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Z. S. Wang

Publications and source records attributed to Z. S. Wang.

18 recordsLinked to original sources
arXiv2024

Intrinsic pinning of FeSe$_1$$_-$$_x$S$_x$ single crystals probed by torque magnetometry

Intrinsic pinning is caused by natural pinning centers that occur because of the modulation of the order parameter or weak superconducting layers. Early work has shown that intrinsic pinning generates a high pinning force and critical current density in some layered oxide superconductors. Studying the intrinsic pinning of superconductors is crucial for both fundamental studies and potential applications. Herein, we use torque magnetometry to study angle-resolved in-plane and out-of-plane magnetic torque for a series of high-quality FeSe$_1$$_-$$_x$S$_x$ single crystals. A fourfold torque signal was observed when the magnetic field was within the \textit{ab} plane. We interpret that this fourfold in-plane irreversible torque is from the intrinsic pinning due to combined effects of gap nodes/minimum and twin domains. Additionally, we attributed the observed out-of-plane torque peaks to intrinsic pinning due to the layered structure.

cond-mat.supr-con↗
arXiv2021

Disorder-robust high-field superconducting phase of FeSe single crystals

When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesize FeSe single crystals with different levels of disorder. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field \textit{B}$_{\rm{c}2}$ was obtained by both resistivity and magnetic torque measurements over a wide range of temperatures, which went as low as $\sim$0.5 K, and magnetic fields, which went up to $\sim$38 T along the \textit{c} axis and in the \textit{ab} plane. In the high-field region parallel to the \textit{ab} plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust against disorder. This result suggests that the high-field SC phase in FeSe is not a conventional FFLO state.

cond-mat.supr-con↗
arXiv2019

Enhancement of the effective mass at high magnetic fields in CeRhIn$_5$

The Kondo-lattice compound CeRhIn$_5$ displays a field-induced Fermi surface reconstruction at $B^*\approx30$ T, which occurs within the antiferromagnetic state, prior to the quantum critical point at $B_{c0}\approx50$ T. Here, in order to investigate the nature of the Fermi surface change, we measured the magnetostriction, specific heat, and magnetic torque of CeRhIn$_5$ across a wide range of magnetic fields. Our observations uncover the field-induced itineracy of the $4f$ electrons, where above $B_{\rm onset}\approx17$ T there is a significant enhancement of the Sommerfeld coefficient, and spin-dependent effective cyclotron masses determined from quantum oscillations. Upon crossing $B_{\rm onset}$, the temperature dependence of the specific heat also shows distinctly different behavior from that at low fields. Our results indicate that the Kondo coupling is remarkably robust upon increasing the magnetic field. This is ascribed to the delocalization of the $4f$ electrons at the Fermi surface reconstruction at $B^*$.

cond-mat.str-el↗
arXiv2018

Characterization of Pd/Y multilayers with B4C barrier layers using GIXR and x-ray standing wave enhanced HAXPES

Pd/Y multilayers are high reflectance mirrors designed to work in the 7.5-11 nm wavelength range. Samples, prepared by magnetron sputtering, are deposited with or without B4C barrier layers located at the interfaces of Pd and Y layers to reduce interdiffusion, which is expected by calculating mixing enthalpy of Pd and Y. Grazing incident x-ray reflectometry is used to characterize these multilayers. B4C barrier layers are found effective on reducing the Pd-Y interdiffusion. Details of the composition of the multilayers are revealed by hard x-ray photoemission spectroscopy under x-ray standing waves effect. It consists in measuring the photoemission intensity from samples that perform an angular scan in the region corresponding to the multilayer period and the incident photon energy according to the Bragg law. The experimental result indicates that Pd does not chemically react with B nor C at the Pd-B4C interfaces while Y does at the Y-B4C interfaces. The formation of Y-B or Y-C chemical compound can be the reason why the interfaces are stabilized. By comparing the experimentally obtained angular variation of the characteristic photoemission with the theoretical calculation, the depth distribution of each component element can be interpreted.

physics.app-ph↗
arXiv2016

eXTP -- enhanced X-ray Timing and Polarimetry Mission

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of about 0.9 m^2 and 0.6 m^2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering <180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of about 3.4 m^2, between 6 and 10 keV, and spectral resolution <250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm^2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees FoV. The eXTP international consortium includes mostly major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

astro-ph.IM↗
arXiv2013

Longitudinal Schottky spectra of a bunched Ne10+ ion beam at the CSRe

The longitudinal Schottky spectra of a radio-frequency (RF) bunched and electron cooled 22Ne10+ ion beam at 70 MeV/u have been studied by a newly installed resonant Schottky pick-up at the experimental cooler storage ring (CSRe), at IMP. For an RF-bunched ion beam, a longitudinal momentum spread of has been reached with less than 107 stored ions. The reduction of momentum spread compared with coasting ion beam was observed from Schottky noise signal of the bunched ion beam. In order to prepare the future laser cooling experiment at the CSRe, the RF-bunching power was modulated at 25th, 50th and 75th harmonic of the revolution frequency, effective bunching amplitudes were extracted from the Schottky spectrum analysis. Applications of Schottky noise for measuring beam lifetime with ultra-low intensity of ion beams are presented, and it is relevant to upcoming experiments on laser cooling of relativistic heavy ion beams and nuclear physics at the CSRe.

