The search for superconductivity in new and unexpected structures has been ongoing since the initial discovery in Leiden over 9 decades ago. Though the successes are few the rewards are great. Our meeting here today is a direct result of Bednorz and Mueller's discovery of cuprate superconductivity . The questions which have arisen as a result of this single discovery have uncovered inadequacies of theory and stimulated new ways of thinking. Understanding the mechanism(s) of high temperature superconductivity is among the foremost challenges of theoretical and experimental research today . Searching for new superconductors has always been a fruitful research enterprise, and as we see, there are new opportunities for doing so today. For more than 4 decades after the initial discovery there was no microscopic theory (the most outstanding theorists from Heisenberg down tried and failed to come up with a satisfactory theory) and the experimental basis for understanding the underlying mechanisms was inadequate. It must have been a surprise for Kamerlingh Onnes, after taking care to use the purest Hg he could obtain in the investigation that led to the discovery of superconductivity, to find that ordinary solder was also superconducting. In 1932 Meissner discovered barely metallic copper sulfide was superconducting, while high conductivity copper itself was not superconducting. These puzzles and others like it suggested that a comprehensive search for new superconductors might reveal a pattern of occurrence that would reveal clues, and prompted John Hulm and Bernd Matthias, with encouragement from Enrico Fermi  in 1951 to undertake a full-scale effort to find new superconductors. This was a propitious time for such an undertaking for a number of reasons. Today parallel reasons exist.
We report results of low temperature thermodynamic and transport measurements of Pb1-xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately 1.5 %. The material superconducts for x > 0.3 %, with a maximum Tc of 1.5 K for the highest Tl concentrations. All superconducting samples exhibit an anomalous resistivity upturn at low temperatures, whereas non-superconducting samples (x < 0.3%) do not. The temperature and field dependence of this resistivity upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature TK ~ 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high Tc values.
Efforts to grow high quality films of YBCO on Si have been complicated by factors discussed in Ref. 1, chief among
them being the reaction between YBCO and Si, which is damaging even at 550 C. This is well below the customary
temperatures for YBCO film growth. To avoid the reaction problem, epitaxial YBCO films were grown on Si (100) using an
intermediate buffer layer of yttria-stabilized zirconia (YSZ).2 Both layers are grown via an entirely in situ process by pulsed
laser deposition (PLD). Although the buffer layer prevents reaction, another problem arises; the large difference in thermal
expansion coefficients between silicon and YBCO causes strain at room temperature. Thin (<500 A) YBCO films are unrelaxed
and under tensile strain with a distorted unit cell. Thicker films are cracked and have poorer electrical properties. The thermal
strain may be reduced by growing on silicon-on-sapphire (SOS) rather than silicon.3 This allows the growth of films of
arbitrary thickness. Ion channeling reveals a high degree of crystalline perfection with a channeling minimum yield for Ba as
low as 12% on either silicon or SOS. The normal state resistivity is 250-300 i-cm at 300 K; the critical temperature, Tc
(R=0), is 86-88 K with a transition width (ATc) of I K. Critical current densities (J)°f 2x107 A/cm2 at 4.2 K and >2x106
A/cm2 at 77 K have been achieved. In addition, the surface resistance of a YBCO film on SOS was measured against Nb at 4.2
K. At 10 GHz, a value of 45 was obtained. This compares favorably to values reported for LaAlO3.
Application of this technology to produce reaction patterned microstrip lines has been tested.4 This was done by ion
milling away portions of the YSZ buffer layer prior to the YBCO deposition. YBCO landing on regions of exposed Si reacts
to form an insulator. This technique was used to make 3 micron lines 1.5 mm long. The resulting structure had a Jc of
l.6xl06 A/cm2 at 77 K. Isolation of separate structures exceeded 20 M. Several advantages of this technique are that no
solvents, etchants or photoresist come into contact with the YBCO, hence this technique has a potential for operational-asgrown
In summary, it is now possible to produce YBCO films with structural and DC electrical properties which rival the
most optimized c-axis epitaxial YBCO films on MgO, SrTiO3 and LaAlO3. Preliminary measurements of microwave
properties appear promising.
We thank Bruce Lairson for help obtaining magnetization data and Richard Johnson, Steve Ready and Lars-Erik Swartz
for technical assistance. This work benefits from AFOSR (F49620-89-C-0017). DBF received support from NSF (DMR-
8822353). DKF acknowledges the AT&T scholarship.
Single target in situ sputter deposition using 9O off-axis geometry has made it possible to study various physical
properties of superconducting films of YBaCu3O'x under well controlled conditions. The superconducting DC and high
frequency properties, and the normal state properties can all be optimized under conditions which do not necessarily produce
the highest Tc's. Surface resistances at 4.2 K and 10 GHz are reproducibly less than 20 iQ. Correlations between the
processing parameters, DC properties, microstructures and high frequency properties of these films as they are presently
known are summarized.
Conference Committee Involvement (1)
Strongly Correlated Electron Materials: Physics and Nanoengineering
31 July 2005 | San Diego, California, United States