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We report on high temperature superconductor (HTS) DC current leads developed for the specific purpose of delivering small currents (

Magnesium diboride (MgB2) has a transition temperature of (Tc) ~40 K, i.e., about 4 times as high as that of niobium (Nb). We have been evaluating MgB2 as a candidate material for radio-frequency (RF) cavities for future particle accelerators. Studies in the last 3 years have shown that it could have about one order of magnitude less RF surface resistance (Rs) than Nb at 4 K. A power dependence test using a 6 GHz TE011 mode cavity has shown little power dependence up to ~12 mT (120 Oe), limited by available power, compared to other high- materials such as YBCO. A recent study showed, however, that the power dependence of Rs is dependent on the coating method. A film made with on-axis pulsed laser deposition (PLD) has showed rapid increase in compared to the film deposited by reactive evaporation method. This paper shows these results as well as future plans.

We have grown and optimized several thin-film materials similar to YBCO, but with Y replaced by an alternative rare earth element. Because many of these elements sublime rather than melt readily, compositional control of these materials during deposition can be more difficult than for YBCO, which has a relatively wide process window. However, using the deposition technique of reactive coevaporation (RCE), we are able to grow high quality, epitaxial (RE)Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// films on 2''-diameter substrates. We will report on the growth conditions, crystal structure, and T/sub c/ values of these films. We will also discuss the Q values of lumped-element microwave resonators patterned from these films. We show that the excellent RF properties of our films make them suitable for use in wireless communications applications.

We have grown MgB/sub 2/ films using the deposition technique of reactive evaporation. This method allows high-quality, large-area, double-sided films to be grown on a large variety of substrate materials. These films are therefore well suited for applications, several of which may exploit the low-loss high-frequency properties of MgB/sub 2/, including passive microwave resonators and filters, nuclear magnetic resonance coils, and RF cavities for particle accelerators. Determining the surface resistance and nonlinear properties of our films can thus give us an idea of their suitability for these applications, as well as tell us something about their fundamental superconducting nature. We have begun measuring the microwave properties of our films using both a parallel plate technique and by patterning the films into stripline resonators and lumped-element resonators and filters. The films display very low surface resistance values.

We use terahertz time domain spectroscopy to examine finite size effects on the optical phonon modes in SrTiO3 thin films. In temperature-dependent measurements we find a near absence of mode softening in the TO1 phonon frequency. Furthermore we see an increase in the soft mode frequency with reduced thickness. Both of these results correlate well with the reduced dielectric response observed for nanoscale ferroelectric systems.

High performance HTS microwave filters stand to benefit from the capability to electrically tune their frequency of operation. This function may be performed with tunable dielectric materials such as SrTiO/sub 3/ (STO). We have optimized the growth of STO thin films by pulsed laser ablation and by reactive coevaporation. Loss tangents on the order of 0.001 at 65 K are possible, while maintaining acceptable tunability. We have integrated STO films into lumped-element YBCO microwave resonators and filters in order to tune their center frequencies. We describe methods of doing so, the effect of coupling strength, and the performance of these devices.

Tunable ferroelectric thin films such as SrTiO/sub 3/ (STO) are promising for a variety of rf applications requiring tunability. We describe the growth and characterization of STO thin films including their dielectric properties, and summarize attempts at reducing the loss tangent of these films. We also discuss their integration with high-temperature superconductor (HTS) microwave filters for trimming purposes.

The microstructures of YBCO/N/YBCO ramp-edge junctions have been studied by transmission electron microscopy and related to supercurrent transport properties which depend on the barrier layer N, the interfacial structures and defect structures in the multilayer epitaxial devices. Three different types of junction materials were investigated: Metallic oxide barriers, CaRuO{sub 3} and SrRuO{sub 3}; barriers isostructural to YBCO, Co-doped YBCO and Co-doped PrBa{sub 2}Cu{sub 3}O{sub 7}; and 'interface engineered' barriers, formed by plasma treatment of the YBCO ramp-edge. Metallic oxide barriers are characterized by high steps and strong variations in local barrier width. The observed topologies are consistent with an island growth mode for both CaRuO{sub 3} and SrRuO{sub 3}. The metallic oxide barriers are associated with interfacial strain fields that are believed to be the cause for interfacial oxygen depletion in YBCO and the observed excess normal-state resistance. A number of structural defects and deviations from perfect epitaxy have been observed. Most disruptive to the integrity of the multilayer structures and transport properties is the nucleation of a-axis YBCO grains at steep barrier steps and within the YBCO layers. The barrier layers in isostructural junctions are well structured with a high degree of interfacial coherence and for the most part,more » Cu-O planes are continuous across the interfaces. In contrast to heterostructured metallic oxide barriers, isostructural junctions contain few extraneous defects, such as a-axis grains at the barrier layer, moreover, the second YBCO layer is of good quality due to the perfect epitaxy between the materials. Among the junctions investigated the interface engineered junctions have shown the best electromagnetic properties. Their structure is characterized by narrow (2-3 nm) barriers that are continuous and crystalline. No significant interfacial strains and structural disorder were observed. This and a constant barrier thickness appear responsible for obtaining good uniformity of electric transport properties. The narrow pinhole-free barrier, coupled with excellent epitaxy and few defects, yields good reproducibility and a range of properties suitable for practical applications.« less

