Crucible aperture: an effective way to reduce source oxidation in oxide molecular beam epitaxy process

Growing multi-elemental complex-oxide structures using an MBE (Molecular Beam Epitaxy) technique requires precise control of each source flux. However, when the component elements have significantly different oxygen affinities, maintaining stable fluxes for easily oxidizing elements is challenging because of a source oxidation problem. Here, using Sr as a test source, we show that a crucible aperture insert scheme significantly reduces the source oxidation in an oxide-MBE environment. The crucible aperture insert was shaped like a disk with a hole at the center and was mounted inside the crucible; it blocks most of the oxygen species coming to the source, thus reducing the source oxidation. However, the depth of the aperture disk was critical for its performance; an ill-positioned aperture could make the flux stability even worse. With an optimally positioned aperture insert, the crucible exhibited more than four times improvement in Sr flux stability, compared to a conventional, non-apertured crucible.Comment: 13 pages, 3 figure

Efficient resistive memory effect on SrTiO3 by ionic-bombardment

SrTiO3 is known to exhibit resistive memory effect either with cation-doping or with high-temperature thermal reduction. Here, we add another scheme, ionic-bombardment, to the list of tools to create resistive memory effect on SrTiO3 (STO). In an Ar-bombarded STO crystal, two orders of resistance difference was observed between the high and low resistive states, which is an order of magnitude larger than those achieved by the conventional thermal reduction process. One of the advantages of this new scheme is that it can be easily combined with lithographic processes to create spatially-selective memory effect.Comment: 14 pages, 4 figure

Thickness-dependent, tunable anomalous Hall effect in hydrogen-reduced PdCoO2_2 thin films

It was recently reported that hydrogen-reduced PdCoO$_2$ films exhibit strong perpendicular magnetic anisotropy (PMA) with sign tunable anomalous Hall effect (AHE). Here, we provide extensive thickness-dependent study of this system, and show that the electronic and magnetic properties are strongly dependent on the thickness and annealing conditions. Below a critical thickness of 25 nm, AHE shows clear PMA with hysteresis, and its sign changes from positive to negative, and back to positive as the annealing temperature increases from 100 $^\circ$C to 400 $^\circ$C. Beyond the critical thickness, both PMA and AHE hysteresis disappear and the AHE sign remains positive regardless of the annealing parameters. Our results show that PMA may have a large role on AHE sign-tunability and that below the critical thickness, competition between different AHE mechanisms drives this sign change

A new type of cyclotron resonance from charge-impurity scattering in the bulk-insulating Bi2_2Se3_3 thin films

We have utilized time-domain terahertz spectroscopy to investigate the low frequency Drude response of bulk-insulating topological insulator Bi$_2$Se$_3$ films grown on buffer layers under a magnetic field up to 7 Tesla. With both field and frequency dependence, such experiments measure the mobility and carrier density of multiple surface conduction channels simultaneously. We observe sharp cyclotron resonances (CRs), and it is consistent with conduction from two different topological surface states (TSSs) on top and bottom of the films. The CR sharpens at high fields, an effect that we attribute to an electron-impurity scattering. This work shows that cyclotron resonance is a powerful tool to study many-body interactions in topological materials.Comment: 5 pages, 3 figures. arXiv admin note: text overlap with arXiv:1502.0457