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Dirac semimetals are classified into different phases based on the types of Dirac fermions. Tuning the transition among different types of Dirac fermions in one system remains a challenge. Recently, KMgBi was predicted to be located at a critical state in which various types of Dirac fermions can be induced owing to the existence of a flatband. Here, we carried out systematic studies on the electronic structure of KMgBi single crystals by combining angle-resolve photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. The flatband was clearly observed near the Fermi level. We also revealed a small bandgap of ∼20 meV between the flatband and the conduction band. These results demonstrate the critical states of KMgBi that transition among various types of Dirac fermions can be tuned in one system. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. ) and Na3Bi (refs , ), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena. [ABSTRACT FROM AUTHOR]

We have investigated the reduction of initial C[sub 60] electronic structure under the influence of a growing number of attached fluorines in C[sub 60]F[sub 48], C[sub 60]F[sub 36], C[sub 60]F[sub 24], and C[sub 60]F[sub 18]. The reduction of the low-energy π-electron subsystem reveals itself in lowering of the optical absorption in the visible region and in gradual blueshift of the luminescence. The absorption of C[sub 60]F[sub 24] and C[sub 60]F[sub 18] films strongly resembles that of C[sub 60], thus showing the similarity of electronic structures of C[sub 60] and these compounds. This resemblance disappears in the absorption spectra of C[sub 60]F[sub 48] and C[sub 60]F[sub 36]. The absorption of these compounds looks like that of C[sub 6]H[sub 6], because the π-bonds are isolated. The luminescence of C[sub 60]F[sub 18] film has an unexpected two-component structure, with the first component close to the C[sub 60] luminescence spectra, and the second one lying approximately in the same region with C[sub 60]F[sub 36]. We attribute this splitting of luminescence spectra to the two different channels for relaxation of the electronic excitation. The first one is correlated with the fluorinated part of the C[sub 60]F[sub 18] molecule, the second one with the part free of fluorine atoms. [ABSTRACT FROM AUTHOR]

The possibility of using graphane monolayer crystals as an electrode material is becoming popular. Graphane is stable at room temperature and has a large surface area, but its chemical inertness hinders its direct interactions with Li ions. In this study, we performed density functional theory calculations to study the energetic stability and structural and electronic properties of Li on graphane with various CH divacancy configurations (v12, v13, and v14). The results show that the adsorption of the Li atom reduces the formation energy of the CH divacancy configurations. The Li-v12 is most stable with the highest binding energy of 3.25 eV/Li and relaxes to in-plane with other C atoms. Altering the Li charge state to have Li−1-v12 or Li+1-v12 affects the energetic stability and electronic characters of Li-v12. The Li−1-v12 (Li+1-v12) slightly (greatly) reduces the binding force between the Li and v12 configuration, and furthermore it improves (deteriorates) the conductivity of the structure. Further investigation of graphane with vacancies is encouraged due to these intriguing observations, as it holds promise for potential utilization as an electrode material. [ABSTRACT FROM AUTHOR]

The spatially resolved ARPES (nanoARPES) is a development of conventional ARPES technique achieved with the focusing of light on the sample into the spot with submicron sizes. This development is used in research of essentially small samples, for example, heterostructures build of flakes of 2D materials, micro-crystals or polycrystalline samples, different crystal termination, domains in electronic structure. ANTARES is delivering the nanoARPES technique to user community since 2010, up to now the instrument was used with samples of various types, and that was useful to accumulate the specific experience. In this paper we report the current layout and actual performance of the ANTARES instrument at Synchrotron SOLEIL, as well as the most typical application areas. The most important and recent upgrades include focusing optics and in-operando setup. The new optical units deliver the increased flux on sample as well as the option to vary the photon energy. The in-operando setup offers the option of electrical connection to the sample being studied with nanoARPES, that promises various applications, where the most demanded now is the control of charge carrier density with applied voltage. This "gate-doping" is often applied to the heterostructures of 2D materials, that could be designed and build on purpose. The 2D heterostructures is probably the most typical field of application of the instrument, with or without in-operando option. At the same time the use case of the ANTARES instrument is still in the development by the realisation of new kinds of samples and of the new types of the experiments. • NanoARPES is a technique providing the angles resolved photoemission with spatial resolution. • ANTARES is a nanoARPES instrument launched about 10 years ago among the first instruments. • The instrument is delivering performance adequate for studies in various research areas. • Research of 2D materials and heterostructures, topological materials, superconductors, and other subjects. • In-operando electrical connections to the sample under measurement provides opportunity to study micro-devices. [ABSTRACT FROM AUTHOR]

