Photoemission Spectroscopy

We have investigated the lowest binding-energy electronic structure of the model cuprate Sr2CuO2Cl2 using angle resolved photoemission spectroscopy (ARPES). Our data from about 80 cleavages of Sr2CuO2Cl2 single crystals give a comprehensive, self-consistent picture of the nature of the first electron-removal state in this model undoped CuO2-plane cuprate. Firstly, we show a strong dependence on the polarization of the excitation light which is understandable in the context of the matrix element governing the photoemission process, which gives a state with the symmetry of a Zhang-Rice singlet. Secondly, the strong, oscillatory dependence of the intensity of the Zhang-Rice singlet on the exciting photon-energy is shown to be consistent with interference effects connected with the periodicity of the crystal structure in the crystallographic c-direction. Thirdly, we measured the dispersion of the first electron-removal states along Γ →(π, π) and Γ →(π, 0), the latter being controversial in the literature, and have shown that the data are best fitted using an extended t-J-model, and extract the relevant model parameters. An analysis of the spectral weight of the first ionization states for different excitation energies within the approach used by Leung et al. (Phys. Rev. B 56, 6320 (1997)) results in a strongly photon-energy dependent ratio between the coherent and incoherent spectral weight. The possible reasons for this observation and its physical implications are discussed.

1 Recent theories and experiments have suggested that strong spin-orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the so-called topological insulator, which can show macroscopic entanglement effects . Such systems feature two-dimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe strongly interacting particles .

We report the first observation of a topological surface state on the (111) surface of the ternary chalcogenide TlBiSe 2 by angle-resolved photoemission spectroscopy. By tuning the synchrotron radiation energy we reveal that it features an almost ideal Dirac cone with the Dirac point well isolated from bulk continuum states. This suggests that TlBiSe 2 is a promising material for realizing quantum topological transport.

1 When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect [1, 2] dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the extreme quantum limit [3, 4, 5], which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields [6]. Bulk Bi 1−x Sb x single crystals are predicted to be prime candidates [7, 8] for one such unusual Hall phase of matter known as the topological insulator [9,. The hallmark of a topological insulator is the existence of metallic surface states that are higher dimensional analogues of the edge states that characterize a quantum spin Hall insulator [3, 4, 5, 6, 7, 8, 9,. In addition to its interesting boundary states, the bulk of Bi 1−x Sb x is predicted to exhibit three-dimensional Dirac particles , another topic of heightened current interest following the new findings of two-dimensional graphene and charge quantum Hall fractionalization observed in pure bismuth . However, despite numerous transport and magnetic measurements on the Bi 1−x Sb x family since the 1960s , no direct evidence of either topological quantum Hall-like states or bulk Dirac particles has ever been found. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy (IPEM-ARPES), we report the direct observation of massive Dirac particles in the bulk of Bi 0.9 Sb 0.1 , locate the Kramers' points at the sample's boundary and provide a comprehensive mapping of the topological Dirac insulator's gapless surface modes. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the much sought exotic "topological metal" [9,. They also suggest that this material has potential application in developing next-generation quantum computing devices that may incorporate "light-like" bulk carriers and topologically protected spin-textured edge-surface currents.

We show that the strongly spin-orbit coupled materials Bi 2 Te 3 and Sb 2 Te 3 and their derivatives belong to the Z 2 topological-insulator class. Using a combination of first-principles theoretical calculations and photoemission spectroscopy, we directly show that Bi 2 Te 3 is a large spin-orbit-induced indirect bulk band gap ( $ 150 meV) semiconductor whose surface is characterized by a single topological spin-Dirac cone. The electronic structure of self-doped Sb 2 Te 3 exhibits similar Z 2 topological properties. We demonstrate that the dynamics of spin-Dirac fermions can be controlled through systematic Mn doping, making these materials classes potentially suitable for topological device applications.

