Hard X-ray Photoemission Spectroscopy Research Articles

X-ray spectroscopic study ofBaFeO3thin films: AnFe4+ferromagnetic insulator

We investigated the electronic and magnetic properties of fully oxidized ${\mathrm{BaFeO}}_{3}$ thin films, which show ferromagnetic-insulating properties with cubic crystal structure, by hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD). We analyzed the results with configuration-interaction (CI) cluster-model calculations for ${\mathrm{Fe}}^{4+}$, which showed good agreement with the experimental results. We also studied ${\mathrm{SrFeO}}_{3}$ thin films, which have an ${\mathrm{Fe}}^{4+}$ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that ${\mathrm{BaFeO}}_{3}$ thin films are insulating with large magnetization ( 1.$7{\ensuremath{\mu}}_{B}$/formula unit) under $\ensuremath{\sim}1$ T, using valence-band HAXPES and Fe $2p$ XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe $2p$ core-level HAXPES and Fe $2p$ XAS spectra of ${\mathrm{BaFeO}}_{3}$ and ${\mathrm{SrFeO}}_{3}$ thin films are quite similar, we compared the insulating ${\mathrm{BaFeO}}_{3}$ to metallic ${\mathrm{SrFeO}}_{3}$ thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of ${\mathrm{BaFeO}}_{3}$ is dominated by ${d}^{5}\underline{L}\phantom{\rule{0.222222em}{0ex}}(\underline{L}$: ligand hole) configuration due to the negative charge transfer energy, and that the band gap has significant O $2p$ character. We revealed that the differences of the electronic and magnetic properties between ${\mathrm{BaFeO}}_{3}$ and ${\mathrm{SrFeO}}_{3}$ arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.

The Hard X-ray Photoemission Spectroscopy Facility at CLS: performance and first results

Photoemission spectroscopy (PES) has been used widely to study the electronic structure of valence and core levels. However, conventional PES is surface-sensitive. To probe the interface and bulk properties of materials, hard X-ray photoemission spectroscopy (HXPES) has received increasing interest in the last decade, because of the deep probing ability of photoelectrons with higher kinetic energies (2–10 keV). Recently, a HXPES system was developed at the Canadian Light Source, using the high-energy version of a R4000 electron analyzer-based spectrometer connected to a medium-energy beamline, the soft X-ray microcharacterization beamline (SXRMB). Excellent performance of the beamline and the spectrometer is demonstrated herein using Au Fermi and 4f core lines; and the controlled probing depth of HXPES at SXRMB is demonstrated by tuning the photon energy (2–9 keV) in the study of a series of SiO2/SiC multilayer samples. Combined with the high-resolution X-ray absorption spectroscopy available at the SXRMB, the HXPES offers a powerful nondestructive technique for studying bulk properties of various materials.

Electronic structure of C and N co-doped TiO2: A combined hard x-ray photoemission spectroscopy and density functional theory study

We have studied the electronic structure of C and N co-doped TiO2 using hard x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Our results reveal overlap of the 2p states of O, N, and C in the system which shifts the valence band maximum towards the Fermi level. Combined with optical data we show that co-doping is an effective route for band gap reduction in TiO2. Comparison of the measured valence band with theoretical photoemission density of states reveals the possibility of C on Ti and N on O site.

Effect of doping on surface reactivity and conduction mechanism in samarium-doped ceria thin films.

A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

Photoemission and DMFT study of electronic correlations inSrMoO3: Effects of Hund's rule coupling and possible plasmonic sideband

We investigate the electronic structure of a perovskite-type Pauli paramagnet SrMoO3 (t2g2) thin film using hard x-ray photoemission spectroscopy and compare the results to the realistic calculations that combine the density functional theory within the local-density approximation (LDA) with the dynamical-mean field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially-filled t2g bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. The interpretation is supported by additional model DMFT calculations including Hund's rule coupling, that show renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around -2 eV that is not present in the LDA+DMFT. We interpret this weight as a plasmon satellite, which is supported by measured Mo, Sr and Oxygen core-hole spectra that all show satellites at this energy.

Charge-order melting in charge-disproportionated perovskite CeCu3Fe4O12.

