arXiv subjects

Peng Zou

Publications and source records attributed to Peng Zou.

At least 55 records · Page 3Linked to original sources

arXiv2020

Dynamical structure factors of a two-dimensional Fermi superfluid within random phase approximation

Based on random phase approximation (RPA), we numerically calculate dynamical structure factors of a balanced two-dimensional (2D) Fermi superfluid, and discuss their energy, momentum and interaction strength dependence in the 2D BEC-BCS crossover. At a small transferred momentum, a stable Higgs mode is observed in the unitary 2D Fermi superfluid gas where the particle-hole symmetry is not satisfied. Stronger interaction strength will make the visibility of the dispersion of Higgs mode harder to be observed. We also discuss the dimension effect and find that the signal of the Higgs mode in two dimension is more obvious than that in 3D case. At a large transferred momentum regime, stronger interaction strength will induce the weight of the molecules excitation increasing, while in verse the atomic one decreasing, which shows the pairing information of Fermi superfluid. The theoretical results qualitatively agree with the corresponding Quantum Monte Carlo data.

cond-mat.quant-gas↗

arXiv2020

Angular Stripe Phase in Spin-Orbital-Angular-Momentum Coupled Bose Condensates

We propose that novel superfluid with supersolid-like properties - angular stripe phase - can be realized in a pancake-like spin-1/2 Bose gas with spin-orbital-angular-momentum coupling. We predict a rich ground-state phase diagram, including the vortex-antivortex pair phase, half-skyrmion phase, and two different angular stripe phases. The stripe phases feature modulated angular density-density correlation with sizable contrast and can occupy a relatively large parameter space. The low-lying collective excitations, such as the dipole and breathing modes, show distinct behaviors in different phases. The existence of the novel stripe phase is also clearly indicated in the energetic and dynamic instabilities of collective modes near phase transitions. Our predictions of the angular stripe phase could be readily examined in current cold-atom experiments with $^{87}$Rb and $^{41}$K.

cond-mat.quant-gas↗

arXiv2020

On Age and Value of Information in Status Update Systems

Motivated by the inherent value of packets arising in many cyber-physical applications (e.g., due to precision of the information content or an alarm message), we consider status update systems with update packets carrying values as well as their generation time stamps. Once generated, a status update packet has a random initial value and a deterministic deadline after which it is not useful (ultimate staleness). In our model, value of a packet decreases in time (even after reception) starting from its generation to ultimate staleness when it vanishes. The value of information (VoI) at the receiver is additive in that the VoI is the sum of the current values of all packets held by the receiver. We investigate various queuing disciplines under potential dependence between value and service time and provide closed form expressions for average VoI at the receiver. Numerical results illustrate the average VoI for different scenarios and the contrast between average age of information (AoI) and average VoI.

cs.IT↗

arXiv2020

Maintaining Information Freshness in Power-Efficient Status Update Systems

This paper is motivated by emerging edge computing systems which consist of sensor nodes that acquire and process information and then transmit status updates to an edge receiver for possible further processing. As power is a scarce resource at the sensor nodes, the system is modeled as a tandem computation-transmission queue with power-efficient computing. Jobs arrive at the computation server with rate $λ$ as a Poisson process with no available data buffer. The computation server can be in one of three states: (i) OFF: the server is turned off and no jobs are observed or processed, (ii) ON-Idle: the server is turned on but there is no job in the server, (iii) ON-Busy: the server is turned on and a job is processed in the server. These states cost zero, one and $p_c$ units of power, respectively. Under a long-term power constraint, the computation server switches from one state to another in sequence: first a deterministic $T_o$ time units in OFF state, then waiting for a job arrival in ON-Idle state and then in ON-Busy state for an independent identically distributed compute time duration. The transmission server has a single unit data buffer to save incoming packets and applies last come first serve with discarding as well as a packet deadline to discard a sitting packet for maintaining information freshness, which is measured by the Age of Information (AoI). Additionally, there is a monotonic functional relation between the mean time spent in ON-Busy state and the mean transmission time. We obtain closed-form expressions for average AoI and average peak AoI. Our numerical results illustrate various regimes of operation for best AoI performances optimized over packet deadlines with relation to power efficiency.

