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P. Zhang

Publications and source records attributed to P. Zhang.

At least 37 records · Page 2Linked to original sources
arXiv2025

Missing odd-order Shapiro steps do not uniquely indicate fractional Josephson effect

Topological superconductivity is expected to spur Majorana zero modes -- exotic states that are also considered a quantum technology asset. Fractional Josephson effect is their manifestation in electronic transport measurements, often under microwave irradiation. A fraction of induced resonances, known as Shapiro steps, should vanish, in a pattern that signifies the presence of Majorana modes. Here we report patterns of Shapiro steps expected in topological Josephson junctions, such as the missing first Shapiro step, or several missing odd-order steps. But our junctions, which are InAs quantum wells with Al contacts, are studied near zero magnetic field, meaning that they are not in the topological regime. We also observe other patterns such as missing even steps and several missing steps in a row, not relevant to topological superconductivity. Potentially responsible for our observations is rounding of not fully developed steps superimposed on non-monotonic resistance versus voltage curves, but several origins may be at play. Our results demonstrate that any single pattern, even striking, cannot uniquely identify topological superconductivity, and a multifactor approach is necessary to unambiguously establish this important phenomenon.

cond-mat.mes-hall↗
arXiv2025

Development and Characterization of a High-Resolution and High-Sensitivity Collinear Resonance Ionization Spectroscopy Setup

With the recent implementation of a radio-frequency quadrupole (RFQ) cooler-buncher and a multi-step laser resonance ionization technique, our previously developed collinear laser spectroscopy setup has been successfully upgraded into a fully functional collinear resonance ionization spectroscopy system. The new system was fully characterized using a bunched ion beam at 30~keV, during which hyperfine structure spectra of $^{85,87}$Rb isotopes were measured. An overall efficiency exceeding 1:200 (one resonant ion detected for every 200 ions after the RFQ cooler-buncher) was achieved while maintaining a spectral resolution of 100 MHz. Under these conditions, the extracted hyperfine structure parameters and isotope shift for $^{85,87}$Rb show excellent agreement with the literature values. These results demonstrate the system's capability to perform high-resolution and high-sensitivity laser spectroscopy of neutron-rich Rb isotopes, which are expected to be produced at the Beijing Radioactive Ion-beam Facility at a rate of approximately 100 particles per second.

physics.ins-det↗
arXiv2025

Source location and evolution of a multi-lane type II radio burst

Shocks in the solar corona can accelerate electrons that in turn generate radio emission known as type II radio bursts. The characteristics and morphology of these radio bursts in the dynamic spectrum reflect the evolution of the shock itself, together with the properties of the local corona where it propagates. In this work, we study the evolution of a complex type II radio burst showing a multi-lane structure, to find the locations where the radio emission is produced and relate them to the properties of the local environment. Using radio imaging, we track separately each lane composing the type II burst and relate the position of the emission to the properties of the ambient medium such as density, Alfven speed, and magnetic field. We show that the radio burst morphology in the dynamic spectrum changes with time and it is related to the complexity of the local environment. The initial stage of the radio emission show a single lane in the spectrum, while the latter stages of the radio signature evolve in a multi-lane scenario. The radio imaging reveals how the initial stage of the radio emission separates with time into different locations along the shock front as the density and orientation of the magnetic field change along the shock propagation. At the time where the spectrum shows a multi-lane shape, we found a clear separation of the imaged radio sources. By combining radio imaging with the properties of the local corona, we described the evolution of a type II radio burst and, for the first time, identified three distinct radio emission regions above the CME front. Two regions were located at the flanks, producing earlier radio emission than the central position, in accordance with the complexity of density and Alfven speed values in the regions where radio emission is generated. This unprecedented observation provides new insights into the nature of multi-lane type II bursts.

astro-ph.SR↗
arXiv2025

Imaging and spectropolarimetric observations of a band-split type II solar radio burst with LOFAR

