arXiv subjects

W. G. Yang

Publications and source records attributed to W. G. Yang.

12 recordsLinked to original sources

arXiv2025

Discern Misclassified flat-spectrum radio quasars from low-frequency peaked BL Lacertae objects

A sample of 312 low-frequency peaked BL Lacertae objects (LBLs) and 694 flat spectrum radio quasars (FSRQs) with the parameters both redshift and $γ$-ray photon spectral index ($Γ_γ$) is compiled from the active galactic nuclei (AGNs) Catalog Data Release 2 (4LAC-DR2) from Fermi-LAT. The multi-wavelength data of the sample sources are downloaded from the Space Science Data Center (SSDC), and then match the corresponding gamma-ray data from 4FGL-DR2. The synchrotron radiation peak frequency and Compton dominance (CD) parameters of the sources are obtained by using a log-parabolic to fit the average-state multi-wavelength spectral energy distribution. A support vector machine (SVM) in the $\log L_γ$-$Γ_γ$ frame is utilized to delineate the optimal boundary between FSRQs and LBLs sources. The 1$σ$ position of the Gaussian fitting on the histograms of the $Γ_γ$, $\log ν^{syn}_{peak}$, and CD parameter distributions are also introduced. In the criterion, 25 FSRQ candidates are selected from LBL sample sources. The optical spectral identification result confirms that 8 out of 13 candidate sources available with the optical spectral data exhibit the relationship of $EW > 5 \mathring{\mathrm{A}}$.

astro-ph.HE↗

arXiv2019

Experimental evidence of crystal symmetry protection for the topological nodal line semimetal state in ZrSiS

Tunable symmetry breaking plays a crucial role for the manipulation of topological phases of quantum matter. Here, through combined high-pressure magneto-transport measurements, Raman spectroscopy, and X-ray diffraction, we demonstrate a pressure-induced topological phase transition in nodal-line semimetal ZrSiS. Symmetry analysis and first-principles calculations suggest that this pressure-induced topological phase transition may be attributed to weak lattice distortions by non-hydrostatic compression, which breaks some crystal symmetries, such as the mirror and inversion symmetries. This finding provides some experimental evidence for crystal symmetry protection for the topological semimetal state, which is at the heart of topological relativistic fermion physics.

cond-mat.mtrl-sci↗

arXiv2019

Pressure-induced superconductivity and topological phase transitions in the topological nodal-line semimetal SrAs3

Topological nodal-line semimetals (TNLSMs) are materials whose conduction and valence bands cross each other, meeting a topologically-protected closed loop rather than discrete points in the Brillouin zone (BZ). The anticipated properties for TNLSMs include drumhead-like nearly flat surface states, unique Landau energy levels, special collective modes, long-range Coulomb interactions, or the possibility of realizing high-temperature superconductivity. Recently, SrAs3 has been theoretically proposed and then experimentally confirmed to be a TNLSM. Here, we report high-pressure experiments on SrAs3, identifying a Lifshitz transition below 1 GPa and a superconducting transition accompanied by a structural phase transition above 20 GPa. A topological crystalline insulator (TCI) state is revealed by means of density functional theory (DFT) calculations on the emergent high-pressure phase. As the counterpart of topological insulators, TCIs possess metallic boundary states protected by crystal symmetry, rather than time reversal. In consideration of topological surface states (TSSs) and helical spin texture observed in the high-pressure state of SrAs3, the superconducting state may be induced in the surface states, and is most likely topologically nontrivial, making pressurized SrAs3 a strong candidate for topological superconductor.

cond-mat.supr-con↗

arXiv2017

Separation of Electron and Hole Dynamics in the Semimetal LaSb

We report investigations on the magnetotransport in LaSb, which exhibits extremely large magnetoresistance (XMR). Foremost, we demonstrate that the resistivity plateau can be explained without invoking topological protection. We then determine the Fermi surface from Shubnikov - de Haas (SdH) quantum oscillation measurements and find good agreement with the bulk Fermi pockets derived from first principle calculations. Using a semiclassical theory and the experimentally determined Fermi pocket anisotropies, we quantitatively describe the orbital magnetoresistance, including its angle dependence. We show that the origin of XMR in LaSb lies in its high mobility with diminishing Hall effect, where the high mobility leads to a strong magnetic field dependence of the longitudinal magnetoconductance. Unlike a one-band material, when a system has two or more bands (Fermi pockets) with electron and hole carriers, the added conductance arising from the Hall effect is reduced, hence revealing the latent XMR enabled by the longitudinal magnetoconductance. With diminishing Hall effect, the magnetoresistivity is simply the inverse of the longitudinal magnetoconductivity, enabling the differentiation of the electron and hole contributions to the XMR, which varies with the strength and orientation of the magnetic field. This work demonstrates a convenient way to separate the dynamics of the charge carriers and to uncover the origin of XMR in multi-band materials with anisotropic Fermi surfaces. Our approach can be readily applied to other XMR materials.

cond-mat.mtrl-sci↗

arXiv2017

Pressure-tuning of bond-directional exchange interactions and magnetic frustration in hyperhoneycomb iridate $β$-$\mathrm{Li_2IrO_3}$

