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The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently discovered family of topological kagome metals with a rich phenomenology of strongly correlated electronic phases including charge order and superconductivity. Understanding how the singularities inherent to the kagome electronic structure are linked to the observed many-body phases is a topic of great interest and relevance. Here, we combine angle-resolved photoemission spectroscopy and density functional theory to reveal multiple kagome-derived van Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and analyze their contribution to electronic symmetry breaking. Intriguingly, the vHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate from their pure and mixed sublattice characters, respectively. This twofold vHs is unique property of the kagome lattice, and its flavor critically determines the pairing symmetry and ground states emerging in AV3Sb5 series. We establish that, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome band and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to set the stage for electronic symmetry breaking. The former band exhibits pronounced Fermi surface nesting, while the latter contributes via higher-order vHs. Our work reveals the essential role of kagome-derived vHs for the collective phenomena realized in the AV3Sb5 family, paving the way to a deeper understanding of strongly correlated topological kagome systems., Comment: Submitted to Nature Physics on May 28th, 2021. A revised version will appear in Nature Physics

Key to our understanding of how electrons behave in crystalline solids is the band structure that connects the energy of electron waves to their wavenumber. Even in phases of matter with only short-range order (liquid or amorphous solid), the coherent part of electron waves still has a band structure. Theoretical models for the band structure of liquid metals were formulated more than five decades ago1–15, but, so far, band-structure renormalization and the pseudogap induced by resonance scattering have remained unobserved. Here we report the observation of the unusual band structure at the interface of a crystalline insulator (black phosphorus) and disordered dopants (alkali metals). We find that a conventional parabolic band structure of free electrons bends back towards zero wavenumber with a pseudogap of 30–240 millielectronvolts from the Fermi level. This is wavenumber renormalization caused by resonance scattering, leading to the formation of quasi-bound states in the scattering potential of alkali-metal ions. The depth of this potential tuned by different kinds of disordered alkali metal (sodium, potassium, rubidium and caesium) allows the classification of the pseudogap of p-wave and d-wave resonance. Our results may provide a clue to the puzzling spectrum of various crystalline insulators doped by disordered dopants16–20, such as the waterfall dispersion observed in copper oxides. A back-bending band structure and an emerging pseudogap are observed at the interface between a crystalline solid (black phosphorus) and disordered alkali-metal dopants.

We propose a method to fabricate the chemical p-n junction in wafer-scale epitaxial graphene. In the case of Au intercalation in between graphene and SiC(0001), there exist two structurally distinct phases that result in p-type and n-type doping in the graphene layer, respectively. In the process of in situ Au deposition on our samples, we used a shadow mask to form a sharp junction of different Au coverage. The intercalation of Au atoms induced by thermal annealing leads to the abrupt p-n junction in the graphene layer, which is characterized by angle-resolved photoemission spectroscopy. This p-n junction of graphene is abrupt in the scale comparable to the beam size of approximately 50 μm. This p-n junction of graphene is expected to be atomically abrupt, since there exist only two structurally distinct phases by self-assembly. The proposed method may be useful not only to fabricate a wafer-scale p-n junction of graphene, but also for a fundamental study on atomically abrupt graphene p-n junction.

In kagome metals AV3Sb5 (A = K, Rb, Cs), three-dimensional charge order (3D-CO) is the primary instability that sets the stage for other collective orders to emerge, including unidirectional stripe order, orbital flux order, electronic nematicity, and superconductivity. Here, we use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order (3D-CO) in AV3Sb5 and its interplay with superconductivity. Our approach is based on identifying an unusual splitting of kagome bands induced by 3D-CO, which provides a sensitive way to refine the spatial charge patterns in neighboring kagome planes. We found a marked dependence of the 3D-CO structure on composition and doping. The observed difference between CsV3Sb5 and the other compounds potentially underpins the double-dome superconductivity in CsV3(Sb,Sn)5 and the suppression of Tc in KV3Sb5 and RbV3Sb5. Our results provide fresh insights into the rich phase diagram of AV3Sb5., 20 pages, 5 figures

In the mechanism of two-dimensional (2D) superconductivity in doped graphene, it was predicted that the presence of dopant bands plays a key role. However, it has been challenging to fabricate an ordered phase of alkali metals on graphene owing to its van-der-Waals nature. We systematically study the phase formation of Rb on graphene epitaxially grown on H-passivated SiC(0001). We found a range of control parameters that stabilize the well-ordered 2 × 2 phase (RbC8), as confirmed by in-situ low-energy electron diffraction. Angle-resolved photoemission (ARPES) spectra taken from the 2 × 2 phase show a folded band of graphene and a parabolic band of Rb. In the vicinity of the Fermi level, the self-energy extracted from ARPES data reveals a clear signature of electronic coupling to the in-plane and out-of-plane phonon modes of C and Rb atoms. The electronic coupling of graphene to the in-plane vibration mode of Rb atoms is identified as a key factor for the enhancement of electron-phonon coupling.

