BaDElPh Highlights

Using experimental and theoretical methods, we show that graphene can be magnetized by coupling to a ferromagnetic Co film through a Au monolayer. The presence of dislocation loops under graphene leads to a ferrimagnetic (FIM) ordering of moments in the two C sublattices. The band gap of ∼80 meV at the Dirac point is revealed by ARPES and DFT methods, and it is shown that the gap has a magnetic nature and exists for ferrimagnetic ordering. Tight-binding calculation results are in good agreement with the spin-ARPES data, showing the possibility for the synthesized system to implement the circular dichroism Hall effect.

We have studied the band structure of epitaxially grown graphene on SiC and its hole-doped compositions by angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). Linearly s- and p-polarized ARPES reveal the dispersive Dirac bands within the anomalous regime which was hidden due to the finite momentum broadening and the proximity of the bands near the Dirac point. No energy gap is observed at the K point of the Brillouin zone even after boron substitution up to 5% indicating that the gap, if there is any, is below the detection level. The similar scenario near the Dirac point in the doped and undoped cases manifests the robustness of the protection of internal symmetries in the studied doping regime. The band renormalization is found to become more prominent with the increase in hole doping.

These results suggest graphene on SiC as a platform for a new paradigm where properties (mobility, density, etc.) of the Dirac fermions can be tuned significantly to realize exotic science and advanced technology.

Using angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED), together with density-functional theory (DFT) calculation, we report the formation of charge density wave (CDW) and its interplay with the Kondo effect and topological states in CeSbTe. The CDW is driven by the electron-phonon coupling (EPC) from the well-nested 2D Fermi surface (FS). The measured CDW gap is large and up to ~0.3 eV, thus explaining the robust CDW order up to very high temperatures. The gap is roughly isotropic in momentum space, except near the X point where the imperfect FS nesting leads to an energy gap less than 0.1 eV. The gap opening leads to a reduced density of states (DOS) near the Fermi level (EF), which correspondingly suppresses the many-body Kondo effect, leading to very localized 4f electrons at 20 K and above.

The topological Dirac cone at the X point is found to remain gapless inside the CDW phase. Our results provide evidence for the competition between CDW and the Kondo effect in a Kondo lattice system. The robust CDW order in CeSbTe and related compounds provide an opportunity to search for the long-sought-after axionic insulator.

We have investigated the growth of a few-layer Cs quantum well on top of a bilayer graphene substrate grown on top of an Ir(111). ARPES reveals four new parabolic electronic bands centered at Γ which arise from the growth of Cs quantum wells. The Cs layers grown on the bilayer graphene substrate have an in-plane strain of 11%. Cs grows in this multi-layered way rather than clustering up, because of its low cohesive energy, favoring the adhesion to the graphene surface. Investigating the electronic structure of the resulting material, reveals the Cs quantum wells to be good approximations of 2D free electron gases. In the future, Cs could be used in electronic structure engineering, and could act as a buffer layer to grow other metals on top, establishing a method to form interfaces

between van-der-Waals layered materials and metallic thin sheets.

Using chemical doping via deposition of a large amount of Cs on a monolayer of graphene grown on an Ir(111) surface, a flat band right at the Fermi energy has been introduced in large samples with size >1 cm². ARPES reveals a strong n-type doping of the graphene close to the saddle point instability. Additionally, the graphene bands are zone-folded into a 2x2 superstructure. This is in line with the observed Cs derived partially filled electron-bands, which are also observed in this 2x2 superstructure state. A flat electronic band is visible in the region of reciprocal space where Cs and graphene bands interact. Combining the experimental results with theoretical calculations, it is shown that the interaction of the doped graphene bands with the alkali metal bands from the Cs

We have predicted by ab initio calculations and further confirmed using various experimental techniques, including high-resolution angle-resolved photoemission spectroscopy (ARPES), the realization of an antiferromagnetic (AFM) topological insulator (TI) in the layered van der Waals compound MnBi₂Te₄. This is the first observation of an intrinsic magnetic topological insulator, i.e. a stoichiometric well ordered magnetic compound. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi₂Te₄ exhibits a large bandgap in the topological surface state. A number of fundamental phenomena are expected to be eventually observed, including quantized magnetoelectric coupling and axion electrodynamics. Other exotic phenomena could become accessible at temperatures significantly higher than those achieved to date on magnetic topological insulators created by the doping of non-magnetic topological insulators with 3d transition metal elements, such as the quantum anomalous Hall effect and chiral Majorana fermions.

