Controlling the carrier concentration of the high temperature superconductor Bi2Sr2CaCu2O{8+x} in Angle Resolved Photoemission Spectroscopy (ARPES) experiments
A. D. Palczewski, T. Kondo, J. S. Wen, G. Z. J. Xu, G. Gu, A. Kaminski
We study the variation of the electronic properties at the surface of a high temperature superconductor as a function of vacuum conditions in angle resolved photoemission spectroscopy (ARPES) experiments. Normally, under less than ideal vacuum conditions the carrier concentration of Bi2Sr2CaCu2O{8+x} (Bi2212) increases with time due to the absorption of oxygen from CO2 and CO molecules that are prime contaminants present in ultra high vacuum (UHV) systems. We find that in a high quality vacuum environment at low temperatures, the surface of Bi2212 is quite stable (the carrier concentration remains constant), however at elevated temperatures the carrier concentration decreases due to the loss of oxygen atoms from the Bi-O layer. These two effects can be used to control the carrier concentration in-situ. Our finding opens the possibility of studying the electronic properties of the cuprates as a function of doping across the phase diagram on the same piece of sample (i.e. with the same impurities and defects). We envision that this method could be utilized in other surface sensitive techniques such as scanning tunneling microscopy/spectroscopy.
Considering the electron states inside and outside the solid, we derive a formula of photoemission intensity. A general theoretical way to determine electronic structures of solids from ARPES experiments is outlined. It is shown that the spectral function inside the solids cannot be measured directly by ARPES, the effects of free electron states on the electronic structure observed by ARPES measurements must be considered, and the results from ARPES experiments cannot be understood until these results have been made consistent with a theoretical calculation.
Present-day angle-resolved photoemission spectroscopy (ARPES) has offered a tremendous advance in the understanding of electron energy spectra in cuprate superconductors and some related compounds. However, in high magnetic field, magnetic quantum oscillations at low temperatures indicate the existence of small electron (hole) Fermi pockets seemingly missing in ARPES of hole (electron) doped cuprates. Here ARPES and quantum oscillations are reconciled in the framework of an impurity band in the charge-transfer Mott-Hubbard insulator.
Angle-resolved photoemission spectroscopy (ARPES) presents significant simplications in analyzing strictly two-dimensional (2D) materials, but even the most anisotropic physical systems display some residual three-dimensionality. Here we demonstrate how this third dimension manifests itself in ARPES spectra of quasi-2D materials by considering the example of the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8}$ (Bi2212). The intercell, interlayer hopping, which is responsible for $k_z$-dispersion of the bands, is found to induce an irreducible broadening to the ARPES lineshapes with a characteristic depende…
Both electronic Raman scattering (ERS) and angle-resolved photoemission spectra (ARPES) revealed two energy scales for the gap in different momentum spaces in the cuprates. However, the interpretations were different, and the gap values were also different in two experiments. In order to clarify the origin of these discrepancies, we directly compared ERS and ARPES by calculating ERS from the experimental data of ARPES through the Kubo formula. The calculated ERS spectra were in good agreement with the experimental results except for the B$_{1g}$ peak energies. The doping-dependent B$_{2g}$ pea…
We report angle-resolved photoemission spectra (ARPES) from the Fermi energy ($E_F$) over a large area of the ($k_x,k_y$) plane using 21.2 eV and 32 eV photons in two distinct polarizations from an optimally doped single crystal of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212), together with extensive first-principles simulations of the ARPES intensities. The results display a wide-ranging level of accord between theory and experiment and clarify how myriad Fermi surface (FS) maps emerge in ARPES under various experimental conditions. The energy and polarization dependences of the ARPES matrix elemen…
