arXiv subjects

Z. J. Li

Publications and source records attributed to Z. J. Li.

At least 19 recordsLinked to original sources

arXiv2026

MIGHTEE/COSMOS-3D: The discovery of three spectroscopically confirmed radio-selected star-forming galaxies at z=4.9-5.6

Radio observations offer a dust-independent probe of star formation and active galactic nucleus (AGN) activity, but sufficiently deep data are required to access the crossover luminosity between these processes at high redshift ($z>4.5$). We present three spectroscopically confirmed high-redshift radio sources (HzRSs) detected at 1.3 GHz at $z=4.9$-$5.6$, with radio luminosities spanning $L_{\rm 1.3 \, GHz}\approx2$-$5\times10^{24} \, \rm W \, Hz^{-1}$. These sources were first identified as high-redshift candidates through spectral energy distribution (SED) fitting of archival Hubble, JWST NIRCam+MIRI, and ground-based photometry, and then spectroscopically confirmed via the $\rm H\,α$ emission line using wide-field slitless spectroscopy from JWST COSMOS-3D. The star formation rates (SFRs) measured from SED fitting, the $\rm H\,α$ flux, and the 1.3 GHz luminosity, span $\sim100$-$1800\, M_{\odot} \, \rm yr^{-1}$, demonstrating broad agreement between these SFR tracers. We find that these three sources lie either on or $0.5$-1.0 dex above the star-forming main sequence at $z=4$-6 and have undergone a recent burst of star formation. The sources have extended rest-UV/optical morphologies with no evidence for a dominant point source component, indicating that an AGN is unlikely to dominate their rest-UV and optical emission. Two of the sources have complex, multi-component rest-frame UV/optical morphologies, suggesting that their starbursts may be triggered by merging activity. These HzRSs open up a new window towards probing radio emission powered by star formation alone at $z> 4.5$, representing a remarkable opportunity to begin tracing star formation, independent of dust, in the early Universe.

astro-ph.GA↗

arXiv2026

Correlation between superfluid density and transition temperature in infinite-layer nickelate superconductor $Nd_{1-x}Sr_xNiO_2$

A strong correlation between zero-temperature superfluid density ($ρ_{s0}$) and transition temperature ($T_c$) is considered as a hallmark of unconventional superconductivity. However, their relationship has yet to be unveiled in nickelates due to sample inhomogeneity. Here we perform local susceptometry on an infinite-layer nickelate superconductor $Nd_{0.8}Sr_{0.2}NiO_2$. The sample shows inhomogeneous superfluid density and $T_c$ on micron-scale. The spatial statistics for different scan areas reveal a linear dependence of local $T_c$ on $ρ_{s0}$ for $T_c$>8 K and a sub-linear one for $T_c$<8 K. Remarkably, the overall relationship is reminiscent of that reported in overdoped cuprate superconductors, hinting at a close connection between them.

cond-mat.supr-con↗

arXiv2025

Pure nematic transition inside the superconducting dome of iron chalcogenide superconductor FeSe$_{1-x}$Te$_x$

Nematicity and magnetism are prevalent orders in high transition temperature (Tc) superconductors, coexisting in the parent compound of most material families. Quantum fluctuations of nematicity or spin orders are both plausible candidates for mediating unconventional Cooper pairing. Identifying the sole effect of a nematic quantum critical point (QCP) on the emergence of superconducting dome without interference of spin fluctuations is therefore highly desirable. The iron chalcogenide superconductor FeSe exhibits pure nematicity without any magnetic ordering. A nematic quantum phase transition can be induced by Te substitution but experimental study of such transition is so far limited to its normal state. By performing local susceptometry on composition-spread FeSe$_{1-x}$Te$_x$ films ($0 < x < 1$) using scanning Superconducting Quantum Interference Device (sSQUID) microscopy, we investigate the superfluid density ($ρ_s$) across the pure nematic transition in extremely fine steps of $Δx$ = 0.0008. The temperature dependence of $ρ_s$ changes from the form of anisotropic pairing on the nematic side to an isotropic one across the critical doping $x_c$. The power-law dependence of gap anisotropy on $|x - x_c|$ provides evidence for nematic quantum criticality under the superconducting dome. The low-temperature $ρ_s$ scales linearly with Tc in the nematic phase $x < x_c$, whereas the gap amplitude, maximized at $x_c$, determines the Tc for $x>x_c$. Our results establish a pure nematic QCP in FeSe$_{1-x}$Te$_x$, separating two superconducting orders with distinct pairing boosted by nematic quantum fluctuations.