cs.IT↗

arXiv2020

Genomic Problems Involving Copy Number Profiles: Complexity and Algorithms

Recently, due to the genomic sequence analysis in several types of cancer, the genomic data based on {\em copy number profiles} ({\em CNP} for short) are getting more and more popular. A CNP is a vector where each component is a non-negative integer representing the number of copies of a specific gene or segment of interest. In this paper, we present two streams of results. The first is the negative results on two open problems regarding the computational complexity of the Minimum Copy Number Generation (MCNG) problem posed by Qingge et al. in 2018. It was shown by Qingge et al. that the problem is NP-hard if the duplications are tandem and they left the open question of whether the problem remains NP-hard if arbitrary duplications are used. We answer this question affirmatively in this paper; in fact, we prove that it is NP-hard to even obtain a constant factor approximation. We also prove that the parameterized version is W[1]-hard, answering another open question by Qingge et al. The other result is positive and is based on a new (and more general) problem regarding CNP's. The \emph{Copy Number Profile Conforming (CNPC)} problem is formally defined as follows: given two CNP's $C_1$ and $C_2$, compute two strings $S_1$ and $S_2$ with $cnp(S_1)=C_1$ and $cnp(S_2)=C_2$ such that the distance between $S_1$ and $S_2$, $d(S_1,S_2)$, is minimized. Here, $d(S_1,S_2)$ is a very general term, which means it could be any genome rearrangement distance (like reversal, transposition, and tandem duplication, etc). We make the first step by showing that if $d(S_1,S_2)$ is measured by the breakpoint distance then the problem is polynomially solvable.

cs.DS↗

arXiv2020

Data-driven MHD Simulation of the Formation and Initiation of a Large-scale Pre-flare Magnetic Flux Rope in Solar Active Region 12371

Solar eruptions are the most powerful drivers of space weather. To understand their cause and nature, it is crucial to know how the coronal magnetic field evolves before eruption. Here we study the formation process of a relatively large-scale magnetic flux rope (MFR) in active region NOAA~12371 that erupts with a major flare and coronal mass ejection on 2015 June 21. A data-driven numerical magnetohydrodynamic model is employed to simulate three-dimensional coronal magnetic field evolution of one-day duration before the eruption. Comparison between the observed features and our modeled magnetic field discloses how the pre-eruption MFR forms. Initially, the magnetic field lines were weakly twisted as being simple sheared arcades. Then a long MFR was formed along the polarity inversion line due to the complex photospheric motion, which is mainly shearing rather than twisting. The presence of the MFR is evidenced by a coherent set of magnetic field lines with twist number above unity. Below the MFR a current sheet is shown in the model, suggesting that tether-cutting reconnection plays a key role in the MFR formation. The MFR's flux grows as more and more field lines are twisted due to continuous injection of magnetic helicity by the photospheric motions. Meanwhile, the height of the MFR's axis increases monotonely from its formation. By an analysis of the decay index of its overlying field, we suggest that it is because the MFR runs into the torus instability regime and becomes unstable that finally triggers the eruption.

astro-ph.SR↗

arXiv2020

Continuous Null-Point Magnetic Reconnection Builds Up a Torus Unstable Magnetic Flux Rope Triggering the X9.3 Flare in Solar Active Region~12673

Two X-class solar flares occurred on 2017 September 6 from active region NOAA 12673: the first one is a confined X2.2 flare, and it is followed only $\sim 3$ hours later by the second one, which is the strongest flare in solar cycle 24, reaching X9.3 class and accompanied with a coronal mass ejection. Why these two X-class flares occurred in the same position with similar magnetic configurations, but one is eruptive while the other is not? Here we track the coronal magnetic field evolution via nonlinear force-free field extrapolations from a time sequence of vector magnetograms with high cadence. A detailed analysis of the magnetic field shows that a magnetic flux rope (MFR) forms and grows gradually before the first flare, and shortly afterwards, the MFR's growth is significantly enhanced with a much faster rise in height, from far below the threshold of torus instability to above it, while the magnetic twist only increases mildly. Combining EUV observations and the magnetic field extrapolation, we found that overlying the MFR is a null-point magnetic topology, where recurrent brightening is seen after the first flare. We thus suggest a scenario to interpret the occurrence of the two flares. The first flare occurred since the MFR reached a high enough height to activate the null point, and its continuous expansion forces the null-point reconnection recurrently. Such reconnection weakens the overlying field, allowing the MFR to rise faster, which eventually crosses the threshold of torus instability and triggers the second, eruptive flare.

astro-ph.SR↗

arXiv2019

Optimizing Information Freshness Through Computation-Transmission Tradeoff and Queue Management in Edge Computing

