Collective motion of chiral particles in complex noise environments.

Collective motion of chiral particles in complex noise environments is investigated based on the Vicsek model. In the model, we added chirality, along with complex noise, affecting particles clustering motion. Particles can only avoid noise interference in a specific channel, and this consideration is more realistic due to the complexity of the environment. Via simulations, we find that the channel proportion, p, critically influences chiral particle synchronization. Specifically, we observe a disorder-order transition at critical [Formula: see text], only when [Formula: see text], the system can achieve global synchronization. Combined with our definition of spatial distribution parameter and observation of the model, the reason is that particles begin to escape from the noise region under the influence of complex noise. In addition, the value of [Formula: see text] increases linearly with velocity, while it decreases monotonically with the increase in chirality and interaction radius. Interestingly, an appropriate noise amplitude minimizes [Formula: see text]. Our findings may inspire novel strategies to manipulate self-propelled particles of distinct chirality to achieve desired spatial migration and global synchronization.

Transition in electronic and magnetic properties of transition metal embedded semimetallic B-graphyne.

Spintronics is extremely important in the future development of information technology. Notably, two-dimensional carbon materials with atomically thick and p-electron systems have great potential for application in ultrathin spintronic devices. B-graphyne (B-GY) is a recently proposed two-dimensional carbon allotrope with double Dirac cones. It is a promising nanomaterial for high-speed spintronic devices due to its ultra-high Fermi velocity and thermodynamic stability. We tune the electronic and magnetic properties of B-GY by doping 3d transition metals (TM) (Cr, Mn, Fe, Co, Ni) based on first-principles calculations. After doping, TM forms strong covalent bonds (Fe, Co, Ni) and ionic bonds (Cr, Mn) with adjacent C atoms. The system of TM-doped B-GY (TM@B-GY) is transformed from a semimetal for B-GY to a metal (Cr, Mn, Fe, Co), but Ni@B-GY is still semimetal. Among them, Co@B-GY is approximately a half-metal. Moreover, TM (except Ni) can induce the magnetism of B-GY to undergo spin splitting. The TM d-orbitals are strongly coupled to the C p-orbitals, which play an important role in inducing magnetism. The results show that the tunable electronic and magnetic properties of TM@B-GY are promising as a high-speed spintronic device. Our research helps advance the study of semimetallic carbon allotropes in the field of spintronics.

First-principles investigation of CO and CO2 adsorption on pristine and Fe-doped planar carbon allotrope net-Y.

A new planar carbon allotrope named net-Y with periodic four-six-eight-membered carbon rings has been synthesized in this study. In this paper, the adsorption properties of CO and CO2 on the pristine net-Y and two Fe-doped net-Y surfaces were studied using first-principles calculations. Using adsorption energy, charge analysis, adsorption distance, deformation charge density and density of states, it can be found that the adsorption of CO and CO2 on the pristine net-Y surface appears as physisorption with weak interactions. Based on this, in order to enhance the adsorption strength of CO and CO2 on the surface of net-Y, Fe-doping is performed on net-Y. The calculation results after doping show that the doping of Fe atoms at different positions can significantly improve the adsorption strength of the two gas molecules. After CO and CO2 are adsorbed on the doped net-Y surface, the adsorption energy is greatly improved compared with the pristine adsorption energy, which makes the two gas molecules form a chemical bond with the substrate. Subsequently, the results of charge density and density of states further prove that the adsorption system shows chemisorption with strong interactions. Comparing doped and pristine adsorption, after Fe doping, the adsorption of CO and CO2 is stronger, which makes it possible to use this material effectively for the adsorption and removal of CO and CO2 gases.

Ni17 W3 -W Interconnected Hybrid Prepared by Atmosphere- and Thermal-Induced Phase Separation for Efficient Electrocatalysis of Alkaline Hydrogen Evolution.

