W. D. Wise

Charge-density-wave origin of cuprate checkerboard visualized by scanning tunnelling microscopy

The checkerboard pattern observed in high-temperature superconductors by scanning tunnelling microscopy is widespread, but what does it mean? And what does it say about the mysterious ’pseudogap’?

Fermi Surface and Pseudogap Evolution in a Cuprate Superconductor

Under the Dome The superconducting transition temperature Tc of copper oxides has a dome-shaped dependence on chemical doping. Whether there is a quantum critical point (QCP) beneath the dome, and whether it is related to the enigmatic pseudogap, has been heavily debated. Two papers address this question in two different families of Bi-based cuprates. In (Bi,Pb)2(Sr,La)2CuO6+δ, He et al. (p. 608) found that the Fermi surface (FS) undergoes a topological change as doping is increased, which points to the existence of a QCP at a doping close to the maximum in Tc, seemingly uncorrelated with the pseudogap. Fujita et al. (p. 612) studied a range of dopings in Bi2Sr2CaCu2O8+δ to find an FS reconstruction simultaneous with the disappearance of both rotational and translational symmetry breaking, the latter of which has been associated with the pseudogap. These findings point to a concealed QCP. Scanning tunneling microscopy is used to provide evidence for a quantum critical point beneath the superconducting dome. The unclear relationship between cuprate superconductivity and the pseudogap state remains an impediment to understanding the high transition temperature (Tc) superconducting mechanism. Here, we used magnetic field–dependent scanning tunneling microscopy to provide phase-sensitive proof that d-wave superconductivity coexists with the pseudogap on the antinodal Fermi surface of an overdoped cuprate. Furthermore, by tracking the hole-doping (p) dependence of the quasi-particle interference pattern within a single bismuth-based cuprate family, we observed a Fermi surface reconstruction slightly below optimal doping, indicating a zero-field quantum phase transition in notable proximity to the maximum superconducting Tc. Surprisingly, this major reorganization of the system’s underlying electronic structure has no effect on the smoothly evolving pseudogap.

Imaging nanoscale Fermi-surface variations in an inhomogeneous superconductor

A variant on Fourier-transform scanning tunnelling spectroscopy enables spatial variations in the Fermi surface of bismuth-based cuprate superconductors to be probed. This technique reveals that these variations take place over nanometre distances. Particle–wave duality suggests we think of electrons as waves stretched across a sample, with wavevector k proportional to their momentum. Their arrangement in ‘k-space’, and in particular the shape of the Fermi surface, where the highest-energy electrons of the system reside, determine many material properties. Here we use a novel extension of Fourier-transform scanning tunnelling microscopy to probe the Fermi surface of the strongly inhomogeneous Bi-based cuprate superconductors. Surprisingly, we find that, rather than being globally defined, the Fermi surface changes on nanometre length scales. Just as shifting tide lines expose variations of water height, changing Fermi surfaces indicate strong local doping variations. This discovery, unprecedented in any material, paves the way for an understanding of other inhomogeneous characteristics of the cuprates, such as the pseudogap magnitude, and highlights a new approach to the study of nanoscale inhomogeneity in general.

An auxiliary capacitor based ultrafast drive circuit for shear piezoelectric motors

Shear piezoelectric motors frequently require large voltage changes on very short time scales. Since piezos behave electrically as capacitors, this requires a drive circuit capable of quickly sourcing or sinking a large amount of current at high voltages. Here we describe a novel circuit design using a high voltage amplifier, transistor switching stage, and auxiliary capacitor. This circuit can drive piezoelectric motors at higher speeds and lower costs than conventional methods and with greater flexibility for computer automation. We illustrate its application in a controller for a scanning tunneling microscope coarse approach mechanism and discuss other possible applications and modifications of this circuit.

Suppression of Superfluid Density and the Pseudogap State in the Cuprates by Impurities

We use scanning tunneling microscopy (STM) to study magnetic Fe impurities intentionally doped into the high-temperature superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}. Our spectroscopic measurements reveal that Fe impurities introduce low-lying resonances in the density of states at Ω_{1}≈4  meV and Ω_{2}≈15  meV, allowing us to determine that, despite having a large magnetic moment, potential scattering of quasiparticles by Fe impurities dominates magnetic scattering. In addition, using high-resolution spatial characterizations of the local density of states near and away from Fe impurities, we detail the spatial extent of impurity-affected regions as well as provide a local view of impurity-induced effects on the superconducting and pseudogap states. Our studies of Fe impurities, when combined with a reinterpretation of earlier STM work in the context of a two-gap scenario, allow us to present a unified view of the atomic-scale effects of elemental impurities on the pseudogap and superconducting states in hole-doped cuprates; this may help resolve a previously assumed dichotomy between the effects of magnetic and nonmagnetic impurities in these materials.