Critical Momentum Research Articles

A Lower Bound on the Critical Momentum of an Impurity in a Bose–Einstein Condensate

A Lower Bound on the Critical Momentum of an Impurity in a Bose–Einstein Condensate

Deep learning-based prediction of initiation jet momentum ratio in jet-induced oblique detonations

Oblique detonation, with its attributes of self-ignition, rapid heat release, and high thermal cycle efficiency, has garnered significant attention. It is crucial to explore methods that actively control the detonation at a shorter distance under less compressive conditions, and wall jets for active control stands out. However, when the jet momentum ratio excessively exceeds the critical value, it not only leads to resource wastage but may also compromise the outlet thrust performance. Thus, this paper presents a novel deep learning-based method for predicting the jet momentum ratio to ensure detonation with a sufficient margin while minimizing its value across various flight conditions. Firstly, the proposed methodology utilizes a deep neural network (DNN) to classify detonation status under different operating conditions. Building upon the concept of incremental learning, knowledge acquired by the classification network is repurposed to identify the transition point between detonation initiation failure and success. To address potential initiation failure issues in practical applications when seeking the critical jet momentum ratio, we elevate the threshold for the probability of successful initiation, defining such the ratio as the "robust jet momentum ratio". The framework developed in this study enables rapid identification of the robust jet momentum ratio within 0.07 s. To improve the model's inadequate learning of the discontinuous features associated with initiation/initiation failure transitions, data augmentation techniques are employed. The improved model demonstrates efficient determination under two-dimensional variables of flight altitude and Mach number. This study addresses a key challenge in jet prediction by devising an adaptive strategy for tuning jet momentum ratios according to varying flight conditions. This work bears significant engineering application value in the realm of hypersonic propulsion technology.

Cash Cows and Dogs with Dentures: Prostheses for Animals in Nineteenth-Century British Culture

Abstract This article shows how the history of prostheticized animals is much deeper and richer than one might think. Investigating the interest in fitting animals with prostheses throughout the nineteenth century in Britain, this article explores the emergence of animal prostheses in bovine agriculture through to their use in late Victorian pet keeping. The key concern of this piece is the varied responses to prostheticized animals in print culture. As this article shows through an analysis of magazine and newspaper contributions, reactions to animal prostheses varied from admiration and amazement to annoyance and disgust. By analysing this coverage, the article reveals how representations of animal prostheses brought into collision a surprising variety of important social discourses, including those related to economic productivity, prosthesis access, food adulteration, professional recognition, social norms, class hierarchies, and gender relations. Animal prostheses proved a novel yet apt image that distilled a wide variety of societal concerns. Redirecting the critical momentum established by recent scholarship that unites disability studies and critical animal studies, this article uses nineteenth-century animal prostheses as a case study to demonstrate how the concepts of disability, ableism, classism, and misogyny have been extended by humans across species boundaries and how our approaches to nonhuman care are impacted by them. It therefore establishes a new field of inquiry within Victorian studies for scholarship on the interface between discourses of disability and nonhuman animals.

Biorthogonal Dynamical Quantum Phase Transitions in Non-Hermitian Systems.

By utilizing biorthogonal bases, we develop a comprehensive framework for studying biorthogonal dynamical quantum phase transitions in non-Hermitian systems. With the help of the previously overlooked associated state, we define the automatically normalized biorthogonal Loschmidt echo. This approach is capable of handling arbitrary non-Hermitian systems with complex eigenvalues and naturally eliminates the negative value of Loschmidt rate obtained without the biorthogonal bases. Taking the non-Hermitian Su-Schrieffer-Heeger model as a concrete example, a 1/2 change of dynamical topological order parameter in biorthogonal bases is observed which is not shown in self-normal bases. Furthermore, we discover that the periodicity of biorthogonal dynamical quantum phase transitions depends on whether the two-level subsystem at the critical momentum oscillates or reaches a steady state.

