Recently, a novel 4
D
Einstein–Gauss–Bonnet gravity was formulated by Glavan and Lin (Phys Rev Lett 124(8):081301, 2020). Although whether the theory is well defined is currently debatable, the ...spherically symmetric black hole solution is still meaningful and worthy of study. In this paper, we study the geodesic motions in the spacetime of the spherically symmetric black hole solution. First of all, we find that a negative GB coupling constant is allowable, as in which case the singular behavior of the black hole can be hidden inside the event horizon. Then we calculate the innermost stable circular orbits for massive particles, which turn out to be monotonic decreasing functions of the GB coupling constant. Furthermore, we study the unstable photon sphere and shadow of the black hole. It is interesting to find that the proposed universal bounds on black hole size in Lu and Lyu (Phys Rev D 101(4):044059, 2020) recently can be broken when the GB coupling constant takes a negative value.
A
bstract
We consider the relation between SYK-like models and vector models by studying a toy model where a tensor field is coupled with a vector field. By integrating out the tensor field, the toy ...model reduces to the Gross-Neveu model in 1 dimension. On the other hand, a certain perturbation can be turned on and the toy model flows to an SYK-like model at low energy. A chaotic-nonchaotic phase transition occurs as the sign of the perturbation is altered. We further study similar models that possess chaos and enhanced reparameterization symmetries.
Developing highly active nonprecious electrocatalysts with superior durability for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial to improve the efficiency ...of overall water splitting but remains challenging. Here, a novel superhydrophilic Co4N‐CeO2 hybrid nanosheet array is synthesized on a graphite plate (Co4N‐CeO2/GP) by an anion intercalation enhanced electrodeposition method, followed by high‐temperature nitridation. Doping CeO2 into Co4N can favor dissociation of H2O and adsorption of hydrogen, reduce the energy barrier of intermediate reactions of OER, and improve the compositional stability, thereby dramatically boosting the HER performance while simultaneously inducing enhanced OER activity. Furthermore, the superhydrophilic self‐supported electrode with Co4N‐CeO2 in situ grown on the conductive substrate expedites electron conduction between substrate and catalyst, promotes the bubble release from electrode timely and impedes catalyst shedding, ensuring a high efficiency and stable working state. Consequently, the Co4N‐CeO2/GP electrode shows exceptionally low overpotentials of 24 and 239 mV at 10 mA cm−2 for HER and OER, respectively. An alkaline electrolyzer by using Co4N‐CeO2/GP as both the cathode and anode requires a cell voltage of 1.507 V to drive 10 mA cm−2, outperforming the Pt/C||RuO2 electrolyzer (1.540 V@10 mA cm−2). More significantly, the electrolyzer has extraordinary long‐term durability at a large current density of 500 mA cm−2 for 50 h, revealing its potential in large‐scale applications.
A superhydrophilic Co4N‐CeO2/graphite plate self‐supported electrode is designed and synthesized via an electrodeposition method, followed by high‐temperature selective nitridation. The coupling of CeO2 into Co4N favors water dissociation and hydrogen adsorption, reduces the energy barrier of intermediate reactions of oxygen evolution reaction, and enhances the compositional stability. Therefore, the Co4N‐CeO2/graphite plate‐based electrolyzer shows outstanding overall water splitting activity with superior stability.
We study gauge theories between two parallel boundaries with non-trivial boundary conditions, which serve as a toy model for black hole background with two boundaries near the horizon and infinite, ...aiming for a better understanding of the Bekenstein–Hawking entropy. The new set of boundary conditions allows boundary modes and non-local modes that interplay between the two boundaries. Those boundary modes and Wilson lines stretched between the two boundaries are carefully analyzed and are confirmed as physical variables in the phase space. Along with bulk fluctuation modes and topological modes, the partition function and entropy of all physical modes are evaluated via Euclidean path integral. It is shown that there are transitions between the dominance of different modes as we vary the temperature. The boundary fluctuation modes whose entropy is proportional to the volume dominate at high temperatures, and the boundary-area scaled boundary modes and Wilson lines are the more important at low temperatures. At super-low temperatures, when all the fluctuation modes die off, we see the topological modes whose entropy is the logarithm of the length scales of the system. The boundary modes and non-local modes should have their counterparts in a black hole system with similar boundary conditions, which might provide important hints for black hole physics.
The electrochemical nitrogen reduction reaction (NRR) offers an energy‐saving and environmentally friendly approach to produce ammonia under ambient conditions. However, traditional catalysts have ...extremely poor NRR performances because of their low activity and the competitive hydrogen evolution reaction. The high catalytic activity of nanoporous gold (NPG) and the hydrophobicity and molecular concentrating effect of the zeolitic imidazolate framework‐8 (ZIF‐8) were incorporated in the NPG@ZIF‐8 nanocomposite so that the ZIF‐8 shell could weaken hydrogen evolution and retard reactant diffusion. A highest Faradaic efficiency of 44 % and an excellent rate of ammonia production of (28.7±0.9) μg h−1 cm−2 were achieved, which are superior to traditional gold nanoparticles and NPG. Moreover, the composite catalyst shows high electrochemical stability and selectivity (98 %). The superior NRR performance makes NPG@ZIF‐8 one of the most promising water‐based NRR electrocatalysts for ammonia production.
