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Using angle-resolved photoemission spectroscopy and band structure calculation, we have investigated the three-dimensional nature and the orbital character of the low-energy electronic structure in the layered ternary boride ${\mathrm{AlMn}}_{2}{\mathrm{B}}_{2}$, in which ferromagnetic Mn $ab$ layers couple antiferromagnetically along the $c$ axis (N\'eel temperature ${\mathrm{T}}_{\mathrm{N}}\ensuremath{\sim}320\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). The calculation indicates that electronic bands in the vicinity of the Fermi energy ${E}_{\mathrm{F}}$ are dominated by the Mn $3d$ orbitals. The ${e}_{g}$ orbitals especially contribute high density of states (DOS) with large effective electron masses near ${E}_{\mathrm{F}}$. The calculated bands are renormalized by a factor of $\ensuremath{\sim}1.5$ to match the overall experimental observations, indicating moderate electronic correlation. Considering that the neighbor compound ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, with one more $3d$ electron, is ferromagnetic, the high DOS near ${E}_{\mathrm{F}}$ from the different $3d$ orbitals should be associated with the different magnetic orders along the special directions in these layered ternary borides. On the other hand, our studies of electronic structure of ${\mathrm{AlMn}}_{2}{\mathrm{B}}_{2}$ would further prompt its potential applications as a layered room-temperature antiferromagnetic material.

Quantum materials with layered kagome structures have drawn considerable attention due to their unique lattice geometry, which gives rise to flat bands together with Dirac-like dispersions. Recently, vanadium-based materials with layered kagome structures were discovered to be topological metals, which exhibit charge density wave (CDW) properties, significant anomalous Hall effect, and unusual superconductivity at low temperatures. Here, we employ angle-resolved photoemission spectroscopy to investigate the electronic structure evolution upon the CDW transition in a vanadium-based kagome material RbV_{3}Sb_{5}. The CDW phase transition gives rise to a partial energy gap opening at the boundary of the Brillouin zone and, most importantly, the emergence of new van Hove singularities associated with large density of states, which are absent in the normal phase and might be related to the superconductivity observed at lower temperatures. Our work sheds light on the microscopic mechanisms for the formation of the CDW and superconducting states in these topological kagome metals.

Electron-boson interaction is fundamental to a thorough understanding of various exotic properties emerging in many-body physics. In photoemission spectroscopy, photoelectron emission due to photon absorption would trigger diverse collective excitations in solids, including the emergence of phonons, magnons, electron-hole pairs, and plasmons, which naturally provides a reliable pathway to study electron-boson couplings. While fingerprints of electron-phonon/-magnon interactions in this state-of-the-art technique have been well investigated, much less is known about electron-plasmon coupling, and direct observation of the band renormalization solely due to electron-plasmon interactions is extremely challenging. Here by utilizing integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy, we discover the long sought-after pure electron-plasmon coupling-induced low-lying plasmonic-polaron replica bands in epitaxial semimetallic SrIrO$_3$ films, in which the characteristic low carrier concentration and narrow bandwidth combine to provide a unique platform where the electron-plasmon interaction can be investigated kinematically in photoemission spectroscopy. This finding enriches the forms of electron band normalization on collective modes in solids and demonstrates that, to obtain a complete understanding of the quasiparticle dynamics in 5d electron systems, the electron-plasmon interaction should be considered on equal footing with the acknowledged electron-electron interaction and spin-orbit coupling., 34 pages, 4 figures

Angle-resolved photoemission spectroscopy (ARPES) is one of the most powerful experimental techniques in condensed matter physics. Synchrotron ARPES, which uses photons with high flux and continuously tunable energy, has become particularly important. However, an excellent synchrotron ARPES system must have features such as a small beam spot, super-high energy resolution, and a user-friendly operation interface. A synchrotron beamline and an endstation (BL03U) were designed and constructed at the Shanghai Synchrotron Radiation Facility. The beam spot size at the sample position is 7.5 (V) μm × 67 (H) μm, and the fundamental photon range is 7–165 eV; the ARPES system enables photoemission with an energy resolution of 2.67 meV at 21.2 eV. In addition, the ARPES system of this endstation is equipped with a six-axis cryogenic sample manipulator (the lowest temperature is 7 K) and is integrated with an oxide molecular beam epitaxy system and a scanning tunneling microscope, which can provide an advanced platform for in situ characterization of the fine electronic structure of condensed matter.

