Photoemission Spectroscopy Measurements Research Articles

Coexistence of Kondo effect and non trivial Berry phase in Gd doped Bi2Se3: an ARPES and magneto-transport study

Abstract The presence of magnetic impurities in Topological Insulators (TI) can disrupt their Time Reversal Symmetry (TRS) and lead to the emergence of an energy gap. This study delves into the energy band structure and the Kondo effect through the introduction of Gadolinium(Gd) magnetic perturbations (at levels of x = 0.1, 0.16) into a pure Bi2Se3 single crystal. In the case of the Bi1.9Gd0.1Se3 (5%) single crystal, the Kondo effect becomes observable at temperatures below 50K. However, the unaltered parent and Bi1.84Gd0.16Se3 (8%) exhibit typical metallic behavior. The pure sample displays the highest magnetoresistance (MR) of around 225% and demonstrates quantum oscillations driven by a nontrivial berry phase. The sample doped with 5% Gd undergoes a transition from negative magnetoresistance (NMR) to positive magnetoresistance (PMR) due to a presence of mixed magnetic state resulting from the opening of a gap at the Dirac point. This gap opening is confirmed through Angle-Resolved Photoemission Spectroscopy (ARPES) measurements. Thermoelectric properties are assessed across all prepared samples. The undoped sample displays the highest Seebeck coefficient and power factor (PF) values of -398.02 µV/K and 6.83 mW/mK2, respectively, at room temperature. These values are notably high for thermoelectric applications at room temperature.

Magnetic topological Weyl fermions in half-metallic In2CoSe4

Magnetic Weyl semimetals (WSMs) have recently attracted much attention due to their potential in realizing strong anomalous Hall effects. Yet, how to design such systems remains unclear. Based on first-principles calculations, we show here that the ferromagnetic half-metallic compound In2CoSe4has several pairs of Weyl points and is hence a good candidate for magnetic WSM. These Weyl points would approach the Fermi level gradually as the HubbardUincreases, and finally disappear after a critical valueUc. The range of the HubbardUthat can realize the magnetic WSM state can be expanded by pressure, manifesting the practical utility of the present prediction. Moreover, by generating two surface terminations at Co or In atom after cleaving the compound at the Co-Se bonds, the nontrivial Fermi arcs connecting one pair of Weyl points with opposite chirality are discovered in surface states. Furthermore, it is possible to observe the nontrivial surface state experimentally, e.g. angle-resolved photoemission spectroscopy measurements. As such, the present findings imply strongly a new magnetic WSM which may host a large anomalous Hall conductivity.

Enhanced Weyl semimetal signature in Co3Sn2S2 Kagome ferromagnet by chlorine doping

Weyl fermions are chiral massless fermions with exotic properties. In the first established magnetic Weyl semimetal, Co3Sn2S2, a giant anomalous Hall effect has been observed, while its Fermi energy remaining 60 meV from the Weyl points. Shifting the Fermi energy closer to the Weyl points may assist in the identification of Weyl Fermion related transport signatures. Here we show that effective chlorine doping has resulted in a shift of the Fermi energy by 15 meV towards the Weyl points, which is confirmed by a combination of the systematic angular-resolved photoemission spectroscopy measurements and density function theory calculations. A five-fold reduction in resistivity is observed in the ferromagnetic phase, accompanied by a pronounced magnetoresistance of over 150%. The anomalous Hall conductivity shows a peak of 1680 Scm−1 at 40 K, which is 30% higher than the undoped sample due to a stronger Weyl point contribution. This work demonstrates the essential role of doping in Co3Sn2S2 for an enhanced Weyl semimetal signature.

Topological Superconductivity in Heavily Doped Single-Layer Graphene.

