The experimental finding of an ubiquitous kink in the nodal direction of angle-resolved photoemission spectroscopies of superconducting cuprates has been reproduced theoretically. Our model is built upon the Migdal-Eliashberg theory for the electron self-energy within the phonon-coupling scenario. Following this perturbative approach, a numerical evaluation of the bare band dispersion energy in terms of the electron-phonon coupling parameter $λ$ allows a unified description of the nodal-kink effect. Our study reveals that distinction between $λ$ and the technically defined mass-enhancement parameter $λ^{*}$ is relevant for the quantitative description of data, as well as for a meaningful interpretation of previous studies.
A remarkable agreement between theory and experiment has been achieved for different samples and at different doping levels. The full energy spectrum is covered in the case of LSCO, Bi2212 and overdoped Y123. In the case of underdoped Y123, the model applies to the low energy region (close to Fermi level).
The theoretical model for the quasiparticle self-energy in HTSC is proposed, which is based on the forward scattering peak in the electron-phonon (EPI) interaction. By assuming that EPI dominates, the model explains qualitatively and in a consistent way the recent ARPES results. The latter show a kink in the normal state quasiparticle energy at 70 meV in the nodal direction, which is (surprisingly) not shifted in the superconducting (SC) state, while the singularity at 40 meV in the anti-nodal direction is shifted by the SC gap. The model predicts a dip-hump structure in the spectral function,…
We apply the new-generation ARPES methodology to the most widely studied cuprate superconductor YBCO. Considering the nodal direction, we found noticeable renormalization effects known as "kinks" both in the quasiparticle dispersion and scattering rate, the bilayer splitting and evidence for strong interband scattering -- all the characteristic features of the nodal quasiparticles detected earlier in BSCCO. The typical energy scale and the doping dependence of the "kinks" clearly point to their intimate relation with the spin-1 resonance seen in the neutron scattering experiments. Our findings…
One of the possible mechanisms of high Tc superconductivity is Cooper pairing with the help of bosons, which change the slope of the electronic dispersion as observed by photoemission. Giustino et al. calculated that in the high temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations (phonons) should have a negligible effect on photoemission spectra and concluded that phonons do not play an important role. We show that the calculations employed by Giustino et al. fail to reproduce huge influence of electron-phonon coupling on important phonons observed in experiments. Thus one w…
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$) nodal direction, accompanied by a peak-dip-hump feature in the photoemission spectra. This indicates that the nodal electrons are coupled to collective excitations (bosons) in electron-doped superconductors, with the phonons as the most likely candidate of the boson. This finding has established a universality of nodal electron c…
Eliashberg theory generalized for the account of the electron-hole nonequivalence and electron correlations in the vertex function is used. The phonon contribution to the nodal electron Green function in cuprates is viewed. At non- zero temperatures the singularities (kinks) in the frequency behavior of a real and imaginary part of an electron nodal Green function, and also in the nodal part of the density of the electron states modified by an electron-phonon interaction are studied. It is shown that near the optimal doping both the low-energy and high-energy nodal Green function kinks and als…
We report the first measurement of the optical phonon dispersion in optimally doped single layer Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a strong softening of the Cu-O bond stretching phonon at about q=(0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A direct comparison with angle-resolved photoemission spectroscopy measurements taken on the same sample, revealed an excellent agreement in terms of energy and momentum between the ARPES nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where…
High-$T_c$ cuprate superconductors are characterized by a strong momentum-dependent anisotropy between the low energy excitations along the Brillouin zone diagonal (nodal direction) and those along the Brillouin zone face (antinodal direction). Most obvious is the d-wave superconducting gap, with the largest magnitude found in the antinodal direction and no gap in the nodal direction. Additionally, while antinodal quasiparticle excitations appear only below $T_c$, superconductivity is thought to be indifferent to nodal excitations as they are regarded robust and insensitive to $T_c$. Here we r…