${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, a layered ferromagnet with a Curie temperature near room temperature (${T}_{c}\ensuremath{\sim}270$ K), has attracted growing attention recently in condensed-matter physics and materials science. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide evidence of a linear band and a saddlelike flat band in the vicinity of the Fermi energy (${E}_{F}$) in ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, consisting of slabs of ${\mathrm{Fe}}_{2}{\mathrm{B}}_{2}$ atoms separated by layers of Al. The anomalous scattering rate of the linear band varies linearly with the kinetic energy up to 120 meV, showing non-Fermi-liquid behavior consistent with T-linear dependence of resistivity. The flat band with ${d}_{xy}$ orbital character and with strong ${k}_{y}$ dispersion may be associated with spin orientation along the $a$ axis in layered ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$. Electronic and magnetic correlation can be modulated by tuning the flat band near ${E}_{F}$ in layered ternary borides. Our findings have important implications to exploit emergent physics and quantum phases in $3d$ transition metals.

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\ensuremath{-}4.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.2\ensuremath{-}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (the ranges correspond to the minimum and maximum value of the coupling energy in the ${k}_{x}{k}_{y}$ plane). The $25%\ensuremath{-}30%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with $s$-wave-like gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers further support of $s$-wave type of the order parameter. We find that a $d$-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

Graphene with a Dirac cone-like electronic structure has been extensively studied because of its novel transport properties and potential application for future electronic devices. For epitaxially grown graphene, the process conditions and the microstructures are strongly dependent on various substrate materials with different lattice constants and interface energies. Utilizing angle-resolved photoemission spectroscopy, here we report an investigation of the electronic structure of single-crystalline graphene grown on Cu/Ni (111) alloy film by chemical vapor deposition. With a relatively low growth temperature, graphene on Cu/Ni (111) exhibits a Dirac cone-like dispersion comparable to that of graphene grown on Cu (111). The linear dispersions forming Dirac cone are as wide as 2 eV, with the Fermi velocity of approximately 1.1×106 m/s. Dirac cone opens a gap of approximately 152 meV at the binding energy of approximately 304 meV. Our findings would promote the study of engineering of graphene on different substrate materials.

Using angle-resolved photoemission spectroscopy, we study the low-energy electronic structure of a layered ternary telluride EuSbTe3 semiconductor. It is found that the photoemission constant energy contours can be well described by the simple two-parameter (t perp and t para ) tight-binding model based on the Te orbitals in square-net planes of EuSbTe 3 , suggesting its Te 5p orbitals dominated low-lying electronic structure, which is reminiscent of other rare-earth tritellurides. However, a possible charge-density-wave gap of 80 meV is found to persist in 300 K, which renders the unexpected semiconducting properties in EuSbTe 3 . Moreover, we reveal an extra band gap occurring around 200 meV below the Fermi level at low temperatures, which can be attributed to the interaction between the main and folded bands due to lattice scatterings. Our findings provide the first comprehensive understanding of the electronic structure of layered ternary tellurides, which lays the basis for future research on these compounds.

We report on local magnetization, tunnel diode oscillators, and specific-heat measurements in a series of Ba(NixFe1−x)2As2 single crystals (0.26≤x≤0.74). We show that the London penetration depth λ(T)=λ(0)+Δλ(T) scales as λ(0)∝1/Tc^0.85±0.2, Δλ(T)∝T^2.3±0.3 (for T≤T_c/3) and $\partial\Delta\lambda/\partial T^2\propto 1/T_c^{2.8\pm0.3}$ in both underdoped and overdoped samples. Moreover, the slope of the upper critical field ($H'_{c2}=-(dH_{c2}/dT)_{|T\rightarrow T_c}$) decreases with $T_c$ in overdoped samples but increases with decreasing $T_c$ in underdoped samples. The remarkable variation of $\lambda(0)$ with $T_c$ and the non exponential temperature dependence of $\Delta\lambda$ clearly indicates that pair breaking effects are important in this system. We show that the observed scalings strongly suggest that those pair breaking effects could be associated with quantum fluctuations near 3D superconducting critical points.

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$. For the insulating Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$, neutron polarization analysis and single-crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ block antiferromagnetic structure. For superconducting samples (${T}_{c}=30$ K), we find that in addition to the tetragonal $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$ is phase separated with coexisting $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ and $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structures.