physics.acc-ph↗
arXiv2011

Fusion of bogoliubons in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and similarity of energy scales in high temperature superconductors

Usually the superconducting pairing is considered to modify electronic states only in a narrow momentum range close to the Fermi surface. Here we present a direct experimental observation of fusion of Bogoliubov dispersion branches originating from the antipodal Fermi crossings by means of angle-resolved photoemission spectroscopy (ARPES). Uncommon discernibility and brightness of bogoliubons' fusion stems from comparability of the superconducting gap magnitude and the distance from the Fermi level to the band's top, and strong electron scattering on a mode with similar energy. Such similarity of the electronic and pairing energy scales seems to be a persistent associate of high-temperature superconductivity (HTSC) rather than just a mere coincidence.

cond-mat.supr-con↗
arXiv2010

Quantum mechanical photon-count formula derived by entangled state representation

By introducing the thermo entangled state representation, we derived four new photocount distribution formulas for a given density operator of light field. It is shown that these new formulas, which is convenient to calculate the photocount, can be expressed as such integrations over Laguree-Gaussian function with characteristic function, Wigner function, Q-function, and P-function, respectively.

quant-ph↗
arXiv2008

Anisotropic phase diagram and strong coupling effects in Ba1-xKxFe2As2

We present a thermodynamic study of the phase diagram of single-crystal Ba1-xKxFe2As2 using specific heat measurements. In zero-magnetic field a clear step in the heat capacity of deltaC/Tc = 0.1 J/f.u.K2 is observed at Tc = 34.6K for a sample with x = 0.4. This material is characterized by extraordinarily high slopes of the upper critical field of dHc2,c/dT= -6.5 T/K and dHc2,ab/dT= -17.4 T/K and a surprisingly low anisotropy of gamma ~ 2.6 near Tc. A consequence of the large field scale is the effective suppression of superconducting fluctuations. Using thermodynamic relations we determine Ginzburg-Landau parameters of kappac ~ 100 and kappaab ~ 260 identifying Ba1-xKxFe2As2 as extreme type-II. The large value of the normalized discontinuity of the slopes of the specific heat at Tc, (Tc/deltaC)times delta(dC/dT)~ 6 indicates strong coupling effects in Ba1-xKxFe2As2.

cond-mat.supr-con↗
arXiv1999

Kaon squeeze-out in heavy ion reactions

The squeeze-out phenomenon of $K^+$ and $K^-$ mesons, i.e. the azimuthal asymmetry of $K^+$ and $K^-$ mesons emitted at midrapidity in heavy ion reactions, is investigated for beam energies of 1-2 A.GeV. It is found that the squeeze-out signal is strongly affected by in-medium potentials of these mesons. The repulsive $K^+$-nucleus potential gives rise to a pronounced out-of-plane emission of $K^+$'s at midrapidity. With the $K^+$ potential we reproduce well the experimental data of the $K^+$ azimuthal distribution. It is found that the attractive $K^-$-nucleus potential cancels to a large extent the influence of rescattering and reabsorption of the $K^-$ mesons on the projectile and target residuals (i.e. shadowing). This results in an azimuthally isotropic emission of the midrapidity $K^-$ mesons with transverse momentum up to 0.8 GeV/c. Since it is well accepted that the shadowing alone would lead to a significant out-of-plane preference of particle emission, in particular at high transverse momenta, the disappearance of the out-of-plane preference for the $K^-$ mesons can serve as an unambiguous signal of the attractive $K^-$ potential. We also apply a covariant formalism of the kaon dynamics to the squeeze-out phenomenon. Discrepancies between the theory and the experiments and possible solutions are discussed.

nucl-th↗
arXiv1999

$J/ψ$ normal and anomalous suppressions in a hadron and string cascade model

A mechanism for the effective dissociation of a $c\bar{c}$ pair in the colour electric field of strings is introduced into a hadron and string cascade model, i.e. JPCIAE, which is based on the LUND model, simulating ultra-relativistic nucleus - nucleus collisions. This new mechanism together with the known mechanism of nuclear absorption (both baryons and mesons) could reproduce fairly the data of the normal and anomalous $J/ψ$ suppressions in minimum bias pA, AB (with light projectile), and Pb + Pb collisions at 200 A GeV/c. However the impact parameter (E_T) dependence of the $J/ψ$ suppression factor, both, in S + U and Pb + Pb reactions at 200 A GeV/c and 158 A GeV/c, respectively, is not well reproduced. We also tested the additional mechanism of the energy degradation of leading particles, with which both, the normal and anomalous $J/ψ$ suppressions in minimum bias pA, AB, and Pb + Pb collisions and the E_T dependence of the $J/ψ$ suppression factor are better reproduced.

