Ø. Fischer

Magnetic susceptibility of (rare earth) x Mo6S8

We have studied the susceptibility of all (RE)xMo6S8 compounds between 1.3 and 300 K (RE = rare earth). In the two superconducting compounds Ho1.2Mo6S8 and Dy1.2Mo6S8 magnetic ordering is expected in the temperature range 0.1–1 K. The magnetic transition in CeMo6S8 at2.25 K is of antiferromagnetic nature. A second anomaly at 1.6 K in this compound may announce a low-temperature ferromagnetic state.

Imaging of granular high‐Tc thin films using a scanning tunnelling microscope with large scan range

STM images are presented in the micron scale, taken in UHV with a single‐tube scanner STM with SEM control of the tip position. For calibration, a carbon grid was used from which the coupling of x and y scan axes has been determined as well as the piezo‐sensitivity factors. Images taken of a YBa2Cu3O7‐δ, film display the granular structure of the crystallites formed during the post‐evaporation annealing. A direct comparison of STM scans with SEM micrographs was made which demonstrates the complementary information obtained by the two methods. The obtained resolution is compared to the tip shape which becomes crucial for the imaging of corrugated surfaces on the micron scale. With a simple geometric model, an attempt has been made to reconstruct the surface topography from STM scans based on the knowledge of the tip shape.

In situ thin films of YBa2Cu3O7 prepared by single target DC-magnetron sputtering

Abstract We have prepared in situ thin films of YBa 2 Cu 3 O 7 by single target dc reactive magnetron sputtering. By a delicate adjustment of the oxygen partial pressure as well as the total sputtering pressure, we can grow stoichiometric films using a stoichiometric target. This method gives highly reproducible results and the films have optimum superconducting properties without the need for any post-annealing treatment. The films are highly textured with the c -axis perpendicular to the plane of the substrate for films grown on unoriented ZrO 2 and a mixed c -axis and a -axis orientation on (100) SrTiO 3 . Zero resistances are obtained between 85 and 88 K with onsets between 90 and 91 K. The residual resistivity ratios R(RT) R(100 K) have values between 2.5 and 3 and the resistivity at 100 K is 60 μΩ cm for films on SrTiO 3 . SEM photos reveal much smoother surfaces than post-annealed films. We also obtained RHEED streaks, indicative of an oriented atomically smooth surface, on a 250–300 A thick layer on SrTio 3 .

Microstructure of YBa2Cu3O7/PrBa2Cu3O7 superlattices deposited on (1 0 0) SrTiO3 single crystals

Abstract High-quality, c -oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices have been grown on (1 0 0) SrTiO 3 substrates. TEM cross sections revealed that the sharpness of the (1 0 0) interfaces was of the order of the lattice parameter (1 nm). The films were deposited at 790°C and no evidence of Y/Pr interdiffusion could be detected. The orthorhombic (1 1 0) twins were 20 nm wide and the orthorhombic distortion in the YBa 2 Cu 3 O 7 layer yielded a ( b − a )/ b ratio of 1.6%. Surface steps and inclined surface areas at the SrTiO 3 YBa 2 Cu 3 O 7 interface were imaged by high-resolution TEM. Apart from a 2–6 nm wide layer at the interface, which contained extended defects accounting for the mismatch due to the imperfect surface, the film grew perfectly on top of this layer. (0 0 1) lattice planes passed continuously from regions with perfect substrate surfaces to regions containing a larger density of surface steps.

Tc variation in PbMo6S8: A critical analysis and a comparison with pure phases

Abstract Different methods were employed to synthesize a series of high quality PbMo 6 S 8 Chevrel-phase compounds. All the samples were characterized by means of X-ray diffraction, Auger electron spectroscopy, specific heat, acsusceptibility, resistivity, scanning electron spectroscopy and microprobe analysis. A marked decrease in the superconducting transition temperature (T c ) and the hexagonal lattice constant ratio (c/a) were noticed on the samples prepared in quartz tubes. Based on a simple correlation between T c and c/a ratio, we present some of the significant structure-property relations and phase purity of the samples. This interdependence is discussed mainly in terms of oxygen content in the phase. For instance, oxygen-free samples have T c invariably above 14.5 K. Interestingly, the optimal T c is found in samples having rather different, from each other, crystallographic parameters, but with a same c/a ratio. On the other hand, the wide variation of T c and structural parameters of PbMo 6 S 8 observed in the present investigation and those of many earlier works are explained by the presence of varying amounts of oxygen in the ternary phase. These results are checked in parallel with the oxygen-doped (PbMo 6 S 8−x O x ; x ≥ 0.2) samples.

