E. W. Hudson

Imaging the effects of individual zinc impurity atoms on superconductivityin Bi2Sr2CaCu2O8+|[delta]|

Although the crystal structures of the copper oxide high-temperature superconductors are complex and diverse, they all contain some crystal planes consisting of only copper and oxygen atoms in a square lattice: superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substituting a single impurity atom for a copper atom strongly perturbs the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. This has provided the motivation for several experimental and theoretical studies. Scanning tunnelling microscopy (STM) is an ideal technique for the study of such effects at the atomic scale, as it has been used very successfully to probe individual impurity atoms in several other systems. Here we use STM to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi2Sr2CaCu 2O8+δ. We find intense quasiparticle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within ∼15 Å of the scattering sites. Imaging of the spatial dependence of the quasiparticle density of states in the vicinity of the impurity atoms reveals the long-sought four-fold symmetric quasiparticle ‘cloud’ aligned with the nodes of the d-wave superconducting gap which is believed to characterize superconductivity in these materials.

Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x.

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are ‘doped’ away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers (‘holes’) into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of ∼14 Å. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

3He refrigerator based very low temperature scanning tunneling microscope

We describe the design and development of a scanning tunneling microscope (STM) which can operate at temperatures down to 240 mK and in magnetic fields up to 7 T with high spatial and energy resolution. The compact and rigid STM head is mounted directly on a low vibration, single shot, 3He refrigerator. This refrigerator can be operated at its base temperature continuously for several days before the 3He needs to be recondensed. The system is equipped with a sample transport manipulator from room temperature, and a cleavage device at low temperature, so that the cryogenic ultrahigh vacuum condition inside the cryostat can be utilized. A superconducting magnet provides a magnetic field of up to 7 T at the sample along the STM tip direction. Test results have shown that, at the base temperature, this instrument has better than 0.5 pm z-direction resolution in imaging mode, and better than 20 μV energy resolution in spectroscopy mode.

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’?

Vacuum tunneling of superconducting quasiparticles from atomically sharp scanning tunneling microscope tips

We report on the study of atomically sharp superconducting tips for scanning tunneling microscopy and spectroscopy. The results clearly show vacuum tunneling of superconducting quasiparticles from atomically sharp tips. Observed deviations of the energy gap of the superconducting tip from its bulk value are attributed to the proximity effect. We show that a combination of a superconducting tip and an atomic resolution scanning tunneling microscope provides a means of achieving very high resolution local spectroscopy. We also discuss how this combination paves the way for a number of important applications.

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.

Dopant clustering, electronic inhomogeneity, and vortex pinning in iron-based superconductors

We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr0.75K0.25Fe2As2 with T-c = 32 K. We find that the low-T cleaved surface is dominated by a half Sr/K termination with 1 x 2 ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by sigma/(Delta) over bar = 16% on a similar to 3 nm length scale, with average 2 (Delta) over bar /k(B)T(c) = 3.6 in the weak-coupling limit. The vortex core size provides a measure of the superconducting coherence length xi = 2.3 nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.

Imaging and identification of atomic planes of cleaved Bi2Sr2CaCu2O8+δ by high resolution scanning tunneling microscopy

Imaging of the surface of a cleaved Bi2Sr2CaCu2O8+δ (BSCCO) single crystal with a scanning tunneling microscope reveals a series of repeating terraces, whose separations are then used to identify the atomic planes which are exposed. On each of the exposed planes, the incommensurate modulation is also clearly resolved with atomic resolution. The measured separations between the terraces lead to the deduction that any atomic layer can be exposed by mechanical cleavage of BSCCO. We, therefore, suggest that the identity of atomic planes, and the direction of tunneling, should always be taken into consideration when interpreting tunneling spectra obtained on such cleaved BSCCO crystals.

Search for superconductivity in lithium

We report on the results of a search for superconductivity in Li. We find no evidence for the predicted transition to superconductivity at any temperature down to 5 mK in magnetic fields down to 0.4 μT. However, an unexpected Curie–Weiss temperature dependence in the magnetic susceptibility is observed. We discuss the possibility that this signal arises from the Li itself, the possibility that it arises from Kondo behavior, and the implications of the effect for the predicted Tcof Li.