Yingfei Chen

Large low-field magnetoresistance observed in twinned La2∕3Ca1∕3MnO3 thin films epitaxially grown on yttria-stabilized zirconia-buffered silicon on insulator substrates

La2∕3Ca1∕3MnO3 thin films have been grown on yttria-stabilized zirconia (YSZ) buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved for the YSZ layer, the top manganite layer was multioriented in plane, with a coexistence of cube-on-cube and cube-on-diagonal epitaxial structures. Due to a combined influence from the magnetocrystalline, shape, and stain-induced magnetic anisotropy, in zero field and low temperatures the local spin orientation varies across the large-angle grain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunneling was observed. The film shows a resistance change of ∼20% in a magnetic field <1000Oe at 50K, which is promising for real applications.

Epitaxial growth of Y Ba2Cu3O7−δ thin films on silicon-on-insulator substrates by pulsed laser deposition

High quality superconducting YBa2Cu3O7-delta (YBCO) thin films have been successfully deposited on silicon-on-insulator (SOI) substrates buffered by an yttria-stabilized zirconia (YSZ) layer using the in situ pulsed laser deposition (PLD) method. X-ray diffraction (XRD) theta-2theta-scan and phi-scan, and reflection high energy electron diffraction (RHEED), indicate that both the YSZ and YBCO thin films are of high crystallinity and good lattice orientation. Atomic force microscopy (AFM) shows that the root mean square roughness R-q of YSZ thin films is less than 0.9 nm in an area of I mum x I mum, and that of YBCO thin films is about 3 nm in an area of 2 /tm x 2 mum. The zero-resistance temperature T-C0 of the YBCO/YSZ/SOI multilayers is up to 88 K, measured by a four-probe method. However, scanning electron microscopy (SEM) demonstrates many clear cracks in YBCO thin films due to the lattice constant and thermal expansion coefficient mismatch between SOI and YBCO. The crack width changes with the thickness of YBCO thin films.

Electron microscopy of step-flow grown c-axis YBa2Cu3O7−δ thin films on vicinal substrates

Investigation by transmission electron microscopy and high-resolution transmission electron microscopy was carried out on YBa2Cu3O7−δ thin films grown on vicinal SrTiO3 substrates by pulsed laser deposition. The c-axis YBa2Cu3O7−δ films grown on 1°–3° miscut substrates displayed a step-flow crossover to three-dimensional island growths, and on 6°–15° miscut substrates displayed a complete step-flow growth. Perfect heteroepitaxial growth of Ba0.1Sr0.9TiO3/YBa2Cu3O7−δ (BST/YBCO) on 1.2° miscut (001) LaAlO3, has been achieved by modification of the deposition conditions. The capacitance–voltage properties of a BST/YBCO capacitor were measured under a dc bias of ±30 V and 1 MHz frequency at 77 K, and the dielectric constant, tunability and figure of merit were found to be 1200, 60% and 133, respectively. These values show that the BST films have potential application at liquid nitrogen temperature in electrically tunable microwave devices.

Atomic Force Microscopy of Surface Reconstructed SrTiO3 Vicinal Substrates for Epitaxial Growth of YBa2Cu3O7-δ Thin Films

SrTiO3(001) vicinal substrates of miscut 1 degrees, 3 degrees, and 6 degrees toward the (001) plane were annealed at 800-1000 degrees C in pure flowing O-2 for 6 h and examined by atomic force microscopy. The high temperature annealed vicinal SiTiO3 (001) displayed arrays of straight steps and smooth terraces. The step heights vary from about 2 to 6 unit cell depending on the annealing temperatures. At 1000 degrees C, step bunching becomes obvious. The height steps of 1 unit cell are difficult to be observed by our atomic force microscopy. The YBa2Cu3O7-delta thin film grows epitaxially on the step-terrace surface with c-axis orientation by magnetron sputtering in the two-dimensional island mode. Prospect of step-flow growth is being discussed.

Control of the growth mode of epitaxial c-axis YBa2Cu3O7-δ thin films by vicinal (001) SrTiO3 substrates

C-axis YBa2Cu3O7-delta (YBCO) thin films have been grown epitaxially in the step-flow mode with the increase of vicinal angle of the SrTiO3 substrates by pulsed laser deposition. However surface hollows/cracks occurred with increasing angle, which disrupted the smoothness of the film surfaces. B reducing the oxygenate pressure during the tetragonal-orthogonal structural phase transition, the YBCO films with smooth surface was formed, but the films are superconducting at 60 K. These films have good metallic property hence they can be employed as electrodes for ferroelectric hetero structures

Detection of nuclear magnetic resonance in the microtesla range using a high Tc dc-SQUID

We have detected the ultra-low field nuclear magnetic resonance signal from water samples using a high-Tc dc-SQUID sensor. The measurements were carried out in a homemade magnetically shielded room. Resonance spectra of 1H from tap water and other substance samples were obtained in the field range from 7-110μT corresponding to resonance frequency 300-4.68kHz. Two kind of experimental systems were built, the first one is a directly coupled system, its signal to noise ratio in a single-shot measurement is around 4 for about 15 ml water. The second one used a Cu coil to transfer the flux to the SQUID sensor. Signal to noise ratio was improved to about 20 in a single-shot measurement for 5ml water, which benefits from the improvement of coupling efficiency. The effect of residual gradient in the magnetically shielded room was also investigated. J-coupling of 2,2,2-Trifluoroethyl alcohol was measured, the peaks are consistent with high field results.