A relationship between the energy of the highest occupied molecular orbital (HOMO) and the oxidation potential of molecular organic semiconductors is presented. Approximating molecules as dipoles consisting of a positively charged ion core surrounded by an electron cloud, the HOMO energy (E HOMO ) is calculated as that required to separate these opposite charges in a neutral organic thin film. Furthermore, an analysis of image charge forces on spherical molecules positioned near a conductive plane formed by the electrode in an electrochemical cell is shown to explain the observed linear relationship between E HOMO and the oxidation potential. The E HOMO Õs of a number of organic semiconductors commonly employed in thin film electronic devices were determined by ultraviolet photoemission spectroscopy, and compared to the relative oxidation potential (V CV ) measured using pulsed cyclic voltammetry, leading to the relationship E HOMO = À(1.4 ± 0.1) · (qV CV ) À (4.6 ± 0.08) eV, consistent with theoretical predictions.

The unusual transport properties of graphene are the direct consequence of a peculiar band structure near the Dirac point. We determine the shape of the π bands and their characteristic splitting, and find the transition from two-dimensional to bulk character for 1 to 4 layers of ...

Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO3) is the bedrock of the emerging field of oxide electronics. SrTiO3 is the preferred template to create exotic two-dimensional (2D) phases of electron matter at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. Here we show, using angle-resolved photoemission spectroscopy (ARPES), that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3, independent of bulk carrier densities over more than seven decades, including the undoped insulating material. This 2DEG is confined within a region of ~5 unit cells with a sheet carrier density of ~0.35 electrons per a^2 (a is the cubic lattice parameter). We unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. The similarity of this 2DEG with those reported in SrTiO3-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices, and a novel route to generate 2DEGs at surfaces of transition-metal oxides.

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,NЈ-bis-͑1-naphthyl͒-N,NЈ-diphenyl1-1,1-biphenyl1-4,4Ј-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly͑3,4-ethylenedioxythiophene͒/ poly͑styrenesulfonate͒. Although both electrode surfaces have a similar work function (ϳ5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with highspeed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr 2 O 3 has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr 2 O 3 single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

Topological insulators (TI) are novel quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem facing the current TI materials is the existence of significant bulk conduction. Here we show the band structure engineering of TIs by molecular beam epitaxy growth of (Bi 1-x Sb x ) 2 Te 3 ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi 1-x Sb x ) 2 Te 3 , indicating the robustness of bulk Z 2 topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behave like one quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new TI devices with well-established semiconductor technology.

This paper reports the synthesis and detailed characterization of graphite thin films produced by thermal decomposition of the (0001) face of a 6H-SiC wafer, demonstrating the successful growth of single crystalline films down to approximately one graphene layer. The growth and characterization were carried out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality were monitored by low-energy electron diffraction, and the thickness of the sample was determined by core level x-ray photoelectron spectroscopy. High-resolution angle-resolved photoemission spectroscopy shows constant energy map patterns, which are very sharp and fully momentum-resolved, but nonetheless not resolution limited. We discuss the implications of this observation in connection with scanning electron microscopy data, as well as with previous studies.

The atomic and electronic structure of polymer films undergoes deep modifications during high energy (keV-MeV) ion irradiation, from molecular solid to amorphous material. At low energy density (1022–1024) typical effects include chain scissions, crosslinks, molecular emission and double bonds formation. In hydrocarbon polymer (polystyrene, polyethylene) the main effect of irradiation is the formation of new bonds as detected by molecular weight distribution, solubility and optical measurements. Moreover the concentration of trigonal carbon (sp2) in the polymer changes with ion fluence (1011–1014) and stabilizes to a value of 20% independently on the initial chemical structure of the irradiated sample. Photoemission spectroscopy shows an evolution of valence band states from localized to extended states. At high energy density (1024–1026) the irradiated polymer continues to evolve showing spectroscopic characteristics close to those of hydrogenated amorphous carbon. Trigonal carbon concentration changes with ion fluence (1014–1016) reaching the steady state value of 60% and the hydrogen concentration decreases to 20%. Moreover the values of the optical gap (2.5–0.5 eV) suggest the presence of medium range order in the obtained hydrogenated amorphous carbon. These values are consistent with the formation of graphitic clusters, whose size goes from 5 Å to 20 Å by changing the ion fluence (or energy density).

