Elettra-Sincrotrone Trieste S.C.p.A. website uses session cookies which are required for users to navigate appropriately and safely. Session cookies created by the Elettra-Sincrotrone Trieste S.C.p.A. website navigation do not affect users' privacy during their browsing experience on our website, as they do not entail processing their personal identification data. Session cookies are not permanently stored and indeed are cancelled when the connection to the Elettra-Sincrotrone Trieste S.C.p.A. website is terminated.
More info
OK

BaDElPh Highlights

High quality MoS2 monolayers on graphene/Ir(111)


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 ED is evaluated to be at −0.25 eV compared to −0.1 eV in the pristine case. Notably, ARPES does not show any hybridization

between MoS2 and graphene bands which supports the idea that MoS2 is weakly interacting with graphene. This can explain the observed narrow photoluminescence (PL) and the absence of the expected quenching of the PL intensity on a metallic surface.

Retrieve article
Narrow photoluminescence and Raman peaks of epitaxial MoS2 on graphene/Ir(111)
N. Ehlen, J. Hall, B.V. Senkovskiy, M. Hell, J. Li, A. Hermann, D. Smirnov, A. Fedorov, V.Yu. Voroshnin, G. Di Santo, L. Petaccia, T. Michely, and A. Grüneis,
2D Mater. 6, 011006 (2019).
doi: 10.1088/2053-1583/aaebd3

This work was selected as an Elettra Top Story.


Attenuation lengths of low-energy electrons in solids

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 (EgCO= 2.5 eV) insulator CoO a factor of six is found. For the large gap (EgMgO= 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. A favorable comparison of our results with a

band structure calculation for MgO highlights how the absorptive part of the dielectric function can be directly correlated with the measured effective attenuation length at very low energy.

Retrieve articles
Effective attenuation lengths of low energy electrons in MgO thin films
S. Iacobucci, F. Offi, P. Torelli, L. Petaccia,
J. Electron Spectr. Rel. Phenom. 233, 1 (2019).
doi: 10.1016/j.elspec.2019.03.002

The attenuation length of low energy electrons in Yb
F. Offi, S. Iacobucci, L. Petaccia, S. Gorovikov, P. Vilmercati, A. Rizzo, A. Ruocco, A. Goldoni, G. Stefani, G. Panaccione,
J. Phys.: Condens. Matter 22, 305002 (2010).
doi: 10.1088/0953-8984/22/30/305002

Attenuation lengths of low-energy electrons in solids: The case of CoO
F. Offi, S. Iacobucci, P. Vilmercati, A. Rizzo, A. Goldoni, M. Sacchi, G. Panaccione,
Phys. Rev. B 77, 201101(R) (2008).
doi: 10.1103/PhysRevB.77.201101



Free surfaces recast superconductivity in few-monolayer MgB2


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.

These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.

Retrieve article
Free surfaces recast superconductivity in few-monolayer MgB2: Combined first-principles and ARPES demonstration
J. Bekaert, L. Bignardi, A. Aperis, P. van Abswoude, C. Mattevi, S. Gorovikov, L. Petaccia, A. Goldoni, B. Partoens, P.M. Oppeneer, F.M. Peeters, M.V. Milošević, P. Rudolf, C. Cepek,
Sci. Rep. 7, 14458 (2017).
doi: 10.1038/s41598-017-13913-z


Spin-orbit coupling induced gap in graphene


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.

Retrieve articles
Spin−orbit coupling induced gap in graphene on Pt(111) with intercalated Pb monolayer
I.I. Klimovskikh, M.M. Otrokov, V.Yu. Voroshnin, D. Sostina, L. Petaccia, G. Di Santo, S. Thakur, E.V. Chulkov, A.M. Shikin,
ACS Nano 11, 368 (2017).
doi: 10.1021/acsnano.6b05982

Reply to "Comment on 'ibid.'"
ACS Nano 11, 10630 (2017).
doi: 10.1021/acsnano.7b06779

This paper was selected as an Elettra Top Story.


Li-doped metallic black phosphorous


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 pz states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at Tc = 17K for LiP8 stoichiometry. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a

superconducting phase in phosphorene and few-layer black phosphorus.