A novel quadruple perovskite oxide CeCu3Fe4O12 has been synthesized under high-pressure and high-temperature conditions of 15 GPa and 1473 K. (57)Fe Mössbauer spectroscopy displays a charge disproportionation transition of 4Fe(3.5+) → 3Fe(3+) + Fe(5+) below ∼270 K, whereas hard X-ray photoemission and soft X-ray absorption spectroscopy measurements confirm that the Ce and Cu valences are retained at approximately +4 and +2, respectively, over the entire temperature range measured. Electron and X-ray diffraction studies reveal that the body-centered cubic symmetry (space group Im3̅, No. 204) is retained at temperatures as low as 100 K, indicating the absence of any types of charge-ordering in the charge-disproportionated CeCu3Fe4O12 phase. The magnetic susceptibility and neutron powder diffraction data illustrate that the antiferromagnetic ordering of Fe ions is predominant in the charge-disproportionated CeCu3Fe4O12 phase. These findings suggest that CeCu3Fe4O12 undergoes a new type of electronic phase in the ACu3Fe4O12 series and that the melting of the charge-ordering in CeCu3Fe4O12 is caused by the substantial decrease in the Fe valence and the resulting large deviation from the ideal abundance ratio of Fe(3+):Fe(5+) = 1:1 for rock-salt-type charge-ordering.

Narrow bandgap in β-BaZn₂As₂ and its chemical origins.

β-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that β-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of β-BaZn2As2 (previously reported value ~0.2 eV) is 1 order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of β-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the temperature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemical bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the nonbonding Ba 5d(x(2)-y(2)) orbitals form an unexpectedly deep conduction band minimum (CBM) in β-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between β-BaZn2As2 and LaZnAsO.

Facile route for the preparation of ordered intermetallic Pt3Pb–PtPb core–shell nanoparticles and its enhanced activity for alkaline methanol and ethanol oxidation

Pt3Pb(core)–PtPb(shell) intermetallic compound nanoparticles (NPs) were synthesized on carbon black (CB) by converting nanocrystalline Pt to an ordered intermetallic compound with the reduction of Pb ions. The Pt3Pb–PtPb core–shell NPs were characterized by analyzing their crystal structures with powder X-ray diffraction (pXRD), hard X-ray photoemission spectroscopy (HX-PES), and transmission electron microscopy (TEM). The synthesized NPs exhibited enhanced catalytic activity and relatively stable cycle performance towards methanol (MeOH) and ethanol (EtOH) oxidation in an alkaline aqueous solution. The improved catalytic performance of the Pt3Pb–PtPb core–shell NPs might be attributed to both the enhancement of EtOH dehydrogenation and the higher concentration of surface OHads at lower potential on the modified PtPb surface in the Pt3Pb–PtPb core–shell NPs.

Band alignment at memristive metal-oxide interfaces investigated by hard x-ray photoemission spectroscopy

Nonvolatile memory elements, known as memristors, have recently received an attention boost by introducing transition-metal oxide /transition-metal bilayer structures. These structures enable the engineering of the electrical properties of the devices and the removal of unwanted forming steps though a redox-reaction at the interface. This work provides much needed information for the understanding of memristive behavior in such devices by providing the first systematic study of the electronic band alignment at the interface, which plays essential role in charge carrier transport.

Investigation of Cu-poor and Cu-rich Cu(In,Ga)Se2/CdS interfaces using hard X-ray photoelectron spectroscopy

Cu-poor and Cu-rich Cu(In,Ga)Se2 (CIGSe) absorbers were used as substrates for the chemical bath deposition of ultrathin CdS buffer layers in the thickness range of a few nanometers in order to make the CIGSe/CdS interface accessible by hard X-ray photo-emission spectroscopy. The composition of both, the absorber and the buffer layer as well as the energetics of the interface was investigated at room temperature and after heating the samples to elevated temperatures (200°C, 300°C and 400°C). It was found that the amount of Cd after the heating treatment depends on the near surface composition of the CIGSe absorber. No Cd was detected on the Cu-poor surface after the 400°C treatment due to its diffusion into the CIGSe layer. In contrast, Cd was still present on the Cu-rich surface after the same treatment at 400°C.

Detection of Effect of Strain on the Valence Band Structure of SiGe by HXPES with High Spatial Resolution

We have investigated the effect of the uniaxial strain on the binding energy of valence band top, Si 1s and Ge 2p core level by Hard X-ray Photoemission Spectroscopy (HXPES) with the high spatial resolution. By combining HXPES with the high spatial resolution using K-B mirrors and uniaxially strained SiGe samples fabricated by selective ion-implantation, we succeeded in observation of the effect of uniaxial strain on the valence band structure. We have found that the energy difference between Ge 2p and valence band top in the case of uniaxial strain is smaller than that in the case of biaxial strain.

