Youguo Shi

A ferroelectric-like structural transition in a metal

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. 3). The transition involves a continuous shift in the mean position of Li(+) ions on cooling below 140 K. Its discovery realizes the scenario described in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals.

Superconductivity emerging from a suppressed large magnetoresistant state in tungsten ditelluride

The recent discovery of large magnetoresistance in tungsten ditelluride provides a unique playground to find new phenomena and significant perspective for potential applications. The large magnetoresistance effect originates from a perfect balance of hole and electron carriers, which is sensitive to external pressure. Here we report the suppression of the large magnetoresistance and emergence of superconductivity in pressurized tungsten ditelluride via high-pressure synchrotron X-ray diffraction, electrical resistance, magnetoresistance and alternating current magnetic susceptibility measurements. Upon increasing pressure, the positive large magnetoresistance effect is gradually suppressed and turned off at a critical pressure of 10.5 GPa, where superconductivity accordingly emerges. No structural phase transition is observed under the pressure investigated. In situ high-pressure Hall coefficient measurements at low temperatures demonstrate that elevating pressure decreases the population of hole carriers but increases that of the electron ones. Significantly, at the critical pressure, a sign change of the Hall coefficient is observed.

Observation of Fermi arc and its connection with bulk states in the candidate type-II Weyl semimetal WTe2

Chenlu Wang, Yan Zhang, Jianwei Huang, Simin Nie, Guodong Liu1,∗, Aiji Liang, Yuxiao Zhang, Bing Shen, Jing Liu, Cheng Hu, Ying Ding, Defa Liu, Yong Hu, Shaolong He, Lin Zhao, Li Yu, Jin Hu, Jiang Wei, Zhiqiang Mao, Youguo Shi, Xiaowen Jia, Fengfeng Zhang, Shenjin Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Hongming Weng, Xi Dai, Zhong Fang, Zuyan Xu, Chuangtian Chen and X. J. Zhou1,5,∗ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, USA Military Transportation University, Tianjin 300161, China. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China. These people contributed equally to the present work. ∗Corresponding author: gdliu arpes@iphy.ac.cn, XJZhou@iphy.ac.cn.

Evidence of Topological Surface State in Three-Dimensional Dirac Semimetal Cd3As2

The three-dimensional topological semimetals represent a new quantum state of matter. Distinct from the surface state in the topological insulators that exhibits linear dispersion in two-dimensional momentum plane, the three-dimensional semimetals host bulk band dispersions linearly along all directions. In addition to the gapless points in the bulk, the three-dimensional Weyl/Dirac semimetals are also characterized by “topologically protected” surface state with Fermi arcs on their surface. While Cd3As2 is proposed to be a viable candidate of a Dirac semimetal, more investigations are necessary to pin down its nature. In particular, the topological surface state, the hallmark of the three-dimensional semimetal, has not been observed in Cd3As2. Here we report the electronic structure of Cd3As2 investigated by angle-resolved photoemission measurements on the (112) crystal surface and detailed band structure calculations. The measured Fermi surface and band structure show a good agreement with the band structure calculations with two bulk Dirac-like bands approaching the Fermi level and forming Dirac points near the Brillouin zone center. Moreover, the topological surface state with a linear dispersion approaching the Fermi level is identified for the first time. These results provide experimental indications on the nature of topologically non-trivial three-dimensional Dirac cones in Cd3As2.

Raman scattering investigation of large positive magnetoresistance material WTe2

We have performed polarized Raman scattering measurements on WTe2, for which an extremely large positive magnetoresistance has been reported recently. We observe 5 A1 phonon modes and 2 A2 phonon modes out of 33 Raman active modes, with frequencies in good accordance with first-principles calculations. The angular dependence of the intensity of the peaks observed is consistent with the Raman tensors of the C2v point group symmetry attributed to WTe2. Although the phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling, the intensity of the A1 phonon mode at 160.6 cm−1 shows an unconventional decrease with temperature decreasing, for which the origin remains unclear.

