arXiv subjects

Z. W. Li

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

At least 19 recordsLinked to original sources

arXiv2025

Dichotomy in Low- and High-energy Band Renormalizations in Trilayer Nickelate $La_{4}Ni_{3}O_{10}$: a Comparison with Cuprates

Band renormalizations comprise crucial insights for understanding the intricate roles of electron-boson coupling and electron correlation in emergent phenomena such as superconductivity. In this study, by combining high-resolution angle-resolved photoemission spectroscopy and theoretical calculations, we systematically investigate the electronic structure of the trilayer nickelate superconductor $La_{4}Ni_{3}O_{10}$ at ambient pressure. We reveal a dichotomy in the electronic band renormalizations of $La_{4}Ni_{3}O_{10}$ in comparison to cuprate superconductors. At a high energy scale of hundreds of meV, its band structure is strongly renormalized by electron correlation effect enhanced by Hund coupling. The resultant waterfall-like dispersions resemble the high-energy kinks in cuprate superconductors. However, at low energy scales of tens of meV, the dispersive bands are nearly featureless and devoid of any resolvable electron-boson interactions, in drastic contrast to the low-energy kinks observed in cuprates and other correlated 3d transition-metal compounds. The dichotomic band renormalizations highlight the disparity between nickelate and cuprate superconductors and emphasize the importance of strong electron-correlation in the superconductivity of Ruddlesden-Popper phase nickelates.

cond-mat.str-el↗

arXiv2025

New insight into the Rapid Burster by Insight-HXMT

We report the timing and spectral analyses upon of the type II X-ray bursts from the Rapid Burster (MXB 1730--335) observed by Insight-HXMT and Swift/XRT. By stacking the long-duration bursts, we find for the first time that the hard X-rays are lagging than the soft X-rays by 3 seconds. However, such a lag is not visible for the short-duration bursts, probably because of the poor statistics. For all bursts the energy spectrum is found to be non-thermal, thanks to the broad band coverage of Insight-HXMT. These findings put new insights into the type-II bursts and require a temporally showing-up corona for possible interpretation.

astro-ph.HE↗

arXiv2024

Superconductivity Discovered in Niobium Polyhydride at High Pressures

Niobium polyhydride was synthesized at high pressure and high temperature conditions by using diamond anvil cell combined with in situ high pressure laser heating techniques. High pressure electric transport experiments demonstrate that superconducting transition occurs with critical temperature(Tc) 42 K at 187 GPa. The shift of Tc as function of external applied magnetic field is in consistent to the nature of superconductivity while the upper critical field at zero temperature Hc2(0) is estimated to~16.8 Tesla while the GL coherent length ~57 angstrom is estimated. The structure investigation using synchrotron radiation implies that the observed superconductivity may come from Fm-3m phase of NbH3.

cond-mat.supr-con↗

arXiv2024

Superconductivity with Tc 116K discovered in antimony polyhydrides

Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was found that the antimony polyhydride samples show superconducting transition with critical temperature Tc 116 K at 184 GPa. The investigation of SC at magnetic field revealed that the superconducting coherent length ~40 angstroms based on Ginzburg Landau (GL) equation. Antimony polyhydride superconductor has the second highest Tc in addition to sulfur hydride among the polyhydrides of elements from main group IIIA to VIIA in periodic table.

cond-mat.supr-con↗

arXiv2024

Correlated Electronic Structure and Density-Wave Gap in Trilayer Nickelate La4Ni3O10

The discovery of pressurized superconductivity at 80 K in La3Ni2O7 officially brings nickelates into the family of high-temperature superconductors, which gives rise to not only new insights but also mysteries in the strongly correlated superconductivity. More recently, the sibling compound La4Ni3O10 was also shown to be superconducting below about 25 K under pressure, further boosting the popularity of nickelates in the Ruddlesden-Popper phase. In this study, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structures of La4Ni3O10 at ambient pressure. We reveal a high resemblance of La4Ni3O10 with La3Ni2O7 in the orbital-dependent fermiology and electronic structure, suggesting a similar electronic correlation between the two compounds. The temperature-dependent measurements imply an orbital-dependent energy gap related to the density-wave transition in La4Ni3O10. By comparing the theoretical pressure-dependent electronic structure, clues about the superconducting high-pressure phase can be deduced from the ambient measurements, providing crucial information for deciphering the unconventional superconductivity in nickelates.

cond-mat.supr-con↗

arXiv2023

Superconductivity Observed in Tantalum Polyhydride at High Pressure

We report experimental discovery of tantalum polyhydride superconductor. It was synthesized at high pressure and high temperature conditions using diamond anvil cell combined with in-situ high pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature Tc was found to be ~30 K at 197 GPa in the sample that was synthesized at the same pressure with ~2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that supports the superconductivity nature. The upper critical field at zero temperature μ0Hc2(0) of the superconducting phase is estimated to be ~20 T that corresponds to GL coherent length ~40 angstroms. Our results suggest that the superconductivity may arise from I-43d phase of TaH3. It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transitional metals.