We explore the response of Ir $5d$ orbitals to pressure in $β$-$\mathrm{Li_2IrO_3}$, a hyperhoneycomb iridate in proximity to a Kitaev quantum spin liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit (SO) interaction in $5d$ orbitals, pushes $β$-$\mathrm{Li_2IrO_3}$ further away from the pure $J_{\rm eff}=1/2$ limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to realization of a QSL state in this hyperhoneycomb iridate at high pressures.

cond-mat.str-el↗

arXiv2017

Reemergence of high-Tc superconductivity in the (Li1-xFex)OHFe1-ySe under high pressure

The pressure-induced reemergence of the second high-Tc superconducting phase (SC-II) in the alkali-metal intercalated AxFe2-ySe2 (A = K, Rb, Cs, Tl) remains an enigma and proper characterizations on the superconducting- and normal-state properties of the SC-II phase were hampered by the intrinsic inhomogeneity and phase separation. To elucidate this intriguing problem, we performed a detailed high-pressure magnetotransport study on the recently discovered (Li1-xFex)OHFe1-ySe single crystals, which have high Tc~40 K and share similar Fermi surface topology as AxFe2-ySe2, but are free from the sample complications. We found that the ambient-pressure Tc~41 K is suppressed gradually to below 2 K upon increasing pressure to Pc ~5 GPa, above which a SC-II phase with higher Tc emerges and the Tc increases progressively to above 50 K up to 12.5 GPa. Interestingly, our high-precision resistivity data enable us to uncover the sharp transition of the normal state from a Fermi liquid for SC-I phase (0 < P < 5 GPa) to a non-Fermi-liquid for SC-II phase (P > 5GPa). In addition, the reemergence of high-Tc SC-II phase is found to accompany with a concurrent enhancement of electron carrier density. Since high-pressure structural study based on the synchrotron X-ray diffraction rules out the structural transition below 10 GPa, the observed SC-II phase with enhanced carrier density should be ascribed to an electronic origin associated with a pressure-induced Fermi surface reconstruction.

cond-mat.supr-con↗

arXiv2016

Observation of two distinct superconducting domes under pressure in tetragonal FeS

As the simplest iron-based superconductor, FeSe forms a tetragonal structure with transition temperature Tc ~ 8 K. With assistance of pressure, or other techniques, Tc can be greatly enhanced, even to above liquid nitrogen temperature. The newly discovered superconducting tetragonal FeS (Tc ~ 4.5 K), a sulfide counterpart of FeSe, promotes us on its high pressure investigation. The transport and structure evolution of FeS with pressure have been studied. A rapid suppression of Tc and vanishing of superconductivity at 4.0 GPa are observed, followed by a second superconducting dome with a 30% enhancement in maximum Tc. An onsite tetragonal to hexagonal phase transition occurs around 7.0 GPa, followed by a broad pressure range of phase coexistence. The residual deformed tetragonal phase is considered as the source of second superconducting dome. The observation of two superconducting domes in iron-based superconductors poses great challenges for understanding their pairing mechanism.

cond-mat.supr-con↗

arXiv2015

High-pressure study of the Weyl semimetal NbAs

We performed a series of high-pressure synchrotron X-ray diffraction (XRD) and resistance measurements on the Weyl semimetal NbAs. The crystal structure remains stable up to 26 GPa according to the powder XRD data. The resistance of NbAs single crystal increases monotonically with pressure at low temperature. Up to 20 GPa, no superconducting transition is observed down to 0.3 K. These results show that the Weyl semimetal phase is robust in NbAs, and applying pressure is not a good way to get a topological superconductor from a Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2014

Superconductivity in Strong Spin Orbital Coupling Compound Sb2Se3

Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. Here, we report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at ~3 GPa which should be related to the topological quantum transition. The superconducting transition temperature (TC) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. High pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of TC slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.

cond-mat.supr-con↗

arXiv2013

Superconductivity in Topological Insulator Sb2Te3 Induced by Pressure

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

cond-mat.mtrl-sci↗

arXiv2013

Superconductivity of topological insulator Bi2Se3 at high pressures

The pressure induced superconductivity and structural evolution for Bi2Se3 single crystal have been studied. The emergence of superconductivity with onset transition temperature (Tc) about 4.4K is observed around 12GPa. Tc increases rapidly to the highest 8.5K at 16GPa, decreases to 6.5K at 21GPa, then keep almost constant. It is found that Tc versus pressure is closely related to the carrier density which increases by more than two orders of magnitude from 2GPa to 23GPa. High pressure synchrotron radiation measurements reveal structure transitions occur around 12GPa, 20GPa, and above 29GPa, respectively. A phase diagram of superconductivity versus pressure is obtained.

cond-mat.supr-con↗

arXiv2011

Pressure induced Superconductivity in Topological Compound Bi2Te3

Bi2Te3 compound has been theoretically predicted (1) to be a topological insulator, and its topologically non-trivial surface state with a single Dirac cone has been observed in photoemission experiments (2). Here we report that superconductivity (Tc^~3K) can be induced in Bi2Te3 as-grown single crystal (with hole-carriers) via pressure. The first-principles calculations show that the electronic structure under pressure remains to be topologically nontrivial, and the Dirac-type surface states can be well distinguished from bulk states at corresponding Fermi level. The proximity effect between superconducting bulk states and Dirac-type surface state could generate Majorana fermions on the surface. We also discuss the possibility that the bulk state could be a topological superconductor.

cond-mat.supr-con↗