The three-dimensional（3D） Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be v...The three-dimensional（3D） Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A3Bi（A = Na, K, Rb）. Here we report comprehensive high-resolution angle-resolved photoemission（ARPES） measurements on the two cleaved surfaces,（001） and（100）, of Na3Bi. On the（001） surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the kx–ky plane and by varying the photon energy to get access to different out-of-plane kzs. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the（100） surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the（100） plane. We directly observe two isolated 3D Dirac nodes on the（100） surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ~150 me V before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na3Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the3 D Dirac cones, on the possible formation of surface reconstruction of the（001） surface, and on the issue of basic Brillouin zone selection for the（100） surface.展开更多
We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and u...We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and undoped FeSe/SrTiO3 films with 1, 2 and 20 layers. While the electronic structure near tile Brillouin zone center F varies dramatically among different materials, the electronic structure near the Brillouin zone corners （M points）, as well as their temperature dependence, are rather similar. The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the dxz and dyz bands below the nematie transition temperature. A clear relation is observed between the band splitting magnitude arid the onset temperature of nematicity. Our results may shed light on the origin of nematicity, its effect on the electronic structures, and its relation with superconductivity in the iron-based superconductors.展开更多
High-resolution angle-resolved photoemission measurements are carried out on transition metal dichalcogenide PdTe2 that is a superconductor with a Tc at 1.7K. Combined with theoretical calculations, we discover for th...High-resolution angle-resolved photoemission measurements are carried out on transition metal dichalcogenide PdTe2 that is a superconductor with a Tc at 1.7K. 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 ehalcogenides.展开更多
Dirac semimetals are materials in which the conduction and the valence bands have robust crossing points protected by topology or symmetry. Recently, a new type of Dirac semimetals, so called the Dirac line-node semim...Dirac semimetals are materials in which the conduction and the valence bands have robust crossing points protected by topology or symmetry. Recently, a new type of Dirac semimetals, so called the Dirac line-node semimetals （DLNSs）, have attracted a lot of attention, as they host robust Dirac points along the one-dimensional （1D） lines in the Brillouin zone （BZ）. In this work, using angle-resolved photoemission spectroscopy （ARPES） and first-principles calculations, we systematically investigated the electronic structures of non-symmorphic ZrSiS crystal where we clearly distinguished the surface states from the bulk states. The photon-energy-dependent measurements further prove the existence of Dirac line node along the X-R direction. Remarkably, by in situ surface potassium doping, we clearly observed the different evolutions of the bulk and surface electronic states while proving the robustness of the Dirac line node. Our studies not only reveal the complete electronic structures of ZrSiS, but also demonstrate the method manipulating the electronic structure of the compound.展开更多
As a van der Waals ferromagnet with high Curie temperature,Fe_(5-x)GeTe_(2) has attracted tremendous interests recently.Here,using high-resolution angle-resolved photoemission spectroscopy(ARPES),we systematically inv...As a van der Waals ferromagnet with high Curie temperature,Fe_(5-x)GeTe_(2) has attracted tremendous interests recently.Here,using high-resolution angle-resolved photoemission spectroscopy(ARPES),we systematically investigated the electronic structure of Fe_(5-x)GeTe_(2) crystals and its temperature evolution.Our ARPES measurement reveals two types of band structures from two different terminations with slight kz evolution.Interestingly,across the ferromagnetic transition,we observed the merging of two split bands above the Curie temperature,suggesting the band splitting due to the exchange interaction within the itinerant Stoner model.Our results provide important insights into the electronic and magnetic properties of Fe_(5-x)GeTe_(2) and the understanding of magnetism in a two-dimensional ferromagnetic system.展开更多