The significant progress in angle-resolved photoemission spectroscopy (ARPES) in last three decades has elevated it from a traditional band mapping tool to a precise probe of many-body interactions and dynamics of quasiparticles in complex quantum systems. The recent developments of deep ultraviolet (DUV, including ultraviolet and vacuum ultraviolet) laser-based ARPES have further pushed this technique to a new level. In this paper, we review some latest developments in DUV laser-based photoemission systems, including the super-high energy and momentum resolution ARPES, the spin-resolved ARPES…
In the pseudogap state the spectrum of the autocorrelation of angle resolved photoemission (AC-ARPES) data of Bi2212 presents non-dispersive peaks in momentum space which compare well with those responsible of the checkerboard pattern found in the density of states by Scanning Tunneling Microscopy. This similarity suggests that the checkerboard pattern originates from peaks in the joint density of states, as the dispersive peaks found in the superconducting state do. Here we show that the experimental AC-ARPES spectrum can be reproduced within a model for the pseudogap with no charge-ordering …
We have analyzed the quasiparticle dispersion and ARPES-spectral density for Sr2CuO2Cl2 and Ca2CuO2Cl2 antiferromagnetic insulators along basic symmetric directions of the Brillouin zone (BZ) in a framework of an extended tight binding method (ETBM) with explicit account for intracell strong electron correlations. The quasiparticle dispersion is in a good agreement with ARPES- data. At the top of valence band we found a narrow impurity-like virtual level with the spectral weight proportional to the concentration of spin fluctuations. A pseudogap between the virtual level and the top of the val…
The angle-resolved photoemission (ARPES) technique has been developed rapidly over the last decay, accompanied by the improvement of energy and momentum resolutions. This technique has been established as the most powerful tool to investigate the high Tc cuprate superconductors. We review recent ARPES data on the cuprates from a theoretical point of view, with emphasis on the systematic evolution of the spectral weight near the momentum (pi,0) from insulator to overdoped systems. The effects of charge stripes on the ARPES spectra are also reviewed. Some recent experimental and theoretical effo…
We show theoretically that double photoemission (2$e$-ARPES) may be used to identify the pairing state in superconductors in which the Cooper pairs have a nonzero center-of-mass momentum, ${\bf q}_{cm}$. We theoretically evaluate the 2$e$ ARPES counting rate, $P^{(2)}$, for the cases of a $d_{x^2-y^2}$-wave superconductor, a pair-density-wave (PDW) phase, and a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase. We show that $P^{(2)}$ provides direct insight into the center-of-mass momentum and spin state of the superconducting condensate, and thus can distinguish between these three different super…
In the course of seeking the microscopic mechanism of superconductivity in cuprate high temperature superconductors, the pseudogap phase\textemdash the very abnormal 'normal' state on the hole-doped side\textemdash has proven to be as big of a quandary as superconductivity itself. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for assessing the momentum-dependent phenomenology of the pseudogap, and recent technological developments have permitted a more detailed understanding. This report reviews recent progress in understanding the relationship between superconductivity …
We develop a novel self-consistent approach for studying the angle resolved photoemission spectra (ARPES) of a hole in the t-J-Holstein model giving perfect agreement with numerically exact Diagrammatic Monte Carlo data at zero temperature for all regimes of electron-phonon coupling. Generalizing the approach to finite temperatures we find that the anomalous temperature dependence of the ARPES in undoped cuprates is explained by cooperative interplay of coupling of the hole to magnetic fluctuations and strong electron-phonon interaction.
We develop a path-integral theory to study the angle-resolved photoemission spectra (ARPES) of high-T_c superconductors based on a two-dimensional model for the CuO2 conduction plane, including both electron-electron (e-e) and electron-phonon (e-ph) interactions. Comparing our result with the experimental one of Bi2Sr2CaCu2O8, we find that the experimentally observed isotopic band shift in ARPES is due to the off-diagonal quadratic e-ph coupling, whereas the presence of e-e repulsion partially suppresses this effect.