The Effect of Low Frequency External Field Disturbance on the SQUID Based Ultra-Low Field NMR Measurements

We have carried out NMR experiments in the microtesla range using a high-Tc DC-SQUID sensor. The measurements were carried out in a simple magnetically shielded room. Resonance spectra of 1H nuclei from tap water and other substance samples were obtained in the field range 7-70 μT. The effect of frequency, strength, direction and phase of a low frequency external disturbance field on the NMR spectra was investigated. A sinusoidal field was applied along or perpendicular to the measurement field to simulate the external disturbance. The results were compared with the numerical calculations based on the Bloch equation and good agreement was obtained. In all cases, the field disturbance less than a few Hz are proved to influence the NMR spectra more severely, suppressing the resonance peak and driving it into split bands. On the other hand, the influence of the disturbance field at the frequency range around 50 Hz on the resonance peak position and profile is small, even though some additional small peaks appear in the spectra due to frequency mixing. The effect is more significant when the disturbance field is along the direction of the measurement field. The phase difference between the disturbance field and the free-induction-decay signal could change the NMR peak position and profile drastically.

Preparation and superconducting properties of double-sided Tl-2212 thin films on LaAlO3 substrates with low temperature and long time annealing

Double-sided Tl2Ba2CaCu2O8 (Tl-2212) thin films have been prepared on lanthanum aluminate (LaAlO3) substrates by a two-step method in which Tl-free amorphous Ba2CaCu2Ox., precursor films were first deposited on the substrates by pulsed laser deposition (PLD) and the subsequent thalliation, crystallization and oriented growth of the superconducting Tl-2212 films were obtained in a long-duration annealing process in argon atmosphere at temperature around 730-750degreesC. The films were characterized by means of x-ray diffraction (XRD), electron simulated x-ray microanalysis in the energy dispersive mode (EDX), scanning electron microscopy (SEM) and transport measurements. The results show that the as-prepared films are predominately single phase, except for the presence of some submicron particles on the film surface. The films show epitaxial growth with c-axis perpendicular to the substrate surface, as confirmed by XRD. The full width at half maximum (FWHM) of the (0012) peak determined from the rocking curves is 0.92degrees and 1.21degrees for the two sides of a double-sided Tl-2212 film, respectively. The zero-resistance transition temperature (T-C0) is 108.8 K for one side and 106.7 K for the other side of the film. The critical current density (J(C)) is in excess of 106 A cm(-2) for both sides of the films.

Ferroelectric/superconductor heterostructures

Abstract The reoccurrence of cracks have been found on the surfaces of epitaxial c-axis YBa2Cu3O7-δ (YBCO) films grown on the vicinal SrTiO3 substrates by pulsed laser deposition. At an optimum oxygen annealing condition, there are always cracks on the step-terrace surface of the YBCO films. Crack formation is attributed to the confinement of {110} mirror twinning induced by the anisotropic vicinal interfacial strain and the stress induced by the tetragonal-to-orthorhombic(T-O) phase transition of YBCO film. By use of vicinal SrTiO3 substrates of with small tilting angle(1°−3°) from (001) toward [010] and reduction of the oxidation pressure to 80Pa for T-O phase transition YBCO films with smooth surface was formed, but the films are superconducting at 60K.

Fabrication of Al/AlOx/Al Josephson junctions and superconducting quantum circuits by shadow evaporation and a dynamic oxidation process

Besides serving as promising candidates for realizing quantum computing, superconducting quantum circuits are one of a few macroscopic physical systems in which fundamental quantum phenomena can be directly demonstrated and tested, giving rise to a vast field of intensive research work both theoretically and experimentally. In this paper we report our work on the fabrication of superconducting quantum circuits, starting from its building blocks: Al/AlOx/Al Josephson junctions. By using electron beam lithography patterning and shadow evaporation, we have fabricated aluminum Josephson junctions with a controllable critical current density (jc) and wide range of junction sizes from 0.01 μm2 up to 1 μm2. We have carried out systematical studies on the oxidation process in fabricating Al/AlOx/Al Josephson junctions suitable for superconducting flux qubits. Furthermore, we have also fabricated superconducting quantum circuits such as superconducting flux qubits and charge-flux qubits.