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We have undertaken a detailed study of the Cr surface and Cr/MgO interface electronic properties by symmetry- and angle-resolved photoemission spectroscopy on a Cr(001) surface covered or not with 1.2 monolayer MgO. For the bare Cr(001) film, the experimental bulk dispersions are in good agreement with the theoretical band structure, and characteristic antiferromagnetic features like the antiferromagnetic gap at the X point are observed. However, by covering the Cr film by 1.2 monolayer MgO, the observed experimental dispersions are strongly perturbed as a result of the existence of metal-induced gap states in the MgO film. By carrying out normal emission measurements, we detect two 2D localized states for the Cr(001) surface that persist at the Cr/MgO interface: a Δ1 state at very low binding energy, which is usually associated with Cr surface magnetism, and an additional resonant Δ5 state, which shall play a crucial role for transport and coupling in Cr/MgO epitaxial systems. [ABSTRACT FROM AUTHOR]

We report the existence of metallic two-dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating TiO2 anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of dxy orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nanoengineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase. [ABSTRACT FROM AUTHOR]

Fe1-xCox epitaxial layers were grown by molecular beam epitaxy and controlled using in situ characterization tools (electron diffraction, x-ray photoelectron, and Auger spectroscopies) in order to use these alloys in magnetic tunnel junctions. Thick films are shown to be BCC up to x = 70%, whereas thin films grown on Fe(001) are BCC over the whole Co concentration range up to a critical thickness. Fe/Co intermixing is also examined in detail. The tunnel magnetoresistance and conductance of MgO-based magnetic tunnel junctions built with BCC Fe1-xCox electrodes show peculiar voltage dependencies. Their interpretation relies on spin- and symmetry-resolved photoemission measurements and ab initio calculations. The bulk and interfacial occupied states detected by photoemission are compared to the calculated ones. Experimental results and ab initio calculations are in good agreement. Finally, the experimental conductance curves are compared to the autoconvolution of the Δ1 density of states in which both bulk and interface states are taken into account. [ABSTRACT FROM AUTHOR]

The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few 1011 cm-2. When the growth temperature is high (T ≈ 1500-1550 °C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered (T ≈ 1450-1500 °C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm2/Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed. [ABSTRACT FROM AUTHOR]

We investigate with angle-resolved photoelectron spectroscopy the changes of the Fermi surface and the main bands from the paramagnetic state to the antiferromagnetic (AFM) state occurring below 72 K in Fe1.06Te. The evolution is completely different from that observed in Fe pnictides, as nesting is absent. The AFM state is a rather good metal, in agreement with our magnetic band structure calculation. On the other hand, the paramagnetic state is very anomalous with a large pseudogap of ~65 meV on the electron pocket that closes in the AFM state. We discuss this behavior in connection with spin fluctuations existing above the magnetic transition and the correlations predicted in the spin-freezing regime of the incoherent metallic state. [ABSTRACT FROM AUTHOR]

We have investigated the electronic band structure of Pt-induced nanowires on Ge(001) [PtJGe(001) NWs] by angle-resolved photoelectron spectroscopy at 6K. Single-domain samples of Pt/Ge(001) NWs were prepared on vicinal Ge(001) substrates. One of the bands disperses only in the nanowire direction, and its Fermi surface consists of strictly straight lines, indicating that it is an ideal one-dimensional metallic state. An energy distribution curve of the one-dimensional band is explained by a Fermi-Dirac-type spectral shape, which is against both a Luttinger liquid and Peierls instability schemes. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe1-xCox)2As2 as a function of Co content and temperature. We show that there is a k-dependent energy shift compared to density functional calculations, which is large below 100 K at low Co contents and reduces the volume of hole and electron pockets by a factor 2. This k shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent fluctuations. [ABSTRACT FROM AUTHOR]

Mn5Ge3(001) thin films grown on Ge(111)-c(2×8) reconstructed surfaces were studied by angle-resolved photoemission using synchrotron radiation in the 14-94 eV photon energy range. The results obtained in the ΓALM plane and in the ΓAHK plane are in agreement with simulations starting with band structure calculations based on the density functional theory. This provides a unique validation of band structure calculations for a proper description of the electronic properties of Mn5Ge3. Only the spectral feature very close to the Fermi level cannot be well explained by the simulation. This departure is discussed in terms of the three-dimensional nature of the sample and of correlation effects. [ABSTRACT FROM AUTHOR]

