Shuheng H. Pan

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.

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.

A low-temperature ultrahigh-vacuum scanning tunneling microscope with rotatable magnetic field

We present a new design of a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope setup with a combination of a solenoid and a split-pair magnet. The scanning tunneling microscope can be operated at temperatures down to 8 K and in a rotatable magnetic field of up to 1 T. Magnetic fields of up to 7 T perpendicular and 2 T parallel to the sample surface can be applied. The UHV part of the system allows in situ preparation and low energy electron diffraction/Auger analysis of samples. First topographic and spectroscopic measurements on p-InAs(110) are presented.

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.

Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2

For a type-II superconductor, when the applied magnetic field is higher than the lower critical value Hc1, the magnetic flux will penetrate into the superconductor and form quantized vortices, which usually are arranged in an Abrikosov lattice. For the newly discovered iron pnictide superconductors, previous measurements have shown that, in electron-doped BaFe2As2, the vortices form a highly disordered structure. In addition, the density of states (DOS) within the vortex cores1 do not exhibit the Andreev bound states in conventional superconductors. In this Letter, we report the observation of a triangular vortex lattice and the Andreev bound states in hole-doped BaFe2As2 by using a low temperature scanning tunneling microscope (STM). Detailed study of the vortex cores reveals that the spectrum of the Andreev bound states inside the vortex core exhibits a distinct spatial evolution: at the center of the vortex core, it appears as a single peak at 0.5 mV below the Fermi-energy; away from the core center, it gradually evolves into two sub-peaks and they eventually fade out. The drastic differences between the vortex cores of the electron-doped and hole-doped counterparts are illusive to the pairing mechanism of the iron pnictide superconductors.

Observation of a robust zero-energy bound state in iron-based superconductor Fe(Te,Se)

The symmetry of Cooper pairs in iron-based superconductors is an issue under continued investigation. A scanning tunnelling study of Fe(Te,Se) reveals a robust zero-energy bound state, providing evidence for a non-trivial pairing symmetry.

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.

In-situ formation of BSCCO thin films by plasma assisted thermal evaporation

Thin films of the superconductor Bi-Sr-Ca-Cu-O (BSCCO) have been prepared by thermal evaporation in an evaporator featuring an RF-excited oxygen plasma generator. Formation of the 2212 phase is obtained in situ, as confirmed by X-ray analysis. The films require postannealing, however, in order to exhibit a superconducting transition. Specifically, postanneals are required to obtain critical temperatures of up to 75 K; 1330 Pa is the minimum annealing pressure. The authors report the study of the superconducting properties as a function of oxygen annealing pressure as well as a characterization of the oxygen plasma. An investigation of the surface morphology was performed using a force microscopy (AFM) and a tunneling microscopy (STM). The latter clearly shows terraces and steps with a height of 1.5 nm, or multiples thereof, corresponding to one-half of the c-axis lattice constant of the 2212 compound. The AFM, on the other hand, shows a drastic difference between as-grown and annealed films, even when the former are sometimes insulating.

Experimental investigation of the electronic structure of Ca

We performed a combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy study of the electronic structure of electron-doped Ca0.83La0.17Fe2As2. A surface reconstruction associated with the dimerization of As atoms is observed directly in the real space, as well as the consequent band folding in the momentum space. Besides this band folding effect, the Fermi surface topology of this material is similar to that reported previously for BaFe1.85Co0.15As2, with Gamma-centered hole pockets quasi-nested to M-centered electron pockets by the antiferromagnetic wave vector. Although no superconducting gap is observed by ARPES possibly due to low superconducting volume fraction, a gap-like density of states depression of 7.7 +/- 2.9 meV is determined by scanning tunneling microscopy.

_{0.83}

Abstract We have previoslly reported a 1.36-nm DOS modulation in the CuO chains on the surface of cold-cleaved, atomically-resolved YBCO 1,2 . We have recently completed new experiments in which we obtained detailed spectroscopic data on the same crystal plane, thereby confirming and extending our earlier work. Here we present data showing twin boundaries and steps, and focusing on the effects of these structures on the CuO chain DOS modulations.