The existence of superconductivity (SC) appears to be established in both twisted and nontwisted graphene multilayers. However, whether their building block, single-layer graphene (SLG), can also host SC remains an open question. Earlier theoretical works predicted that SLG could become a chiral d-wave superconductor driven by electronic interactions when doped to its van Hove singularity, but questions such as whether the d-wave SC survives the strong band renormalizations seen in experiments, its robustness against the source of doping, or if it will occur at any reasonable critical temperature (Tc) have remained difficult to answer, in part due to uncertainties in model parameters. Furthermore, doping of graphene beyond its van Hove singularity remained experimentally challenging and was not demonstrated until recently. In this study, we n dope SLG past the van Hove singularity by employing Tb intercalation and derive structural models from angle-resolved photoemission spectroscopy measurements. We adopt a reliable numerical framework based on a random-phase approximation technique to investigate the emergence of unconventional SC in the heavily doped monolayer. We predict that robust d + id topological SC could arise in SLG doped by Tb, with a Tc up to 600 mK. We also employ first-principles calculations to investigate the possibility of realizing d-wave SC with other dopants, such as Li or Cs. We find that dopants that change the lattice symmetry of SLG are detrimental to the d-wave state. The stability of the d-wave SC predicted here in Tb-doped SLG could provide a valuable insight for guiding future experimental efforts aimed at exploring topological superconductivity in monolayer graphene.

Photoemission spectroscopy of battery materials

Recognized by the 2019 Nobel Prize in Chemistry, rechargeable lithium-ion battery (LIB) has become a world-revolutionary technology. Further developments of LIB-based and “beyond LIBs” regarding capacity, cycle life, and safety are intimately associated with the fundamental understanding of chemical compositions, structures, physical properties of electrodes and electrolytes, and other related components. The time-evolving snapshots of the dynamical processes occurring during the battery operation can help design better strategies to prevent the formation of uncontrolled interphase layers, dendrites, electrode/electrolyte decompositions, and generation of gases. Photoemission spectroscopy (PES) has become one of the important techniques for understanding the aforementioned aspects. However, many potential pitfalls and cautions need to be considered from sample preparation, during PES measurements, to data analyses. Although the primary focus of this article is not to evaluate the PES technique itself, we first introduce a minimal set of fundamental concepts to minimize misinterpretation arising from the physics of PES. Subsequently, we examine studies that utilize PES techniques to determine chemical compositions of solid- and liquid-state battery materials, energy level diagrams that bridge different terminologies between PES and electrochemistry, along with the theoretical aspects of PES evolving from first-principle calculations to machine learning. Toward the end of this review, we outline potential future research directions.

UOTe: Kondo-Interacting Topological Antiferromagnet in a Van der Waals Lattice.

Since the initial discovery of 2D van der Waals (vdW) materials, significant effort has been made to incorporate the three properties of magnetism, band structure topology, and strong electron correlations-to leverage emergent quantum phenomena and expand their potential applications. However, the discovery of a single vdW material that intrinsically hosts all three ingredients has remained an outstanding challenge. Here, the discovery of a Kondo-interacting topological antiferromagnet is reported in the vdW 5f electron system UOTe. It has a high antiferromagnetic (AFM) transition temperature of 150K, with a unique AFM configuration that breaks the combined parity and time reversal (PT) symmetry in an even number of layers while maintaining zero net magnetic moment. This angle-resolved photoemission spectroscopy (ARPES) measurements reveal Dirac bands near the Fermi level, which combined with the theoretical calculations demonstrate UOTe as an AFM Dirac semimetal. Within the AFM order, the presence of the Kondo interaction is observed, as evidenced by the emergence of a 5f flat band near the Fermi level below 100 K and hybridization between the Kondo band and the Dirac band. The density functional theory calculations in its bilayer form predict UOTe as a rare example of a fully-compensated AFM Cherninsulator.

Molecular beam epitaxy and band structures of type-II antiferromagnetic semiconductor EuTe thin films

The rare-earth Eu-based compounds with a unique half-filled 4f orbital have attracted an amount of research interest recently. Here, we synthesized EuTe(001) single-crystal thin films on SrTiO3(001) substrate via molecular beam epitaxy (MBE). The scanning tunneling microscopy and x-ray diffraction results indicate that the grown EuTe thin films orientated as EuTe[100]//SrTiO3[110] in plane. In the angle-resolved photoemission spectroscopic (ARPES) measurements, the grown EuTe films show a semiconductive band structure with the valence band maximum lying on the center point of the Brillouin zone. The bandgap size of EuTe was further identified by the optical transmission spectra as 2.2 eV. The antiferromagnetic transition temperature of the grown EuTe film is 10.5 K measured by a superconductive quantum interference device (SQUID). Our results provide important information on the fundamental electronic structures for the further research and applications of the Eu-based compounds.