nucl-th↗
arXiv1998

Chiral kaon dynamics in heavy ion collisions

The influence of the chiral mean field on the collective motion of kaons in relativistic heavy ion reactions at SIS energies is investigated. We consider three types of collective motion, i.e. the transverse flow, the out-of-plane flow (squeeze-out) and the radial flow. The kaon dynamics is thereby described with a relativistic mean field as it originates form chiral lagrangiens. For the $K$ mesons inside the nuclear medium we adopt a covariant quasi-particle picture including scalar and vector fields and compare this to a treatment with a static potential like force. The comparison to the available data ($K^+$) measured by FOPI and KaoS strongly favor the existence of an in-medium potential. However, using full covariant dynamics makes it more difficult to describe the data which might indicate that the mean field level is not sufficient for a reliable description of the kaon dynamics.

nucl-th↗
arXiv1998

Lambda collective flow in heavy ion reactions

Collective flow of Lambda hyperons in heavy ion reactions at SIS energies is investigated. It is found that a $Λ$ mean field constructed on the basis of the quark model leads to a good description of the experimental data of the in-plane transverse flow of $Λ$'s. The attractive mean field can also give rise to an additional "virtual" $Λ$ radial flow directed inwards, which is reflected by a "concave" structure of the transverse mass spectrum of the $Λ$ hyperons emitted at midrapidity. The $Λ$ radial flow is found to exhibit a strong mass dependence: The flow is visible in the Ni+Ni system, but is strongly reduced in the system of Au on Au.

nucl-th↗
arXiv1998

Consequences of covariant kaon dynamics in heavy ion collisions

The influence of the chiral mean field on the kaon dynamics in heavy ion reactions is investigated. Inside the nuclear medium the kaons are described as dressed quasi-particles carrying effective masses and momenta. A momentum dependent part of the interaction which resembles a Lorentz force originates from spatial components of the vector field and provides an important contribution to the in-medium kaon dynamics. This contribution is found to counterbalance the influence of the vector potential on the $K^+$ in-plane flow to a strong extent. Thus it appears to be difficult to restrict the in-medium potential from the analysis of the corresponding transverse flow.

nucl-th↗
arXiv1998

Radial flow of kaon mesons in heavy ion reactions

This work investigates the collective motion of kaons in heavy ion reactions at SIS energies (about 1-2 GeV/nucleon). A radial collective flow of $K^+$ mesons is predicted to exist in central Au + Au collisions, which manifests in a characteristic "shoulder-arm" shape of the transverse mass spectrum of the midrapidity $K^+$ mesons. The $K^+$ radial flow arises from the repulsive $K^+$ mean field in nuclear matter. In spite of a strong reabsorption and rescattering the attractive $K^-$ mean field leads as well to a collective radial flow of $K^-$ mesons. The $K^-$ radial flow, however, is different from that of $K^+$ mesons and can be observed by a characteristic "concave" structure of the transverse mass spectrum of the $K^-$ mesons emitted at midrapidity. The kaon radial flows can therefore serve as a novel tool for the investigation of kaon properties in dense nuclear matter.

nucl-th↗
arXiv1997

Role of isospin dependent mean field in pion production in heavy ion reactions

The importance of a isospin dependent nuclear mean field (IDMF) in regard to the pion production mechanism is studied for the reaction $Au+Au$ at 1 GeV/nucleon using the Quantum Molecular Dynamics (QMD) model. In particular, the effect of the IDMF on pion spectra and the charged pion ratio are analyzed. It is found that the inclusion of a IDMF considerably suppresses the low$-p_t$ pions, thus, leading to a better agreement with the data on pion spectra. Moreover, the rapidity distribution of the charged pion ratio appears to be sensitive to the isospin dependence of the nuclear mean field.

nucl-th↗
arXiv1997

Role of the Coulomb interaction in the flow and the azimuthal distribution of kaons from heavy ion reactions

Coulomb final-state interaction of positive charged kaons in heavy ion reactions and its impact on the kaon transverse flow and the kaon azimuthal distribution are investigated within the framework of QMD (Quantum Molecular Dynamics) model. The Coulomb interaction is found to tend to draw the flow of kaons away from that of nucleons and lead to a more isotropic azimuthal distribution of kaons in the target rapidity region. The recent FOPI data have been analyzed by taking into accout both the Coulomb interaction and a kaon in-medium potential of the strong interaction. It is found that both the calculated kaon flows with only the Coulomb interaction and with both the Coulomb interaction and the strong potential agree within the error bars with the data. The kaon azimuthal distribution exhibits asymmetries of similar magnitude in both theoretical approaches. This means, the inclusion of the Coulomb potential makes it more difficult to extract information of the kaon mean field potential in nuclear matter from the kaon flow and azimuthal distribution data.

nucl-th↗