Semiconductor devices rely on the charge and spin of electrons, but there is another electronic degree of freedom called pseudospin in a two-level quantum system1 such as a crystal consisting of two sublattices2. A potential way to exploit the pseudospin of electrons in pseudospintronics3–5 is to find quantum matter with tunable and sizeable pseudospin polarization. Here, we propose a bipolar pseudospin semiconductor, where the electron and hole states have opposite net pseudospin polarization. We experimentally identify such states in anisotropic honeycomb crystal—black phosphorus. By sublattice interference of photoelectrons, we find bipolar pseudospin polarization greater than 95% that is stable at room temperature. This pseudospin polarization is identified as a consequence of Dirac cones merged in the highly anisotropic honeycomb system6,7. The bipolar pseudospin semiconductor, which is a pseudospin analogue of magnetic semiconductors, is not only interesting in itself, but also might be useful for pseudospintronics. Anisotropic honeycomb crystal of black phosphorous is found to have pseudospin polarization greater than 95% at room temperature, attributed to the merging of Dirac cones. This bipolar pseudospin semiconductor may be useful for pseudospintronics.

A longstanding question in the field of solid-state physics is the renormalization of quasiparticles induced by the effect of electronic correlations. We employ angle-resolved photoemission spectroscopy to study quasiparticle renormalizations in graphene whose electron density can be modulated by the deposition of alkali metals. The evolution of ARPES spectra taken over a wide range of the Rb density reveals a discontinuity in the electron compressibility, which is identified as due to the phase formation of Rb atoms on graphene. We found a clear satellite feature of Dirac cones formed by correlations effects, and its coupling strength is enhanced with the phase formation of Rb atoms. Our results suggest that in graphene doped by alkali metals the phase formation of dopants should be carefully considered in a quantitative analysis on band renormalizations.

A key to understand how electrons behave in crystalline solids is the band structure that connects the energy of electron waves to their wavenumber (k). Even in the phase of matter with only short-range order (liquid), the coherent part of electron waves still possesses a band structure. Theoretical models for the band structure of liquid metals were formulated more than 5 decades ago1-15, but so far, it has remained unobserved experimentally. Here, we reveal the band structure of liquid metals using the interface between liquid dopants (alkali metals) and a crystalline insulator (black phosphorus). We find that the conventional parabolic band structure of free electrons bends back towards zero k with the isotropic pseudogap of 30-240 meV from the Fermi level. This is the k renormalization caused by resonance scattering that leads to the formation of quasi-bound states in the scattering potential of liquid alkali-metal ions. The depth of this potential tuned by different kinds of alkali metal (Na, K, Rb, and Cs) allows us to classify the pseudogap of p-wave and d-wave resonance. Our results provide a key clue to the pseudogap phase of various materials16-20, a common aspect of which is the crystalline insulator doped by disordered (liquid) dopants.

Key to our understanding of how electrons behave in crystalline solids is the band structure that connects the energy of electron waves to their wavenumber. Even in phases of matter with only short-range order (liquid or amorphous solid), the coherent part of electron waves still has a band structure. Theoretical models for the band structure of liquid metals were formulated more than five decades ago

We present an overview on the electronic band structure of surface-doped black phosphorus. Angle-resolved photoemission spectroscopy data show that the in situ deposition of potassium atoms on the surface of single-crystalline black phosphorus modulates the band gap in the wide range of 0.0–0.6 eV. At zero band gap, the surface layers of black phosphorus become a Dirac semimetal whose band dispersion is highly anisotropic, linear in armchair and quadratic in zigzag directions. In light of theoretical band calculations, we elucidate the mechanism of these band modifications as the giant Stark effect due to strong vertical electric fields induced by potassium atoms.