The two-dimensional (2D) superconducting state is a fragile state of matter susceptible to quantum phase fluctuations. Although superconductivity has been observed in ultrathin metal films down to a few layers 1-10 , it is still not known whether a single layer of ordered metal atoms, which represents the ultimate 2D limit of a crystalline film, could be superconducting. Here we report scanning tunnelling microscopy measurements on single atomic layers of Pb and In grown epitaxially on Si(111) substrate, and demonstrate unambiguously that superconductivity does exist at such a 2D extreme. The film shows a superconducting transition temperature of 1.83 K for an atom areal density n = 10.44 Pb atoms nm −2 , 1.52 K for n = 9.40 Pb atoms nm −2 and 3.18 K for n = 9.40 In atoms nm −2 , respectively. We confirm the occurrence of superconductivity by the presence of superconducting vortices under magnetic field. In situ angle-resolved photoemission spectroscopy measurements reveal that the observed superconductivity is due to the interplay between the Pb-Pb (In-In) metallic and the Pb-Si (In-Si) covalent bondings.

Topological insulators in three dimensions are nonmagnetic insulators that possess metallic surface states (SSs) as a consequence of the nontrivial topology of electronic wavefunctions in the bulk of the material. They are the first known examples of topological order in bulk solids. We review the basic phenomena and experimental history, starting with the observation of topological insulator behavior in BixSb1-x by angle and spin-resolved photoemission spectroscopy (spin-ARPES) and continuing through measurements on other materials and by other probes. A self-contained introduction to the single-particle theory is then given, followed by the many-particle definition of a topological insulator as a material with quantized magnetoelectric polarizability. The last section reviews recent work on strongly correlated topological insulators and new effects that arise from the proximity effect between a topological insulator and a superconductor. Although this article is not intended to be a comprehensive review of what is already a rather large field, we hope that it serves as a useful introduction, summary of recent progress, and guideline to future directions.

A term first coined by Mott back in 1968 a `pseudogap' is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales (`two-gap scenario') and charge density waves patterns in the cuprates. Despite many excellent reviews on the pseudogap phenomenon in HTSC, published from its very discovery up to now, the mechanism of the pseudogap and its relation to superconductivity are still open questions. The present review represents a contribution dealing with the pseudogap, focusing on results from angle resolved photoemission spectroscopy (ARPES) and ends up with the conclusion that the pseudogap in cuprates is a complex phenomenon which includes at least three different `intertwined' orders: spin and charge density waves and preformed pairs, which appears in different parts of the phase diagram. The density waves in cuprates are competing to superconductivity for the electronic states but, on the other hand, should drive the electronic structure to vicinity of Lifshitz transition, that could be a key similarity between the superconducting cuprates and iron based superconductors. One may also note that since the pseudogap in cuprates has multiple origins there is no need to recoin the term suggested by Mott.

Intense femtosecond (10 215 s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion 1,2,3 . This technique is particularly useful in the study 4,5 and in the control 6 of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time-and angleresolved photoemission spectroscopy is in this context a direct and comprehensive, energy-and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom 7-10 . Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy 10,11 . Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time-and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe 2 (refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solidstate transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems 14 , we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.

We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO0.9F0.1 single crystals. We find that the Γ hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 ± 1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value -consistent with an isotropic or anisotropic s-wave symmetry of the order parameter. This is a significant departure from the situation in the cuprates, pointing to the possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.

