Resolved Photoemission Spectroscopy Research Articles

Thermal- and air- stability of the compositional variants of van der Waals Pt-Telluride thin films probed by high resolution photoemission spectroscopy

The Pt-Te compositional phase diagram consists of at least three different compositional line phases (PtTe2, Pt3Te4, and Pt2Te2) that can be described as layered van der Waals materials. This presents challenges in controlling the composition of ultrathin Pt-telluride 2D materials by physical vapor deposition methods. Here we show by temperature programmed synchrotron photoemission spectroscopy that the different phases have varying thermal stability in vacuum. This enables the synthesis of these materials by preparation of PtTe2 films at low growth temperatures and subsequent vacuum annealing to ∼ 350 °C for Pt3Te4, or ∼ 400 °C for Pt2Te2. Such prepared phases are characterized by high resolution core level spectroscopy to provide reference spectra for these materials. Moreover, the chemical stability of these materials was tested by exposure to oxygen and air. Even after prolonged air exposure only the surface Te layer was modified by oxygen chemical bonds that caused a 3-eV shift to higher binding energy of the Te-3d core levels. However, these oxygen species could be desorbed by vacuum annealing at 280 °C and pristine Pt-telluride samples can be re-established. This shows the excellent chemical stability of these materials, important for practical applications.

Nodeless electron pairing in CsV3Sb5-derived kagome superconductors.

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order and lattice geometry1-9. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive10-17. In particular, consensus on the electron pairing symmetry has not been achieved so far18-20, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic and orbital-independent superconducting gap in the momentum space of two exemplary CsV3Sb5-derived kagome superconductors-Cs(V0.93Nb0.07)3Sb5 and Cs(V0.86Ta0.14)3Sb5-using ultrahigh-resolution and low-temperature angle-resolved photoemission spectroscopy. Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Our comprehensive characterizations of the superconducting gap provide indispensable information on the electron pairing symmetry of kagome superconductors, and advance our understanding of the superconductivity and intertwined electronic orders in quantum materials.

Coexistence of Strong and Weak Topological Orders in a Quasi-One-Dimensional Material.

Topological materials have broad application prospects in quantum computing and spintronic devices. Among them, dual topological materials with low dimensionality provide an excellent platform for manipulating various topological states and generating highly conductive spin currents. However, direct observation of their topological surface states still lacks. Here, we reveal the coexistence of the strong and weak topological phases in a quasi-one-dimensional material, TaNiTe_{5}, by spin- and angle- resolved photoemission spectroscopy. The surface states protected by weak topological order forms Dirac-node arcs in the vicinity of the Fermi energy, providing the opportunity to develop spintronics devices with high carrier density that is tunable by bias voltage.

Electronic structure studies on single crystalline Nd2PdSi3, an exotic Nd-based intermetallic: evidence for Nd 4f hybridization

In the series R2PdSi3, Nd2PdSi3 is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4f localization. Here, we have studied the electronic structure of single crystalline Nd2PdSi3 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Theoretical results obtained for the effective on-site Coulomb energy of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4d–Nd 4f hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4f–Si 3p states, which is possibly responsible for the ferromagnetism in Nd compound. Nd 3d core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4f states with the conduction electrons possibly responsible for the exoticity of this compound.

In situ investigation of interfacial properties of Sb2Se3 heterojunctions

Antimony selenide (Sb2Se3), emerging as a promising photovoltaic material, has achieved over 9% efficiency within only 6 years. Various kinds of buffer materials are employed for Sb2Se3 solar cells to construct heterojunctions with distinctive device performance. Herein, we introduce in situ high resolution photoemission spectroscopy (HRPES) to investigate the interfacial properties between Sb2Se3 and three types of widely adopted buffer layers: CdS, ZnO, and TiO2. HRPES results and theoretical thermodynamic calculations reveal that in the initial stage, the deposited Sb2Se3 reacts with buffer materials in terms of activity in the following order: CdS ≥ ZnO > TiO2. Distinct transition layers are formed at CdS/Sb2Se3 and ZnO/Sb2Se3 interfaces, whereas it is nearly absent at TiO2/Sb2Se3. Our results suggest that the CdS/Sb2Se3 heterojunction shows spike-like conduction band offsets (CBOs), whereas ZnO/Sb2Se3 demonstrates a cliff-like CBO, and TiO2/Sb2Se3 is almost flat. The transition layers and band alignments at the interface could be the reasons for the stability and performance of Sb2Se3 photovoltaic devices with different buffer materials. Our investigation deepens the understanding of Sb2Se3 heterojunction formation and can benefit further development of Sb2Se3 thin-film solar cells.

