arXiv subjects

Z. W. Li

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

49 records · Page 3Linked to original sources

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

Chaotic dynamics in spin-vortex pairs

We report on spin-vortex pair dynamics measured at temperatures low enough to suppress stochastic core motion, thereby uncovering the highly non-linear intrinsic dynamics of the system. Our analysis shows that the decoupling of the two vortex cores is resonant and can be enhanced by dynamic chaos. We detail the regions of the relevant parameter space, in which the various mechanisms of the resonant core-core dynamics are activated. We show that the presence of chaos can reduce the thermally-induced spread in the switching time by up to two orders of magnitude.

cond-mat.mes-hall↗

arXiv2018

Antiferromagnetic correlations in the metallic strongly correlated transition metal oxide LaNiO$_3$

The material class of rare earth nickelates with high Ni$^{3+}$ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO$_3$ thin films has also triggered intensive research efforts. LaNiO$_3$ seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimetre-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimetre-sized LaNiO$_3$ single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni$^{2+}$ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

cond-mat.str-el↗

arXiv2017

Single crystal growth and physical properties of 50% electron doped rhodate Sr$_{1.5}$La$_{0.5}$RhO$_{4}$

Centimeter-sized single crystals of Sr$_{1.5}$La$_{0.5}$RhO$_{4}$ were grown by the floating zone method at oxygen pressures of 20~bar. The quality of our single crystals was confirmed by X-ray Laue, powder and single crystal X-ray diffraction, neutron and X-ray absorbtion spectroscopy measurements. At $\sim$50% electron doping we observe RhO$_3$ octahedral rotations of $\sim$8.2$^{\circ}$ within the octahedral basal plane which are incompatible with space group I4/mmm. Our single crystal was further characterized by susceptibility, electrical transport and, finally, specific heat measurements showing a temperature dependent Debye temperature.

cond-mat.str-el↗

arXiv2016

Incommensurate spin correlations in highly oxidized cobaltates La$_{2-x}$Sr$_{x}$CoO$_{4}$

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2-xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2-xSrxCoO4.

cond-mat.str-el↗

arXiv2016

Electronic and magnetic nano phase separation in cobaltates La$_{2-x}$Sr$_{x}$CoO$_4$

The single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble the ones of the famous high-temperature superconducting cuprates. Here, we present an overview of our most recent studies of the spin and charge correlations in floating-zone grown cobaltate single crystals. We find that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. We also point out the difference between nano phase separation and conventional phase separation.

cond-mat.str-el↗

arXiv2015

Hour-glass magnetic excitations induced by nanoscopic phase separation in cobalt oxides La$_{2-x}$Sr$_x$CoO$_4$

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

cond-mat.str-el↗

arXiv2014

k=0 magnetic structure and absence of ferroelectricity in SmFeO3

SmFeO3 has attracted considerable attention very recently due to the reported multiferroic properties above room-temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveals a clear sign for magneto-elastic coupling at the Néel-temperature of ~675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

cond-mat.str-el↗

arXiv2014

Gorenstein Projective Modules Over Triangular Matrix Rings

We study totally acyclic complexes of projective modules over triangular matrix rings and then use it to classify Gorenstein projective modules over such rings. We also use this classification to obtain some information concerning Cohen-Macaulay finite and virtually Gorenstein triangular matrix artin algebras.

math.RT↗

arXiv2014

Spin-density-wave-induced anomalies in the optical conductivity of AFe2As2, (A=Ca, Sr, Ba) single-crystalline iron pnictides

We report the complex dielectric function of high-quality $A\textrm{Fe}_2\textrm{As}_2$, ($A$=Ca, Sr, Ba) single crystals with $T_{\mathrm{N}}\approx150\ \textrm{K}$, $200\ \textrm{K}$, and $138\ \textrm{K}$, respectively, determined by broadband spectroscopic ellipsometry at temperatures $10\leq T\leq300\ \textrm{K}$ and wavenumbers from $100\ \textrm{cm}^{-1}$ to $52000\ \textrm{cm}^{-1}$. In $\textrm{CaFe}_2\textrm{As}_2$ we identify the optical spin-density--wave gap $2Δ_{\mathrm{SDW}}\approx1250\ \textrm{cm}^{-1}$. The $2Δ_{\mathrm{SDW}}/(k_{\mathrm{B}}T_{\mathrm{N}})$ ratio, characterizing the strength of the electron-electron coupling in the spin-density--wave state, amounts to $\approx12$ in $\textrm{CaFe}_2\textrm{As}_2$, significantly larger than the corresponding values for the $\textrm{SrFe}_2\textrm{As}_2$ and $\textrm{BaFe}_2\textrm{As}_2$ compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both $\textrm{SrFe}_2\textrm{As}_2$ and $\textrm{CaFe}_2\textrm{As}_2$ and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in $\textrm{CaFe}_2\textrm{As}_2$ at a temperature $T^*\approx80\ \textrm{K}$, well below the Néel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. The temperature dependence of the infrared phonons reveals clear anomalies at the corresponding Néel temperatures of the investigated compounds. In $\textrm{CaFe}_2\textrm{As}_2$, the phonons exhibit signatures of SDW fluctuations above $T_N$ and some evidence for anomalies at $T^*$. (full abstract in PDF)

cond-mat.supr-con↗

arXiv2004

Large LFMR observed in twinned La2/3Ca1/3MnO3 thin films epitaxially grown on YSZ-buffered SOI substrates

La2/3Ca1/3MnO3 thin films have been grown on yttria-stabilized zirconia (YSZ) buffered silicon-on-insulator (SOI) substrate by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved for the YSZ layer, the top manganite layer was multi-domain-oriented, with a coexistence of cube-on-cube and cube-on-diagonal epitaxy. Due to a combined influence from the magnetocrystalline anisotropy and the magnetoelastic anisotropy, in zero field the local spin orientation varies across the twin boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunnelling was observed. The film shows a resistance change of ~20% in a magnetic field <1000 Oe at 50 K, which is promising for real applications.

cond-mat.str-el↗

arXiv2003

Evolution of In-Plane Magnetic Anisotropy In Sputtered FeTaN/TaN/FeTaN Sandwich Films

FeTaN/TaN/FeTaN sandwich films, FeTaN/TaN and TaN/FeTaN bilayers were synthesized by using RF magnetron sputtering. The magnetic properties, crystalline structures, microstructures and surface morphologies of the as-deposited samples were characterized using angle-resolved M-H loop tracer, VSM, XRD, TEM, AES and AFM. An evolution of the in-plane anisotropy was observed with the changing thickness of the nonmagnetic TaN interlayer in the FeTaN/TaN/FeTaN sandwiches, such as the easy-hard axis switching and the appearing of biaxial anisotropy. It is ascribed to three possible mechanisms, which are interlayer magnetic coupling, stress, and interface roughness, respectively. Interlayer coupling and stress anisotropies may be the major reasons to cause the easy-hard axis switching in the sandwiches. Whereas, magnetostatic and interface anisotropies may be the major reasons to cause biaxial anisotropy in the sandwiches, in which magnetostatic anisotropy is the dominant one.

cond-mat.mtrl-sci↗