M. Nantoh

Cryogenic Scanning Tunneling Microscopy and Spectroscopy on the Cleaved Surface of Bi-Sr-Ca-Cu-O Down to 4.2 K

Atomic resolution has been achieved by a scanning tunneling microscopy (STM) system for the first time on an oxide superconductor at cryogenic temperatures. The atomic corrugation with a periodicity of 0.35 nm and the modulation along the b-axis with a superlattice periodicity of about 2.7 nm have both been clearly observed on the cleaved a-b plane of a Bi-Sr-Ca-Cu-O single crystal. Furthermore, it is found that some features of the spectrum, including the shape of the superconducting gap, depended significantly on the distance between the probe tip and sample surface. Wide scattering of the results so far reported for various types of tunneling methods can be interpreted in terms of this distance dependence of the tunneling spectrum. We conclude that the superconducting gap structure in the spectrum is by far different from the prediction of the BCS weak-coupling theory.

Preparation and nanoscale characterization of highly stable YBa2Cu3O7-δ thin films

High-quality c-axis-oriented YBa2Cu3O7-δ (YBCO) films were prepared using the laser ablation method. Films with a full width at half-maximum of the (005) X-ray diffraction peak as narrow as 0.1° and an χmin of Rutherford backscattering spectrometry as low as 3.5% were obtained by optimizing the experimental conditions. The atomic image and superconducting gap at 4.2 K were simultaneously observed for the first time on these high-quality YBCO films by scanning tunneling microscopy and scanning tunneling spectroscopy. Such observations, even after the film was stored in air for three weeks, indicates that YBCO film is sufficiently chemically and physically stable to prevent significant deterioration in superconductivity at the film surface.

Atomic site tunneling spectroscopy on oxide superconductors

Abstract Atomic site tunneling spectroscopy has been applied to bulk single crystal of Bi2Sr2CaCu2Oy (BSCCO) and laser-ablated thin film of YBa2Cu3Oy (YBCO). The tunneling spectrum on the cleaved surface of BSCCO showed both semiconducting and superconducting behaviors, depending on the tip-sample distance. This observation is consistent with the quasi two-dimensional layered model. In both systems, clear superconducting gap structures with quite low zero bias conductances were observed, suggesting the s-wave like symmetry of the order parameter.

Cryogenic scanning tunneling microscopy/spectroscopy on the (001) surfaces of YBa2Cu3Oy epitaxial thin films

As‐grown surfaces of c‐axis‐oriented laser‐ablated YBa2Cu3Oy (YBCO) films have been investigated by scanning tunneling microscopy and spectroscopy (STM/STS) at 4.2 K. The square atomic arrangement with an average spacing of 0.4 nm was observed on the specimens even after exposure to ambient atmosphere, indicating that the (001) surface of the YBCO thin film was stable and free from severe contamination or chemical reaction. The STS observations revealed the semiconductive nature of the surface with a band gap of 0.1 eV. However, the tunneling spectrum varied its shape depending on the tip‐to‐sample distance and a clear superconducting gap structure with 2Δ∼40 meV appeared when the STM tip was closer to the surface than the usual position of the scanning mode, suggesting that the semiconductive layer is confined in the surface region of atomic size thickness.

Cryogenic scanning tunneling spectroscopy on oxide superconductors

Abstract Anisotropic electronic nature of Bi 2 Sr 2 CaCu 2 O y was investigated by scanning tunneling microscopy/spectroscopy (STM/STS) at various temperatures. From the STS measurements on the edge and cleaved surface, it was concluded that the BiO layer is essentially insulative even below T c . However, the superconducting gap structure with 2Δ/k B T c ≈10 was occasionally observed on the as-grown single crystals, possibly due to the surface variation.

Cryogenic scanning tunneling microscopy/spectroscopy on the (110) surfaces of YBa2Cu3Oy epitaxial thin films

Abstract As-grown surfaces of (110) oriented laser-ablated YBa 2 Cu 3 O y films have been investigated by scanning tunneling microscopy and spectroscopy, for the first time, at room temperature and 4.2 K. We found semi-macro scale surface structures along the c -axis, which were composed of (110) top planes and (100)/(010) lateral planes. The orthorhombic atomic lattice image, corresponding to Cu atom arrangement of both CuO 2 double layers and CuO chain layers alternately, was observed on the (110) top plane at 4.2 K. Furthermore, broad swell-and-depression structures were observed as stripes perpendicular to the c -axis on the top plane. The STS results indicated superconducting nature on the swells and semiconducting nature on the depressions.

Superconducting gap observed by the atomic-site tunneling spectroscopy on YBCO and BSCCO

Abstract Atomic site tunneling spectroscopy has been applied to bulk single crystal of BSCCO and laser-ablated thin film of YBCO. The tunneling spectrum on the cleaved surface of BSCCO showed both semiconducting and superconducting behaviors, depending on the tip-sample distance. This observation is consistent with the quasi two-dimensional layered model. In both systems, clear superconducting gap structures with quite low zero bias conductances were observed, suggesting the s -wave like symmetry of the order parameter.

STM/STS study of single crystalline Bi2Sr2CaCu2Oy (2): Electronic nature of the BiO layer as a function of oxygen content

Abstract The electronic nature of the BiO layer in Bi 2 Sr 2 CaCu 2 O y was studied as a function of oxygen content by STM/STS at room temperature and cryogenic temperatures down to 4.2 K. The conductance (dI/dV) vs V curve taken on the cleaved surface exhibited relatively large energy gap of ∼0.1 eV, but no BCS-like peak was observed. This result suggests that the BiO layer is essentially insulative even below Tc. Furthermore, the specimens annealed under oxidized atmosphere tended to show metallic tunneling behavior. It seems that the incorporation of oxygen content in the BiO layer increases the density of states in the vicinity of the Fermi level.

Atomic site tunneling spectroscopy on high-Tc superconductors

Abstract Superconducting gap structures of Bi2Sr2CaCu2Oy (BSCCO) and YBa2Cu3Oy (YBCO) have been probed by scanning tunneling microscopy (STM) at cryogenic temperatures. The tunneling conductance curves observed on bulk single crystals of BSCCO and epitaxial thin films of YBCO revealed clear overshooting peaks and flat bottom regions around V=0 with quite low zero-bias conductances of ≈1%. Since the electron tunneling process in STM is essentially incoherent, the present observation is favored by the s-wave pairing mechanism. However, the conductance curves were found to be substantially smeared in comparison with the conventional excitation spectra predicted in the BCS (isotropic s-wave) superconductors, suggesting gap anisotropy.

Nanoscale structural variation observed on the vicinal SrTiO3(001) surface

The vicinal (001) surface of a Nb-doped SrTiO3 single crystal has been investigated by scanning tunneling microscopy and low energy electron diffraction. The stepped surface prepared by annealing in ultrahigh vacuum at 250 °C exhibits a complex atomic structure composed of four types of reconstructions, which shows short-range variation within nanoscale regions. SrO layers show a c(6×2) structure being stable up to 1000 °C, while √13×√13-R33.7°, c(√13×√13)-R33.7°, and c(√2×√18)-R45° structures are formed on TiO2 layers, which disappear at 450–750 °C followed by the formation of 2×2 and √5×√5-R26.6° structures. These results indicate instability of the reconstructions on the TiO2 terminated surface due to the variation in Sr adatom density caused by multikinetic processes, in contrast to the thermodynamically stable SrO terminated surface.