The structure of graphite oxide (GO) has been systematically studied using various tools such as SEM, TEM, XRD, Fourier transform infrared spectroscopy (FT-IR), X-ray photoemission spectroscopy (XPS), 13 C solidstate NMR, and O K-edge X-ray absorption near edge structure (XANES). The TEM data reveal that GO consists of amorphous and crystalline phases. The XPS data show that some carbon atoms have sp 3 orbitals and others have sp 2 orbitals. The ratio of sp 2 to sp 3 bonded carbon atoms decreases as sample preparation times increase. The 13 C solid-state NMR spectra of GO indicate the existence of sOH and sOs groups for which peaks appear at 60 and 70 ppm, respectively. FT-IR results corroborate these findings. The existence of ketone groups is also implied by FT-IR, which is verified by O K-edge XANES and 13 C solid-state NMR. We propose a new model for GO based on the results; sOs, sOH, and sCdO groups are on the surface.

We use angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increases with underdoping - a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near nodal regime where a coherent peak in the spectrum can be observed, does not increase with less doping - a behavior not observed in the single particle spectra before. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high-Tc superconductivity.

We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi 2 Se 3 from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.

We have investigated nitrogen doping effects on the structure and crystallinity of bamboo-shaped multiwalled carbon nanotubes ͑BS-MWNTs͒ by means of x-ray photoemission spectroscopy ͑XPS͒ and transmission electron microscopy. By controlling the NH 3 /C 2 H 2 flow ratio during the chemical vapor deposition, the nitrogen concentrations of 0.4% to 2.4% were obtained. According to the XPS measurements, the increasing nitrogen concentration gave rise to an increase of the N-sp 3 C bonds as well as the deterioration of the crystallinity of the BS-MWNTs. Besides, the N-sp 3 C bonds were found to prevail over the N-sp 2 C bonds above 5% nitrogen concentration. At higher nitrogen concentrations, the BS-MWNTs showed shorter compartment distances, presumably due to the suppressed surface diffusion of carbon on the catalyst particles.

This paper reports on the bulk properties of the quaternary Heusler alloy Co 2 Mn 1−x Fe x Si with the Fe concentration x =0,1/2,1. All samples, which were prepared by arc melting, exhibit L2 1 long-range order over the complete range of Fe concentration. The structural and magnetic properties of the Co 2 Mn 1−x Fe x Si Heusler alloys were investigated by means of x-ray diffraction, high-and low-temperature magnetometry, Mössbauer spectroscopy, and differential scanning calorimetry. The electronic structure was explored by means of highenergy photoemission spectroscopy at about 8 keV photon energy. This ensures true bulk sensitivity of the measurements. The magnetization of the Fe-doped Heusler alloys is in agreement with the values of the magnetic moments expected for a Slater-Pauling-like behavior of half-metallic ferromagnets. The experimental findings are discussed on the basis of self-consistent calculations of the electronic and magnetic structure. To achieve good agreement with experiment, the calculations indicate that on-site electron-electron correlation must be taken into account, even at low Fe concentration. The present investigation focuses on searching for the quaternary compound where the half-metallic behavior is stable against outside influences. Overall, the results suggest that the best candidate may be found at an iron concentration of about 50%.

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Recently, high resolution angle-resolved photoemission spectroscopy has been used to determine the detailed momentum dependence of the superconducting gap in the high temperature superconductor Bi-2212. In this paper, we first describe tight binding fits to the normal state dispersion and superlattice modulation effects. We then discuss various theoretical models in light of the gap measurements. We find that the simplest model which fits the data is the anisotropic s-wave gap cos(k x) cos(k y), which within a one-band BCS framework suggests the importance of next near neighbor Cu-Cu interactions. Various alternative interpretations of the observed gap are also discussed, along with the implications for microscopic theories of high temperature superconductors.

