arXiv subjects

Z. G. Wang

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

At least 19 recordsLinked to original sources

arXiv2023

Optimization of WLS fiber readout for the HERD calorimeter

A novel 3-D calorimeter, composed of about 7500 LYSO cubes, is the key and crucial detector of the High Energy cosmic-Radiation Detection (HERD) facility to be installed onboard the China Space Station. Energy deposition from cosmic ray in each LYSO cube is translated by multiple wavelength shifting (WLS) fibers for multi-range data acquisition and real-time triggering. In this study, various methods of surface finish and encapsulation of the LYSO cube were investigated to optimize the amplitude from the WLS fiber end with the aim of improving the signal-to-noise ratio of Intensified scientific CMOS (IsCMOS) collection. The LYSO cube with five rough surfaces and a specular reflector achieves the maximum amplitude at the low-range fiber end, which is increased by roughly 44% compared to the polished cube with PTFE wrapping. The non-uniformity of amplitude at different positions on the LYSO cube surface was measured by X-ray and the positional correlation factor was derived for the entire cube. A simulation based on HERD CALO was conducted, which revealed that both the LYSO cube with five rough surfaces and the cube with rough bottom face exhibit superior energy resolution for electrons compared to the other two configurations.

astro-ph.IM↗

arXiv2022

Investigation of wetting layers in InAs/GaAs self-assembled nanostructures with reflectance difference spectroscopy

As a system with both profound physics and promising application potentials, the strain-induced self-assembled semiconductor nanostructures have been investigated for decades of years. The optical and electrical properties of this system are mainly determined by the nanostructures, but can be greatly affected by the generally existent two dimensional structures from which the nanostructures evolve, the so called wetting layer (WL). The WL configurations can be varied with different growth conditions, and further influence the nanostructure morphology and properties. This is a reviewing article introducing some recent progresses in investigating the evolution of WLs in InAs/GaAs system with reflectance difference spectroscopy. Different kinds of WL evolution processes in Stranski-Krastanov growth mode are introduced. The segregation and desorption of indium atoms and the effect of growth interruption are mentioned. The existence and the evolution process of wetting layers in droplet epitaxy method are also discussed.

cond-mat.mtrl-sci↗

arXiv2021

Band-selective Holstein polaron in Luttinger liquid material A0.3MoO3 (A = K, Rb)

(Quasi-)one-dimensional systems exhibit various fascinating properties such as Luttinger liquid behavior, Peierls transition, novel topological phases, and the accommodation of unique quasiparticles (e.g., spinon, holon, and soliton, etc.). Here we study molybdenum blue bronze A0.3MoO3 (A = K, Rb), a canonical quasi-one-dimensional charge-density-wave material, using laser-based angle-resolved photoemission spectroscopy. Our experiment suggests that the normal phase of A0.3MoO3 is a prototypical Luttinger liquid, from which the charge-density-wave emerges with decreasing temperature. Prominently, we observe strong renormalizations of band dispersions, which is recognized as the spectral function of Holstein polaron derived from band-selective electron-phonon coupling in the system. We argue that the strong electron-phonon coupling plays a dominant role in electronic properties and the charge-density-wave transition in blue bronzes. Our results not only reconcile the long-standing heavy debates on the electronic properties of blue bronzes but also provide a rare platform to study novel composite quasiparticles in Luttinger liquid materials.

cond-mat.str-el↗

arXiv2021

Orbital-dependent modulation of the superconducting gap in uniaxially strained Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$

Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ($|Δ_k|$). Here, we report the measurement of a strain-induced gap modulation ($\partial|Δ_k|$) in uniaxially strained Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ utilizing angle-resolved photoemission spectroscopy and $in$-$situ$ strain-tuning. We found that the uniaxial strain drives Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ into a nematic superconducting state which breaks the four-fold rotational symmetry of the superconducting pairing. The superconducting gap increases on the $d_{yz}$ electron and hole pockets while it decreases on the $d_{xz}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The $d_{xz}$ and $d_{yz}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intra-orbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $\partial|Δ_k|$ as an effective way to characterize the superconducting pairing from a perturbation perspective.

cond-mat.supr-con↗

arXiv2021

Metal-Insulator Transition and Emergent Gapped Phase in the Surface-Doped 2D Semiconductor 2H-MoTe$_2$

