High Energy Dispersions in Bi2Sr2CaCu2O8 High Temperature Superconductor from Laser-Based Angle-Resolved Photoemission
Wentao Zhang, Guodong Liu, Jianqiao Meng, Lin Zhao, Haiyun Liu, Xiaoli Dong, Wei Lu, J. S. Wen, Z. J. Xu, G. D. Gu, T. Sasagawa, Guiling Wang, Yong Zhu, Hongbo … 展开作者
Wentao Zhang, Guodong Liu, Jianqiao Meng, Lin Zhao, Haiyun Liu, Xiaoli Dong, Wei Lu, J. S. Wen, Z. J. Xu, G. D. Gu, T. Sasagawa, Guiling Wang, Yong Zhu, Hongbo Zhang, Yong Zhou, Xiaoyang Wang, Zhongxian Zhao, Chuangtian Chen, Zuyan Xu, X. J. Zhou
Super-high resolution laser-based angle-resolved photoemission (ARPES) measurements have been carried out on the high energy electron dynamics in Bi2Sr2CaCu2O8 (Bi2212) high temperature superconductor. Momentum dependent measurements provide new insights on the nature of high energy kink at 200~400 meV and high energy dispersions. The strong dichotomy between the MDC- and EDC-derived bands is revealed which raises critical issues about its origin and which one represents intrinsic band structure. The MDC-derived high energy features are affected by the high-intensity valence band at higher binding energy and may not be intrinsic.
Comment on "Circular Dichroism in the Angle-Resolved Photoemission Spectrum of the High-Temperature Bi2Sr2CaCu2O8 Superconductor: Can These Measurements Be Interpreted as Evidence for Time-Reversal Symmetry Breaking?"
We review angle resolved photoemission spectroscopy (ARPES) results on the high Tc superconductors, focusing primarily on results obtained on the quasi-two dimensional cuprate Bi2Sr2CaCu2O8 and its single layer counterpart Bi2Sr2CuO6. The topics treated include the basics of photoemission and methodologies for analyzing spectra, normal state electronic structure including the Fermi surface, the superconducting energy gap, the normal state pseudogap, and the electron self-energy as determined from photoemission lineshapes.
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 analyzed scanning tunneling spectra of two electron-doped cuprates Pr0.88LaCe0.12CuO4 (Tc = 21 K and 24 K) and compared them with tunneling spectrum of hole-doped La1.84Sr0.16CuO4 and effective electron-boson spectral function of hole-doped La1.97Sr0.03CuO4 (extracted from angle-resolved photoemission spectrum). We have also analyzed tunneling spectra and angle-resolved photoemission spectra for hole-doped Bi2Sr2CaCu2O8. These results unambiguously rule out magnetic pairing mechanism in both electron- and hole-doped cuprates and support polaronic/bipolaronic superconductivity in hole-d…
Recent developments in the techniques of ultrafast pump-probe photoemission have made possible the search for collective modes in strongly correlated systems out of equilibrium. Including inelastic scattering processes and a retarded interaction, we simulate time- and angle- resolved photoemission spectroscopy (trARPES) to study the amplitude mode of a d-wave superconductor, a collective mode excited through the nonlinear light-matter coupling to the pump pulse. We find that the amplitude mode oscillations of the d-wave order parameter occur in phase at a single frequency that is twice the qua…
Based on results from femtosecond time-resolved photoemission, we compare three different methods for determination of the electron-phonon coupling constant λ in Eu and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small λ < 0.2. This makes simple electron-phonon mediated superconductivity unlikely in these compounds.
We have investigated the electronic structures of newly discovered superconductor FeSe1-x by bulk-sensitive photoemission spectroscopy (PES). The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF) and it decreases steeply toward EF . Compared with results of band structure calculations, narrowing the Fe 3d band width and the energy shift of the band toward EF are found, suggesting a mass enhancement due to the weak electron correlation effect. Meanwhile, Fe 2p core-level PES reveals a strong itinerant character of Fe 3d electrons. These features are very similar to …
We conclude that arguments of Norman et al. in their Comment do not provide a significant basis for their claim that the geometric mechanism for explaining the observations reported by them is not viable. More generally, our study highlights the importance of assessing structural issues before invoking exotic mechanisms for explaining unusual spectroscopic observations, especially in complex materia.
