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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.

Since the discovery of superconductivity at elevated temperatures in the copper oxide materials there has been a considerable effort to find universal trends and correlations amongst physical quantities, as a clue to the origin of the superconductivity. One of the earliest patterns that emerged was the linear scaling of the superfluid density (rho(s)) with the superconducting transition temperature (T(c)), which marks the onset of phase coherence. This is referred to as the Uemura relation, and it works reasonably well for the underdoped materials. It does not, however, describe optimally doped (where T(c) is a maximum) or overdoped materials. Similarly, an attempt to scale the superfluid density with the d.c. conductivity (sigma(dc)) was only partially successful. Here we report a simple scaling relation (rho(s) proportional, variant sigma(dc)T(c), with sigma(dc) measured at approximately T(c)) that holds for all tested high-T(c) materials. It holds regardless of doping level, nature of dopant (electrons versus holes), crystal structure and type of disorder, and direction (parallel or perpendicular to the copper-oxygen planes).

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.

Angle-resolved photoemission spectroscopy on optimally doped Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+delta) uncovers a coupling of the electronic bands to a 40 meV mode in an extended k-space region away from the nodal direction, leading to a new interpretation of the strong renormalization of the electronic structure seen in Bi2212. Phenomenological agreements with neutron and Raman experiments suggest that this mode is the B(1g) oxygen bond-buckling phonon. A theoretical calculation based on this assignment reproduces the electronic renormalization seen in the data.

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We report a high-resolution angle-resolved photoemission spectroscopic study of the electron-doped ( n-type) cuprate superconductor Nd1.85Ce0.15CuO4. We observe regions along the Fermi surface where the near- E(F) intensity is suppressed and the spectral features are broad in a manner reminiscent of the high-energy "pseudogap" in the underdoped p-type (hole doped) cuprates. However, instead of occurring near the (pi,0) region, as in the p-type materials, this pseudogap falls near the intersection of the underlying Fermi surface with the antiferromagnetic Brillouin zone boundary.

In this article we show, using scanning tunneling spectroscopy, the existence of static striped density of electronic states in nearly optimally doped Bi2Sr2CaCu2O8؉␦ in zero field. This modulation is aligned with the CuOO bonds, with a periodicity of four lattice constants, and exhibits features characteristic of a two-dimensional system of line objects. We further show that the density of states modulation manifests itself as a shift of states from above to below the superconducting gap. The fact that a single energy scale (i.e., the gap) appears for both superconductivity and stripes suggests that these two effects have the same origin.O ne of the most important outstanding problems in condensed-matter physics is determination of the ground state of a strongly correlated electron system, in particular, hightemperature superconductors (HTSCs). For these materials, theoretical (1-7) and experimental (8-14) evidence has been mounting in support of the possibility that their ground state exhibits spin and charge density waves, which are primarily one-dimensional (i.e., stripes) and either compete with or promote superconductivity. Coexistence of charge or spin density waves and superconductivity has been reported previously in the lower Tc materials (9)(10)(11)14) and in the presence of large magnetic fields (12, 13). The absence of data showing stripes in the higher Tc materials in zero field has lent support to the idea that stripes are competing with high-temperature superconductivity. However, here we show, using scanning tunneling spectroscopy, the existence of static striped density of electronic states in nearly optimally doped Bi 2 Sr 2 CaCu 2 O 8ϩ␦ in zero field. This modulation is aligned with the CuOO bonds, with a periodicity of four lattice constants, and exhibits features characteristic of a two-dimensional system of line objects. We further show that the density-of-states (DOS) modulation manifests itself as a shift of states from above to below the superconducting gap. The fact that a single energy scale (i.e., the gap) appears for both superconductivity and stripes suggests that these two effects have the same origin. Scanning tunneling spectroscopy allows one to measure, on an atomic scale, the electronic DOS. This measurement of the local DOS (LDOS) makes it a powerful tool for investigating correlated electron systems. In this study we used Bi 2 Sr 2 CaCu 2 O 8ϩ␦ because it cleaves easily, yielding large, stable, atomically flat surfaces and has a high Tc (90 K). The single crystals of Bi 2 Sr 2 CaCu 2 O 8ϩ␦ used in this study, grown by a floating-zone method, were annealed to be slightly overdoped, yielding a superconducting transition temperature of 86 K. The samples were cleaved (between the BiO planes) at room temperature in a vacuum of Ͼ10 Ϫ9 torr (1 torr ϭ 133 Pa) and then transferred in Ͻ1 min to the low-temperature scanning tunneling microscope, where cryopumping yields an orders of magnitude better vacuum. Data were taken at 8 K with an iridium tip at a sample bias of Ϫ200 mV and a se...