Angle-resolved photoemission spectroscopy (ARPES) reveals no hybridization between electronic states of graphene and MoS₂ monolayer grown by molecular beam epitaxy (MBE), through coevaporation of Mo and S from pyrite, on graphene/Ir(111). The valence band (VB) maximum of MoS₂ appears at the K-point of the Brillouin zone consistent with monolayer MoS₂, while for bilayer MoS₂ the VB maximum is at the Γ point. The splitting of the VB at the MoS₂ K-point due to spin–orbit interaction is clearly seen in the high resolution scan shown in the inset of the figure. The fit to the energy distribution curve from a cut through the MoS₂ K-point reveals a band splitting due to spin–orbit coupling (SOC) of 144 meV. Interestingly, graphene is more hole doped than it was before MoS₂ growth, the Dirac-point binding energy E_D is evaluated to be at −0.25 eV compared to −0.1 eV in the pristine case. Notably, ARPES does not show any hybridization

The effective attenuation length (EAL) of low-energy electrons in oxides (CoO, MgO) and in a rare earth film (Yb) is investigated by photoemission spectroscopy experiments (5<hv<28 eV). In all cases the EAL is found to increase when lowering the electron energy, but the experimental values are smaller than expected from the predictive formula, i.e. the so-called “universal curve”. In particular, for the Yb case the experimental EAL is found to be a factor of four smaller than those predicted from the “universal curve”, while for the small-gap (E_g^CO= 2.5 eV) insulator CoO a factor of six is found. For the large gap (E_g^MgO= 7.77 eV) insulator MgO, instead, a steep increase in the EAL for energies smaller than the insulator optical gap of MgO is found. The comparison of the experimental results with calculated optical properties available in the literature suggests that, for energies lower than 20 eV, the relevant scattering mechanisms are described by the imaginary part of the dielectric function, accounting in particular for the steep increase of the EAL for energies smaller than the insulator band gap.
We point out that the bulk sensitivity in low-energy photoemission may be lower than the expectation and it may strongly depend on the investigated material, with the further warning that theoretical efforts have to be undertaken for a clear interpretation of the photon energy dependence of the LEPES spectra.

We have investigated the role of the film thickness in determining the superconducting phase transition in thin layers of MgB₂, epitaxially grown on Mg(0001), via angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations.
We show that surface states in few-monolayer MgB₂ make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. We predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB₂.

By means of angle-resolved photoemission spectroscopy (ARPES), we show that intercalation of a Pb monolayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of about 200 meV at the Dirac point of graphene. Spin-resolved photoemission (SRPES) measurements confirm the splitting to be of a spin−orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall (QSH) state in graphene, proposed by Kane and Mele [Phys. Rev. Lett. 95, 226801 (2005)]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.

Using angle-resolved photoemission spectroscopy (ARPES) we show that the surface of black phosphorus (bP) can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding p_z states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at T_c = 17K for LiP₈ stoichiometry. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a

We have investigated the giant Rashba-Dresselhaus spin-splitting of the electronic band structure of the centrosymmetric bulk BaNiS₂ crystal by means of angle-resolved photoemission spectroscopy (ARPES) supported by ab initio calculations. The system is composed by light elements so the spin-orbit coupling could not account for the measured remarkable band splitting of ΔE ≈ 150 meV. This  is  explained  by a  huge staggered crystal field of 1.4 V/Å, produced by a gliding plane symmetry, that breaks inversion symmetry at the Ni site. This  unexpected  result  in  the  absence  of  heavy elements demonstrates an effective mechanism of Rashba coupling

