לוח שנה משולב

. In 1949, Fermi proposed a mechanism for the heating of particles in cosmic rays. He suggested that on average, charged particles gain energy from collisions with moving magnetic mirrors since they hit the mirrors more frequently with heads on collisions. Fermi, Ulam and their followers modeled this problem by studying the energy gain of particles moving in billiards with slowly moving boundaries. Until 2010 several examples of such oscillating billiards leading to power-law growth of the particles averaged energy were studied. In 2010 we constructed an oscillating billiard which produces exponential in time growth of the particles energy. The novel mechanism which leads to such an exponential growth is robust and may be extended to arbitrary dimension. Moreover, the exponential rate of the energy gain may be predicted by utilizing adiabatic theory and probabilistic models. The extension of these results to billiards with mixed phase space leads to the development of adiabatic theory for non-ergodic systems. Finally, such accelerators lead to a faster energy gain in open systems, when particles are allowed to enter and exit them through a small hole. The implications of this mechanism on transport in extended systems and on equilibration of energy in closed systems like "springy billiards" will be discussed. The latter application provides a key principle: to achieve ergodicity in slow-fast systems in the adiabatic limit, the fast subsystems should NOT be ergodic.

Bacterial resistance to antibiotics is a major concern worldwide, leading to an extensive search for alternative drugs. Promising candidates are antimicrobial peptides, innate immunity molecules, which were shown to be highly efficient against multidrug resistant bacteria. Therefore, it is essential to study bacterial resistance mechanisms against them. In Salmonella Typhimurium (S.Typhimurium), a pathogenic bacterium that causes inflammation of the gastrointestinal tract, resistance to antimicrobial peptide is mainly mediated by surface modifications. These reduce the molecular interactions between the bacterial surface and the peptides. Searching for new resistance mechanisms to antimicrobial peptides, we revealed two novel strategies that evolved in a S. Typhimurium resistant line. One involves mutations in the AcrAB-TolC efflux pump and the second is acquired by the loss of the periplasmic chaperone Skp. Our data provide a deeper understanding on the role of the AcrAB-TolC system and Skp in S. Typhimurium.

During the last decade, seismic sensing with optical fibers has become a reality. By analyzing the effect of seismic deformation on the fiber’s optical response, state-of-the-art Distributed Acoustic Sensing (DAS) now offers a 1-meter sensor resolution for tens of kilometers of fiber. In other words, a single DAS system can record up to 40,000 data channels at once – two orders of magnitude more than the entire earthquake-monitoring seismic network in Israel.

In this talk, I will first introduce the underlying operating principles of DAS acquisition. These measurements are very different from conventional seismic sensors and need to be analyzed accordingly. Subsequently, most of the talk will revolve around DAS applications in various scenarios.

We use the ambient seismic field, recorded on a standard telecommunication fiber deployed around the Stanford campus, to analyze subsurface properties. We also acquired DAS data from a downhole fiber deployed in the SAFOD well and utilized it to reconstruct the earth’s structure and detect earthquakes. Finally, we study DAS data from an unconventional gas field and show how to conduct a simple analysis that unveils reservoir properties.

This seminar consists of two parts. The first part is related to the investigation of mechanism of tunable domain wall (DW) conductivity in the ferroelectric LiNbO3 thin films with sub-µm thickness, Using a combination of scanning transmission electron microscopy (STEM) and local probe techniques we generate and delineate the electrically-charged 180º DWs and test their conducting behavior using local probe spectroscopy and imaging under electrical bias. More importantly, electrical tunability of DW conductivity by sub-coercive voltage is realized through the changes in DW conformity. The obtained results provide tangible evidence that the charged DWs can be used as multilevel logic elements in analog computing devices.
The second part discusses the dynamic switching behavior in the HfO2-based films investigated by a combination of local probe microscopy and pulse switching techniques. Application of HfO2-based materials to ferroelectric memory and logic devices has generated considerable interest as they allow overcoming significant problems associated with poor compatibility of perovskite ferroelectrics with CMOS processing. High-resolution studies of the time- and field-dependent evolution of the domain structure in La:HfO2 thin film capacitors provides an insight into the mechanism of imprint - one of the main degradation effects hindering integration of ferroelectric HfO2 into CMOS-compatible memory technology.