We use micro-angle-resolved photoemission spectroscopy (micro-ARPES) to investigate chiral charge density waves (CDWs) in 4Hb-TaS2 with micron-scale spatial resolution. In the 1T layers of 4Hb-TaS2, we uncover coexisting left- and right-handed CDW domains and resolve four distinct spectral patterns arising from the interplay of chirality and rotational stacking. In contrast, bulk 1T-TaS2 exhibits uniform chirality. In addition, 4Hb-TaS2 shows negligible out-of-plane dispersion of the 1T-derived bands, in contrast to the large interlayer coupling observed in bulk 1T-TaS2. Density Functional Theory (DFT) calculations corroborate this picture, revealing that the interlayer interaction of the chiral order in 4Hb-TaS2 is nearly two orders of magnitude weaker than in the 1T polytype. Our findings establish 4Hb-TaS2 as a quasi-two-dimensional platform for exploring tunable chiral CDW phenomena.

We use micro-angle-resolved photoemission spectroscopy (micro-ARPES) to investigate chiral charge density waves (CDWs) in 4Hb-TaS2 with micron-scale spatial resolution. In the 1T layers of 4Hb-TaS2, we uncover coexisting left- and right-handed CDW domains and resolve four distinct spectral patterns arising from the interplay of chirality and rotational stacking. In contrast, bulk 1T-TaS2 exhibits uniform chirality. In addition, 4Hb-TaS2 shows negligible out-of-plane dispersion of the 1T-derived bands, in contrast to the large interlayer coupling observed in bulk 1T-TaS2. Density Functional Theory (DFT) calculations corroborate this picture, revealing that the interlayer interaction of the chiral order in 4Hb-TaS2 is nearly two orders of magnitude weaker than in the 1T polytype. Our findings establish 4Hb-TaS2 as a quasi-two-dimensional platform for exploring tunable chiral CDW phenomena.

Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a conclusion about their origin. In CsPbBr3, we uncover both key signatures: Upon cooling, orthorhombic replica bands emerge which had eluded earlier studies. This includes an extra valence band maximum at Γ which can even be distinguished at room temperature. Most importantly, band narrowing along Γ-M, a splitting at Γ, and the lifting of degeneracy between nonequivalent M points, all of several 100 meV, become apparent. Temperature-dependent x-ray diffraction, used as input for band structure calculations, links these effects directly to tilts and rotations of the PbBr6 octahedra. Our results uncover a strong electron-lattice interaction which is at the heart of so-far unresolved questions concerning polaronic transport, dynamic disorder, and exciton trapping in halide perovskites.

Halide perovskites are a promising class of materials for optoelectronic and photovoltaic applications, exhibiting high power conversion efficiency due to strong light absorption and long carrier diffusion lengths. While various aspects of their crystal and electronic structure have been studied, we identify a fundamental property previously overlooked that may significantly impact their efficiency. We demonstrate that halide perovskites realize a three-dimensional (3D) Lieb lattice, giving rise to a gapped 3D Dirac cone of spin-1 fermions. This leads to a fivefold reduction in effective mass compared to a conventional cubic structure and suppressed carrier backscattering due to Klein tunneling. Our conclusions are supported by band structure calculations and angle-resolved photoemission spectroscopy from CsPbBr_3 and CsSnBr_3. In particular, we reveal the transformation of the flat band of the Lieb lattice and the emergence of a dark corridor effect in photoemission from the Dirac cone, which increases as the band gap is decreased from CsPbBr_3 to CsSnBr_3.

Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow bandgap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. It is demonstrated that dispersions measured along and perpendicular to the phosphorene chains self-assembled on Ag(111) reveal pronounced electronic confinement resulting in a 1D band, flat and dispersionless perpendicular to the chain direction in momentum space. Density functional theory calculations reproduce the 1D band for the experimentally determined structure. It is shown that phosphorene chains aligned equiprobably to three directions of the Ag(111) surface can be characterized by angle-resolved photoemission spectroscopy because the three rotational variants are separated in the angular domain. A semiconductor-to-metal phase transition is predicted upon increasing the density of the chain array, a promising approach to band structure engineering.