Depth-resolved core level spectroscopy of noncentrosymmetric solid BiPd

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and topological phase of matter. Employing high resolution photoemission spectroscopy with photon energies ranging from hard x-ray to extreme ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derived the escape depths close to the extreme surface sensitivity and find that the photon energies used for high resolution measurements such as ARPES fall in the surface sensitive regime. In addition, we discover deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such paradigm shift in core level spectroscopy can be attributed to the absence of center of symmetry and spin-orbit interactions.

Observation of bulk states and spin-polarized topological surface states in transition metal dichalcogenide Dirac semimetal candidate NiTe2

Using spin- and angle- resolved photoemission spectroscopy (spin-ARPES) together with ${\it ab~initio}$ calculations, we demonstrate the existence of a type-II Dirac semimetal state in NiTe$_2$. We show that, unlike PtTe$_2$, PtSe$_2$, and PdTe$_2$, the Dirac node in NiTe$_2$ is located in close vicinity of the Fermi energy. Additionally, NiTe$_2$ also hosts a pair of band inversions below the Fermi level along the $\Gamma-A$ high-symmetry direction, with one of them leading to a Dirac cone in the surface states. The bulk Dirac nodes and the ladder of band inversions in NiTe$_2$ support unique topological surface states with chiral spin texture over a wide range of energies. Our work paves the way for the exploitation of the low-energy type-II Dirac fermions in NiTe$_2$ in the fields of spintronics, THz plasmonics and ultrafast optoelectronics.

Origin of electronic localization in metal-insulator transition of phase change materials

Tellurium based phase change materials are unique 3D-solids proposed to undergo Anderson type metal-insulator transition. However, the origin of this transition is not unambiguously understood. Here, we report combined high energy resolution photoemission spectroscopy and high k-resolution X-ray diffraction measurements on a reversibly phase switched Ge2Sb2Te5 film. The results resolve the ambiguity between previous spectroscopic data and the proposed theoretical model for the origin of Anderson localization in these materials. Furthermore, by switching between the metallic state to insulating and back to metallic, we probe the electronic structure evolution in the phase change material.

Spin-polarized resonant surface state in (1 1 1) Sm1−xGdxAl2, a zero-magnetization ferromagnet

The electronic structure of (1 1 1) Sm1−xGdxAl2, a zero-magnetization ferromagnet, is investigated by angle- and spin- resolved photoemission spectroscopy. An intense electron pocket strongly localized around and close to the Fermi level is observed and analyzed in detail. Its various characteristics, combined with electronic structure calculations, reveal a resonant surface state of 5d character and Λ1 symmetry, likely built on bulk states developing around L points. It exhibits moreover a low temperature positive spin polarization at the Fermi level, of strong interest for spin-dependent transport properties in Sm1−xGdxAl2-based spintronic devices.

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.

Amplitude mode oscillations in pump-probe photoemission spectra from a d -wave superconductor

Recent developments in the techniques of ultrafast pump-probe photoemission have made possible the search for collective modes in strongly correlated systems out of equilibrium. Including inelastic scattering processes and a retarded interaction, we simulate time- and angle- resolved photoemission spectroscopy (trARPES) to study the amplitude mode of a d-wave superconductor, a collective mode excited through the nonlinear light-matter coupling to the pump pulse. We find that the amplitude mode oscillations of the d-wave order parameter occur in phase at a single frequency that is twice the quasi-steady-state maximum gap size after pumping. We comment on the necessary conditions for detecting the amplitude mode in trARPES experiments.

