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Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas–Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high- [Formula: see text] dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

International audience; The large tolerance of hybrid perovksites to the trapping of electrons by defects is a key asset in photovoltaic applications. Here, the ionic surface terminations of CH3NH3PbI3 are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterize the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. When the orientation of organic cations is frozen in the orthorhombic phase, the positively charged termination induces a massive accumulation of excited electrons at the surface of the sample. Conversely, no electron accumulation is observed in the tetragonal phase. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ϵ = 120.

International audience; Self-assembled nanolines are attractive to build the technological devices of next generation, but characterizing their electronic properties is often difficult to achieve. In this work we employ angle-resolved photoemission spectroscopy and density functional theory to clarify the electronic structure exhibited by self-assembled Bi nanolines grown on the InAs(100) surface. A surface resonance associated to the reconstructed (4 2) surface is visible in the photoemission spectra before and after the formation of the Bi nanolines. This demonstrates that Bi deposition does not necessarily drive a transition to an unreconstructed surface in the substrate, which is contrary to what was reported in previous studies. In addition, experiment and theory show the presence of a flat band located in the band gap of InAs, just above the valence band maximum. This flat band is associated to the Bi nanolines and possesses a strong orbital character, consistent with its unidimensional nature. These spectral features suggest that Bi nanolines on InAs(100) may have a strongly polarized conductivity, which makes them suitable to be exploited as nanowires in nanotechnology. The coexistence with an accumulation layer suggests an even farther functionalization.

International audience; Quasi two-dimensional hybrid organic-inorganic perovskites (HOIPs) have been rediscovered recently for photovoltaics due to a higher stability than other HOIPs. We focus here on the electronic structure of the 2D perovskite (C6H5C2H4NH3)2PbI4. We perform an experimental k-resolved determination of the valence and conduction bands by angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES). The experimental and theoretical dispersions are compared. The valence band width is in agreement with that of renormalized theoretical bands, while no significative renormalization is observed for the conduction band. The effect of the spin-orbit coupling in the conduction band is also experimentally observed.

The low-energy electronic structure, including the Fermi surface topology, of the itinerant metamagnet $$\hbox {Sr}_{{4}}\hbox {Ru}_{{3}}\hbox {O}_{{10}}$$ Sr 4 Ru 3 O 10 is investigated for the first time by synchrotron-based angle-resolved photoemission. Well-defined quasiparticle band dispersions with matrix element dependencies on photon energy or photon polarization are presented. Four bands crossing the Fermi-level, giving rise to four Fermi surface sheets are resolved; and their complete topography, effective mass as well as their electron and hole character are determined. These data reveal the presence of kink structures in the near-Fermi-level band dispersion, with energies ranging from 30 to 69 meV. Together with previously reported Raman spectroscopy and lattice dynamic calculation studies, the data suggest that these kinks originate from strong electron–phonon coupling present in $$\hbox {Sr}_{{4}}\hbox {Ru}_{{3}}\hbox {O}_{{10}}$$ Sr 4 Ru 3 O 10 . Considering that the kink structures of $$\hbox {Sr}_{{4}}\hbox {Ru}_{{3}}\hbox {O}_{{10}}$$ Sr 4 Ru 3 O 10 are similar to those of the other three members of the Ruddlesden Popper structured ruthenates, the possible universality of strong coupling of electrons to oxygen-related phonons in $$\hbox {Sr}_{n+1}\hbox {Ru}_{{n}}\hbox {O}_{3n+1}$$ Sr n + 1 Ru n O 3 n + 1 compounds is proposed.

The atomic structure and electronic properties of monolayer graphene on a curved multi-nano-vicinal Pt (1 1 1) substrate are investigated with a low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Despite graphene grows continuously over the terraces and step bunching areas, the spatial periodicity also varies by varying the vicinal angle of the substrate. This superperiodicity has been evidenced by splitting of first order Pt spots in LEED and from STM. The photoemission spectroscopy unravels that the linearly dispersing π band of graphene is affected by the step periodicity as evidenced by the opening of bandgaps. The gaps opening occurs at the intersection of the main graphene band with Umklapp bands due to the superperiodicity of the one-dimensional nanostructured substrate. The energy and momentum locations of the minigaps change with the superperiodicity, which is related to the spatial periodicity and vicinal angle of the substrate. Our results show a simple way to tune the electronic properties of epitaxial graphene by tuning the substrate vicinality.