cond-mat.supr-con↗

arXiv2022

Record High Tc Element Superconductivity Achieved In Titanium

It is challenging to search for high Tc superconductivity (SC) in transition metal elements wherein d electrons are usually not favored by conventional BCS theory. Here we report discovery of surprising SC up to 310 GPa with Tc above 20 K in wide pressure range from 108 GPa to 240 GPa in titanium. The maximum Tc^onset above 26 K and zero resistance Tc^zero of 21 K are record high values hitherto achieved among element superconductors. The Hc2(0) is estimated to be about 32 Tesla with coherence length 32 angstrom. The results show strong s-d transfer and d-band dominance, indicating correlation driven contributions to high Tc SC in dense titanium. This finding is in sharp contrast to the theoretical predications based on pristine electron-phonon coupling scenario. The study opens a fresh promising avenue for rational design and discovery of high Tc superconductors among simple materials via pressure tuned unconventional mechanism.

cond-mat.supr-con↗

arXiv2022

Superconductivity above 80 K in polyhydrides of hafnium

Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC was investigated by in-situ high pressure resistance measurements in applied magnetic fields. A superconducting transition with onset Tc ~83 K was observed at 243 GPa. The upper critical field Hc2(0) was estimated to be 24 Tesla by GL theory and the consequent superconducting coherent length to be ~37 angstrom. Our results suggest that the superconducting phase is from C2/m-HfH14. This is the first 5d transition metal polyhydride superconductor with Tc above the liquid nitrogen temperature.

cond-mat.supr-con↗

arXiv2022

Superconductivity in Zirconium Polyhydrides with Tc above 70 K

We report the experimental discovery of superconductivity with critical temperature Tc above 70 K in zirconium polyhydrides, the record high so far for 3d transition metal hydrides. The superconducting zirconium polyhydrides are synthesized at the extreme conditions above 200 GPa & 2000 K by using diamond anvil cell combined with in-situ laser heating. Superconductivity was observed in the samples maintained at the same pressure upon quenched from heating.

cond-mat.supr-con↗

arXiv2022

Peculiar disk behaviors of the black hole candidate MAXI J1348-630 in the hard state observed by Insight-HXMT and Swift

We present a spectral study of the black hole candidate MAXI J1348-630 during its 2019 outburst, based on monitoring observations with Insight-HXMT and Swift. Throughout the outburst, the spectra are well fitted with power-law plus disk-blackbody components. In the soft-intermediate and soft states, we observed the canonical relation L ~ T_in^4 between disk luminosity L and peak colour temperature T_in, with a constant inner radius R_in (traditionally identified with the innermost stable circular orbit). At other stages of the outburst cycle, the behaviour is more unusual, inconsistent with the canonical outburst evolution of black hole transients. In particular, during the hard rise, the apparent inner radius is smaller than in the soft state (and increasing), and the peak colour temperature is higher (and decreasing). This anomalous behaviour is found even when we model the spectra with self-consistent Comptonization models, which take into account the up-scattering of photons from the disk component into the power-law component. To explain both those anomalous trends at the same time, we suggest that the hardening factor for the inner disk emission was larger than the canonical value of ~1.7 at the beginning of the outburst. A more physical trend of radii and temperature evolution requires a hardening factor evolving from ~3.5 at the beginning of the hard state to ~1.7 in the hard intermediate state. This could be evidence that the inner disk was in the process of condensing from the hot, optically thin medium and had not yet reached a sufficiently high optical depth for its emission spectrum to be described by the standard optically-thick disk solution.

astro-ph.HE↗

arXiv2021

Search for Gamma-Ray Bursts and Gravitational Wave Electromagnetic Counterparts with High Energy X-ray Telescope of \textit{Insight}-HXMT