cond-mat.supr-con↗

arXiv2024

Observation of vortex stripes in UTe$_2$

Quantum vortices are fundamentally important for properties of superconductors. In conventional type-II superconductor they determine the magnetic response of the system and tend to form regular lattices. UTe$_2$ is a recently discovered heavy fermion superconductor exhibiting many anomalous macroscopic behaviors. However, the question whether it has a multicomponent order parameter remains open. Here, we study magnetic properties of UTe$_2$ by employing scanning superconducting quantum interference device microscopy. We find vortex behavior which is very different from that in ordinary superconductors. We imaged vortices generated by cooling in magnetic field applied along different crystalline directions. While a small out-of-plane magnetic field produces typical isolated vortices, higher field generates vortex stripe patterns which evolve with vortex density. The stripes form at different locations and along different directions in the surface plane when the vortices are crystalized along the crystalline b or c axes. The behavior is reproduced by our simulation based on an anisotropic two-component order parameter. This study shows that UTe$_2$ has a nontrivial disparity of multiple length scales, placing constraints on multicomponent superconductivity. The tendency of vortex stripe formation and their control by external field may be useful in fluxonics applications.

cond-mat.supr-con↗

arXiv2024

Observation of single-quantum vortex splitting in the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductor

Since their theoretical discovery more than a half-century ago, vortices observed in bulk superconductors have carried a quantized value of magnetic flux determined only by fundamental constants. A recent experiment reported 'unquantized' quantum vortices carrying the same fraction of flux quantum in Ba$_{0.23}$K$_{0.77}$Fe$_2$As$_2$ in a small temperature range below its superconducting critical temperature ($T_C$). Here, we use scanning superconducting quantum interference device (sSQUID) microscopy with improved sensitivity to investigate the genesis of fractional vortices in Ba$_{0.23}$K$_{0.77}$Fe$_2$As$_2$. We report the direct observation of a single-flux quantum vortex splitting into two different fractions with increasing temperature. The flux of the two fractions has opposite dependence on temperature, while the total flux sums up to one flux quantum despite their spatial separation. Overall, our study shows the existence of different fractional vortices and their stability in temperature ranging from 0.1 to 0.99 $T_C$. Besides the implications of this observation for the fundamental question of quantum vorticity, the discovery of these objects paves the way for the new platform for anyon quasiparticles and applications for fractional fluxonics.

cond-mat.supr-con↗

arXiv2023

Observation of long-range ferromagnetism via anomalous supercurrents in a spin-orbit coupled superconductor

Conventional superconductors naturally disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. In superconductors with strong Rashba spin-orbit coupling, impurity magnetic moments induce supercurrents through the spin-galvanic effect. As a result, long-range ferromagnetic interaction among the impurity moments may be mediated through such anomalous supercurrents in a similar fashion as in itinerant ferromagnets. Fe(Se,Te) is such a superconductor with topological surface bands, previously shown to exhibit quantum anomalous vortices around impurity spins. Here, we take advantage of the flux sensitivity of scanning superconducting quantum interference devices to investigate superconducting Fe(Se,Te) in the regime where supercurrents around impurities overlap. We find homogeneous remanent flux patterns after applying a supercurrent through the sample. The patterns are consistent with anomalous edge and bulk supercurrents generated by in-plane magnetization, which occur above a current threshold and follow hysteresis loops reminiscent of those of a ferromagnet. Similar long-range magnetic orders can be generated by Meissner current under a small out-of-plane magnetic field. The magnetization weakens with increasing temperature and disappears after thermal cycling to above superconducting critical temperature; further suggesting superconductivity is central to establishing and maintaining the magnetic order. These observations demonstrate surface anomalous supercurrents as a mediator for ferromagnetism in a spin-orbit coupled superconductor, which may potentially be utilized for low-power cryogenic memory.

cond-mat.supr-con↗

arXiv2023

A self-sustaining mechanism for Internal Transport Barrier formation in HL-2A tokamak plasmas