Semiconductor devices rely on the charge and spin of electrons, but there is another electronic degree of freedom called pseudospin in a two-level quantum system

Copper thiocyanate (CuSCN) is known as a promising hole transport layer in organic photovoltaics (OPVs) due to its good hole conduction and exciton blocking abilities with high transparency. Despite its successful device applications, the origin of its hole extraction enhancement in OPVs has not yet been understood. Here, we investigated the electronic structure of CuSCN and the energy level alignment at the poly(3-hexylthiophene-2,5-diyl) (P3HT)/CuSCN/ITO interfaces using ultraviolet photoelectron spectroscopy. The band-tail states of CuSCN close to the Fermi level (EF) were observed at 0.25 eV below the EF, leading to good hole transport. The CuSCN interlayer significantly reduces the hole transport barrier between ITO and P3HT due to its high work function and band-tail states. The barrier reduction leads to enhanced current density-voltage characteristics of hole-dominated devices. These results provide the origin of hole-extraction enhancement by CuSCN and insights for further application.

Two-dimensional (2D) crystals have emerged as a class of materials with tunable carrier density1. Carrier doping to 2D semiconductors can be used to modulate many-body interactions2 and to explore novel composite particles. The Holstein polaron is a small composite particle of an electron that carries a cloud of self-induced lattice deformation (or phonons)3-5, which has been proposed to play a key role in high-temperature superconductivity6 and carrier mobility in devices7. Here we report the discovery of Holstein polarons in a surface-doped layered semiconductor, MoS2, in which a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently8-11. Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons12-18. The short-range nature of electron-phonon (e-ph) coupling in MoS2 can be explained by its valley degeneracy, which enables strong intervalley coupling mediated by acoustic phonons. The coupling strength is found to increase gradually along the superconducting dome up to the intermediate regime, which suggests a bipolaronic pairing in the 2D superconductivity.

We report the realization of novel symmetry-protected Dirac fermions in a surface-doped two-dimensional (2D) semiconductor, black phosphorus. The widely tunable band gap of black phosphorus by the surface Stark effect is employed to achieve a surprisingly large band inversion up to ∼0.6 eV. High-resolution angle-resolved photoemission spectra directly reveal the pair creation of Dirac points and their movement along the axis of the glide-mirror symmetry. Unlike graphene, the Dirac point of black phosphorus is stable, as protected by space-time inversion symmetry, even in the presence of spin-orbit coupling. Our results establish black phosphorus in the inverted regime as a simple model system of 2D symmetry-protected (topological) Dirac semimetals, offering an unprecedented opportunity for the discovery of 2D Weyl semimetals.

van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8-2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.

A widely tunable band gap in few-layer K-doped black phosphorus is realized by in situ deposition of potassium atoms.

Functionally graded materials have composition gradients from one end to the other as the result of a gradual transition of the properties of different materials. The residual stress caused by the difference of coefficient of thermal expansion can be minimized using functionally graded material. Therefore, the gradient of the coefficient of thermal expansion should vary according to the compositional gradient. In this study, the coefficient of thermal expansion of each compositional layer of Ni-Al 2 O 3 functionally graded material was measured using a dilatometer. These measurements provided the material properties required to calculate the residual stress, using three-dimensional modeling for accurately predicting crack positions, since it is difficult to measure residual stress experimentally. The measurement results showed the gradual increase of the coefficient of thermal expansion from Al 2 O 3 -rich composition to Ni-rich composition. Finally, the results of calculating residual stresses using the measured coefficient of thermal expansion showed that the crack positions were predicted more accurately than those using the coefficient of thermal expansion calculated by the linear rule of mixtures. This was because the measured values include the effect of porosity of the composite, whereas the linear rule of mixtures cannot account for the porosity of each layer.

With the recent emphasis on the importance of successfully joining materials, researchers have tried to join metals and ceramics with different coefficients of thermal expansion (CTEs) by using the functionally graded material (FGM) method. This involves inserting interlayers with composition gradients that range from one material to the other, thereby minimizing the stress caused by differences in CTE values. In this study, the FGM that included 10 layers of Ni-Al 2 O 3 with eight inter-layers was studied. Previous studies have focused on controlling the composition of inter-layers and optimizing the dispersion process to prevent cracks. Thermal stress was reduced by varying the weights of the inter-layers and increasing the green-body density by using several powder sizes. The powders were well-dispersed during fabrication by using simultaneous dispersion and dry processes followed by a cold isostatic press (CIP) and pressure-less sintering in an inert atmosphere. As a result, a crack-free Ni-Al 2 O 3 FGM joint was obtained. The residual stress in each layer was calculated to predict cracks using ANSYS simulation and maximum principal stress criterion; experimental values matched simulation results. In addition, an oriented Vickers indentation test was used to assess the quality of the joint. Crack-paths were not deflected across the interface, indicating good bond strength between interfaces. Sample density was measured using the Archimedes method; the sintered joint was less dense than its theoretical density but was denser than the results obtained by using previous methods.