We review a number of experimental techniques that are beginning to reveal fine details of the bosonic spectrum α 2 F (Ω) that dominates the interaction between the quasiparticles in high temperature superconductors. Angle-resolved photo emission (ARPES) shows kinks in electronic dispersion curves at characteristic energies that agree with similar structures in the optical conductivity and tunnelling spectra. Each technique has its advantages. ARPES is momentum resolved and offers independent measurements of the real and imaginary part of the contribution of the bosons to the self energy of the quasiparticles. The optical conductivity can be used on a larger variety of materials and with the use of maximum entropy techniques reveals rich details of the spectra including their evolution with temperature and doping. Scanning tunnelling spectroscopy offers spacial resolution on the unit cell level. We find that together the various spectroscopies, including recent Raman results, are pointing to a unified picture of a broad spectrum of bosonic excitations at high temperature which evolves, as the temperature is lowered into a peak in the 30 to 60 meV region and a featureless high frequency background in most of the materials studied. This behaviour is consistent with the spectrum of spin fluctuations as measured by magnetic neutron scattering. However, there is evidence for a phonon contribution to the bosonic spectrum as well.

We present the first angle-resolved photoemission study of Na 0:7 CoO 2 , the host material of the superconducting Na x CoO 2 nH 2 O series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard U. The quasiparticle band crosses the Fermi level from M toward ÿ suggesting a negative sign of effective single-particle hopping t eff (about 10 meV) which is on the order of magnetic exchange coupling J in this system. Quasiparticles are well defined only in the T-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of a strongly interacting quantum system.

We present an ab initio numerical many-body GW calculation of the band plot in free-standing graphene. We consider the full ionic and electronic structure introducing e-e interaction and correlation effects via a self-energy containing non-hermitian and dynamical terms. With respect to the density-functional theory local-density approximation, the Fermi velocity is renormalized with an increase of 17%, in better agreement with the experiment. Close to the Dirac point the linear dispersion is modified by the presence of a kink, as observed in angle-resolved photoemission spectroscopy. We demonstrate that the kink is due to low-energy π → π * single-particle excitations and to the π plasmon. The GW self-energy does not open the band gap.

A residual linear term is observed in the thermal conductivity of optimally-doped Bi2Sr2CaCu2O8 at very low temperatures whose magnitude is in excellent agreement with the value expected from Fermi-liquid theory and the d-wave energy spectrum measured by photoemission spectroscopy, with no adjustable parameters. This solid basis allows us to make a quantitative analysis of thermodynamic properties at low temperature and establish that thermally-excited quasiparticles are a significant, perhaps even the dominant mechanism in suppressing the superfluid density in cuprate superconductors Bi2Sr2CaCu2O8 and YBa2Cu3O7.

Rhodium single crystals show a surprisingly wide range of reconstructions induced by light elements such as oxygen and nitrogen, as well a number of chemical reactions of fundamental chemical importance. In this review, we present and critically discuss the current state of knowledge of the interaction of oxygen, nitrogen and mixed O+N layers with such Rh surfaces, emphasising structural aspects and their impact on the surface reactivity. On the basis of the available experimental results we will elucidate some general trends, which shed light on the possible driving forces behind the diffusive and displacive reconstructions induced by adsorbed atomic oxygen and nitrogen. More attention will be paid to the reconstructive interactions on a Rh(1 1 0) surface.

The electronic structure of Bi(001) ultrathin films (thickness 7 bilayers) on Si111-7 7 was studied by angle-resolved photoemission spectroscopy and first-principles calculations. In contrast with the semimetallic nature of bulk Bi, both the experiment and theory demonstrate the metallic character of the films with the Fermi surface formed by spin-orbit-split surface states (SSs) showing little thickness dependence. Below the Fermi level, we clearly detected quantum well states (QWSs) at the M point, which were surprisingly found to be non-spin-orbit split; the films are ''electronically symmetric'' despite the obvious structural nonequivalence of the top and bottom interfaces. We found that the SSs hybridize with the QWSs near M and lose their spin-orbit-split character.