We have performed angle- and spin-resolved photoemission measurements on Fe3O4(0 0 1)/MgO(0 0 1). Despite intrinsic electron-lattice (polaron) interaction a Fe t2g band dispersion is observed, as well as a Fermi surface. The states close to the Fermi level show a spin polarization up to -- 72% as measured with a 4.65 eV laser light. All the data can be simulated starting from GGA + U bulk band-structure calculations and taking into account the polaron and initial-state lifetime broadening involved in the photoemission process. This conciliates the electronic structure of Fe3O4 with a band model and confirms the half-metallic nature of Fe3O4. [ABSTRACT FROM AUTHOR]

We investigate the quasiperiodic crystal (LaS)1.196 VS2 by angle and time resolved photoemission spectroscopy. The dispersion of electronic states is in qualitative agreement with the band structure calculated for the VS2 slab without incommensurate distortion. Nonetheless, the spectra display a temperature dependent pseudogap instead of quasiparticle crossing. The sudden photoexcitation at 50K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable time scale. We attribute this surprisingly short time scale to a very strong electron-phonon coupling to the incommensurate distortion. This result sheds light on the electronic localization arising in aperiodic structures and quasicrystals. [ABSTRACT FROM AUTHOR]

We investigate the bismuth (111) surface by means of time and angle resolved photoelectron spectroscopy. The parallel detection of the surface states below and above the Fermi level reveals a giant anisotropy of the spin-orbit spitting. These strong deviations from the Rashba-like coupling cannot be treated in k · p perturbation theory. Instead, first principles calculations could accurately reproduce the experimental dispersion of the electronic states. Our analysis shows that the giant anisotropy of the spin-orbit splitting is due to a large out-of plane buckling of the spin and orbital texture. [ABSTRACT FROM AUTHOR]

We investigate the structural and electronic properties of strained Ge/Si(111)-(5×5) surfaces by means of scanning tunneling microscopy and high-resolution angle-resolved photoemission spectroscopy. The homogeneous (5×5) reconstructed overlayers are characterized by three electronic surface states, similar to the Si(111)-(7×7) surface. The dispersion of the dangling bond related surface state exhibits the same periodicity as that of the (5×5) reconstruction. Moreover, a careful analysis of the shape and width of this surface state provides striking evidence of electron-phonon coupling at low temperatures. By considering the spectral function within a simple Debye model, we determine both the Debye energy and the electron-phonon coupling strength. The latter value is further confirmed by analyzing the temperature-dependent phonon broadening of the dangling bond related surface state linewidth. [ABSTRACT FROM AUTHOR]

We study, using angle-resolved photoemission spectroscopy, the two-dimensional electron gas (2DEG) at the surface of KTa03 (KTO), a wide-gap insulator with strong spin-orbit coupling (SOC). We find that this 2DEG is a genuinely different physical state with respect to the bulk: the orbital symmetries of its subbands are entirely reconstructed and their masses are renormalized. This occurs because the values of the SOC, the Fermi energy, and the subband splittings become comparable in the 2DEG. Additionally, we identify an F-center-like heavy band resulting from the polar nature of the KTO surface. [ABSTRACT FROM AUTHOR]

In combining spin- and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe1-xCox(001)/MgO/Fe1-xCox(001) magnetic tunnel junctions. Contrary to previous theoretical predictions, we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic % as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Δ1 symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe1-xCox(001) free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system. [ABSTRACT FROM AUTHOR]

In this work we use low-energy electron microscopy, x-ray photoemission electron microscopy, and x-ray photoelectron spectroscopy to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC(OOO¯1) rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020°C. In a sequence of depositions, we have been able to intercalate ~6 ML of Si into the graphene-SiC interface. [ABSTRACT FROM AUTHOR]

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2√3 x 2√3R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects. [ABSTRACT FROM AUTHOR]

We present a detailed comparison of the electronic structure of BaFe2As2 in its paramagnetic and antiferromagnetic (AFM) phases through angle-resolved photoemission studies. Using different experimental geometries, we resolve the full elliptic shape of the electron pockets, including parts with dxy symmetry along its major axis that are usually missing. This allows us to precisely define how the hole and electron pockets are nested and how the different orbitals evolve at the transition. We conclude that the imperfect nesting between hole and electron pockets explains rather well the formation of gaps and residual metallic droplets in the AFM phase, provided the relative parity of the different bands is taken into account. Beyond this nesting picture, we observe shifts and splittings of numerous bands at the transition. We show that the splittings are surface sensitive and probably not a reliable signature of the magnetic order. On the other hand, the shifts indicate a significant redistribution of the orbital occupations at the transition, especially within the dxz/dyz system, which we discuss. [ABSTRACT FROM AUTHOR]