WTe2 has attracted a great deal of attention because it exhibits extremely large and non-saturating magnetore- sistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-b...WTe2 has attracted a great deal of attention because it exhibits extremely large and non-saturating magnetore- sistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe2. This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concen- tration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range,and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe2. We identify a fiat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a fiat band can play an important role in dictating the transport properties of WTe2. Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe2.展开更多
Cux Bi2Se3 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 supercondu...Cux Bi2Se3 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 Dirae 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.g K. The relation between the upper critical field He2 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 Cuo.3Bi2Se3 superconductors up to room temperature. These observations provide useful information for further study of this possible candidate for topological superconductors.展开更多
High-resolution laser-based angle-resolved photoemission （ARPES） measurements are carried out on Sb（111） single crystal. Two kinds of Fermi surface sheets are observed, which are derived from the topological surfa...High-resolution laser-based angle-resolved photoemission （ARPES） measurements are carried out on Sb（111） single crystal. Two kinds of Fermi surface sheets are observed, which are derived from the topological surface states： one small hexagonal electron-like Fermi pocket around F point and the other six elongated lobes of holelike Fermi pockets around the electron pocket. Clear Rashba-type band splitting due to the strong spin-orbit coupling is observed to be anisotropic in the momentum space. Our super-high-resolution AFtPES measurements reveal no obvious kink in the surface band dispersions, indicating a weak electron-phonon interaction in the surface states. In particular, the electron scattering rate for these topological surface states is nearly a constant over a large energy window near the Fermi level that is unusual in terms of the conventional picture.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.11574367)the National Basic Research Program of China(Grant Nos.2013CB921700,2013CB921904,and 2015CB921300)+2 种基金the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB07020300)The synchrotron radiation experiments have been done under the HiSOR Proposal numbers12-B-47 and 13-B-16
文摘The three-dimensional（3D） Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A3Bi（A = Na, K, Rb）. Here we report comprehensive high-resolution angle-resolved photoemission（ARPES） measurements on the two cleaved surfaces,（001） and（100）, of Na3Bi. On the（001） surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the kx–ky plane and by varying the photon energy to get access to different out-of-plane kzs. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the（100） surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the（100） plane. We directly observe two isolated 3D Dirac nodes on the（100） surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ~150 me V before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na3Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the3 D Dirac cones, on the possible formation of surface reconstruction of the（001） surface, and on the issue of basic Brillouin zone selection for the（100） surface.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11190022,11334010 and 11534007the National Basic Research Program of China under Grant No 2015CB921000the Strategic Priority Research Program(B)of Chinese Academy of Sciences under Grant No XDB07020300
文摘We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and undoped FeSe/SrTiO3 films with 1, 2 and 20 layers. While the electronic structure near tile Brillouin zone center F varies dramatically among different materials, the electronic structure near the Brillouin zone corners （M points）, as well as their temperature dependence, are rather similar. The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the dxz and dyz bands below the nematie transition temperature. A clear relation is observed between the band splitting magnitude arid the onset temperature of nematicity. Our results may shed light on the origin of nematicity, its effect on the electronic structures, and its relation with superconductivity in the iron-based superconductors.
基金the National Natural Science Foundation of China under Grant Nos 11190022,11274359 and 11422428the National Basic Research Program of China under Grant Nos 2011CB921703,2011CBA00110,2011CBA00108 and 2013CB921700the Strategic Priority Research Program(B)of the Chinese Academy of Sciences under Grant Nos XDB07020300 and XDB07020100
文摘High-resolution angle-resolved photoemission measurements are carried out on transition metal dichalcogenide PdTe2 that is a superconductor with a Tc at 1.7K. 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 ehalcogenides.