Edge computing applications typically require generated data to be preprocessed at the source and then transmitted to an edge server. In such cases, transmission time and preprocessing time are coupled, yielding a tradeoff between them to achieve the targeted objective. This paper presents analysis of such a system with the objective of optimizing freshness of received data at the edge server. We model this system as two queues in tandem whose service times are independent over time but the transmission service time is monotonically dependent on the computation service time in mean value. This dependence captures the natural decrease in transmission time due to lower offloaded computation. We analyze various queue management schemes in this tandem queue where the first queue has a single server, Poisson packet arrivals, general independent service and no extra buffer to save incoming status update packets. The second queue has a single server receiving packets from the first queue and service is memoryless. We consider the second queue in two forms: (i) No data buffer and (ii) One unit data buffer and last come first serve with discarding. We analyze various non-preemptive as well as preemptive cases. We perform stationary distribution analysis and obtain closed form expressions for average age of information (AoI) and average peak AoI. Our numerical results illustrate analytical findings on how computation and transmission times could be traded off to optimize AoI and reveal a consequent tradeoff between average AoI and average peak AoI.

cs.IT↗

arXiv2019

A Study of Pre-Flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes

Magnetic flux ropes (MFRs) are thought to be the central structure of solar eruptions, and their ideal MHD instabilities can trigger the eruption. Here we performed a study of all the MFR configurations that lead to major solar flares, either eruptive or confined, from 2011 to 2017 near the solar disk center. The coronal magnetic field is reconstructed from observed magnetograms, and based on magnetic twist distribution, we identified the MFR, which is defined as a coherent group of magnetic field lines winding an axis with more than one turn. It is found that 90% of the events possess pre-flare MFRs, and their three-dimensional structures are much more complex in details than theoretical MFR models. We further constructed a diagram based on two parameters, the magnetic twist number which controls the kink instability (KI), and the decay index which controls the torus instability (TI). It clearly shows lower limits for TI and KI thresholds, which are $n_{\rm crit} = 1.3$ and $|T_w|_{\rm crit} = 2$, respectively, as all the events above $n_{\rm crit}$ and nearly 90% of the events above $|T_w|_{\rm crit}$ erupted. Furthermore, by such criterion, over 70% of the events can be discriminated between eruptive and confined flares, and KI seems to play a nearly equally important role as TI in discriminating between the two types of flare. There are more than half of events with both parameters below the lower limits, and 29% are eruptive. These events might be triggered by magnetic reconnection rather than MHD instabilities.

astro-ph.SR↗

arXiv2019

A Statistical Study of Solar Filament Eruptions That Forms High-Speed Coronal Mass Ejections

Coronal mass ejections (CMEs) play a decisive role in driving space weather, especially, the fast ones (e.g., with speeds above $800$~km~s$^{-1}$). Understanding the trigger mechanisms of fast CMEs can help us gaining important information in forecasting them. The filament eruptions accompanied with CMEs provide a good tracer in studying the early evolution of CMEs. Here we surveyed 66 filament-accompanied fast CMEs to analyse the correlation between the trigger mechanisms, namely either magnetic reconnection or ideal MHD process, associated flares, and CME speeds. Based on the data gathering from SDO, GONG and STEREO, we find that: (1) Active region (AR) filament and intermediate filaments (IFs) eruptions show a higher probability for producing fast CMEs than quiet Sun (QS) filaments, while the probability of polar crown (PC) filament eruptions is zero in our statistic; (2) AR filament eruptions that produce fast CMEs are more likely triggered by magnetic reconnection, while QS and IFs are more likely triggered by ideal MHD process; (3) For AR filaments and IFs, it seems that the specific trigger mechanism does not have a significant influence on the resulted CME speeds, while for the QS filaments, the ideal MHD mechanism can more likely generate a faster CME; (4) Comparing with previous statistic study, the onset heights of filament eruptions and the decay indexes of the overlying field show some differences: for AR filaments and IFs, the decay indexes are larger and much closer to the theoretical threshold, while for QS filaments, the onset heights are higher than those obtained in previous results.

astro-ph.SR↗

arXiv2019

Relative Age of Information: A New Metric for Status Update Systems

In this paper, we introduce a new data freshness metric, relative Age of Information (rAoI), and examine it in a single server system with various packet management schemes. The (classical) AoI metric was introduced to measure the staleness of status updates at the receiving end with respect to their generation at the source. This metric addresses systems where the timings of update generation at the source are absolute and can be designed separately or jointly with the transmission schedules. In many decentralized applications, transmission schedules are blind to update generation timing, and the transmitter can know the timing of an update packet only after it arrives. As such, an update becomes stale after a new one arrives. The rAoI metric measures how fresh the data is at the receiver with respect to the data at the transmitter. It introduces a particularly explicit dependence on the arrival process in the evaluation of age. We investigate several queuing disciplines and provide closed form expressions for rAoI and numerical comparisons.