Extended defects in YBa2Cu3O7/PrBa2Cu3O7 superlattices deposited on (1 0 0) MgO single crystals

Abstract Mixed a - and c -oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices have been grown on (1 0 0) MgO substrates. These superlattices were analyzed by transmission electron microscopy. The interfaces between the various layers are sharp (sharpness ≈ 1 nm) and show no evidence for Y/Pr interdiffusion. In cross sections, a -oriented nuclei 25 nm in diameter were observed at the MgO interface. The volume fraction of a -oriented grains increases with the distance from the interface. At the surface the 115 nm thick film is almost completely a -oriented and consist of grains 150 nm in size. The planar defects in these grains were studied by high- resolution transmission electron microscopy and were due to additionally inserted CuO layers.

Study of field‐emitting microstructures using a scanning tunneling microscope

Field emission from broad metal cathodes is known to be strongly enhanced at a small number of emitting sites per cm2 compared to the expected Fowler–Nordheim emission from ideal, flat surfaces. We have operated a scanning tunneling microscope (STM) in the field emission regime (typical tip voltage: +80 V) and measured the local field emission strengths and variations on niobium samples. With a modulation technique, which is an adaptation of the standard work function measurement, maps of the field enhancement factor β have been obtained. An example of an emission site is presented where STM topograph and β map are compared with a secondary electron microscope image and with field emission data obtained in a standard way using high‐voltage anodes. This demonstrates the capability of a scanning tunneling microscope to localize enhanced field emission sites (with typical β values of 50 in the present work) with high spatial resolution and to study surfaces down to the limit β=1.

Artificially prepared YBa2Cu3O7 PrBa2Cu3O7 superlattices: Growth and superconducting properties

Abstract We report on the growth and superconducting properties of artificially prepared YBa 2 Cu 3 O 7 PrBa 2 Cu 3 O 7 ( YBCO PrBCO ) superlattices. We have grown these superlattices by single target dc planar magnetron sputtering using the high sputtering pressure technique to obtain the in situ growth with the correct stoichiometry. We find in these modulated structures that the superconducting critical temperature, Tc, depends markedly on the individual YBCO thickness in the multilayer and on the insulating PrBCO separation thickness. For series of samples with constant 12A or 24A layers of YBCO (one or two unit cells), Tc initially decreases with increasing thickness of PrBCO, d-PrBCO, and then saturates for d-PrBCO larger than 60–70A. We discuss this behavior in terms of a progressive decoupling of the ultrathin YBCO layers in the multilayer. Resistively measured parallel critical fields show, for structures with thick PrBCO layers, a transition from a flux flow regime for thick YBCO layers to a behavior with only very weak magnetic field induced broadening of the resistive transitions for thin YBCO layers. For multilayers with individual YBCO layers of 48A or 96A thickness we find a linear behavior of the resistively determined critical field with a slope of ≅7.5T/K. When the individual YBCO layer thickness is reduced to 24A practically no effect of a 9T magnetic field could be observed. These different regimes are discussed in terms of the particular vortex structures in the multilayers.

A low-temperature scanning tunneling microscope with in-situ sample cleaving

Abstract We present the design of a scanning tunneling microscope (STM) that is operated in an ultra-high vacuum (UHV) chamber at room temperature and can be lowered into a standard helium crystat and cooled with helium exchange gas to low temperatures. Central to the design of our system is a compact concentric tube scanning tunneling microscope in which the central tube serves as the scanner and the outer tube allows linear inertial translation of the sample by slip-stick motion. The fact that no mechanical connections are needed to approach the sample and tip allows great flexibility in the design of the system and the vibration isolation of the STM. Samples are cleaved in-situ in the UHV chamber. The stability of the instrument is demonstrated with images of both graphite and the high-temperature superconductor Bi2Sr2Ca1Cu2O8 obtained in UHV (P = 2×10-10 Torr) at room temperature.

Epitaxial high Tc cuprate superlattices: A study of the thermal flux motion

Abstract Epitaxial c-axis YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PrBCO) superlattices grown by single target dc planar magnetron sputtering are used as a model system to investigate the nature of flux motion in high T c superconductors (HTS). When a magnetic field H is applied parallel to the c-axis we find that, for sufficiently thick PrBCO layers, the activation energy for flux motion U is proportional to the YBCO thickness. This allows us to extract a lower limit, about 500A, for the correlation lenght along the vortices in pure YBCO. We find also for thin YBCO layers that U∼lnH which could be the signature that the dissipation process is related to the creation in the 2-D vortex lattice of dislocations pairs. When the field is applied in the a-b plane we find for thin YBCO layers separated by thick enough PrBCO layers that the resistive transition becomes field independent. Measurements on a 24A/96A YBCO/PrBCO multilayer up to 20T do not show any broadening of the resistive transition. We also find that the activation energy for flux motion decreases with decreasing the YBCO thickness and is insensitive to magnetic field below the H c1 of the individual YBCO layers.