Investigations on Tl-2223 thin films fabricated through ultrasonic spray pyrolysis under oxygen deficient conditions

In this paper we have shown that polycrystalline films corresponding to Tl-2223 phase can be grown by employing high thalliation temperatures and short thalliation times. Ultrasonically deposited precursor films corresponding to Ba₂Ca_2.2Cu_3.3O_x(Ag_y) have been thalliated under high vacuum (∼ 10^-5 torr) at 890°C to obtain single phase Tl-2223 films. An off-stoichiometric and unreacted pellet of composition Tl_2.05Ba₂Ca₂Cu₃O_z has been used as source of Tl. We have shown that oxygen ambient is not necessary for the growth of Tl-2223 phase. The as-thalliated films have T_c’s in the range 123 K ±0.70 K. TheT _c has been found to be independent of the addition of AgNO₃ to the precursor. The zero field transportJ _c has been observed to be > 1.2 X 10⁵ A/cm² at 77 K. NearT _c (110 K-122 K),J _c has been observed to follow the power lawJ _c ∞ (1-T/T _c )_p,p 2. A power law withp tt 1.4 has been observed for the temperature range 70 K-110 K. An optimum doping of Ag has been observed to induce about 25% increase inJ _c and it also leads to uniform and enlarged grain growth. The surface morphology of Ag free samples contains plate like grains having arbitrary shapes. In contrast to this 0.35 Ag doped sample exhibits nearly rectangular plate like grains     

Universal behavior of3He-4He mixture films near the Kosterlitz-Thouless transition

We report measurements of the convective thermal conductance of³He-⁴He mixture films near the Kosterlitz-Thouless transition. The thickness of our⁴He films is 14.7 and 19.1 Å above the inert layer and the³He concentration ranged from 0.033 to almost 2%. The thermal response is tested for the critical behavior as observed in pure films, and it is found to be preserved in the mixture films case. However, the parametersb, D/a ² andT _c exhibit a strong dependence on the concentration. The mixture film conductance at fixedT-T _c is found to decrease upon addition of³He implying a decrease in the 2D correlation length. Mixture films thus exhibit 2D behavior over a narrower temperature range than pure films. Further, for temperaturesT _c , the largest measurable conductance decreases sharply with the addition of³He. We attribute this behavior to a³He-⁴He scattering mechanism and a³He induced free-vortex density.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7– (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (31) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Preparation of high-T c superconducting film in the system of Bi-Sr-Ca-Cu-O from sol-gel process and its properties

High-T _c superconducting Bi₂Sr₂CaCu₂O_x films withT _c ^off =80 K were prepared by the dipping method of sol-gel processing using inorganic salts. The influence of the preparation conditions on the superconducting properties of the derived material is reviewed. Bi, Sr, Ca and Cu nitrates were used as raw materials. Glycerol was used as solvent. The thickness of films made by the dip method was about 0.5 m. The films were crystallized by heat-treatment at 830°C for 10 min.T _c ^off of films was 80 K andJ _c at 77 K was more than 8 kA cm^–2. Synthesis of high-T _c superconducting films was very easy and the crystallization of films was possible with a relatively low heat-treatment temperature.     

Nanodimensional effects in the structure of epitaxial YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">y</Emphasis></Subscript> films

We have studied the structure of YBa₂Cu₃O_7−y (YBCO) high-T _c superconductor films obtained by deposition for various times under otherwise identical conditions. Thin YBCO films prepared by the nonaxial laser shadow deposition technique are epitaxial and have microstrains below 1×10⁻³. The main factor determining variations of the unit cell parameter c in the films is the difference of the crystal block size 〈D〉 in the direction normal to the film surface, which reflects the nanodimensional effects known in oxides.     

Effect of Sputtering Parameters on Superconducting Properties of GdBaCuO Epitaxial Thin Film Deposited by ICP Method

Thin films of GdBaCuO (GBCO) have been deposited in situ onto LaAlO₃ single crystal substrates by inverted cylindrical sputtering pattern (ICP). The superconductive properties of the thin films' dependence on the substrate temperature and sputtering pressure have been systematically investigated. By optimization of the deposition parameter, high-quality c-axis epitaxial GBCO thin films of T _c0>92 K were reproducibly grown. The T _c of the best sample is as high as 93.2 K. Upon changing the target composition to GdBa₂Cu₄O _y (Gd₁₂₄), it was observed that the samples always show some a-axis oriented films, implying that excess copper would favor a-axis growth in thin films. The superconductivity of the thin films under higher substrate temperature (T _s>800°C) was clearly improved by the procedure of special post-oxygenization at 400°C with an ozone atmosphere. This is very useful for preparing large-area thin films of GBCO.     