Unexpected band structure changes within the higher-temperature antiferromagnetic state of CeBi

The interest in the rare-earth monopnictides was boosted after the discovery of unconventional surface-state pairs in antiferromagnetically ordered NdBi. In contrast to other materials in which such states were reported, CeBi is known to have multiple antiferromagnetic phases. In this study, we perform angle-resolved photoemission spectroscopy (ARPES) measurements in conjunction with density functional theory (DFT) calculations to investigate the evolution of the electronic structure of CeBi upon a series of antiferromagnetic (AFM) transitions. We find evidence for a new AFM transition in addition to two previously known from transport studies. We demonstrate the development of an additional Dirac state in the ( + − + − ) ordered phase and a transformation of unconventional surface-state pairs in the ( + + − − ) ordered phase. This revises the phase diagram of this intriguing material, where there are now three distinct AFM states below TN in zero magnetic field instead of two as it was previously thought.

Observation of Highly Spin-Polarized Dangling Bond Surface States in Rare-Earth Pnictide Tellurides.

To generate and manipulate spin-polarized electronic states in solids are crucial for modern spintronics. The textbook routes employ quantum well states or Shockley/topological type surface states whose spin degeneracy is lifted by strong spin-orbit coupling and inversion symmetry breaking at the surface/interface. The resultant spin polarization is usually truncated because of the intertwining between multiple orbitals. Here a unique type of surface states is realized, namely, dangling bond surface states in a family of ternary rare-earth pnictide tellurides RePnTe (Re = La, Gd, Ce; Pn = Sb, Bi), with robust band structure and sizeable spin splitting. Spin and angle-resolved photoemission spectroscopy measurements reveal high spin polarization and distinct spin-momentum locking texture, which, according to the theoretical analysis, arise from local site asymmetry and surface-purified spin-orbital texture. The work extends the so-called "hidden spin polarization" from the bulk to the surface, presenting an intriguing spin-orbital-momentum-layer locking phenomenon, which may shed lights on potential spintronic applications.

Observation of doping-induced spin–orbit coupling in gold cluster assembly to carrier-tuneable semiconductors and Schottky behaviors

The doping influenced carrier dynamics to maneuvering n-type to p-type semiconducting gold cluster assembly have been assessed. The resonance photoemission spectroscopic measurements corroborate an incremental rise in the density of states at the valence edge, the overlap of valence states due to doping, and the presence of distinct spin–orbit splitting and coupling in manganese-doping (Au7Mn) compared to gold clusters (Au8), originating from the relativistic effect resulted in a semiconducting property. The work function dependent current–voltage characteristics in metal–semiconductor configuration show Ohmic and Schottky behaviors assorting p-type carriers in Au7Mn in contrast to the n-type Au8 and are presenting an atomic cluster based fast electronic device.

Observation of kagome-like bands in two-dimensional semiconducting Cr8Se12

The kagome lattice is a versatile platform for investigating correlated electronic states. However, its realization in two-dimensional (2D) semiconductors for tunable device applications is still challenging. An alternative strategy to create kagome-like bands is to realize a coloring-triangle (CT) lattice in semiconductors through a distortion of a modified triangular lattice. Here, we report the observation of low-energy kagome-like bands in a semiconducting 2D transition metal chalcogenide—Cr8Se12 with a thickness of 7 atomic layers—which exhibits a CT lattice and a bandgap of 0.8 eV. The Cr-deficient layer beneath the topmost Se-full layer is partially occupied with 2/3 occupancy, yielding a √3 × √3 Cr honeycomb network. Angle-resolved photoemission spectroscopy measurements and first-principles investigations reveal the surface kagome-like bands near the valence band maximum, which are attributed to topmost Se pz orbitals modulated by the honeycomb Cr.