The electronic structure of Na2Ti2Sb2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES). The obtained band structure and Fermi surface agree well with the band structure calculation of Na2Ti2Sb2O in the non-magnetic state, which indicating that there is no magnetic order in Na2Ti2Sb2O and the electronic correlation is weak. Polarization dependent ARPES results suggest the multi-band and multi-orbital nature of Na2Ti2Sb2O. Photon energy dependent ARPES r…
We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission (ARPES). From detailed low-energy electron diffraction measurements and an analysis of the ARPES polarization-dependence, we show that these pockets are not intrinsic, but arise from multiple overlapping superstructure replicas of the main and shadow bands. We further demonstrate that the hole pockets reported recently from ARPES [Meng et al, Nature 462, 335 (2009)] have a similar structural origin, and are inconsistent with an intrinsic hole pocket associat…
We present a soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichiometric pnictide superconductor LaRu2P2. The observed electronic structure is in good agreement with density functional theory (DFT) calculations. However, it is significantly different from its counterpart in high-temperature superconducting Fe-pnictides. In particular the bandwidth renormalization present in the Fe-pnictides (~2 - 3) is negligible in LaRu2P2 even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu2P2 has a different origi…
Expansion of Momentum Space and Full 2$π$ Solid Angle Photoelectron Collection in Laser-Based Angle-Resolved Photoemission Spectroscopy by Applying Sample Bias
Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Technical Institute of Physics and Chemistry, Chinese Academy of Sciences（中国科学院化学物理研究所）；Songshan Lake Materials Laboratory（松山湖材料实验室）
Recently, angle-resolved photoemission spectroscopy (ARPES) has revealed a dispersion anomaly at high binding energy near 0.3-0.5eV in various families of the high-temperature superconductors. For further studies of this anomaly we present a new two-dimensional fitting-scheme and apply it to high-statistics ARPES data of the strongly-overdoped Bi2Sr2CuO6 cuprate superconductor. The procedure allows us to extract theself-energy in an extended energy and momentum range. It is found that the spectral function of Bi2Sr2CuO6 can be parameterized using a small set of tight-binding parameters and a w…
We show that in the angle resolved photoemission spectroscopy (ARPES) near-surface induced fields can be useful for disentangling the surface and bulk related emission. The jump of the dielectric function at the interface results in a nonzero term $\operatorname{div}\textbf{A}$ in the photoemission matrix element. The term happens to be significant approximately within the first unit cell and leads to the circular dichroism for the states localized therein. As an example we use ARPES spectra of an YBa$_2$Cu$_3$O$_{7-δ}$ crystal to distinguish between the overdoped surface related component and…
We consider photoemission from cuprate high-temperature superconductors taking into account joint relaxation of strongly coupled fields: a field of correlated electrons and phonon field. Such relaxation occurs in systems with predominantly long-range electron-phonon interaction at sufficiently high carrier concentration due to coexistence of autolocalized and delocalized carriers. A simple method to calculate analytically a high-energy part of the ARPES spectrum arising is proposed. The approach suggested yields all the high-energy spectral features like broad bands of Gaussian shape and verti…
Angle-resolved photoemission spectroscopy (ARPES) is considered as the only experimental tool from which the momentum distribution of both the superconducting and pseudo-gap can be quantitatively derived. The binding energy of the leading edge of the photoemission spectrum, usually called the leading edge gap (LEG), is the model-independent quantity which can be measured in the modern ARPES experiments with the very high accuracy--better than 1 meV. This, however, may be useless as long as the relation between the LEG and the real gap is unknown. We present a systematic study of the LEG as a f…
Knowledge of the gap function is important to understand the pairing mechanism for high-temperature ($T_\mathrm{c}$) superconductivity. However, Fourier transform scanning tunneling spectroscopy (FT STS) and angle-resolved photoemission spectroscopy (ARPES) in the cuprates have reported contradictory gap functions, with FT-STS results deviating strongly from a canonical $d_{x^2-y^2}$ form. By applying an "octet model" analysis to autocorrelation ARPES, we reveal that a contradiction occurs because the octet model does not consider the effects of matrix elements and the pseudogap. This reaffirm…
The angle-resolved photoemission spectra of the superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ have been investigated both experimentally and theoretically. Our results explain the previously obscured origins of all salient features of the ARPES response of this paradigm pnictide compound and reveal the origin of the Lifshitz transition. Comparison of calculated ARPES spectra with the underlying DMFT band structure shows an important impact of final state effects, which results for three-dimensional states in a deviation of the ARPES spectra from the true spectral function. In particular, the ap…