The electron-doped infinite-layer superconductor Sr0.9La0.1CuO2 is studied with x-ray photoemission spectroscopy (XPS). A nonaqueous chemical etchant is shown to effectively remove contaminants and to yield surfaces from which signals intrinsic to the superconductor dominate. These data are compared to measurements from hole-doped La1.85Sr0.15CuO4, from undoped La2CuO4, and from electron-doped Nd1.85Ce0.15CuO4-d. The Cu 2p core level is consistent with a lower value of the O 2p to Cu 3d charge transfer energy D than in hole-doped cuprates. A clear Fermi edge is observed in the valence band reg…
In this paper, we review some of the work our group has done in the past few years to obtain the electron self-energy of high temperature superconductors by analysis of angle-resolved photoemission data. We focus on three examples which have revealed: (1) a d-wave superconducting gap, (2) a collective mode in the superconducting state, and (3) pairing correlations in the pseudogap phase. In each case, although a novel result is obtained which captures the essense of the data, the conventional physics used leads to an incomplete picture. This indicates that new physics needs to be developed to …
Using the non-equilibrium Keldysh formalism, we solve the equations of motion for electron-phonon superconductivity, including an ultrafast pump field. We present results for time-dependent photoemission spectra out of equilibrium which probes the dynamics of the superconducting gap edge. The partial melting of the order by the pump field leads to oscillations at twice the melted gap frequency, a hallmark of the Higgs or amplitude mode. Thus the Higgs mode can be directly excited through the nonlinear effects of an electromagnetic field and detected without any additional symmetry breaking.
We performed high-resolution angle-resolved photoemission spectroscopy on triple-layered high-Tc cuprate Bi2Sr2Ca2Cu3O10+delta. We have observed the full energy dispersion (electron and hole branches) of Bogoliubov quasiparticles and determined the coherence factors above and below EF as a function of momentum from the spectral intensity as well as from the energy dispersion based on BCS theory. The good quantitative agreement between the experiment and the theoretical prediction suggests the basic validity of BCS formalism in describing the superconducting state of cuprates.
Cobalt oxide superconductor NaxCoO2.yH2O is studied by angle-resolved photoemission spectroscopy (ARPES). We report the Fermi surface (FS) topology and electronic structure near the Fermi level (EF) of NaxCoO2.yH2O. Our result indicates the presence of the hexagonal FS centered at G point, while the small pocket FSs along G-K direction are absent similar to NaxCoO2. The top of the eg' band, which is expected in band calculations to form the small pocket FSs, extends to within 20 - 50 meV below EF, more closer to EF than in NaxCoO2. We discuss its possible role in superconductivity, comparing w…
We present the development of a high-precision sample-position-autocorrection system for photoemission experiments. A binocular vision method based on image pattern matching calculations was realized to track the sample position with an accuracy better than 1 \um, which was much smaller than the spot size of the incident laser. We illustrate the performance of the sample-position-autocorrection system with representative photoemission data on the topological insulator Bi$_2$Se$_3$ and an optimally-doped cuprate superconductor \Bi. Our method provides new possibilities for studying the temperat…
We review recent work on the theory for pump/probe photoemission spectroscopy of electron-phonon mediated superconductors in both the normal and the superconducting states. We describe the formal developments that allow one to solve the Migdal-Eliashberg theory in nonequilibrium for an ultrashort laser pumping field, and explore the solutions which illustrate the relaxation as energy is transferred from electrons to phonons. We focus on exact results emanating from sum rules and approximate numerical results which describe rules of thumb for relaxation processes. In addition, in the supercondu…
Nodal quasiparticles of Bi2Sr2CaCu2O8 have been studied by angle-resolved photoemission spectroscopy with high momentum and energy resolution. Low-energy tunable photons have enabled us to resolve a small nodal bilayer splitting, unmasking the intrinsic single-particle scattering rate. The nodal scattering rate is abruptly suppressed upon the superconducting transition, and shows a linear energy dependence at low energies, indicating the nontrivial effect of elastic scatterings on the quasiparticles. With increasing energy, the antibonding-band scattering rate becomes higher than the bonding o…
We have investigated the low-energy electronic structure of the strongly correlated one-dimensional copper oxide chain compound SrCuO2 by angle resolved photoemission as a function of excitation energy. In addition to the prominent spinon-holon continuum we observe a peaklike and dispersive feature at the zone boundary. By fine-tuning the experimental parameters we are able to monitor the full holon branch and to directly measure the electronic hopping parameter with unprecedented accuracy.