基金Project supported by the National Key R&D Program of China(Grant No.2017YFA0305400)Chinese Academy of Science–Shanghai Science Research Center(Grant No.CAS-SSRC-YH-2015-01)+2 种基金the National Natural Science Foundation of China(Grant No.11674229)the Engineering and Physical Sciences Research Council Platform(Grant No.EP/M020517/1)the Hefei Science–Center Chinese Academy of Sciences(Grant No.2015HSC-UE013)
文摘Dirac semimetals are materials in which the conduction and the valence bands have robust crossing points protected by topology or symmetry. Recently, a new type of Dirac semimetals, so called the Dirac line-node semimetals （DLNSs）, have attracted a lot of attention, as they host robust Dirac points along the one-dimensional （1D） lines in the Brillouin zone （BZ）. In this work, using angle-resolved photoemission spectroscopy （ARPES） and first-principles calculations, we systematically investigated the electronic structures of non-symmorphic ZrSiS crystal where we clearly distinguished the surface states from the bulk states. The photon-energy-dependent measurements further prove the existence of Dirac line node along the X-R direction. Remarkably, by in situ surface potassium doping, we clearly observed the different evolutions of the bulk and surface electronic states while proving the robustness of the Dirac line node. Our studies not only reveal the complete electronic structures of ZrSiS, but also demonstrate the method manipulating the electronic structure of the compound.
基金the National Key R&D Program of China(Grant No.2017YFA0305400)。
文摘As a van der Waals ferromagnet with high Curie temperature,Fe_(5-x)GeTe_(2) has attracted tremendous interests recently.Here,using high-resolution angle-resolved photoemission spectroscopy(ARPES),we systematically investigated the electronic structure of Fe_(5-x)GeTe_(2) crystals and its temperature evolution.Our ARPES measurement reveals two types of band structures from two different terminations with slight kz evolution.Interestingly,across the ferromagnetic transition,we observed the merging of two split bands above the Curie temperature,suggesting the band splitting due to the exchange interaction within the itinerant Stoner model.Our results provide important insights into the electronic and magnetic properties of Fe_(5-x)GeTe_(2) and the understanding of magnetism in a two-dimensional ferromagnetic system.
基金Supported by the National Natural Science Foundation of China under Grant No 11574367the National Basic Research Program of China under Grant Nos 2013CB921904 and 2015CB921300+2 种基金the National Key Research and Development Program of China under Grant No 2016YFA0300600the Strategic Priority Research Program(B)of the Chinese Academy of Sciences under Grant No XDB07020300the US Department of Energy under Grant No DE-SC0014208
文摘WTe2 has attracted a great deal of attention because it exhibits extremely large and non-saturating magnetore- sistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe2. This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concen- tration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range,and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe2. We identify a fiat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a fiat band can play an important role in dictating the transport properties of WTe2. Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe2.
基金the National Natural Science Foundation of China under Grant Nos 11190022,91021006 and 11374338the National Basic Research Program of China under Grant Nos 2011CB921703,2011CBA00110 and 2013CB921700the Strategic Priority Research Program(B) of the Chinese Academy of Sciences under Grant No XDB07020300
文摘Cux Bi2Se3 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 Dirae 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.g K. The relation between the upper critical field He2 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 Cuo.3Bi2Se3 superconductors up to room temperature. These observations provide useful information for further study of this possible candidate for topological superconductors.
基金Supported by the National Natural Science Foundation of China under Grant Nos 91021006 and 11174346, and the National Basic Research Program of China under Grant Nos 2011CB921703, 2013CB921700 and 2013CB921904.
文摘High-resolution laser-based angle-resolved photoemission （ARPES） measurements are carried out on Sb（111） single crystal. Two kinds of Fermi surface sheets are observed, which are derived from the topological surface states： one small hexagonal electron-like Fermi pocket around F point and the other six elongated lobes of holelike Fermi pockets around the electron pocket. Clear Rashba-type band splitting due to the strong spin-orbit coupling is observed to be anisotropic in the momentum space. Our super-high-resolution AFtPES measurements reveal no obvious kink in the surface band dispersions, indicating a weak electron-phonon interaction in the surface states. In particular, the electron scattering rate for these topological surface states is nearly a constant over a large energy window near the Fermi level that is unusual in terms of the conventional picture.
