface peptide hydrolysis and no detectable N-acetyl polysaccharide hydrolysis, together with the high percentage of hydrolyzable amino acids compared to hydrolyzable aminosugars present in the HMW DON, pointed to the protein fraction as the more likely source of nitrogen used for growth. Whether or not nitrogen scavenging from protein is a common mechanism in phytoplankton is at present unknown but needs to be investigate
Mechanical Flip-Chip for Ultra-High Electron Mobility Devices
Electrostatic gates are of paramount importance for the physics of devices
based on high-mobility two-dimensional electron gas (2DEG) since they allow
depletion of electrons in selected areas. This field-effect gating enables the
fabrication of a wide range of devices such as, for example, quantum point
contacts (QPC), electron interferometers and quantum dots. To fabricate these
gates, processing is usually performed on the 2DEG material, which is in many
cases detrimental to its electron mobility. Here we propose an alternative
process which does not require any processing of the 2DEG material other than
for the ohmic contacts. This approach relies on processing a separate wafer
that is then mechanically mounted on the 2DEG material in a flip-chip fashion.
This technique proved successful to fabricate quantum point contacts on both
GaAs/AlGaAs materials with both moderate and ultra-high electron mobility.Comment: 5 pages, 3 figure
Fluctuation-Dissipation-Theorem violation during the formation of a colloidal-glass
The relationship between the conductivity and the polarization noise is
measured in a gel as a function of frequency in the range . It is
found that at the beginning of the transition from a fluid like sol to a solid
like gel the fluctuation dissipation theorem is strongly violated. The
amplitude and the persistence time of this violation are decreasing functions
of frequency. At the lowest frequencies of the measuring range it persists for
times which are about 5% of the time needed to form the gel. This phenomenology
is quite close to the recent theoretical predictions done for the violation of
the fluctuation dissipation theorem in glassy systems.Comment: 6 pages + 4 figure
Inner ear ossification and mineralization kinetics in human embryonic development - microtomographic and histomorphological study.
Little is known about middle and inner ear development during the second and third parts of human fetal life. Using ultra-high resolution Microcomputed Tomography coupled with bone histology, we performed the first quantitative middle and inner ear ossification/mineralization evaluation of fetuses between 17 and 39 weeks of gestational age. We show distinct ossification paces between ossicles, with a belated development of the stapes. A complete cochlear bony covering is observed within the time-frame of the onset of hearing, whereas distinct time courses of ossification for semicircular canal envelopes are observed in relation to the start of vestibular functions. The study evidences a spatio-temporal relationship between middle and inner ear structure development and the onset of hearing and balance, critical senses for the fetal adaptation to birth
- …