We examine the effect on the superconducting transition temperature (Tc) of chemical inhomogeneities in Bi2Sr2CuO 6+δ and Bi2Sr2CaCu2O 8+δ single crystals. Cation disorder at the Sr crystallographic site is inherent in these materials and strongly affects the value of Tc. Partial substitution of Sr by Ln (Ln = La, Pr, Nd, Sm, Eu, Gd, and Bi) in Bi2Sr1.6Ln0.4CuO 6+δ results in a monotonic decrease of Tc with increasing ionic radius mismatch. By minimizing Sr site disorder at the expense of Ca site disorder, we demonstrate that the Tc of Bi2Sr2CaCu2O 8+δ can be increased to 96 K. Based on these results we discuss the effects of chemical inhomogeneity in other bulk high-temperature superconductors.

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Understanding the role of competing states in the cuprates is essential for developing a theory for high-temperature superconductivity. We report angle-resolved photoemission spectroscopy experiments which probe the 4a0 x 4a0 charge-ordered state discovered by scanning tunneling microscopy in the lightly doped cuprate superconductor Ca2-xNaxCuO2Cl2. Our measurements reveal a marked dichotomy between the real- and momentum-space probes, for which charge ordering is emphasized in the tunneling measurements and photoemission is most sensitive to excitations near the node of the d-wave superconducting gap. These results emphasize the importance of momentum anisotropy in determining the complex electronic properties of the cuprates and places strong constraints on theoretical models of the charge-ordered state.

A catalyst functions by stabilizing reaction intermediates, usually through surface adsorption. In the oxygen evolution reaction (OER), surface oxygen adsorption plays an indispensable role in the electrocatalysis. The relationship between the adsorption energetics and OER kinetics, however, has not yet been experimentally measured. Herein we report an experimental relationship between the adsorption of surface oxygen and the kinetics of the OER on IrO(110) epitaxially grown on a TiO(110) single crystal. The high quality of the IrO film grown using molecular-beam epitaxy affords the ability to extract the surface oxygen adsorption and its impact on the OER. By examining a series of electrolytes, we find that the adsorption energy changes linearly with pH, which we attribute to the electrified interfacial water. We support this hypothesis by showing that an electrolyte salt modification can lead to an adsorption energy shift. The dependence of the adsorption energy on pH has implications for the OER kinetics, but it is not the only factor; the dependence of the OER electrocatalysis on pH stipulates two OER mechanisms, one operating in acidic solution and another operating in alkaline solution. Our work points to the subtle adsorption-kinetics relationship in the OER and highlights the importance of the interfacial electrified interaction in electrocatalyst design.

We present an angle-resolved photoemission doping dependence study of the n-type cuprate superconductor Nd(2-x)Ce(x)CuO(4+/-delta), from the half-filled Mott insulator to the T(c) = 24 K superconductor. In Nd2CuO4, we reveal the charge-transfer band for the first time. As electrons are doped into the system, this feature's intensity decreases with the concomitant formation of near- E(F) spectral weight. At low doping, the Fermi surface is an electron-pocket (with volume approximately x) centered at (pi,0). Further doping leads to the creation of a new holelike Fermi surface (volume approximately 1+x) centered at (pi,pi). These findings shed light on the Mott gap, its doping evolution, as well as the anomalous transport properties of the n-type cuprates.

The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and of the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to √ 2× √ 2 surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the α, β, and γ sheets.PACS numbers: 74.25.Jb, 74.70.Ad, 79.60.Bm Angle resolved photoemission spectroscopy (ARPES) has proven itself to be an extremely powerful tool in studying the electronic structure of correlated electron systems. In particular, in the case of the hightemperature superconductors, it has been very successful in measuring the superconducting gap, determining the symmetry of the order parameter, and characterizing the pseudo-gap regime [1]. On the other hand one of the fundamental issues, namely the determination of the Fermi surface (FS) topology, has been controversial, as in the case of Bi 2 Sr 2 CaCu 2 O 8 , raising doubts concerning the reliability of the ARPES results. A similar controversy has also plagued the fermiology of Sr 2 RuO 4 . In this context, the latter system is particularly interesting because it can also be investigated with de Haas-van Alphen (dHvA) experiments, contrary to the cuprates, thus providing a direct test for the reliability of ARPES.Whereas dHvA analysis, in agreement with LDA bandstructure calculations [2,3], indicates two electronlike FS (β, and γ) centered at the Γ point, and a hole pocket (α) at the X point [4-6], early ARPES measurements suggested a different picture: one electronlike FS (β) at the Γ point and two hole pockets (γ, and α) at the X point [7,8]. The difference comes from the detection by ARPES of an intense, weakly dispersive feature at the M point just below E F , that was interpreted as an extended van Hove singularity (evHs) pushed down below E F by electron-electron correlations [7,8]. With the evHs below E F , rather than above (LDA band-structure calculations place it 60 meV above E F [2,3]), the γ pocket is converted from electronlike to holelike. The existence of the evHs was questioned in a later photoemission paper [9], where it was suggested that dHvA and ARPES results could be reconciled by assuming that the feature detected by ARPES at the M point was due to a surface state (SS). Recently, two possible explanations were proposed: first, the evHs at the M point could be only slightly above E F (e.g., 10 meV), so that considerable spectral weight would be detected just below E F [10]. Alternatively, ARPES could be probing ferromagnetic (FM) correlations reflected by the existence of two different γ-FS (hole and electronlike, respectively, for majority and minority spin direction), which escaped detection in dHvA experiments [11]. Lastly, the surface reconstruction as detected by LEED, which...