Magneto-plasmon of AA-stacked bilayer graphene nanoribbons at finite temperature

Magneto-band structures of AA-stacked bilayer graphene nanoribbons with armchair and zigzag (BLAGNR and BLZGNR) edges are calculated by the pz-orbital tight-binding model. At zero field, BLAGNR is a semiconductor or metal determined by its width. Magnetic field can reduce band-gap and exhibit crossing bands. For metallic BLZGNRs, crossing bands shift and parabolic bands are separated more widely with magnetic field; however, partial flat bands keep unchanged. Temperature significantly increases plasmon frequency and strength for semiconducting BLAGNRs, but slightly affects those of metallic BLAGNRs and BLZGNRs. Magnetic field, for gapped BLAGNRs, can effectively reduce the threshold temperature for inducing plasmon. Whether magneto-plasmon frequency and critical momentum of dispersion relation increase or decrease is strongly sensitive to nanoribbon’s geometry. Field-modulated plasmons with a wide range of frequency could provide potential applications in waveguides and optical sensors.

The state of emergency medicine in Greece: at critical momentum

Greece is a parliamentary republic in southeastern Europe populated by over 10 million permanent residents: 9 million reside on the mainland, with almost 4 million in the greater Athens area. The remaining 1 million populate the over 1200 Greek islands. In addition, more than 160,000 asylum-seekers reached Greece in 2022, and more than 25 million tourists have visited Greece in the last two years. Modern Greek Emergency Medicine (EM) is now in its 4th decade. The Greek government has focused the last few years on enhancing the quality of emergency services provided in public hospitals. Emergency Departments (EDs) are being modernized, undergraduate medical education gradually incorporates EM, and a specialty training program in emergency nursing has been established. However, the late recognition of the critical importance of EM as a specialty in Greece has resulted in the subsequent need to create three alternative pathways to EM, none of which are direct from residency. The first is a 24-month Emergency Medicine fellowship after completing a residency in another specialty and then passing the national exam. The second is for physicians who have worked in a public hospital ED (Gr: Ethniko Systima Ygeias (ESY) ESY for at least three years and successfully passed the national exam. The third, which no longer exists, is a ‘grandfather’ pathway for those physicians who worked in an ESY ED for five years prior to the creation of the fellowship training program. As a result, there is a critical shortage of EM-trained physicians, resulting in most care being provided by physicians without formal training in EM. This is further confounded by the country’s challenging geography, with frequent air transfers from the islands to mainland hospitals. Creating an EM Residency training program is a critical next step to overcoming many of the challenges facing EM provision in Greece today: it would address the shortage of EM-trained providers, decrease the need for costly ground and air transfers, and improve the quality of emergency care throughout Greece.

Korea's Practice on Refugee Protection: Focusing on the Refugee Act and Relevant Court Cases

The Republic of Korea became the first country in East Asia to enact its own refugee law, which has been in force since 2013. While the enactment of the new Refugee Act provided a critical momentum for Korea's refugee policy to shift from mere immigration control to a more human rights–based approach, Korea has often come under fire for its low rate of refugee admittance. As the number of refugee applicants in Korea has drastically surged in recent years, there remain important issues such as improving the efficiency of the refugee admissions process. This article explores the key provisions of Korea's Refugee Act along with recent legislative proposals to amend the Refugee Act, and it highlights some court decisions related to refugee issues. By analyzing the relevant law, court cases, and government policies of Korea, this article explores the possibility of setting Korea up as a good exemplar of upholding the values of refugee protection in the Asian region.

Effect of film thickness and temperature on condensation and momentum accommodation at the liquid–vapor methane interphase in contact with a quartz substrate

In this paper, we used the Molecular Dynamics method to simulate the equilibrium vapor–liquid methane in contact with a solid quartz substrate and study the condensation process and momentum exchange at the atomic level. A large potential cutoff radius is used to determine the vapor and liquid densities at good accuracy. By tracking the motions of fluid molecules exchanged between the vapor and liquid phases and carrying out statistical analysis of residence time, penetration depth and especially velocity correlation, the mass and the velocity accommodation coefficients can be determined at the same time. The latter is based on the assumption that the reflected events take place over a short time, near the interface and thus atoms velocities are correlated while the condensation/evaporation events are not. The calculated coefficients are sensitive to the film thickness due to the presence of the quartz substrate and the layering effects. As temperature increases, atoms condense less and reflect more and the reflection is more diffusive due to increasing collision rate. Near the critical temperature, both the condensation and momentum accommodation coefficients vary significantly.