The formation of Internal Transport Barrier (ITB) is studied in HL-2A plasmas by means of nonlinear gyrokinetic simulations. A new paradigm for the ITB formation is proposed in which different physics mechanisms play a different role depending on the ITB formation stage. In the early stage, fast ions, introduced by Neutral Beam Injection (NBI) ion system, are found to stabilize the thermal-ion-driven instability by dilution, thus reducing the ion heat fluxes and finally triggering the ITB. Such dilution effects, however, play a minor role after the ITB is triggered as electromagnetic effects are dominant in the presence of established high pressure gradients. We define the concept of ITB self-sustainment, as the low turbulence levels found within the fully formed ITB are consequences of large scale zonal flows, which in turn are fed by a non-linear interplay with large scale high frequency electromagnetic perturbations destabilized by the ITB itself.

physics.plasm-ph↗

arXiv2022

Suppression and revival of superconducting phase coherence in monolayer FeSe/SrTiO$_3$

Monolayer FeSe grown on SrTiO$_3$ (FeSe/STO) is an interfacial high temperature superconductor distinctively different from bulk FeSe. Due to the fragility of this two-dimensional system in the atmosphere, the investigation of its intrinsic superconductivity and intertwined orders has largely been limited to surface-sensitive charge probes compatible with ultra-high vacuum environment. However, the superconducting phase coherence of the interface is challenging to probe. Here, we perform in-situ mutual inductance in ultra-high vacuum on FeSe/STO in combination with band mapping by angle-resolved photoemission spectroscopy (ARPES). We find that even though the monolayer showed a gap-closing temperature above 50 K, surprisingly no diamagnetism is visible down to 5 K. This is the case for few-layer FeSe/STO until it exceeds a critical number of 5 layers where diamagnetism suddenly appears. But the superfluid density does not saturate down to the base temperature in these thick samples. On the other hand, the suppression of diamagnetism in the few-layer FeSe/STO can be lifted by depositing a FeTe layer on top. The superconducting transition is much sharper than that in the thick FeSe/STO. However, Tc and superfluid density both decrease with increasing FeTe thickness. Shining ultraviolet light on the FeTe/FeSe/STO heterostructure enhances Tc similarly independent of the FeSe thickness, showing that the diamagnetism originates at the FeSe/STO interface. Our observation may be understood by a scenario in which interfacial superconducting phase coherence is highly anisotropic.

cond-mat.supr-con↗

arXiv2019

Insight-HXMT observations of 4U~1636-536: Corona cooling revealed with single short type-I X-ray burst

Corona cooling was detected previously from stacking a series of short type-I bursts occurred during the low/had state of atoll outburst. Type-I bursts are hence regarded as sharp probe to our better understanding on the basic property of the corona. The launch of the first Chinese X-ray satellite Insight-HXMT has large detection area at hard X-rays which provide almost unique chance to move further in this research field. We report the first detection of the corona cooling by Insight-HXMT from single short type-I burst showing up during {\bf flare} of 4U 1636-536. This type-I X-ray burst has a duration of $\sim$13 seconds and hard X-ray shortage is detected with significance 6.2~$σ$ in 40-70 keV. A cross-correlation analysis between the lightcurves of soft and hard X-ray band, shows that the corona shortage lag the burst emission by 1.6 $\pm$1.2~s. These results are consistent with those derived previously from stacking a large amount of bursts detected by RXTE/PCA within a series of {\bf flares} of 4U 1636-536. Moreover, the broad bandwidth of Insight-HXMT allows as well for the first time to infer the burst influence upon the continuum spectrum via performing the spectral fitting of the burst, which ends up with the finding that hard X-ray shortage appears at around 40 keV in the continuum spectrum. These results suggest that the evolution of the corona along with the outburst{\bf /flare} of NS XRB may be traced via looking into a series of embedded type-I bursts by using Insight-HXMT.

astro-ph.HE↗

arXiv2019

Timing analysis of 2S 1417-624 observed with NICER and Insight-HXMT

We present a study of timing properties of the accreting pulsar 2S 1417-624 observed during its 2018 outburst, based on Swift/BAT, Fermi/GBM, Insight-HXMT and NICER observations. We report a dramatic change of the pulse profiles with luminosity. The morphology of the profile in the range 0.2-10.0keV switches from double to triple peaks at $\sim2.5$ $\rm \times 10^{37}{\it D}_{10}^2\ erg\ s^{-1}$ and from triple to quadruple peaks at $\sim7$ $\rm \times 10^{37}{\it D}_{10}^2\ erg\ s^{-1}$. The profile at high energies (25-100keV) shows significant evolutions as well. We explain this phenomenon according to existing theoretical models. We argue that the first change is related to the transition from the sub to the super-critical accretion regime, while the second to the transition of the accretion disc from the gas-dominated to the radiation pressure-dominated state. Considering the spin-up as well due to the accretion torque, this interpretation allows to estimate the magnetic field self-consistently at $\sim7\times 10^{12}$G.