The electronic structure of heavily overdoped Bi2Sr2CaCu2O 8+δ is investigated by angle-resolved photoemission spectroscopy. The long-sought bilayer band splitting in this two-plane system is observed in both normal and superconducting states, which qualitatively agrees with the bilayer Hubbard model calculations. The maximum bilayer energy splitting is about 88 meV for the normal state feature, while it is only about 20 meV for the superconducting peak. This anomalous behavior cannot be reconciled with the quasiparticle picture.PACS numbers: 71.18.+y, 74.72.Hs, 79.60.Bm High temperature superconductors (HTSC's), as doped Mott insulators, show strong doping dependent behavior. The underdoped regime of the HTSC's is characterized by its unconventional properties, such as the pseudogap and non-Fermi liquid transport behavior. On the other hand, the overdoped regime is considered to be more "normal", partly because of the absence of a pseudogap and more Fermi liquid-like behavior. It is very challenging and important for HTSC theories to be able to explain the phenomenology in both regimes. Angle resolved photoemission spectroscopy (ARPES), one of the most direct probes of the electronic structure, has contributed greatly to the understanding of the electronic structure of the HTSC's. However, most systems studied by ARPES have either low T c 's (below 40K for La 2−x Sr x CuO 4+δ (LSCO), and Bi 2 Sr 2 CuO 6+δ (Bi2201)), or doping limitations (only up to slightly overdoping for Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) and YBa 2 Cu 3 O 7−y (YBCO)). For a complete understanding, it is very important to study the heavily overdoped systems, especially Bi2212, which is the most studied system by ARPES.Recent advances in high pressure annealing techniques have made it possible to synthesize heavily overdoped Bi2212. In this paper, we report ARPES measurements of the electronic structure of heavily overdoped Bi2212. We show that the long-sought bilayer band splitting (BBS) exists for both normal and superconducting states of this material over large fraction of the Brillouin zone. The detection of the BBS, which has been predicted by band structure calculations [1,2], but not observed in earlier ARPES data [3], enables us to address several important issues. First, it provides a very detailed test for the theoretical calculations, with our experimental results favoring the bilayer Hubbard model [4] over LDA calculations [1,2]. Second, it shows the novel result that the bilayer splitting energy in the superconducting state is only about 23% of the normal state splitting. Third, it provides an explanation for the detection of a "peak-diphump" structure in the normal state of heavily overdoped samples [5,6].Heavily overdoped Bi2212 samples (T C (onset) = 65 K, ∆T C (10% ∼ 90%) = 3 K, denoted as OD65) were synthesized by annealing floating-zone-grown single crystals under oxygen pressure P O2 = 300 atm at 300• C for two weeks, and characterized by various techniques. Magnetic susceptibility measurements show that the presence of a s...

The evolution of Ca2−xNaxCuO2Cl2 from Mott insulator to superconductor was studied using angle-resolved photoemission spectroscopy. By measuring both the excitations near the Fermi energy as well as non-bonding states, we tracked the doping dependence of the electronic structure and the chemical potential with unprecedented precision. Our work reveals failures in the conventional quasiparticle theory, including the broad lineshapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wavevector with the behavior of the chemical potential.PACS numbers: 74.20. Rp, 74.25.Jb, A central intellectual issue in the field of hightemperature superconductivity is how an antiferromagnetic insulator evolves into a superconductor. In principle, the ideal tool to address this problem is angleresolved photoemission spectroscopy (ARPES), which can directly extract the single-particle excitations. Despite the interest in this doping induced crossover, there continues to be a lack of experimental consensus, perhaps the most prominent example being the controversy over the chemical potential, µ. Over the past fifteen years, there have been conflicting claims of µ either being pinned in mid-gap or shifting to the valence / conduction band upon carrier doping [1,2,3,4,5,6]. The inability of photoemission spectroscopy to provide a logically consistent understanding of this fundamental thermodynamic quantity has been a dramatic shortcoming in the field.In this paper, we present a new procedure to quantify µ with unprecedented precision by ARPES, while allowing simultaneous high resolution measurements on the low energy states. These measurements allow us to make major conceptual advances in addressing the doping evolution. We find that the long standing confusion over µ stems from the manner in which quasiparticle-like (QP) excitations in the doped samples emerge from the unusually broad features in the undoped insulator. Our work reveals inconsistencies in the conventional framework that considers the main peak in the insulator spectrum to represent the QP pole. On the one hand, we find that µ changes in a manner consistent with an approximate rigid band shift; on the other hand, this shift appears to occur within the apparent Mott gap of the parent insulator. We show that this ostensible paradox can be naturally explained if one uses a model based on Franck-Condon-like broadening (FCB) where the quasiparticle residue, Z, is vanishingly small near half filling. This also reconciles existing puzzles regarding the insulator and the lightly doped compounds, and naturally ties the behavior of µ to low energy features such as the Fermi wavevector, k F , and the quasiparticle velocity v F .Ca 2−x Na x CuO 2 Cl 2 is an ideal system to address the doping evolution of the cuprates. The stoichiometric parent compoun...