Temperature and spectral dependences of persistent photoconductivity in YBa2Cu3O x thin films

Temperature and spectral dependences of photoinduced changes of resistance were measured in YBa₂Cu₃O _x thin films with oxygen content ranging as 6.35 <x < 6.75. The absolute value of efficiency of initiation of photoinduced changes decreases with increase in oxygen content, but the position of peaks in the spectral dependence does not change with a change ofx. Temperature dependences of efficiency have an anomaly atT∽220 K, which is present in all the samples studied, and correlates with anomalies observed by other experimental techniques. Qualitatively similar temperature and spectral dependences of efficiency for the samples in both the insulating and metallic phases may be considered as an indication that the persistent photoconductivity effect in YBCO on both sides of the metal-insulator transition has a common origin.     

Recrystallization of YBa2Cu3Ox superconductors in vacuum

YBa₂Cu₃O _x ceramics were recrystallized in vacuum at high temperatures. Recrystallized layers consisting of small grains were observed near the surfaces of the original large-grain 1 2 3 ceramics. The small grains consisted of transformation twins and were identified to orthorhombic 1 2 3 using X-ray diffraction. As vacuum annealing time increased, the thickness of the recrystallized layer increased. The relationship between the thickness and the annealing time showed a linear relationship and an effective diffusion coefficient of 6.25 × 10^–10cm²s^–1. The recrystallized layer showed a critical temperature of 90 K.     

Submillimeter surface impedance and relaxation: A case study of quasi-two-dimensional YBa2Cu3Ox

Surface impedance measurements in the normal and superconducting state are an excellent method to study the conduction electron dynamics in metals. This holds especially in the relaxation range, i.e., for distances traveled in one r.f. periods=υ _F/ω(υ_f is the Fermi velocity) being smaller or of the order of the penetration depth λ and mean free pathl. For materials withυ _F<-10⁷ cm/sec the relaxation range is easily accessible forf>0.1 THz. Then, in the normal state, relaxation defines the surface impedance with a penetration depth approaching the London penetration depth λ_L, andR≈μ ₀λ_l/2τ as surface resistance allowing a measure of λ_L and relaxation time τ(T, ω). In the superconducting state the photon interaction scales withξ _F/λ_L=l/γ (ξ _f is the dimension of Cooper pairs for l→∞) and causes at low frequencies an absorption rate growing withγ, which is decreasing withξ _F/l. The rate increase proportional toγ turns to a decrease above 0.1 THz, being accompanied by a decrease ofγ with frequency which is stronger for largeγ and smallξ _F/l. These characteristic dependences allow a measurement of material parameters, anisotropy, and dynamics of electrons. To evaluate the consequences of theâ, b, and?-direction anisotropy, the integral kernels for normal and superconducting surface impedances in its nonintegrated, angle-dependent form are presented, analyzed, and compared with impedance measurements above 0.1 THz of YBa₂Cu₃O_x.     

Microstructure of plasma-sprayed YBa2Cu3Ox high-temperature superconductors

The arc plasma spraying process has been applied to deposit YBaCuO superconductors on to steel substrates. Detached, as-sprayed coatings were heat treated at 950 °C in air in order to restore superconductivity. Oxygen annealing at temperatures 400–500 °C has been used as the final stage in material preparation. The changes in the microstructure of the material during heat treatment have been followed with the aid of scanning electron and optical microscopy. The phase transformations resulting from heat treatment have been studied by differential thermal analysis and X-ray diffraction.     

Variations of internal friction in YBa2Cu3Ox superconductors

The internal friction (Q ^–1) spectrum of YBa₂Cu₃O_x superconducting ceramic exhibits several peaks. It has been confirmed that the high-temperature peak (around 240 K) depends on structural changes and varies during subsequent cycles of cooling and heating.Q ^–1 conductivity, X-ray spectra and the shielding effect have been measured on several samples having different superconducting properties obtained by various thermal treatments. Splitting is a characteristic feature of the high-temperature internal friction peak of the sample which exhibits good superconducting properties. In the case of the specimen exhibiting the worst properties the peaks decrease and overlap. In both cases an increase can be observed of this peak with the number of thermal cycles. After ageing at 470 K, the high-temperature peak disappears. Subsequent thermal cycles slightly recover it. Hysteresis of the Young modulus is also observed. The results are interpreted as transition of the 04 oxygen atom between two energy minima in the O4-Cu-O4 chain.     