Several angle resolved photoemission spectroscopy (ARPES) studies reveal a poorly nested Fermi surface of LiFeAs, far away from a spin density wave instability, and clear-cut superconducting gap anisotropies. On the other hand a very different, more nested Fermi surface and dissimilar gap anisotropies have been obtained from quasiparticle interference (QPI) data, which were interpreted as arising from intraband scattering within hole-like bands. Here we show that this ARPES-QPI paradox is completely resolved by interband scattering between the hole-like bands. The resolution follows from an ex…
The joint density of states (JDOS) of Bi2Sr2CaCu2O8+x is calculated by evaluating the autocorrelation of the single particle spectral function A(k,omega) measured from angle resolved photoemission spectroscopy (ARPES). These results are compared with Fourier transformed (FT) conductance modulations measured by scanning tunneling microscopy (STM). Good agreement between the two experimental probes is found for two different doping values examined. In addition, by comparing the FT-STM results to the autocorrelated ARPES spectra with different photon polarization, new insight on the form of the S…
The electronic structures of the iron-based superconductors have been intensively studied by using angleresolved photoemission spectroscopy (ARPES). A considerable amount of research has been focused on the LaFeAsO family, showing the highest transition temperatures, where previous ARPES studies have found much larger Fermi surfaces than bulk theoretical calculations would predict. The discrepancy has been attributed to the presence of termination-dependent surface states. Here, using photoemission spectroscopy with a sub-micron focused beam spot (nano-ARPES) we have successfully measured the …
We present a numerical study of the isotope effect on the angle resolved photoemission spectra (ARPES) in the undoped cuprates. By the systematic-error-free Diagrammatic Monte Carlo method, the Lehman spectral function of a single hole in the tt't''-J model in the regime of intermediate and strong couplings to optical phonons is calculated for normal and isotope substituted systems. We found that the isotope effect is strongly energy-momentum dependent, and is anomalously enhanced in the intermediate coupling regime while it approaches to that of the localized hole model in the strong coupling…
We investigated the low-energy electronic structure of Sr2RuO4 by angle-resolved photoemission spectroscopy (ARPES) focusing, in particular, on the controversial issues of the Fermi surface (FS) topology and of the extended van Hove singularity (evHs). A detailed study of temperature and photon energy dependence demonstrates the surface-state nature of the electronic band responsible for the evHs, and may suggest the presence of ferromagnetic correlations at the surface. In addition, we detected a replica of the primary FS that indicates a (sqrt2 x sqrt2) surface reconstruction, as confirmed b…
The design and performance of the first vacuum ultra-violet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994 eV and bandwidth of 0.26 meV is achieved from the second harmonic generation using a novel non-linear optical crystal KBe2BO3F2 (KBBF). The new VUV laser-based ARPES system exhibits superior performance, including super-high energy resolution better than 1 meV, high momentum resolution, super-high photon flux and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high…
Detecting the full photoemission cone from laser-based ARPES experiments by leveraging deflector technology
Institut National de la Recherche Scientifique - Énergie Matériaux Télécommunications（国立科学研究研究所 - 能源材料电信）；McGill University（麦吉尔大学）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Brookhaven National Laboratory（布鲁克海文国家实验室）
In a recent preprint Campuzano et al.(cond-mat/0309402), have questioned the validity of our ARPES results (cond-mat/0305179) contradicting the interpretation of earlier photoemission experiments in terms of the time-reversal symmetry breaking in Bi2212. Here we highlight the principal results of our study and refute all the criticism.
The angle-resolved photoemission spectroscopy (ARPES) is one powerful experimental technique to study the electronic structure of materials. As many electron materials show unusual many-body correlations, the technique to detect directly these many-body correlations will play important roles in the study of their many-body physics. In this article, we propose a technique to detect directly the two-particle correlations, a coincidence ARPES (cARPES) where two incident photons excite two respective photoelectrons which are detected in coincidence. While the one-photon-absorption and one-photoele…
Using angle-resolved photoemission spectroscopy it is revealed that in the vicinity of optimal doping the electronic structure of La2-xSrxCuO4 cuprate undergoes an electronic reconstruction associated with a wave vector q_a=(pi, 0). The reconstructed Fermi surface and folded band are distinct to the shadow bands observed in BSCCO cuprates and in underdoped La2-xSrxCuO4 with x <= 0.12, which shift the primary band along the zone diagonal direction. Furthermore the folded bands appear only with q_a=(pi, 0) vector, but not with q_b= (0, pi). We demonstrate that the absence of q_b reconstruction i…
Recent angle-resolved photoemission (ARPES) studies of the high Tc superconductors are reviewed. Amongst the topics discussed are: the spectral function interpretation of ARPES data and sum rules; studies of the momentum distribution and the Fermi surface (FS); dispersion of electronic states, flat bands and superlattice effects; unusual lineshapes and their temperature dependence; the question of bilayer splitting; detailed studies of the superconducting gap and its anisotropy; and, finally, studies of the pseudogap and evolution of the FS with doping in the underdoped materials.