Artificially created two-dimensional (2D) interfaces or structures are ideal for seeking exotic phase transitions due to their highly tunable carrier density and interfacially enhanced many-body interactions. Here, we report the discovery of a metal-insulator transition (MIT) and an emergent gapped phase in the metal-semiconductor interface that is created in 2H-MoTe$_2$ via alkali-metal deposition. Using angle-resolved photoemission spectroscopy, we found that the electron-phonon coupling is strong at the interface as characterized by a clear observation of replica shake-off bands. Such strong electron-phonon coupling interplays with disorder scattering, leading to an Anderson localization of polarons which could explain the MIT. The domelike emergent gapped phase could then be attributed to a polaron extended state or phonon-mediated superconductivity. Our results demonstrate the capability of alkali-metal deposition as an effective method to enhance the many-body interactions in 2D semiconductors. The surface-doped 2H-MoTe$_2$ is a promising candidate for realizing polaronic insulator and high-$T_c$ superconductivity.

cond-mat.supr-con↗

arXiv2020

Doping-controlled transition from excitonic insulator to semimetal in Ta$_2$NiSe$_5$

Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein Condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating method of excitonic condensation. In this paper, we report the realization of a doping-controlled EI-to-semi-metal transition in Ta$_2$NiSe$_5$ using $in$-$situ$ potassium deposition. Combining with angle-resolved photoemission spectroscopy (ARPES), we delineate the evolution of electronic structure through the EI transition with unprecedented precision. The results not only show that Ta$ _2 $NiSe$ _5 $ (TNS) is an EI originated from a semi-metal non-interacting band structure, but also resolve two sequential transitions, which could be attributed to the phase-decoherence and pair-breaking respectively. Our results unveil the Bardeen-Cooper-Schrieffer (BCS)-BEC crossover behavior of TNS and demonstrate that its band structure and excitonic binding energy can be tuned precisely via alkali-metal deposition. This paves a way for investigations of BCS-BEC crossover phenomena, which could provide insights into the many-body physics in condensed matters and other many-body systems.

cond-mat.str-el↗

arXiv2020

Band insulator to Mott insulator transition in 1T-TaS$_2$

1T-TaS$_2$ undergoes successive phase transitions upon cooling and eventually enters an insulating state of mysterious origin. Some consider this state to be a band insulator with interlayer stacking order, yet others attribute it to Mott physics that support a quantum spin liquid state.Here, we determine the electronic and structural properties of 1T-TaS$_2$ using angle-resolved photoemission spectroscopy and X-Ray diffraction. At low temperatures, the 2$π$/2c-periodic band dispersion, along with half-integer-indexed diffraction peaks along the c axis, unambiguously indicates that the ground state of 1T-TaS$_2$ is a band insulator with interlayer dimerization. Upon heating, however, the system undergoes a transition into a Mott insulating state, which only exists in a narrow temperature window. Our results refute the idea of searching for quantum magnetism in 1T-TaS$_2$ only at low temperatures, and highlight the competition between on-site Coulomb repulsion and interlayer hopping as a crucial aspect for understanding the material's electronic properties.

cond-mat.str-el↗

arXiv2020

Anomalous spectral weight transfer in the nematic state of iron-selenide superconductor

Nematic phase intertwines closely with high-Tc superconductivity in iron-based superconductors. Its mechanism, which is closely related to the pairing mechanism of superconductivity, still remains controversial. Comprehensive characterization of how the electronic state reconstructs in the nematic phase is thus crucial. However, most experiments focus only on the reconstruction of band dispersions. Another important characteristic of electronic state, the spectral weight, has not been studied in details so far. Here, we studied the spectral weight transfer in the nematic phase of FeSe$_{0.9}$S$_{0.1}$ using angle-resolved photoemission spectroscopy and in-situ detwinning technique. There are two elliptical electron pockets overlapping with each other orthogonally at the Brillouin zone corner. We found that, upon cooling, one electron pocket loses spectral weight and fades away, while the other electron pocket gains spectral weight and becomes pronounced. Our results show that the symmetry breaking of electronic state is manifested by not only the anisotropic band dispersion but also the band-selective modulation of spectral weight. Our observation completes our understanding of the nematic electronic state, and put strong constraints on the theoretical models. It further provide crucial clues to understand the gap anisotropy and orbital-selective pairing in iron-selenide superconductors.

cond-mat.supr-con↗

arXiv2020

Momentum-resolved measurement of electronic nematic susceptibility in the FeSe$_{0.9}$S$_{0.1}$ superconductor

