Showing papers by "Guodong Liu published in 2015"
TL;DR: The electronic structure of Cd3As2 is investigated by angle-resolved photoemission measurements on the crystal surface and detailed band structure calculations and the topological surface state with a linear dispersion approaching the Fermi level is identified for the first time.
Abstract: 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.
169 citations
TL;DR: In this paper, a cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently by introducing the d-metal Ti into NiMn alloys, and strong ferromagnetism is established by further doping Co atoms into the B2 type antiferromagnetic Ni-Mn-Ti austenite.
Abstract: Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.
131 citations
TL;DR: In this article, a cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently by introducing the d-metal Ti into NiMn alloys, and strong ferromagnetism is established by further doping Co atoms into the B2 type antiferromagnetic Ni-Mn-Ti austenite.
Abstract: Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases, but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.
127 citations
TL;DR: In this paper, high resolution angle-resolved photoemission measurements have provided direct evidence on the existence of strongly anisotropic Dirac cones in three-dimensional topological insulators and AMnBi2.
Abstract: The Dirac materials, such as graphene and three-dimensional topological insulators, have attracted much attention because they exhibit novel quantum phenomena with their low energy electrons governed by the relativistic Dirac equations One particular interest is to generate Dirac cone anisotropy so that the electrons can propagate differently from one direction to the other, creating an additional tunability for new properties and applications While various theoretical approaches have been proposed to make the isotropic Dirac cones of graphene into anisotropic ones, it has not yet been met with success There are also some theoretical predictions and/or experimental indications of anisotropic Dirac cone in novel topological insulators and AMnBi2 (A = Sr and Ca) but more experimental investigations are needed Here we report systematic high resolution angle-resolved photoemission measurements that have provided direct evidence on the existence of strongly anisotropic Dirac cones in SrMnBi2 and CaMnBi2 Distinct behaviors of the Dirac cones between SrMnBi2 and CaMnBi2 are also observed These results have provided important information on the strong anisotropy of the Dirac cones in AMnBi2 system that can be governed by the spin-orbital coupling and the local environment surrounding the Bi square net
93 citations
TL;DR: In this paper, a series of unprecedentedly wide Curie-temperature windows (CTWs) between 40 and 450 K are realized by employing the isostructural alloying principle for the strongly coupled magnetstructural phase transitions in a single host system, providing a design platform for magneto-multifunctional multiferroic alloys that can be manipulated in a quite large temperature space in various scales and patterns, as well as by multiple physical fields.
Abstract: A series of unprecedentedly wide Curie-temperature windows (CTWs) between 40 and 450 K are realized by employing the isostructural alloying principle for the strongly coupled magnetostructural phase transitions in a single host system. The CTWs provide a design platform for magneto-multifunctional multiferroic alloys that can be manipulated in a quite large temperature space in various scales and patterns, as well as by multiple physical fields.
77 citations
TL;DR: In this article, the existence of topologically nontrivial surface state with Dirac cone in PbTe2 superconductor was discovered for the first time, where the Dirac point of the surface state in PdTe2 lies deeply below the Fermi level at 1.75 eV binding energy.
Abstract: High-resolution angle-resolved photoemission measurements are carried out on transition metal dichalcogenide PdTe2 that is a superconductor with a Tc at 1.7 K. Combined with theoretical calculations, we discover for the first time the existence of topologically nontrivial surface state with Dirac cone in PbTe2 superconductor. It is located at the Brillouin zone center and possesses helical spin texture. Distinct from the usual three-dimensional topological insulators where the Dirac cone of the surface state lies at the Fermi level, the Dirac point of the surface state in PdTe2 lies deeply below the Fermi level at ~1.75 eV binding energy and is well separated from the bulk states. The identification of topological surface state in PdTe2 superconductor deeply below the Fermi level provides a unique system to explore new phenomena and properties and opens a door for finding new topological materials in transition metal chalcogenides.
56 citations
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TL;DR: In this paper, the existence of topologically nontrivial surface state with Dirac cone in PbTe2 superconductor was discovered for the first time, where the Dirac point of the surface state in PdTe2 lies deep below the Fermi level at ~1.75 eV binding energy.
Abstract: High resolution angle-resolved photoemission measurements have been carried out on transition metal dichalcogenide PdTe2 that is a superconductor with a Tc at 1.7 K. Combined with theoretical calculations, we have discovered for the first time the existence of topologically nontrivial surface state with Dirac cone in PbTe2 superconductor. It is located at the Brillouin zone center and possesses helical spin texture. Distinct from the usual three-dimensional topological insulators where the Dirac cone of the surface state lies at the Fermi level, the Dirac point of the surface state in PdTe2 lies deep below the Fermi level at ~1.75 eV binding energy and is well separated from the bulk states. The identification of topological surface state in PdTe2 superconductor deep below the Fermi level provides a unique system to explore for new phenomena and properties and opens a door for finding new topological materials in transition metal chalcogenides.