Chemically exfoliated MoS2 layers: Spectroscopic evidence for the semiconducting nature of the dominant trigonal metastable phase

A trigonal phase existing only as small patches on chemically exfoliated few layer, thermodynamically stable 1H phase of MoS2 is believed to influence critically properties of MoS2 based devices. This phase has been most often attributed to the metallic 1T phase. We investigate the electronic structure of chemically exfoliated MoS2 few layered systems using spatially resolved (lesser than 120 nm resolution) photoemission spectroscopy and Raman spectroscopy in conjunction with state-of-the-art electronic structure calculations. On the basis of these results, we establish that the ground state of this phase is a small gap (~90 meV) semiconductor in contrast to most claims in the literature; we also identify the specific trigonal (1T') structure it has among many suggested ones.

Observation of pseudogap in MgB2

We investigate the electronic structure of a specially prepared highly dense conventional high temperature superconductor, MgB2, employing high resolution photoemission spectroscopy. The spectral evolution close to the Fermi energy is commensurate to BCS descriptions as expected. However, the spectra in the wider energy range reveal the emergence of a pseudogap much above the superconducting transition temperature indicating an apparent departure from the BCS scenario. The energy scale of the pseudogap is comparable to the energy of the phonon mode responsible for superconductivity in MgB2 and the pseudogap can be attributed to the effect of electron–phonon coupling on the electronic structure. These results reveal a scenario of the emergence of the superconducting gap within an electron–phonon coupling induced pseudogap and have significant implications in the study of high temperature superconductors.

Effects of K adsorption on the CO-induced restructuring of Co(11-20)

The location of potassium (K) on Cobalt (Co) and its effect on adsorption and adsorption-induced surface restructuring is important for understanding the deactivation of Co Fischer-Tropsch catalysts and the nature of the active surface. Co(11-20) restructures by anisotropic migration of Co atoms upon CO exposure. Deposition of sub-monolayer amounts of K on Co(11-20) and the effect on the CO-induced restructuring were therefore investigated using scanning tunneling microscopy (STM), high resolution photoemission spectroscopy (HR-PES), and density functional theory calculations (DFT). The combined STM and DFT results suggest that the preferred adsorption site for K at low coverage is in the vicinity of step edges. DFT also found that diffusion of K along the [0001] direction, in between the zigzag rows of the topmost Co layer is facile. The restructuring under CO exposure with K pre-adsorbed proceeded on the terraces rather than from the step edges, in a slower and more disordered manner. HR-PES showed that the amount of CO adsorbed at saturation significantly decreased with predeposited K. The obstructed migration of Co atoms across the surface may be important in understanding why very low amounts of K on supported Co catalysts significantly reduces the activity towards hydrogenation of CO.

Enhanced surface transfer doping of diamond by V2O5 with improved thermal stability

Surface transfer doping of hydrogen-terminated diamond has been achieved utilising V2O5 as a surface electron accepting material. Contact between the oxide and diamond surface promotes the transfer of electrons from the diamond into the V2O5 as revealed by the synchrotron-based high resolution photoemission spectroscopy. Electrical characterization by Hall measurement performed before and after V2O5 deposition shows an increase in hole carrier concentration in the diamond from 3.0 × 1012 to 1.8 × 1013 cm−2 at room temperature. High temperature Hall measurements performed up to 300 °C in atmosphere reveal greatly enhanced thermal stability of the hole channel produced using V2O5 in comparison with an air-induced surface conduction channel. Transfer doping of hydrogen-terminated diamond using high electron affinity oxides such as V2O5 is a promising approach for achieving thermally stable, high performance diamond based devices in comparison with air-induced surface transfer doping.

Fast One-Pot Synthesis of MoS2/Crumpled Graphene p-n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production.

Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS2 nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV-visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS2 nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS2 nanohybrids are seven times more efficient with respect to single MoS2 because of the formation of local p-n MoS2/N-doped graphene nanojunctions, which allow an efficient charge carrier separation.

Anomalies of a topologically ordered surface.

Bulk insulators with strong spin orbit coupling exhibit metallic surface states possessing topological order protected by the time reversal symmetry. However, experiments show vulnerability of topological states to aging and impurities. Different studies show contrasting behavior of the Dirac states along with plethora of anomalies, which has become an outstanding problem in material science. Here, we probe the electronic structure of Bi2Se3 employing high resolution photoemission spectroscopy and discover the dependence of the behavior of Dirac particles on surface terminations. The Dirac cone apex appears at different binding energies and exhibits contrasting shift on Bi and Se terminated surfaces with complex time dependence emerging from subtle adsorbed oxygen-surface atom interactions. These results uncover the surface states behavior of real systems and the dichotomy of topological and normal surface states important for device fabrication as well as realization of novel physics such as Majorana Fermions, magnetic monopole, etc.

