Angle-resolved photoemission study of NbGeSb with non-symmorphic symmetry

We investigate the electronic structure of NbGeSb with non-symmorphic symmetry.We employ angle-resolved photoemission spectroscopy(ARPES)to observe and identify the bulk and surface states over the Brillouin zone.By utilizing high-energy photons,we identify the bulk Fermi surface and bulk nodal line along the direction X–R,while the Fermi surface of the surface state is observed by using low-energy photons.We observe the splitting of surface bands away from the high-symmetry point X.The density functional theory calculations on bulk and 1 to 5-layer slab models,as well as spin textures of NbGeSb,verify that the band splitting could be attributed to the Rashba-like spin–orbit coupling caused by space-inversion-symmetry breaking at the surface.These splitted surface bands cross with each other,forming two-dimensional Weyl-like crossings that are protected by mirror symmetry.Our findings provide insights into the two-dimensional topological and symmetry-protected band inversion of surface states.

Negligible normal fluid in superconducting state of heavily overdoped Bi_(2)Sr_(2)CaCu_(2)O_(8+δ) detected by ultra-low temperature angle-resolved photoemission spectroscopy

In high temperature cuprate superconductors,it was found that the superfluid density decreases with the increase of hole doping.One natural question is whether there exists normal fluid in the superconducting state in the overdoped region.In this paper,we have carried out high-resolution ultra-low temperature laser-based angle-resolved photoemission measurements on a heavily overdoped Bi2212 sample with a T_(c) of 48 K.We find that this heavily overdoped Bi2212 remains in the strong coupling regime with 2Δ_(0)/(k_(B)T_(c))=5.8.The single-particle scattering rate is very small along the nodal direction(~5 meV) and increases as the momentum moves from the nodal to the antinodal regions.A hard superconducting gap opening is observed near the antinodal region with the spectral weight at the Fermi level fully suppressed to zero.The normal fluid is found to be negligibly small in the superconducting state of this heavily overdoped Bi2212.These results provide key information to understand the high T_(c) mechanism in the cuprate superconductors.

Visualizing the electronic structure of kagome magnet LuMn6Sn6 by angle-resolved photoemission spectroscopy

Searching for the dispersionless flat band(FB)in quantum materials,especially in topological systems,becomes an interesting topic.The kagome lattice is an ideal platform for such exploration because the FB can be naturally induced by the underlying destructive interference.Nevertheless,the magnetic kagome system that hosts the FB close to the Fermi level(EF)is exceptionally rare.Here,we study the electronic structure of a kagome magnet LuMn6Sn6 by combining angleresolved photoemission spectroscopy and density functional theory calculations.The observed Fermi-surface topology and overall band dispersions are similar to previous studies of the XMn6Sn6(X=Dy,Tb,Gd,Y)family of compounds.We clearly observe two kagome-derived FBs extending through the entire Brillouin zone,and one of them is located just below EF.The photon-energy-dependent measurements reveal that these FBs are nearly dispersionless along the kz direction as well,supporting the quasi-two-dimensional character of such FBs.Our results complement the XMn6Sn6 family and demonstrate the robustness of the FB features across this family.

Optical manipulation of the topological phase in ZrTe_(5) revealed by time-and angle-resolved photoemission

High-resolution time-and angle-resolved photoemission measurements were conducted on the topological insulator ZrTe_(5).With strong femtosecond photoexcitation,a possible ultrafast phase transition from a weak to a strong topological insulating phase was experimentally realized by recovering the energy gap inversion in a time scale that was shorter than 0.15 ps.This photoinduced transient strong topological phase can last longer than 2 ps at the highest excitation fluence studied,and it cannot be attributed to the photoinduced heating of electrons or modification of the conduction band filling.Additionally,the measured unoccupied electronic states are consistent with the first-principles calculation based on experimental crystal lattice constants,which favor a strong topological insulating phase.These findings provide new insights into the longstanding controversy about the strong and weak topological properties in ZrTe_(5),and they suggest that many-body effects including electron–electron interactions must be taken into account to understand the equilibrium weak topological insulating phase in ZrTe_(5).