astro-ph.HE↗

arXiv2019

Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT

We report on analysis of observations of the bright transient X-ray pulsar \src obtained during its 2017-2018 giant outburst with Insight-HXMT, \emph{NuSTAR}, and \textit{Swift} observatories. We focus on the discovery of a sharp state transition of the timing and spectral properties of the source at super-Eddington accretion rates, which we associate with the transition of the accretion disk to a radiation pressure dominated (RPD) state, the first ever directly observed for magnetized neutron star. This transition occurs at slightly higher luminosity compared to already reported transition of the source from sub- to super-critical accretion regime associate with onset of an accretion column. We argue that this scenario can only be realized for comparatively weakly magnetized neutron star, not dissimilar to other ultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further evidence for this conclusion is provided by the non-detection of the transition to the propeller state in quiescence which strongly implies compact magnetosphere and thus rules out magnetar-like fields.

astro-ph.HE↗

arXiv2019

The brightening of the pulsar wind nebula of PSR B0540--69 after its spin-down rate transition

It is believed that an isolated pulsar loses its rotational energy mainly through a relativistic wind consisting of electrons, positrons and possibly Poynting flux\cite{Pacini1973,Rees1974,Kennel1984}. As it expands, this wind may eventually be terminated by a shock, where particles can be accelerated to energies of X-ray synchrotron emission, and a pulsar wind nebula (PWN) is usually detectable surrounding a young energetic pulsar\cite{Pacini1973,Rees1974,Kennel1984}. However, the nature and/or energetics of these physical processes remain very uncertain, largely because they typically cannot be studied in a time-resolved fashion. Here we show that the X-ray PWN around the young pulsar PSR B0540--69 brightens gradually up to 32$\pm8\%$ over the mean previous flux, after a sudden spin-down rate ($\dotν$) transition (SRT) by $\sim36\%$\ in December 2011, which has very different properties from a traditional pulsar glitch\cite{Marshall2015}. No evidence is seen for any change in the pulsed X-ray emission. We conclude that the SRT results from a sudden change in the pulsar magnetosphere that increases the pulsar wind power and hence the PWN X-ray emission. The X-ray light curve of the PWN suggests a mean life time of the particles of $397\pm374$\,days, corresponding to a magnetic field strength of $0.78_{-0.28}^{+4.50}$\,mG in the PWN.

astro-ph.HE↗

arXiv2019

Photoemission Spectroscopic Evidence for the Dirac Nodal Line in Monoclinic Semimetal SrAs$_3$

Topological nodal-line semimetals with exotic quantum properties are characterized by symmetry-protected line-contact bulk band crossings in the momentum space. However, in most of identified topological nodal-line compounds, these topological non-trivial nodal lines are enclosed by complicated topological trivial states at the Fermi energy ($E_F$), which would perplex their identification and hinder further applications. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide compelling evidence for the existence of Dirac nodal-line fermions in the monoclinic semimetal SrAs$_3$, which are close to $E_F$ and away from distraction of complex trivial Fermi surfaces or surface states. Our calculation indicates that two bands with opposite parity are inverted around \emph{Y} near $E_F$, which results in the single nodal loop at the $Γ$-\emph{Y}-\emph{S} plane with a negligible spin-orbit coupling effect. We track these band crossings and then unambiguously identify the complete nodal loop quantitatively, which provides a critical experimental support to the prediction of nodal-line fermions in the CaP$_3$ family of materials. Hosting simple topological non-trivial bulk electronic states around $E_F$ and no interfering with surface states on the natural cleavage plane, SrAs$_3$ is expected to be a potential platform for topological quantum state investigation and applications.

cond-mat.mes-hall↗

arXiv2019

In-orbit demonstration of X-ray pulsar navigation with the Insight-HXMT satellite

In this work, we report the in-orbit demonstration of X-ray pulsar navigation with Insight-Hard X-ray Modulation Telescope (Insight-HXMT), which was launched on Jun. 15th, 2017. The new pulsar navigation method 'Significance Enhancement of Pulse-profile with Orbit-dynamics' (SEPO) is adopted to determine the orbit with observations of only one pulsar. In this test, the Crab pulsar is chosen and observed by Insight-HXMT from Aug. 31th to Sept. 5th in 2017. Using the 5-day-long observation data, the orbit of Insight-HXMT is determined successfully with the three telescopes onboard - High Energy X-ray Telescope (HE), Medium Energy X-ray Telescope (ME) and Low Energy X-ray Telescope (LE) - respectively. Combining all the data, the position and velocity of the Insight-HXMT are pinpointed to within 10 km (3 sigma) and 10 m/s (3 sigma), respectively.