The electronic structure of a Bi 0.91 Sb 0.09 single crystal was elucidated at a bisectrix ( 11 2 ¯ ) surface by angle-resolved photoemission spectroscopy using microbeam synchrotron radiation. A Dirac-cone-like dispersion of the surface state was observed at the Γ ¯ point. The detected surface-state band likely corresponds to the one observed in previous quantum transport research (Taskin and Ando, 2009) [14].

International audience; One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi 2 Te 3 , we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time-and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states.

Producing a usable semiconducting form of graphene has plagued the development of graphene electronics for nearly two decades. Now that new preparation methods have become available, graphene's intrinsic properties can be measured and the search for semiconducting graphene has begun to produce results. This is the case of the first graphene "buffer" layer grown on SiC(0001) presented in this work. We show, contrary to assumptions of the last forty years, that the buffer graphene layer is not commensurate with SiC. The new modulated structure we've found resolves a long standing contradiction where ab initio calculations expect a metallic buffer, while experimentally it is found to be a semiconductor. Model calculations using the new incommensurate structure show that the semiconducting $\pi$-band character of the buffer comes from partially hybridized graphene incommensurate boundaries surrounding unperturbed graphene islands., Comment: 17 pages, 4 figures, 1 table, 47 references, supplemental material: 15 pages, 4 figures

The Kondo ground state can arise in surface-supported organometallic systems, such as the 3d transition metal phthalocyanines, as a result of the hybridization of the 3d localized orbitals with the underlying metal substrate electrons. Low-temperature scanning tunneling spectroscopy can identify the Kondo states as a zero-bias anomaly in the differential conductance curves, often localized in close proximity with the 3d metal center. However, the information on the symmetry of the Kondo state, on the exact shape of the spectral function, and on the magnetic state is generally missing from the experimental data. We apply complementary techniques, i.e., scanning tunneling spectroscopy, resonant photoemission spectroscopy, and X-ray magnetic circular dichroism, to identify the Kondo state, the orbital symmetry, and the magnetic moment of Fe-Phthalocyanine chains formed by two structural phases with increasing molecular density: first ×5 and at higher coverage ×7 FePc ordered chain structures assembled on the Au(110) surface. The experimental data suggest the presence of a Kondo state only in the ×5 phase, at lower molecular density, with a Kondo temperature of about 60 K, having parent states with the dxz, dyz symmetry, that cancels the magnetic moment of the molecules at low temperatures.

A synergistic effect between strong electron correlation and spin-orbit interaction (SOI) has been theoretically predicted to result in a new topological state of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001), using angle-resolved photoelectron spectroscopy. The SS showed temperature-dependent reconstruction corresponding with the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c-f hybridization was metallic, forming a closed Fermi contour surrounding $\bar{\Gamma}$ on the surface Brillouin zone and agreeing with the theoretically expected behavior for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect., Comment: 15 pages, 4 figures

A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metal's electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.

Detailed surface electronic structure on the Bi/InSb(001) surface hosting Tomonaga–Luttinger liquid was observed by angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation. ARPES energy distribution curves (EDCs) around the Fermi level indicated the two separate features dispersing with different Fermi velocity but approaching each other. Two possible scenarios, elemental excitations from separate spin/charge terms and those from independent two surface bands, are discussed based on the experimental results.

International audience; We investigate black phosphorus by time- and angle-resolved photoelectron spectroscopy. The electrons excited by 1.57 eV photons relax down to a conduction band minimum within 1 ps. Despite the low band gap value, no relevant amount of carrier multiplication could be detected at an excitation density 3–6 × 1019 cm–3. In the thermalized state, the band gap renormalization is negligible up to a photoexcitation density that fills the conduction band by 150 meV. Astonishingly, a Stark broadening of the valence band takes place at an early delay time. We argue that electrons and holes with a high excess energy lead to inhomogeneous screening of near surface fields. As a consequence, the chemical potential is no longer pinned in a narrow impurity band.