We introduce a simple phenomenological form for the self-energy which allows us to extract important information from angle resolved photoemission data on the high Tc superconductor Bi2212. First, we find a rapid suppression of the single particle scattering rate below Tc for all doping levels. Second, we find that in the overdoped materials the gap Delta at all k-points on the Fermi surface has significant temperature dependence and vanishes near Tc. In contrast, in the underdoped samples such behavior is found only at k-points close to the diagonal. Near (pi,0), Delta is essentially T-indepe…
We demonstrate that the photo-electron counting rate, $P^{(2)}$, measured in two electron coincidence spectroscopy (2$e$-ARPES) experiments, provides unprecedented insight into the nature of topological superconductivity. In particular, we show that the spin dependence of $P^{(2)}$ allows one to detect superconducting spin-triplet correlations that are induced in a topological superconductor even in the absence of an associated triplet superconducting order parameter. This ability to detect spin-triplet correlations allows one to distinguish between two recently proposed scenarios for the micr…
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) has been performed on the single- to triple-layered Bi-family high-{\it T$_c$} superconductors (Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4}$, $\it{n}$=1-3). We found a sharp quasiparticle peak as well as a pseudogap at the Fermi level in the triple-layered compound. Comparison among three compounds has revealed a universal rule that the characteristic energies of superconducting and pseudogap behaviors are scaled with the maximum {\it T$_c$}.
The self-energy $Σ({\bf k},ω)$, the fundamental function which describes the effects of many-body interactions on an electron in a solid, is usually difficult to obtain directly from experimental data. In this paper, we show that by making certain reasonable assumptions, the self-energy can be directly determined from angle resolved photoemission data. We demonstrate this method on data for the high temperature superconductor $Bi_2Sr_2CaCu_2O_{8+x}$ (Bi2212) in the normal, superconducting, and pseudogap phases.
Angle-resolved photoemission spectroscopy is performed on single crystals of the single-layer high-Tc superconductor Bi(2)Sr(2-x)La(x)CuO(6+d) at optimal doping (x=0.4) in order to study in great detail the Zhang-Rice (ZR) singlet band at the Fermi level. Besides the high crystal quality the advantages of a single-layer material are the absence of bilayer effects and the distinct reduction of thermal broadening. Due to the high energy and angle resolution and, most important, due to the controlled variation of the polarization vector of the synchrotron radiation the emission from the ZR single…
Our valence band photoelectron spectroscopic studies show a temperature dependent spectral weight transfer near the Fermi level in the Fe-based superconductor FeSe1-xTex. Using theoretical band structure calculations we have shown that the weight transfer is due to the temperature induced changes in the Fe(Se,Te)4 tetrahedra. These structural changes lead to shifts in the electron occupancy from the xz/yz and x2-y2 orbitals to the 3z2-r2 orbitals indicating a temperature induced crossover from a metallic state to an Orbital Selective Mott (OSM) Phase. Our study presents the observation of a te…
High resolution laser-based angle-resolved photoemission measurements have been carried out on Bi2Sr2CuO6+d superconductor covering a wide doping range from heavily underdoped to heavily overdoped samples. Two obvious energy scales are identified in the nodal dispersions: one is the well-known 50-80 meV high energy kink and the other is <10 meV low energy kink. The high energy kink increases monotonously in its energy scale with increasing doping and shows weak temperature dependence, while the low energy kink exhibits a non-monotonic doping dependence with its coupling strength enhanced sharp…
We show that the so-called kinks and waterfalls observed in angle-resolved photoemission spectra of La2-xSrxCuO4, a prototypical high-Tc superconducting cuprate, result from the coupling of quasiparticles with two distinct nearly critical collective modes with finite characteristic wave vectors, typical of charge and spin fluctuations near a stripe instability. Both phonon-like charge and spin collective modes are needed to account for the kinked quasiparticle dispersions. This clarifies the long-standing question whether kinks are due to phonons or spin waves and the nature of the bosonic med…