Using angle resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi2Se3 as a function of water vapor exposure. We find that a surface reaction with water induces a band bending shifting the Dirac point deep into the occupied states and creating quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se-abstraction leaving positively charged vacancies at the surface. Due to the presence of water vapor, a similar effect takes place when Bi2Se3 crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi2Se3 band structure. Topological insulators (TIs) are a recently discovered new class of materials with a strong spin-orbit coupling which distinguish themselves from other materials through fundamental symmetry considerations . The narrow band gap semiconductor Bi 2 Se 3 serves as a simple model system for TIs hosting a single topologically non-trivial surface state . The TIs bear great potential for the observation of exotic phenomena, such as, e. g., Majorana fermions, unconventional superconductivity, magnetic monopoles . A hallmark of the TIs is the robustness of the surface state to external perturbations. This is a consequence of the time-reversal symmetry, which requires the surface state bands to be spin degenerate at time reversal invariant points in the surface Brillouin zone. Together with the non-trivial band topology, this dictateson a fundamental level -that the surface state be metallic even in the presence of small perturbations in the Hamiltonian. It has been shown already that the topologically nontrivial surface state (TSS) of Bi 2 Se 3 is robust against different kinds of adsorbates. Depending on the adsorbate atoms or molecules, they induce an n-type or p-type doping in the surface state as a consequence of a band bending induced at the surface . An important test for the suitability in device applications is the robustness of the TSS under ambient conditions. While oxygen has been shown to induce a p-type doping , the effect of moisture in the air, i. e. water vapor, on the TSS in Bi 2 Se 3 is still unknown. Water is known to react with selenides to form hydrogen selenide gas . This point is particularly important as the surface is not just decorated with adsorbates, but in the case of water it is modified through a chemical reaction. Here, we show using angular resolved photoemission spectroscopy (ARPES) that water vapor reacts with the Bi 2 Se 3 surface inducing an n-type doping of the surface. As a result, up to three quantum well states (QWS) with a strong Rashba-type splitting are formed that coexist with the robust topological state. By tuning the exposure to water vapor we can directly control the electronic structure of the Bi 2 Se 3 surface. We also show that a similar effect related to a reaction of the surface with water molecules can be observed when Bi 2 Se 3 crystal is exposed to residual gas in vacuum or cleaved in air. Thus, our findings can explain previously ob-served time-dependent effects in the surface electronic structure of Bi 2 Se 3 .

We have investigated the effects of doping on a single layer of graphene using angle-resolved photoemission spectroscopy. We show that many-body interactions severely warp the Fermi surface, leading to an extended van Hove singularity (EVHS) at the graphene M point. The ground state properties of graphene with such an EVHS are calculated, analyzing the competition between a magnetic instability and the tendency towards superconductivity. We find that the latter plays the dominant role as it is enhanced by the strong modulation of the interaction along the Fermi line, leading to an energy scale for the onset of the pairing instability as large as 1 meV when the Fermi energy is sufficiently close to the EVHS.

A pairing gap and coherence are the two hallmarks of superconductivity. In a classical BCS superconductor they are established simultaneously at T c . In the cuprates, however, an energy gap (pseudogap) extends above T c [1, 2, 3,. The origin of this gap is one of the central issues in high temperature superconductivity. Recent experimental evidence demonstrates that the pseudogap and the superconducting gap are associated with different energy scales . It is however not clear whether they coexist independently or compete . In order to understand the physics of cuprates and improve their superconducting properties it is vital to determine whether the pseudogap is friend or foe of high temperature supercondctivity . Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that the pseudogap and high temperature superconductivity represent two competing orders. We find that there is a direct correlation between a loss in the low energy spectral weight due to the pseudogap and a decrease of the coherent fraction of paired electrons. Therefore, the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space where the superconducting gap is largest. This leads to a very unusual state in the underdoped cuprates, where only part of the Fermi surface develops coherence.

28. The same approach to d-screening applied to bulk line widths reduces them by a factor of ε ϭ 2.4 for Cu. A recent first-principles calculation has shown that d-electrons decrease the line width of bulk states in Cu by a factor of 2.4, in excellent agreement with this model (33).

