We present the development of a time- and angle-resolved photoemission spectroscopy based on a Yb-based femtosecond laser and a hemispherical electron analyzer. The energy of the pump photon is tunable between 1.4 and 1.9 eV, and the pulse duration is around 30 fs. We use a KBe$_2$BO$_3$F$_2$ non-linear optical crystal to generate probe pulses, of which the photon energy is up to 6.7 eV, and obtain an overall time resolution of 1 ps and energy resolution of 18 meV. In addition, $β$-BaB$_2$O$_4$ crystals are used to generate alternative probe pulses at 6.05 eV, giving an overall time resolution of 130 fs and energy resolution of 19 meV. We illustrate the performance of the system with representative data on several samples (Bi$_2$Se$_3$, YbCd$_2$Sb$_2$, FeSe).
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.
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.
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.
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.
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…
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.
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.
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…
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.
We present a study of the tetragonal to collapsed-tetragonal transition of CaFe2As2 using angle-resolved photoemission experiments and dynamical mean field theory-based electronic structure calculations. We observe that the collapsed-tetragonal phase exhibits reduced correlations and a higher coherence temperature due to the stronger Fe-As hybridization. Furthermore, a comparison of measured photoemission spectra and theoretical spectral functions shows that momentum-dependent corrections to the density functional band structure are essential for the description of low-energy quasiparticle dis…
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.
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.
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.
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…
By comparing single-particle spectral functions of t-J and Hubbard models with recent angle-resolved photoemission (ARPES) results for LSCO and Nd-LSCO, we can decide where holes go as a function of doping, and more specifically, which type of stripe (bond-, site-centered) is present in these materials at a given doping. For dopings greater than about 12% our calculation shows furthermore that the holes prefer to proliferate out of the metallic stripes into the neighboring antiferromagnetic domains. The spectra were calculated by a cluster perturbation technique, for which we present an altern…
We performed polarization- and photon-energy-dependent angle-resolved photoemission spectroscopy of a slightly overdoped iron pnictide superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the three-dimensional electronic structure including its orbital characters at the Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and $d_{xy/x^2-y^2}$ orbitals were observed but $d_{z^2}$ hole FS, which has nodes according to a theory of the spin-fluctuation superconductivity mechanism, did not appear. These results suggest that no node will appear at hole FSs at the zone center.
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…
The three-dimensional electronic structure and Ce 4f electrons of the heavy fermion superconductor CePt2In7 is investigated. Angle-resolved photoemission spectroscopy using variable photon energy establishes the existence of quasi-two and three dimensional Fermi surface topologies. Temperature-dependent 4d-4f on-resonance photoemission spectroscopies reveal that heavy quasiparticle bands begin to form at a temperature well above the characteristic (coherence) temperature T*. T* emergence may be closely related to crystal electric field splitting, particularly the low-lying heavy band formed by…
We report on experimental data of the three-dimensional bulk Fermi surfaces of the layered strongly correlated Ca1.5Sr0.5RuO4 system. The measurements have been performed by means of hn-depndent bulk-sensitive soft x-ray angle-resolved photoemission technique. Our experimental data evinces the bulk Fermi surface topology at kz~0 to be qualitatively different from the one observed by surface-sensitive low-energy ARPES. Furthermore, stronger kz dispersion of the circle-like gamma Fermi surface sheet is observed compared with Sr2RuO4. Thus in the paramagnetic metal phase, Ca1.5Sr0.5RuO4 compound …
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…
Using methods made possible by recent advances in photoemission technology, we perform an indepth line-shape analysis of the angle-resolved photoemission spectra of the electron doped (n-type) cuprate superconductor Nd_1.85Ce_0.15CuO_4. Unlike for the p-type materials, we only observe weak mass renormalizations near 50-70 meV. This may be indicative of smaller electron-phonon coupling or due to the masking effects of other interactions that make the electron-phonon coupling harder to detect. This latter scenario may suggest limitations of the spectral function analysis in extracting electronic…
Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba1-xKxFe2As2 (Tc = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X-symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in G-X and G-Y directions.
We report the observation and systematic investigation of the space charge effect and mirror charge effect in photoemission spectroscopy. When pulsed light is incident on a sample, the photoemitted electrons experience energy redistribution after escaping from the surface because of the Coulomb interaction between them (space charge effect) and between photoemitted electrons and the distribution of mirror charges in the sample (mirror charge effect). These combined Coulomb interaction effects give rise to an energy shift and a broadening which can be on the order of 10 meV for a typical third-…
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 have performed high-resolution angle-resolved photoemission spectroscopy on single-layered cuprate Bi$_2$Sr$_2$CuO$_6$ to clarify the origin of the pseudogap. By using various photon energies, we have succeeded in directly observing two different pseudogaps with two different energy scales which coexist in the antinodal region: one reflects the $d_{x^2-y^2}$-wave pairing strength while the other has a larger energy scale suggesting an origin distinct from superconductivity. The observed two-pseudogap behavior provides a key to fully understand the pseudogap phenomena in cuprates.