Epitaxial growth and band offsets of β -(ScxGa1− x)2O3 thin films grown on (100) β -Ga2O3 substrate

β-(ScxGa1−x)2O3 (x = 0–0.36) thin films were epitaxially grown on (100) β-Ga2O3 substrates by oxygen-radical-assisted pulsed-laser deposition. β-(ScxGa1−x)2O3 epilayers were coherently strained up to x = 0.30, although the presence of a structural disorder was implied when x > 0.2. The bandgap energies measured by reflection electron energy loss spectroscopy increased from 4.56 to 5.25 eV with increasing Sc content. In β-(ScxGa1−x)2O3 epilayers, a slightly negative bandgap bowing behavior with a bowing parameter of −0.4 eV was observed, resulting in a larger bandgap increase than in β-(AlxGa1−x)2O3 epilayers with identical x. X-ray photoemission spectroscopy measurement revealed that the valence-band and conduction-band offsets of β-(Sc0.17Ga0.83)2O3 epilayer with respect to β-Ga2O3 were 0.0 and 0.3 eV, respectively.

Peeling-induced interfacial roughness and charging for enhanced triboelectric power generation

To date, there have been a lot of efforts to enhance the conversion efficiency of triboelectric nanogenerators (TENGs) via physical and chemical modifications of the polymer surface. Herein, we report that a facile peeling of polymer from a substrate can enhance the triboelectric power output. The peeling of spin-coated polydimethylsiloxane (PDMS) from glass and polytetrafluoroethylene (PTFE) substrates has revealed a considerable increase in interfacial roughness and lateral friction. Surface-sensitive X-ray photoemission spectroscopy and non-contact current measurements have also revealed the transfer of counter-material and interfacial charging. The surface roughness of peeled PDMS is significant for the PTFE substrate, while the surface charge is significant for the glass substrate. The triboelectric output power density for peeled PDMS from the substrate is similar, 10.3 µW/cm2, but significantly larger than that for as-grown PDMS (2.2 µW/cm2). The peeling strength of PDMS significantly increases for glass compared to PTFE after the oxygen plasma treatment of substrates. The triboelectric charge of peeled PDMS from a plasma-treated glass substrate is almost 1.3 times larger than that from a PTFE substrate. This work implies that peeling polymer from a rough substrate with a large adhesion force would be a facile and effective way to increase the triboelectric power output, without any delicate surface treatments.

Ferroelectric Semimetals with α-Bi/SnSe van der Waals Heterostructures and Their Topological Currents.

We show that proximity effects can be utilized to engineer van der Waals heterostructures (vdWHs) displaying semimetallic spin-ferroelectricity locking, where ferroelectricity and semimetallic spin states are confined to different layers, but are correlated by means of proximity effects. Our findings are supported by first principles calculations involving α-Bi/SnSe bilayers. We show that such systems support ferroelectrically switchable nonlinear anomalous Hall effect originating from large Berry curvature dipoles as well as direct and inverse spin Hall effects with giant bulk spin-charge interconversion efficiencies. The giant efficiencies are consequences of the proximity-induced semimetallic nature of low energy electron states, which are shown to behave as two-dimensional pseudo-Weyl fermions by means of symmetry analysis and first principles calculations as well as direct angle-resolved photoemission spectroscopy measurements.

Magnetotransport and angle-resolved photoemission spectroscopy of MnSb12Te19: a new member of MnSb2nTe3n+1 family

The quest for intrinsically ferromagnetic topological materials is a focal point in the study of topological phases of matter, as intrinsic ferromagnetism plays a vital role in realizing exotic properties such as the anomalous Hall effect (AHE) in quasi-two-dimensional materials, and this stands out as one of the most pressing concerns within the field. Here, we investigate a novel higher order member of the MnSb2nTe3n+1family, MnSb12Te19, for the first time combining magnetotransport and angle-resolved photoemission spectroscopy (ARPES) measurements. Our magnetic susceptibility experiments identify ferromagnetic transitions at temperatureTc= 18.7 K, consistent with our heat capacity measurements (T= 18.8 K). The AHE is observed for the field along thec-axis belowTc. Our study of Shubinikov-de-Haas oscillations provides evidence for Dirac fermions withπBerry phase. Our comprehensive investigation reveals that MnSb12Te19exhibits a FM ground state along with AHE, and hole-dominated transport properties consistent with ARPES measurements.