The development of efficient and stable noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) in alkaline media is still a challenge. Herein, a hybrid material formed by the interconnection of Ni17 W3 intermetallic compound with metallic W is demonstrated for HER. The Ni17 W3 -W hybrid is prepared by the atmosphere- and thermal-induced phase-separation strategy from a single-phase precursor (NiWO4 ), which gives Ni17 W3 -W hybrid abundant and tight interfaces. The theoretical calculation manifests that Ni17 W3 shows more optimized energetics for adsorbed H atom, while W has lower energy barrier for water dissociation, and the synergistic effect between them is believed to facilitate the HER kinetics. Moreover, Ni17 W3 presents a proper adsorption strength for both adsorbed OH and H, and thus Ni17 W3 may also act as a high HER catalyst by itself. As a result, the Ni17 W3 -W hybrid demonstrates high activity and durability for HER in liquid alkaline electrolyte; the electrolyzer assembled by Ni17 W3 -W hybrid and Ni-Fe-layered double hydroxide (LDH) as, respectively, the cathode and anode electrocatalysts presents superior performance to Pt/C-IrO2 benchmark. In addition, the Ni17 W3 -W hybrid also works well in the water electrolyzer based on solid hydroxide exchange membrane. The present work provides a promising pathway to the design of high-performance electrocatalysts.

Retraction Note: Contrasting the complexity of the climate of the past 122,000 years and recent 2000 years.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Photogalvanic Effect in Nitrogen-Doped Monolayer MoS2 from First Principles.

We investigate the photogalvanic effect in nitrogen-doped monolayer molybdenum disulfide (MoS2) under the perpendicular irradiation, using first-principles calculations combined with non-equilibrium Green function formalism. We provide a detailed analysis on the behavior of photoresponse based on the band structure and in particular the joint density of states. We thereby identify different mechanisms leading to the existence of zero points, where the photocurrent vanishes. In particular, while the zero point in the linear photovoltaic effect is due to forbidden transition, their appearance in the circular photovoltaic effect results from the identical intensity splitting of the valance band and the conduction band in the presence of Rashba and Dresslhaus spin-orbit coupling. Furthermore, our results reveal a strong circular photogalvanic effect of nitrogen-doped monolayer MoS2, which is two orders of magnitude larger than that induced by the linearly polarized light.

Mixing and demixing of binary mixtures of polar chiral active particles.

We study a binary mixture of polar chiral (counterclockwise or clockwise) active particles in a two-dimensional box with periodic boundary conditions. Besides the excluded volume interactions between particles, the particles are also subjected to the polar velocity alignment. From the extensive Brownian dynamics simulations, it is found that the particle configuration (mixing or demixing) is determined by the competition between the chirality difference and the polar velocity alignment. When the chirality difference competes with the polar velocity alignment, the clockwise particles aggregate in one cluster and the counterclockwise particles aggregate in the other cluster; thus, the particles are demixed and can be separated. However, when the chirality difference or the polar velocity alignment is dominant, the particles are mixed. Our findings could be used for the experimental pursuit of the separation of binary mixtures of chiral active particles.

Contrasting the complexity of the climate of the past 122,000 years and recent 2000 years.

The complexity of the climate of the past 122;000 years and recent 2000 years was investigated by analyzing the δ 18 O records of ice cores based on the sample entropy (SampEn) method and Lempel-Ziv (LZ) complexity. In using SampEn method, the climate complexity is measured by the sample entropy, which is a modified approximate entropy defined in terms of the occurring probability of new modes in a record. A larger sample entropy reflects a higher probability to spot a new mode in the data, and in this sense signals a larger complexity of the sample. The δ 18 O record of the past 122,000-year is found to have smaller SampEn than the recent 2000-year. This result suggests that the climate of the past 122;000-year has less complexity than that of the recent 2000 years, even though the record for the former exhibits stronger fluctuations and multifractality than the latter. This diagnosis is additionally supported by calculations of LZ complexity, which has smaller value for the record of the past 122;000 years than the recent 2000 years. Our theoretical findings may further contribute to ongoing explorations into the nonlinear statistical character of the climate change.

Contrasting scaling properties of interglacial and glacial climates.

Understanding natural climate variability is essential for assessments of climate change. This is reflected in the scaling properties of climate records. The scaling exponents of the interglacial and the glacial climates are fundamentally different. The Holocene record is monofractal, with a scaling exponent H∼0.7. On the contrary, the glacial record is multifractal, with a significantly higher scaling exponent H∼1.2, indicating a longer persistence time and stronger nonlinearities in the glacial climate. The glacial climate is dominated by the strong multi-millennial Dansgaard-Oeschger (DO) events influencing the long-time correlation. However, by separately analysing the last glacial maximum lacking DO events, here we find the same scaling for that period as for the full glacial period. The unbroken scaling thus indicates that the DO events are part of the natural variability and not externally triggered. At glacial time scales, there is a scale break to a trivial scaling, contrasting the DO events from the similarly saw-tooth-shaped glacial cycles.