Angle-resolved photoemission spectroscopy study on iron-based superconductors

Angle-resolved photoemission spectroscopy （ARPES） has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.

Common Electronic Features and Electronic Nematicity in Parent Compounds of Iron-Based Superconductors and FeSe/SrTiO_3 Films Revealed by Angle-Resolved Photoemission Spectroscopy

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.

Identification of Topological Surface State in PdTe2 Superconductor by Angle-Resolved Photoemission Spectroscopy

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.

Growth of High-Quality Superconducting FeSe0.5Te0.5 Thin Films Suitable for Angle-Resolved Photoemission Spectroscopy Measurements via Pulsed Laser Deposition

High-quality superconducting FeSe0.5 Te0.5 films are epitaxiMly grown on different substrates by using the pulsed laser deposition method. By measuring the transport properties and surface morphology of films grown on single- crystal substrates of Al2O3 （0001）, SrTiO3 （001）, and MgO （001）, as well as monitoring the real-time growth process on MgO substrates with reflection high energy electron diffraction, we find the appropriate parameters for epitaxial growth of high-quality FeSe0.5 Te0.5 thin films suitable for angle-resolved photoemission spectroscopy measurements. We further report the angle-resolved photoemission spectroscopy characterization of the super- conducting films. The clearly resolved Fermi surfaces and the band structure suggest a sample quality that is as good as that of high-quality single-crystals, demonstrating that the pulsed laser deposition method can serve as a promising technique for in situ preparation and manipulation of iron-based superconducting thin films, which may bring new prosperity to angle-resolved photoemission spectroscopy research on iron-based superconductors.

High-resolution angle-resolved photoemission study of oxygen adsorbed Fe/MgO(001)

We have investigated the electronic states of clean Fe(001) and oxygen adsorbed Fe(001)–p(1 × 1)-O films epitaxially grown on MgO(001) substrates by means of polarization-dependent angle-resolved photoemission spectroscopy(ARPES)and extensive density-functional theory(DFT) calculations. The observed Fermi surfaces and band dispersions of pure Fe near the Fermi level were modified upon oxygen adsorption. By the detailed comparison of ARPES and DFT results of the oxygen adsorbed Fe surface, we have clarified the orbital-dependent p–d hybridization in the topmost and second Fe layers.Furthermore, the observed energy levels and Fermi wave numbers for the oxygen adsorbed Fe surface were deviated from the DFT calculations depending on the orbital characters and momentum directions, indicating an anisotropic interplay of the electron correlation and p–d hybridization effects in the surface region.

Flat band in hole-doped transition metal dichalcogenide observed by angle-resolved photoemission spectroscopy

Layered transition metal dichalcogenides(TMDCs)gained widespread attention because of their electron-correlationrelated physics,such as charge density wave(CDW),superconductivity,etc.In this paper,we report the high-resolution angle-resolved photoemission spectroscopy(ARPES)studies on the electronic structure of Ti-doped 1T-Ti_(x)Ta_(1-x)S_(2) with different doping levels.We observe a flat band that originates from the formation of the star of David super-cell at the x=5%sample at the low temperature.With the increasing Ti doping levels,the flat band vanishes in the x=8%sample due to the extra hole carrier.We also find the band shift and variation of the CDW gap caused by the Ti-doping.Meanwhile,the band folding positions and the CDW vector g_(CDW)intact.Our ARPES results suggest that the localized flat band and the correlation effect in the 1T-TMDCs could be tuned by changing the filling factor through the doping electron or hole carriers.The Ti-doped 1T-Ti_(x)Ta_(1-x)S_(2) provides a platform to fine-tune the electronic structure evolution and a new insight into the strongly correlated physics in the TMDC materials.

