James N. Eckstein

Magnetoelastic coupling and magnetic anisotropy in La0.67Ca0.33MnO3 films

We report the results of magnetization measurements on pseudomorphic (fully strained) c-axis oriented colossal magnetoresistance manganite thin films grown by molecular beam epitaxy. We observe uniaxial magnetic anisotropy (hard axis/easy plane) with the easy plane being the film plane. Within the plane a weaker biaxial anisotropy is observed with [100] (Mn–O bond direction) easy axes. The magnetization dependence of the uniaxial anisotropy constant follows the predicted magnetization dependence of the magnetostriction constants within single-ion models indicating that the anisotropy energy is dominated by strain-induced anisotropy from the lattice constant mismatch with the SrTiO3 substrate. These results indicate a magnetostriction constant λ100≈+7×10−5, and an induced orbital moment of at least 0.02μb/Mn ion. We predict that by appropriate substrate selection an equilibrium out-of-plane magnetization can be produced.

Anisotropic magnetoresistance in tetragonal La1-xCaxMnOδ thin films

We have fabricated thin films of La1−xCaxMnOδ with tetragonal symmetry. For low temperatures and magnetic fields the measured magnetoresistance is anisotropic: initially positive for applied magnetic field perpendicular to the film plane and negative for field applied parallel to the film plane. At high temperatures the magnetoresistance is negative for all fields and field orientations. We also observe an in‐plane magnetoresistance anisotropy with an angular dependence corresponding to that observed in transition metal ferromagnets. We suggest an interpretation requiring a substantial spin‐orbit interaction in the material.

Superconductivity in epitaxial Bi2Sr2CuO6/Bi2Sr2CaCu2O8 superlattices: The superconducting behavior of ultrathin cuprate slabs

Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), we have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-Tc superconductor Bi2Sr2CaCu2O8 alternate with up to five such layers of the low-Tc Bi2Sr2Cu1O6 phase. In all these superlattices we foundTc to be essentially equal to that of the high-Tc Bi2Sr2CaCu2O8 phase itself, which indicates that this cuprate is a 2D superconductor insofar as the interslab coupling plays at best a secondary role. Furthermore, it is demonstrated thatTc need not be reduced at heterostructure interfaces.

Superconducting oxide multilayers and superlattices: Physics, chemistry, and nanoengineering

Abstract A technique has been developed for synthesis of heterostructures containing various Bi- and Dy-based cuprates and other complex oxides, with a high level of control of deposition of individual atomic monolayers as well as capability for atomic-layer engineering. It produces multilayers and superlattices which display striking long-range crystalline order and atomically abrupt interfaces; this enables deposition of one-unit-cell thick 2212 layers which display HTSC, and of barriers less than 1 nm thick with no pinholes over macroscopic areas. Employing Dy-doped 1278 barriers, trilayer Josephson junctions with I c R n =5−7 mV have been fabricated. Phase locking is seen in stacked junctions. Tunneling transport (but no supercurrent) is observed in junctions with insulating titanate barriers. Modulation doping is attained by omitting or inserting entire monolayers, and artificial metastable high-T c compounds have been synthesized in this way. Finally, long-range proximity effects are seen in 2212-2201-2212 trilayers.

Atomic-level engineering of cuprates and manganites

Abstract Using ALL-MBE, we have engineered a novel, artificial (metastable) cuprate superconductor, BiSr2Ca7−xDyxCu8O19+y, in which only the central Ca layer is doped by Dy. This provides, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer (only few A thick), and the top superconducting electrode, thus constituting an artificial intra-cell Josephson junction. In this way, we have fabricated the first high-TC tunnel (SIS) junctions. They exhibit significant Josephson supercurrents and very sharp quasiparticle tunneling I-V characteristics. Next, single-crystal thin films of manganites grown by ALL-MBE have shown remarkable anisotropy of colossal magnetoresistance (CMR). We have also fabricated lateral superlattices with alternating stripes (nano-wires) of La1−xCaxMnO3 and La1−xSrxMnO3, few tens of nanometers wide. They show substantial in-plane anisotropy in transport properties, and a fivefold increase in low-field sensitivity. Our most recent results include successful growth of La1−xSrxMnO3 on top of Bi2Sr2CaCu2O8, and vice versa.

Optical wavelength shifting by traveling-wave electrooptic modulation

Optical wavelength shifting of 1.054-nm laser pulses in excess of +or- 10 wavenumbers (+or- 300 GHz) was demonstrated. The wavelength shifter consists of synchronous microwave and optical waveguides fabricated monolithically on LiNbO/sub 3/. An optical pulse experiences a constant refractive index gradient that travels with the pulse and causes the wavelength shift.<<ETX>>

Atomic-layer fabrication of high-Tc tunnel junctions

Abstract Using ALL-MBE, we have engineered a novel, metastable cuprate superconductor, BiSr 2 Ca 7− x Dy x Cu 8 O 19+ y , in which only the central Ca layer is doped by Dy. This provides, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer (only few A thick), and the top superconducting electrode, thus constituting an artificial intra-cell Josephson junction. In this way, we have fabricated the first high- T c tunnel (SIS) junctions. They exhibit very sharp quasiparticle tunneling I–V characteristics, consistent with tunneling between 2D superconductors with d-wave pairing.

Transport in atomically engineered BiSrCaCuO multilayers

We have utilized atomic layer deposition to synthesize single crystal thin films of cuprate superconductors, as well as a variety of superlattices and multilayer heterostructures with atomically abrupt interfaces. For example, we made trilayer structures with the top and the bottom BiSrCaCuO electrodes separated by titanate barriers that were only 4 Å thick and yet free of pinholes over macroscopic areas. This unique synthetic capability made it possible to study transport in thec-axis direction under controlled and systematically varied conditions. Taken together, these experiments provide a picture of thec-axis transport in 2201, 2212, and various heterostructures under study. In particular, they suggest presence of a large density of localized states near the Fermi level and thus support a two-component description of the electron system in these compounds.

Measurements ofHc2(T) in Bi-Sr-Cu-O

Hc2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.Hc2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.

Cuprate trilayer c-axis tunnelling heterostructures

Trilayer tunneling structures consisting of cuprate electrodes and titanate barriers were grown by atomic layer-by-layer molecular beam epitaxy and processed into c-axis transport samples. Barriers of SrTiO/sub 3/ and related titanates with thicknesses ranging from 4 /spl Aring/ to 28 /spl Aring/ (one to seven unit cells of the titanate) were grown. While no supercurrent was observed for even the thinnest barrier, the zero bias resistance was an exponential function of barrier thickness for samples with five or fewer titanate unit cell barriers, indicating tunneling transport. Each additional titanate unit cell caused the zero bias resistance to increase by one order of magnitude. A detailed investigation of the properties of the cuprate layers immediately adjacent to the titanate layers revealed that they were depleted of charge carriers and exhibited variable range hopping transport. Thus the electron states in these layers were localized. The trilayer transport process is modeled as one phonon assisted tunneling between localized states.<<ETX>>