Preparation and densification of superconducting YBa2Cu3Ox ceramics

The solid-state reaction method to form the superconducting oxide YBa₂Cu₃O _x was studied. It was found that the starting cupric and yttrium components accelerated the decomposition of the BaCO₃ component. At a constant heating rate of 10 ° Cmin^–1 in thermogravimetric analysis, the temperature of complete decomposition,T _f, was lowered from greater than 1000 ° C in pure BaCO₃ to between 915 and 985 ° C. The effectiveness in decreasingT _f can be ranked in the order of oxalate, carbonate and oxide. The highest sintered density achieved in this study was 6.03 g cm^–3 (/_th = 95%) at 990 ° C and 5.85 g cm^–3 (/_th = 92%) at 960 ° C. The source of cupric ion had the largest effect on densification. The use of cupric carbonate resulted in a consistently high Archimedes density of about 6.00gcm^–3 and large dimensional shrinkage of about 20% at 990 ° C for 12h. In contrast, the use of cupric oxide gave the lowest density and smallest shrinkage. Within the same powder lot, higher sintered density and smaller dimensional shrinkage were observed in samples with higher initial green density and compaction pressure. However, the data suggested that the enhanced densification and higher density achieved by the use of cupric carbonate and oxalate cannot be accounted for by the different physical characteristics of the powders and the mechanics of powder compaction, measured collectively by the green density.     

Polymer coating for filamentary YBa2Cu3Ox superconductors

Polymer coating for filamentary YBa₂Cu₃O _x superconductors was examined. The precursor filaments were prepared by solution spinning through aqueous poly(vinyl alcohol) solution containing mixed acetates of Y, Ba and Cu. The as-drawn filaments were heated to remove volatile components and to generate a superconducting phase. The synthesis of polyamide acid for the polymer coating was made. The filamentary superconductors were coated with the polyamide acid and cured to convert to the polyimide. The surface of the sample was an insulator. The critical current (I _c) value of the filamentary sample at 77 K was preserved by the polyimide coating. Moreover, the I _c for the polyimide-coated sample was maintained after holding at room temperature for more than 6 months. After thermal cycling between 77 K and room temperature 10 times, the I _c for the coated sample did not deteriorate, whereas the I _c for the sample without polymer coating disappeared. Polymer coating using epoxy resin applied to the filamentary superconductors was also examined.     

Bond-Stretching Anomalies in YBa2Cu3Ox: a DFT Study

In the past few years, there has been growing evidence for unusual behavior of phonon modes related to the Cu–O bond stretching vibrations in several members of the high-T _c cuprates, which suggest a peculiar interaction between electronic and lattice degrees of freedom. Here, we analyze some of these anomalies in the light of density-functional calculations of phonon dispersions for optimally doped YBa₂Cu₃O₇ and for underdoped YBa₂Cu₃O_6.5 in the ortho-II structure with the goal to clarify which of the observed properties can not be accounted for by a local-density type screening, and thus may be seen as fingerprints for more subtle electron-correlation effects.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Glassy behavior of low-temperature energy relaxation in YBa2Cu3O7 and YBa2Cu3O6

Measuring the power release after rapid cooling a YBa₂Cu₃O₇ sample (m=42.85 g, T_c=91 K) from the equilibrium temperature T₁ (2.35 KT₁15.1 K) to T₀=1.5 K, we observed a time dependence typical of a glass: is proportional to t^–1. The results allow us to determine the linear term of the heat capacity (0.8 mJ/mole · K²) due to the two-level systems. While the low-temperature heat capacity anomaly noticeably decreases, the power release is essentially unchanged after oxygen reduction of the sample.     