Characterization of Chemical Bonding Features and Interfacial Reactions in Ge-MIS Structure with HfO2/TaGexOy Dielectric Stack

As means to control interface reactions between HfO2 and Ge(100), chemical vapor deposition (CVD) of ultrathin Ta-rich oxide using tris(tert-butoxy)(tert-butylimido)tantalum (Ta-TTT) on chemically-cleaned Ge(100) has been conducted prior to atomic-layer controlled CVD of HfO2 using tetrakis (ethylmethylamino) hafnium (TEMA-Hf) and O3. An impact of post metallization annealing (PMA) in N2 ambience on chemical structure and electrical properties for TiN-gate Ge-MIS structure has been investigated. Hard x-ray photo-emission spectroscopy (HAXPES) measurements under synchrotron radiation with the excitation energy of 7939 eV show that good thermal stability of MIS structure against PMA at temperature of 400 °C. Diffusion of Ge atoms into stack dielectrics and TiN was observed after PMA at temperatures over 500 °C. As for the electrical properties of MIS capacitors, PMA at 300 °C was found to be effective to reduce the hysteresis due to charge trap and gate leakage current through the dielectrics. It was also found that the relative dielectric constant of interfacial TaGexOy thin layer was ~ 9, and calculated equivalent SiO2 thickness (CET) of 1.0 nm can be achieved for the sample with 3 nm thick HfO2/1nm-thick TaGexOy stack dielectrics.

Characterization of Chemical Bonding Features and Interfacial Reactions in Ge-MIS Structure with HfO2/TaGexOy Dielectric Stack

Control of the chemical reactions at both high-k/Ge and metal/high-k interfaces is of great importance to realize the high quality Ge-MIS capacitor with a thinner EOT. So far, we have fabricated Ge-MIS structures with HfO2/TaGexOy gate stack, in which TaGexOy is used as an interfacial control layer between HfO2 and Ge substrate from the viewpoint of relatively high dielectric constant (ε = ~9ε0), and CET of 1.4 nm has been obtained [1]. As for the metal/dielectric interface, it has been clarified that thermally grown GeO2 are reduced depending on element metals formed by thermal evaporation, and this reduction reaction reflects the Gibbs free energy difference [2, 3]. In this work, we mainly focused on the chemical reactions in the TiN/HfO2/TaGexOygate stack caused by post metallization annealing (PMA), and the chemical structures of the MIS samples were examined by using hard x-ray photoemission spectroscopy (HAXPES) excited synchrotron radiation (hν = 7939 eV).A 2 nm-thick TaOx layer was deposited on wet-cleaned p-type Ge(100) surface at 400 ºC in a layer-by-layer fashion in which Ta-TTT was introduced intermittently using a N2 bubbling method. Subsequently, a ~2 nm-thick HfO2 layer was deposited at 280 ºC by an ALD method using TEMA-Hf and O3. Then, the post deposition annealing (PDA) was performed at 400 °C in diluted O2 ambience to densify the dielectrics. A formation of the TaGexOy interfacial layer by PDA was confirmed from XPS analyses. After that, a TiN thin layer with a typical thickness of ~15 nm was deposited by RF sputtering. In some samples, PMA in the temperature range from 300 to 600 °C was carried out in N2ambience at atmospheric pressure.Figure 1 shows the HAXPES core-line signals taken for TiN/HfO2/TaGexOy/Ge structure before and after PMA. Although a slight increase in the chemically shifted Ge 2p3/2 signals due to the incorporation of Ge atoms into high-k dielectric layers was observed after PMA at 400 °C, change in the chemical structure near the interface between TiN and HfO2/TaGexOy dielectrics were hardly detected. Even in the reference sample of TiN/GeO2(21nm)/Ge structure, chemical bonding features at the top interface show a good thermal stability against PMA at 400 °C. After the PMA at temperatures over 500 ºC, chemically shifted Ge 2p3/2 signals were drastically increased. From the photoelectron take-off angle dependence of core-line signals for the sample after PMA at 600 °C, the formation of Ge oxide component was confirmed at the sample surface. In contrast, significant diffusion of Hf atoms into TiN was hardly detected. Electrical properties of MIS capacitors also show a significant increase in the gate leakage current and a decrease of accumulation capacitance by PMA at temperatures over 500 ºC. These results suggest that dissociation of Ge-O bonds proceeds in the region near the TiN at temperatures over 500 ºC and it makes a degradation of the electrical properties.AcknowledgementsThis work was supported in part by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration. The Ta-TTT precursor was provided by TOSOH Corporation. A part of this work was supported by Research Institute for Nano-device and Bio Systems (RNBS), Hiroshima University, Japan. HAXPES experiments were done at a beam line of BL47XU in SPring-8 with the approval of JASRI as a Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science, and Technology (Proposal No. 2013A1696/BL47XU). We would like to thank Prof. S. Higashi (Hiroshima Univ.) and Dr. E. Ikenaga (JASRI) for the fruitful discussion and contribution in assisting the experiments.