Gray tin, also known as $\ensuremath{\alpha}$-Sn, can be turned into a three-dimensional topological insulator (3D-TI) by strain and finite-size effects. Such room-temperature 3D-TI is peculiarly interesting for spintronics due to the spin-momentum locking along the Dirac cone (linear dispersion) of the surface states. Angle-resolved photoemission spectroscopy (ARPES) has been used to investigate the dispersion close to the Fermi level in thin (001)-oriented epitaxially strained films of $\ensuremath{\alpha}$-Sn for different film thicknesses as well as for different capping layers (Al, ${\mathrm{AlO}}_{x}$, and MgO). Indeed a proper capping layer is necessary to be able to use $\ensuremath{\alpha}$-Sn surface states for spintronic applications. In contrast with free surfaces or surfaces coated with Ag, coating the $\ensuremath{\alpha}$-Sn surface with Al or ${\mathrm{AlO}}_{x}$ leads to a drop in the Fermi level below the Dirac point, and an important consequence for electronic transport is the presence of bulk states at the Fermi level. $\ensuremath{\alpha}$-Sn films coated by ${\mathrm{AlO}}_{x}$ are studied by electrical magnetotransport: Despite magnetotransport properties of the bulk electronic states of the ${\mathrm{\ensuremath{\Gamma}}}_{8}$ band playing an important role as suggested by ab initio calculations, there is clear evidence of surface states revealed by Shubnikov--de Haas oscillations corresponding to the ARPES observation.

Author(s): Kuo, CT; Lin, SC; Conti, G; Pi, ST; Moreschini, L; Bostwick, A; Meyer-Ilse, J; Gullikson, E; Kortright, JB; Nemsak, S; Rault, JE; Le Fevre, P; Bertran, F; Santander-Syro, AF; Vartanyants, IA; Pickett, WE; Saint-Martin, R; Taleb-Ibrahimi, A; Fadley, CS | Abstract: A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO2 layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to Bi2Sr2CaCu2O8+δ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large c represent a promising future direction.

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the $\overline{M}$ valley. Subsequently, the electronic cooling is dictated by Fr\"ohlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\overline{M}$ valley could travel through a multilayer flake of InSe with a lateral size of 1 $\ensuremath{\mu}\mathrm{m}$. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

International audience; Under-doped La-214 cuprates show a charge-and spin-modulation known as " stripes " [1]. These stripe modulations are (quasi)-static close to 1/8 hole doping where superconductivity is suppressed. The pseudo-gap phase of other cuprate compounds recently also revealed charge modulation, but interpreted rather as a charge density wave (CDW) [2, 3, 4], that possibly competes with superconductivity. In this context, to better understand the interplay between the stripe phase and the superconductivity, we use angle-resolved photoemission spectroscopy to study the electronic band structure and gap in La-214 cuprates near 1/8 doping (La 2−x−y Nd y Sr x CuO 4 (x = 0.12; y = 0.0 & 0.4)) and compare with the previous results in the same system [5] and La 1.86 Ba 0.14 CuO 4 [6]. Our data shows a loss of spectral intensity towards the end of the Fermi arcs, that is possibly due to a strong renormalisation, as already pointed out elsewhere * matteo.dastuto@neel.cnrs.fr-Institut Néel CNRS-25, av des Martyrs-38042 Grenoble cedex 9; tel: (+33)(0)4 76 88 12 84 [5], with a noisy but still measurable gap. On the nodal direction no gap is observed within our statistics, but a sizeable decrease in intensity with temperature. Moreover, we do not see any shadow band, but our Fermi surface can be well modelled with a single electron band calculation in the tight binding approximation, even very close to the 1/8 doping La 2−x−y Nd y Sr x CuO 4 with and without Nd substitution.

We present a time- and angle-resolved photoemission study of the transition-metal dichalcogenide ${\mathrm{WTe}}_{2}$, a candidate type-II Weyl semimetal exhibiting extremely large magnetoresistence. Using femtosecond light pulses, we characterize the unoccupied states of the electron pockets above the Fermi level. Following the ultrafast carrier relaxation in distinct parts of the Brillouin zone, we report remarkably similar decay dynamics for electrons and holes. Our results confirm that charge compensation between electron and hole pockets---a key effect to explain the nonsaturating magnetoresistance of this material---is a distinctive feature of ${\mathrm{WTe}}_{2}$ even in an out-of-equilibrium regime.