We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the…
Recent photoemission work on underdoped cuprates between Tc and T*, the pseudogap temperature, are interpreted within the framework of a previously developed negative-U scenario. From this exposition, incorporating a developed 2q diagonal striping model, detailed understanding follows of the ARPES results, together with certain STM and far infrared optical data. Identification of the origin of the magnetic circular dichroism and related effects exhibited by such materials below T* also appears to have been achieved.
Its direct momentum sensitivity confers to angle-resolved photoemission spectroscopy (ARPES) a unique perspective in investigating the superconducting gap of multi-band systems. In this review we discuss ARPES studies on the superconducting gap of the high-temperature Fe-based superconductors. We show that while Fermi-surface-driven pairing mechanisms fail to provide a universal scheme for the Fe-based superconductors, theoretical approaches based on short-range interactions lead to a more robust and universal description of superconductivity in these materials. Our findings are also discussed…
Central issues in the electronic structure of underdoped cuprate superconductors are to clarify the shape of the Fermi surfaces and the origin of the pseudogap. On the basis of the model proposed by Kamimura and Suwa, which bears important features originating from the interplay of Jahn-Teller physics and Mott physics, the feature of Fermi surfaces in underdoped cuprates is the presence of Fermi pockets constructed from doped holes under the coexistence of a metallic state and a local antiferromagnetic order. Below $T_{\rm c}$, the holes on Fermi pockets form Cooper pairs with d-wave symmetry …
The discovery of quantum oscillations in the normal-state electrical resistivity of YBa2Cu3O6.5 provides the first evidence for the existence of Fermi surface (FS) pockets in an underdoped cuprate. However, the pockets' electron vs. hole character, and the very interpretation in terms of closed FS contours, are the subject of considerable debate. Angle-resolved photoemission spectroscopy (ARPES), with its ability to probe electronic dispersion as well as the FS, is ideally suited to address this issue. Unfortunately, the ARPES study of YBa2C3O7-d (YBCO) has been hampered by the technique's sur…
A theory of angle-resolved photoemission (ARPES) in doped cuprates and other charge-transfer Mott insulators is developed taking into account the realistic (LDA+U) band structure, (bi)polaron formation due to the strong electron-phonon interaction, and a random field potential. In most of these materials the first band to be doped is the oxygen band inside the Mott-Hubbard gap. We derive the coherent part of the ARPES spectra with the oxygen hole spectral function calculated in the non-crossing (ladder) approximation and with the exact spectral function of a one-dimensional hole in a random po…
We report the first measurement of the optical phonon dispersion in optimally doped single layer Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a strong softening of the Cu-O bond stretching phonon at about q=(0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A direct comparison with angle-resolved photoemission spectroscopy measurements taken on the same sample, revealed an excellent agreement in terms of energy and momentum between the ARPES nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where…
We measured the electronic structure of an iron arsenic parent compound LaFeAsO using angle resolved photoemission spectroscopy (ARPES). By comparing with a full-potential Linear Augmented PlaneWave calculation we show that the extra large Gamma hole pocket measured via ARPES comes from electronic structure at the sample surface. Based on this we discuss the strong polarization dependence of the band structure and a temperature-dependent hole-like band around the M point. The two phenomena give additional evidences for the existence of the surface-driven electronic structure.
Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct methods of studying the electronic structure of solids. By measuring the kinetic energy and angular distribution of the electrons photoemitted from a sample illuminated with sufficiently high-energy radiation, one can gain information on both the energy and momentum of the electrons propagating inside a material. This is of vital importance in elucidating the connection between electronic, magnetic, and chemical structure of solids, in particular for those complex systems which cannot be appropriately described within …
We study how time- and angle-resolved photoemission (tr-ARPES) reveals the dynamics of BCS-type, s-wave superconducting systems with time-varying order parameters. Approximate methods are discussed, based on previous approaches to either optical conductivity or quantum dot transport, in order to enable computationally efficient prediction of photoemission spectra. One use of such predictions is to enable extraction of the underlying order parameter dynamics from experimental data, which is topical given the rapidly growing use of tr-ARPES in studying unconventional superconductivity. The metho…
We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic structure of Ba(Fe1-x-yCoxMny)2As2 for x=0.06 and 0<=y <=0.07. From ARPES we derive that the substitution of Fe by Mn does not lead to hole doping, indicating a localization of the induced holes. An evaluation of the measured spectral function does not indicate a diverging effective mass or scattering rate near optimal doping. Thus the present ARPES results indicate a continuous evolution of the quasiparticle interaction and therefore question previous quantum critical s…
Angle-resolved photoemission spectroscopy (ARPES) has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. Here from the ARPES perspective, we briefly review the main results from our group in the recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.
One of the interesting features of the photoemission spectra of the high temperature cuprate superconductors is the presence of a large signal (referred to as the "background'') in the unoccupied region of the Brillouin zone. Here we present data indicating that the origin of this signal is extrinsic and is most likely due to strong scattering of the photoelectrons. We also present an analytical method that can be used to subtract the background signal.
We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO_{0.7} and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO rev…
For solid-state materials, the electronic structure, E(k), is critical in determining a crystal's physical properties. By experimentally detecting the electronic structure, the fundamental physics can be revealed. Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for directly observing the electronic structure with energy- and momentum-resolved information. Over the past decades, major improvements in the energy and momentum resolution, alongside the extension of ARPES observables to spin (SpinARPES), micrometer or nanometer lateral dimensions (MicroARPES/NanoARPES), an…
Angle resolved photoemission spectroscopy (ARPES) has been commonly applied to evaluate the shape of Fermi surfaces by employing simple criteria for the determination of the Fermi vector k_F parallel to the surface such as maximum photoemission intensity at the Fermi level or discontinuity in the momentum distribution function. Here we show that these criteria may lead to large uncertainties in particular for narrow band systems. We develop a reliable method for the determination of Fermi vectors employing high resolution ARPES at different temperatures. The relevance and accuracy of the metho…
The electronic structure near the Fermi level (EF) of Ba1-xKxFe2As2 (BaK122 ; x = 0.2 - 0.7) is studied using laser ultrahigh-resolution angle-resolved photoemission spectroscopy(ARPES). For the optimally doped case of x = 0.4, we clearly observe two peaks below Tc in the ARPES spectra at a binding energies (BE) of 5 meV and 13meV. The former is assigned to a superconducting (SC) coherence peak since it appears and evolves below the bulk SC transition at Tc (= 36 K), accompanying a gap opening centered at EF. In contrast, the latter peak, which appears below ~ 90 K without any gap formation, i…
We study the superconducting(SC)-gap anisotropy of the Γ-centered hole Fermi surface in optimally doped FeTe_{0.6}Se_{0.4} (T_c = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (ARPES). We observe sharp superconducting coherence peaks at T = 2.5 K. In contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a \cos(4φ) modulation of the SC-gap anisotropy. The observed SC-gap anisotropy strongly indicates that the pairing interaction is not a conventional phonon-mediated isotropic one. Instead, the results suggest the impo…
In this paper we apply the emerging- consensus understanding of the fermionic self energy deduced from angle resolved photoemisssion spectroscopy (ARPES) experiments to deduce the implications for orbital diamagnetism in the underdoped cuprates. Many theories using many different starting points have arrived at a broadened BCS-like form for the normal state self energy associated with a d-wave excitation gap, as is compatible with ARPES data. Establishing compatibility with the f-sum rules, we show how this self energy, along with the constraint that there is no Meissner effect in the normal p…