astro-ph.IM↗

arXiv2018

Insight-HXMT observations of the New Black Hole Candidate MAXI J1535-571: timing analysis

We present the X-ray timing results of the new black hole candidate (BHC) MAXI J1535-571 during its 2017 outburst from Hard X-ray Modulation Telescope (\emph{Insight}-HXMT) observations taken from 2017 September 6 to 23. Following the definitions given by \citet{Belloni2010}, we find that the source exhibits state transitions from Low/Hard state (LHS) to Hard Intermediate state (HIMS) and eventually to Soft Intermediate state (SIMS). Quasi-periodic oscillations (QPOs) are found in the intermediate states, which suggest different types of QPOs. With the large effective area of \emph{Insight}-HXMT at high energies, we are able to present the energy dependence of the QPO amplitude and centroid frequency up to 100 keV which is rarely explored by previous satellites. We also find that the phase lag at the type-C QPOs centroid frequency is negative (soft lags) and strongly correlated with the centroid frequency. By assuming a geometrical origin of type-C QPOs, the source is consistent with being a high inclination system.

astro-ph.HE↗

arXiv2018

Time evolution of the X-ray and gamma-ray fluxes of the Crab pulsar

We studied the evolution of the X-ray and gamma-ray spectra of the Crab pulsar utilizing the 11-year observations from the Rossi X-ray Timing Explorer (RXTE) and 9-year observations from the Fermi Gamma-ray Space Telescope (FGST). By fitting the spectrum of each observation, we obtained the corresponding flux, and then analysed the long term evolution of the X-ray (or gamma-ray) luminosities as well as their correlations with the spin down power of the pulsar. The X-ray flux in 5-60 keV obtained by the Proportional Counter Array (PCA) of RXTE decreases with a rate of (-2.4+/-0.4)*10^(-14) erg cm^(-2) s^(-1) per day. The X-ray flux in 15-250 keV obtained by the High Energy X-ray Timing Experiment (HEXTE) of RXTE and the gamma-ray flux in 0.1-300 GeV by the Large Area Telescope (LAT) onboard FGST show similar decreasing trend, but are unsignificant statistically. The 5--60 keV X-ray luminosity L_(X) is correlated with the spin down power L_(sd) by L_(X) propto L_(sd)^(1.6+/-0.3), which is similar to the statistical results for young pulsars.

astro-ph.HE↗

arXiv2015

Electronic structure of a superconducting topological insulator Sr-doped Bi2Se3

Using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, the atomic and low energy electronic structure of the Sr-doped superconducting topological insulators (SrxBi2Se3) was studied. Scanning tunneling microscopy shows that most of the Sr atoms are not in the van der Waals gap. After Sr doping, the Fermi level was found to move further upwards when compared with the parent compound Bi2Se3, which is consistent with the low carrier density in this system. The topological surface state was clearly observed, and the position of the Dirac point was determined in all doped samples. The surface state is well separated from the bulk conduction bands in the momentum space. The persistence of separated topological surface state combined with small Fermi energy makes this superconducting material a very promising candidate for the time reversal invariant topological superconductor

cond-mat.supr-con↗

arXiv2014

Fully Gapped s-wave-like Superconducting State and Electronic Structures in the Ir0.95Pd0.05Te2 Single Crystals with Strong Spin-orbital Coupling

Due to the large spin-orbital coupling in the layered 5d-transition metal chalcogenides compound, the occurrence of superconductivity in Ir2-xPdxTe2 offers a good chance to search for possible topological superconducting states in this system. We did comprehensive studies on the superconducting properties and electronic structures of single crystalline Ir0.95Pd0.05Te2 samples. The superconducting gap size, critical fields and coherence length along different directions were experimentally determined. Macroscopic bulk measurements and microscopic low temperature scanning tunneling spectroscopy results suggest that Ir0.95Pd0.05Te2 possesses a BCS-like s-wave state. No sign of zero bias conductance peak were found in the vortex core at 0.4K.

cond-mat.supr-con↗