Type II solar radio bursts are generated by electrons accelerated by coronal shock waves. They appear in dynamic spectra as lanes drifting from higher to lower frequencies at the plasma frequency and its harmonic. These lanes can often be split into two or more sub-bands that have similar drift rates. This phenomenon is called band-splitting, and its physical origins are still under debate. Our aim is to investigate the origin of band-splitting using novel imaging and spectropolarimetric observations of a type II solar radio burst from the Low Frequency Array (LOFAR). We used LOFAR imaging at multiple frequencies and time steps to track the locations of the radio sources corresponding to the two components of the band-split emission lane. In addition, we estimated the degree of circular polarisation (dcp) for both components using LOFAR's full Stokes dynamic spectra. From the imaging of the type II burst, we found two close but clearly separated emission regions clustered over several frequencies spanning each split band. One emission region corresponds to the lower frequency band and the other to the higher frequency band of the split lane. Using the full Stokes dynamic spectra, we also found the dcp to be very similar for both bands. The two distinct emission regions suggest that the split bands originate from two separate regions at the shock. The similar values of dcp for both sub-bands correspond to similar values of magnetic field strength in the two regions and indicate little to no change in the emission region plasma. Thus, our findings are in contradiction with previous theories, which have suggested that split bands originate in the same region but upstream and downstream of the shock. Instead, our results suggest that both bands originate in two separate upstream regions since we find a clear separation in locations and no magnetic compression.

astro-ph.SR↗
arXiv2025

Evidence of mini-jet emission in a large emission zone from a magnetically-dominated gamma-ray burst jet

The second brightest GRB in history, GRB230307A, provides an ideal laboratory to study the mechanism of GRB prompt emission thanks to its extraordinarily high photon statistics and its single episode activity. Here we demonstrate that the rapidly variable components of its prompt emission compose an overall broad single pulse-like profile. Although these individual rapid components are aligned in time across all energy bands, this overall profile conspires to show a well-defined energy-dependent behavior which is typically seen in single GRB pulses. Such a feature demonstrates that the prompt emission of this burst is from many individual emitting units that are casually linked in a emission site at a large distance from the central engine. Such a scenario is in natural consistency with the internal-collision-induced magnetic reconnection and turbulence framework, which invokes many mini-jets due to local magnetic reconnection that constantly appear and disappear in a global magnetically-dominated jet.

astro-ph.HE↗
arXiv2025

New insight into the hard X-ray emission influenced by the type-\uppercase\expandafter{\romannumeral1} bursts observed by Insight-HXMT during outburst of 4U 1636--536

By analyzing the data from Insight-HXMT and NICER, we can determine the evolution of the significance of the hard shortage in 4U 1636--536 with its spectral state, as well as the evolution of the fraction of deficit with energy. Additionally, we investigate the possible geometry and evolution of the corona in 4U 1636-536 by combining our findings with the results of spectral analysis. We find that during the soft state, the significance of possible hard X-ray shortage in bursts is almost zero. However, in the hard state, some bursts exhibit significant shortages (>3 $σ$), while others do not. We attempt to establish a correlation between the significance of the hard X-ray shortage and the spectral parameters, but the data quality and the limited number of bursts prevent us from finding a strong correlation. For bursts with insignificant shortages in the soft state, their fraction of the deficit remains small. However, in the hard state, the fraction of deficit for all bursts increases with energy, regardless of the significance of the shortage of individual bursts. For bursts during the hard state, we investigate the evolution of the fraction of deficit during the bursts by stacking the peaks and decays of the bursts, respectively, and find that as the flux of the bursts decreases, the energy corresponding to the maximum of the fraction of deficit becomes progressively higher. We explore the possible geometry and evolution of the corona clued by the evolution of the fraction of deficit, which is obtained from the spectral and temporal analysis.

astro-ph.HE↗
arXiv2025

Resolving spatial and temporal shock structures using LOFAR observations of type II radio bursts

