1,721,296 research outputs found
Self-Adapted Floquet Dynamics of Ultracold Bosons in a Cavity
No full textIncludes supplementary materialFloquet dynamics of a quantum system subject to periodic modulations of system parameters provides a powerful tool for engineering new quantum matter with exotic properties. While system dynamics is significantly altered, the periodic modulation itself is usually induced externally and independent of Floquet dynamics. Here we propose a new type of Floquet physics for a Bose-Einstein condensate (BEC) subject to a shaken lattice generated inside a cavity, where the shaken lattice and atomic Floquet bands are mutually dependent, resulting in self-adapted Floquet dynamics. In particular, the shaken lattice induces Floquet quasienergy bands for the BEC, whose backaction leads to a self-adapted dynamical normal-superradiant phase transition for the shaken lattice. Such self-adapted Floquet dynamics shows two surprising and unique features: (i) The normal-superradiant phase transition possesses a hysteresis even without atom interactions. (ii) The dynamical atom-cavity steady state could exist at free energy maxima. The atom interactions strongly affect the phase transition of the BEC from zero to finite momenta. Our results provide a powerful platform for exploring self-adapted Floquet physics, which may open an avenue for engineering novel quantum materials. © 2018 American Physical Society.This work is supported by AFOSR (Grant No. FA9550-16-1-0387), the National Science Foundation (Grant No. PHY-1505496), and ARO (Grant No. W911NF-17-1-0128).School of Natural Sciences and Mathematic
Where is the cosmic-ray modulation boundary of the heliosphere?
No full textThe intensity of Galactic cosmic rays in the heliosphere is modulated by solar activities. The outer boundary where the solar modulation begins has always been a subject matter of debate in the cosmic-ray and heliophysics community. Various experimental methods and theoretical model calculations have been used to determine the boundary. Although the heliopause was always suspected to be the boundary, it is only until very recently after Voyager 1 had crossed the heliopause did we confirm that the boundary is indeed the heliopause. In this paper, we use a model simulation and detailed Voyager observation of cosmic rays at the heliopause crossing to show that the modulation boundary, in fact, is a fraction of an AU beyond the heliopause. Such a conclusion requires a very low turbulence level of the interstellar magnetic field in the outer heliosheath. According to the quasi-linear theory, a low level of turbulence should result in a very large diffusion coefficient parallel to the magnetic field and a very small perpendicular diffusion coefficient. For the first time, we are confident that Voyager 1 has obtained the truly pristine local interstellar cosmic-ray spectra down to the energies below 1 MeV. The cosmic-ray intensity is rapidly filtered by a thin layer of the interstellar magnetic field immediately outside of the heliopause. Its filtration amount depends on the conditions of magnetic fieldturbulence on the both sides of the heliopause, thus making it solar-cycle dependent as wel
Stability boundary analysis of grid-forming and grid-following inverters
No full textStability issues and oscillations associated with inverter-based resources (IBRs) are of increasing concern in inverter-dominated power systems. This study explores the stability boundary of grid-following (GFL) and grid-forming (GFM) inverters and performs a sensitivity analysis for the most relevant control and grid parameters. A stability boundary tracking (SBT) algorithm is proposed, which can identify the stability boundary relating up to three parameters of interest for both GFL and GFM inverters. This boundary is then illustrated as two-dimensional stability curve or three-dimensional stability surface to facilitate understanding the impact of certain parameters on the stability margin. A comprehensive sensitivity analysis of different controller parameters as well as power factor and line R/X ratio draws parallels on their impact to GFL and GFM stability
Spin-Tensor–Momentum-Coupled Bose-Einstein Condensates