We have performed in-situ angle-resolved photoemission spectroscopy on the electron doped cuprate superconductor La2-xCexCuO4 (x = 0.1) using high-quality films grown by molecular beam epitaxy. The obtained photoemission spectra were free from dirt peaks. In addition, the time evolution of the spectra due to impurity adsorption is rather gradual, and the sample surface is stable even at ambient temperature, which is in contrast with previous reports on bulk Nd2-xCexCuO4 (NCCO) single crystals. The energy distribution curves show clear dispersions near the Fermi energy (EF) with some EF crossin…
In order to study the possible superconductivity at the polar surfaces of 1111-type iron-based superconductors, which is doped with a large amount of holes in spite of the electron doping in bulk materials, we have performed angle-resolved photoemission spectroscopy (ARPES) studies on superconducting PrFeAsO$_{1-y}$~crystals. We have indeed observed the opening of a superconducting gap on surface-derived hole pockets as well as on a bulk-derived hole pocket. The superconducting gap is found to open on the surface-derived hole pockets below the bulk $T_c$, which suggests that the surface superc…
We present the first angle-resolved photoemission measurement on the single-layer Hg-based cuprate, HgBa$_2$CuO$_{4+δ}$ (Hg1201). A quasi-particle peak in the spectrum and a kink in the band dispersion around the diagonal of the Brillouin zone are observed, whereas no structure is detected near the Brillouin zone boundary. To search for a material-dependent trend among hole-doped cuprates, including Hg1201, we use a tight-binding model to fit their Fermi surfaces. We find a positive correlation between the $T_{c,\mathrm{max}}$ and $t'/t$, consistent with theoretical predictions.
We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (pi, 0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to be anisotropic, with the largest value near the intersecting point of the Fermi surface and the AF zone boundary. In addition, we observed that the QP peak disappears around the Neel temperature (TN) while the AF pseudogap is gradually filled up at …
We use microprobe Angle-Resolved Photoemission Spectroscopy to study the Fermi surface and band dispersion of the CuO2 planes in the high temperature superconductor, YBa2Cu4O8. We find a strong in-plane asymmetry of the electronic structure between directions along a and b axes. The saddle point of the antibonding band lies at a significantly higher energy in the a direction (pi,0) than the b direction (0,pi), whereas the bonding band displays the opposite behavior. We demonstrate that the abnormal band shape is due to a strong asymmetry of the bilayer band splitting, likely caused by a non-tr…
We have performed x-ray photoemission spectroscopy on the system of noncentrosymmetric superconductor, Li$_2$(Pd$_x$Pt$_{1-x}$3)B. For Li$_2$Pt$_3$B, we found 2 major peaks with 2 other weak components, and the band calculations were in agreement with the observation. The assignment of valence band features using the calculated partial density of states determined that Pt 5d and B 2p contribute to the density of states at the Fermi level. The effect of antisymmetric spin-orbit coupling on the band structure might have been probed, and the analysis on the effect of Pt incorporation into the sys…
The angle-resolved photoemission spectra of the superconductor (Ba$_{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…
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.
Rigorous relations for Gutzwiller projected BCS states are derived. The obtained results do not depend on the details of model systems, but solely on the wave functions. Based on the derived relations, physical consequences are discussed for strongly correlated superconducting states such as high-$T_{\rm C}$ cuprate superconductors.
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.
This paper summarizes experimental results presented at the international conference honoring Prof. C.N. Yang's 80th birthday. I show seven examples that illustrate how one can use angle-resolved photoemission spectroscopy to gain insights into the many-body physics responsible for the rich phase diagram of cuprate superconductors. I hope to give the reader a snapshot of the evolution of this experimental technique from a tool to study chemical bonds and band structure to an essential many-body spectroscopy for one of the most important physics problems of our time.