Interplay between disorder and electronic correlations in compositionally complex alloys

Owing to their exceptional mechanical, electronic, and phononic transport properties, compositionally complex alloys, including high-entropy alloys, represent an important class of materials. However, the interplay between chemical disorder and electronic correlations, and its influence on electronic structure-derived properties, remains largely unexplored. This is addressed for the archetypal CrMnFeCoNi alloy using resonant and valence band photoemission spectroscopy, electrical resistivity, and optical conductivity measurements, complemented by linear response calculations based on density functional theory. Utilizing dynamical mean-field theory, correlation signatures and damping in the spectra are identified, highlighting the significance of many-body effects, particularly in states distant from the Fermi edge. Electronic transport remains dominated by disorder and potentially short-range order, especially at low temperatures, while visible-spectrum optical conductivity and high-temperature transport are influenced by short quasiparticle lifetimes. These findings improve our understanding of element-specific electronic correlations in compositionally complex alloys and facilitate the development of advanced materials with tailored electronic properties.

Hole-Carrier-Dominant Transport in 2D Single-Crystal Copper.

In 2D noble metals like copper, the carrier scattering at grain boundaries has obscured the intrinsic nature of electronic transport. However, it is demonstrated that the intrinsic nature of transport by hole carriers in 2D copper can be revealed by growing thin films without grain boundaries. As even a slight deviation from the twin boundary is perceived as grain boundaries by electrons, it is only through the thorough elimination of grain boundaries that the hidden hole-like attribute of 2D single-crystal copper can be unmasked. Two types of Fermi surfaces, a large hexagonal Fermi surface centered at the zone center and the triangular Fermi surface around the zone corner, tightly matching to the calculated Fermi surface topology, confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and vivid nonlinear Hall effects of the 2D single-crystal copper account for the presence of hole carriers experimentally. This breakthrough suggests the potential to manipulate the majority carrier polarity in metals by means of grain boundary engineering in a 2D geometry.

Characterization of MoS2:Nb sputtered thin films. An application as hole transport layer in Cu2ZnSnS4/Si tandem solar cells

MoS2:Nb films deposited by radio-frequency magnetron sputtering are investigated in view of their application in infrared (IR)-transparent contacts for tandem photovoltaic devices.This material is already known to give a good electrical contact with p-type chalcogenide semiconductors, which are typically grown onto opaque molybdenum metallic contacts and a MoS2 layer spontaneously forms at the back interface during the on-top semiconductor growth. Our study explores a different approach, which involves the direct growth of IR-transparent MoS2:Nb films via sputtering inside complete photovoltaic devices, like Cu2ZnSnS4 (CZTS)-based single junction solar cells and CZTS/Si tandem devices. Films deposited at different sputtering pressures are compared by analysing their microstructure, morphology, chemical composition, optical and electrical properties. The effects of post-deposition sulfurization treatments are also investigated. We find that MoS2:Nb films deposited at around 0.1 Pa exhibit compactness but show a notable sulfur deficit ([S]/[Mo]≈1.4), a significant sub-bandgap optical absorptance and lack of crystallinity. Increasing the Ar pressure to 1 Pa raises the [S]/[Mo] ratio to 2.2, yielding crystalline films with good IR-transparency, although with a porous morphology. Despite 0.5 wt% Nb-doping of the sputtering target, the as-deposited films demonstrate n-type conductivity likely due to uncontrolled impurities and intrinsic defects. Ultraviolet Photoemission Spectroscopy measurements suggest that films’ work function higher than 5 eV can be obtained with a post-deposition sulfurization, making these materials suitable as Hole Transport Layer in photovoltaic applications. A similar increase in work function is expected in the CZTS/MoS2 junctions, since the sputtered MoS2:Nb films undergo a sulfurization process needed to obtain the overlying polycrystalline CZTS absorber.CZTS solar cells produced with sputtered MoS2 and Transparent Conductive Oxides contacts on glass substrates, despite plagued by severe adhesion problems, show the potentiality to give efficiencies comparable to reference devices with standard Mo back contact. Fabrication of CZTS/Si tandem devices on textured silicon bottom cells yields a maximum efficiency of 4.4 %, primarily hindered by the low quality of the CZTS film on textured substrates. Nonetheless, optoelectronic characterizations based on both spectrophotometric and quantum efficiency measurements confirm a good IR transparency of the MoS2-based intermediate contacts and the desired electrical behaviour.

Quantum-Confined Lifshitz Transition on Weyl Semimetal Td-MoTe2.

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal Td-MoTe2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.