Ratchet effect and amplitude dependence of phase locking in a two-dimensional Frenkel-Kontorova model.

We demonstrate the ratchet and phase locking effects in a two-dimensional overdamped Frenkel-Kontorova model with a square symmetric periodic substrate when both a longitudinal dc drive and a circular ac drive are applied. Besides the harmonic steps, the large half integer steps can also clearly be seen in the longitudinal (x) direction. These half integer steps are directly correlated to the appearance of positive and negative ratchet effects in the transverse (y) direction due to the symmetry breaking in the combination of the dc and ac drives. The angle between the net displacement and the longitudinal direction is analytically obtained in a single period of the ac drive. In the examination of the amplitude dependence of the ac drive, the maxima decrease monotonically with the amplitude, while the anomalies occur for the critical depinning force and the harmonic steps due to the spatial symmetry breaking of orbits in the presence of the ac drive.

Rectified brownian transport in corrugated channels: Fractional brownian motion and Lévy flights.

We study fractional brownian motion and Lévy flights in periodic corrugated channels without any external driving forces. From numerical simulations, we find that both fractional gaussian noise and Lévy-stable noise in asymmetric corrugated channels can break thermodynamical equilibrium and induce directed transport. The rectified mechanisms for fractional brownian motion and Lévy flights are different. The former is caused by non-uniform spectral distribution (low or high frequencies) of fractional gaussian noise, while the latter is due to the nonthermal character (occasional long jumps) of the Lévy-stable noise. For fractional brownian motion, average velocity increases with the Hurst exponent for the persistent case, while for the antipersistent case there exists an optimal value of Hurst exponent at which average velocity takes its maximal value. For Lévy flights, the group velocity decreases monotonically as the Lévy index increases. In addition, for both cases, the optimized periodicity and radius at the bottleneck can facilitate the directed transport. Our results could be implemented in constrained structures with narrow channels and pores where the particles undergo anomalous diffusion.

Existence and stability of the resonant phenomena in the dc- and ac-driven overdamped Frenkel-Kontorova model with the incommensurate structure.

Dynamical mode-locking phenomena in the incommensurate structures of the dc- and ac-driven overdamped Frenkel-Kontorova model are studied by molecular-dynamics simulations. The obtained results have shown that Shapiro steps exhibit significantly different amplitude and frequency dependence from the one observed in the commensurate structures. Due to the incommensurability of the system the special symmetry of the motion of particles is broken, and in the amplitude dependence of Shapiro steps, this will result in the appearance of anomalies and deviation from the well-known Bessel-like behavior. In the frequency or period dependence, oscillations have been observed in the high-amplitude limit; however, they exhibit strong anomalies compared with those in the commensurate structures. The oscillatory behavior and the anomalies have been also be revealed in the (F(ac),F) and the (ν(0),F) phase diagrams where several phases are defined.

Friction phenomena and phase transition in the underdamped two-dimensional Frenkel-Kontorova model.

Locked-to-sliding phase transition has been studied in the driven two-dimensional Frenkel-Kontorova model with the square symmetric substrate potential. It is found that as the driving force increases, the system transfers from the locked state to the sliding state where the motion of particles is in the direction different from that of driving force. With the further increase in driving force, at some critical value, the particles start to move in the direction of driving force. These two critical forces, the static friction or depinning force, and the kinetic friction force for which particles move in the direction of driving force have been analyzed for different system parameters. Different scenarios of phase transitions have been examined and dynamical phases are classified. In the case of zero misfit angle, the analytical expressions for static and kinetic friction force have been obtained.

Transition from the exhibition to the nonexhibition of negative differential thermal resistance in the two-segment Frenkel-Kontorova model.

