Strange Matter: a state before black hole

Normal baryonic matter inside an evolved massive star can be intensely compressed by gravity after a supernova. General relativity predicts formation of a black hole if the core material is compressed into a singularity, but the real state of such compressed baryonic matter (CBM) before an event horizon of black hole appears is not yet well understood because of the non-perturbative nature of the fundamental strong interaction. Certainly, the rump left behind after a supernova explosion could manifest as a pulsar if its mass is less than the unknown maximum mass, $M_{\rm max}$. It is conjectured that pulsar-like compact stars are made of strange matter (i.e., with 3-flavour symmetry), where quarks are still localized as in the case of nuclear matter. In principle, different manifestations of pulsar-like objects could be explained in the regime of this conjecture. Besides compact stars, strange matter could also be manifested in the form of cosmic rays and even dark matter.Comment: 20 pages, 3 figures, contribution to "Centennial of general relativity - A celebration

Creep behavior of natural fiber reinforced polymer composites

Creep behavior of natural fiber/polymer composites (NFPCs) was studied in response to the increasing application of this material as structural building products. Factors that influence creep behavior of the composites were investigated by analyzing creep curves of several different NFPC systems, which were designed for overall performance of the composites. Among different models, the 4-element Burgers type was mostly used for quantitative characterization of the creep curves to compare the properties of different composites. The parameters from the 4-element Burgers model were easily interpretable due to their physical meanings. Generalized Burgers models provided better fit by introducing extra Kelvin units, but they are more complicated. Indexed Burgers models performed better for creep curves within the primary stage in terms of both characterization and prediction. Creep prediction was attempted through two approaches: modeling and accelerated testing. Burgers models were proven unsuitable for long-term prediction if the creep test time was not long enough. Comparatively, the indexed Burgers and 2-parameter power law models performed better for prediction purposes. Accelerated creep tests were conducted at higher temperatures, and smooth curves were obtained based on the time-temperature superposition (TTS) principle. The accuracy of long-term prediction was unable to be evaluated due to the lack of long-term experimental data. Several factors were shown to affect the creep resistance of NFPCs. These include polymer matrix type, natural fiber loading, additives, temperature, and weathering treatment. PVC had higher creep resistance than HDPE, and HDPE showed better creep resistance than ultra-high molecular weight polyethylene (UHMWPE). Introducing engineering plastics to form microfibrils in HDPE matrix improved its creep performance. Certain recycled plastics had smaller creep deformation than the corresponding virgin resin. Adding natural fibers into polymer matrix greatly enhanced its creep resistance. The effect of a coupling agent on creep property of NFPCs was dependent on its modulus and coupling effect. UVA, an ultrafine titanium dioxide, slightly reduced the creep deformation of HDPE composites at a low loading level. Higher temperatures led to not only larger instantaneous deformations, but also to higher long-term creep rates. Weathering treatment also affected the creep properties of polymer and NFPCs

Self-current induced spin-orbit torque in FeMn/Pt multilayers

Extensive efforts have been devoted to the study of spin-orbit torque in ferromagnetic metal/heavy metal bilayers and exploitation of it for magnetization switching using an in-plane current. As the spin-orbit torque is inversely proportional to the thickness of the ferromagnetic layer, sizable effect has only been realized in bilayers with an ultrathin ferromagnetic layer. Here we demonstrate that, by stacking ultrathin Pt and FeMn alternately, both ferromagnetic properties and current induced spin-orbit torque can be achieved in FeMn/Pt multilayers without any constraint on its total thickness. The critical behavior of these multilayers follows closely three-dimensional Heisenberg model with a finite Curie temperature distribution. The spin torque effective field is about 4 times larger than that of NiFe/Pt bilayer with a same equivalent NiFe thickness. The self-current generated spin torque is able to switch the magnetization reversibly without the need for an external field or a thick heavy metal layer. The removal of both thickness constraint and necessity of using an adjacent heavy metal layer opens new possibilities for exploiting spin-orbit torque for practical applications.Comment: 28 pages, 5 figure