Motivated by the observation in copper-oxide high-temperature superconductors, we investigate the appearance of kinks in the electronic dispersion due to coupling to phonons for a system with strong electronic repulsion. We study a Hubbard model supplemented by an electron-phonon coupling of Holstein type within Dynamical Mean Field Theory (DMFT) utilizing Numerical Renormalization Group as impurity solver. Paramagnetic DMFT solutions in the presence of large repulsion show a kink only for large values of the electron-phonon coupling $λ$ or large doping and, contrary to the conventional electr…
The high-energy kink or the waterfall effect seen in the photoemission spectra of the cuprates is suggestive of the coupling of the quasiparticles to a high energy bosonic mode with implications for the mechanism of superconductivity. Recent experiments however indicate that this effect may be an artifact produced entirely by the matrix element effects, i.e. by the way the photoemitted electron couples to the incident photons in the emission process. In order to address this issue directly, we have carried out realistic computations of the photo-intensity in ${\rm Bi_2Sr_2CaCu_2O_8}$ (Bi2212) …
The improved resolution of laser-based angle-resolved photoemission spectroscopy (ARPES) allows reliable access to fine structures in the spectrum. We present a systematic, doping-dependent study of a recently discovered low-energy kink in the nodal dispersion of Bi2Sr2CaCu2O8+d (Bi-2212), which demonstrates the ubiquity and robustness of this kink in underdoped Bi-2212. The renormalization of the nodal velocity due to this kink becomes stronger with underdoping, revealing that the nodal Fermi velocity is non-universal, in contrast to assumed phenomenology. This is used together with laser-ARP…
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…
Despite over two decades of intense research efforts, the origin of high-temperature superconductivity in the copper oxides remains elusive. Angle-resolved photoemission experiments (ARPES) revealed a kink in the dispersion relations (energy vs. wavevector) of electronic states in the cuprates at binding energies of 50-80 meV, and raised the hope that this anomaly could be key to understanding high-temperature superconductivity. The kink is often interpreted in terms of interactions between the electrons and a bosonic field. While there is no consensus on the nature of the bosons or even wheth…
We use time- and angle-resolved photoemission to measure the nodal non-equilibrium electronic states in various dopings of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We find that the initial pump-induced transient signal of these ungapped states is strongly affected by the onset of the superconducting gap at $T_c$, superconducting pairing fluctuations at $T_p$, and the pseudogap at $T^*$. Moreover, $T_p$ marks a suggestive threshold in the fluence-dependent transient signal, with the appearance of a critical fluence below $T_p$ that corresponds to the energy required to break apart all Cooper pairs. These…
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…
Samples of Nd(2-x)Ce(x)CuO(4), an electron-doped high temperature superconducting cuprate (HTSC), near optimal doping at x = 0.155 were measured via angle resolved photoemission (ARPES). We report a renormalization feature in the self energy ("kink") in the band dispersion at 50 - 60 meV present in nodal and antinodal cuts across the Fermi surface. Specifically, while the kink had previously only been seen in the antinodal region, it is now observed also in the nodal region, reminiscent of what has been observed in hole-doped cuprates.
The origin of a ubiquitous bosonic coupling feature in the photoemission spectra of high-Tc cuprates, an energy-momentum dispersion 'kink' observed at ~70 meV binding energy, remains a two-decade-old mystery. Understanding this phenomenon requires an accurate description of the coupling between the electron and some collective modes. We report here ab initio calculations based on GW perturbation theory and show that correlation-enhanced electron-phonon interaction in cuprates gives rise to the strong kinks, which not only explains quantitatively the observations but provides new understanding …
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…
