Arunava Gupta

Colossal magnetoresistance of 1 000 000‐fold magnitude achieved in the antiferromagnetic phase of La1−xCaxMnO3

Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3 system. At 125 K, the CMR of the La0.5Ca0.5MnO3 reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated.

Superconducting mercury-based cuprate films with a zero-resistance transition temperature of 124 Kelvin.

The synthesis of high-quality films of the recently discovered mercury-based cuprate films with high transition temperatures has been plagued by problems such as the air sensitivity of the cuprate precursor and the volatility of Hg and HgO. These processing difficulties have been circumvented by a technique of atomic-scale mixing of the HgO and cuprate precursors, use of a protective cap layer, and annealing in an appropriate Hg and O2 environment. With this procedure, a zero-resistance transition temperature as high as 124 kelvin in c axis-oriented epitaxial HgBa2CaCu2O6+δ films has been achieved.

Gas‐phase oxidation chemistry during pulsed laser deposition of YBa2Cu3O7−δ films

The gas‐phase processes relevant for oxidation during growth of YBa2Cu3O7−δ (YBCO) films by pulsed laser deposition in different oxidizing ambients are discussed using a phenomenological model. The model consists of an accounting for the neutral species generated from the target during laser ablation, and their attenuation due to reactive and elastic scattering during transport, to determine the flux of neutral cation and anion species incident onto the substrate. In particular, the oxygen (O and O2), cation (Y, Ba, and Cu), and diatomic oxide fluxes in the presence of O2, O3, N2O, or NO2 as background oxidizer gases are qualitatively analyzed based on the known photochemical and electron‐impact dissociation, and oxidation reactions of these gases. The relative effectiveness of these oxidizing agents, as judged from their propensity to produce diatomic oxides and atomic oxygen in the gas phase, is found to be in qualitative agreement with the previously observed trends in their oxidation behavior as deduc...

Design and applications of a scanning SQUID microscope

The scanning SQUID (Superconducting Quantum Interference Device) microscope is an extremely sensitive instrument for imaging local magnetic fields. The authors describe one such instrument which combines a novel pivoting lever mechanism for coarse-scale imaging with a piezoelectric tube scanner for fine-scale scans. The magnetic field sensor is an integrated miniature SQUID magnetometer. This instrument has a demonstrated magnetic field sensitivity of <10{sup {minus}6} gauss/{radical}Hz at a spatial resolution of {approximately}10 {micro}m. The design and operation of this scanning SQUID microscope are described, and several illustrations of the capabilities of this technique are presented. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, is shown. The use of this instrument for the first observation of half-integer flux quanta, in tricrystal thin-film rings of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, is described. The half-integer flux quantum effect is a general test of the symmetry of the superconducting order parameter. One such test rules out symmetry-independent mechanisms for the half-integer flux quantum effect, and proves that the order parameter in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} has lobes and nodes consistent with d-wave symmetry.

Magnetotransport in doped manganate perovskites

Recent progress in oxide perovskite thin-film technology has led to the discovery of a large negative magnetoresistance at room temperature in doped manganate perovskite thin films. These films may have potentials for magnetic sensing applications. In this paper we review the basic phenomena and physics of magnetotransport in this class of materials. We also discuss our recent demonstration of a large low-field magnetoresistance effect, and the associated challenges that lie ahead.

Temperature distribution during heating using a high repetition rate pulsed laser

A general equation has been derived for computing the temperature distribution produced in a substrate heated by a high repetition rate pulsed laser. The theoretical predictions have been compared with experimental results obtained for etching of Mn‐Zn ferrite in KOH solution using a copper vapor laser. The effects of the laser power and substrate scan speed on the temperature distribution have been investigated, and the predicted melt‐zone boundaries have been compared with the experimentally observed width, depth, and shape of the etched grooves.

ATOMIC SCALE OXIDE SUPERLATTICES GROWN BY RHEED CONTROLLED PULSED LASER DEPOSITION

By depositing thin films under conditions where intensity oscillations are observed in RHEED (reflection high-energy electron diffraction) spots, unit cell level multilayers of SrTiO 3 /BaTiO 3 structures have been grown by pulsed laser ablation. High resolution TEM (transmission electron microscopy) and STEM (scanning transmission electron microscopy) observations of the deposits show that epitaxial multilayers with layer thicknesses of 1, 2, 4, 8, and 16 unit cells can be grown on [100] orientation SrTiO 3 substrates. The superlattices show partial intermixing of the Sr and Ba for layer thicknesses less than 8 unit cells, but incomplete intermixing occurs even when the layers are only a single unit cell thick. From observations of the degree of intermixing at different depths in the deposit, it was determined that most of the intermixing takes place during deposition and not during subsequent annealing of the deposit. The 16 and 8 unit cell thick BaTiO 3 layers were found to be tetragonal with the c -axis of the layers oriented normal to the substrate but with the a -axis strained to coherently match the SrTiO 3 layers.

Direct laser writing of superconducting patterns of Y1Ba2Cu3O7−δ

Direct argon ion laser writing of 40‐μm‐wide and 3‐μm‐thick superconducting lines of Y1Ba2Cu3O7−δ on magnesium oxide (MgO) substrate is reported. A 514.5 nm laser beam of 300 mW was focused onto a thin sprayed film of the nitrate precursors (Y:Ba:Cu=1:2:3) and scanned over it. The laser beam converted oxides along the irradiation path. The film was then ‘‘developed’’ by rinsing it in ethanol, which removed the unirradiated areas, but left the irradiated areas intact. This was followed by the standard oven annealing procedure in flowing oxygen. The resistivity of the laser‐written lines as a function of temperature showed an onset and completion of the superconducting transition at 82 and 69 K, respectively. The corresponding values of the unpatterned, blanket film were 82 and 77 K, respectively.

Bismuth strontium calcium copper oxide high-Tc superconducting films from nitrate precursors

Abstract Superconducting films of bismuth strontium calcium cuprate were prepared by spray pyrolysis of mixed nitrate solutions onto heated (100) orientated MgO substrates, as well as yttria-stabilized zirconia and SrTiO3. Various compositions and annealing temperatures were explored. Best results −Tc = 82 K, TR=0=77 K, Jc (60 K) = 800 A/cm2—were obtained on MgO for a Bi2.15Sr1.68Ca1.17Cu2Ox composition after brief annealing in air just below its melting point. A mixture of phases with c-axis normal to the substrate is obtained; the “Bi2Sr2CaCu2O8+x” phase predominates in the best samples, while a phase with c 0 =24.4 A (or 48.8 A) is always present. The superconductivity of the films is not significantly degraded by a 3 day immersion in water.

Improved laser-ablated thin films of NdCeCuO by use of N2O

Thin films of the electron‐doped superconductor Nd1.85Ce0.15CuO4−δ were prepared by laser ablation in O2 and N2O ambients. Although films prepared in an O2 ambient need to be reduced to become superconducting, the use of a lower oxygen pressure during deposition produces inferior films. A more metallic character can be obtained by increasing the O2 pressure during deposition. The substitution of N2O for O2 results in films that are superconducting without a separate reduction step, with a sharp transition at about 16 K. A short vacuum anneal at the deposition temperature yields reproducible films with Tc=20 K, without change in the transition width.