We have performed an angle-resolved photoemission study of the two-leg ladder system Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ with $x$= 0 and 11. "Underlying Fermi surfaces" determined from low energy spectral weight mapping indicates the quasi-one dimensional nature of the electronic structure. Energy gap caused by the charge density wave has been observed for $x$=0 and the gap tends to close with Ca substitution. The absence of a quasi-particle peak even in $x$=11 is in contrast to the two-dimensional high-$T_c$ cuprates, implying strong carrier localization related to the hole crystalization.
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$) nodal direction, accompanied by a peak-dip-hump feature in the photoemission spectra. This indicates that the nodal electrons are coupled to collective excitations (bosons) in electron-doped superconductors, with the phonons as the most likely candidate of the boson. This finding has established a universality of nodal electron c…
Techniques in time- and angle-resolved photoemission spectroscopy have facilitated a number of recent advances in the study of quantum materials. We review developments in this field related to the study of incoherent nonequilibrium electron dynamics, the analysis of interactions between electrons and collective excitations, the exploration of dressed-state physics, and the illumination of unoccupied band structure. Future prospects are also discussed.
Lattice contribution to the electronic self-energy in complex correlated oxides is a fascinating subject that has lately stimulated lively discussions. Expectations of electron-phonon self-energy effects for simpler materials, such as Pd and Al, have resulted in several misconceptions in strongly correlated oxides. Here we analyze a number of arguments claiming that phonons cannot be the origin of certain self-energy effects seen in high-$T_c$ cuprate superconductors via angle resolved photoemission experiments (ARPES), including the temperature dependence, doping dependence of the renormaliza…
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…
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…
We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of the high-Tc cuprates La2-xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. We have revealed a systematic doping evolution of the band dispersions and (underlying) Fermi surfaces, pseudogap and quasi-particle features under the influence of strong electron-electron interaction and electron-phonon interaction. The unusual transport and thermodynamic properties are explained by taking into account the pseudogap opening and the Fermi arc formation, due to which the …
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…
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…
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…
To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca2CuO2Cl2 using photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the lineshape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may ha…
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.
Angle-resolved photoemission measurements on the electron-doped cuprate Sm(1.85)Ce(0.15)CuO(4) evidence anisotropic dressing of charge-carriers due to many-body interactions. Most significantly, the scattering rate along the zone boundary saturates for binding energies larger than ~200 meV, while along the diagonal direction it increases nearly linearly with the binding energy in the energy range ~150-500 meV. These results indicate that many-body interactions along the diagonal direction are strong down to the bottom of the band, while along the zone-bounday they become very weak at energies …
Using a mapping from the three-band extended Hubbard model for cuprate superconductors into a generalized t-J model, and exact diagonalization of the latter in a 4x4 cluster, we determine the quasiparticle weight for destruction of a Cu or an O electrons with definite wave vector k. We also derive an approximate but accurate analytical expression which relates the O intensity with the quasiparticle weight in the generalized t-J model, and use it to obtain the O intensity from results of the self-consistent Born approximation. The k-dependence of Cu and O intensities is markedly different. In p…
The sudden photohole of least energy created in the photoemission process is a vibrationally excited state of a small polaron. Therefore the photoemission spectrum in LaMnO3 is predicted to have multiple Franck-Condon vibrational sidebands. This generates an intrinsic line broadening approximately 0.5 eV. The photoemission spectral function has two peaks whose central energies disperse with band width approximately 1.2 eV. Signatures of these phenomena are predicted to appear in angle-resolved photoemission spectra.
We have studied the electronic structure of two Chevrel-phase compounds, Mo$_6$Se$_{7.5}$ and Sn$_{1.2}$Mo$_6$Se$_{7.5}$, by combining photoemission spectroscopy and band-structure calculations. Core-level spectra taken with x-ray photoemission spectroscopy show systematic core-level shifts, which do not obey a simple rigid-band model. The inverse photoemission spectra imply the existence of an energy gap located $\sim 1$ eV above the Fermi level, which is a characteristic feature of the electronic structure of the Chevrel compounds. Quantitative comparison between the photoemission spectra an…
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…
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…
Nonequilibrium calculations in the presence of an electric field are usually performed in a gauge, and need to be transformed to reveal the gauge-invariant observables. In this work, we discuss the issue of gauge invariance in the context of time-resolved angle-resolved pump/probe photoemission. If the probe is applied while the pump is still on, one must ensure that the calculations of the observed photocurrent are gauge invariant. We also discuss the requirement of the photoemission signal to be positive and the relationship of this constraint to gauge invariance. We end by discussing some t…
The implications of the angle-resolved photoemission spectroscopy (ARPES) on the issue of spin-charge separation are discussed from the viewpoint of slave-particle RVB theories. It is pointed out that the non-local phase string is essential to reproduce the two-peak structure experimentally observed in SrCuO_2. The comparison between 1D and 2D cases is also made.