We present detailed first-principles density functional theory-based studies on RbRE2Fe4As4O2 (RE = Sm, Tb, Dy, Ho) hybrid 12442-type iron-based superconducting compounds with particular emphasis on competing magnetic interactions and their effect on possible magneto-structural coupling and electronic structure. The stripe antiferromagnetic (sAFM) pattern across the xy plane emerges as the most favorable spin configuration for all the four compounds, with close competition among the different magnetic orders along the z-axis. The structural parameters, including arsenic heights, Fe-As-Fe angle, and other relevant factors that influence superconducting T c and properties, closely match the experimental values in stripe antiferromagnetic arrangement of Fe spins. Geometry optimization with inclusion of explicit magnetic ordering predicts a spin–lattice coupling for all the four compounds, where a weak magneto–structural transition, a tetragonal-to-orthorhombic structural transition, takes place in the relaxed stripe antiferromagnetic spin configuration. Absence of any experimental evidence of such structural transition is possibly an indication of nematic transition in RE-12442 compounds. As a result of structural distortion, the lattice contracts (expands) along the direction with parallel (anti-parallel) alignment of Fe spins. Introduction of stripe antiferromagnetic order in Fe sub-lattice reconstructs the low-energy band structure, which results in significantly reduced number of bands crossing the Fermi level. Moreover, the dispersion of bands and their orbital characteristics also are severely modified in the stripe antiferromagnetic phase similar to BaFe 2 As 2 . Calculations of exchange parameters were performed for all the four compounds. Exchange coupling along the anti-parallel alignment of Fe spins J 1 a is larger than that for the parallel aligned spins J 1 b . A crossover between the super-exchange-driven in-plane next-nearest-neighbor exchange coupling J 2 and in-plane exchange coupling J 1 a due to lanthanide substitution was found. A large super-exchange-driven next-nearest-neighbor exchange interaction is justified using the construction of 32 maximally localized Wannier functions, where the nearest-neighbor Fe-As hopping amplitudes were found to be larger than the nearest- and the next-nearest-neighbor Fe-Fe hopping amplitudes. We compare the hopping parameters in the stripe antiferromagnetic pattern with non-magnetic configuration, and increased hopping amplitude was found along the anti-parallel spin alignment with more majority-spin electrons in Fe d x z and d x y but not in Fe d y z . On the other hand, the hopping amplitudes are increased in stripe antiferromagnetic phase along the parallel spin alignment with more majority-spin electrons in only Fe d y z . This difference in hopping amplitudes in the stripe antiferromagnetic order enables more isotropic hopping. [ABSTRACT FROM AUTHOR]

Chirality depends on particular symmetries. For crystal structures it describes the absence of mirror planes and inversion centers, and in addition to translations, only rotations are allowed as symmetry elements. However, chiral space groups have additional restrictions on the allowed screw rotations as a symmetry element, because they always appear in enantiomorphous pairs. This study classifies and distinguishes the chiral structures and space groups. Chirality is quantified using Hausdorff distances and continuous chirality measures and selected crystal structures are reported. Chirality is discussed for bulk solids and their surfaces. Moreover, the band structure, and thus, the density of states, is found to be affected by the same crystal parameters as chirality. However, it is independent of handedness. The Berry curvature, as a topological measure of the electronic structure, depends on the handedness but is not proof of chirality because it responds to the inversion of a structure. For molecules, optical circular dichroism is one of the most important measures for chirality. Thus, it is proposed in this study that the circular dichroism in the angular distribution of photoelectrons in high symmetry configurations can be used to distinguish the handedness of chiral solids and their surfaces. [ABSTRACT FROM AUTHOR]

Phase separation in the nanometer- to micrometer-scale is characteristic for correlated materials, for example, high temperature superconductors, colossal magnetoresistance manganites, Mott insulators, etc. Resolving the electronic structure with spatially-resolved information is critical for revealing the fundamental physics of such inhomogeneous systems yet this is challenging experimentally. Here by using nanometer- and micrometer-spot angle-resolved photoemission spectroscopies (NanoARPES and MicroARPES), we reveal the spatially-resolved electronic structure in the stripe phase of IrTe2. Each separated domain shows two-fold symmetric electronic structure with the mirror axis aligned along 3 equivalent directions, and 6 × 1 replicas are clearly identified. Moreover, such electronic structure inhomogeneity disappears across the stripe phase transition, suggesting that electronic phase with broken symmetry induced by the 6 × 1 modulation is directly related to the stripe phase transition of IrTe2. Our work demonstrates the capability of NanoARPES and MicroARPES in elucidating the fundamental physics of phase-separated materials. Obtaining spatially-resolved electronic structure information at the microscale is key to a complete understanding of phase transitions and domain separation in the solid-state. Here, micro- and nanoscale angle-resolved photoemission spectroscopy reveals the electronic structure of domains in the striped phase of IrTe2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Co3Sn2S2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co3Sn2S2 with exchange-correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co3Sn2S2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di- and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co3Sn2S2. [ABSTRACT FROM AUTHOR]