Recently, the time-reversal violation predicted for the pseudogap phase of the cuprates, which was observed by dichroism experiments using Angle-Resolved Photoemission has also been observed by polarized neutron diffraction. Earlier derivation of dichroism in angle resolved photoemission due to time-reversal violation relied on existence of mirror planes in the crystal. Here the theory of the effect is generalized to the case that mirror plane symmetry is weakly violated due to perturbing potentials such as a superstructure.
High-resolution laser-based angle-resolved photoemission measurements have been carried out on the electron-doped (Nd$_{1.85}$Ce$_{0.15}$)CuO$_4$ high temperature superconductor. We have revealed a clear kink at $\sim$60 meV in the dispersion along the (0,0)-($π$,$π$) nodal direction, accompanied by a peak-dip-hump feature in the photoemission spectra. This indicates that the nodal electrons are coupled to collective excitations (bosons) in electron-doped superconductors, with the phonons as the most likely candidate of the boson. This finding has established a universality of nodal electron c…
Electronic evolution from an antiferromagnet to a high-Tc superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca(1.9)Na(0.1)CuO(2)Cl(2) single crystals, which were successfully grown for the first time under high pressures. In this underdoped superconductor, we found clear fingerprints of the parent insulator: a shadow band and a large pseudo-gap. These observations are most likely described by a "chemical potential shift", which contrasts clearly with the prevailing wisdom of the "pinned chemical potential" learned from the prototype La(2-x)Sr(x)CuO(4), demons…
This treatise reviews latest results obtained from angle-resolved photoemission spectroscopy (ARPES) on cuprate superconductors, with a special focus on the electron-phonon interaction. What has emerged is rich information about the anomalous electron-phonon interaction well beyond the traditional views of the subject. It exhibits strong doping, momentum and phonon symmetry dependence, and shows complex interplay with the strong electron-electron interaction in these materials.
Recently there has been an accumulation of experimental evidence in the high temperature superconductors suggesting the relevance of electron-phonon coupling in these materials. These findings challenge some well-held beliefs of what electron-phonon interactions can and cannot do. In this article we review evidence primarily from angle-resolved photoemission (ARPES) measurements which point out the importance of electronic coupling to certain phonon modes in the cuprates.
Superconducting excitations -- Bogoliubov quasiparticles -- are the quantum mechanical mixture of negatively charged electron (-e) and positively charged hole (+e). We propose a new observable for Angular Resolved Photoemission Spectroscopy (ARPES) studies that is the manifestation of the particle-hole entanglement of the superconducting quasiparticles. We call this observable a {\em Bogoliubov angle}. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We show how this quantity can be measured by comparing the ratio of spec…
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.
Angle-resolved photoemission spectra present two challenges for the d-density wave (DDW) theory of the pseudogap state of the cuprates: (1) hole pockets near $(π/2,π/2)$ are not observed, in apparent contradiction with the assumption of translational symmetry breaking, and (2) there are no well-defined quasiparticles at the {\it antinodal} points, in contradiction with the predictions of mean-field theory of this broken symmetry state. Here, we show how these puzzles can be resolved.
We have performed angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2 (Tc = 4.1 K), which is isostructural to the 122-type Fe-pnictide superconductor with the pnictogen atom being replaced with boron. We found that the Fermi level is located at a dip in the density of states (DOS) in contrast to Co-pnictide ferromagnets. This reduction in DOS together with the strong Co 3d-B 2p covalent bonding removes the ferromagnetic order and may cause the superconductivity. The energy bands near the Fermi level show higher three dimensionality and a weaker electron…
Laser-based angle-resolved photoemission measurements with super-high resolution have been carried out on an optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. New high energy features at $\sim$115 meV and $\sim$150 meV, besides the prominent $\sim$70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which can not be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.
