Tl2Ba2CaCu2O8 Films Research Articles

A new technique to achieve thick Tl2Ba2CaCu2O8 films for advanced applications

Tl2Ba2CaCu2O8 (Tl-2212) high-temperature superconductors (HTS) exhibit excellent superconductivity performance as their critical temperature (Tc) is higher than 100 K. However, the emergence of microcracks on the film surface restricts the thickness of the obtained Tl-2212 films, thus greatly limiting their scientific research and engineering application. Here, we fabricate crack-free Tl2Ba2CaCu2O8 (Tl-2212) films with a thickness of up to 5 μm on LaAlO3 (001) substrates synthesized by an improved strategy of a closed system. The impact of the thickness of the Tl-2212 films on their crystallographic orientations, Tc, critical current density (Jc) (0 T), critical current intensity (Ic) (0 T), and microwave surface resistance (Rs) was systemically explored. As the thickness increases, Tc values fluctuate slightly above 100 K while the variation of Jc follows the extended strong pinning theory. Ic of the 4 μm thick film stays around 80 A at 95 K, and the Rs result is less than 1 mΩ at 93 K, indicating their excellent performance at high temperatures. The interaction mechanism between the Jc and Rs of the Tl-2212 film is replenished in light of the quantitative exploration of Jc and Rs. This work on Tl-2212 films will promote their fundamental physics research and guide their potential applications in the electronics and electrical field. It can also serve as a reference for future research on other HTS materials.

Growth and property analysis of large-area epitaxial Tl-2212 high temperature superconducting films

The extensive use of large-area high temperature superconducting (HTS) thin films has emerged in superconducting microwave devices. The interest in Tl-based HTS films arises from their high critical temperature (Tc), allowing the microwave device made on Tl films to work above the liquid nitrogen temperature zone. Here, we introduce a simplified two-step method to fabricate high-quality Tl2Ba2CaCu2O8 (Tl-2212) films on 2-inch LaAlO3 substrates. This method enables the preparation of Tl-2212 films to be easy and repeatable, promoting the theoretical research and practical applications of Tl-based films. Based on the high reproducibility of this fabricating process, we studied the superconducting properties of Tc, the critical current density Jc (77 K, zero field) and surface resistance (Rs) of 2-inch Tl-2212 films. Results show that the Tc values of the sample films prepared at different conditions vary from 99 K to 103 K, depending on the oxygen content; the Jc of the films ranges from 0.7 to 1.8 MA/cm2, relying on the surface roughness. On the basis of the two-fluid model, the influence of Jc on Rs of the Tl-2212 films is incorporated to explain the variation of Rs in different samples.

The Resonant Interaction of Intrinsic Josephson Junctions With Standing Waves

IV-characteristics of shunted microbridges made of Tl2Ba2CaCu2O8 (TBCCO) were measured. The TBCCO films were grown epitaxially on 20° misaligned LaAlO3 substrates and then covered by the gold shunting layer. Each of the microbridges consisted of about 360 shunted intrinsic Josephson junctions. Self-induced resonant particularities with a main period of about 17 mV were found on IV-curves. We showed that these particularities did not depend on the characteristics of the individual junctions and that they were connected with the geometrical dimensions of the system. The origin of particularities was the interaction of Josephson generation with the standing wave appeared in the system. We modeled this interaction considering radiation of junctions into the transmission line and found that the model described the main resonant structure of IV-characteristics. Strong synchronization of junctions appeared in the vicinity of the first particularity.

Hole Concentration Effect on the Microwave Nonlinearity of Tl2Ba2CaCu2O8±δ Superconducting Thin Films

The carrier concentration of Tl2Ba2CaCu2O8 films was modified by annealing in N2 gas. X-ray analysis of the structure and the oxygen content revealed a correspondence between carrier concentration and oxygen depletion. The TC and nonlinear surface impedance was measured using a dielectric resonator and the nonlinearity slope parameter r=ΔXS/ΔRS was found to converge to unity at the critical temperature, indicating a dominance of Josephson fluxon hysteresis on the nonlinearity. Highly inductive nonlinearity was observed in a small range of doping levels between 0.180<p<0.195 holes/Cu, which does not include the optimal doping level of 0.16 holes/Cu.