Research on electron and positron spectrum in the high-energy region based on the gluon condensation model

Electron (positron), proton and nuclei can be accelerated to very high energy by local supernova remnants (SNR). The famous excesses of electron and proton (nuclei) potentially come from such kind of local sources. Recently, the DAMPE experiment measured the electron spectrum (including both electrons and positrons) of cosmic rays with high-accuracy. It provides an opportunity to further explore the excess of electrons. According to the gluon condensation (GC) theory, once GC occurs, huge number of gluons condense at a critical momentum, and the production spectra of electron and proton showing typical GC characteristics. There are exact correlations between the electron and proton spectrum from a same GC process. It is possible to interpret the power-law break of cosmic rays in view of GC phenomenon, and predict one from another based on the relations between electron and proton spectrum. In this work, we point out the potential existence of a second excess in the electron spectrum, the characteristics of this excess is derived from experimental data of proton. We hope that the future DAMPE experiments will confirm the existence of this second excess and support the result of GC model.

Pines’ demon observed as a 3D acoustic plasmon in Sr2RuO4

The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a ‘demon’, could exist in three-dimensional (3D) metals containing more than one species of charge carrier1. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral and do not couple to light, so have never been detected in an equilibrium, 3D metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals2, optical properties of metal nanoparticles3, soundarons in Weyl semimetals4 and high-temperature superconductivity in, for example, metal hydrides3,5–7. Here, we present evidence for a demon in Sr2RuO4 from momentum-resolved electron energy-loss spectroscopy. Formed of electrons in the β and γ bands, the demon is gapless with critical momentum qc = 0.08 reciprocal lattice units and room-temperature velocity v = (1.065 ± 0.12) × 105 m s−1 that undergoes a 31% renormalization upon cooling to 30 K because of coupling to the particle–hole continuum. The momentum dependence of the intensity of the demon confirms its neutral character. Our study confirms a 67-year old prediction and indicates that demons may be a pervasive feature of multiband metals.

Transport Equation Transition Modeling in Cerans For Hypersonic Flows

Scope of present work covers implementation to validation of the Langtry-Menter two equation γ-Reθt Local Correlation based Transition Model (LCTM) in CERANS code for modeling subsonic to hypersonic flow transition. The LCTM is coupled with SST k-ω turbulence model. The governing equations of γ-Reθt model is discretized in finite volume framework similar to the RANS model and implicitized using the point Jacobi method. Transition correlations based on freestream as well as local turbulence intensity and critical momentum thickness Reynolds number were integrated with the model, and the code is validated for several standard transition test cases involving low subsonic to hypersonic high enthalpy flows covering a wide range of turbulent intensities.

Dynamical excitation of spin-orbit coupled spin-1 Bose-Einstein condensate in a narrow box trap

We demonstrate the dynamics of a spin-1 ferromagnetic Bose-Einstein condensate (BEC) with Rashba spin-orbit (SO) coupling in a narrow square box trap when an obstacle potential is moving in it. The ground-state energy would be enhanced with the box trap becomes narrower. From the Bogoliubov analytical excitation spectrum lines, we obtain the dependence of critical excitation velocity and momentum on the trap geometry under different SO coupling. Numerically, when the obstacle moves along the direction of plane wave ground state, in addition to spin-density waves being excited periodically, a long density belt accompanied by density island chain is found ahead of the obstacle. Such density distribution is mainly determined by the narrow box trap. A series of density islands arranged more closely are generated as the obstacle moves along opposite direction. There exists another velocity threshold for different density excitation under a larger SO coupling.