HgBa$_{2}$CuO$_{4+δ}$ (Hg1201) has been shown to be a model cuprate for scattering, optical, and transport experiments, but angle-resolved photoemission spectroscopy (ARPES) data are still lacking owing to the absence of a charge-neutral cleavage plane. We report on progress in achieving the experimental conditions for which quasiparticles can be observed in the near-nodal region of the Fermi surface. The d-wave superconducting gap is measured and found to have a maximum of 39 meV. At low temperature, a kink is detected in the nodal dispersion at approximately 51 meV below the Fermi level, an …
Performing an in-depth analysis of the photoemission spectra along the nodal direction of the high temperature superconductor Bi-2212 we have developed a procedure to determine the underlying electronic structure and established a precise relation of the measured quantities to the real and imaginary parts of the self-energy of electronic excitations. The self-consistency of the procedure with respect to the Kramers-Kronig transformation allows us to draw conclusions on the applicability of the spectral function analysis and on the existence of well defined quasiparticles along the nodal direct…
We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+δ}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($Γ$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energ…
The ARPES spectra in high-Tc superconductors show four distinctive features in the quasiparticle self-energy. All of them can be explained consistently by the theory in which the electron phonon interaction (EPI) with the forward scattering peak dominates over the Coulomb scattering. In particular, this theory explains why there is no shift of the nodal kink at 70 meV in the superconducting state, contrary to the clear shift of the anti-nodal singularity at 40 meV. The theory predicts a ``knee''-like structure of the imaginary part of the self-energy, which is phonon dominated for $ω\approx ω^…
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 analyze the effects of a momentum-dependent self-energy on the photoemission momentum distribution curve (MDC) lineshape, dispersion and linewidth. We illustrate this general analysis by a detailed examination of nodal quasiparticles in high Tc cuprates. We use variational results for the nodal quasiparticle weight Z (which varies rapidly with hole doping x) and the low energy Fermi velocity $v_F^{low}$ (which is independent of x), to show that the high energy MDC dispersion $v_{high} = v_F^{low}/Z$, so that it is much larger than the bare (band structure) velocity and also increases strong…
We present detailed energy dispersions near the Fermi level on the monolayer perovskite ruthenate Sr2RuO4, determined by high-resolution angle-resolved photoemission spectroscopy. An orbital selectivity of the kink in the dispersion of Sr2RuO4 has been found: A kink for the Ru 4d_xy orbital is clearly observed, but not for the Ru 4d_yz and 4d_zx ones. The result provides insight into the origin of the kink.
We performed angle-resolved photoemission spectroscopy with micro-focused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc~1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the G point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication conditio…
The kink in the dispersion and the drop in the width observed by angle-resolved photoemission in the nodal direction of the Brillouin zone of $\mathrm{Bi_{2-x}Pb_xSr_2CaCu_2O_{8+ δ}}$ (abbreviated as (Pb)Bi2212) has attracted broad interest [1-3]. Surprisingly optimally lead-doped (Pb)Bi2212 with $\mathrm{T_C>89K}$ as well as the shadow band were not investigated so far, although the origin of the kink and the drop is still under strong debate. In this context a resonant magnetic-mode scenario and an electron-phonon coupling scenario are discussed controversially. Here we analyze the relevant …
Using a precursor superconductivity scenario for the cuprates we present a theory for the temperature dependent behavior of the spectral gaps associated with four distinct spectroscopies: angle resolved photoemission (ARPES), differential conductance $dI/dV$, quasi-particle interference spectroscopy, and the autocorrelated ARPES pattern. We find good agreement for a range of existing experiments and make predictions for others. Our theory, which incorporates the necessary (observed) contrast between the nodal and anti-nodal response, shows how different nodal gap shapes are associated with the…
We investigate the electronic dispersion of high-Tc superconductor on the basis of the two-dimensional three-band Hubbard model with the electron-phonon interaction together with the strong electron-electron interaction. In our model, it is shown across the hole-doped region of high-Tc superconductor that the electron-phonon interaction makes a dispersion kink, observed along the nodal direction, and that the small isotope effect appears on the electronic dispersion.
