Angle-resolved Photoemission Spectroscopy Spectra Research Articles

Br-Vacancies Induced Variable Ranging Hopping Conduction in High-Order Topological Insulator Bi4Br4.

The defects have a remarkable influence on the electronic structures and the electric transport behaviors of the matter, providing the additional means to engineering their physical properties. In this work, a comprehensive study on the effect of Br-vacancies on the electronic structures and transport behaviors in the high-order topological insulator Bi4Br4 is performed by the combined techniques of the scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and physical properties measurement system along with the first-principle calculations. The STM results show the defects on the cleaved surface of a single crystal and reveal that the defects are correlated to the Br-vacancies with the support of the simulated STM images. The role of the Br-vacancies in the modulation of the band structures has been identified by ARPES spectra and the calculated energy-momentum dispersion. The relationship between the Br-vacancies and the semiconducting-like transport behaviors at low temperature has been established, implying a Mott variable ranging hopping conduction in Bi4Br4. The work not only resolves the unclear transport behaviors in this matter, but also paves a way to modulate the electric conduction path by the defects engineering.

Electronic structure and effective mass of pristine and Cl-doped CsPbBr3

Organic–inorganic lead halide perovskites (LHPs) have attracted great interest owing to their outstanding optoelectronic properties. Typically, the underlying electronic structure would determinate the physical properties of materials. But as for now, limited studies have been done to reveal the underlying electronic structure of this material system, comparing to the huge amount of investigations on the material synthesis. The effective mass of the valance band is one of the most important physical parameters which plays a dominant role in charge transport and photovoltaic phenomena. In pristine CsPbBr3, the Fröhlich polarons associated with the Pb–Br stretching modes are proposed to be responsible for the effective mass renormalization. In this regard, it would be very interesting to explore the electronic structure in doped LHPs. Here, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on both pristine and Cl-doped CsPbBr3. The experimental band dispersions are extracted from ARPES spectra along both Γ¯ – M¯ – Γ¯ and X¯ – M¯ – X¯ high symmetry directions. DFT calculations are performed and directly compared with the ARPES data. Our results have revealed the band structure of Cl-doped CsPbBr3 for the first time, which have also unveiled the effective mass renormalization in the Cl-doped CsPbBr3 compound. Doping dependent measurements indicate that the chlorine doping could moderately tune the renormalization strength. These results will help understand the physical properties of LHPs as a function of doping.

Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of AB-Stacked Crystals.

We investigate the electronic structure of 2H-NbS_{2} and h-BN by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the k_{z}=π/c plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra. Our results show that the near-surface electronic structure can be qualitatively different from the bulk electronic structure due to partially broken nonsymmorphic symmetries.

Neural-Network-Based Methods for ARPES Data Processing (Review Article)

In recent years, many developed upgrades of angle-resolved photoemission spectroscopy (ARPES) have significantly increased the amount of the obtained data. In this article, we briefly review the methods of processing of ARPES spectra with the use of convolutional neural networks (CNNs). In addition, we have made a short checkup of the potential application of CNNs that outperforms the existing methods or gives the possibility to achieve previously unachievable results.

An algorithm for subtraction of doublet emission lines in angle-resolved photoemission spectroscopy

Plasma discharge lamps are widely utilized in the practice of angle-resolved photoemission spectroscopy (ARPES) experiments as narrow-linewidth ultraviolet photon sources. However, many emission lines such as Ar-I, Ne-I, and Ne-II have closely spaced doublet emission lines, which result in superimposed replica on the measured ARPES spectra. Here, we present a simple method for subtracting the contribution of these doublet emission lines from photoemission spectra. Benchmarking against ARPES spectra of well-characterized 2D materials, we demonstrate that this algorithm manages to subtract the doublet signal and reproduce the key features of the monochromated He-Iα spectra in a physically sound manner that reliably reproduces quantifiable dispersion relations and quasiparticle lifetimes.