Detailed electronic structure of three-dimensional Fermi surface and its sensitivity to charge density wave transition in ZrTe3 revealed by high resolution laser-based angle-resolved photoemission spectroscopy

The detailed information of the electronic structure is the key to understanding the nature of charge density wave （CDW） order and its relationship with superconducting order in the microscopic level. In this paper, we present a high resolution laser-based angle-resolved photoemission spectroscopy （ARPES） study on the three-dimensional （3D） hole-like Fermi surface around the Brillouin zone center in a prototypical quasi-one-dimensional CDW and superconducting system ZrTe3. Double Fermi surface sheets are clearly resolved for the 3D hole-like Fermi surface around the zone center. The 3D Fermi surface shows a pronounced shrinking with increasing temperature. In particular, the quasiparticle scattering rate along the 3D Fermi surface experiences an anomaly near the charge density wave transition temperature of ZrTe3 - 63 K）. The signature of electron-phonon coupling is observed with a dispersion kink at -20 meV; the strength of the electron-phonon coupling around the 3D Fermi surface is rather weak. These results indicate that the 3D Fermi surface is also closely connected to the charge-density-wave transition and suggest a more global impact on the entire electronic structure induced by the CDW phase transition in ZrTe3.

High-resolution angle-resolved photoemission study of large magnetoresistance topological semimetal CaAl4

Extremely large magnetoresistance(XMR)has been explored in many nonmagnetic topologically nontrivial/trivial semimetals,while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures.In this paper,the magnetoresistance origin of single crystal CaAl4 with C2/m structure at low temperature is investigated,exhibiting unsaturated magnetoresistance of~3000%at 2.5 K and 14 T as the fingerprints of XMR materials.By the combination of ARPES and the first-principles calculations,we elaborate multiband features and anisotropic Fermi surfaces,which can explain the mismatch of isotropic two-band model.Although the structural phase transition from I4/mmm to C2/m has been recognized,the subtle impact on electronic structure is revealed by our ARPES measurements.Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl4,our findings report CaAl4 as a new reference material for exploring the XMR phenomena.

Evidence of Electron-Hole Imbalance in WTe2 from High-Resolution Angle-Resolved Photoemission Spectroscopy

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.

Electronic structure of transition metal dichalcogenides PdTe_2 and Cu_(0.05)PdTe_2 superconductors obtained by angle-resolved photoemission spectroscopy

The layered transition metal chalcogenides have been a fertile land in solid state physics for many decades. Various MX2-type transition metal dichalcogenides, such as WTe2, IrTe2, and MoS2, have triggered great attention recently, either for the discovery of novel phenomena or some extreme or exotic physical properties, or for their potential applications. PdTe2 is a superconductor in the class of transition metal dichalcogenides, and superconductivity is enhanced in its Cu- intercalated form, Cuo.05PdTe2. It is important to study the electronic structures of PdTe2 and its intercalated form in order to explore for new phenomena and physical properties and understand the related superconductivity enhancement mecha- nism. Here we report systematic high resolution angle-resolved photoemission （ARPES） studies on PdTe2 and Cuo.05PdTe2 single crystals, combined with the band structure calculations. We present in detail for the first time the complex multi-band Fermi surface topology and densely-arranged band structure of these compounds. By carefully examining the electronic structures of the two systems, we find that Cu-intercalation in PdTe2 results in electron-doping, which causes the band structure to shift downwards by nearly 16 meV in Cuo.05PdTe2. Our results lay a foundation for further exploration and investigation on PdTe2 and related superconductors.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8

We utilize high-resolution resonant angle-resolved photoemission spectroscopy(ARPES)to study the band structure and hybridization effect of the heavy-fermion compound Ce2 IrIn8.We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh^40 K,which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization.However,the Fermi vector and the Fermi surface volume have little change around Tcoh,which challenges the widely believed evolution from a hightemperature small Fermi surface to a low-temperature large Fermi surface.Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Angle-resolved photoemission spectroscopy in Hubbard model based on cluster perturbation

Angle-resolved photoemission spectra(ARPES) are calculated in the Hubbard model by using cluster perturbation method. It is found that in a cluster of 12 sites, the local density of states displays the phase transition from normal conductor to Mott insulator with the increase of the electron-electron coupling. We show that a pseudogap develops from the metallic phase to the insulating phase. Evidence of spin-charge separation is also verified in the calculated single particle spectral functions.