Trions, three-body bound states composed of an exciton and an additional charge, are typically fragile and require external excitation to form. Here, we report the spontaneous emergence of a stable trion gas at the surface of the layered semiconductor Ta2NiS5, revealed through angle-resolved photoemission spectroscopy. We observe a sharp, highly localized in-gap feature that cannot be explained by conventional band-theory. Instead, we argue that it arises from the formation of negative trions, stabilized by surface-induced band bending and the material's quasi-one-dimensional geometry. Unlike excitons, these trions form without optical pumping and persist at equilibrium, marking a rare example of an interaction-driven surface state in a nominally conventional semiconductor. Our findings establish Ta2NiS5 as a unique platform for exploring many-body physics at surfaces and open new avenues for studying and controlling collective excitations in low-dimensional systems.

Tamai et al. discovered an unusual electronic state near the Fermi level at the interface of Cu(111) and a molecular layer of C60, which was initially attributed to C60-Cu interfacial hybridization [Phys. Rev. B 77, 075134 (2008)]. Later on, Yue et al. suggested that the state was due to the reshaping of a two-dimensional electron gas hosted at the Cu(111) surface, into an artificial graphene with Dirac cones by cutting out muffin-tin potentials of adsorbed fullerene molecules [Phys. Rev. B 102, 201401(R) (2020)]. In the present paper, we introduce a different explanation using angle-resolved photoemission and show that the observed conical bands in the C60/Cu(111) system are neither Dirac cones nor hybridization states. Rather, they are formed by umklapp scattering of photoelectrons emitted from bulk and surface bands of Cu(111) and diffracted on the (4 x 4) fullerene superstructure. The circular contours near the Fermi level, which resemble the low-energy part of graphene Dirac cones, are a result of the backfolding of the bulk band of copper, while the triangular silhouettes observed at higher binding energies, which mimic the threefold symmetric higher-binding-energy part of graphene Dirac cones, are due to an umklapp effect governed by highly coherent photoelectron diffraction of steeply dispersing Cu(111) sp-type surface resonances. We also used density functional theory to study the be-havior of the two-dimensional electron gas localized within the honeycomb net created by the electron-exclusive potentials of C60 and demonstrate doping above the Fermi level (p doping). The results show no presence of the electronic structure of artificial graphene in C60/Cu(111).

Two-dimensional (2D) Dirac materials are electronically and structurally very sensitive to proximity effects. We demonstrate, however, the opposite effect: that the deposition of a monolayer 2D material could exercise a substantial influence on the substrate electronic structure. Here we investigate TiC(111) and show that a graphene overlayer produces a proximity effect, changing the Fermi surface topology of the TiC from six electron pockets to one hole pocket on the depth of several atomic layers inside the substrate. In addition, the graphene electronic structure undergoes an extreme modification as well. While the Dirac cone remains gapless, it experiences an energy shift of 1.0 eV beyond what was recently achieved for the Lifshitz transition of overdoped graphene. Due to this shift, the antibonding pi* band at the (M) over bar point becomes occupied and observable by photoemission.

Colossal negative magnetoresistance is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. In contrast, topological semimetals show large but positive magnetoresistance, originated from the high-mobility charge carriers. Here, we show that in the highly electron-doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb0.11Te1.90 hosts multiple charge density wave modulation vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor metal-like transition, which results in a colossal negative magnetoresistance. Moreover, signatures of the coupling between the charge density wave and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow band gap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. Phosphorene atomic chains self-assembled on Ag(111) have recently been characterized structurally but were found by angle-resolved photoemission (ARPES) to be electronically 2D. We show that these chains, although aligned equiprobably to three <$1 \bar{1} 0$> directions of the Ag(111) surface, can be characterized by ARPES because the three rotational variants are separated in the angular domain. The dispersion of the phosphorus band measured along and perpendicular to the chains reveals pronounced electronic confinement resulting in a 1D band, flat and dispersionless perpendicular to the chain direction in momentum space. Our density functional theory calculations reproduce the 1D band for the experimentally determined structure of P/Ag(111). We predict a semiconductor-to-metal phase transition upon increasing the density of the chain array so that a 2D structure would be metallic.