The oxidation of Cu0.7Zn0.3(111) exposed to O2 at low pressure (7.0 × 10− 7 mbar) and higher pressure (1 mbar) both at 400 K has been investigated by photoelectron spectroscopy. The results evidenced a preferential oxidation of Zn at the surface in agreement with the literature and Zn segregation to the surface. Moreover, a systematic fitting procedure of high-resolution spectra enabled us to decompose the Zn 2p core-level peak into metallic and oxide components in order to follow the growth of the oxide. By combining these results with careful analysis of the Zn L3M45M45 Auger transition, we evidenced the formation of a discontinuous ZnO layer (3D ZnO islands) in both pressure conditions. Measurements of the O 1s peak for two photon energies using synchrotron radiation allowed us to identify interface and bulk components. Changes in the Zn 3d level and valence band during the initial steps of oxidation were also followed. We show that not only thermodynamics but also kinetic effects have to be considered to describe this complex oxidation process. The importance of the role played by the structural quality of the sample on the composition, growth kinetic and structure of the surface oxide layer is evidenced.

The present study gives experimental evidence of two-dimensional electronic states localized in subsurface layers of a Ge(1 1 1) substrate, in which a monolayer of Pb adsorbed on a Ge(1 1 1) surface is used as a template. We observed three pairs of the subsurface states by angle-resolved photoelectron spectroscopy, which are closely related to bulk heavy-hole, light-hole and spin–orbit split-off bands of Ge(1 1 1). The heavy-hole- and light-hole-derived bands show the Rashba-type band crossing at the Γ ¯ point. A density functional theory calculation suggests that the branches of these bands exhibit the Rashba-type spin splitting. On the other hand, we observed peculiar spin polarization for the subsurface states attributed to the bulk spin–orbit split-off band by spin- and angle-resolved photoelectron spectroscopy, where spin polarization of each branch is inverted with respect to Γ ¯ and the degeneracy, expected for a normal Rashba spin-split surface state is lifted at Γ ¯ .

The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) is difficult to realize in quantum materials because, unlike in ultracold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal, Fe1+y Se x Te1-x , by tuning the Fermi energy eF via chemical doping, which permits us to systematically change Δ/eF from 0.16 to 0.50, where Δ is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap, and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multiband superconductors, which go beyond those addressed in the context of cold atoms.

International audience; The epitaxial graphene buffer layer on the Si face of hexagonal SiC shows a promising band gap, of which the precise origin remains to be understood. In this work, we correlate the electronic to the atomic structure of the buffer layer by combining angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and high-resolution scanning transmission electron microscopy (HR-STEM). We show that the band structure in the buffer has an electronic periodicity related to the structural periodicity observed in STM images and published X-ray diffraction. Our HR-STEM measurements show the bonding of the buffer layer to the SiC at specific locations separated by 1.5 nm. This is consistent with the quasi 6 x 6 periodic corrugation observed in the STM images. The distance between buffer C and SiC is 1.9 Å in the bonded regions and up to 2.8 Å in the decoupled regions, corresponding to a 0.9 Å corrugation of the buffer layer. The decoupled regions are sp2 hybridized. Density functional tight binding (DFTB) calculations demonstrate the presence of a gap at the Dirac point everywhere in the buffer layer, even in the decoupled regions where the buffer layer has an atomic structure close to that of graphene. The surface periodicity also promotes band in the superperiodic Brillouin zone edges as seen by photoemission and confirmed by our calculations.