At a temperature of roughly 1 K, Sr 2 RuO 4 undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state has not even been understood after 25 years of study. More recently, it has been found that critical temperatures can be enhanced by the application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an "instability" approach and seek divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show that in the unstrained compound, the singlet and triplet instabilities of the normal Fermi liquid phase are closely spaced. Under uniaxial strain, electrons residing on all orbitals contributing to the Fermiology become more coherent, while the electrons of the Ru- d x y character become heavier, and the electrons of the Ru- d x z , y z characters become lighter. In the process, Im χ (q , ω) increases rapidly around q   =   (0.3 , 0.3 , 0) 2 π / a and q   =   (0.5 , 0.25 , 0) 2 π / a , while it gets suppressed at all other commensurate vectors, in particular at q   =   0 , which is essential for spin-triplet superconductivity. We observe that the magnetic anisotropy under strain drops smoothly, which is concomitant with the increment in singlet instability. Thus, the triplet superconducting instability remains the lagging instability of the system, and the singlet instability enhances under strain, leading to a large energy-scale separation between these competing instabilities. However, since this happens even without spin-orbit coupling, we believe it is primarily the enhancement in the spin fluctuation glue around quasi-anti-ferromagnetic vectors that drives the Cooper pairing instead of the magnetic anisotropy. At large strain, an instability to a spin density wave overtakes the superconducting one. The analysis relies on a high-fidelity, ab initio description of the one-particle properties and two-particle susceptibilities, based on the quasiparticle self-consistent GW approximation augmented by dynamical mean field theory. This approach is described and its high fidelity confirmed by comparing to observed one- and two-particle properties. [ABSTRACT FROM AUTHOR]

Abstract—: This paper presents a theoretical study of the effects of crystal structure and Mn and Co substitutions for Cd on the electronic structure of the topological material Cd3As2. We have carried out density functional theory calculations of the band structure and density of states in tetragonal and cubic Cd3As2, as well as in Cd3 -xMnxAs2 and Cd3 -xCoxAs2 crystals. The results indicate that the band structure of the cubic Cd3As2 phase differs significantly from that of a Dirac semimetal, characteristic of the tetragonal phase. It has also been shown that, after Co substitution for 1/24 of the Cd atoms, the structure of the density of 3d electron states is similar to that of the density of states in the magnetic semiconductor Cd3 -xMnxAs2, with a characteristic minimum at the Fermi energy. At the same time, in the case of analogous Mn substitution for Cd, the density of d-electron states has no such minimum. [ABSTRACT FROM AUTHOR]

We have successfully synthesized niobium monophosphide and tantalum monophosphide crystals by a chemical vapor transport technique. We report resistivity vs. temperature of both materials in the temperature range from 2 K to 300 K.We have also performed electronic structure calculations and present the band structure and density of states of these two compounds. The calculations show that both compounds are semimetals, as their conduction and valence bands overlap near the Fermi energy. [ABSTRACT FROM AUTHOR]

We report an angle-resolved photoemission spectroscopy study of a ThCr2Si2-type superconductor YFe2Ge2 (Tc ≈ 1.8 K) in the normal state. The resolved low-energy band structure mainly consists of several flat Fe bands, while the renormalization factors of different bands range from 1.1 to 2.8. Around the Fermi level, these bands contribute to a high density of states, which may account for the unusually high Sommerfeld coefficient Y. Besides, a 30 meV kink is observed, which is probably induced by electron-phonon interactions. Our results indicate that YFe2Ge2 is a moderately correlated compound, while electron-phonon coupling should be taken into account to better understand its properties. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. ) and Na3Bi (refs , ), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena. [ABSTRACT FROM AUTHOR]