Unveiling the driving force for a phase transition is normally difficult when multiple degrees of freedom are strongly coupled. One example is the nematic phase transition in iron-based superconductors. Its mechanism remains controversial due to a complex intertwining among different degrees of freedom. In this paper, we report a method for measuring the nematic susceptibly of FeSe$_{0.9}$S$_{0.1}$ using angle-resolved photoemission spectroscopy (ARPES) and an $in$-$situ$ strain-tuning device. The nematic susceptibility is characterized as an energy shift of band induced by a tunable uniaxial strain. We found that the temperature-dependence of the nematic susceptibility is strongly momentum dependent. As the temperature approaches the nematic transition temperature from the high temperature side, the nematic susceptibility remains weak at the Brillouin zone center while showing divergent behavior at the Brillouin zone corner. Our results highlight the complexity of the nematic order parameter in the momentum space, which provides crucial clues to the driving mechanism of the nematic phase transition. Our experimental method which can directly probe the electronic susceptibly in the momentum space provides a new way to study the complex phase transitions in various materials.

cond-mat.supr-con↗

arXiv2020

Isostructural spin-density-wave and superconducting gap anisotropies in iron-arsenide superconductors

When passing through a phase transition, electronic system saves energy by opening energy gaps at the Fermi level. Delineating the energy gap anisotropy provides insights into the origin of the interactions that drive the phase transition. Here, we report the angle-resolved photoemission spectroscopy (ARPES) study on the detailed gap anisotropies in both the tetragonal magnetic and superconducting phases in Sr$_{1-x}$Na$_x$Fe$_2$As$_2$. First, we found that the spin-density-wave (SDW) gap is strongly anisotropic in the tetragonal magnetic phase. The gap magnitude correlates with the orbital character of Fermi surface closely. Second, we found that the SDW gap anisotropy is isostructural to the superconducting gap anisotropy regarding to the angular dependence, gap minima locations, and relative gap magnitudes. Our results indicate that the superconducting pairing interaction and magnetic interaction share the same origin. The intra-orbital scattering plays an important role in constructing these interactions resulting in the orbital-selective magnetism and superconductivity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2018

Electrical initialization of electron and nuclear spins in a single quantum dot at zero magnetic field

The emission of circularly polarized light from a single quantum dot relies on the injection of carriers with well-defined spin polarization. Here we demonstrate single dot electroluminescence (EL) with a circular polarization degree up to 35% at zero applied magnetic field. The injection of spin polarized electrons is achieved by combining ultrathin CoFeB electrodes on top of a spin-LED device with p-type InGaAs quantum dots in the active region. We measure an Overhauser shift of several $μ$eV at zero magnetic field for the positively charged exciton (trion X$^+$) EL emission, which changes sign as we reverse the injected electron spin orientation. This is a signature of dynamic polarization of the nuclear spins in the quantum dot induced by the hyperfine interaction with the electrically injected electron spin. This study paves the way for electrical control of nuclear spin polarization in a single quantum dot without any external magnetic field.

cond-mat.mes-hall↗

arXiv2015

Study of helium irradiation induced hardening in MNHS steel

A recently developed reduced activation ferritic/martensitic steel MNHS was irradiated with 200keV He ions to a fluence of 1E21ions/m^2 at 450 celsius degree and 1E20ions/m^2 at 300 celsius degree and 450 celsius degree, respectively. The irradiation hardening of the steel was investigated by nanoindentation measurements combined with transmission electron microscopy (TEM) analysis. Dispersed barrier-hardening (DBH) model was applied to predict the hardness increments based on TEM analysis. The predicted hardness increments are consistent with the values obtained by nanoindentation tests. It is found that dislocation loops and He bubbles are hard barriers against dislocation motion and they are the main contributions to He irradiation-induced hardening of MNHS steel. The obstacle strength of He bubbles is stronger than the obstacle strength of dislocation loops.

cond-mat.mtrl-sci↗

arXiv2014

Monte Carlo Simulation of HERD Calorimeter

The High Energy cosmic-Radiation Detection (HERD) facility onboard China's Space Station is planned for operation starting around 2020 for about 10 years. It is designed as a next generation space facility focused on indirect dark matter search, precise cosmic ray spectrum and composition measurements up to the knee energy, and high energy gamma-ray monitoring and survey. The calorimeter plays an essential role in the main scientific objectives of HERD. A 3-D cubic calorimeter filled with high granularity crystals as active material is a very promising choice for the calorimeter. HERD is mainly composed of a 3-D calorimeter (CALO) surrounded by silicon trackers (TK) from all five sides except the bottom. CALO is made of 9261 cubes of LYSO crystals, corresponding to about 55 radiation lengths and 3 nuclear interaction lengths, respectively. Here the simulation results of the performance of CALO with GEANT4 and FLUKA are presented: 1) the total absorption CALO and its absorption depth for precise energy measurements (energy resolution: 1\% for electrons and gamma-rays beyond 100 GeV, 20\% for protons from 100 GeV to 1 PeV); 2) its granularity for particle identification (electron/proton separation power better than $10^{-5}$); 3) the homogenous geometry for detecting particles arriving from every unblocked direction for large effective geometrical factor ($>$3 ${\rm m}^{2}{\rm sr}$ for electron and diffuse gamma-rays, $>$2 $ {\rm m}^{2}{\rm sr}$ for cosmic ray nuclei); 4) expected observational results such as gamma-ray line spectrum from dark matter annihilation and spectrum measurement of various cosmic ray chemical components.