cs.IT↗

arXiv2019

Trading Off Computation with Transmission in Status Update Systems

This paper is motivated by emerging edge computing applications in which generated data are pre-processed at the source and then transmitted to an edge server. In such a scenario, there is typically a tradeoff between the amount of pre-processing and the amount of data to be transmitted. We model such a system by considering two non-preemptive queues in tandem whose service times are independent over time but the transmission service time is dependent on the computation service time in mean value. The first queue is in M/GI/1/1 form with a single server, memoryless exponential arrivals, general independent service and no extra buffer to save incoming status update packets. The second queue is in GI/M/1/2* form with a single server receiving packets from the first queue, memoryless service and a single data buffer to save incoming packets. Additionally, mean service times of the first and second queues are dependent through a deterministic monotonic function. We perform stationary distribution analysis in this system and obtain closed form expressions for average age of information (AoI) and average peak AoI. Our numerical results illustrate the analytical findings and highlight the tradeoff between average AoI and average peak AoI generated by the tandem nature of the queueing system with dependent service times.

cs.IT↗

arXiv2019

The Tandem Duplication Distance is NP-hard

In computational biology, tandem duplication is an important biological phenomenon which can occur either at the genome or at the DNA level. A tandem duplication takes a copy of a genome segment and inserts it right after the segment - this can be represented as the string operation $AXB \Rightarrow AXXB$. For example, Tandem exon duplications have been found in many species such as human, fly or worm, and have been largely studied in computational biology. The Tandem Duplication (TD) distance problem we investigate in this paper is defined as follows: given two strings $S$ and $T$ over the same alphabet, compute the smallest sequence of tandem duplications required to convert $S$ to $T$. The natural question of whether the TD distance can be computed in polynomial time was posed in 2004 by Leupold et al. and had remained open, despite the fact that tandem duplications have received much attention ever since. In this paper, we prove that this problem is NP-hard. We further show that this hardness holds even if all characters of $S$ are distinct. This is known as the exemplar TD distance, which is of special relevance in bioinformatics. One of the tools we develop for the reduction is a new problem called the Cost-Effective Subgraph, for which we obtain W[1]-hardness results that might be of independent interest. We finally show that computing the exemplar TD distance between $S$ and $T$ is fixed-parameter tractable. Our results open the door to many other questions, and we conclude with several open problems.

cs.DS↗

arXiv2019

Waiting before Serving: A Companion to Packet Management in Status Update Systems

In this paper, we explore the potential of server waiting before packet transmission in improving the Age of Information (AoI) in status update systems. We consider a non-preemptive queue with Poisson arrivals and independent general service distribution and we incorporate waiting before serving in two packet management schemes: M/GI/1/1 and M/GI/1/$2^*$. In M/GI/1/1 scheme, the server waits for a deterministic time immediately after a packet enters the server. In M/GI/1/$2^*$ scheme, depending on idle or busy system state, the server waits for a deterministic time before starting service of the packet. In both cases, if a potential newer arrival is captured existing packet is discarded. Different from most existing works, we analyze AoI evolution by indexing the incoming packets, which is enabled by an alternative method of partitioning the area under the evolution of instantaneous AoI to calculate its time average. We obtain expressions for average and average peak AoI for both queueing disciplines with waiting. Our numerical results demonstrate that waiting before service can bring significant improvement in average age, particularly, for heavy-tailed service distributions. This improvement comes at the expense of an increase in average peak AoI. We highlight the trade-off between average and average peak AoI generated by waiting before serving.

cs.IT↗

arXiv2019

Can Injection Model Replenish the Filaments in Weak Magnetic Environment?