The formation of graphene by chemical vapor deposition (CVD) on single crystal Ir(111) films, grown heteroepitaxially on Si(111) wafers with yttria stabilized zirconia (YSZ) buffer layers, has been investigated by ARPES and other spectroscopic and microscopic techniques. Our results highlight the excellent crystalline quality of graphene, comparable to graphene prepared on Ir(111) bulk single crystals. This synthesis route allows for inexpensive growth on standardized disposable substrates, suitable for both optical and electron spectroscopic characterization, which meets the needs of many researchers in the field. Furthermore, this fabrication technique can potentially be scaled towards larger output and, using Si

We have performed an high-resolution ARPES investigation to try to find an electron donor for graphene that is capable of inducing strong electron–phonon coupling and superconductivity. Experiments were carried out on Cs, Rb, K, Na, Li, and Ca doped graphene. For each dopant we determine the full electronic band structure, the Eliashberg function, and the superconducting critical temperature T_c from the spectral function. From a self-energy analysis we find an anisotropic electron-phonon coupling parameter for the KΓ (KM) high-symmetry directions in momentum space, respectively. Interestingly, the high-energy part of the Eliashberg function which relates to graphene's optical phonons is equal in both directions but only in KM does appear an additional low-energy peak for all dopants with an energy and intensity that depend on the dopant atom. The low energy and high intensity of this peak are crucially important for achieving superconductivity, with Ca being the most promising candidate for realizing superconductivity in graphene.

Angle-resolved photoemission spectroscopy at low photon energy reveals a temperature-dependent evolution of the CuO₂ plane band dispersion and apparent Fermi surface pockets in underdoped Bi₂Sr_2-xLa_xCuO_6+δ (Bi2201), which is directly associated with an hitherto-undetected evolution of the incommensurate superstructure periodicity below 130K. Surprisingly, this effect is limited to the surface (ARPES-LEED), with no corresponding temperature evolution in the bulk (XRD-REXS). These findings point to a surface-enhanced incipient charge-density-wave instability, driven by Fermi surface nesting. This discovery is of critical importance in the interpretation of single-particle spectroscopy data, and establishes

We show with angle-resolved photoemission spectroscopy that a new dispersionless band appears in the electronic structure of hydrogenated K-doped graphene (H-graphene) which is extended over the whole Brillouin zone. Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene’s doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed of hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by x-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the π* resonance. The spatial extension of H defects, their preferential adsorption patterns on graphene, and local electronic structure was also investigated by high-resolution scanning tunneling microscopy and spectroscopy.

Polarization-dependent ARPES measurements performed at low photon energy along high-symmetry directions in the Brillouin zone made it possible to get new insight into the role of the five Fe 3d orbitals in the complex electronic structure of Ba(Fe_1−xCo_x)₂As₂ close to the Fermi level, and in particular, into the nature of the holelike pockets. Two distinct inner α pockets could be disentangled, suggesting that their origin is probably due to hybridized d_xz and d_yz, while the complex outer β pocket is mainly of d_z² nature.
The observation of momentum- and band-dependent superconducting gaps allowed us to detect the gap opening across T_c for both α and β pockets. These results confirm that the superconducting order parameter is consistent with a s± symmetry, and suggest that multiple harmonic terms may be included in its description.

We present a low photon energy angle resolved photoemission study of V₂O₃, a prototype system for the observation of Mott transitions in correlated materials. We show that the spectral features corresponding to the quasiparticle peak in the metallic phase present a marked wave vector dependence, with a stronger intensity along the GZ direction. The analysis of their intensity for different probing depths shows the existence of a characteristic length scale for the attenuation of coherent electronic excitations at the surface. This length scale, which is larger than the thickness of the surface region as normally defined for noncorrelated electronic states, is found to increase when approaching the Mott transition. These results are in agreement with the behavior of quasiparticles at

surfaces as predicted by Borghi et al. [Phys. Rev. Lett. 102, 066806 (2009)].

Retrieve article

F. Rodolakis, B. Mansart, E. Papalazarou, S. Gorovikov, P. Vilmercati, L. Petaccia, A. Goldoni, J. P. Rueff, S. Lupi, P. Metcalf, M. Marsi,
Phys. Rev. Lett. 102, 066805 (2009).
doi: 10.1103/PhysRevLett.102.066805

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