Retrieve article
First-principles and angle-resolved photoemission study of lithium doped metallic black phosphorous
A. Sanna, A.V. Fedorov, N.I. Verbitskiy, J. Fink, C. Krellner, L. Petaccia, A. Chikina, D.Yu. Usachov, A. Grüneis, G. Profeta,
2D Mater. 3, 025013 (2016).
doi: 10.1088/2053-1583/3/2/025031


Rashba coupling amplification by a staggered crystal field


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

amplification that may foster spin-orbit band engineering.

Retrieve article
Rashba coupling amplification by a staggered crystal field
D. Santos-Cottin,  M. Casula, G. Lantz, Y. Klein, L. Petaccia, P. Le Fèvre, F. Bertran, E. Papalazarou, M. Marsi, A. Gauzzi,
Nat. Commun. 7, 11258 (2016).
doi: 10.1038/ncomms11258

This paper was selected as an Elettra Top Story.


High-quality graphene on single crystal Ir(111) films on Si(111) wafers


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

wafers, can be more easily implemented into standard Si technology than processes based on other supports.

Retrieve article
High-quality graphene on single crystal Ir(111) films on Si(111) wafers: Synthesis and multi-spectroscopic characterization
C. Struzzi, N.I. Verbitskiy, A.V. Fedorov, A. Nefedov, O. Frank, M. Kalbac, G. Di Santo, M. Panighel, A. Goldoni, J. Gärtner, W. Weber, M. Weinl, M. Schreck, Ch. Wöll, H. Sachdev, A. Grüneis, L. Petaccia,
Carbon 81, 167 (2015).
doi: 10.1016/j.carbon.2014.09.045

Electron-phonon coupling and superconductivity in graphene


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 Tc 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.

Retrieve articles
Observation of a universal donor-dependent vibrational mode in graphene
A.V. Fedorov, N.I. Verbitskiy, D. Haberer, C. Struzzi, L. Petaccia, D. Usachov, O.Y. Vilkov, D.V. Vyalikh, J. Fink, M. Knupfer, B. Büchner, A. Grüneis,
Nature Commun. 5, 3257 (2014).
doi: 10.1038/ncomms4257

This paper was selected as an Elettra Top Story.

Anisotropic Eliashberg function and electron-phonon coupling in doped graphene
D. Haberer, L. Petaccia, A.V. Fedorov, C.S. Praveen, S. Fabris, S. Piccinin, O. Vilkov, D.V. Vyalikh, A.  Preobrajenski, N.I. Verbitskiy, H. Shiozawa, J. Fink, M. Knupfer, B. Büchner, A. Grüneis,
Phys. Rev. B 88, 081401(R) (2013).
doi: 10.1103/PhysRevB.88.081401

Surface-enhanced CDW instability in underdoped Bi(2201)-cuprates


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 Bi2Sr2-xLaxCuO6+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

the surface of cuprates and other complex oxides as a rich playground for the study of electronically soft phases.

Retrieve article
Surface-enhanced charge-density-wave instability in underdoped Bi2Sr2-xLaxCuO6+δ
J.A. Rosen, R. Comin, G. Levy, D. Fournier, Z.-H. Zhu, B. Ludbrook, C.N. Veenstra, A. Nicolaou, D. Wong, P. Dosanjh, Y. Yoshida, H. Eisaki, G.R. Blake, F. White, T.T.M. Palstra, R. Sutarto, F. He, A. Fraño Pereira, Y. Lu, B. Keimer, G. Sawatzky, L. Petaccia, A. Damascelli,
Nature Commun. 4, 1977 (2013).
doi: 10.1038/ncomms2977

This paper was selected as an Elettra Top Story.

Origin of electron accumulation in In2O3


Angle-resolved photoemission spectroscopy at low photon energy reveals the presence of a two-dimensional electron gas at the surface of In2O3(111). Quantized subband states arise within a confining potential well associated with surface electron accumulation. Coupled Poisson-Schrödinger calculations suggest that downward band bending for the conduction band must be much bigger than band bending in the valence band. Surface oxygen vacancies acting as doubly ionized shallow donors are shown to provide the free electrons within this accumulation layer. Identification of the origin of electron accumulation in transparent oxides has significant implications  in the realization of devices based on these compounds.

Retrieve article
Microscopic Origin of Electron Accumulation in In2O3
K.H.L. Zhang, R.G. Egdell, F. Offi, S. Iacobucci, L. Petaccia, S. Gorovikov, P.D.C. King,
Phys. Rev. Lett. 110, 056803 (2013).
doi: 10.1103/PhysRevLett.110.056803

This paper was selected as an Elettra Top Story.