Thermodynamics of oxygen uptake for the YBa2Cu3Ox superconductor

Thermogravimetric analysis and pressure (p) monitoring in known volume systems are used to give phase diagram information (e.g. vant Hoff plots) for roughly 6<x<7 in YBa₂Cu₃O_x. As an example we find for O₂ desorption for the important composition x = 6.8, beyond which superconductivity with transition temperature, T_s > 90 K becomes essentially independent of x, ΔH^o = 43.4 ±1 kcal/mo (O₂) and ΔS^o = 48.6 ±1 cal/Kmo (O₂). ΔS^o stays roughly constant for x > 6.7 but decreases for x < 6.7 indicating increased disorder on the O substructure. p (x) curves do not show phase separation which corresponds to above critical behavior, but given indications for limiting compositions near x = 6 and x = 7. Accordingly a model is proposed in which these compositions correspond to valencies of Cu⁺ and Cu³⁺ respectively for the symmetric Cu site (Ba coordinated) while the asymmetric Cu (Ba and Y coordinated) is taken to be 2+ over this whole range in x.     

Compositional variations in the bulk of YBa2Cu3Ox thick films

Thick films of YBa₂Cu₃O_x (123) prepared on a number of substrate materials, namely Si, Al₂O₃, SrTiO₃, MgO and YSZ, have been investigated for the compositional variations/impurity phases present, microstructure, grain orientation etc. Energy-dispersive X-ray and X-ray diffraction analyses for as-prepared films of two different thicknesses, 2–3 m and 10–15 m, and after their partial and complete etching, are reported. The results show that the film-substrate reaction dominates up to 2–3 m thickness of a film. The decomposition of 123 material at the processing temperature and the substrate-film reaction together are seen to govern the crystallization behaviour, compositional variations and the superconducting properties of the films. In various cases, the films have been found to undergo superconducting phase transitions at temperatures between 66 and 89.5 K.     

Hydrostatic and uniaxial pressure effect on Tc of YBa2Cu3Ox

The Variation of the transition temperature T_c of YBa₂Cu₃O_x with hydrostatic He-gas pressure depends on the oxygen content x. The pressure effect dT_c/dp increases from small negative values at x=7 to dT_c/dp=7.4 K/GPa at x=6.7. For oxygen contents below x=6.7 dT_c/dp drops to 3 K/GPa and remains nearly constant¹. The charge transfer model² cannot explain the drop at x=6.7.Thermal expansion measurements on YBa₂Cu₃O_x indicated that the uniaxial pressure effects along the three crystal axes are different^3,4 To investigate the uniaxial pressure effects inductively an experimental setup was constructed. The T_c-change of several YBa₂Cu₃O_x single crystals with different oxygen contents has been investigated under pressure along the c-axis. To avoid oxygen ordering processes the samples were held below 105 K during the measurements. The results of uniaxial pressure measurements in c-axis direction fit to former uniaxial pressure data^3,4,5 and are explained within the charge transfer model. Hydrostatic pressure data¹² of overdoped samples fit to the same curve. However, this is not the case for underdoped samples. From this we conclude that only a part of the hydrostatic pressure effect can be explained by charge transfer in the underdoped region. The remaining part can be ascribed to uniaxial pressure effects along the a- and b-axis.     

Photoinduced Changes of the In-Plane and Out-Of-Plane Electronic Properties in YBa2Cu3Ox

We report on the photoinduced changes of the electronic properties of YBa₂Cu₃O_x (x 6.6) thin films, focussing on the electrical anisotropy in the normal and the superconducting state. The latter was determined from magnetoconductivity measurements, performed before and after light excitation, and fitting the experimental results to the Aslamazov–Larkin theory of superconducting order-parameter fluctuations. The normal-state anisotropy _ab/_c was calculated from resistance measurements carried out on samples grown on vicinal substrates. We observed that the photodoping process enhanced superconducting coherence lengths _c(0) and _ab(0), but reduced the superconducting anisotropy _ab(0)/_c(0) at all studied temperatures. In contrast, _ab/_c was enhanced by photodoping at low temperatures, but reduced at high temperature. Our results strongly suggest that two different mechanisms contribute to the photodoping process, with one affecting only the normal-state properties.     