Topologically protected electron states arranged artificially in real space form a highly tunable emergent atomic chain., Engineered lattices in condensed matter physics, such as cold-atom optical lattices or photonic crystals, can have properties that are fundamentally different from those of naturally occurring electronic crystals. We report a novel type of artificial quantum matter lattice. Our lattice is a multilayer heterostructure built from alternating thin films of topological and trivial insulators. Each interface within the heterostructure hosts a set of topologically protected interface states, and by making the layers sufficiently thin, we demonstrate for the first time a hybridization of interface states across layers. In this way, our heterostructure forms an emergent atomic chain, where the interfaces act as lattice sites and the interface states act as atomic orbitals, as seen from our measurements by angle-resolved photoemission spectroscopy. By changing the composition of the heterostructure, we can directly control hopping between lattice sites. We realize a topological and a trivial phase in our superlattice band structure. We argue that the superlattice may be characterized in a significant way by a one-dimensional topological invariant, closely related to the invariant of the Su-Schrieffer-Heeger model. Our topological insulator heterostructure demonstrates a novel experimental platform where we can engineer band structures by directly controlling how electrons hop between lattice sites.

We present an experimental study describing the effects of surface termination on the electronic structure of the natural topological superlattice phase ${\mathrm{Sb}}_{2}\mathrm{Te}$. Using scanning angle-resolved photoemission microscopy, we consistently find various nonequivalent regions on the same surface after cleaving various ${\mathrm{Sb}}_{2}\mathrm{Te}$ single crystals. We were able to identify three distinct terminations characterized by different Sb/Te surface stoichiometric ratios and with clear differences in their band structure. For the dominating Te-rich termination, we also provide a direct observation of the excited electronic states and of their relaxation dynamics by means of time-resolved angle-resolved photoemission spectroscopy. Our results clearly indicate that the surface electronic structure is strongly affected by the bulk properties of the superlattice.

Motivated by the different behaviour of Sn/Si(1 1 1) and Sn/Ge(1 1 1) in their metal-insulator transition, we have explored the possibility of growing Sn on an ultra-thin Ge layer strained on top of a Si(1 1 1) substrate. We have demonstrated by scanning tunneling microscopy and low energy electron diffraction that a ( 2 3 × 2 3 ) R30° reconstruction can be stabilized under adequate growth conditions. The size of the reconstructed domains increases progressively up to a coverage of 1.3 monolayer of Sn, as determined by a combined study of scanning tunneling microscopy and core level spectroscopy. This coverage differs from that of Sn/Si(1 1 1) and Sn/Ge(1 1 1) exhibiting Mott phases. Angle resolved photoemission shows that the highly strained reconstruction is a band insulator, with a surface state dispersing roughly between 1300 and 2300 meV of binding energy.

We discuss the application of time-resolved ultrafast angle resolved photoelectron spectroscopy to the study of photoexcited topological insulators. Measurements performed on the prototype material Bi2Te3 clearly show that all the main processes involved in the ultrafast surface carrier dynamics of topological insulators can be clearly observed and quantitatively analyzed. The comparison with other experimental results shows that the relative position of surface and bulk conduction bands with respect to the system Fermi level play an essential role in the recombination processes following ultrafast optical excitation.

Spintronics evolves along new paths involving non-magnetic materials having large spin-obit coupling, typically 5d metals, allowing for example large spin-to-charge current conversion (spin Hall and Rashba-Edelstein effects). These heavy metals have other effects: in proximity of magnetic thin films they can burst out the Dzyaloshinskii-Moriya interaction leading to the stabilization of chiral magnetic structures. Another source of recent interest relies on “non-trivial topologies”, either of the band structure of the topological insulators, or of the spin textures in magnetic thin films. We will discuss our recent progress to control the topological textures known as skyrmions in multilayers made of heavy metals and magnetic layers. Aiming at using skyrmions as magnetic bits in “racetrack memory” structures, one of the present challenges is to efficiently move skyrmions with dimensions of a few tens of nanometers. The topology of these magnetic structures imposes peculiar dynamics, interesting both in fundamental and applied perspectives. Simulations indicate that spin-orbit torques, through the absorption of the spin current generated by a nearby layer, should be the most efficient method. The conducting surfaces of topological insulators at which the carriers’ spin and momentum are locked, can display better spin-to-charge conversion than what is found using heavy metals. However, the control of the interfaces is crucial to conserve the Dirac cone and the associated spin-momentum locking. We demonstrate by ARPES and spin pumping experiments how the properties of the α-Sn thin film topological insulator are preserved and can be used for spintronics, maybe to move skyrmions!