Impurity state in H-Graphene


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.

Retrieve articles
Direct observation of a dispersionless impurity band in hydrogenated graphene
D. Haberer, L. Petaccia, M. Farjam, S. Taioli, S. A. Jafari, A. Nefedov, W. Zhang, L. Calliari, G. Scarduelli, B. Dora, D. V. Vyalikh, T. Pichler, Ch. Wöll, D. Alfè, S. Simonucci, M. S. Dresselhaus, M. Knupfer, B. Büchner, A. Grüneis,
Phys. Rev. B 83, 165433 (2011).
doi: 10.1103/PhysRevB.83.165433

Probing local hydrogen impurities in quasi-free-standing graphene
M. Scheffler, D. Haberer, L. Petaccia, M. Farjam, R. Schlegel, D. Baumann, T. Hänke, A. Grüneis, M. Knupfer, C. Hess, B. Büchner,
ACS Nano 6, 10590 (2012).
doi: 10.1021/nn303485c

Gap opening and orbital character in Ba(Fe1-xCox)2As2


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(Fe1−xCox)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 dxz and dyz, while the complex outer β pocket is mainly of dz2 nature.
The observation of momentum- and band-dependent superconducting gaps allowed us to detect the gap opening across Tc 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.

Retrieve articles
Orbital nature of the hole-like Fermi surface in superconducting Ba(Fe1−xCox)2As2
B. Mansart, V. Brouet, E. Papalazarou, M. Fuglsang Jensen, L. Petaccia, S. Gorovikov, A.N. Grum-Grzhimailo, F. Rullier-Albenque, A. Forget, D. Colson, M. Marsi,
Phys. Rev. B 83, 064516 (2011).
doi: 10.1103/PhysRevB.83.064516

Opening of the superconducting gap in the hole pockets of Ba(Fe1−xCox)2As2 as seen via angle-resolved photoelectron spectroscopy
B. Mansart, E. Papalazarou, M. Fuglsang Jensen, V. Brouet, L. Petaccia, L. de Medici, G. Sangiovanni, F. Rullier-Albenque, A. Forget, D. Colson, M. Marsi,
Phys. Rev. B 85, 144508 (2012).
doi: 10.1103/PhysRevB.85.144508

Surface-umklapp in ARPES at an organic-metal interface

We show that angle-resolved photoemission performed using low-energy photons on an organic-metal interface allows to clearly distinguish genuine interface states from features of substrate photoelectrons diffracted by the molecular lattice. As a model system an ordered monolayer of Zn-phthalocyanine is used as a diffraction lattice to probe the electronic band structure of a Ag(110) substrate. Photoemission close to normal emission geometry reveals strongly dispersive features absent in the pristine substrate spectra. Density functional theory modeling helped identifying these as bulk sp direct transitions undergoing surface-umklapp processes. The present results establish the important role of final-state diffraction effects in photoemission experiments at organic-inorganic interfaces. Moreover, this phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large π-conjugated organic molecules. Retrieve articles
Final-state diffraction effects in angle-resolved photoemission at an organic-metal interface
F.C. Bocquet, L. Giovanelli, P. Amsalem, L. Petaccia, D. Topwal, S. Gorovikov, M. Abel, N. Koch, L. Porte, A. Goldoni, J.-M. Themlin,
Phys. Rev. B 84, 241407(R) (2011).
doi: 10.1103/PhysRevB.84.241407

Interpretation of valence band photoemission spectra at organic-metal interfaces
L. Giovanelli, F.C. Bocquet, P. Amsalem, H.-L. Lee, M. Abel, S. Clair, M. Koudia, T. Faury, L. Petaccia, D. Topwal, E. Salomon, T. Angot, A.A. Cafolla, N. Koch, L. Porte, A. Goldoni, J.-M. Themlin,
Phys. Rev. B 87, 035413 (2013).
doi: 10.1103/PhysRevB.87.035413


Quasiparticles at the Mott Transition in V2O3

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


Quasiparticles at the Mott Transition in V2O3: Wave Vector Dependence and Surface Attenuation

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
See accompanying Physics Synopsis

Last Updated on Monday, 25 March 2019 16:23