500–600 K Dilatation Anomaly and Tc Increase in YBa2Cu3Ox

YBa₂Cu₃O _x bulk ceramics have been simultaneously investigated by the thermal expansion and the acoustic emission methods in the 500–600 K temperature range. An error of the differential dilatometer does not exceed 10^?7. At T = 555 K, the dilatation anomaly accompanied by the acoustic emission has been observed and measured. Based on this measured anomaly, a value of Δc = ?0.003 μm has been determined. And based on this Δc size the stoichiometric coefficient x = 7.01 has been calculated too. The possible mechanism for T _c increase in YBa₂Cu₃O _x as a result of closing b axis by the absorbed oxygen atoms during the samples' heating through 500–600 K has been discussed.     

Synthesis of a phase-pure orthorhombic YBa2Cu3Ox under low oxygen pressure

Reaction of Y₂O₃, BaCo₃ and CuO for 4 h at 800 °C in flowing O₂ with a total pressure of about 2.7 × 10² Pa, followed by cooling in O₂ at ambient pressure, has produced phase-pure orthorhombic YBa₂Cu₃O_x. Keeping the ratio of O₂ to evolved CO₂ above 50 was necessary to ensure phase purity. The resultant powder yielded pressed and inserted pellets with improved superconducting properties.     

Transformation of the YBa2Cu3Ox superconducting phase to an insulator-nonsuperconducting phase

Three YBa₂Cu₃O _x samples (A, B, and C) were prepared under similar conditions. The starting materials were Y₂O₃ (99.99%), CuO (99.999%), and BaCO₃ (99.999%). All the specimens demonstrated a Meissner effect at liquid-nitrogen temperature. From temperature-dependent resistivity measurements, theT _c for the three samples was found to be aroundT _c =93 K, in agreement with other reports [1].     

Effect of Photodoping on the Fiske Resonances of YBa2Cu3O x Grain Boundary Josephson Junctions

We have expanded our studies on illuminated YBa₂Cu₃O _x grain boundary Josephson junctions (GBJJ) which show both dc Josephson properties (Fraunhofer pattern) and ac Josephson properties (Fiske resonance). Illuminating GBJJs with visible light changes the Josephson coupling. This change is characterized by an increase of the critical current and a large shift in the voltage position of the Fiske resonances. This effect is due to persistent photoinduced superconductivity (PPS) of the oxygen-depleted YBa₂Cu₃O _x barrier, similar to the PPS found in illuminated oxygen-deficient YBa₂Cu₃O _x thin films. From Fiske resonance experiments in GBJJs of different lengths, it is possible to study the velocity of the electromagnetic wave in the barrier and its change after illumination. Information on the parameters of the barrier, before and after illumination, is obtained from this study.     

Thermodynamic study of high-temperature superconductors YBa2Cu3Ox and Bi2Sr2CaCu2Oy

S. V. Starodubtsev Physicotechnical Institute, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 62, No. 1, pp. 5–9, January, 1992.     

Growth and characterization of YBa2Cu3Ox and NdBa2Cu3Ox superconducting thin films by mist microwave-plasma chemical vapor deposition using a CeO2 buffer layer

Superconducting thin films of YBa₂Cu₃O_x (YBaCuO) and NdBa₂Cu₃O_x (NdBaCuO) were grown by mist microwave-plasma chemical vapor deposition (MPCVD) using a CeO₂ buffer layer on a MgO (001) substrate. In this method, the CeO₂ buffer layer was deposited on the MgO (001) substrate at 1173 K by MPCVD. YBaCuO and NdBaCuO films were then grown at 1073 K and 1223 K, respectively. The T_c (zero resistance) values of the YBaCuO and NdBaCuO films obtained with a CeO₂ buffer layer were 90.1 K and 94.1 K, respectively, about 10 K higher than those without a CeO₂ buffer layer. The surface roughness of the films was less than 5 nm in each case. The interface between the substrate and the grown layer was confirmed to be extremely sharp by Auger profile analysis.