Detection of Effect of Strain on the Valence Band Structure of SiGe by HXPES with High Spatial Resolution

Since the aggressive scaling of the Si-based metal–oxide–semiconductor field effect transistors (MOSFET) is reaching its limits, the strained-Ge and III–V semiconductors are being studied extensively as one of the alternative channel materials due to their higher carrier mobility [1,2]. Recently, the channel strain engineering has become an essential technology for the future complementary metal oxide semiconductor. The process-induced local strain is currently the mainstream technology [3]. We reported that a selective-ion-implantation technique was developed as a novel method of creating uniaxially strained Si/Ge heterostructures [4]. We investigated the effect of the strain on the binding energy of valence band top, Si 1s and Ge 2p core level by Hard X-ray photoemission spectroscopy (HXPES) with the high spatial resolution.The sample was prepared as follows. A SiO2 film was firstly deposited as a mask on a Si(100) substrate by plasma-enhanced chemical vapor deposition (PECVD), followed by the patterning of stripes with a standard photolithography process. Ar+ ion were selectively implanted into the Si substrate through the SiO2 mask windows at an energy of 25 keV with a dose of 1×1015 cm-2. The line widths of the implanted and unimplanted regions were fixed to be 20 and 2μm, respectively. A SiGe layer with a Ge content of 26% and thickness around 70 nm was pseudomorphically grown on the selectively ion-implanted Si substrate. Subsequently, postgrowth annealing was carried out to promote strain relaxation at 900°C. The hard X-ray (7.94 keV photons) excited Si 1s, Ge 2p, and valence band spectra arising from the sample were measured at photoelectron take-off angle (TOA) of 85 degrees with the energy resolution of 100 meV at undulator beam line (BL47XU at SPring-8 (JASRI, Proposal No. 2012B1269)) using high resolution electron energy analyzer R-4000 with the spatial resolution [11(|)0] direction is about 0.8µm [5]. The Si 1s photoelectron spectra are changing in the position of a sample as shown in Fig. 1 (a). From Fig. 1 (b) and (c), the binding energies of Si 1s and Ge 2p core level are reduced at the position of 8, 30, and 52µm. Here, spectra are shown in figures as a parameter at the position of the [11(|)0] direction. On the other hand, the change of the binding energy of valence band top is less than that of peak of Si 1s and Ge 2p as shown in Fig. 1(d). The comparison of the position of the valence band top in the case in relation to the binding energy of Ge 2p is shown in Fig. 2. Focus on the position of the valence band top, the position of the valence band of the uniaxial strained SiGe is lower than that in the case of the biaxial strained SiGe (50% relaxation rate). The change in the valence band position due to the difference in stress is consistent with the calculated results [6].Acknowledgments We acknowledge Prof. N. Usami and Y. Hoshi for their supports for the MBE growth. This work was partly supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2009-2013, by Grant-in-Aid for Scientific Research (Grant No. 25600079) from MEXT, Japan, and by the Strategic Information and Communications R&D Promotion Programme (SCOPE) from MIC, Japan.[1] ITRS 2012, (http://www.itrs.net)[2] M. Heyns, et al.: MRS Bull. 34 (2009) 485.[3] M. L. Lee, et al.: J. Appl. Phys. 97 (2005) 011101.[4] K. Sawano, et al.: Applied Physics Express 1 (2008) 121401.[5] E. Ikenaga,et al.: ELSPEC-46121 ; No. of Pages 8. [6] Y. Sun, et al.: Appl. Phys. 101 (2007) 104503.