From Petitions to Demands, Restitution to Representation

From Petitions to Demands, Restitution to Representation

Lifetime of excitations in atomic and molecular Bose-Einstein condensates

Recent experimental progress has produced Molecular Superfluids (MSF) in thermal equilibrium; this opens the door to a new class of experiments investigating the associated thermodynamic and dynamical responses. We review the theoretical picture of the phase diagram and quasiparticle spectrum in the Atomic Superfluid (ASF) and MSF phases. We further compute the parametric dependence of the quasiparticle lifetimes at one-loop order. In the MSF phase, the $U(1)$ particle number symmetry breaks to $\mathbb{Z}_2$ and the spectrum exhibits a gapless Goldstone mode in addition to a gapped $\mathbb{Z}_2$-protected atom-like mode. In the ASF phase, the $U(1)$ symmetry breaks completely, leaving behind a Goldstone mode and an unprotected gapped mode. In both phases, the Goldstone mode decays with a rate given by the celebrated Belyaev result, as in a single component condensate. In the MSF phase, the gapped mode is sharp up to a critical Cherenkov momentum beyond which it emits phonons. In the ASF phase, the gapped mode decays with a constant rate even at small momenta. These decay rates govern the spectral response in microtrap tunneling experiments and lead to sharp features in the transmission spectrum of atoms fired through molecular clouds.

Review on Recent Strategies for Integrating Energy Storage Systems in Microgrids

Energy security and the resilience of electricity networks have recently gained critical momentum as subjects of research. The challenges of meeting the increasing electrical energy demands and the decarbonisation efforts necessary to mitigate the effects of climate change have highlighted the importance of microgrids for the effective integration of renewable energy sources. Microgrids have been the focus of research for several years; however, there are still many unresolved challenges that need to be addressed. Energy storage systems are essential elements that provide reliability and stability in microgrids with high penetrations of renewable energy sources. This study provides a systematic review of the recent developments in the control and management of energy storage systems for microgrid applications. In the early sections, a summary of the microgrid topologies and architectures found in the recent literature is given. The main contributions and targeted applications by the energy storage systems in the microgrid applications is defined for each scenario. As various types of energy storage systems are currently being integrated for the reliable operation of the microgrids, the paper analyses the properties and limitations of the solutions proposed in the recent literature. The review that was carried out shows that a hybrid energy storage system performs better in terms of microgrid stability and reliability when compared to applications that use a simple battery energy storage system. Therefore, a case study for a DC microgrid with a hybrid energy storage system was modelled in MATLAB/Simulink. The presented results show the advantages of hybrid energy storage systems in DC microgrids.

The gluon condensation in hadron collisions

Gluons may converge to a stable state at a critical momentum in hadrons. This gluon condensation is predicted by a nonlinear QCD evolution equation. We review the understanding of the gluon condensation and present a clear physical picture that produces the gluon condensation from the colour glass condensate. We summarize the applications of the GC effect in the p−p(A) collisions and predict that the p−Pb and Pb−Pb collisions at the LHC are close to the energy region of the gluon condensation. We warn that for the next generation of hadron colliders with the increasing of the collision energy, the extremely strong gamma-rays will be emitted in a narrow space of the accelerator due to the gluon condensation effect. Such artificial mini gamma-ray bursts in the laboratory may damage the detectors.