A fundamental part of the puzzle of unconventional superconductivity in the Fe-based superconductors is the understanding of the magnetic and nematic instabilities of the parent compounds. The issues of which of these can be considered the leading instability, and whether weak- or strong-coupling approaches are applicable, are both critical and contentious. Here, we revisit the electronic structure of BaFe2As2 using angle-resolved photoemission spectroscopy (ARPES). Our high-resolution measurements of samples "detwinned" by the application of a mechanical strain reveal a highly anisotropic 3D Fermi surface in the low-temperature antiferromagnetic phase. By comparison of the observed dispersions with ab initio calculations, we argue that overall it is magnetism, rather than orbital/nematic ordering, which is the dominant effect, reconstructing the electronic structure across the Fe 3d bandwidth. Finally, using a state-of-the-art nano-ARPES system, we reveal how the observed electronic dispersions vary in real space as the beam spot crosses domain boundaries in an unstrained sample, enabling the measurement of ARPES data from within single antiferromagnetic domains, and showing consistence with the effective mono-domain samples obtained by detwinning. [ABSTRACT FROM AUTHOR]

We calculated the band structures of a variety of N- and S-doped graphenes in order to understand the effects of the N and S dopants on the graphene electronic structure using density functional theory (DFT). Band-structure analysis revealed energy band upshifting above the Fermi level compared to pristine graphene following doping with three nitrogen atoms around a mono-vacancy defect, which corresponds to p-type nature. On the other hand, the energy bands were increasingly shifted downward below the Fermi level with increasing numbers of S atoms in N/S-co-doped graphene, which results in n-type behavior. Hence, modulating the structure of graphene through N- and S-doping schemes results in the switching of "p-type" to "n-type" behavior with increasing S concentration. Mulliken population analysis indicates that the N atom doped near a mono-vacancy is negatively charged due to its higher electronegativity compared to C, whereas the S atom doped near a mono-vacancy is positively charged due to its similar electronegativity to C and its additional valence electrons. As a result, doping with N and S significantly influences the unique electronic properties of graphene. Due to their tunable band-structure properties, the resulting N- and S-doped graphenes can be used in energy and electronic-device applications. In conclusion, we expect that doping with N and S will lead to new pathways for tailoring and enhancing the electronic properties of graphene at the atomic level. [ABSTRACT FROM AUTHOR]

van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of (PbSe)1.16(TiSe2)m (m=1, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the TiSe2 layers is halved from m=2 to m=1, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the m=2 compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10 K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs. [ABSTRACT FROM AUTHOR]

Recently, commercially available copper foil has become an efficient and inexpensive catalytic substrate for scalable growth of large-area graphene films for fundamental research and applications. Interestingly, despite its hexagonal honeycomb lattice, graphene can be grown into large aligned square-shaped sheets on copper foils. Here, by applying angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES) to study the three-dimensional electronic structures of square graphene sheets grown on copper foils, we verified the high quality of individual square graphene sheets as well as their merged regions (with aligned orientation). Furthermore, by simultaneously measuring the graphene sheets and their substrate copper foil, we not only established the (001) copper surface structure but also discovered that the square graphene sheets’ sides align with the ⟨110⟩ copper direction, suggesting an important role of copper substrate in the growth of square graphene sheets—which will help the development of effective methods to synthesize high-quality large-size regularly shaped graphene sheets for future applications. This work also demonstrates the effectiveness of micro-ARPES in exploring low-dimensional materials down to atomic thickness and sub-micron lateral size (e.g., besides graphene, it can also be applied to transition metal dichalcogenides and various van der Waals heterostructures) [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh2Si2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh2Si2 (RE = rare-earth elements) compounds are discussed. [ABSTRACT FROM AUTHOR]

PtBi2 with a layered hexagonal crystal structure was recently reported to exhibit an unconventional large linear magnetoresistance, while the mechanism involved is still elusive. Using high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on its bulk and surface electronic structure. Through careful comparison with first-principle calculations, our experiment distinguishes the low-lying bulk bands from entangled surface states, allowing the estimation of the real composition of samples. We find significant electron doping in PtBi2, implying a substantial Bi-deficiency-induced disorder therein. Intriguingly, we discover a Dirac-cone-like surface state on the boundary of the Brillouin zone, which is identified as an accidental Dirac band without topological protection. Our findings exclude linear band dispersion in the quantum limit as the cause of the unconventional large linear magnetoresistance but give support to the classical disorder model from the perspective of the electronic structure. [ABSTRACT FROM AUTHOR]

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 meV 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 the 3D 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. [ABSTRACT FROM AUTHOR]