Growth of Tl-2212 Films on CeO&lt;sub&gt;2&lt;/sub&gt;-Buffered Sapphire Substrates

Tl2Ba2CaCu2O8 (Tl-2212) films were prepared on r-cut sapphire substrates buffered with CeO2 by dc magnetron sputtering and post-annealing method. The in situ two-temperature process was used to grow CeO2 buffer layers. XRD and AFM measurements showed that CeO2 films deposited at temperature of 370-470°C were excellent c-axis orientation and had smooth surface. SEM observations demonstrated that the Tl-2212 films’ surface morphology was changed from condensing crystal structure to plate-like structure when the CeO2 deposition temperature was increased. The best Tl-2212 film’s critical temperature Tc can reach to 108.3 K, and critical current density Jc obtained at 5.33 MA/cm2 (77 K, 0 T).

Thallium Based Superconducting Thin Films Synthesized by RF-Sputtering and &lt;i&gt;Ex Situ&lt;/i&gt; Thallination

Thallium based superconducting thin films of TlBa2CaCu2O7 (Tl-1212), Tl2Ba2CaCu2O8 (Tl-2212) and Tl2Ba2Ca2Cu3O10 (Tl-2223) of about 100-125 nm in thickness have been prepared on (001) LaAlO3 single crystal substrate using rf-sputtering deposition of Ba2Ca2Cu3O7 precursor films and ex-situ thallination in sealed quartz tube under different atmospheres (oxygen, argon and nitrogen). We have studied phases formation as function of partial pressure of Tl2O which may be adjusted by varying thallium source content TlxBa2Ca2Cu3Oz (x = 1.5, 1.6, 1.7, 1.8, 1.9, 2.0) and atmosphere pressure at ambient temperature (300, 500 mbar). The influence of the temperature and time of thallination (60 min to 1400 min) is also presented. The synthesized films were characterised by optical microscopy, atomic force microscopy AFM, AC susceptibility and XRD measurements. The XRD measurement shows that the obtained films contain several superconducting phases and are strongly c-axis oriented. The properties of the resulting films are critically dependant on the microstructures developed during thallination process. Thallium source content as well as the time of thallination and partial atmosphere pressure at room temperature play an important role in the obtained film’s quality and phase’s purity.

HTS filter subsystem for future mobile communication system

A high-temperature superconducting (HTS) filter subsystem consisting of a 10-pole HTS filter with group delay self-equalization, a cryogenic low noise amplifier (LNA), and a cooling device is presented in this paper. The HTS filter has a 0.51% fractional bandwidth at 1955 MHz, simulated in Sonnet, and is fabricated using double-sided Tl2Ba2CaCu2O8 films on 0.5 mm thick LaAlO3 substrate. Measured responses show that the minimum insertion loss of the filter is 0.16 dB, and the out-of-band rejection is better than 75 dB. The slope is 27 dB MHz−1 at the low band edge and 22 dB MHz−1 at the upper band edge. The HTS filter subsystem has a gain of 19.3 dB and a noise figure (NF) of 0.8 dB. Furthermore, the phase distortion of HTS filters is considered, and an efficient design approach for self-equalized HTS filters is proposed.

The design of a linear phase superconducting filter with quasi-elliptic response

This paper presents the design of a linear phase superconducting filter with quasi-ellipticresponse. The coupling structure of the filter contains two trisections and one quadruplet.The two trisections are applied to generate two independent transmission zeros for highselectivity and are realized by meandered open-loop microstrip resonators; the quadrupletis applied for phase equalization and realized by four novel L-shaped microstrip resonators.The filter is designed at 1950 MHz with a bandwidth of 20 MHz. It is fabricated on aLaAlO3 wafer withdouble-coated Tl2Ba2CaCu2O8 films. The filter shows good selectivity together with flat group delay over 80% of thepassband both in the simulation and measurements.

A study on the microwave responses of YBCO and TBCCO thin films by coplanar resonator technique

YBa2Cu3O7(YBCO) thin films have been prepared by thermal coevaporation on LaAlO3(LAO) substrates, and Tl2Ba2CaCu2O8(TBCCO) thin films are synthesized by magnetron sputtering method on LAO substrates. The transition temperature Tc is90 K for YBCO/LAO and 104 K for TBCCO/LAO. Microwave responses of the films are studied systematically by coplanar resonator technique. Energy gaps of the films obtained are Δ0 = 1.04kBTc for YBCO films and Δ0 = 0.84kBTc for TBCCO films by analysing the temperature dependence of resonant frequencies of coplanar resonator. Penetration depth at 0 K λ0 = 198 nm for YBCO films and λ0 = 200 nm for TBCCO films could also be obtained by using the weak coupling theory and two fluid theory. Results of penetration depth and energy gap confirm the weak coupling properties of the films. In addition, microwave surface resistances Rs of YBCO/LAO and TBCCO/LAO are also investigated by analysing the quality factor and insert loss of the coplanar resonator. Surface resistance of TBCCO/LAO is less than that of YBCO/LAO, so that TBCCO/LAO films may have more potential applications.