Angle resolved photoemission spectroscopy (ARPES) has been commonly applied to evaluate the shape of Fermi surfaces by employing simple criteria for the determination of the Fermi vector k_F parallel to the surface such as maximum photoemission intensity at the Fermi level or discontinuity in the momentum distribution function. Here we show that these criteria may lead to large uncertainties in particular for narrow band systems. We develop a reliable method for the determination of Fermi vectors employing high resolution ARPES at different temperatures. The relevance and accuracy of the metho…
We have performed an angle-resolved photoemission study of the electron-doped high-Tc superconductors (HTSCs) Sm_{1.85}Ce_{0.15}CuO_{4} (SCCO) and Eu_{1.85}Ce_{0.15}CuO_{4} (ECCO). Around the nodal point (pi/2, pi/2), we observed an energy gap at the Fermi level (EF) for both samples, resulting from the strong effects of antiferromagnetism. The magnitude of this gap in ECCO is larger than that in SCCO, implying that the effects of antiferromagnetism in ECCO are stronger than that in SCCO. Also, we quantitatively confirmed the effects of antiferromagnetism by the band dispersions to tight-bindi…
We introduce a formalism for calculating dynamic response functions using experimental single particle Green's functions derived from angle resolved photoemission spectroscopy (ARPES). As an illustration of this procedure we estimate the dynamic spin response of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We find good agreement with superconducting state neutron data, in particular the $(π,π)$ resonance with its unusual `reversed magnon' dispersion. We anticipate our formalism will also be of useful in interpreting results from other spectroscopies, such as optical and Raman resp…
Although nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in KxFe2-ySe2, whether its pairing symmetry is s-wave or nodeless d-wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in KxFe2-ySe2, which favors the s-wave pairing symmetry.
Time- and angle-resolved photoemission spectroscopy has played an important role in revealing the non-equilibrium electronic structures of solid-state materials. The implementation of high harmonic generation to obtain a higher photon energy also allows us to investigate the wide Brillouin zone on a time scale below 100 fs. In this article, we review our recent studies using high-harmonic-generation-laser-based time- and angle-resolved photoemission spectroscopy to study a variety of quantum materials. We reveal many unprecedented phenomena in each system and highlight some representative resu…
We amend ab initio strongly-correlated band structures by taking into account the band-tailing phenomenon in doped charge-transfer Mott-Hubbard insulators. We show that the photoemission from band tails accounts for sharp "quasi-particle" peaks, rapid loss of their intensities in some directions of the Brillouin zone ("Fermi-arcs") and high-energy "waterfall" anomalies as a consequence of matrix-element effects of disorder-localised states in the charge-transfer gap of doped cuprates.
Techniques in time- and angle-resolved photoemission spectroscopy have facilitated a number of recent advances in the study of quantum materials. We review developments in this field related to the study of incoherent nonequilibrium electron dynamics, the analysis of interactions between electrons and collective excitations, the exploration of dressed-state physics, and the illumination of unoccupied band structure. Future prospects are also discussed.
High resolution angle-resolved photoemission measurements have been carried out on (Sr,K)Fe$_2$As$_2$ superconductor (Tc=21 K). Three hole-like Fermi surface sheets are clearly resolved for the first time around the Gamma point. The overall electronic structure shows significant difference from the band structure calculations. Qualitative agreement between the measured and calculated band structure is realized by assuming a chemical potential shift of -0.2 eV. The obvious band renormalization suggests the importance of electron correlation in understanding the electronic structure of the Fe-ba…
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 have studied the electronic structure of $β$-Na$_{0.33}$V$_2$O$_5$, which becomes a superconductor under pressure, by angle-resolved photoemission spectroscopy. Clear band dispersions is observed only along the chain direction, indicating the quasi-one-dimensional (1D) electronic structure. The spectra of the V 3d band are dominated by a Gaussian-like broad feature at $\sim$ 1 eV below the Fermi level ($E_F$) with negligible intensity at $E_F$, which we attribute to strong electron-phonon coupling as in other 1D polaronic metals. In the momentum space, the spectra show a maximum intensity a…
In order to determine the orbital characters on the various Fermi surface pockets of the Fe-based superconductors Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ and FeSe$_{0.45}$Te$_{0.55}$, we introduce a method to calculate photoemission matrix elements. We compare our simulations to experimental data obtained with various experimental configurations of beam orientation and light polarization. We show that the photoemission intensity patterns revealed from angle-resolved photoemission spectroscopy measurements of Fermi surface mappings and energy-momentum plots along high-symmetry lines exhibit asymmet…