Type‐II Dirac Nodal Lines in a Double‐Kagome‐Layered Semimetal

AbstractLorentz‐violating type‐II Dirac nodal line semimetals (DNLSs), hosting curves of band degeneracy formed by two dispersion branches with the same sign of slope, represent a novel state of matter. While being studied extensively in theory, convincing experimental evidence of type‐II DNLSs remain elusive. Recently, vanadium‐based kagome materials have emerged as a fertile ground to study the interplay between lattice symmetry and band topology. This work studies the low‐energy band structure of double‐kagome‐layered CsV8Sb12 and identifies it as a scarce type‐II DNLS protected by mirror symmetry. This work observes multiple DNLs consisting of type‐II Dirac cones close to or almost at the Fermi level via angle‐resolved photoemission spectroscopy (ARPES), which provides an electronic explanation for the nonsaturating magnetoresistance effect as observed. First‐principles theory analyses show that spin‐orbit coupling only opens a small gap, resulting in effectively gapless ARPES spectra, yet generating large spin Berry curvature. These type‐II DNLs, together with the interaction between a low‐energy van Hove singularity and quasi‐one‐dimensional band as observed in the same material, suggest CsV8Sb12 as an ideal platform for exploring novel transport properties.

Electronic structure and anharmonic phonon mode in BaIr2Ge7 with two-dimensional Ba-Ge networks studied by photoemission spectroscopy

We report the electronic structure of ${\mathrm{BaIr}}_{2}{\mathrm{Ge}}_{7}$ with two types of cage structure by means of angle-resolved photoemission spectroscopy (ARPES) and hard x-ray photoemission spectroscopy. ARPES spectra reveal the three-dimensional and multiband Fermi surfaces (FSs) originating from the hybridized Ir $5d$ and Ge $4p$ orbitals. The observed FSs show ${C}_{2}$ symmetry, reflecting the orthorhombic $Ammm$ crystal structure of ${\mathrm{BaIr}}_{2}{\mathrm{Ge}}_{7}$. The temperature dependence of the ARPES spectra exhibits the thermal spectral broadening, and the width of the spectral peak shows a concave-downward behavior with temperature. Considering the effect of anharmonic phonon modes, we have reproduced the temperature dependence of the electrical resistivity as well as the thermal spectral broadening. The resultant renormalized phonon frequencies ${\ensuremath{\omega}}_{\mathrm{r}0}^{(1)}=146.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${\ensuremath{\omega}}_{\mathrm{r}0}^{(2)}=70.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ are comparable to the Einstein temperatures estimated from the previous specific heat measurement. Our results suggest the existence of the weak anharmonic phonon modes in ${\mathrm{BaIr}}_{2}{\mathrm{Ge}}_{7}$.

Temperature dependence of the superconducting gap of single-layer FeSe/SrTiO3 : Direct comparison between transport and spectroscopic measurements

We studied the superconducting properties of the one-unit cell FeSe grown on high-doped ${\mathrm{SrTiO}}_{3}$(001) (STO) substrate by in situ transport and angle-resolved photoemission spectroscopy (ARPES) measurements. By comparing with previous data taken for the low-doped STO substrate [Phys. Rev. Lett. 124, 227002 (2020)], it was revealed that the onset of superconductivity occurs at $\ensuremath{\sim}40$ K irrespective of the STO doping level. Furthermore, from the temperature dependence of the critical current, we were able to deduce the superconducting gap that is relevant in the transport phenomena, which gradually increases by cooling the sample down from 40 K. In contrast, the coherent peak emerges at 60 K that likely corresponds to the Cooper pair formation, while the leading edge shift near the Fermi level is clearly observed at 40 K in the ARPES spectra. We speculate that the STO surface presumably acts as an effective ``disorder'' in this system and the Cooper pairs are preformed well above the onset of the resistance drop.

Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures

In van der Waals heterostructures, the relative alignment of bands between layers, and the resulting band hybridisation, are key factors in determining a range of electronic properties. This work examines these effects for heterostructures of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), an ubiquitous combination given the role of hBN as an encapsulating material. By comparing results of density functional calculations with experimental angle-resolved photoemission spectroscopy (ARPES) results, we explore the hybridisation between the valence states of the TMD and hBN layers, and show that it introduces avoided crossings between the TMD and hBN bands, with umklapp processes opening ‘ghost’ avoided crossings in individual bands. Comparison between density functional theory (DFT) and ARPES spectra for the MoSe2/hBN heterostructure shows that the valence bands of MoSe2 and hBN are significantly further separated in energy in experiment as compared to DFT. We then show that a novel scissor operator can be applied to the hBN valence states in the DFT calculations, to correct the band alignment and enable quantitative comparison to ARPES, explaining avoided crossings and other features of band visibility in the ARPES spectra.

Electronic transport studies of Ag-doped Bi2Se3 topological insulator

The structural, magnetotransport, and angle-resolved photoemission spectroscopy (ARPES) of Ag-doped Bi2Se3 single crystals are presented. Temperature dependent resistivity exhibits metallic behavior with a slope change above 200 K for Ag-doped Bi2Se3. The magnetoresistance shows positive quadratic dependence at low fields satisfying Kohler's rule. Hall resistivity measurement shows that electrons are dominant charge carriers. Furthermore, these results agree well with the ARPES spectra observed at T = 20 K, where the Fermi level lies inside the bulk conduction band. The Dirac point of the topological surface states is shifted toward higher binding energy (∼0.12 eV) for Ag-doped samples as compared to pristine Bi2Se3.

Probing topological Floquet states in WSe2 using circular dichroism in time- and angle-resolved photoemission spectroscopy

Observing signatures of light-induced topological Floquet states in materials has been shown to be very challenging. Angle-resolved photoemission spectroscopy (ARPES) is well suited for the investigation of Floquet physics, as it allows to directly probe the dressed electronic states of driven solids. Depending on the system, scattering and decoherence can play an important role, hampering the emergence of Floquet states. Another challenge is to disentangle Floquet side bands from laser-assisted photoemission (LAPE), since both lead to similar signatures in ARPES spectra. Here, we investigate the emergence of Floquet state in the transition metal dichalcogenide 2H-WSe2, one of the most promising systems for observing Floquet physics. We discuss how the topological Floquet state manifests in characteristic features in the circular dichroism in photoelectron angular distributions (CDAD) that is determined by the transient band structure modifications and the associated texture of the orbital angular momentum. Combining highly accurate modeling of the photoemission matrix elements with an ab initio description of the light-matter interaction, we investigate regimes which can be realized in current state-of-the-art experimental setups. The predicted features are robust against scattering effects and are expected to be observed in forthcoming experiments.

Facet-dependent Electronic Quantum Diffusion in the High-Order Topological Insulator Bi4Br4

We apply the magnetoconductivity characteristic and angle-resolved photoemission spectroscopy (ARPES) to visualize the evolution of surface-selected electronic features of quasi-one-dimensional material ${\mathrm{Bi}}_{4}{\mathrm{Br}}_{4}$. The transport measurements indicate the quantum-interference correction to conductivity possesses a symbolic spin-rotational characteristic correlated to the value of Berry phase with the effects of weak localization and weak antilocalization for (001) and (100) surfaces, respectively. The ARPES spectra provide the experimental evidence for a quasi-one-dimensional surface state at the side (100) surface and an anisotropic massive Dirac surface state at the top (001) surface, respectively, which is highly coincided with the angle-dependent scaling behavior of magnetoconductivity. Our results reveal the facet-dependent electronic quantum diffusion in quasi-one-dimensional ${\mathrm{Bi}}_{4}{\mathrm{Br}}_{4}$, stimulating further investigations of this topological class and applications of the feasible technologies of topological spintronics.