Collisionless shocks are one of the most powerful particle accelerators in the Universe. In the heliosphere, type II solar radio bursts are signatures of electrons accelerated by collisionless shocks launched at the Sun. Spectral observations of these bursts show a variety of fine structures often composing multiple type II lanes. The origin of these lanes and structures is not well understood and has been attributed to the inhomogeneous environment around the propagating shock. Here, we aim to determine the large-scale local structures near a coronal shock wave using high-resolution radio imaging observations of a complex type II radio burst observed on 3 October 2023. By using inteferometric imaging from the Low Frequency Array (LOFAR), combined with extreme ultraviolet observations, we investigate the origin of multiple type II lanes at low frequencies (30--80~MHz) relative to the propagating shock wave. We identify at least three radio sources at metric wavelengths corresponding to a multi-lane type II burst. The type II burst sources propagate outwards with a shock driven by a coronal mass ejection. We find a double radio source that exhibits increasing separation over time, consistent with the expansion rate of the global coronal shock. This suggests that the overall shock expansion is nearly self-similar, with acceleration hotspots forming at various times and splitting at a rate proportional to the shock's expansion. Our results show the importance of increased spatial resolution in determining either the small-scale spatial properties in coronal shocks or the structuring of the ambient medium. Possible shock corrugations or structuring of the upstream plasma at the scale of 10$^5$~km can act as hotspots for the acceleration of suprathermal electrons.

astro-ph.SR↗
arXiv2024

Evidence of $ϕ$0-Josephson junction from skewed diffraction patterns in Sn-InSb nanowires

We study Josephson junctions based on InSb nanowires with Sn shells. We observe skewed critical current diffraction patterns: the maxima in forward and reverse current bias are at different magnetic flux, with a displacement of 20-40 mT. The skew is greatest when the external field is nearly perpendicular to the nanowire, in the substrate plane. This orientation suggests that spin-orbit interaction plays a role. We develop a phenomenological model and perform tight-binding calculations, both methods reproducing the essential features of the experiment. The effect modeled is the $ϕ$0-Josephson junction with higher-order Josephson harmonics. The system is of interest for Majorana studies: the effects are either precursor to or concomitant with topological superconductivity. Current-phase relations that lack inversion symmetry can also be used to design quantum circuits with engineered nonlinearity.

cond-mat.mes-hall↗
arXiv2024

Search for solar axions by Primakoff effect with the full dataset of the CDEX-1B Experiment

We present the first limit on $g_{Aγ}$ coupling constant using the Bragg-Primakoff conversion based on an exposure of 1107.5 kg days of data from the CDEX-1B experiment at the China Jinping Underground Laboratory. The data are consistent with the null signal hypothesis, and no excess signals are observed. Limits of the coupling $g_{Aγ}<2.08\times10^{-9}$ GeV$^{-1}$ (95\% C.L.) are derived for axions with mass up to 100 eV/$c^2$. Within the hadronic model of KSVZ, our results exclude axion mass $>5.3~\rm{eV}/c^2$ at 95\% C.L.

hep-ex↗
arXiv2024

Flattening-off of droplet bouncing trend under high ambient gas pressures

It was previously observed that colliding liquid droplets in a gaseous medium tend to bounce off at elevated gas pressure up to about 12 atm. In this letter, we extended the droplet collision experiment to up to 41 atm for the first time and reported a noticeable discovery that the tendency is flattened off at higher pressures. The colliding droplets stop bouncing but start to coalesce beyond a critical Weber number, which increases with pressure but tends to a limit value at 21 atm and above. A scaling analysis taking into account the gas-film dynamics, the rarefied gas effects, and van der Waals force well correlates with the experimental discovery.

physics.flu-dyn↗
arXiv2024

α_s from DIS data with large $x$ resummations

The deep inelastic scattering data on the nucleon F_2 structure function, accumulated by BCDMS, SLAC and NMC collaborations in fixed-target experiments, are analyzed in the non-singlet approximation within the frameworks of both conventional \overline{\mathbf{MS}} scheme as well as those with resummations of logarithms at large Bjorken x values. The use of the latter is important because they greatly modify the values of the twist four corrections while leaving a strong coupling constant almost intact.

hep-ph↗
arXiv2023

Large Second-Order Josephson Effect in Planar Superconductor-Semiconductor Junctions