No full textIncludes supplementary materialThe recent experimental realization of spin-orbit coupling for ultracold atomic gases provides a powerful platform for exploring many interesting quantum phenomena. In these studies, spin represents the spin vector (spin 1/2 or spin 1) and orbit represents the linear momentum. Here we propose a scheme to realize a new type of spin-tensor-momentum coupling (STMC) in spin-1 ultracold atomic gases. We study the ground state properties of interacting Bose-Einstein condensates with STMC and find interesting new types of stripe superfluid phases and multicritical points for phase transitions. Furthermore, STMC makes it possible to study quantum states with dynamical stripe orders that display density modulation with a long tunable period and high visibility, paving the way for the direct experimental observation of a new dynamical supersolidlike state. Our scheme for generating STMC can be generalized to other systems and may open the door for exploring novel quantum physics and device applications.Air Force Office of Scientific Research (FA9550-16-1-0387), National Science Foundation (PHY-1505496), and Army Research Office (W911NF- 17-1-0128).School of Natural Sciences and Mathematic
A numerical simulation of cosmic-ray modulation near the heliopause
Full text linkBased on a hybrid galactic cosmic-ray transport model, which incorporated MHD global heliospheric data into
Parker’s cosmic-ray transport equation, we studied the behavior of the transport of galactic cosmic rays and the
corresponding gradients in their flux near the heliopause (HP). We found that, (1) by increasing the ratio of the
parallel diffusion coefficient to the perpendicular diffusion coefficient in the interstellar magnetic field of the outer
heliosheath, the simulated radial flux near the HP increases as well. As the ratio multiplying factor reached 1010,
the radial flux experienced a sudden jump near the HP, similar to what Voyager 1 observed in 2012. (2) The effect
of changing the diffusion coefficients’ ratio on the radial flux variation depends on the energy of the cosmic rays,
the lower the energy, the more pronounced the effect is. (3) The magnitude of the diffusion coefficients also affect
the radial flux near the HP, the modulation beyond the HP varies by adjusting the magnitude multiplying facto
Modeling of DC-Side Dynamics in PV/Battery Grid-forming Inverter Systems
No full textThe DC-side dynamics of two-stage grid-forming (GFM) inverters are often neglected or oversimplified in power system studies, although they play a vital role in stability. Detailed models for the primary source and power converter are not practical for complexity reasons, leading usually to DC side representations that omit critical aspects such as the operational limits. To bridge this gap between accuracy and simplicity, this paper proposes for the first time an equivalent model for the DC side dynamics of GFM systems that captures all important dynamics and limits in a simple formulation. The model supports either batteries or photovoltaic arrays as a primary source and is easily parameterizable with minimal information, thus proving a useful tool for grid stability studies. Time-domain simulations validate the proposed equivalent model
A Numerical Study of Forbush Decreases with a 3D Cosmic-Ray Modulation Model Based on an SDE Approach
Full text linkBased on the reduced diffusion mechanism for producing Forbush decreases (Fds) in the heliosphere, we constructed a three-dimensional (3D) diffusion barrier, and by incorporating it into a stochastic differential equation (SDE) based time-dependent, cosmic-ray transport model, a 3D numerical model for simulating Fds is built and applied to a period of relatively quiet solar activity. This SDE model generally corroborates previous Fd simulations concerning the effects of the solar magnetic polarity, the tilt angle of the heliospheric current sheet (HCS), and cosmic-ray particle energy. Because the modulation processes in this 3D model are multi-directional, the barrier's geometrical features affect the intensity profiles of Fds differently. We find that both the latitudinal and longitudinal extent of the barrier have relatively fewer effects on these profiles than its radial extent and the level of decreased diffusion inside the disturbance. We find, with the 3D approach, that the HCS rotational motion causes the relative location from the observation point to the HCS to vary, so that a periodic pattern appears in the cosmic-ray intensity at the observing location. Correspondingly, the magnitude and recovery time of an Fd change, and the recovering intensity profile contains oscillation as well. Investigating the Fd magnitude variation with heliocentric radial distance, we find that the magnitude decreases overall and, additionally, that the Fd magnitude exhibits an oscillating pattern as the radial distance increases, which coincides well with the wavy profile of the HCS under quiet solar modulation condition
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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