Electronic and vibrational properties of bulk Cr2Ge2Te6 from first-principles calculations

We use density functional theory based ab initio calculations to investigate the structural, vibrational, magnetic, and electronic properties of the layered ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) that has attracted attention for potential spintronic applications. We optimize the structure for a fixed $c/a$ ratio. Our results are in excellent agreement with experimental data on structure, phonons, and electronic properties. The use of van der Waals interactions and relativistic spin-orbit coupling yields accurate lattice constants and interlayer distances. Simultaneously, the adopted theoretical methods lead to a rigorous description of the vibrational normal modes, as well as the valence bands, that are in excellent agreement with Raman spectroscopy and angle-resolved photoemission spectroscopy spectra, respectively. The magnetic moment is slightly overestimated, and the magnetic anisotropy has the correct sign but is greater in magnitude than that in experiment. We find that inclusion of the on-site Coulomb repulsion on $\mathrm{Cr}\text{\ensuremath{-}}3d$ orbitals worsens agreement with experiment, in contrast to previous studies. Our results provide a path toward ab initio analysis of magnetic heterostructures based on CGT.

Magnetotransport and ARPES studies of the topological insulators Sb2Te3 and Bi2Te3 grown by MOCVD on large-area Si substrates

Recently, the topological insulators (TIs) antimony telluride (Sb2Te3) and bismuth telluride (Bi2Te3) are attracting high interest for applications based on spin-charge interconversion mechanisms. Aiming to make a step toward the technology transfer, it is of major importance to achieve and investigate epitaxial quality-TIs on large area Si-based substrates. In view of that, we report here magnetotransport and angle-resolved photoemission spectroscopy (ARPES) studies on Sb2Te3 and Bi2Te3 thin films grown by metal organic chemical vapor deposition (MOCVD) on top of 4″ Si(111) substrates. Clear weak antilocalization (WAL) effects are observed in both TIs, proving the existence of quantum transport mechanism, and the data are successfully interpreted in the framework of the Hikami–Larkin–Nagaoka model. Further, by dedicated magnetotransport experiments, it has been confirmed that the investigated WAL originates from two-dimensional (2D) topological states. ARPES has been performed ex-situ, and in both TIs the gapless Dirac cones have been observed and attributed to the topological surface states. Combining the proofs of the existence of quantum 2D transport as deduced from the analysis of the magnetoconductance curve with the direct observation of the Dirac-like band structure revealed by the ARPES spectra, it is possible to unambiguously confirm the topological nature of our Sb2Te3 and Bi2Te3 thin films. The results obtained on thin films grown by MOCVD on 4’’ Si(111) substrate mark an important step towards the technology transfer of the topological insulators studied in this work.

Electronic properties of TaAs2 topological semimetal investigated by transport and ARPES

We have performed electron transport and angle-resolved photo-emission spectroscopy (ARPES) measurements on single crystals of transition metal dipnictide TaAs2 cleaved along the (01) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov–de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between magnetic field and the [01] direction. The results indicate elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved (01) surface and it was found that bulk states pockets at constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level (FL) is increased, which is due to a small n-doping of the samples. This shifts the FL closer to the Dirac point, allowing investigating the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

Hidden self-energies as origin of cuprate superconductivity revealed by machine learning

Experimental data are the source of understanding matter. However, measurable quantities are limited and theoretically important quantities are sometimes hidden. Nonetheless, recent progress of machine-learning techniques opens possibilities of exposing them only from available experimental data. In this paper, after establishing the reliability of the method in various careful benchmark tests, the Boltzmann-machine method is applied to the angle-resolved photoemission spectroscopy spectra of cuprate high temperature superconductors, Bi$_2$Sr$_2$CuO$_{6+\delta}$ (Bi2201) and Bi$_2$Sr$_2$CaCuO$_{8+\delta}$ (Bi2212). We find prominent peak structures both in normal and anomalous self-energies, but they cancel in the total self-energy making the structure apparently invisible, while the peaks make universally dominant contributions to superconducting gap, hence evidencing the signal that generates the high-$T_{\rm c}$ superconductivity. The relation between superfluid density and critical temperature supports involvement of universal carrier relaxation associated with dissipative strange metals, where enhanced superconductivity is promoted by entangled quantum-soup nature of the cuprates. The present achievement opens avenues for innovative machine-learning spectroscopy method to reveal fundamental properties hidden in direct experimental accesses.

Rotational Resonances and Regge-like Trajectories in Lightly Doped Antiferromagnets.

