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We report on the fabrication and measurement of a novel type of quantum dot device in which the shape and size of the dot can be controlled. The device consists of four uniformly spaced quantum point contacts which can be biased to produce quantum dots ranging in size from 600 nm to below 200 nm. The capacitances of the gates controlling the dot are small. Measurements were made at a temperature of 0.3 K for three different dot sizes. Devices were fabricated in three lithography steps, with x‐ray nanolithography used to define the final gate layer. The device exhibits conductance maxima whose spacing depends on which quantum point contacts are used to define the quantum dot.

Critical currents and upper critical fields were measured on reactively sputtered AgMo 6 S 8 films with critical temperatures of up to 9.2K. X-ray measurements on these films, which were grown on sapphire substrates held at temperatures between 800°C and 1000°C, show a preferential orientation of the grains with the 001 direction parallel to the surface of the substrate. The critical field in the perpendicular direction is about 11 tesla when extrapolated to zero temperature. Critical current densities 1×108A/m2were obtained at 6 tesla and 1.2K. Polycrystalline AgMo 6 S 8 , prepared by annealing Ag/Mo multilayers in sealed quartz tubes which contain MoS 2 powder, are used as a comparison to investigate the influence of crystalline order on the critical currents and critical fields of Chevrel phases. We find that although the critical field is larger for the polycrystalline sample, the critical currents are much larger for the preferentially oriented samples.

Spin-polarized tunneling is used to measure the energy difference delta between spin-up and spin-down electrons in a magnetic field in thin films of superconducting Al. delta decreases from 2..mu../sub B/H at low temperature and field to 0.8 (2..mu../sub B/H) as the phase boundary T/sub c/(H) is approached. This change in delta provides the first measurement of the l = 0 antisymmetric Fermi-liquid parameter G/sup 0/ in a real metal and the first direct evidence for the renormalization of the Pauli field in high-field superconductivity.

The upper critical fields ${B}_{c2}(T)$ were measured up to 28 T for ${R}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$, where $R=\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{u},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{D}\mathrm{y},\phantom{\rule{0ex}{0ex}}\mathrm{H}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{n}\mathrm{d}\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m}$, and for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$. The midpoints ${(\frac{d{B}_{c2}}{\mathrm{dT}})}_{T={T}_{c}}$ cluster about (2.8\ifmmode\pm\else\textpm\fi{}0.2) T/K, the ${T}_{c}'\mathrm{s}=(93\ifmmode\pm\else\textpm\fi{}2)$ K show a slight systematic decrease from Nd to Tm, and the extrapolated values of ${B}_{c2}(0)$ cluster about (160\ifmmode\pm\else\textpm\fi{}20) T. The wide range of normal-state resistivities indicates that they are not intrinsic to the superconductor. The anisotropy $A=\frac{{B}_{c2\ensuremath{\parallel}}}{{B}_{c2}\ensuremath{\perp}}$ is estimated to be less than 12 based on the data for the resistive transition. Criteria for the midpoint resistive transition are also reexamined.

Thin films of the Chevrel phase superconductor AgMo6S8 have been prepared by simultaneous sputtering of silver and molybdenum onto sapphire substrates held at temperatures between 800°C and 1000°C using H2S as a reactive gas. The best films have a superconducting critical temperature of 9.2 K and a transition width of 0.2 K. Depending on the deposition conditions the microstructure can be varied from films that are smooth on a sub-micron scale to films that have a grain size of fractions of a micron. The smooth films have a resistivity comparable to that measured for single crystals and a resistance ratio of twenty. The films with smaller grains resistivities which are anorder of magnitude higher and show very little temperature dependence. Critical field measurements of the films with high transition temperatures give a slope of the upper critical field of 1.7 T/K and a critical field extrapolated to zero temperature of 14 T.

By electron tunneling we have studied the superconducting properties of granular aluminum as one approaches the metal-insulator transition. We find that with increasing sample resistivity, the superconducting energy-gap edge broadens because of lifetime effects until that broadening becomes comparable to the gap. At this point, superconductivity disappears.

