Chris Chen

Low‐frequency optical response in epitaxial thin films of La0.67Ca0.33MnO3 exhibiting colossal magnetoresistance

We report measurements of the low‐frequency optical response in epitaxial thin films of La0.67Ca0.33MnO3 in the temperature range covering both the ferromagnetic metallic state and the paramagnetic insulating state. We observe a bolometric optical response in fully oxygenated films of the above composition as well as in oxygen deficient films. In both types of samples, the optical response is thermal in nature as indicated by its proportionality with the temperature derivative of the resistance dR/dT.

ION IMPLANTATION INDUCED ENHANCEMENT OF MAGNETORESISTANCE IN LA0.67CA0.33MNO3

Thin films of colossal magnetoresistance (MR) material, La0.67Ca0.33MnO3, were implanted with different doses (1011–1015 ions/cm2) of 200 keV Ar+ ions. The implanted samples were examined by ion channeling and x‐ray diffraction techniques. The channeling results clearly showed that the magnitude of the induced lattice disorder did not increase greatly for implantation doses up to 5×1013 ions/cm2. In this low dose implantation regime, the magnetoresistance {MR=[R(0)−R(H)]/R(0)} increased by 50%, the peak resistivity went up by two orders of magnitude, and the magnetoresistivity peak temperature decreased by 130 K compared to the original, unimplanted sample. For doses ≥5×1013 ions/cm2, the damage was significant and caused the sample to become semiconducting.

Fabrication of high-temperature superconducting Josephson junctions on substrates patterned by focused ion beam

High aspect ratio trenches were patterned on (100) LaAlO3 substrates by focused ion-beam milling prior to film deposition. Subsequent YBCO films deposited by pulsed laser deposition were found to break naturally across these trenches. In situ Au was then deposited to connect the separated YBCO electrodes. Junctions were then patterned across the trenches to form a superconductor/normal/superconductor configuration. The resistively shunted junctions exhibited supercurrents up to 86 K. Under microwave irradiation, the junctions exhibited a number of clear Shapiro steps in the I–V characteristics. The transverse magnetic-field diffraction pattern indicted uniform current distribution across the junction.

Enhanced magnetoresistance in YBa2Cu3O7/Nd0.7Sr0.3MnO3 heterostructures using magnetic flux focusing

Enhanced low field magnetoresistance (MR) in Nd0.7Sr0.3MnO3 (NSMO) has been achieved by using high‐Tc superconducting flux focusing devices made of YBa2Cu3O7(YBCO)/NSMO heterostructures. Two kinds of flux focusing schemes, gap, and washer devices, have been used where an applied magnetic field is expelled from patterned YBCO pads and focused onto a bridge of NSMO film. Using such a magnetic lens, at temperatures below 77 K ∼800% enhancement in MR=−(RH−R0)/R0 was observed for different samples. This corresponds to more than 10% change in the MR of NSMO in the presence of 200 G. For the maximum MR response in NSMO, the YBCO/NSMO heterostructures were deposited on (100) LaAlO3 substrate by pulsed laser deposition in situ so that the peak resistance temperature (Tp) of NSMO was below the superconducting transition temperature (Tc) of YBCO.

Combined method of focused ion beam milling and ion implantation techniques for the fabrication of high temperature superconductor Josephson junctions

We have studied the Tc degradation of epitaxial YB2Cu3O7−x (YBCO) films on (100) LaAlO3 substrates implanted with 100 keV O+ ions at different conditions. The influence of Au mask thickness and the implantation doses on the film characteristics have been investigated systematically. YBCO bridges have been modified by local oxygen ion implantation at optimal condition through a narrow trench in an Au/photoresist mask, which was fabricated formed by focused ion beam milling and reactive ion etching. The critical current and normal resistance of the modified bridges were found to be characteristic of superconductor/normal/superconductor Josephson junction behavior. Microwave irradiation of the junctions resulted in Shapiro steps in the I–V characteristics.

Fabrication of high-temperature superconductor Josephson junctions by focused ion beam milling

We have fabricated high-critical-temperature Josephson junctions using a focused ion beam (FIB) milling to define the gap, in a-axis oriented YBa2Cu3O7 (YBCO) films. A good quality normal metal/YBCO interface is obtained by in situ Au deposition. The effect of FIB milling has been studied with different milling conditions and the resulting devices exhibit different I–V characteristics, i.e., flux flow or superconductor/normal/superconductor Josephson junction behavior. The junctions exhibit Shapiro steps in their I–V characteristics under microwave irradiation. The critical current and normal resistance product (IcRn) of the order of 1 mV has been obtained. We also show that there is a voltage modulation in the superconducting quantum interference devices as a function of an applied magnetic flux up to 80 K.