Understanding the nature of charge carriers in doped Mott insulators holds the key to unravelling puzzling properties of strongly correlated electron systems, including cuprate superconductors. Several theoretical models suggested that dopants can be understood as bound states of partons, the analogues of quarks in high-energy physics. However, direct signatures of spinon-chargon bound states are lacking, both in experiment and theory. Here we propose a rotational variant of angle-resolved photo-emission spectroscopy (ARPES) and calculate rotational spectra numerically using the density-matrix renormalization group. We identify long-lived rotational resonances for an individual dopant, which we interpret as a direct indicator of the microscopic structure of spinon-chargon bound states. Similar to Regge trajectories reflecting the quark structure of mesons, we establish a linear dependence of the rotational energy on the superexchange coupling. The rotational peaks we find are strongly suppressed in standard ARPES spectra, but we suggest a multiphoton extension of ARPES which allows us to access rotational spectra. Our findings suggest that multiphoton spectroscopy experiments should provide new insights into emergent universal features of strongly correlated electron systems.

Photoemission signature of momentum-dependent hybridization in CeCoIn5

Hybridization between $f$ electrons and conduction bands ($c$-$f$ hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of CeCoIn$_5$ heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of $T=6$ K. The used photon energy of 122 eV corresponds to Ce $4d$-$4f$ resonance. Calculations carried out with relativistic multiple scattering Korringa-Kohn-Rostoker method and one-step model of photoemission yielded realistic simulation of the ARPES spectra indicating that Ce-In surface termination prevails. Surface states, which have been identified in the calculations, contribute significantly to the spectra. Effects of the hybridization strongly depend on wave vector. They include a dispersion of heavy electrons and bands gaining $f$-electron character when approaching Fermi energy. We have also observed a considerable variation of $f$-electron spectral weight at $E_F$, which is normally determined by both matrix element effects and wave vector dependent $c$-$f$ hybridization. Fermi surface scans covering a few Brillouin zones revealed large matrix element effects. A symmetrization of experimental Fermi surface, which reduces matrix element contribution, yielded a specific variation of $4f$-electron enhanced spectral intensity at $E_F$ around $\bar{\Gamma}$ and $\bar{M}$ points. Tight-binding approximation calculations for Ce-In plane provided the same universal distribution of $4f$-electron density for a range of values of the parameters used in the model.

Semimetallic nature of and magnetic polarons in EuB$$_6$$ studied using angle-resolved photoemission spectroscopy

Temperature-dependent angle-resolved photoemission spectroscopy (ARPES) was carried out on single-crystalline EuB6 samples. By measuring ARPES spectra in an extended Brillouin zone, a B 2p hole pocket centered at the X point is clearly observed, thus proving the semimetallic nature of EuB6. Below the Curie temperature TC, ARPES spectra show two B 2p bands of which separation is due to an exchange interaction between local Eu 4f and itinerant B 2p electrons. The exchange splitting becomes smaller as the temperature increases and disappears well above TC. Additionally, a diffuse structure near the Fermi level survives just above TC. Such behavior is well described by Monte Carlo simulations of a Kondo lattice model, thus supporting the formation of magnetic polarons in EuB6, which accounts for the resistivity upturn near above TC when lowering the temperature.

Kagome-like structure of germanene on Al(111)

The structure of a 3 \ifmmode\times\else\texttimes\fi{} 3 reconstruction formed by Ge deposition onto Al(111) [Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge] was investigated by coaxial impact collision ion scattering spectroscopy, which is one of the most surface-sensitive structural analysis techniques. It has been revealed that Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge forms a structure with 10 Ge atoms in a unit cell, similar to the recently reported ``kagome-like'' silicene on Al(111) rather than the previously reported honeycomb lattice. A surface band structure of the Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge has also been investigated by angle-resolved photoemission spectroscopy (ARPES). The calculated band structure of our proposed model based on a density functional theory well reproduces ARPES spectra, which also indicates that Al(111)3 \ifmmode\times\else\texttimes\fi{} 3-Ge forms a kagome-like structure.