S. H. Liou

Surface electronic phase transition in colossal magnetoresistive manganese perovskites: La0.65Sr0.35MnO3

We have observed a distinct surface phase transition for an important class of giant magnetoresistance materials [La1−xSrxMnO3(x=0.35)]. The surface phase transition occurs at 240 K compared to 370 K for the bulk and is fundamentally different. In the bulk, a ferromagnetic metal to paramagnetic bad-metal transition occurs, while the lower-temperature surface transition is from an insulator to a semimetal. The surface of this manganese perovskite is electronically and compositionally quite different from the bulk with important implications for the behavior of artificially grown layered transition-metal oxides and for the use of surface sensitive techniques to probe the bulk.

Preparation of high Tc Tl‐Ba‐Ca‐Cu‐O thin films by pulsed laser evaporation and Tl2O3 vapor processing

Tl‐Ba‐Ca‐Cu‐O superconducting thin films with zero‐resistance temperatures up to 115 K have been prepared using a Tl2 O3 vapor process on Ba‐Ca‐Cu‐O precursor thin films. The Ba‐Ca‐Cu‐O thin films were made by laser deposition on Y‐stabilized ZrO2 substrates. This technique minimizes problems caused by the toxicity of Tl2 O3, and its subsequent decomposition to the volatile and toxic Tl2 O upon heating. Therefore, it may have practical application in the fabrication of high Tc Tl‐Ba‐Ca‐Cu‐O superconducting thin‐film devices.

Preparation of high T/sub c/ Tl-Ba-Ca-Cu-O thin films by pulsed laser evaporation and Tl/sub 2/O/sub 3/ vapor processing

Tl‐Ba‐Ca‐Cu‐O superconducting thin films with zero‐resistance temperatures up to 115 K have been prepared using a Tl2 O3 vapor process on Ba‐Ca‐Cu‐O precursor thin films. The Ba‐Ca‐Cu‐O thin films were made by laser deposition on Y‐stabilized ZrO2 substrates. This technique minimizes problems caused by the toxicity of Tl2 O3, and its subsequent decomposition to the volatile and toxic Tl2 O upon heating. Therefore, it may have practical application in the fabrication of high Tc Tl‐Ba‐Ca‐Cu‐O superconducting thin‐film devices.

Highly oriented Tl2Ba2Ca2Cu3O10 thin films by pulsed laser evaporation

We have fabricated superconducting thin films on MgO(100) substrates with nearly pure Tl2Ba2Ca2Cu3O10 (2:2:2:3) phase using pulsed laser evaporation and post‐annealing. The films had c axes perpendicular to the substrates. Superconducting films with onset temperatures of 125 K and zero resistance at 110 K were obtained. X‐ray microprobe fluorescence measurements indicate that a typical composition of films is Tl0.66Ba1.77Ca1.46Cu3Ox, which is low in Tl compared to that expected for the 2:2:2:3 phase. A typical grain size is greater than 10 μm as revealed by scanning electron microscopy.

Epitaxial Tl2Ba2Ca2Cu3O10 on (0,0,1) LaAlO3 substrates

Abstract Transmission electron microscopy of sputter deposited films of Tl 2 Ba 2 Ca 2 Cu 3 O 10 on LaAlO 3 show good film epitaxy and good film crystallinity at the film-substrate interface. Two samples, deposited under different sputtering conditions, yielded different critical current densities. The microstructure of the two samples is also very different.

The microstructure of (Hg,T1)-based high temperature superconducting films on LaAlO3 substrates

Abstract The results of transmission electron microscopy (TEM) and X-ray microanalysis of a (Hg, Tl)Ba2Cu3O8+x (1223) thin film on a LaAlO3 substrate are presented. The film was highly textured but polycrystalline, with c - axis oriented perpendicular to the substrate surface. The film/substrate interface was mostlsharp, but in some areas a 4–5 nm thick intermediate layer, of the (Hg,Tl)Ba2Ca1Cu2O6+x (1212) phase and an amorphous phase, was observed. CuOx, BaCuO2 and Ca-rich precipitates were found in the film. The intergrowth of other members of (Hg,Tl)BaCaCuO compound family, in the form of stacking faults and ordered structures, were found to be common. Microstructural evidence suggested that texture was induced by surface energy/growth anisotropy, and initiated at the substrate surface more than at the free surface.

Pulsed Laser Evaporation of Tl-Ba-Ca-Cu-O Films

Pulsed Laser Evaporation (PLE) has been shown to produce superconducting films of excellent quality. We will be discussing the results obtained from the PLE of Tl-Ba-Ca-Cu-O using a frequency doubled Nd:YAG laser operating at 532 mn. Films were deposited on SrTiO3, MgO, yttrium stabilized ZrO2, and polycrystalline Al2O3. Nearly single phase films of Tl2Ba2Ca2Cu3O10 on MgO were routinely obtained. The best films exhibited a superconducting transition onset temperature of about 125K and zero resistance at 110K. The films had a c-axis orientation perpendicular to the substrates. X-ray microprobe fluorescence measurements indicate that a typical composition of the films is Tl0.66Ba1.77Ca1.46Cu3Ox, which is low in Ti compared to that expected for the 2:2:2:3 phase. The typical grain size is greater than 10 μm as revealed by scanning electron microscopy.

Synthesis and thermal stabilization of nearly single phase Hg(Tl)Ba2Ca2Cu3O8+δ superconductors

Abstract Nearly single phase Hg(Tl)Ba 2 Ca 2 Cu 3 O 8+δ superconductors have been synthesized. The samples annealed in O 2 had a zero resistace temperature Tc=134K. The onset temperature of Hg(Tl)-1223 phase decomposition was high up to 825°C. The vibration of oxygens shifted to higher frequency (596 cm −1 ). Addition of thallium rised the vibration frequency of oxygens and stabilized the crystal structure of Hg-1223 phase.

Pulsed Laser Deposition of HTSC Thallium Films

Pulsed laser deposition is a technique commonly used to deposit high quality thin films of high temperature superconductors. We discuss the results obtained when this technique is applied to the deposition of Tl-CA-Ba-Cu-O thin films using a frequency doubled Nd:YAG laser operating at 532 nm and an excimer laser operating at 248 nm. Films with onset temperatures of 125 K and zero resistance temperatures of 110 K deposited on (100) oriented MgO from a composite T12Ca2Ba2Cu3Ox target were obtained at both wavelengths upon appropriate post deposition annealing. We will discuss the microstructure and composition as a function of laser wavelength and annealing conditions.

Hall effect studies of the mixed state of c-axis-oriented superconducting films with K

The temperature and the magnetic-field dependences of both the Hall resistivity and the longitudinal resistivity in the mixed state of c-axis-oriented superconducting films with K have been measured. It is found that the Hall resistivity shows a sign anomaly and scales with the longitudinal resistivity as with and a magnetic-field-independent coefficient K.