Colossal magnetoresistance (MR) is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. Though topological semimetals also show large MR, its origin and nature are completely different. Here, we show that in the highly electron doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb$_{0.11}$Te$_{1.90}$ hosts multiple charge density wave (CDW) modulation-vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor-metal-like transition, which results in a colossal negative MR. Moreover, signatures of the coupling between the CDW and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

The electronic structure and chemical composition of the n-GaAs surface after implantation of N2+ ions with energy Ei = 3000 eV and fluence Q - 3 x 1015 cm-2 were studied by synchrotron-based X-ray photoelectron spectroscopy to clarify effects of low-energy nitrogen ion implantation on A3B5 semiconductor surfaces. Conversion of the conductivity type and creation of a p-n structure on the n-GaAs surface were revealed under N2+ ion irradiation. The conductivity type transformation was shown to occur due to pure mechanical action of nitrogen ions, resulting in formation of Ga anti-site acceptors. The - 10 nm-thick p-layer obtained consisted of concentrated GaAs1-xNx (x - 0.1) alloy whose bandgap width is known to be essentially narrower compared to the pristine GaAs semiconductor. Therefore, the structure formed is a nano-heterosctructure incorporating the semiconductor alloy whose properties are considered to be attractive for infrared applications. The obtained GaAsN electron binding energies (EB (N1s) = 397.0 eV, EB (Ga3d) = 19.42 eV and EB (As3d) = 41.25 eV) can be used for identification of the GaAsN alloy phase. An approach to 3D p-n structures formation was suggested using nitrogen ion beam without wet lithography.

Cleaning the n-GaAs surface with low-energy Ar+ ions, required in X-ray photoelectron spectroscopy (XPS), has been recently shown to drastically change the core-level binding energies (BEs) of the irradiated surface layer, which prevents the diagnostics of the n-GaAs based semiconductors by the ordinary XPS. Synchrotron-based XPS measurements and modeling of As3d and Ga3d photoemission spectra for an Ar+ - etched n-GaAs wafer made it possible to find conditions for XPS testing the unmodified deep n-GaAs bulk. XPS resolution should be better than 0.5 eV and Ar+ ion energy should be less than 0.5 keV. Control of the Ga3d and As3d BEs in unmodified n-GaAs is important for chemical analysis since they are close to Ga2O3 and elemental arsenic BEs, respectively, which appear in oxidation, nitridation and other important chemical processes.

Few-layer graphene on β-SiC(001) functionalized with phenazine dye Neutral Red by means of diazonium chemistry has been studied using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, photoemission electron microscopy, scanning tunneling microscopy, and density functional theory calculations. The experimental data reveal the formation of a composite phenazine dye/graphene structure with a large energy gap. The molecules in this structure can be oriented both parallel and perpendicular to the graphene surface. According to scanning tunneling spectroscopy and theoretical calculations, the density of electron states in different surface areas depends on the local short-range order and the molecules’ environment. On the other hand, the photoemission spectroscopy study shows that the bottom layers of the few-layer graphene remain intact, which inherently makes the synthesized layered composite a low-dimensional metal/semiconductor heterostructure. In addition, photoemission electron microscopy imaging shows a high homogeneity of the dye-modified graphene on a micrometer scale.

Artificial graphene based on molecular networks enables the creation of novel 2D materials with unique electronic and topological properties. Landau quantization has been demonstrated by CO molecules arranged on the two-dimensional electron gas on Cu(111) and the observation of electron quantization may succeed based on the created gauge fields. Recently, it was reported that instead of individual manipulation of CO molecules, simple deposition of nonpolar C-60 molecules on Cu(111) and Au(111) produces artificial graphene as evidenced by Dirac cones in photoemission spectroscopy. Here, we show that C-60-induced Dirac cones on Au(111) have a different origin. We argue that those are related to umklapp diffraction of surface electronic bands of Au on the molecular grid of C-60 in the final state of photoemission. We test this alternative explanation by precisely probing the dimensionality of the observed conical features in the photoemission spectra, by varying both the incident photon energy and the degree of charge doping via alkali adatoms. Using density functional theory calculations and spin-resolved photoemission we reveal the origin of the replicating Au(111) bands and resolve them as deep leaky surface resonances derived from the bulk Au sp-band residing at the boundary of its surface projection. We also discuss the manifold nature of these resonances which gives rise to an onion-like Fermi surface of Au(111).

