T. Haage

Direct scanning tunneling microscopy observation of non‐unit‐cell growth of YBa2Cu3O7−δ thin films

The surface morphology of thin YBa2Cu3O7−δ films grown on LaSrAlO4 (001) and SrTiO3 (001) by pulsed laser deposition was studied by scanning tunneling microscopy (STM). STM was carried out in ultra high vacuum without prior exposure of the films to air. In the STM images, stacks of terraces which are separated by steps of various heights were observed. Some steps were shorter than the YBa2Cu3O7−δ unit cell height c≊1.2 nm. We present evidence of the fact that the YBa2Cu3O7−δ thin film surface exhibits different terminations. The microstructure revealed from the STM images implies a non‐unit‐cell growth mechanism.

Substrate-mediated anisotropy of transport properties in YBa2Cu3O7−δ thin films

Abstract We have generated an almost periodic step structure of c-axis-oriented YBa 2 Cu 3 O 7−δ films grown on 10° miscut SrTiO 3 (001) substrates. Combined scanning tunneling microscopy and cross-sectional transmission electron microscopy studies reveal that this growth morphology is linked to an anisotropic defect microstructure. We present evidence that translational boundaries contribute to strong flux pinning in our films. The resistivity and critical current density in the miscut-grown YBa 2 Cu 3 O 7−δ films have been found to be anisotropic.

Raman spectra of ultrathin YBaCuO7−δ films

Thin films of YBa2Cu3O7−δ (YBCO) on LaSrAlO4 (001) substrates with various thicknesses have been investigated by Raman spectroscopy. The Raman spectra of the films have been obtained by properly subtracting the spectrum of the substrate from the recorded spectra. This data anaysis enables us to determine the Raman spectra of YBCO films with thicknesses down to 12 nm (about 10 unit cells). The evolution of the peak positions and the linewidths of the B1g mode with thickness allow us to conclude that the residual strains in these films are negligibly small within the measured thickness range. The influence of the birefringence on measurement of the degree of epitaxy by Raman has been studied. With the reduction of film thickness the degree of epitaxy is slightly increased in these films.

Periodic step structure of clean and YBa2Cu3O7-δ-covered vicinal SrTiO3(001) surfaces

Abstract We have studied the structure of a stable vicinal SrTiO 3 (001) surface by UHV scanning tunneling microscopy. We find that the regular nanoscale step structure on the vicinal surface strongly influences the growth of c -axis-orientedYBa 2 Cu 3 O 7 − δ (YBCO) thin films. It generates an almost periodic surface structure of the YBCO films. This substrate-mediated control of the film growth is particularly important, because the resultant YBCO microstructure leads to an in-plane anisotropy of the transport properties in the high-temperature superconducting material.

Anisotropic defect structure and transport properties of YBa2Cu3O7−δ films on vicinal SrTiO3(001)

The microstructure of YBa2Cu3O7−δ thin films grown on vicinal SrTiO3(001) has been studied as a function of the vicinal angle by x-ray diffraction using the two-dimensional q-scan technique. Our results reveal a strong correlation between the miscut of a SrTiO3(001) substrate and the anisotropic defect structure of the film. Furthermore, we observed an anisotropy of the corresponding critical current density up to 4.6 depending on the angle of miscut.

Anisotropic flux pinning in thin YBCO-films by substrate modifications

Abstract The magnetooptical Faraday effect is an excellent tool to investigate the spatially resolved flux density distribution and, by a numerical inversion of the Biot–Savart law, the local current density in a superconducting film with a spatial resolution better than 10 μm [Ch. Jooss, R. Warthmann, A. Forkl, H. Kronmuller, Physica C 299 (1998) 215]. Irradiating the surface of SrTiO 3 (001) substrates with a focused ion beam in a line pattern leads to an anisotropy of the flux and current density distribution in a well-defined area. While we measure an enhancement of j c parallel to the lines, a reduction of the value of the current perpendicular to the lines is found. This is in qualitative agreement with experiments on YBCO films grown on vicinal substrate surfaces [T. Haage, J. Zegenhagen, J.Q. Li, H.-U. Habermeier, M. Cardona, Ch. Jooss, R. Warthmann, A. Forkl, H. Kronmuller, Phys. Rev. B 56 (1997) 8404] and based on the data gained on these systems, a pinning model for the films on the irradiated substrates is suggested.

Substrate-mediated control of the microstructure of YBa2Cu3O7−δ thin films

Abstract We have studied the surface of vicinal SrTiO3(001) substrates and thereon deposited YBa2Cu3O7−δ (YBCO) thin films by UHV-scanning tunneling microscopy (STM). We discuss the influence of the uniformity of substrate terraces on film growth. STM images indicate, that a combination of island and step-flow growth occurs on 1.2° miscut SrTiO3. In particular, a regular nanoscale step structure of a 10° miscut SrTiO3 surface generates an almost periodic surface structure of the YBCO films. Our findings indicate that a substrate-mediated control of the surface structure of YBCO films can be achieved.

Tc reduction and related phases at early stages of YBaCuO7-δ film growth on LaSrAlO4

Abstract The temperature-dependence of the resistivity of YBa 2 Cu 3 O 7-δ (YBCO) films grown on LaSrAlO 4 (LSAO) substrate with thicknesses from 12 to 240 nm were studied systematically. It was found that a drastic reduction of T c occurs at a thickness of about 60 nm, which is much larger than the value (10 nm) for YBCO films on SrTiO 3 . These ultra-thin films were also characterised by Raman, STM, TEM and grazing-incidence X-ray diffraction (GIXRD). With a film thickness below 60 nm Raman spectra revealed an abnormal enhancement of the intensity of Cu Cu vibration mode, which indicates the formation of oxygen deficient phases. Additional evidences were provided by GIXRD and TEM. The formation of oxygen deficient microdomain phases near the interface explains the observed early suppression of T c in YBCO on LSAO as compared to YBCO on STO.

Growth-induced in plane anisotropy of flux-line pinning in YBa2Cu3O7 thin films

Abstract Recent progress in tailoring periodic step structures in vicinal SrTiO3 single crystal substrate surfaces enable a systematic study of the interplay of growth mediated defect structures and transport properties in HTS thin films. UHV annealed (001) oriented SrTiO3 substrates with a 10° miscut towards the [010] direction show after a UHV annealing a regular nanoscale step structure which generates an almost periodic terrace-like surface structure of the YBa2Cu3O7 film. Cross-sectional as well as planar view TEM reveals an anisotropic defect structure arising from the nanoscale step structure of the substrate surface. Transport measurements and magneto-optical analysis of the flux penetration show a growth-induced in plane anisotropy and an anisotropic enhancement of the critical current up to values close to the depairing limit.

Nanoscale engineering: Tailored transport properties by self-organization in YBa2Cu3O7-δ thin films

Abstract We have grown YBa2Cu3O7-δ (YBCO) films on vicinal SrTiO3(001) surfaces. Scanning tunneling microscopy reveals that their morphology is reminiscent of the regular array of steps on the substrate surface. The resultant film microstructure leads to a pronounced in-plane anisotropy of the resistivity and the critical current density jc. We observed a substantial enhancement of jc up to 8 × 1011A/m2 in the nanoscale engineered films.