Ba3LnInS6 (Ln = Pr, Sm, Gd, Yb) and Ba2LnGaS5 (Ln = Pr, Nd): syntheses, structures, and magnetic and optical properties.

Six new quaternary rare-earth sulfides Ba(3)LnInS(6) (Ln = Pr, Sm, Gd, Yb) and Ba(2)LnGaS(5) (Ln = Pr, Nd) have been synthesized for the first time. Ba(3)LnInS(6) (Ln = Pr, Sm, Gd, Yb) belong to the centrosymmetric space group R3[overline]c of the trigonal system. The structures contain infinite one-dimensional anionic chains (1)(∞)[LnInS(6)](6-), which are built from face-sharing LnS(6) distorted triangular prisms and InS(6) octahedra. Ba(2)LnGaS(5) (Ln = Pr, Nd) crystallize in the centrosymmetric space group I4/mcm of the tetragonal system. Their structures consist of (BaLn)S layers built from (BaLn)S(8) bicapped trigonal prisms. These layers are stacked perpendicular to the c axis and further connected by GaS(4) tetrahedra to form a three-dimensional framework with channels occupied by Ba(2+) cations. As deduced from magnetic susceptibility measurements on Ba(2)NdGaS(5), it is paramagnetic and obeys the Curie-Weiss law. Besides, the band gap of Ba(2)NdGaS(5) is determined to be about 2.12(2) eV.

Syntheses, Structures, Optical and Magnetic Properties of Ba2MLnSe5 (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er)

The twelve quaternary rare-earth selenides Ba(2)MLnSe5 (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er) have been synthesized for the first time. The compounds Ba(2)GaLnSe(5) (Ln = Y, Nd, Sm, Gd, Dy, Er) are isostructural and crystallize in a new structure type in the centrosymmetric space group P ̅1 of the triclinic system while the isostructural compounds Ba(2)InLnSe(5) (Ln = Y, Nd, Sm, Gd, Dy, Er) belong to the Ba(2)BiInS(5) structure type and crystallize in the noncentrosymmetric space group Cmc2(1) of the orthorhombic system. The structures contain infinite one-dimensional anionic chains (1)(∞)[GaLnSe(5)](4-) and (1)(∞)[InLnSe(5)](4-), and both chains are built from LnSe(6) octahedra and MSe(4) (M = Ga, In) tetrahedra in the corresponding selenides. As deduced from the diffuse reflectance spectra, the band gaps of most Ba(2)MLnSe(5) (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er) compounds are around 2.2 eV. The magnetic susceptibility measurements on Ba(2)GaGdSe(5) and Ba(2)InLnSe(5) (Ln = Nd, Gd, Dy, Er) indicate that they are paramagnetic and obey the Curie-Weiss law, while the magnetic susceptibility of Ba(2)InSmSe(5) deviates from the Curie-Weiss law as a result of the crystal field splitting. Furthermore, Ba(2)InYSe(5) exhibits a strong second harmonic generation response close to that of AgGaSe(2), when probed with the 2090 nm laser as fundamental wavelength.

Evidence for Half-Metallicity in n-type HgCr2Se4.

High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport, and Andreev reflection spectroscopy. In the ferromagnetic ground state, the saturation magnetic moment of each unit cell corresponds to an integer number of electron spins (3  μB/Cr3+), and the Hall effect measurements suggest n-type charge carriers. Spin polarizations as high as 97% were obtained from fits of the differential conductance spectra of HgCr2Se4/Pb junctions with the modified Blonder-Tinkham-Klapwijk theory. The temperature and bias-voltage dependencies of the subgap conductance are consistent with recent theoretical calculations based on spin active scatterings at a superconductor-half-metal interface. Our results suggest that n-HgCr2Se4 is a half-metal, in agreement with theoretical calculations that also predict undoped HgCr2Se4 is a magnetic Weyl semimetal.

Spin texture in type-II Weyl semimetal WTe 2

We determine the band structure and spin texture of

Giant semiclassical magnetoresistance in high mobility TaAs2 semimetal

{\mathrm{WTe}}_{2}