Recently the graphene/SiC interface has emerged as a versatile platform for the epitaxy of otherwise unstable, monoelemental, two-dimensional (2D) layers via intercalation. Intrinsically capped into a van der Waals heterostructure with overhead graphene, they compose a new class of quantum materials with striking properties contrasting their parent bulk crystals. Intercalated silver presents a prototypical example where 2D quantum confinement and inversion symmetry breaking entail a metal-to-semiconductor transition. However, little is known about the associated unoccupied states and coherent control of the Fermi level position across the bandgap would be desirable. Here, we n-type dope a graphene/2D-Ag/SiC heterostack via in situ potassium deposition and probe its band structure by means of synchrotron-based angle-resolved photoelectron spectroscopy. While the induced carrier densities on the order of 1014 cm−2 are not yet sufficient to reach the onset of the silver conduction band, the band alignment of graphene can be tuned relative to the rigidly shifting Ag valence band and substrate core levels. We further demonstrate an ordered potassium adlayer (2 × 2 relative to graphene) with free-electron-like dispersion, suppressing plasmaron quasiparticles in graphene via enhanced metalization of the heterostack. Our results establish surface charge-transfer doping as an efficient handle to tune band alignment and electronic properties of a van der Waals heterostructure assembled from graphene and a novel type of monolayered quantum material.

The formation of large polarons has been proposed as reason for the high defect tolerance, low mobility, low charge carrier trapping, and low nonradiative recombination rates of lead halide perovskites. Recently, direct evidence for large-polaron formation has been reported from a 50% effective mass enhancement in angle-resolved photoemission of CsPbBr_{3} over theory for the orthorhombic structure. We present in-depth band dispersion measurements of CsPbBr_{3} and GW calculations, which lead to similar effective masses at the valence band maximum of 0.203±0.016 m_{0} in experiment and 0.226 m_{0} in orthorhombic theory. We argue that the effective mass can be explained solely on the basis of electron-electron correlation and large-polaron formation cannot be concluded from photoemission data.

Efficient control over the grain boundaries (GBs) is a vital aspect in optimizing the graphene growth conditions. A number of methods for visualization of GBs were developed for graphene grown on weakly interacting surfaces. Here, we utilize oxygen intercalation to reveal GBs and study their morphology for graphene strongly bound to the cobalt surface. We demonstrate that upon the intercalation of oxygen, GBs in polycrystalline graphene become easily detectable due to graphene cracking and selective oxidation of the substrate, thus giving a direct insight into the graphene micro- and nanostructure by means of different electron microscopy methods, including scanning electron microscopy, photoemission microscopy and low-energy electron microscopy.

Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface Bi2Se3/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface Bi2Se3/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of Bi2Se3/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by Eu3+, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of mz,spin = 7 μB/Eu 2+, in good agreement with the SQUID data and the expected theoretical moment of Eu2+. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K.

New developments in the field of topological matter are often driven by materials discovery. In the last few years, large efforts have been performed to classify all known inorganic materials with respect to their topology. Unfortunately, a large number of topological materials suffer from non-ideal band structures. For example, topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. This leaves just a handful of materials that are intensively studied. Finding strategies to design new topological materials is a solution. Here we introduce a new mechanism that is based on charge density waves and non-symmorphic symmetry to design an idealized topological semimetal. We then show experimentally that the antiferromagnetic compound GdSb0.46Te1.48 is a nearly ideal topological semimetal based on the proposed mechanism. Its highly unusual transport behavior points to a thus far unknown regime, in which Dirac carriers with Fermi energy very close to the node gradually localize in the presence of lattice and magnetic disorder.