We report the crystal structure and magnetic properties of Zn 1−x Co x O (0 x 0.10) nanoparticles synthesized by heating metal acetates in organic solvent. The nanoparticles were crystallized in the wurtzite ZnO structure after annealing in air and in a forming gas (Ar95% + H5%). The x-ray diffraction and x-ray photoemission spectroscopy (XPS) data for different Co content show clear evidence for the Co 2+ ions in tetrahedral symmetry, indicating the substitution of Co 2+ in the ZnO lattice. However, samples with x = 0.08 and higher cobalt content also indicate the presence of Co metal clusters. Only those samples annealed in the reducing atmosphere of the forming gas, that showed the presence of oxygen vacancies, exhibited ferromagnetism at room temperature. The air annealed samples remained non-magnetic down to 77 K. The essential ingredient in achieving room temperature ferromagnetism in these Zn 1−x Co x O nanoparticles was found to be the presence of additional carriers generated by the presence of the oxygen vacancies.

Two-photon photoemission (2PP) spectra of TiO 2 ͑110͒ surfaces are measured for the nearly perfect surface, and surfaces modified by introduction of defects and adsorbed molecules. Defects are generated on nearly perfect surfaces by three methods: electron irradiation, annealing in vacuum, and Ar + sputtering. Nearly perfect or damaged surfaces can be further modified by adsorption of O 2 or H 2 O molecules. 2PP spectroscopy is used to systematically investigate the work function change due to the presence of defects or adsorbates. 2PP spectroscopy detects both surface and bulk oxygen vacancy defects. We find from the results on oxygen adsorption that oxygen vacancies created by electron irradiation are localized on the surface and may be removed by O 2 adsorption at 100 K. The surface defects are substantially different from those created by annealing or by ion sputtering where vacancies in the subsurface region are proposed. We find that O 2 acts as an acceptor molecule on surface defect states whereas H 2 O acts as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of H 2 O adsorbed on TiO 2 surface is derived to be 0.5 D positive outward. We also find an unoccupied electronic state 2.45 eV above the Fermi level that appears at submonolayer coverage of H 2 O, which we tentatively assign to charge transfer from surface titanium ions to the surface-adsorbed H 2 O molecules or OH ligands.

We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a Bi 2 Te 3 substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and Bi 2 Te 3. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one-and two-dimensional edge states are coexisting at the surface.

Angle-resolved photoemission spectroscopy (ARPES) on optimally doped Bi2Sr2Ca0.92Y0.08Cu2O 8+δ uncovers a coupling of the electronic bands to a 40 meV mode in an extended k-space region away from the nodal direction, leading to a new interpretation of the strong renormalization of the electronic structure seen in Bi2212. Phenomenological agreements with neutron and Raman experiments suggest that this mode is the B1g oxygen bond-buckling phonon. A theoretical calculation based on this assignment reproduces the electronic renormalization seen in the data.

A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction 1 . Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs' 2 . This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity 3 or by invoking orders which are natural competitors of d-wave superconductors 4,5 . Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 . Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state.

The phase diagram of a correlated material is the result of a complex interplay between several degrees of freedom, providing a map of the material's behavior. One can understand (and ultimately control) the material's ground state by associating features and regions of the phase diagram, with specific physical events or underlying quantum mechanical properties. The phase diagram of the newly discovered iron arsenic high temperature superconductors is particularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca, Sr, Ba, T being transition metals), the simultaneous structural/magnetic phase transition that occurs at elevated temperature in the undoped material, splits and is suppressed by carrier doping, the suppression being complete around optimal doping. A dome of superconductivity exists with apparent equal ease in the orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonal state with no long range magnetic order. The question then is what determines the critical doping at which superconductivity emerges, if the AFM order is fully suppressed only at higher doping values. Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that critical changes in the Fermi surface (FS) occur at the doping level that marks the onset of superconductivity. The presence of the AFM order leads to a reconstruction of the electronic structure, most significantly the appearance of the small hole pockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitz transition, at the onset of superconductivity. Superconductivity and magnetism are competing states in the iron arsenic superconductors. In the presence of the hole pockets superconductivity is fully suppressed, while in their absence the two states can coexist.