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y₁Ba₂Cu₃O_{7 – x} (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a T _c ^onset of 85 K and a T _c of 5 K. The resistance measurements gave a T _c ^onset of 90 K and a T _c of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.     

In-situ Single Chamber Arc Sputtering Process for Ycd0.3Ba2Cu3O7-δ Superconducting Thin Films

Superconducting YCd _0.3 Ba ₂ Cu ₃ O _7- thin films have been deposited in-situ onto single crystal MgO substrates using a DC arc-sputtering process. The depositions were carried out in a single chamber deposition system equipped with two target holders. The films deposited at the optimum condition exhibited strong (00l) orientation with a high peak intensity. The best electrical properties were achieved to be 90K for T _c , 81K for T _zero and the transport critical current density J _c =675 A/ cm ² at 77K and 2.3×10 ³ A/cm ² at 4.2K for the sample deposited at the optimum conditions.     

Quantized Vortices and Diamagnetic Precursor to the Meissner State in High-T c Superconductors

We report the magnetic imaging for underdoped and optimally-dopedLa_2–x Sr _x CuO₄ (LSCO) thin films on single substrates and nearly optimallydoped YBa₂Cu₃O_7–x (YBCO) thin films on tricrystal substrates in the temperature range both below and above T _c using scanning SQUID microscopy. Below T _c, clear integer- and half-integer quantized vortices were observable. Above T _c, however, the inhomogeneous diamagnetic domains appeared. The local diamagnetic domains that led to the Meissner state were found in the broad temperature range for underdoped samples and in the narrow limited temperature range for optimally-doped samples. The results provide evidence that local diamagnetic domains are closely related to the pseudogap state. The continuous connection of the domain state above T _c with the state of a half-integer vortex at the tricritical point in the YBCO film below T _c also indicates that the diamagnetic domains are also closely related to the occurrence of d_x ^{2-y 2}-wave superconductivity.     

Growth of highT c Bi-Sr-Ca-Cu-O thick films

Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyT _c (R=0) increased from 77 to 103 K as the annealing duration was increased after whichT _c decreased. Kinetics of the growth of highT _c phase is discussed in the light of our results.     

Investigation of the properties of superconducting niobium nitride films

Niobium nitride films with a value for T_c of up to 17.3 K have been prepared by reactive magnetron and diode sputtering in Ar and N₂ gas mixture. Alteration of T_c and N₂ ion implantation into NbN films was studied. It is shown that small doses of implanted ions cause an abrupt decrease of T_c to 12.8 K. Annealing at 900°C restores the high T_c By the tunnel effect, the magnitude of the energy gap is 3.05 Me V at 1.56 K for NbN with T_c = 17.1 K and $\text{22/kT}{}_{\mathrm{c}}\text{= 4.14}$. The existence of an undersurface layer has been discovered, whose width is of the order of the coherence length, with a lower T_c, of 12.0–12.8 K. The superconductivity of this layer above T_c is due to the proximity effect. NbN tunnel junctions are made with a density of the Josephson current i=(1.7-2.87) · 10³ A cm^?2.     

Thermal resistance between liquid3He and copper potassium tutton salt

The thermal resistance between liquid³He and copper potassium tutton salt (CPS) has been measured through its magnetic ordering temperature (T _c=29.6 mK). The thermal resistanceR for pure³He has a broad minimum near 60 mK and increases continuously throughT _cwith decreasing temperature, except for a dip atT _c. BelowT _c,R is proportional toT ^–1.5. Effects of⁴He coating have been studied by stepwise addition of⁴He into liquid³He. The thermal resistance increased drastically for the liquid containing 150 ppm⁴He and more for 95%⁴He. By sudden depressurization of the liquid³He containing 480 ppm⁴He, a considerable decrease ofR was observed. SinceR for pure³He was much smaller than the calculated Kapitza resistance, the present experimental results indicate the existence of surface magnetic coupling between liquid³He and CPS.     

An Improved Procedure for Preparing Epitaxial Tl–2201 Films on Single Crystal LaAlO3 and SrTiO3

Epitaxial Tl-2201 films are prepared on single crystal LaAlO ₃ and SrTiO ₃ by thallization of thallium-free precursor films made by laser ablation. Thallization is carried out in two consecutive steps. In the first step, at 720 °C, a smooth and epitaxial film is produced. The second thallization, at 820 °C, improved film crystallinity and the T _c . The films were characterized by X-ray diffraction and by resistance and susceptibility measurements. For a film on single crystal LaAlO ₃ , the FWHM of the scan (0,0,10) was 0.27°. T _c was 84K while J _c , reached 1.6×10 ⁶ A/ cm ² at 77K. A film on single crystal SrTiO ₃ exhibited somewhat lower T _c (78 K) while J _c was much smaller (9.5×10 ³ A/cm ² at 70.4K).     