Nonlinear surface impedance of Tl2Ba2CaCu2O8 thin films as a function of temperature, frequency, and magnetic field

YBa 2 Cu 3 O 7 epitaxial film quality has reached a stage where rf applications at T⩽80 K are carried through. Higher-operating temperatures or better power handling are very much desired. Tl2Ba2CaCu2O8 (TBCCO) is an excellent candidate with its higher-transition temperature Tc≈100–105 K. We study the rf field dependence of surface impedance Zs of epitaxial TBCCO films using dielectric resonators with TE011-modes at 8.5 and 15.3 GHz at 4.2 K&amp;lt;T&amp;lt;100 K and at 0.01 mT⩽Hrf⩽10 mT in the frequency and time domain. High-quality films show surface resistances at low fields Rs(77 K) superior to YBCO and promising power handling with a nonlinearity onset above turn-up field μ0H*⩾3 mT. Defective TBCCO films show anomalous nonlinear behavior δRs(Hrf)∝√Hrf for μ0Hrf⩽μ0H*∼0.3–1 mT, extremely pronounced at T&amp;lt;40 K, indicating the presence of very weak links yielding flux flow, followed at higher fields μ0Hrf≳0.5 mT by hysteresis losses with a linear field dependence δRs(Hrf)∝Hrf. Better quality TBCCO films show δRs(Hrf)∝Hrf2 up to fields of about 5–10 mT. The δRs(Hrf)∝Hrfn (n=1,2) dependencies together with ratios r(T)=δXs/δRs∼1–10 hint to hysteresis losses of Josephson fluxons enhanced by slits or holes in the films.

A metal-organic chemical vapor deposition approach to double-sided Tl2Ba2Ca1Cu2O8superconducting films on LaAlO3(100) substrates

Double-sided Tl2Ba2CaCu2O8 (Tl-2212) films for device applications have been fabricated through thallium vapor diffusion into BaCaCuOF matrices deposited by a novel metal-organic chemical vapor deposition (MOCVD) process onto 10 × 10 mm2 LaAlO3 (100) substrates. The present approach uses a multimetal molten single source in a monocomponent hot-wall MOCVD reactor whose geometry allows simultaneous and uniform film growth on both sides of the substrate. Both sides are epitaxially coated by homogeneous and monophasic Tl-2212 films, which, even more importantly, have comparable superconducting and transport properties.

TlBaCaCuO-(2212) thin films on lanthanum aluminate and sapphiresubstrates for microwave filters

High-quality Tl2Ba2CaCu2O8 films for microwave filterapplication are prepared on both sides of 2 inch lanthanum aluminatesubstrates as well as on double-sided CeO2 coated r-cut sapphiresubstrates. The films have thicknesses as high as 1 µm in the case oflanthanum aluminate substrates. On CeO2 buffered sapphire substrates itis possible to prepare crack-free superconducting films as thick as 400 nm.For both substrate materials we reach high critical current densities ofJc>2×106 A cm-2 at 77 K. The microwave surfaceresistances Rs were measured to fall short of 100 µΩ at 5.6 GHzand 77 K in the low-field case, comparable to the best films made ofYBa2Cu3O7-x material. The films were able to tolerate highmicrowave power levels corresponding to magnetic field amplitudes ofBRF>2 mT. The critical temperature exceeds 102 K. This excellentperformance is maintained even after several thermal cycles. The films wereused to produce four-pole band-pass filters for the C-band which wereoperated without any detectable loss in their transfer characteristics upto temperatures of about 85 K. This is the first time that large area (2inch) double-sided Tl2Ba2CaCu2O8 films have been prepared onsapphire with excellent critical current density in conjunction with lowsurface resistance.

Magnetically tunable Tl 2 Ba 2 CaCu 2 O 8 -basedmicrostrip ring resonators

The tunability by means of an external magnetic field and its impact on microwave losses of microstrip ring resonators based on Tl2Ba2CaCu2O8 films combined with ferrite plane elements has been studied. At 2.4 GHz maximum tunability of ~10% with Q ≃ 450 was obtained. At 11.4 GHz the achieved frequency shift was 200 MHz with a Q-factor of 2500, the best reported to date for such a tunable structure.