astro-ph.IM↗

arXiv2014

Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2

The formation of nano-hillocks on CaF2 crystal surfaces by individual ion impact has been studied using medium energy (3 and 5 MeV) highly charged ions (Xe19+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy while for swift heavy ions a minimum electronic energy loss is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via electronic energy loss the potential energy threshold for hillock production can be substantially lowered. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, as demonstrated when plotting the results in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to case where kinetic and potential energies are deposited into the surface.

cond-mat.mtrl-sci↗

arXiv2013

Identification of a $9/2^-$[505] isomer in the neutron-rich $^{193}$Os nucleus

The neutron rich nucleus $^{193}$Os was produced in the $^{192}$Os($^{7}$Li,$^{6}$Li)$^{193}$Os reaction. An isomeric state based on the $9/2^-$[505] nilsson orbital was identified in the present work. Half-life of the isomeric state was extracted and discussed in terms of the $K$ quantum number. Level scheme built on the isomeric state was proposed based on the experimental data.

nucl-ex↗

arXiv2011

Influences of Si sheet doping densities on the morphological, conductive and optical characteristics of InAs/GaAs quantum dots

The influences of Si sheet doping levels on the properties of InAs/GaAs quantum dots (QDs) are investigated by atomic force microscopy (AFM) and photoluminescence (PL). AFM measurements reveal that Si sheet doping doesn't change the morphology of InAs QDs. Conductive AFM exhibits a quick current decrease when the Si doping density reaches 5\times1011cm-2. PL measurements show that the Si doping can significantly enhance the PL intensity. The PL peak intensity of InAs QDs doped to 5\times1011cm-2 is increased about thirty-five times from that of the undoped ones at 300K. The results observed here can be explained by a supposed positive-charged, strain-relaxed Si-doped thin InAs layer inside the InAs QDs.

cond-mat.mes-hall↗

arXiv2008

Strong decays $B_{s0} \to B_s π$ and $B_{s1} \to B^*_s π$ with light-cone QCD sum rules

In this article, we calculate the strong coupling constants $g_{B_{s0} B_s η}$ and $g_{B_{s1} B^*_s η}$ with the light-cone QCD sum rules. Then we take into account the small $η-π^0$ transition matrix according to Dashen's theorem, and obtain the small decay widths for the isospin violation processes $B_{s0}\to B_sη\to B_sπ^0$ and $B_{s1}\to B_s^*η\to B_s^*π^0$. We can search the strange-bottomed $(0^+,1^+)$ mesons $B_{s0}$ and $B_{s1}$ in the invariant $B_s π^0$ and $B^*_s π^0$ mass distributions respectively.

hep-ph↗

arXiv2008

Structure of the $(0^+,1^+)$ mesons $B_{s0}$ and $B_{s1}$, and the strong coupling constants $g_{B_{s0} B K}$ and $g_{B_{s1} B^* K}$

In this article, we take the point of view that the bottomed $(0^+,1^+)$ mesons $B_{s0}$ and $B_{s1}$ are the conventional $b\bar{s}$ meson, and calculate the strong coupling constants $g_{B_{s0} B K}$ and $g_{B_{s1} B^* K}$ with the light-cone QCD sum rules. The numerical values of strong coupling constants $g_{B_{s1} B^* K}$ and $g_{B_{s0} B K}$ are very large, and support the hadronic dressing mechanism. Just like the scalar mesons $f_0(980)$, $a_0(980)$, $D_{s0}$ and axial-vector meson $D_{s1}$, the $(0^+,1^+)$ bottomed mesons $B_{s0}$ and $B_{s1}$ may have small $b\bar{s}$ kernels of the typical $b\bar{s}$ meson size, the strong couplings to the hadronic channels (or the virtual mesons loops) may result in smaller masses than the conventional $b\bar{s}$ mesons in the potential quark models, and enrich the pure $b\bar{s}$ states with other components.

hep-ph↗