Robert L. Sandstrom

Lanthanum gallate substrates for epitaxial high‐temperature superconducting thin films

We demonstrate that lanthanum gallate (LaGaO3) has considerable potential as an electronic substrate material for high‐temperature superconducting films. It provides a good lattice and thermal expansion match to YBa2Cu3O7−x, can be grown in large crystal sizes, is compatible with high‐temperature film processing, and has a reasonably low dielectric constant (e≂25) and low dielectric losses. Epitaxial YBa2Cu3O7−x films grown on LaGaO3 single‐crystal substrates by three techniques have zero resistance between 87 and 91 K.

Room-temperature ferromagnetic nanotubes controlled by electron or hole doping.

Nanotubes and nanowires with both elemental (carbon or silicon) and multi-element compositions (such as compound semiconductors or oxides), and exhibiting electronic properties ranging from metallic to semiconducting, are being extensively investigated for use in device structures designed to control electron charge. However, another important degree of freedom—electron spin, the control of which underlies the operation of ‘spintronic’ devices—has been much less explored. This is probably due to the relative paucity of nanometre-scale ferromagnetic building blocks (in which electron spins are naturally aligned) from which spin-polarized electrons can be injected. Here we describe nanotubes of vanadium oxide (VOx), formed by controllable self-assembly, that are ferromagnetic at room temperature. The as-formed nanotubes are transformed from spin-frustrated semiconductors to ferromagnets by doping with either electrons or holes, potentially offering a route to spin control in nanotube-based heterostructures.

Crystallography and microstructure of Y1Ba2Cu3O9−x, a perovskite‐based superconducting oxide

We have investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x‐ray diffraction. Y1Ba2Cu3O9−x is a distorted, oxygen‐defect perovskite with ordering of the yttrium and barium ions. Its unit cell is orthorhombic with space group Pmm2 and lattice parameters a=3.893 A, b=11.688 A, and c=3.820 A. The structure is heavily twinned on {101} type planes, possibly due to a tetragonal‐to‐orthorhombic transition above room temperature.

Reliable single‐target sputtering process for high‐temperature superconducting films and devices

We report a simple, single‐target magnetron sputtering process for films of high‐temperature superconductors involving an off‐axis sputtering geometry. The process lends itself both to film growth with high‐temperature post‐anneals and to low‐temperature in situ film growth. The post‐anneal process routinely yields YBa2Cu3O7−x films on SrTiO3 substrates that are fully superconducting at 86–89 K. Current densities at 77 K range from 104 to 8×105 A/cm2. A single‐level superconducting quantum interference device (dc SQUID), made by photolithographically patterning a low current density film, has a flux noise level at 77 K of 3×10−4 Φ0/(Hz)1/2 at 20 Hz, dominated by low‐frequency noise associated with flux motion in the film.

Design, fabrication, and performance of integrated miniature SQUID susceptometers

The design, construction, and performance of miniature SQUID (superconducting quantum interference device) susceptometers is discussed. Spins (in units of mu /sub beta /) per square root Hz has been identified as an important figure of merit. Simple expressions for S/sub n/ (spin sensitivity) in miniature SQUID susceptometers are developed and the implications of dimensional scaling explored. The details of several existing and proposed designs are reviewed, including versions that utilize commercial SQUIDs. With thin-film DC SQUIDs, S/sub n/ values of a few thousand spins/ square root Hz have already been obtained, and it is projected that values of a few hundred will be achieved soon. >

Low‐frequency noise in low 1/f noise dc SQUID’s

We demonstrate that the low‐frequency noise in our edge junction dc superconducting quantum interference devices, with a basic 1/f flux noise of 2×10−12 Φ20/Hz at 1 Hz, can all be accounted for in terms of junction critical current fluctuations. A novel modulation readout scheme is able to cancel the effect of junction critical current fluctuations and reduce our total noise to 1×10−12 Φ20/Hz at 0.1 Hz, a level that is three times lower than the lowest flux noise ever previously reported at this frequency.

Low-noise modular microsusceptometer using nearly quantum limited dc SQUIDs

A flexible combination of superconducting integrated circuits was used to construct a low‐temperature magneto‐optic microsusceptometer utilizing a dc superconducting quantum inteference device (SQUID) detector operating near the quantum limit (coupled energy sensitivity of 1.7ℏ). Miniature pick‐up loop assemblies on transparent substrates were joined by superconducting interchip connections to a thin‐film dc SQUID, which is in turn read out by a second dc SQUID connected to room‐temperature electronics. Measurements on an 8.5‐μm‐diam titanium dot evaporated directly into the pick‐up loop demonstrate a spin sensitivity of ∼103 spins/(Hz)1/2 at T=290 mK.

Josephson integrated circuit process for scientific applications

We have developed and are regularly practicing a seven mask-level Josephson integrated circuit fabrication process tailored to dc SQUID requirements and intended for SQUID studies and other scientific applications of Josephson technology. The process incorporates low capacitance Nb/Nb 2 O 5 /PbAuIn edge junctions, PdAu shunt resistors, and a wiring pitch of 5 μm for the SQUID input coil level (which is PbAuIn). The junctions can be made as small as 2μm by 0.3μm, with a capacitance (including parasitics) of ∼0.14 pF. This process yields stable and reliable junctions and integrated circuits.

Wet etch process for patterning insulators suitable for epitaxial high Tc superconducting thin film multilevel electronic circuits

We describe a wet etch process for patterning insulators suitable for multilayer epitaxial highTc superconductor‐insulator‐superconductor structures down to micronmeter‐scale dimensions. A solution of 7% HF in water gives convenient etch rates for SrTiO3 and MgO insulators (about 1500 A/min for single crystals), and easily stops on thin high Tc superconducting layers, due to the high selectivity of this etchant between these insulators and the cuprate superconductors. Using entirely wet etching patterning processes, we have fabricated 5‐turn (20‐turn) coils with zero resistance at 89 K (79 K) and critical currents at 77 K of 2.5 mA (6 μA).

Current-induced local oxidation of metal films: Mechanism and quantum-size effects

A novel route is introduced for oxidizing thin metal films with nanometer-scale resolution. By locally subjecting Ti and Nb films to high in-plane current densities, metal-oxide tunneling barriers are formed in a self-limiting fashion. The oxidation is triggered by current-induced atomic rearrangements and local heating. At the final stages of the barrier formation, when only atomic-scale channels remain unoxidized, the oxidation rate decreases drastically while the conductance drops in steps of about 2e2/h. This behavior gives evidence of ballistic transport and a superior stability of such metallic nanowires against current-induced forces compared with the bulk metal.