Graphene stacked in a Bernal configuration (60° relative rotations between sheets) differs electronically from isolated graphene due to the broken symmetry introduced by interlayer bonds forming between only one of the two graphene unit cell atoms. A variety of experiments have shown that non-Bernal rotations restore this broken symmetry; consequently, these stacking varieties have been the subject of intensive theoretical interest. Most theories predict substantial changes in the band structure ranging from the development of a Van Hove singularity and an angle-dependent electron localization that causes the Fermi velocity to go to zero as the relative rotation angle between sheets goes to zero. In this work we show by direct measurement that non-Bernal rotations preserve the graphene symmetry with only a small perturbation due to weak effective interlayer coupling. We detect neither a Van Hove singularity nor any significant change in the Fermi velocity. These results suggest significant problems in our current theoretical understanding of the origins of the band structure of this material. [ABSTRACT FROM AUTHOR]

The electronic structure of the reentrant superconductor Eu(Fe0.86Ir0.14)2As2 (Tc=22 K) with coexisting ferromagnetic order (TM=18 K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of Eu(Fe0.86Ir0.14)2As2. The near-EF Fe-3d-derived band dispersions near the Γ and X high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at T=5 K, we estimate an essentially isotropic s-wave gap (Δ∼5.25±0.25 meV), indicative of strong-coupling superconductivity with 2Δ/kBTc≃5.8. The gap gets closed at temperatures T≥10 K, and this is attributed to the resistive phase which sets in at TM=18 K due to the Eu2+-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound. [ABSTRACT FROM AUTHOR]

We report the existence of a two-dimensional electron system (2DES) at the (001) surface of CaTiO3. Using angle-resolved photoemission spectroscopy, we find a hybridization between the dxz and dyz orbitals, not observed in the 2DESs at the surfaces of other ATiO3 perovskites, e.g., SrTiO3 or BaTiO3. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital energy order, and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic CaTiO3 results from the rotation of the oxygen octahedra, which can also occur at the interface of oxide heterostructures to compensate strain. More generally, the control of the orbital energy order in 2DES by choosing different A-site cations in perovskites offers a gateway toward 2DESs in oxide heterostructures beyond SrTiO3. [ABSTRACT FROM AUTHOR]

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO3. By means of angle-resolved photoemission spectroscopy (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range where the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose-dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the ARPES spectrum in the limit of a negligible concentration of vacancies, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated with the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas at the surface of insulating d0 transition metal oxides. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The electronic band structure of heavily boron doped diamond was investigated by angle-resolved photoemission spectroscopy on (100)-oriented epilayers. A unique set of Luttinger parameters was deduced from a comparison of the experimental band structure of metallic diamond along the Ä(GX) and Σ(GK) high-symmetry directions of the reciprocal space, with theoretical band structure calculations performed both within the local density approximation and by an analytical k·p approach. In this way, we were able to describe the experimental band structure over a large three-dimensional region of the reciprocal space and to estimate hole effective masses in agreement with previous theoretical and experimental papers. [ABSTRACT FROM AUTHOR]

Mn5Ge3(001) thin films grown on Ge(111) were studied by angle- and spin-resolved photoemission using synchrotron radiation in the 17-40 eV photon energy range. The photoelectron spectra were simulated starting from a first-principles band-structure calculation for the ground state, using the free-electron approximation for the final states, taking into account photohole lifetime effects and k⊥ broadening plus correlation effects, but ignoring transition matrix elements. The measured spin polarizations for the various k points investigated in the ΓMLA plane of the Brillouin zone are found to be in fair enough agreement with the simulated ones, providing a strong support to the ground-state band-structure calculations. Possible origins for the departures between either simulations and experiments or previous and present experiments are discussed. [ABSTRACT FROM AUTHOR]