We have fabricated all YBCO edge junctions which do not require the deposition of an interlayer or barrier material. Rather, through appropriate high-temperature vacuum annealing and in-situ ion plasma treatments, we have modified the junction interface in such a way as to lead to reliable weak link behavior. These devices display RSJ-type I-V characteristics with excellent magnetic field modulation. I/sub c/R/sub n/ values over the range 0.5 to 3 mV and corresponding R/sub n/A values of 6/spl times/10/sup -8/ to 1.2/spl times/10/sup -9/ /spl Omega/-cm/sup 2/ at 20 K are easily attained by varying the process, which is not possible in most high-T/sub c/ junction technologies. These junctions can also be very uniform and appear to be quite stable. We have observed an unusual response to applied microwave radiation. We have investigated the microstructure of these junctions using transmission electron microscopy (TEM). Results of these analyses lead us to believe these junctions are feasible as the basic components of a high-T/sub c/ circuit technology.

The microstructure of interface engineered ramp-edge Josephson junctions in YBCO has been studied by transmission electron microscopy (TEM). The junctions are distinctly different from the structures found in heterophase SNS junctions. This structural difference can be directly related to the improved electrical properties of interface engineered junctions. The plasma treatment used in the interface engineering method forms a continuous barrier layer of 2 to 3 nm in thickness, much thinner than the deposited heterophase layers in SNS junctions. The layer, which separates the two YBCO layers, is crystalline, but has a crystal structure different from YBCO. The main factors responsible for the improved junction properties and their good reproducibility are: small barrier thickness, small thickness variation, weak interfacial strain fields, flat and nearly defect-free barrier/YBCO interfaces and good epitaxy of the second YBCO layer with few defects. Possible mechanisms for the formation of the large junction resistance are discussed.

A process is provided for fabricating YBa 2 Cu 3 O 7 thin-film edge junctions in which no deposited barrier is employed. These devices display excellent RSJ-type I-V characteristics with values of I c and R n tunable over a useful range for operation of digital circuits.

Magnesium diboride (M g B 2 ) is a superconducting material that has a transition temperature (T c ) of ∼40 K, which is ∼30 K higher than niobium (Nb) that has been used for most superconducting RF cavities in the past decades. Last year, it was demonstrated that the RF surface resistance of M g B 2 can be lower than Nb at 4 K. One of the problems with other high-Tc materials such as YBCO was its rapid increase in RF surface resistance with higher surface magnetic fields. Recently, we have shown that M g B 2 shows little increase in the surface resistance up to ∼120 Oe, equivalent of an accelerating field of ∼3 MV/m. The highest field tested was limited by available power. This result is encouraging and has made us consider fabrication of a cavity coated with M g B 2 and test it. Also, there is a potential that this material has a higher critical magnetic field that enables the cavity to run at a higher gradient than Nb cavities in addition to the possibility of operation at higher temperatures.

Niobium (Nb) Superconducting RF (SRF) cavities have been used or will be used for a number of particle accelerators. The fundamental limit of the accelerating gradient has been thought to be around 50 MV/m due to its RF critical magnetic field of around 200 mT. This limit will prevent new projects requiring higher gradient and compact accelerators from considering SRF structures. There is a theory, however, that promises to overcome this limitation by coating thin (less than the penetration depth) superconductors on Nb. We initiated measurements of critical magnetic fields of Nb coated with various thin film superconductors, starting with MgB2 films deposited using reactive evaporation technique, with the goal to apply this coating to SRF cavities. This paper will present first test results of the RF critical magnetic field of a system consisting of a 10 nm B and a 100 nm MgB2 films deposited on a chemically polished 2-inch single grain Nb substrate.