Superconducting energy gaps and transition temperatures of quench-condensed cadmium and zinc films

The superconducting transition temperatures of Cd and Zn films deposited upon a liquid³He-cooled substrate increase compared to the bulk values. In disordered Cd, values of around 0.9 K are obtained, while correspondingT _c for Zn is about 1.4 K. These values agree well with theoretically predicted ones. For a Cd_0.9Ge_0.1, probably amorphous, film,T _c is further increased to about 1.6 K in approximate agreement with theory. The ratio between the energy gap andT _c , 2(0)/k _B T _c , remains considerably lower than 3.53 in all the disordered films. This, as well as fairly broad transition ranges and the shape of the fluctuation pairing contribution to the conductance, is taken as an indication of structural inhomogeneities of the films. In pure, quenchcondensed Mg films, no superconductivity is detected above 0.35 K.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.     

Superconductivity in transparent zinc-doped In2O3 films having low carrier density

Abstract

Thin polycrystalline zinc-doped indium oxide (In₂O₃–ZnO) films were prepared by post-annealing amorphous films with various weight concentrations x of ZnO in the range 0x 0.06. We have studied the dependences of the resistivity ρ and Hall coefficient on temperature T and magnetic field H in the range 0.5T 300 K, H6 Tfor 350 nm films annealed in air. Films with 0x0.03 show the superconducting resistive transition. The transition temperature T_c is below 3.3 K and the carrier density n is about 10²⁵–10²⁶ m^?3. The annealed In₂O₃–ZnO films were examined by transmission electron microscopy and x-ray diffraction analysis revealing that the crystallinity of the films depends on the annealing time. We studied the upper critical magnetic field H_c2 (T) for the film with x = 0.01. From the slope of dH_c2 /dT, we obtain the coherence length ξ (0) ≈ 10 nm at T = 0 K and a coefficient of electronic heat capacity that is small compared with those of other oxide materials.     

DC-sputtered Bi(Pb)SrCaCuO films on single (MgO) and polycrystalline (MgO,CuO) substrates

Bi(Pb)SrCaCuO films have been deposited on single (MgO) and polycrystalline (poly MgO, CuO) substrates by DC sputtering. All the films became superconducting after post-annealing them in air. The films on single crystal MgO showedT _c onset at 120 K andT _c at 92 K. However, the extrapolated zero resistivity is obtained at 106 K. The ac susceptibility showed an onset at 106 K and a sharp transition at 85 K. The films on poly MgO and poly CuO had aT _c=72 K. The preparation, X-ray diffraction and morphology of these samples are presented.     

Microwave Surface Impedance of Nd-rich Nd 1+x Ba 2−x Cu 3 O 7−δ Thin Films

Surface impedance measurements on highly c-axis epitaxial Nd _1+x Ba _2–x Cu ₃ O ₇ (x=0, 0.09 and 0.12) films grown by d.c. magnetron sputtering on LaAlO ₃ substrates are presented. It is found that the zero temperature London penetration depth correlates well with the critical temperature of the films and with the corresponding number of carriers. The low temperature penetration depth follows a linear T law for optimally doped Nd123 sample and a T ² law in Nd-rich samples. In the case of the heavily underdoped samples (T _c < 60K) the T ² law extends to temperatures higher than T _c/2. The possible role of the Nd/Ba ions substitution on the penetration depth and surface resistance is discussed.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7?δ (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (3∶1) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Surface structure of high-T c Nb3Ge films

We have studied the surface structure of rf-sputtered Nb₃Ge films by means of reflection electron diffraction and Auger electron spectroscopy. It is found that a tetragonal Nb₅Ge₃ phase exists, being partly disordered, at the surface of high-T _c films, whereas A15 phase is predominant in the body of the film as proved by x-ray diffraction. The tetragonal surface phase is replaced by an amorphous phase in thin films of <1000 Å, where reducedT _c 's as compared with those of thick films are found. Single A15 phase occurs at the surface of Ge-deficient films which have lowT _c 's. Auger analysis has shown a pronounced Ge-rich layer with a depth of 60–100 Å, depending on film composition and thickness. At the top of the surface layer, Ge is enriched beyond a composition corresponding to Nb₅Ge₃. It is inferred that the Ge-rich layer is responsible for formation of the tetragonal phase.Work supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.