A BCS-BEC crossover in a multiband superconductor has been observed experimentally using ARPES., The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) is difficult to realize in quantum materials because, unlike in ultracold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal, Fe1+ySexTe1−x, by tuning the Fermi energy εF via chemical doping, which permits us to systematically change Δ/εF from 0.16 to 0.50, where Δ is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap, and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multiband superconductors, which go beyond those addressed in the context of cold atoms.

Surface states of platinum-induced atomic nanowires on a germanium (0 0 1) surface, which shows a structural phase transition at 80 K, were studied by angle-resolved photoelectron spectroscopy (ARPES). We observed four one-dimensional metallic surface states, among which, two bands were reported in our previous study (Yaji et al 2013 Phys. Rev. B 87 241413). One of the newly-found two bands is a quasi-one-dimensional state and is split into two due to the Rashba effect. Photoelectron intensity from one of the spin-polarized branches is reduced at a boundary of the surface Brillouin zone below the phase transition temperature. The reduction of the photoelectron intensity in the low temperature phase is interpreted as the interference of photoelectrons, not as the Peierls instability. We also discuss the low energy properties of the metallic surface states and their spin splitting using high-resolution ARPES with a vacuum ultraviolet laser.

International audience; Graphene nanoribbons are fundamental components to the development of graphenenanoelectronics. At the nanoscale, electronic confinement effects and electronic edge statesbecome essential to the properties of graphene. These effects depend critically on the ribbonwidth and the nature of the ribbon edge, the control of which at the atomic scale is a majorchallenge. Graphene nanoribbons have been largely studied theoretically, experimentally andwith the perspective of electronic applications. We review the basic properties of graphenenanoribbons and recent progress in fabrication processes, focusing on the question of theelectronic gap. We examine top–down and bottom–up approaches to fabricate graphenenanoribbons by lithographic, catalytic cutting, chemical assembly and epitaxial growthmethods and compare their electronic characteristics.

We present results on spin to charge current conversion in experiments of resonant spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) α-Sn. By angle-resolved photoelectron spectroscopy (ARPES), we first check that the Dirac cone (DC) at the α-Sn (0 0 1) surface subsists after covering Sn with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into α-Sn layers induces a lateral charge current that can be ascribed to the inverse Edelstein effect by the DC states. Our observation of an inverse Edelstein effect length much longer than those generally found for Rashba interfaces demonstrates the potential of TIs for the conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of the TI for spin to charge conversion and the conditions to reach it.

International audience; We study with ARPES the renormalization and quasiparticle lifetimes of the $d_{xy}$ and $d_{xz}$/$d_{yz}$ orbitals in two iron pnictides, LiFeAs and Ba($Fe_{0.92}$ $Co_{0.08}$)${}_2As_2$ (Co8). We find that both quantities depend on orbital character rather than on the position on the Fermi Surface (for example hole or electron pocket). In LiFeAs, the renormalizations are larger for dxy, while they are similar on both types of orbitals in Co8. The most salient feature, which proved robust against all the ARPES caveats we could think of, is that the lifetimes for dxy exhibit a markedly different behavior than those for $d_{xz}$/$d_{yz}$. They have smaller values near $E_F$ and exhibit larger ω and temperature dependences. While the behavior of $d_{xy}$ is compatible with a Fermi liquid description, it is not the case for $d_{xz}$/$d_{yz}$. This situation should have important consequences for the physics of iron pnictides, which have not been considered up to now. More generally, it raises interesting questions on how a Fermi liquid regime can be established in a multiband system with small effective bandwidths.

A blueprint for producing scalable digital graphene electronics has remained elusive. Current methods to produce semiconducting-metallic graphene networks all suffer from either stringent lithographic demands that prevent reproducibility, process-induced disorder in the graphene, or scalability issues. Using angle resolved photoemission, we have discovered a unique one dimensional metallic-semiconducting-metallic junction made entirely from graphene, and produced without chemical functionalization or finite size patterning. The junction is produced by taking advantage of the inherent, atomically ordered, substrate-graphene interaction when it is grown on SiC, in this case when graphene is forced to grow over patterned SiC steps. This scalable bottomup approach allows us to produce a semiconducting graphene strip whose width is precisely defined within a few graphene lattice constants, a level of precision entirely outside modern lithographic limits. The architecture demonstrated in this work is so robust that variations in the average electronic band structure of thousands of these patterned ribbons have little variation over length scales tens of microns long. The semiconducting graphene has a topologically defined few nanometer wide region with an energy gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet. This work demonstrates how the graphene-substrate interaction can be used as a powerful tool to scalably modify graphene's electronic structure and opens a new direction in graphene electronics research., Comment: 11 pages, 7 figures