The electronic properties of mixed valence hydrated europium chloride thin film.

We investigate the electronic properties of a model mixed-valence hydrated chloride europium salt by means of high resolution photoemission spectroscopy (HRPES) and resonant photoemission spectroscopy (RESPES) at the Eu 3d → 4f and 4d → 4f transitions. From the HRPES spectra, we have determined that the two europium oxidation states are homogeneously distributed in the bulk and that the hydrated salt film is exempt from surface mixed valence transition. From the RESPES spectra, the well separated resonant contributions characteristic of divalent and trivalent europium species (4f(6) and 4f(7) final states, respectively) are accurately extracted and quantitatively determined from the resonant features measured at the two edges. The partial absorption yield spectra, obtained by integrating the photoemission intensity in the valence-band region, can be well reproduced by atomic multiplet calculation at the M(4,5) (3d-4f) absorption edge and by an asymmetric Fano-like shape profile at the N(4,5) (4d-4f) absorption edge. The ratio of Eu(2+) and Eu(3+) species measured at the two absorption edges matches with the composition of the mixed valence europium salt as determined chemically. We have demonstrated that the observed spectroscopic features of the mixed valence salt are attributed to the mixed-valence ground state rather than surface valence transition. HRPES and RESPES spectra provide reference spectra for the study of europium salts and their derivatives.

Estimate of the Coulomb correlation energy in CeAg2Ge2 from inverse photoemission and high resolution photoemission spectroscopy

The occupied and the unoccupied electronic structure of CeAg2Ge2 single crystal has been studied using high resolution photoemission and inverse photoemission spectroscopy, respectively. High resolution photoemission reveals the clear signature of Ce 4f states in the occupied electronic structure which was not observed clearly in our earlier studies. The Coulomb correlation energy in this system has been determined experimentally from the position of the 4f states above and below the Fermi level. Theoretically, the correlation energy has been determined by using the first principles density functional calculations within the generalized gradient approximations taking into account the strong intra-atomic (on-site) interaction Hubbard Ueff term. The calculated valence band shows minor changes in the spectral shape with increasing Ueff due to the fact that the density of Ce 4f state is narrow in the occupied part and is hybridized with the Ce 5d, Ag 4d and Ge 4p states. On the other hand, substantial changes are observed in the spectral shape of the calculated conduction band with increasing Ueff since the density of Ce 4f state is very large in the unoccupied part, compared to other states. The estimated value of correlation energy for CeAg2Ge2 from the experiment and the theory is ≈ 4.2 eV. The resonant photoemission data are analyzed in the framework of the single-impurity Anderson model which further confirms the presence of the Coulomb correlation energy and small hybridization in this system.

Electronic and structural properties of graphene-based metal-semiconducting heterostructures engineered by silicon intercalation

The initial decoupling of the (6√3×6√3)R30° buffer layer also called zero layer graphene (ZLG) on 6H-SiC(0001) by Si intercalation has been investigated by means of high resolution photoemission spectroscopy (HRPES) and microscopy imaging techniques. A combination of complementary techniques has shown that the annealing above 700°C of amorphous Si deposited on ZLG leads to the diffusion of the silicon over the surface. Two competing processes are then observed. Part of the silicon contributes to a progressive decoupling of the ZLG from the substrate (partial decoupling) while the rest agglomerates at the surface to form oriented silicon clusters. After sequences of Si deposition, followed by annealing at 750°C, complete decoupling is observed into quasi-free standing monolayer (ML) graphene. Investigation of the evolution of the C1s and Si2p core levels during the intermediate states shows that the appearance of the graphene contribution coincides with the creation of an extra SiC bulk component, indicating their electronic decoupling. At partial decoupling of the ZLG, we have the coexistence of structurally linked metal-semiconducting materials presenting mutual electronic interactions and composed of nanometric metal-semiconducting heterojunctions.