Generative Dissensus in a Youth-Led Coalition-Building Enterprise

Context: Rooted in the principles of Youth Participatory Action Research (YPAR), this article explores how a team of youth community activists extended their coalition virtually to produce a bill of demands for structural social change in their city and its surrounding county. Focus of Study: Our inquiry focuses on how the youth’s dialectic goal of arriving at consensus (through debate fiercely, and intentionally, tied to the country’s own internal reckonings) loomed, uncomfortably at times, over the dialogic process of inviting youth from across the city to share experiences, exchange ideas, and build relationships. Setting: The youth’s coalitional work extended outward from the Yamacraw Center, a community literacy center and social justice organization based in a historic coastal city in the southern United States. Participants: The primary team, reported on here, was made up of eight youth researchers and two adult allies/co-researchers responsible for supporting youth as they engaged in YPAR. Research Design: Our study is a reflexive thematic analysis of the interplay between youth during 11 planning and organizing sessions. Data Collection: The data collected for this study are video digital Google Hangout meetings recorded over a five-month period. Findings: Our findings consider key strategies youth exhibited in coming together to hear one another, triage priorities, and carefully attend to the ways they needed to craft their arguments to be taken seriously. Conclusions: We highlight the importance of process in building youth capacity, detail discursive moves youth made to maintain critical momentum, and situate the project within a larger ethos that recognizes the immanent value of cultivating youth capacity to engage in fierce and humanizing exchanges with their peers in pursuit of collective progress.

Hydrodynamic response of an annular swirling liquid sheet surrounding a forced gas jet

Gas–liquid coaxial injectors invariably involve a shear layer between a fast-moving gas stream and a slow-moving liquid stream. This shear layer within the confinement of a recess has a region of absolute instability beyond a critical momentum flux ratio. This causes the spray to exhibit self-pulsation at discrete natural frequencies. We apply sinusoidal acoustic forcing to the gas jet over a range of frequencies and amplitudes to explore the frequency response of the spray. The fluctuating spray width near the injector orifice is measured as a time-resolved tracer characteristic, and its power spectral density is used to determine the spectral response of the spray. The non-pulsating spray that involves a primarily convectively unstable shear layer responds unconditionally to all the forcing frequencies. However, for a self-pulsating spray, when the forcing frequency is far from the natural frequency and both are incommensurate, the spectral response involves both the frequencies, their linear combinations and harmonics representing a state of quasi-periodicity. When the forcing frequency is close enough or the amplitude is high enough, 1 : 1 lock-in is observed where the natural mode is suppressed completely and the spray behaves just like the unforced flow with the peak shifted to the forcing frequency. Combining the experimental observations and application of the van der Pol oscillator model, we could demonstrate the analogous behaviour of this multiphase system with other hydrodynamically self-excited systems with external forcing. The results presented here can prove significant in understanding the dynamics of the atomization process in thermoacoustic coupling and possible control of it.

Reconstructing the dynamical quantum phase transitions via dimensional expansion in a generalized Su-Schrieffer-Heeger model.

We know that a one-dimensional (1D) modulated system can simulate 2D topological states by expanding the dimension. This scenario provides a justifiable avenue to test the dilatation of the dynamical quantum phase transition (DQPT). Through a generalized Su-Schrieffer-Heeger model, we have shown how the Loschmidt echo, Fisher zero, and Dynamical topological order parameter (DTOP) transit from one to two dimensions. Owing to the introduced pseudomomentum, the derivative of the return rate does not always capture the DQPT well, but the Fisher zero and the DTOP can be treated as faithful indicators. A topology-independent parameter will also affect the occurrence of the DQPTs for quenches inside a given phase. Moreover, a comparison with the Haldane model owning the same phase diagram implies that a pair of fixed points will lead to different critical momentum distributions, thus different robustness, further reminding us that the correspondences between the equilibrium and dynamical phases transitions are multifarious.

Intrinsic Superconducting Diode Effect.

Stimulated by the recent experiment [F. Ando et al., Nature (London) 584, 373 (2020).NATUAS0028-083610.1038/s41586-020-2590-4], we propose an intrinsic mechanism to cause the superconducting diode effect (SDE). SDE refers to the nonreciprocity of the critical current for the metal-superconductor transition. Among various mechanisms for the critical current, the depairing current is known to be intrinsic to each material and has recently been observed in several superconducting systems. We clarify the temperature scaling of the nonreciprocal depairing current near the critical temperature and point out its significant enhancement at low temperatures. It is also found that the nonreciprocal critical current shows sign reversals upon increasing the magnetic field. These behaviors are understood by the nonreciprocity of the Landau critical momentum and the change in the nature of the helical superconductivity. The intrinsic SDE unveils the rich phase diagram and functionalities of noncentrosymmetric superconductors.