Oscillations of the Electric-Dipole Echo in Glasses in a Magnetic Field

Using a simple diagram technique we derive the electric-dipole echo amplitude from two-level systems with a quadrupole nuclear moment in glasses in an external magnetic field. We show, that due to the quadrupole moment interaction of a tunneling particle with a gradient of an internal electric field, the echo amplitude experiences oscillations in rather weak magnetic fields. With an increase of the magnetic field, when the Zeeman energy becomes larger than the quadrupole energy splitting, the average echo amplitude increases and saturates (for high magnetic fields) at some level which is above the average level of echo oscillations for small magnetic fields. PACS numbers: 77.22.Ch, 61.43.Fs, 76.60.Gv, 76.60.Lz     

Photorhabdus luminescens TccC3 Toxin Targets the Dynamic Population of F-Actin and Impairs Cell Cortex Integrity

Due to its essential role in cellular processes, actin is a common target for bacterial toxins. One such toxin, TccC3, is an effector domain of the ABC-toxin produced by entomopathogenic bacteria of Photorhabdus spp. Unlike other actin-targeting toxins, TccC3 uniquely ADP-ribosylates actin at Thr-148, resulting in the formation of actin aggregates and inhibition of phagocytosis. It has been shown that the fully modified F-actin is resistant to depolymerization by cofilin and gelsolin, but their effects on partially modified actin were not explored. We found that only F-actin unprotected by tropomyosin is the physiological TccC3 substrate. Yet, ADP-ribosylated G-actin can be produced upon cofilin-accelerated F-actin depolymerization, which was only mildly inhibited in partially modified actin. The affinity of TccC3-ADP-ribosylated G-actin for profilin and thymosin-β4 was weakened moderately but sufficiently to potentiate spontaneous polymerization in their presence. Interestingly, the Arp2/3-mediated nucleation was also potentiated by T148-ADP-ribosylation. Notably, even partially modified actin showed reduced bundling by plastins and α-actinin. In agreement with the role of these and other tandem calponin-homology domain actin organizers in the assembly of the cortical actin network, TccC3 induced intense membrane blebbing in cultured cells. Overall, our data suggest that TccC3 imposes a complex action on the cytoskeleton by affecting F-actin nucleation, recycling, and interaction with actin-binding proteins involved in the integration of actin filaments with each other and cellular elements.     

Modeling TCP SACK performance over wireless channels with semi-reliable ARQ/FEC

Providing reliable data communications over wireless channels is a challenging task because time-varying wireless channel characteristics often lead to bit errors. These errors result in loss of IP packets and, consequently, TCP segments encapsulated into these packets. Since TCP cannot distinguish packet losses due to bit corruption from those due to network congestion, any packet loss caused by wireless channel impairments leads to unnecessary execution of the TCP congestion control algorithms and, hence, sub-optimal performance. Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC) try to improve communication reliability and reduce packet losses by detecting and recovering corrupted bits. Most analytical models that studied the effect of ARQ and FEC on TCP performance assumed that the ARQ scheme is perfectly persistent (i.e., completely reliable), thus a frame is always successfully transmitted irrespective of the number of transmission attempts it takes. In this paper, we develop an analytical cross-layer model for a TCP connection running over a wireless channel with a semi-reliable ARQ scheme, where the amount of transmission attempts is limited by some number. The model allows to evaluate the joint effect of stochastic properties of the wireless channel characteristics and various implementation-specific parameters on TCP performance, which makes it suitable for performance optimization studies. The input parameters include the bit error rate, the value of the normalized autocorrelation function of bit error observations at lag 1, the strength of the FEC code, the persistency of ARQ, the size of protocol data units at different layers, the raw data rate of the wireless channel, and the bottleneck link buffer size.     

Carrier Lifetime Measurements in Long-Wave Infrared InAs/GaSb Superlattices Under Low Excitation Conditions

Minority carrier lifetime in long-wave infrared (LWIR) type?II InAs/GaSb superlattices was studied using the optical modulation response (OMR) technique in wide ranges of excitation and temperature. The measured carrier lifetime was found to increase superexponentially with decreasing excitation power density below the level of 1?mW/cm² to 2?mW/cm². The phenomenon was qualitatively explained by the presence of shallow trapping centers.     

Size Effects on the Mechanical Properties of Nanoporous Graphene Networks

It is essential to understand the size scaling effects on the mechanical properties of graphene networks to realize the potential mechanical applications of graphene assemblies. Here, a “highly dense‐yet‐nanoporous graphene monolith (HPGM)” is used as a model material of graphene networks to investigate the dependence of mechanical properties on the intrinsic interplanar interactions and the extrinsic specimen size effects. The interactions between graphene sheets could be enhanced by heat treatment and the plastic HPGM is transformed into a highly elastic network. A strong size effect is revealed by in situ compression of micro‐ and nanopillars inside electron microscopes. Both the modulus and strength are drastically increased as the specimen size reduces to ≈100 nm, because of the reduced weak links in a small volume. Molecular dynamics simulations reveal the deformation mechanism involving slip‐stick sliding, bending, buckling of graphene sheets, collapsing, and densification of graphene cells. In addition, a size‐dependent brittle‐to‐ductile transition of the HPGM nanopillars is discovered and understood by the competition between volumetric deformation energy and critical dilation energy.     

Impact of traffic localization on communication rates in ad-hoc networks

The effect of traffic distribution on communication rates and receiver complexity in ad-hoc networks is addressed, considering a network with constant density of users and a certain traffic model. Information theoretic upper bounds on communication rates are derived under an assumption that transmitting nodes as well as receiving nodes cooperate. It is shown that for the case of large signal attenuation the bounds hold even when the cooperation among users is limited to a certain region of the network domain. Furthermore, achievability bounds on communication rates are derived. The bounds rely on two proposed local cooperation strategies. A comparison shows that the upper bounds are tight and closely follow the achievability results. Finally, the impact of traffic localization on the receiver complexity is addressed.