We fabricated a crack-free joining of Ni and Al2O3 using a functionally graded method. Because porosity reduction is important, particle size of the Ni and Al2O3 powders was varied to improve green body density and minimize shrinkage during sintering. As a result, crack-free joining of a Ni–Al2O3 sample with 10 layers was obtained. The ANSYS simulation tool was used to calculate residual stress. The hardness and modulus of each graded layer were measured using an Vickers’ indenter. The experimental values matched the simulation results, showing that this analysis is useful when residual stresses are very difficult to measure experimentally.

Joining Si3N4 and Al2O3 using 15 layers has been achieved by a unique approach that introduces SiAlON polytypoids as a functionally graded material (FGM) bonding layer. Previously, the hot press sintering of multilayered FGM with 20 layers, each 500 µm thick, has been achieved successfully. In the present study, the number of layers for FGM was reduced from 20 to 15 to increase optimization. Samples were fabricated by hot pressing at 48 MPa during the temperature ramp to 1650°C and cooling at 2°C/min to minimize residual stresses from sintering. Moreover, a finite element method (FEM) program based on the maximum principal stress theory and the maximum tensile stress theory was applied to design optimized and reduced FGM layers that produced a crack-free joint. The sample had a 3-dimensional cylindrical shape that was transformed to a 2-dimensional axisymmetric mode. By determining the expected thermal stress from the calculated elastic modulus and coefficient of thermal expansion, we were able to predict and prevent damage due to thermal stresses. These analyses are especially useful for FGM samples where it is very difficult to measure the residual stresses experimentally. Finally, oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were deflected at the SiAlON layers, implying weak interfaces. In other areas, cracks were not deflected, implying strong interfaces.

Numerical analysis method was used to reduce the number of functionally graded material (FGM) layers for joining Si3N4-Al2O3 using polytypoid interlayer by estimating the position of crack. In the past, hot press sintering of multi-layered FGMs with 20 layers of thickness 500 mm each have been fabricated successfully. In this paper, thermal residual stresses were calculated using finite element method (FEM) to find the optimized number of layers and its thicknesses of FGM joint. The number of layers for FGM was reduced to 15 layers from 20 layers. Thicknesses were varied to minimize residual stresses within the layers while reducing the number of FGM layers. The damage caused by thermal residual stress was estimated using maximum principal stress theory and maximum tensile stress theory. The calculated maximum stress was found to be axial stress of 430 MPa around 90% 12H/10% Al2O3 area. For each case, calculated strength of each FGM layer by linear rule of mixture was compared with computed thermal residual stresses. Thermal analysis results correctly predicted the position of crack, and this position agreed well with fabricated joints. Therefore, this numerical analysis method can be applied to reduced FGM layers of crack free joint. Finally, new composition profile of crack free joint was proposed using FGM method. [doi:10.2320/matertrans.MRA2007319]

Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices.

Crack-free joining of Si N and Al O using 15 layers has been achieved by a unique approach introducing Sialon polytypoids as a functionally graded materials (FGMs) bonding layer. In the past, hot press sintering of multilayered FGMs with 20 layers of thickness 500 µm each has been fabricated successfully. In this study, the number of layers for FGM was reduced to 15 layers from 20 layers for optimization. For fabrication, model was hot pressed at 38 MPa while heating up to 1700 C, and it was cooled at 2 C/min to minimize residual stress during sintering. Initially, FGM with 15 layers had cracks near 90 wt.% 12H / 10 wt.% Al O and 90 wt.% 12H/10 wt.% Si N layers. To solve this problem, FEM (finite element method) program based on the maximum tensile stress theory was applied to design optimized FGM layers of crack free joint. The sample is 3-dimensional cylindrical shape where this has been transformed to 2-dimensional axisymmetric mode. Based on the simulation, crack-free FGM sample was obtained by designing axial, hoop and radial stresses less than tensile strength values across all the layers of FGM. Therefore, we were able to predict and prevent the damage by calculating its thermal stress using its elastic modulus and coefficient of thermal expansion. Such analyses are especially useful for FGM samples where the residual stresses are very difficult to measure experimentally.