We investigate the current-phase relations of Al/InAs-quantum well planar Josephson junctions fabricated using nanowire shadowing technique. Based on several experiments, we conclude that the junctions exhibit an unusually large second-order Josephson harmonic, the $\sin(2φ)$ term. First, superconducting quantum interference devices (dc-SQUIDs) show half-periodic oscillations, tunable by gate voltages as well as magnetic flux. Second, Josephson junction devices exhibit kinks near half-flux quantum in supercurrent diffraction patterns. Third, half-integer Shapiro steps are present in the junctions. Similar phenomena are observed in Sn/InAs quantum well devices. We perform data fitting to a numerical model with a two-component current phase relation. Analysis including a loop inductance suggests that the sign of the second harmonic term is negative. The microscopic origins of the observed effect remain to be understood. We consider alternative explanations which can account for some but not all of the evidence.

cond-mat.mes-hall↗
arXiv2023

A Phase-resolved View of the Low-frequency Quasiperiodic Oscillations from the Black Hole Binary MAXI J1820+070

Although low-frequency quasiperiodic oscillations (LFQPOs) are commonly detected in the X-ray light curves of accreting black hole X-ray binaries, their origin still remains elusive. In this study, we conduct phase-resolved spectroscopy in a broad energy band for LFQPOs in MAXI J1820+070 during its 2018 outburst, utilizing Insight-HXMT observations. By employing the Hilbert-Huang transform method, we extract the intrinsic quasiperiodic oscillation (QPO) variability, and obtain the corresponding instantaneous amplitude, phase, and frequency functions for each data point. With well-defined phases, we construct QPO waveforms and phase-resolved spectra. By comparing the phase-folded waveform with that obtained from the Fourier method, we find that phase folding on the phase of the QPO fundamental frequency leads to a slight reduction in the contribution of the harmonic component. This suggests that the phase difference between QPO harmonics exhibits time variability. Phase-resolved spectral analysis reveals strong concurrent modulations of the spectral index and flux across the bright hard state. The modulation of the spectral index could potentially be explained by both the corona and jet precession models, with the latter requiring efficient acceleration within the jet. Furthermore, significant modulations in the reflection fraction are detected exclusively during the later stages of the bright hard state. These findings provide support for the geometric origin of LFQPOs and offer valuable insights into the evolution of the accretion geometry during the outburst in MAXI J1820+070.

astro-ph.HE↗
arXiv2023

REWAFL: Residual Energy and Wireless Aware Participant Selection for Efficient Federated Learning over Mobile Devices

Participant selection (PS) helps to accelerate federated learning (FL) convergence, which is essential for the practical deployment of FL over mobile devices. While most existing PS approaches focus on improving training accuracy and efficiency rather than residual energy of mobile devices, which fundamentally determines whether the selected devices can participate. Meanwhile, the impacts of mobile devices' heterogeneous wireless transmission rates on PS and FL training efficiency are largely ignored. Moreover, PS causes the staleness issue. Prior research exploits isolated functions to force long-neglected devices to participate, which is decoupled from original PS designs. In this paper, we propose a residual energy and wireless aware PS design for efficient FL training over mobile devices (REWAFL). REW AFL introduces a novel PS utility function that jointly considers global FL training utilities and local energy utility, which integrates energy consumption and residual battery energy of candidate mobile devices. Under the proposed PS utility function framework, REW AFL further presents a residual energy and wireless aware local computing policy. Besides, REWAFL buries the staleness solution into its utility function and local computing policy. The experimental results show that REW AFL is effective in improving training accuracy and efficiency, while avoiding "flat battery" of mobile devices.

cs.LG↗
arXiv2023

"Smoking gun" signatures of topological milestones in trivial materials by measurement fine-tuning and data postselection