A direct comparison between photoemission measurements and band structure calculations is sometimes tricky. Matrix element effects may affect considerably the spectral weight of the electronic states and prevent the expected translational symmetry of the band structure from being observed. We show how matrix element effects can be qualitatively described to a certain extent by making an analogy between photoemission and low energy electron diffraction. We have tested this approach in two superperiodic systems. We have first explained the intensity distribution in different Brillouin zones of a surface state in Si(1 1 1)-(7 × 7), where the surface state spectral intensity does not exhibit the (7 × 7) symmetry. We have also compared the LEED intensity of superperiodic LEED spots with the energy dependence of bulk bands on a facetted Si surface as measured by photoemission.

The thermal stability of the Co/ β -Si 3 N 4 /Si(111) interface has been studied by high-resolution photoemission spectroscopy in a temperature range extending from room temperature to 650 °C. It is demonstrated the ability of a very thin crystalline buffer layer of silicon nitride to prevent the interfacial reaction between cobalt and silicon at room temperature. The behaviour of the interface at higher temperature shows the formation of cobalt silicides already at 300 °C. Moreover, the presence of new components in the decomposition of the photoemission spectra is discussed in the light of the existing literature.

To examine the influence of the barrier quality in fully epitaxial Fe/MgO/Fe(001) magnetic tunnel junctions (MTJs), we propose to use Fe-V alloys as magnetic electrodes. This leads to a reduced misfit with MgO. We actually observe, by high-resolution electron microscopy (HREM) and local strain measurements, that the misfit dislocations density in the MgO barrier is lower when it is grown on Fe-V(001). This improvement of the crystalline quality of the MgO barrier actually leads to a significant increase of the tunnel magneto-resistance (TMR), despite the loss of spin polarization (SP) in these alloys, which was measured by spin-polarized X-ray photoelectron spectroscopy (SR-XPS).

A 1D metallic surface state was created on an anisotropic InSb(001) surface covered with Bi. Angle-resolved photoelectron spectroscopy (ARPES) showed a 1D Fermi contour with almost no 2D distortion. Close to the Fermi level (E_{F}), the angle-integrated photoelectron spectra showed power-law scaling with the binding energy and temperature. The ARPES plot above E_{F}, obtained thanks to a thermally broadened Fermi edge at room temperature, showed a 1D state with continuous metallic dispersion across E_{F} and power-law intensity suppression around E_{F}. These results strongly suggest a Tomonaga-Luttinger liquid on the Bi/InSb(001) surface.

MnSi has been extensively studied for five decades; nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevents a comprehensive understanding of the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at ${T}_{C}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}40\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, along with the appearance of sharp quasiparticle emission below ${T}_{C}$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by the state-of-the-art quasiparticle self-consistent $\mathit{GW}$ approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below ${T}_{C}$, along with the persistence of the $d$-band splitting well above ${T}_{C}$, at odds with a simple Stoner model for itinerant magnetism, opens the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticle lifetime.

Dynamical phase transitions are a challenge to identify experimentally and describe theoretically. Here, we study a new reconstruction of Sn on silicon and observe a reversible transition where the surface unit cell divides its area by a factor of 4 at 250 degrees C. This phase transition is explained by the 24-fold degeneracy of the ground state and a novel diffusive mechanism, where four Sn atoms arranged in a snakelike cluster wiggle at the surface exploring collectively the different quantum mechanical ground states.

While numerous methods have been proposed to produce semiconducting graphene, a significant bandgap has never been demonstrated. The reason is that, regardless of the theoretical gap formation mechanism, disorder at the sub-nanometer scale prevents the required chiral symmetry breaking necessary to open a bandgap in graphene. In this work, we show for the first time that a 2D semiconducting graphene film can be made by epitaxial growth. Using improved growth methods, we show by direct band measurements that a bandgap greater than 0.5 eV can be produced in the first graphene layer grown on the SiC(0001) surface. This work demonstrates that order, a property that remains lacking in other graphene systems, is key to producing electronically viable semiconducting graphene.