We have performed high resolution angle resolved photoemission (ARPES) studies on electron doped cuprate superconductors Sm2-xCexCuO4 (x=0.10, 0.15, 0.18), Nd2-xCexCuO4 (x=0.15) and Eu2-xCexCuO4 (x=0.15). Imaginary parts of the electron removal self energy show step-like features due to an electron-bosonic mode coupling. The step-like feature is seen along both nodal and anti-nodal directions but at energies of 50 and 70 meV, respectively, independent of the doping and rare earth element. Such energy scales can be understood as being due to preferential coupling to half- and full-breathing mod…
Doping Evolution of Nodal Electron Dynamics in Trilayer Cuprate Superconductor Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy
Here we present the details of a self-consistent procedure of the photoemission data analysis within the self-energy approach introduced in Ref.1 (cond-mat/0405696). We derive the relations of the quasiparticle self-energy with the parameters provided by photoemission spectra, demonstrate self-consistency of the Kramers-Kronig procedure and its robustness in determination of the bare dispersion, examine the possible influence of a particle-hole asymmetry, discuss the necessity of a clear definition of the "kink", and demonstrate the applicability of the developed approach for a couple of sampl…
Using low-photon energy angle-resolved photoemission (ARPES), we study the low-energy dispersion along the nodal (pi, pi) direction in Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveals a novel "kink"-like feature in the real part of the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below E_F. This new kink is strongest below the superconducting critical temperature and weakens substantially as the temperature is raised. A corollary of this finding is that the Fermi …
We report the origin of a high-energy kink structure of heavily electron-doped nonsuperconducting Li$_{1+x}$FeAs observed by three-dimensional angle-resolved photoemission spectroscopy. The $d_{xy}$ orbital at the center of the Brillouin zone is strongly renormalized, indicating stronger electron correlation, exceeding that of stoichiometric LiFeAs despite the fact that the orbital characters of all bands remain unchanged. Two types of kink structure on the $d_{yz}$ band were identified: an isotropic kink at the binding energy of 20 meV, and another kink located at 100 meV observed only in the…
Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in details the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed "70 meV kink" on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below T* and sharpening below Tc, and is the main responsible for the formatio…
The sudden change of the velocity, so-called "kink," of the dispersing peak in angle resolved photoelectron spectroscopy is a well-known feature in the high temperature superconducting cuprates. Currently, the origin of the kink is being much debated, but a consensus has not emerged yet. Here, we present a study of the momentum evolution of the kink structure from the nodal region towards the anti-nodal region, for optimally doped Bi2212 sample. We show that the observed temperature dependence of the kink structure in both regions of the momentum space is consistent with a scenario in which ph…
Ultrahigh resolution angle-resolved photoemission spectroscopy with low-energy photons is used to study the detailed momentum dependence of the well-known nodal "kink" dispersion anomaly of Bi2Sr2CaCu2O8+δ. We find that the kink's location transitions smoothly from a maximum binding energy of about 65 meV at the node of the d-wave superconducting gap to 55 meV roughly one-third of the way to the antinode. Meanwhile, the self-energy spectrum corresponding to the kink dramatically sharpens and intensifies beyond a critical point in momentum space. We discuss the possible bosonic spectrum in ener…
We investigate the electronic dispersion of the high-T$_{\mathrm{c}}$ cuprate superconductors using the fully self-consistent version of the phenomenological model, where charge planar quasiparticles are coupled to spin fluctuations. The inputs we use ---the underlying (bare) band structure and the spin susceptibility $χ$--- are extracted from fits of angle resolved photoemission and inelastic neutron scattering data of underdoped YBa$_{2}$Cu$_{3}$O$_{6.6}$ by T. Dahm and coworkers (T. Dahm et al., Nat. Phys. 5, 217 (2009)). Our main results are: (i) We have confirmed the finding by T. Dahm an…
We show how, within a preformed pair scenario for the cuprate pseudogap, the nodal and antinodal responses in angle resolved photoemission spectroscopy necessarily have very different temperature $T$ dependences. We examine the behavior and the contrasting $T$ dependences for a range of temperatures both below and above $T_c$. Our calculations are based on a fully microscopic $T$-matrix approach for addressing pairing correlations in a regime where the attraction is stronger than BCS and the coherence length is anomalously short. Previously, the distinct nodal and anti-nodal responses have pro…
Electronic band dispersions in copper oxide superconductors have kinks around 70 meV that are typically attributed to coupling of electrons to a bosonic mode. We performed angle resolved photoemission spectroscopy (ARPES) experiments on overdoped cuprate high temperature superconductors to test the relationship between the superconducting transition temperature and electron-bosonic mode coupling. Remarkably, the kinks remain strong in the heavily overdoped region of the doping phase diagram of LSCO, even when the superconductivity completely disappears. This unexpected observation is incompati…
Here we report the first angle-resolved photoemission measurement on nearly optimally-doped multi-layer Tl-based superconducting cuprates (Tl-2212 and Tl-1223) and a comparison study to single layer (Tl-2201) compound. A "kink" in the band dispersion is found in all three compounds but exhibits different momentum dependence for the single layer and multi-layer compounds, reminiscent to that of Bi-based cuprates. This layer number dependent renormalization effect strongly implies that the spin resonance mode is unlikely responsible for the dramatic renormalization effect near the antinodal regi…
Studying the nodal quasiparticles in superconducting cuprates by photoemission with highly improved momentum resolution, we show that a new "kink" feature in the scattering rate is a key to uncover the nature of electron correlations in these compounds. Our data provide evidence that the main doping independent contribution to the scattering can be well understood in terms of the conventional Fermi liquid model, while the additional doping dependent contribution has a magnetic origin. This sheds doubt on applicability of a phonon-mediated pairing mechanism to high temperature superconductors.