Being efficient: A core–shell structure composite of nanoporous gold embedded in a ZIF‐8 shell has been developed. The high catalytic activity of the nanoporous gold and the hydrophobic porous shell of ZIF‐8 results in a superior enhancement of electrochemical nitrogen fixation compared to traditional electrocatalysts, as shown by a Faradaic efficiency of 44 % (see picture, scale bar: 300 nm).
Ischemic stroke leads to severe neurological dysfunction in adults. Hyperbaric oxygen (HBO) induces tolerance to cReperfusion inj/reperfusion (I/R) injury. Therefore, our aims were to investigate ...whether SIRT1 participates in regulatingin the neuro‐protective effect of HBO in a cerebral I/R model and its mechanism. Mice N2a cells were used to construct an oxygen deprivation/reperfusion (OGD/R) model to simulate in vitro brain I/R injury and to evaluate the role of HBO in OGD/R stimulated cells. Cell proliferation was detected using MTT, and apoptosis was determined by flow cytometry. ELISA was used to measure the concentration of TNF‐α, IL‐1β and IL‐6 related inflammatory factors. RT‐qPCR and western blot assays were performed to test the expression of SIRT1. Immunoprecipitation was used to detect acetylation of HMGB1. Expression of SIRT1 was obviously reduced after OGD/R treatment in N2a cells, while SIRT1 was obviously enhanced in HBO treated cells. Moreover, knockdown of SIRT1 induced neuro‐inflammation damage in cells and HBO effectively improved the inflammatory response in OGD/R treated cells by affecting SIRT1 levels. Furthermore, HBO induced the deacetylation of HMGB1 via regulating SIRT1. Interestingly, HBO via regulating the SIRT1‐induced HMGB1 deacetylation and suppressing MMP‐9 improved ischemic brain injury. HBO regulated ischemic brain injury via regulation of SIRT1‐induced HMGB1 deacetylation, making it a potential treatment for ischemic brain injury treatment.
After ischemia/reperfusion treatment, the expression of SIRT1 in N2a cells was significantly reduced, and the expression of SIRT1 in HBO‐treated cells increased. HBO induced the deacetylation of HMGB1 by regulating SIRT1 and localized HMGB1 in the nucleus. HBO could also improve ischemic brain injury by regulating SIRT1‐induced deacetylation of HMGB1 and inhibiting MMP‐9.
A novel 4D Einstein–Gauss–Bonnet gravity was recently formulated by Glavan and Lin Phys. Rev. Lett. 124, 081301 (2020). Although this theory may run into trouble at the level of action or equations ...of motion, the spherically symmetric black hole solution, which can be successfully reproduced in those consistent theories of 4D EGB gravity, is still meaningful and worthy of study. In this paper, we investigate Hawking radiation in the spacetime containing such a de Sitter black hole. Both the greybody factor and the power spectra of the Hawking radiation of the massless scalar are studied numerically for the full range of various parameters, including the GB coupling constant
α
, the cosmological constant
Λ
and the coupling constant related to the scalar filed
ξ
. In particular, we find a negative
α
leads to a larger greybody factor than that of a
α
≥
0
. While, for the power spectra of the Hawking radiation the situation is quite the opposite. The reason is that the temperature of the black hole would be very high when
α
<
0
. Actually, we observe that the temperature would be arbitrarily high when
α
approaches to the lower bound.
For dark compact objects such as black holes or wormholes, the shadow size has long been thought to be determined by the unstable photon sphere (region). However, by considering the asymmetric ...thin-shell wormhole (ATSW) model, we find that the impact parameter of the null geodesics is discontinuous through the wormhole in general and hence we identify novel shadows whose sizes are dependent of the photon sphere in the other side of the spacetime. The novel shadows appear in three cases: (A2) The observer's spacetime contains a photon sphere and the mass parameter is smaller than that of the opposite side; (B1, B2) there' s no photon sphere no matter which mass parameter is bigger. In particular, comparing with the black hole, the wormhole shadow size is always smaller and their difference is significant in most cases, which provides a potential way to observe wormholes directly through Event Horizon Telescope with better detection capability in the future.
Metal–organic frameworks (MOFs) with multiple emission centers are newly emerging as ratiometric sensors owing to their high sensitivity and high selectivity toward a wide range of targeted ...functional species. Energy transfer between the light‐absorbing group and emission centers and between different emission centers is the key to rationally design and synthesize MOF‐based ratiometric sensors. A good match between the energy levels of the light‐absorbing groups and emission centers is the prerequisite for MOF‐based sensors to exhibit multiple emissions, and a good match of the MOF‐based sensors and those of the targeted species can increase the sensitivity and selectivity, but this match is highly challenging to obtain via synthesis. MOFs with multiple emission centers can be produced by functionalizing MOFs with multiple lanthanide centers, organic luminophores, dyes, carbon dots, and other such emissive groups. In this progress report, recent advances in the strategies for synthesizing MOFs with multiple emission centers and their applications for ratiometric sensing of solution conditions, including the pH value, and ion, organic molecule, and biomolecule concentrations, are summarized, as are the related sensing mechanisms.
Recent advances in both the strategies for the synthesis of MOF‐based ratiometric fluorescent sensors and their potential application in sensing pH values, ions, organic molecules, and biomolecules are summarized, with the aim of providing fundamentals for future studies of MOF‐based ratiometric fluorescent sensors.
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