Orbital correlations and dimensional crossover in epitaxial Pr0.5Ca0.5MnO3/La0.5Sr0.5MnO3 superlattices

We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering.

Electronic structure ofCeCuAs2

We study the electronic structure of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$, which is known to show an anomalous negative temperature coefficient of resistivity below room temperature, while the thermopower shows metallic behavior. We carry out hard x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and resonant PES across the Ce $3d\text{\ensuremath{-}}4f$ and Cu $2p\text{\ensuremath{-}}3d$ thresholds to investigate the role of Kondo screening in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$. The Ce $3d$ core-level PES shows the ${f}^{0}$, ${f}^{1}$, and ${f}^{2}$ features, and the Ce $3d\text{\ensuremath{-}}4f$ XAS shows corresponding features due to transitions into the ${f}^{1}$ and ${f}^{2}$ states, as in Kondo systems. The spectral feature assignments are confirmed by single-impurity Anderson model calculations and indicate a nearly trivalent-Ce configuration with small but finite mixed valency. The resonant PES valence band spectra across the Ce $3d\text{\ensuremath{-}}4f$ threshold show an intense Ce $4{f}^{1}$ resonance just below the Fermi level, while the Ce $4{f}^{0}$ feature is observed at a binding energy of 2.5 eV. The obtained $f$ partial density of states shows an unusual pseudogaplike dip at the Fermi level in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$. The results indicate the importance of Kondo screening and a pseudogap in the $f$ partial density of states for the anomalous transport and magnetic properties of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$.

Future directions in standing-wave photoemission

Over the past decade, standing-wave photoemission (SW-XPS) has evolved into a powerful and versatile non-destructive technique for probing element-specific electronic, magnetic, and structural properties of buried layers and interfaces with sub-nanometer depth resolution. In this article, I will discuss several promising future directions in this emergent field stemming from experimental and theoretical studies wherein SW-XPS is combined with other X-ray techniques, such as magnetic circular dichroism (MCD), hard X-ray photoemission spectroscopy (HAXPES), angle-resolved photoemission (ARPES), and photoemission microscopy (PEEM), adding extra dimensions to the measurement and thus widening the scope of scientific and technological questions accessible via the use of standing waves. I will further discuss examples of recently developed methods for X-ray standing-wave data analysis, which yield layer-resolved matrix-element-weighted densities of states at interfaces as well as Ångstrom-level changes in periodicity of synthetic superlattices. Finally, I will explore the possibility of localizing the standing waves near the surface and within a buried layer by the use of aperiodic superlattices, total reflection, and X-ray waveguide effects.

Investigation of group IVA elements combined with HAXPES and first-principles calculations

The core level and valence band spectra of group IVA elements were investigated with hard x-ray photoemission spectroscopy (HAXPES) photon energy of 7.939 keV by bulk sensitive manner. The survey and valance band spectra were presented, relative peaks intensity are discussed by thinking about inelastic mean free path (IMFP) and photoionization cross section of photoelectrons (PICS). In order to understand bulk band structures, valence bands are compared with the calculated ones by considering PICS, IMFP and total energy resolution. The calculated results by GGA, HSE06 and GW0 methods are simply discussed by comparing with experiment spectra.

Ultrathin, epitaxial cerium dioxide on silicon

It is shown that ultrathin, highly ordered, continuous films of cerium dioxide may be prepared on silicon following substrate prepassivation using an atomic layer of chlorine. The as-deposited, few-nanometer-thin Ce2O3 film may very effectively be converted at room temperature to almost fully oxidized CeO2 by simple exposure to air, as demonstrated by hard X-ray photoemission spectroscopy and X-ray diffraction. This post-oxidation process essentially results in a negligible loss in film crystallinity and interface abruptness.

Bulk nature of layered perovskite iridates beyond the Mott scenario: An approach from a bulk-sensitive photoemission study

We present genuine bulk Ir 5d jeff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasiparticle density of states calculated by the local density approximation with dynamical mean-field theory (LDA+DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-low-energy bulk-sensitive photoemission spectroscopy reveals “bad metallic” states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Néel temperature as predicted by the LDA+DMFT.Received 27 October 2013Revised 2 March 2014DOI:https://doi.org/10.1103/PhysRevB.89.121111©2014 American Physical Society