The High Energy X-ray telescope (HE) on-board the Hard X-ray Modulation Telescope (\textit{Insight}-HXMT) can serve as a wide Field of View (FOV) gamma-ray monitor with high time resolution ($μ$s) and large effective area (up to thousands cm$^2$). We developed a pipeline to search for Gamma-Ray Bursts (GRBs), using the traditional signal-to-noise ratio (SNR) method for blind search and the coherent search method for targeted search. By taking into account the location and spectrum of the burst and the detector response, the targeted coherent search is more powerful to unveil weak and sub-threshold bursts, especially those in temporal coincidence with Gravitational Wave (GW) events. Based on the original method in literature, we further improved the coherent search to filter out false triggers caused by spikes in light curves, which are commonly seen in gamma-ray instruments (e.g. \textit{Fermi}/GBM, \textit{POLAR}). We show that our improved targeted coherent search method could eliminate almost all false triggers caused by spikes. Based on the first two years of \textit{Insight}-HXMT/HE data, our targeted search recovered 40 GRBs, which were detected by either \textit{Swift}/BAT or \textit{Fermi}/GBM but too weak to be found in our blind search. With this coherent search pipeline, the GRB detection sensitivity of \textit{Insight}-HXMT/HE is increased to about 1.5E-08 erg/cm$^2$ (200 keV$-$3 MeV). We also used this targeted coherent method to search \textit{Insight}-HXMT/HE data for electromagnetic (EM) counterparts of LIGO-Virgo GW events (including O2 and O3a runs). However, we did not find any significant burst associated with GW events.

astro-ph.HE↗

arXiv2021

HXMT Identification of a non-thermal X-ray burst from SGR J1935+2154 and with FRB 200428

Fast radio bursts (FRBs) are short pulses observed in radio band from cosmological distances. One class of models invoke soft gamma-ray repeaters (SGRs), or magnetars, as the sources of FRBs. Some radio pulses have been observed from some magnetars, however, no FRB-like events had been detected in association any magnetar burst, including one giant flare. Recently, a pair of FRB-like bursts (FRB 200428 hereafter) separated by milliseconds (ms) were detected from the general direction of the Galactic magnetar SGR J1935+2154. Here we report the detection of a non-thermal X-ray burst in the 1-250 keV energy band with the Insight-HXMT satellite, which we identify as emitted from SGR J1935+2154. The burst showed two hard peaks with a separation of 34 ms, broadly consistent with that of the two bursts in FRB 200428. The delay time between the double radio and X-ray peaks is about 8.57 s, fully consistent with the dispersion delay of FRB 200428. We thus identify the non-thermal X-ray burst is associated with FRB 200428 whose high energy counterpart is the two hard peaks in X-ray. Our results suggest that the non-thermal X-ray burst and FRB 200428 share the same physical origin in an explosive event from SGR J1935+2154.

astro-ph.HE↗

arXiv2021

Physical origin of the nonphysical spin evolution of MAXI J1820+070

We report on the Insight-HXMT observations of the new black hole X-ray binary MAXI J1820+070 during its 2018 outburst. Detailed spectral analysis via the continuum fitting method shows an evolution of the inferred spin during its high soft sate. Moreover, the hardness ratio, the non-thermal luminosity and the reflection fraction also undergo an evolution, exactly coincident to the period when the inferred spin transition takes place. The unphysical evolution of the spin is attributed to the evolution of the inner disc, which is caused by the collapse of a hot corona due to condensation mechanism or may be related to the deceleration of a jet-like corona. The studies of the inner disc radius and the relation between the disc luminosity and the inner disc radius suggest that, only at a particular epoch, did the inner edge of the disc reach the innermost stable circular orbit and the spin measurement is reliable. We then constrain the spin of MAXI J1820+070 to be a*=0.2^{+0.2}_{-0.3}. Such a slowly spinning black hole possessing a strong jet suggests that its jet activity is driven mainly by the accretion disc rather than by the black hole spin.

astro-ph.HE↗

arXiv2021

Accretion Torque Reversals in GRO J1008-57 Revealed by Insight-HXMT

GRO J1008-57, as a Be/X-ray transient pulsar, is considered to have the highest magnetic field in known neutron star X-ray binary systems. Observational data of the X-ray outbursts in GRO J1008-57 from 2017 to 2020 were collected by the Insight-HXMT satellite. In this work, the spin period of the neutron star in GRO J1008-57 was determined to be about 93.28 seconds in August 2017, 93.22 seconds in February 2018, 93.25 seconds in June 2019 and 93.14 seconds in June 2020. GRO J1008-57 evolved in the spin-up process with a mean rate of $-(2.10\pm 0.05)\times$10$^{-4}$ s/d from 2009 -- 2018, and turned into a spin down process with a rate of $(6.7\pm 0.6)\times$10$^{-5}$ s/d from Feb 2018 to June 2019. During the type II outburst of 2020, GRO J1008-57 had the spin-up torque again. During the torque reversals, the pulse profiles and continuum X-ray spectra did not change significantly, and the cyclotron resonant scattering feature around 80 keV was only detected during the outbursts in 2017 and 2020. Based on the observed mean spin-up rate, we estimated the inner accretion disk radius in GRO J1008-57 (about 1 - 2 times of the Alfvén radius) by comparing different accretion torque models of magnetic neutron stars. During the spin-down process, the magnetic torque should dominate over the matter accreting inflow torque, and we constrained the surface dipole magnetic field $B\geq 6\times 10^{12}$ G for the neutron star in GRO J1008-57, which is consistent with the magnetic field strength obtained by cyclotron line centroid energy.