Angle-resolved photoemission spectroscopy (ARPES) measurements have established the phenomenon of kink in band dispersion of high-$T_{\rm c}$ cuprate superconductors. However, systematic studies of the kink in electron-doped cuprates are still lacking experimentally. We performed $in$-$situ$ ARPES measurements on ${\rm La}_{2-x}{\rm Ce}_x{\rm CuO}_{4\pmδ}$ (LCCO) thin films over a wide electron doping ($n$) range from 0.05 to 0.23. While the nodal kink is nearly invisible, an antinodal kink around 45 meV, surviving above 200 K, is observed for $n\sim0.05-0.19$, whose position is roughly indepe…
Angle-resolved photoemission data on optimally- and under-doped high temperature superconductors reveal a dichotomy between the nodal and antinodal electronic excitations. In this paper we propose an explanation of this unusual phenomenon by employing the coupling between the quasiparticle and the commensurate/incommensurate magnetic excitations.
We study the systematic doping evolution of nodal dispersions by in-situ angle-resolved photoemission spectroscopy on the continuously doped surface of a high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. We reveal that the nodal dispersion has three segments separated by two kinks, located at ~10 meV and roughly 70 meV, respectively. The three segments have different band velocities and different doping dependence. In particular, the velocity of the high-energy segment increases monotonically as the doping level decreases and can even surpass the bare band velocity. We propose tha…
We present an overview of angle-resolved photoemission spectroscopy (ARPES) studies of high-temperature cuprate superconductors aiming at elucidating the relationship between the superconductivity, the pseudogap, and the Fermi arc. ARPES studies of underdoped samples show a momentum dependence of the energy gap below Tc which deviates from a simple d-wave form, suggesting the coexistence of multiple energy scales in the superconducting state. Hence, two distinct energy scales have been introduced, namely, the gap near the node (characterized by Delta_0) and in the anti-nodal region (characteri…
Recent angle resolved photoemission spectrascope (ARPES) experiments on strongly underdoped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ cuprates have reported an unusual gap in the nodal direction. Transport experiments on these cuprates found variable range hopping behavior observed. These cuprates have both electron and hole doping which has led to proposals that this cuprate is analogous to a partially compensated semiconductors. The nodal gap then corresponds to the Efros-Schklovskii(ES) gap in such semiconductors. We calculate the doping dependence and temperature dependence of a ES gap model and f…
Due to the orthorhombic distortion of the lattice, the electronic hopping integrals along the $a$ and $b$ diagonals, the orthorhombic directions, are slightly different. We calculate their difference in the LDA and find $t_{a}^{\prime}-t_{b}^{\prime}\approx 8 $meV. We argue that electron correlations in the insulating phase of La$_{2-x}$Sr$_{x}$CuO$_{4}$, i. e. at doping $x\leq 0.055,$ dramatically enhance the $(t_{a}^{\prime}-t_{b}^{\prime}) $-splitting between the $a$- and $b$-hole valleys. In particular, we predict that the intensity of both angle-resolved photoemission and of optical absor…
In the present photoemission study of a cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. The incoherent nature of the spectra suggests that the hopping-dominated dispersion occurs possibly with the assistance of local spin correlations. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate in the low energy region. These results recovered significant missing pieces in our cu…
Extending our previous studies we present results for the doping-, momentum-, frequency-, and temperature- dependence of the kink-like change of the quasiparticle velocity resulting from the coupling to spin fluctuations. In the nodal direction a kink is found in both the normal and superconducting state while in the antinodal direction a kink occurs only below $T_c$ due to the opening of the superconducting gap. A pronounced kink is obtained only for hole-doped, but not for electron-doped cuprates and is characteristically different from what is expected due to electron-phonon interaction. We…
The parent compound of the copper-oxide high temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high temperature superconductors and the superconductivity mechanism. Here we report high resolution angle-resolved photoemission measurement on heavily underdoped Bi2Sr2-xLaxCuO6+d system. The electronic structure of the lightly-doped samples exhibit a number of characteristics: existence of an energy gap…