We report the evolution of the surface electronic structure and surface material properties of a topological crystalline insulator (TCI) Pb1-xSnxSe as a function of various material parameters including composition x, temperature T and crystal structure. Our spectroscopic data demonstrate the electronic groundstate condition for the saddle point singularity, the tunability of surface chemical potential, and the surface states' response to circularly polarized light. Our results show that each material parameter can tune the system between trivial and topological phase in a distinct way unlike as seen in Bi2Se3 and related compounds, leading to a rich and unique topological phase diagram. Our systematic studies of the TCI Pb1-xSnxSe are valuable materials guide to realize new topological phenomena., Comment: 10 pages, 7 figures. Expanded version of arXiv:1403.1560

Équipe 102 : Surfaces et Spectroscopies; International audience; Graphene nanoribbons grown on sidewall facets of SiC have demonstrated exceptional quantized ballistic transport up to 15 mu m at room temperature. Angular-resolved photoemission spectroscopy (ARPES) has shown that the ribbons have the band structure of charge neutral graphene, while bent regions of the ribbon develop a bandgap. We present scanning tunneling microscopy and transmission electron microscopy of armchair nanoribbons grown on recrystallized sidewall trenches etched in SiC. We show that the nanoribbons consist of a single graphene layer essentially decoupled from the facet surface. The nanoribbons are bordered by 1-2 nm wide bent miniribbons at both the top and bottom edges of the nanoribbons. We establish that nanoscale confinement in the graphene miniribbons is the origin of the local large band gap observed in ARPES. The structural results presented here show how this gap is formed and provide a framework to help understand ballistic transport in sidewall graphene.

We investigated the effect of H passivation on the growth of Co on Si(1 1 1) by means of core-level photoemission spectroscopy. The Co was deposited simultaneously on both in situ prepared H/Si(1 1 1)-(1×1) and Si(1 1 1)-(7×7) surfaces. This method enabled us to perform a direct comparison of the growth mode on both surfaces: the differences in the evolution of the core-level spectra have been ascribed only to the presence of the hydrogen interlayer. The decomposition of the Si 2p core-level peaks shows that disilicide-like islands appeared on both samples at the very beginning of the growth, but a cobalt-rich phase rapidly arises. The direct comparison between the two systems clearly indicates that the cobalt-rich phase is favored on the hydrogen passivated surface.

The family of hybrid organic–inorganic halide perovskites is in the limelight because of their recently discovered high photovoltaic efficiency. These materials combine photovoltaic energy conversion efficiencies exceeding 22% and low-temperature and low-cost processing in solution; a breakthrough in the panorama of renewable energy. Solar cell operation relies on the excitation of the valence band electrons to the conduction band by solar photons. One factor strongly impacting the absorption efficiency is the band dispersion. The band dispersion has been extensively studied theoretically, but no experimental information was available. Herein, we present the first experimental determination of the valence band dispersion of methylammonium lead halide in the tetragonal phase. Our results pave the way for contrasting the electronic hopping or the electron effective masses in different theories by comparing to our experimental bands. We also show a significant broadening of the electronic states, promoting relaxed conditions for photon absorption, and demonstrate that the tetragonal structure associated to the octahedra network distortion below 50 °C induces only a minor modification of the electronic bands, with respect to the cubic phase at high temperature, thus minimizing the impact of the cubic-tetragonal transition on solar cell efficiencies.

The typical Kondo insulator, YbB12, is a candidate of topological Kondo insulators. To clarify the surface electronic structure of not only bulk but also surface by photoelectron measurements, we developed a method to obtain a clean surface of YbB12(001) by annealing at 1650 K in an ultra-high vacuum. By surface-sensitive photoelectron measurements, it was known that the surface consists of only B atoms without Yb. In the sub-surface region, Yb atoms exist and the temperature dependence of the valence agrees with those of previous HAXPES results. Here we report the cleaning method to obtain well-defined YbB12(001) surfaces and the temperature-dependent Yb valence obtained from Yb 3d core-level and valence photoelectron spectra.