G. F. Virshup

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.

Atomic-layer engineering of cuprate superconductors

A technique for atomic layer-by-layer synthesis of cuprate superconductors and other complex oxides has been developed. Thin films with excellent transport properties and atomically flat surfaces and interfaces are obtained. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by adding or omitting atomic monolayers to create novel compounds, and by doping within specified atomic monolayers to fabricate, for the first time, intra-cell barriers. Apart from manufacturing trilayer Josephson junctions withIcRn>5 mV, this technique enables one to customize both the materials and the devices according to the needs of a specific experiment. A number of fundamental issues, such as the dimensionality of the HTSC state, existence of long-range proximity effects, occurrence of resonant tunneling with a specified number of hops, etc., have been addressed in this way. Synthesis of the first “artificial” metastable HTSC compounds is also reported.

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.

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>>

25.2% efficiency (1‐sun, air mass 0) AlGaAs/GaAs/InGaAsP three‐junction, two‐terminal solar cell

A 25.2% efficiency measured under 1 sun, air mass 0 illumination has been achieved in a two‐terminal AlGaAs/GaAs/InGaAsP three‐junction solar cell. The cascade cell consists of a monolithic AlGaAs (Eg=1.93 eV)/GaAs two‐junction mechanically stacked on an InGaAsP (Eg=0.95 eV) single‐junction cell. The component cell of the AlGaAs/GaAs two‐junction structure were electrically connected using a metal interconnect fabricated during post‐growth processing. To minimize the obscuration effect introduced by the grid lines and metal interconnect, a prismatic cover glass was bonded to the AlGaAs/GaAs cascade cell. The results obtained with this structure represent the highest 1‐sun, air mass 0 efficiency achieved in any solar cell operating under a two‐terminal configuration. The implications of achieving this high efficiency in a two‐terminal, three‐junction solar cell for terrestrial and space applications are described as well.

Temperature coefficients of multijunction solar cells

Temperatures coefficients measured in solar simulators with those measured under AM0 solar illumination are compared to illustrate the challenges in making these measurements. It is shown that simulator measurements of the short-circuit current ( delta J/sub SC// delta T) are inaccurate due to the mismatch between the solar spectrum and the simulators at the bandgaps of the solar cells. Especially susceptible to error is the delta J/sub SC// delta T of cells which are components in monolithic multijunctions solar cells, such as GaAs filtered by 1.93 eV AlGaAs, which has an AM0 coefficient of 6.82 mu A/cm/sup 2// degrees C, compared to a Xenon simulator coefficient of 22.2 mu A/cm/sup 2// degrees C.<<ETX>>

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.

Transmission electron study of heteroepitaxial growth in the BiSrCaCuO system

Films of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} and Bi{sub 2}Sr{sub 2}CuO{sub 6} have been grown using Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely related metastable compounds like Bi{sub 2}Sr{sub 2}Ca{sub 7}Cu{sub 8}O{sub 20} (2278) and rare-earth-doped compounds like Bi{sub 2}Sr{sub 2}Dy{sub {ital x}}Ca{sub 1{minus}{ital x}}Cu{sub 2}O{sub 8} (Dy:2212) to form heterostructures with unique superconducting properties, including superconductor/insulator multilayers and tunnel junctions. Transmission electron microscopy (TEM) has been used to study the morphology and microstructure of these heterostructures. These TEM studies shed light on the physical properties of the films, and give insight into the growth mode of highly anisotropic solids like Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. {copyright} {ital 1996 Materials Research Society.}

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>>

25.2%-efficiency (1-Sun, air mass 0) AlGaAs/GaAs/InGaAsP three-junction, two-terminal solar cell

A 25.2% efficiency measured under 1-Sun, air mass 0 illumination has been achieved in a two-terminal AlGaAs/GaAs/InGaAsP three-junction solar cell. The cascade cell consists of a monolithic AlGaAs (E/sub g/=1.93 eV)/GaAs two-junction mechanically stacked on an InGaAsP (E/sub g/=0.95 eV) single-junction cell. The component cell of the AlGaAs/GaAs two-junction structure were electrically connected using a metal interconnect fabricated during post-growth processing. To minimize the obscuration effect introduced by the grid lines and metal interconnect, a prismatic cover glass was bonded to the AlGaAs/GaAs cascade cell. The results obtained with this structure represent the highest 1-Sun, air mass 0 efficiency achieved to date in any solar cell operating under a two-terminal configuration.<<ETX>>