We investigate the origin of ubiquitous low energy kinks found in Angle Resolved Photoemission (ARPES) experiments in a variety of correlated matter. Such kinks are unexpected from weakly interacting electrons and hence identifying their origin should lead to fundamental insights in strongly correlated matter. We devise a protocol for extracting the kink momentum and energy from the experimental data which relies solely on the two asymptotic tangents of each dispersion curve, away from the feature itself. It is thereby insensitive to the different shapes of the kinks as seen in experiments. Th…
A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to ten times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of th…
We have performed an angle-resolved photoemission spectroscopy (ARPES) study of Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$ (NCCO) in order to elucidate the origin of the high-energy kink (HEK) observed in the high-$T_{\rm c}$ superconductors (HTSCs). The energy scale of the HEK in NCCO is large compared with that in hole-doped HTSCs, consistent with previous ARPES studies. From measurement in a wide momentum region, we have demonstrated that between the hole- and electron-doped HTSCs the energy position of the HEK is shifted approximately by the amount of the chemical potential difference. Also, we have …
Using angle-resolved photoemission spectroscopy with ultimate momentum resolution we have explicitly resolved the bilayer splitting in the nodal direction of Bi-2212. The splitting is observed in a wide doping range and, within the experimental uncertainty, its size does not depend on doping. The value of splitting derived from the experiment is in good agreement with that from band structure calculations which implies the absence of any electronic confinement to single planes within bilayers of Bi-2212. Other consequences of this finding are also discussed.
We report quantum oscillations in underdoped YBa2Cu3O6.56 over a significantly large range in magnetic field extending from 24 to 101 T, enabling three well-spaced low frequencies at 440 T, 532 T, and 620 T to be clearly resolved. We show that a small nodal bilayer coupling that splits a nodal pocket into bonding and antibonding orbits yields a sequence of frequencies, F0 - ΔF, F0, and F0 + ΔF and accompanying beat pattern similar to that observed experimentally, on invoking magnetic breakdown tunneling at the nodes. The relative amplitudes of the multiple frequencies observed experimentally i…
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 …
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…
High resolution angle-resolved photoemission measurements on underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ system show that, at energies below 70 meV, the quasiparticle peak is well defined around the ($π$/2,$π$/2) nodal region and disappears rather abruptly when the momentum is changed from the nodal point to the ($π$,0) antinodal point along the underlying ``Fermi surface''. It indicates that there is an extra low energy scattering mechanism acting upon the antinodal quasiparticles. We propose that this mechanism is the scattering of quasiparticles across the nearly parallel segments of the Fermi sur…
In a conventional framework, superconductivity is lost at a critical temperature (T_c) because, at higher temperatures, gluing bosons can no longer bind two electrons into a Cooper pair. In high-T_c cuprates, it is still unknown how superconductivity vanishes at T_c. We provide evidence that the so-called <~70-meV kink bosons that dress the quasi-particle excitations are playing a key role in the loss of superconductivity in a cuprate. We irradiated a 170-fs laser pulse on Bi2Sr2CaCu2O8+δand monitored the responses of the superconducting gap and dressed quasi-particles by time- and angle-resol…
Lightly-doped La$_{2-x}$Sr$_x$CuO$_4$ in the so-called "insulating" spin-glass/diagonal-stripe phase has been studied by angle-resolved photoemission spectroscopy. A quasi-particle (QP) peak crossing the Fermi level has been observed in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface consistent with the recently observed metallic transport behavior at high temperatures of lightly-doped materials. The spectral weight of the nodal QP smoothly increases with hole doping, corresponding to the $n \sim x$ behavior of the carrier number in the underdoped and li…