31 citations
TL;DR: In this article, the structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction, differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations.
Abstract: The structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction, differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations. Results indicate that all samples undergo a martensitic transition from the Ni2In-type parent phase to TiNiSi-type orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in Mn1−xCoxNiSi (x = 0.2, 0.3, and 0.4) and Mn1−yFeyNiSi (y = 0.26, 0.30, 0.36, 0.46, and 0.55) samples decreases the structural transition temperature and Curie temperature of martensite. The martensite phases show a typical ferromagnetic behavior with saturation field being basically unchanged with increasing Co(Fe) content, while the saturation magnetization shows a decreasing tendency. The theoretically calculated moments are in good agreement with the experimentally measured results. The orbital hybridizations between different 3d elements are analyzed from the dist...
23 citations
TL;DR: In this paper, the anomalous high energy electronic structure in Sr2IrO4 was studied by taking high-resolution angle-resolved photoemission measurements over a wide energy range, and it was shown that the high-energy electronic structures show unusual nearly-vertical bands that extend over a large energy range.
Abstract: The low energy electronic structure of Sr2IrO4 has been well studied and understood in terms of an effective Jeff = 1/2 Mott insulator model. However, little work has been done in studying its high energy electronic behaviors. Here we report a new observation of the anomalous high energy electronic structure in Sr2IrO4. By taking high-resolution angle-resolved photoemission measurements on Sr2IrO4 over a wide energy range, we have revealed for the first time that the high energy electronic structures show unusual nearly-vertical bands that extend over a large energy range. Such anomalous high energy behaviors resemble the high energy waterfall features observed in the cuprate superconductors. While strong electron correlation plays an important role in producing high energy waterfall features in the cuprate superconductors, the revelation of the high energy anomalies in Sr2IrO4, which exhibits strong spin-orbit coupling and a moderate electron correlation, points to an unknown and novel route in generating exotic electronic excitations.
19 citations
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TL;DR: By taking high-resolution angle-resolved photoemission measurements on Sr2IrO4 over a wide energy range, it is revealed for the first time that the high energy electronic structures show unusual nearly-vertical bands that extend over a large energy range.
Abstract: The layered 5d transition metal oxides like Sr2IrO4 have attracted significant interest recently due to a number of exotic and new phenomena induced by the interplay between the spin-orbit coupling, bandwidth W and on-site Coulomb correlation U. In contrast to a metallic behavior expected from the Mott-Hubbard model due to more spatially extended 5d orbitals and moderate U, an insulating ground state has been observed in Sr2IrO4. Such an insulating behavior can be understood by an effective J_eff=1/2 Mott insulator model by incorporating both electron correlation and strong spin-orbital coupling, although its validity remains under debate at present. In particular, Sr2IrO4 exhibits a number of similarities to the high temperature cuprate superconductors in the crystal structure, electronic structure, magnetic structure, and even possible high temperature superconductivity. Here we report a new observation of the anomalous high energy electronic structure in Sr2IrO4. By taking high-resolution angle-resolved photoemission measurements on Sr2IrO4 over a wide energy range, we have revealed that the high energy electronic structures show unusual nearly-vertical bands that extend over a large energy range. Such anomalous high energy behaviors resemble the high energy waterfall features observed in the cuprate superconductors, adding one more important similarity between these two systems. While strong electron correlation plays an important role in producing high energy waterfall features in the cuprate superconductors, the revelation of the high energy anomalies in Sr2IrO4 points to a novel route in generating exotic electronic excitations from the strong spin-orbit coupling and a moderate electron correlation.
17 citations
TL;DR: In this article, a laser-based angle-resolved photoemission measurement on the electronic structure of the CuxBi2Se3 superconductor, and a detailed magneto-resistance measurement in both normal and superconducting states were reported.
Abstract: CuxBi2Se3 is a superconductor that is a potential candidate for topological superconductors. We report our laser-based angle-resolved photoemission measurement on the electronic structure of the CuxBi2Se3 superconductor, and a detailed magneto-resistance measurement in both normal and superconducting states. We find that the topological surface state of the pristine Bi2Se3 topological insulator remains robust after the Cu-intercalation, while the Dirac cone location moves downward due to electron doping. Detailed measurements on the magnetic field-dependence of the resistance in the superconducting state establishes an irreversibility line and gives a value of the upper critical field at zero temperature of ~4000 Oe for the Cu0.3Bi2Se3 superconductor with a middle point Tc of 1.9K. The relation between the upper critical field Hc2 and temperature T is different from the usual scaling relation found in cuprates and in other kinds of superconductors. Small positive magneto-resistance is observed in Cu0.3Bi2Se3 superconductors up to room temperature. These observations provide useful information for further study of this possible candidate for topological superconductors.