Electronic and Phonon Contributions to the Pairing in the Cuprates

Thermal Difference Reflectance (TDR) Spectroscopy has been used to determinethe superconducting gap parameter for several of the superconducting cupratesover a wide range of energies, ω extending from the infrared (0.3 eV)to the ultraviolet (5.3 eV). A contribution to the pairing is found in eachcase from the phonons, and from an electronic excitation with energy thatranges from 1.6 eV and 2.3 eV for the different compounds attributed to thed 9−d 10 L charge-transfer excitation between Cu and O. In every case thereflectance ratio between the superconducting and normal state, Rs/Rnplotted as a function of photon energy can be well described using theEliashberg theory. The theory also predicted a characteristic shape for thelow energy part of such spectra due to the phonons. We report theobservation of this feature in measurements on films of Tl2Ba2CaCu2O8. Wediscuss the significance of the success of the Eliashberg theory inexplaining these results and successfully predicting new effects in thelight of the correlations that had been thought to invalidate such atheoretical approach.

A new hybrid two-zone/crucible furnace process for the growth of epitaxial Tl2Ba2CaCu2O8 superconducting films

Epitaxial Tl2Ba2CaCu2O8 (Tl-2212) films ∼5500 Å thick are grown on LaAlO3(100) substrates using a new hybrid two-zone/crucible furnace process enabling precise control of thallination. This method combines the best features of both conventional crucible and two-zone processing for the first time: superb film properties and reduced handling of hazardous Tl-oxide powders. Single-phase, highly caxis oriented Tl-2212 films are grown with smooth morphology, Meissner transition ∼103 K, and critical current density ∼1.1 × 107 A/cm2 at 5 K for twenty consecutive runs without having to change or add to the Tl-oxide source.

High power microwave circuits and the demand to HTS materials

The paper focuses on the latest progress of the power handling capability of HTS thin film materials and microwave circuits particularly filters. The surface resistance vs. rf magnetic field data measured at 8.14 GHz and different temperatures for YBa2Cu3O7−δ and Tl2Ba2CaCu2O8 films are presented. A 3-pole 6.04 GHz 1.3% filter with polygon-shape resonators and back-side coupling circuits were designed, fabricated and tested up to 74 watts transmitting power at 77 K, which is suitable for applications in the transmitters.

Transport properties of Tl2Ba2CaCu2O8 weak links on LaAlO3 bicrystal substrates

Josephson junctions and dc superconducting quantum interference devices (SQUIDs) have been fabricated in ex situ epitaxial Tl2Ba2CaCu2O8 films on bicrystal LaAlO3 substrates with symmetric 32° [001] tilt grain boundaries. The critical temperature Tc, of the junctions was in the range 105–107 K and the critical current densities at 77 K varied between 3×102 and 3×104 A/cm2, two or three orders of magnitude less than those of the film. The I–V curves are described by a resistively shunted junction model. Close to Tc, the temperature dependence of the critical current was described by (1−T/Tc)2. The flux noise spectra SΦ(f) of dc SQUIDs were measured in the locked-loop regime with constant current bias at temperatures up to 94 K. The white noise level was 50μΦ0/√Hz at 77 K. The crossover frequency to 1/f noise was low, about 5 Hz, and the flux noise level at 1 Hz was 440μΦ0/√Hz.

Tl2Ba2CaCu2O8Superconducting Thin Films: A Comparative Study ofex SituAnnealing Using Amorphous Precursors with Different Thallium Content

The synthesis of Tl2Ba2CaCu2O8(2212) films obtained byex situannealing of amorphous films, deposited at room temperature with a multitarget sputtering system, is described. The growth phenomenon of 2212 films has been extensively studied by annealing precursor thin films of variable thallium content under various O2partial pressures. The best single-phase 2212 films are obtained from stoichiometric amorphous precursor films. Such 2212 films, annealed under low oxygen pressure, exhibit a critical temperature for superconductivityTc(Resistivity = 0) at 110K with a critical current densityJc(77 K) > 106A/cm2. After annealing, the composition of these films, deduced from the backscattering spectra of4He+beam measurements, indicates a formula close to Tl:Ba:Ca:Cu = 2:2:1:2.

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl2Ba2CaCu2O8 films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO3, LaAlO3, and Y-ZrO2 substrates. A critical current density,Jc, of 2×106 A/cm2 at 77 K was measured on LaAlO3. Film morphology was studied by SEM, AFM, and STM.

Hall conductivity in Tl2Ba2CaCu2O8 films in the mixed state.

Hall conductivity in Tl2Ba2CaCu2O8 films in the mixed state.