astro-ph.HE↗

arXiv2021

QPOs and Orbital elements of X-ray binary 4U 0115+63 during the 2017 outburst observed by Insight-HXMT

In this paper, we presented a detailed timing analysis of a prominent outburst of 4U 0115+63 detected by \textit{Insight}-HXMT in 2017 August. The spin period of the neutron star was determined to be $3.61398\pm 0.00002$ s at MJD 57978. We measured the period variability and extract the orbital elements of the binary system. The angle of periastron evolved with a rate of $0.048\pm0.003$ $yr^{-1}$. The light curves are folded to sketch the pulse profiles in different energy ranges. A multi-peak structure in 1-10 keV is clearly illustrated. We introduced wavelet analysis into our data analysis procedures to study QPO signals and perform a detailed wavelet analysis in many different energy ranges. Through the wavelet spectra, we report the discovery of a QPO at the frequency $\sim 10$ mHz. In addition, the X-ray light curves showed multiple QPOs in the period of $\sim 16-32 $ s and $\sim 67- 200 $ s. We found that the $\sim100$ s QPO was significant in most of the observations and energies. There exist positive relations between X-ray luminosity and their Q-factors and S-factors, while the QPO periods have no correlation with X-ray luminosity. In wavelet phase maps, we found that the pulse phase of $\sim 67- 200 $ s QPO drifting frequently while the $\sim 16-32 $ s QPO scarcely drifting. The dissipation of oscillations from high energy to low energy was also observed. These features of QPOs in 4U 0115+63 provide new challenge to our understanding of their physical origins.

astro-ph.HE↗

arXiv2020

A variable ionized disk wind in the black-hole candidate EXO 1846-031

After 34 years, the black-hole candidate EXO 1846-031 went into outburst again in 2019. We investigate its spectral properties in the hard intermediate and the soft states with NuSTAR and Insight-HXMT. A reflection component has been detected in the two spectral states but possibly originating from different illumination spectra: in the intermediate state, the illuminating source is attributed to a hard coronal component, which has been commonly observed in other X-ray binaries, whereas in the soft state the reflection is probably produced by the disk self-irradiation. Both cases support EXO 1846-031 as a low inclination system of ~40 degrees. An absorption line is clearly detected at ~7.2 keV in the hard intermediate state, corresponding to a highly ionized disk wind (log ξ > 6.1) with a velocity up to 0.06c. Meanwhile, quasi-simultaneous radio emissions have been detected before and after the X-rays, implying the co-existence of disk winds and jets in this system. Additionally, the observed wind in this source is potentially driven by magnetic forces. The absorption line disappeared in the soft state and a narrow emission line appeared at ~6.7 keV on top of the reflection component, which may be evidence for disk winds, but data with the higher spectral resolution are required to examine this.

astro-ph.HE↗

arXiv2020

Insight-HXMT observations of Swift J0243.6+6124: the evolution of RMS pulse fractions at super-Eddington luminosity

Based on Insight-HXMT data, we report on the pulse fraction evolution during the 2017-2018 outburst of the newly discovered first Galactic ultraluminous X-ray source (ULX) Swift J0243.6+6124. The pulse fractions of 19 observation pairs selected in the rising and fading phases with similar luminosity are investigated. The results show a general trend of the pulse fraction increasing with luminosity and energy at super-critical luminosity. However, the relative strength of the pulsation between each pair evolves strongly with luminosity. The pulse fraction in the rising phase is larger at luminosity below $7.71\times10^{38}$~erg~s$^{-1}$, but smaller at above. A transition luminosity is found to be energy independent. Such a phenomena is firstly confirmed by Insight-HXMT observations and we speculate it may have relation with the radiation pressure dominated accretion disk.

astro-ph.HE↗

arXiv2020

X-ray reprocessing in accreting pulsar GX 301-2 observed with Insight-HXMT

We investigate the absorption and emission features in observations of GX 301-2 detected with Insight-HXMT/LE in 2017-2019. At different orbital phases, we found prominent Fe Kalpha, Kbeta and Ni Kalpha lines, as well as Compton shoulders and Fe K-shell absorption edges. These features are due to the X-ray reprocessing caused by the interaction between the radiation from the source and surrounding accretion material. According to the ratio of iron lines Kalpha and Kbeta, we infer the accretion material is in a low ionisation state. We find an orbital-dependent local absorption column density, which has a large value and strong variability around the periastron. We explain its variability as a result of inhomogeneities of the accretion environment and/or instabilities of accretion processes. In addition, the variable local column density is correlated with the equivalent width of the iron Kalpha lines throughout the orbit, which suggests that the accretion material near the neutron star is spherically distributed.

astro-ph.HE↗