Visualizing pair formation on the atomic scale in the high-Tc superconductor Bi2Sr2CaCu2O8+d

Pairing of electrons in conventional superconductors occurs at the superconducting transition temperature Tc, creating an energy gap D in the electronic density of states (DOS). In the high-Tc superconductors, a partial gap in the DOS exists for a range of temperatures above Tc. A key question is whether the gap in the DOS above Tc is associated with pairing, and what determines the temperature at which incoherent pairs form. Here we report the first spatially resolved measurements of gap formation in a high-Tc superconductor, measured on Bi2Sr2CaCu2O8+d samples with different Tc values (hole concentration of 0.12 to 0.22) using scanning tunnelling microscopy. Over a wide range of doping from 0.16 to 0.22 we find that pairing gaps nucleate in nanoscale regions above Tc. These regions proliferate as the temperature is lowered, resulting in a spatial distribution of gap sizes in the superconducting state. Despite the inhomogeneity, we find that every pairing gap develops locally at a temperature Tp, following the relation 2D/kBTp = 8. At very low doping (<0.14), systematic changes in the DOS indicate the presence of another phenomenon, which is unrelated and perhaps competes with electron pairing. Our observation of nanometre-sized pairing regions provides the missing microscopic basis for understanding recent reports of fluctuating superconducting response above Tc in hole-doped high-Tc copper oxide superconductors

Nanoscale Proximity Effect in the High Temperature Superconductor Bi-2212

High temperature cuprate superconductors exhibit extremely local nanoscale phenomena and strong sensitivity to doping. While other experiments have looked at nanoscale interfaces between layers of different dopings, we focus on the interplay between naturally inhomogeneous nanoscale regions. Using scanning tunneling microscopy to carefully track the same region of the sample as a function of temperature, we show that regions with weak superconductivity can persist to elevated temperatures if bordered by regions of strong superconductivity. This suggests that it may be possible to increase the maximum possible transition temperature by controlling the distribution of dopants.Comment: To appear in Physical Review Letter

Appearance of fluctuating stripes at the onset of the pseudogap in the high-Tc Superconductor Bi2Sr2CaCu2O8+x

Doped Mott insulators have been shown to have a strong propensity to form patterns of holes and spins often referred to as stripes. In copper-oxides, doping also gives rise to the pseudogap state, which transforms into a high temperature superconductor with sufficient doping or by reducing the temperature. A long standing question has been the interplay between pseudogap, which is generic to all hole-doped cuprates, and stripes, whose static form occurs in only one family of cuprates over a narrow range of the phase diagram. Here we examine the spatial reorganization of electronic states with the onset of the pseudogap state at T* in the high-temperature superconductor Bi2Sr2CaCu2O8+x using spectroscopic mapping with the scanning tunneling microscope (STM). We find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. As expected, the stripe patterns are strongest when the hole concentration in the CuO2 planes is close to 1/8 (per Cu). While demonstrating that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the cuprate phase diagram, our experiments indicate that they are a consequence of pseudogap behavior rather than its cause.Comment: preprint version, 25 pages including supplementary informatio

Evolution of the excitation spectrum of cuprate superconductors with doping and temperature

Understanding the mechanism by which d-wave superconductivity in the cuprates emerges and is optimized by doping a Mott insulator is one of the major outstanding problems in physics. A key unresolved question in this field is how the strength of electron pairing evolves as a function of doping and temperature and whether pairing strength and the Tc of the sample are related, as they are in simple BCS superconductors. To address these questions, we have developed several new experimental techniques with the scanning tunneling microscope to measure the excitation spectra in the cuprates on the atomic scale as a function of doping and temperature. In this thesis, we will describe these techniques as well as a series of new experiments that reveal a surprisingly simple picture of how superconductivity in the cuprates is optimized. We will first show that over a wide range of doping (optimal to overdoped), the pairing gaps in these systems nucleate in nanoscale regions at temperatures above Tc unlike in the conventional superconductors where the superconducting order parameter sets in at the bulk Tc. These regions proliferate as the temperature is lowered, resulting in a spatial distribution of gap sizes in the superconducting state. Analysis of our data shows no correlation between the inhomogeneous pairing gaps and either the energy scale of the boson modes or the strength of the local electron-boson coupling as measured by the local excitation spectra. This spatially inhomogeneous pairing strength is in fact determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 eV) electron-boson interactions are responsible for superconductivity in the cuprates. In contrast, the excitation spectra in the underdoped samples show multiple features that can't be fit to a simple d-wave order parameter. However, these spectra show a universal low energy excitation spectrum, indicating that the pairing strength near the nodes is independent of doping. The transition temperature Tc in this doping regime correlates with the fraction of the Fermi surface over which the samples exhibit the universal d-wave spectrum. Optimal Tc is achieved when all parts of the Fermi surface follow this universal behavior. Increasing temperature above Tc turns the universal spectrum into an arc of gapless excitations, while overdoping breaks down the universal nodal behavior

Enhancement of T-c in EuSr2Cu2+xRu1-xO8-y magnetic superconductor

Partial substitution of Ru by Cu in EuSr2Cu2RuO8 magnetic superconductor enhances the superconducting properties and decreases the magnetic ordering temperature of the Ru sublattice. However, superconductivity is destroyed if Ru replaces Cu. The observed behaviour may be expected for enhanced hole-doping type behaviour in the Cu-O planes which are responsible for superconductivity in these compounds. (C) 2003 Elsevier B.V. All rights reserved