In all iron pnictides, the positions of the ligand alternatively above and below the Fe plane create 2 inequivalent Fe sites. This results in 10 Fe 3d bands in the electronic structure. However, they do not all have the same status for an ARPES experiment. There are interference effects between the 2 Fe that modulate strongly the intensity of the bands and that can even switch their parity. We give a simple description of these effects, notably showing that ARPES polarization selection rules in these systems can not be applied by reference to a single Fe ion. We show that ARPES data for the electron pockets in Ba(Fe0.92Co0.08)2As2 are in excellent agreement with this model as well as with direct calculation of the spectral weight. We observe both the total suppression of some bands and the parity switching of some other bands. Once these effects are properly taken into account, the structure of the electron pockets, as measured by ARPES, becomes very clear and simple. By combining ARPES measurements in different experimental configurations, we clearly isolate each band forming one of the electron pockets. We identify a deep electron band along one ellipse axis with the dxy orbital and a shallow electron band along the perpendicular axis with the dxz/dyz orbitals, in good agreement with band-structure calculations. We show that the electron pockets are warped as a function of kz as expected theoretically, but that they are much smaller than predicted by the calculation. [ABSTRACT FROM AUTHOR]

Novel topological phases of matter are fruitful platforms for the discovery of unconventional electromagnetic phenomena. Higher-fold topology is one example, where the low-energy description goes beyond standard model analogs. Despite intensive experimental studies, conclusive evidence remains elusive for the multigap topological nature of higher-fold chiral fermions. In this Letter, we leverage a combination of fine-tuned chemical engineering and photoemission spectroscopy with photon energy contrast to discover the higher-fold topology of a chiral crystal. We identify all bulk branches of a higher-fold chiral fermion for the first time, critically important for allowing us to explore unique Fermi arc surface states in multiple interband gaps, which exhibit an emergent ladder structure. Through designer chemical gating of the samples in combination with our measurements, we uncover an unprecedented multigap bulk boundary correspondence. Our demonstration of multigap electronic topology will propel future research on unconventional topological responses.

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.

We investigate with angle-resolved photoelectron spectroscopy the changes of the Fermi surface and the main bands from the paramagnetic state to the antiferromagnetic (AFM) state occurring below 72 K in Fe1.06Te. The evolution is completely different from that observed in Fe pnictides, as nesting is absent. The AFM state is a rather good metal, in agreement with our magnetic band structure calculation. On the other hand, the paramagnetic state is very anomalous with a large pseudogap of ~65 meV on the electron pocket that closes in the AFM state. We discuss this behavior in connection with spin fluctuations existing above the magnetic transition and the correlations predicted in the spin-freezing regime of the incoherent metallic state.

Spin-split two-dimensional electronic states have been observed on ultrathin Sn(001) films grown on InSb(001) substrates. Angle-resolved photoelectron spectroscopy (ARPES) performed on these films revealed Dirac-cone-like linear dispersion around the Γ¯ point of the surface Brillouin zone, suggesting nearly massless electrons belonging to 2D surface states. The states disperse across a band gap between bulklike quantum well states in the films. Moreover, both circular dichroism of ARPES and spin-resolved ARPES studies show helical spin polarization of the Dirac-cone-like surface states, suggesting a topologically protected character as in a bulk topological insulator (TI). These results indicate that a quasi-3D TI phase can be realized in ultrathin films of zero-gap semiconductors.

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe(1-x)Co(x))(2)As(2) as a function of Co content and temperature. We show that there is a k-dependent energy shift compared to density functional calculations, which is large below 100 K at low Co contents and reduces the volume of hole and electron pockets by a factor 2. This k shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent fluctuations.

We investigate Ba(Fe0.65Ru0.35)2As2, a compound in which superconductivity appears at the expense of magnetism, by transport measurements and angle resolved photoemission spectroscopy. By resolving the different Fermi surface pockets and deducing from their volumes the number of hole and electron carriers, we show that Ru induces neither hole nor electron doping. However, the Fermi surface pockets are about twice larger than in BaFe2As2. A change of sign of the Hall coefficient with decreasing temperature evidences the contribution of both carriers to the transport. Fermi velocities increase significantly with respect to BaFe2As2, suggesting a reduction of correlation effects.

Previous angle-resolved photoemission spectroscopy experiments in NaxCoO2 reported both a strongly renormalized bandwidth near the Fermi level and moderately renormalized Fermi velocities, leaving it unclear whether the correlations are weak or strong and how they could be quantified. We explain why this situation occurs and solve the problem by extracting clearly the coherent and incoherent parts of the band crossing the Fermi level. We show that one can use their relative weight to estimate self-consistently a quasiparticle weight Z=0.15+/-0.05. We suggest this method could be a reliable way to study the evolution of correlations in cobaltates and for comparison with other strongly correlated systems.

Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.