We describe different approximations associated with employing a constant matrix element for the coupling of light to multiband electrons in the context of time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). In particular, we demonstrate that the constant matrix approximation only holds for one of the coordinate representations, and changing to other bases, requires including nonconstant corrections to the matrix element. We also discuss some simplifying approximations, where a constant matrix element is employed in multiple bases, and the consequences of this further approxima…
Angle-resolved photoemission spectroscopy allows direct visualization and experimental determination of the electronic structure of crystals in the momentum space, including the precise characterization of the Fermi surface and the superconducting order parameter. It is thus particularly suited for investigating multiband systems such as the Fe-based superconductors. In this review, we cover several aspects of these recently discovered materials that have been addressed by this technique, with a special emphasis on their superconducting gap and their Fermi surface topology. We provide sufficie…
Time-resolved photoemission experiments can reveal fascinating quantum dynamics of correlated electrons. However, the thermalization of the electronic system is typically so fast that very short probe pulses are necessary to resolve the time evolution of the quantum state, and this leads to poor energy resolution due to the energy-time uncertainty relation. Although the photoemission intensity can be calculated from the nonequilibrium electronic Green functions, the converse procedure is therefore difficult. We analyze a hypothetical time-resolved photoemission experiment on a correlated elect…
The connection between the Fermi surface and charge-density wave (CDW) order is revisited in 2H-TaSe2. Using angle-resolved photoemission spectroscopy, ab initio band structure calculations, and an accurate tight-binding model, we develop the empirical k-resolved susceptibility function, which we use to highlight states that contribute to the susceptibility for a particular q-vector. We show that although the Fermi surface is involved in the peaks in the susceptibility associated with CDW order, it is not through conventional Fermi surface nesting, but rather through finite energy transitions …
The consistency of angle-resolved photoemission and optical conductivity experiments on high temperature superconductors is examined. In the limit (apparently consistent with angle-resolved photoemission data) of an electron self energy with a weak momentum dependence and a strong frequency dependence formulae are derived which directly related quantities measured in the two experiments. Application of the formuale to optimally and overdoped $Bi_{2}Ca_{2}SrCu_{2}O_{8+δ}$ shows that the total self energy inferred from photoemission measurements cannot be interpreted as a transport scattering ra…
Polarization-controlled synchrotron radiation was used to map the electronic structure of buried conducting interfaces of LaAlO$_3$/SrTiO$_3$ in a resonant angle-resolved photoemission experiment. A strong dependence on the light polarization of the Fermi surface and band dispersions is demonstrated, highlighting the distinct Ti 3d orbitals involved in 2D conduction. Samples with different 2D doping levels were prepared and measured by photoemission, revealing different band occupancies and Fermi surface shapes. A direct comparison between the photoemission measurements and advanced first-prin…
We use angle-resolved photoemission spectroscopy to study heavy fermion superconductor Ce2RhIn8. The Fermi surface is rather complicated and consists of several hole and electron pock- ets. We do not observe kz dispersion of Fermi sheets, which is consistent with 2D character of the electronic structure. Comparison of the ARPES data and band structure calculations points to a localized picture of f electrons. Our findings pave the way for understanding the transport and thermodynamical properties of this material.
Spin- and angle-resolved resonant (Ce $4d\to4f$) photoemission spectra of a monolayer Ce on Fe(110) reveal spin-dependent changes of the Fermi-level peak intensities. That indicate a spin-dependence of $4f$ hybridization and, thus, of $4f$ occupancy and local moment. The phenomenon is described in the framework of the periodic Anderson model by $4f$ electron hopping into the exchange split Fe 3d derived bands that form a spin-gap at the Fermi energy around the $\barΓ$ point of the surface Brillouin zone.
Electronic Coulomb correlations lead to characteristic signatures in the spectroscopy of transition metal pnictides and chalcogenides: quasi-particle renormalizations, lifetime effects or incoherent badly metallic behavior above relatively low coherence temperatures are measures of many-body effects due to local Hubbard and Hund's couplings. We review and compare the results of angle-resolved photoemission spectroscopy experiments (ARPES) and of combined density functional dynamical mean field theory (DFT+DMFT) calculations. We emphasize the doping-dependence of the quasi-particle mass renorma…
In connection with the experiment on oxygen isotope effect of Bi2Sr2CaCu2O8 with the angle-resolved photoemission spectroscopy (ARPES), we theoretically study the isotope-induced band shift in ARPES by the Hartree-Fork and quantum Monte Carlo methods. We find that this band shift can be clarified based on a quadratically coupled electron-phonon (e-ph) model. The large ratio of band shift versus phonon energy change is connected with the softening effect of phonon, and the positive-negative sign change is due to the momentum dependence of the e-ph coupling.
We report the electronic structure of a prototypical valence fluctuation system, YbAl2, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4f bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization s…
In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We use a femtosecond laser pump pulse to perturb superconducting Bi2Sr2CaCu2O8+δ, and study subsequent dynamics using time- and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics reveal marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap is sensitive to the pump intensity and Cooper pairs recombine slowly.…