Exploring the topology of electronic bands is a way to realize new states of matter with possible implications for information technology. Because bands cannot always be observed directly, a central question is how to tell that a topological regime has been achieved. Experiments are often guided by a prediction of a unique signal or a pattern, called "the smoking gun". Examples include peaks in conductivity, microwave resonances, and shifts in interference fringes. However, many condensed matter experiments are performed on relatively small, micron or nanometer-scale, specimens. These structures are in the so-called mesoscopic regime, between atomic and macroscopic physics, where phenomenology is particularly rich. In this paper, we demonstrate that the trivial effects of quantum confinement, quantum interference and charge dynamics in nanostructures can reproduce accepted smoking gun signatures of triplet supercurrents, Majorana modes, topological Josephson junctions and fractionalized particles. The examples we use correspond to milestones of topological quantum computing: qubit spectroscopy, fusion and braiding. None of the samples we use are in the topological regime. The smoking gun patterns are achieved by fine-tuning during data acquisition and by subsequent data selection to pick non-representative examples out of a fluid multitude of similar patterns that do not generally fit the "smoking gun" designation. Building on this insight, we discuss ways that experimentalists can rigorously delineate between topological and non-topological effects, and the effects of fine-tuning by deeper analysis of larger volumes of data.

cond-mat.mes-hall↗
arXiv2023

Precise lifetime measurement of $^4_Λ$H hypernucleus using in-flight $^4$He$(K^-, π^0)^4_Λ$H reaction

We present a new measurement of the $^4_Λ$H hypernuclear lifetime using in-flight $K^-$ + $^4$He $\rightarrow$ $^4_Λ$H + $π^0$ reaction at the J-PARC hadron facility. We demonstrate, for the first time, the effective selection of the hypernuclear bound state using only the $γ$-ray energy decayed from $π^0$. This opens the possibility for a systematic study of isospin partner hypernuclei through comparison with data from ($K^-$, $π^-$) reaction. As the first application of this method, our result for the $^4_Λ$H lifetime, $τ(^4_Λ\mathrm{H}) = 206 \pm 8 (\mathrm{stat.}) \pm 12 (\mathrm{syst.})\ \mathrm{ps}$, is one of the most precise measurements to date. We are also preparing to measure the lifetime of the hypertriton ($^3_Λ$H) using the same setup in the near future.

nucl-ex↗
arXiv2023

Intermittent QPO properties of MAXI J1820+070 revealed by Insight-HXMT

We investigate the dynamical properties of low frequency quasi-periodic oscillations (QPOs) observed from the black hole X-ray binary MAXI J1820+070 during the early part of its 2018 outburst, when the system was in a bright hard state. To this aim, we use a series of observations from the Hard X-ray Modulation Telescope Insight-HXMT, and apply a wavelet decomposition (weighted wavelet Z-transforms) to the X-ray light-curve. We find that the QPO phenomenon is intermittent within each individual observation, with some sub-intervals where the oscillation is strongly detected (high root-mean-square amplitude) and others where it is weak or absent. The average life time of individual QPO segments is ~ 5 oscillation cycles, with a 3 sigma tail up to ~ 20 cycles. There is no substantial difference between the energy spectra during intervals with strong and weak/absent QPOs. We discuss two possible reasons for the intermittent QPO strength, within the precessing jet model previously proposed for MAXI J1820+070. In the rigid precession model, intermittent QPOs are predicted to occur with a coherence Q ~ a few when the disk alignment time-scale is only a few times the precession time-scale. Alternatively, we suggest that changes in oscillation amplitude can be caused by changes in the jet speed. We discuss a possible reason for the intermittent QPO strength, within the precessing jet model previously proposed for MAXI J1820+070: we suggest that changes in oscillation amplitude are caused by changes in the jet speed. We argue that a misaligned, precessing jet scenario is also consistent with other recent observational findings that suggest an oscillation of the Compton reflection component in phase with the QPOs.

astro-ph.HE↗
arXiv2023

Multilinear fractional Calderón-Zygmund operators with Dini type kernel

In this paper, the main purpose is to consider a number of results concerning boundedness of multilinear fractional Calderón-Zygmund operators with kernels of mild regularity. Let $T_α$ be a multilinear fractional Calderón-Zygmund operators of type $ω(t)$ with $ω$ being nondecreasing and $ω\in \text{Dini}(1)$. The end-point weak-type estimates for multilinear operator $T_α$ are obtained. Moreover, some boundedness properties of the multilinear fractional operators are also established on variable exponent Lebesgue spaces.

math.CA↗