An extensive study of the one-dimensional two-segment Frenkel-Kontorova (FK) model reveals a transition from the counterintuitive existence to the ordinary nonexistence of a negative-differential-thermal-resistance (NDTR) regime, when the system size or the intersegment coupling constant increases to a critical value. A "phase" diagram which depicts the relevant conditions for the exhibition of NDTR was obtained. In the existence of a NDTR regime, the link at the segment interface is weak and therefore the corresponding exhibition of NDTR can be explained in terms of effective phonon-band shifts. In the case where such a regime does not exist, the theory of phonon-band mismatch is not applicable due to sufficiently strong coupling between the FK segments. The findings suggest that the behavior of a thermal transistor will depend critically on the properties of the interface and the system size.

Negative differential thermal resistance induced by ballistic transport.

Using nonequilibrium molecular-dynamics simulations, we study the temperature dependence of the negative differential thermal resistance that appears in two-segment Frenkel-Kontorova lattices. We apply the theoretical method based on Landauer equation to obtain the relationship between the heat current and the temperature, which states a fundamental interpretation about the underlying physical mechanism of the negative differential thermal resistance. The temperature profiles and transport coefficients are demonstrated to explain the crossover from diffusive to ballistic transport. The finite-size effect is also discussed.

Scaling and the thermal conductivity of the Frenkel-Kontorova model.

We have studied the dependence of the thermal conductivity kappa on the strength of the interparticle potential lambda and the strength of the external potential beta in the Frenkel-Kontorova model. We found that the functional relation can be expressed in a scaling form, kappa proportional, variantlambda;{32}/beta;{2} . This result is first obtained by nonequilibrium molecular dynamics. It is then confirmed by two analytical methods, the self-consistent phonon theory and the self-consistent stochastic reservoirs method. The thermal conductivity kappa is therefore a decreasing functon of beta and an increasing function of lambda.

PtRu/Ti anodes with varying Pt ratio: Ru ratio prepared by electrodeposition for the direct methanol fuel cell.

PtRu/Ti anodes with varying Pt ratio Ru ratio were prepared by electrodeposition of a thin PtRu catalyst layer onto Ti mesh for a direct methanol fuel cell (DMFC). The morphology and structure of the catalyst layers were analyzed by SEM, EDX and XRD. The catalyst coating layer shows an alloy character. The relative activities of the PtRu/Ti electrodes were assessed and compared in half cell and single DMFC experiments. The results show that these electrodes are very active for the methanol oxidation and that the optimum Ru surface coverage was ca. 9 at.% for DMFC operating at 20 degrees C and 11 at.% at 60 degrees C. The PtRu/Ti anode shows a performance comparable to that of the conventional carbon-based anode in a DMFC operating with 0.25 M or 0.5 M methanol solution and atmosphere oxygen gas at 90 degrees C.

Particle-cluster aggregation on a small-world network.

To describe the aggregation behaviors on substrates with long-range jump paths, a model of particle-cluster aggregation on a two-dimensional small-world network is presented. This model is characterized by two parameters: the clustering exponent alpha and the long-range connection rate phi. The results show that there exists an asymptotic fractal dimension D(max)(f) that depends upon alpha. With decrement of alpha, D(max)(f) varies from 1.7 to 2.0, which corresponds to a crossover from diffusion-limited-aggregation-like to dense growth. The change of the aggregation pattern results from the long-range connection in the network, which reduces the effect of screening during the aggregation. When the system size is not large enough, the effective fractal dimension D(f) depends upon phi because of the finite-size effect. With primitive analysis, we obtain the expression of the effective fractal dimension D(f) with the network parameters alpha and phi.

Critical behavior of efficiency dynamics in small-world networks.

Some dynamical processes in a small-world network shows a critical transition at a finite disorder phi(c) of the network, in contrast with the geometrical properties that exhibit the critical behavior at phi(c)=0. Although it has been pointed out in previous works that the transition is related to the structural properties of the network, it is still not very clear why the transition occurs at phi(c) not equal 0. In this paper we present a simple social model of efficiency dynamics in small-world networks, which also shows a transition at phi(c)>0. We obtain the critical point with phi(c) approximately equal 0.098 from the finite-size analysis. It is found that both the geometrical properties of the network and the specific dynamical characters of the model contribute to the critical transition. This work is useful for understanding this kind of transition occurring in many dynamical processes in small-world networks.