Chemical vapor deposition of high T(sub c) superconducting films in a microgravity environment

Since the discovery of the YBaCuO bulk materials in 1987, Metalorganic Chemical Vapor Deposition (MOCVD) has been proposed for preparing HTSC high T(sub c) films. This technique is now capable of producing high-T(sub c) superconducting thin films comparable in quality to those prepared by any other methods. The MOCVD technique has demonstrated its superior advantage in making large area high quality HTSC thin films and will play a major role in the advance of device applications of HTSC thin films. The organometallic precursors used in the MOCVD preparation of HTSC oxide thin films are most frequently metal beta-diketonates. High T(sub c) superconductors are multi-component oxides which require more than one component source, with each source, containing one kind of precursor. Because the volatility and stability of the precursors are strongly dependent on temperature, system pressure, and carrier gas flow rate, it has been difficult to control the gas phase composition, and hence film stoichiometry. In order circumvent these problems we have built and tested a single source MOCVD reactor in which a specially designed vaporizer was employed. This vaporizer can be used to volatilize a stoichiometric mixture of diketonates of yttrium, barium and copper to produce a mixed vapor in a 1:2:3 ratio respectively of the organometellics. This is accomplished even though the three compounds have significantly different volatilities. We have developed a model which provides insight into the process of vaporizing mixed precursors to produce high quality thin films of Y1Ba2Cu3O7. It shows that under steady state conditions the mixed organometallic vapor must have a stoichiometric ratio of the individual organometallics identical to that in the solid mixture

Vortex Phase Diagram of weakly pinned YBa2_2Cu3_3O7−δ_{7-\delta} for H ∥\parallel c

Vortex phase diagram in a weakly pinned crystal of YBCO for H $\parallel$ c is reviewed in the light of a recent elucidation of the process of `inverse melting' in a Bismuth cuprate system and the imaging of an interface between the ordered and the disordered regions across the peak effect in 2H-NbSe$_2$. In the given YBCO crystal, a clear distinction can be made between the second magnetization peak (SMP) and the peak effect (PE) between 65 K and 75 K. The field region between the peak fields of the SMP (H$^m_{smp}$) and the onset fields of the PE (H$^{on}_{pe}$)is not only continuously connected to the Bragg glass phase at lower fields but it is also sandwiched between the higher temperature vortex liquid phase and the lower temperature vortex glass phase. Thus, an ordered vortex state between H$^m_{smp}$ and H$^{on}_{pe}$ can get transformed to the (disordered) vortex liquid state on heating as well as to the (disordered) vortex glass state on cooling, a situation analogous to the thermal melting and the inverse melting phenomenon seen in a Bismuth cuprate.Comment: Presented in IWCC-200

Vaporization of a mixed precursors in chemical vapor deposition for YBCO films

Single phase YBa2Cu3O7-delta thin films with T(c) values around 90 K are readily obtained by using a single source chemical vapor deposition technique with a normal precursor mass transport. The quality of the films is controlled by adjusting the carrier gas flow rate and the precursor feed rate

Model for the Vaporization of Mixed Organometallic Compounds in the Metalorganic Chemical Vapor Deposition of High Temperature Superconducting Films

A model of the vaporization and mass transport of mixed organometallics from a single source for thin film metalorganic chemical vapor deposition is presented. A stoichiometric gas phase can be obtained from a mixture of the organometallics in the desired mole ratios, in spite of differences in the volatilities of the individual compounds. Proper film composition and growth rates are obtained by controlling the velocity of a carriage containing the organometallics through the heating zone of a vaporizer

Evidence of Electromagnetic Absorption by Collective Modes in the Heavy Fermion Superconductor UBe13

We present results of a microwave surface impedance study of the heavy fermion superconductor UBe13. We clearly observe an absorption peak whose frequency- and temperature-dependence scales with the BCS gap function. Resonant absorption into a collective mode, with energy approximately proportional to the superconducting gap, is proposed as a possible explantation

Comparison of History Effects in Magnetization in Weakly pinned Crystals of high-TcT_c and low-Tc_c Superconductors

A comparison of the history effects in weakly pinned single crystals of a high $T_c$ YBa$_2$Cu$_3$O$_{7 - \delta}$ (for H $\parallel$ c) and a low $T_c$ Ca$_3$Rh$_4$Sn$_{13}$, which show anomalous variations in critical current density $J_c(H)$ are presented via tracings of the minor magnetization hysteresis loops using a vibrating sample magnetometer. The sample histories focussed are, (i) the field cooled (FC), (ii) the zero field cooled (ZFC) and (iii) an isothermal reversal of field from the normal state. An understanding of the results in terms of the modulation in the plastic deformation of the elastic vortex solid and supercooling across order-disorder transition is sought.Comment: Presented in IWCC-200

Vortex phase boundaries from ferromagnetic measurements in a patterned disc array

Using a recently developed broadband microwave measurement technique, we have studied the hysteretic appearance and disappearance with in-plane magnetic field of the uniform ferromagnetic resonance (FMR) mode of a patterned permalloy disk array. The observed features are consistent with our micromagnetic simulations (performed on an infinite array of such disk), which predict that on decreasing the magnetic field from a positively magnetized state at positive fields the array will: (i) pass continuously into a double-vortex state; (ii) followed by a discontinuous transition to a single-vortex state; and finally (iii) discontinuously into a negatively magnetized state at some negative field. The hysteretic counterpart occurs on reversing the field sweep and returning to positive fields. The FMR data are consistent with the hysteretic dc magnetization measurements performed earlier on samples patterned in an identical manner

Measurements of the Pair-Breaking Edge in Superfluid \u3csup\u3e3\u3c/sup\u3eHe-B

We have made the first systematic study of the pair-breaking edge in superfluid 3He-B over a wide range of pressures and frequencies. This direct measurement of the gap enables us to experimentally verify the predictions made for strong-coupling corrections to the gap. In addition, it indirectly lends support to the temperature scale of Greywall

Thermodynamics of the UPt\u3csub\u3e3\u3c/sub\u3e superconducting phase diagram

In this paper we present a thermodynamic analysis of the UPt3 superconducting phase diagram in the H-T plane. The analysis relates the specific-heat jumps to the slopes of the four phase-transition lines. The existing data are found to be in good mutual agreement, and consistent with the assumption that the inner phase-transition line at low field, Tc*, is second order for both Ĥll ĉ and Ĥll ĉ. For Ĥllĉ, the inner phase-transition line at higher field, HFL, is first order albeit with a small latent heat. For Ĥll ĉ, it can be second order