We observed an H$α$ surge occurred in the active region NOAA 12401 on 2015 August 17, and discuss its trigger mechanism, kinematic and thermal properties. It is suggested that this surge is caused by a chromospheric reconnection which ejects cool and dense material with the transverse velocity about 21-28 km s$^{-1}$ and the initial doppler velocity of 12 km s$^{-1}$. This surge is similar to the injection of newly formed filament materials from their footpoints, except that the surge here occurred in a relatively weak magnetic environment of ~100 G. Thus we discuss the possibility of filament material replenishment via the erupting mass in such a weak magnetic field, which is often associated with quiescent filaments. It is found that the local plasma can be heated up to about 1.3 times of original temperature, which results in an acceleration about -0.017 km s$^{-2}$. It can lift the dense material up to 10 Mm and higher with a inclination angle smaller than 50$^{\circ}$, namely typical height of active region filaments. But it can hardly inject the material up to those filaments higher than 25 Mm, namely some quiescent filaments. Thus we think injection model does not work well in the formation of quiescent filaments.

astro-ph.SR↗

arXiv2018

A Two-Step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673

Solar flares are often associated with coronal eruptions, but there are confined ones without eruption, even for some X-class flares. How such large flares occurred and why they are confined are still not well understood. Here we studied a confined X2.2 flare in NOAA 12673 on 2017 September 6. It exhibits two episodes of flare brightening with rather complex, atypical ribbons. Based on topology analysis of extrapolated coronal magnetic field, we revealed that there is a two-step magnetic reconnection process during the flare. Prior to the flare, there is a magnetic flux rope (MFR) with one leg rooted in a rotating sunspot. Neighboring to the leg is a magnetic null-point structure. The sunspot drives the MFR to expand, pushing magnetic flux to the null point, and reconnection is first triggered there. The disturbance from the null-point reconnection triggers the second reconnection, i.e., a tether-cutting reconnection below the rope. However, these two reconnections failed to produce an eruption, because the rope is firmly held by its strapping flux. Furthermore, we compared this flare with an eruptive X9.3 flare in the same region with 2 hours later, which has a similar MFR configuration. The key difference between them is that, for the confined flare, the MFR is fully below the threshold of torus instability, while for the eruptive one, the MFR reaches entirely above the threshold. This study provides a good evidence supporting that reconnection alone may not be able to trigger eruption, rather, MHD instability plays a more important role.

astro-ph.SR↗

arXiv2018

Magnetohydrodynamic Simulation of the X9.3 Flare on 2017 September 6: Evolving Magnetic Topology

Three-dimensional magnetic topology is crucial to understanding the explosive release of magnetic energy in the corona during solar flares. Much attention has been given to the pre-flare magnetic topology to identify candidate sites of magnetic reconnection, yet it is unclear how the magnetic reconnection and its attendant topological changes shape the eruptive structure and how the topology evolves during the eruption. Here we employed a realistic, data-constrained magnetohydrodynamic simulation to study the evolving magnetic topology for an X9.3 eruptive flare that occurred on 2017 September 6. The simulation successfully reproduces the eruptive features and processes in unprecedented detail. The numerical results reveal that the pre-flare corona contains multiple twisted flux systems with different connections, and during the eruption, these twisted fluxes form a coherent flux rope through tether-cutting-like magnetic reconnection below the rope. Topological analysis shows that the rising flux rope is wrapped by a quasi-separatrix layer, which intersects itself below the rope, forming a topological structure known as hyperbolic flux tube, where a current sheet develops, triggering the reconnection. By mapping footpoints of the newly-reconnected field lines, we are able to reproduce both the spatial location and, for the first time, the temporal separation of the observed flare ribbons, as well as the dynamic boundary of the flux rope's feet. Futhermore, the temporal profile of the total reconnection flux is comparable to the soft X-ray light curve. Such a sophisticated characterization of the evolving magnetic topology provides important insight into the eventual understanding and forecast of solar eruptions.

astro-ph.SR↗

arXiv2018

Observation of quantum phase transition in spin-orbital-angular-momentum coupled Bose-Einstein condensate

Orbital angular momentum (OAM) of light represents a fundamental optical freedom that can be exploited to manipulate quantum state of atoms. In particular, it can be used to realize spin-orbital-angular-momentum (SOAM) coupling in cold atoms by inducing an atomic Raman transition using two laser beams with differing OAM. Rich quantum phases are predicted to exist in many-body systems with SOAM coupling. Their observations in laboratory, however, are often hampered by the limited control of the system parameters. In this work we report, for the first time, the experimental observation of the ground-state quantum phase diagram of the SOAM coupled Bose-Einstein condensate (BEC). The discontinuous variation of the spin polarization as well as the vorticity of the atomic wave function across the phase boundaries provides clear evidence of first-order phase transitions. Our results open up a new way to the study of phase transitions and exotic quantum phases in quantum gases.

cond-mat.quant-gas↗