Angle-Resolved and Resonant Photoemission Study of the Valence Bands of α-La(0001) on W(110)

We report a photoelectron spectroscopic study of the valence bands of double hexagonal-close-packed (dhcp) α-La(0001) films epitaxially grown on W(110) at room temperature. The La 5d photoemission cross section in the photon energy region from 20 eV to 130 eV was obtained and the valence-band structure of α-La was determined. Except for 4f-related structures, the valence-band structures of dhcp α-La and dhcp β-Ce were found to resemble each other. From the band structure, the crystal structure of the La film was confirmed. No evidence for the existence of a 5d-like surface state near the Fermi energy at the point of the surface Brillouin zone was obtained and a 6s band bottom was identified.

Ultrafast photoemission electron microscopy:A multidimensional probe of nonequilibrium physics

Exploring the realms of physics that extend beyond thermal equilibrium has emerged as a crucial branch of condensed matter physics research.It aims to unravel the intricate processes involving the excitations,interactions,and annihilations of quasi-and many-body particles,and ultimately to achieve the manipulation and engineering of exotic non-equilibrium quantum phases on the ultrasmall and ultrafast spatiotemporal scales.Given the inherent complexities arising from many-body dynamics,it therefore seeks a technique that has efficient and diverse detection degrees of freedom to study the underlying physics.By combining high-power femtosecond lasers with real-or momentum-space photoemission electron microscopy(PEEM),imaging excited state phenomena from multiple perspectives,including time,real space,energy,momentum,and spin,can be conveniently achieved,making it a unique technique in studying physics out of equilibrium.In this context,we overview the working principle and technical advances of the PEEM apparatus and the related laser systems,and survey key excited-state phenomena probed through this surface-sensitive methodology,including the ultrafast dynamics of electrons,excitons,plasmons,spins,etc.,in materials ranging from bulk and nano-structured metals and semiconductors to low-dimensional quantum materials.Through this review,one can further envision that time-resolved PEEM will open new avenues for investigating a variety of classical and quantum phenomena in a multidimensional parameter space,offering unprecedented and comprehensive insights into important questions in the field of condensed matter physics.

Photoemission study of iron-based superconductor

The iron-based superconductivity （IBSC） is a great challenge in correlated system. Angle-resolved photoemission spectroscopy （ARPES） provides electronic structure of the IBSCs, the pairing strength, and the order parameter symmetry. Here, we briefly review the recent progress in IBSCs and focus on the results from ARPES. The ARPES study shows the electronic structure of ＂122＂, ＂111＂, ＂11＂, and ＂122＂＂ families of IBSCs. It has been agreed that the IBSCs are unconventional superconductors in strong coupling region. The order parameter symmetry basically follows s form with considerable out-of-plane contribution.

A Surface Femtosecond Two-Photon Photoemission Spectrometer for Excited Electron Dynamics and Time-Dependent Photochemical Kinetics

A surface femtosecond two-photon photoemission （2PPE） spectrometer devoted to the study of ultrafast excited electron dynamics and photochemical kinetics on metal and metal oxide surfaces has been constructed. Low energy photoelectrons are measured using a hemispherical electron energy analyzer with an imaging detector that allows us to detect the energy and the angular distributions of the photoelectrons simultaneously. A Mach-Zehnder interferom- eter was built for the time-resolved 2PPE （TR-2PPE） measurement to study ultrafast surface excited electron dynamics, which was demonstrated on the Cu（111） surface. A scheme for measuring time-dependent 2PPE （TD-2PPE） spectra has also been developed for studies of surface photochemistry. This technique has been applied to a preliminary study on the photochemical kinetics on ethanol/TiO2（110）. We have also shown that the ultrafast dynamics of photoinduced surface excited resonances can be investigated in a reliable way by combining the TR-2PPE and TD-2PPE techniques.