Fermi-liquid effects are incorporated into the quasiclassical theory of superconductivity to give a quantitative theory for understanding tunneling experiments on real metals in high magnetic fields. The theory is formulated for arbitrary impurity scattering and anisotropy. Numerical calculations are done for the dirty, isotropic limit. The calculations for low spin-orbit scattering show that the energy difference in a magnetic field between spin-up and spin-down electrons is proportional to the l=0 antisymmetric Fermi-liquid parameter as confirmed by recent experiments. Furthermore, this same Fermi-liquid parameter is shown to renormalize the Pauli-limiting field in the calculation of the upper critical field. Recent experiments on thin films of Al are compared with the theoretical calculations.

Preferentially oriented thin films of the Chevrel‐phase superconductor AgMo6S8 were prepared by reactive sputtering. Ag and Mo were simultaneously sputtered from separate guns onto sapphire substrates held at about 850 °C with H2S gas injected near the substrate. The films have superconducting critical temperatures up to 9.2 K and narrow‐phase transitions. The reactive sputtering process chosen for the preparation of our films makes it possible to change the superconducting properties and the microstructure of the samples in a systematic way by changing individual preparation parameters and to study which of the preparation conditions are the most crucial for the formation of the Chevrel phase. We find that the superconducting transition temperature of the Chevrel phase is very sensitive to both substrate temperature and to the flow of H2S but insensitive to the background pressure in the chamber before deposition. The microstructure can be changed by controlling the H2S pressure. X‐ray measurements show ...

Highly oriented superconducting thin films of Bi-Sr-Ca-Cu-oxides have been reproducibly prepared by reactive magnetically-enhanced triode sputtering using three separate metal targets (Sr, Ca, and Cu-Bi). The as-deposited films had composition ratios of Bi:Sr:Ca:Cu that were very close to either 2:2:1:2 or 2:2:2:3. The films were deposited on SrTiO/sub 3/, MgO, CaF/sub 2/, Al/sub 2/O/sub 3/, and yttria stabilized zirconia (YSZ) substrates and had thicknesses of about 250 nm. After deposition, the films were annealed to 850-900 degrees C in various partial pressures of oxygen (0.05, 0.2 and 1 atm). Films deposited on

Low‐frequency (∼1 Hz) ac loss measurements were made at applied fields up to 20 T with field modulation amplitudes of up to 1 T. Results for the alternative in situ and powder metallurgy (P/M) processed Nb3Sn and Nb–Al wires are presented for single‐strand and multistrand geometries. Loss measurements for commercial continuous‐fiber Nb3Sn wires manufactured by Holec and Supercon also are presented and show that the effective fiber size determined by these ac‐loss measurements corresponds to that measured optically. The effective fiber size for alternative processed wires was intermediate between the wire size and the fiber size, and the losses (and effective fiber size) were reduced by twisting. Multistrand geometries showed further reductions in losses. The smallest effective fiber size at high field was measured for P/M processed Nb–Al. The losses were fully hysteretic and the ac losses were used to determine the critical‐current density at low fields where it was difficult to measure directly.

Two independent methods are used to measure the spin-orbit scattering rate in thin films of Al. Superconducting tunneling gives a rate that agrees reasonably well with normal-state magnetoresistance measurements interpreted by use of weak-localization theory. This agreement is good considering that the theory of superconducting tunneling does not include localization effects.

Superconducting thin films of Bi‐Sr‐Ca‐Cu‐oxides have been reproducibly prepared by reactive magnetically enhanced triode sputtering using three separate metal targets (Sr, Ca, and Cu‐Bi). Oxygen was introduced directly at the substrates during film growth. The films were deposited on ambient temperature substrates [Al2O3, SrTiO3, and yttria‐stabilized zirconia (YSZ)] and had thicknesses of 0.5–1.5 μm. After deposition, the films were annealed at 850–900 °C in flowing oxygen. The electrical resistance of the best films on YSZ decreased approximately linearly with temperature until the onset of superconductivity at about 85 K. Zero resistance was achieved by 74 K. Critical current densities in excess of 5×105 A/cm2 at 4.2 K were also obtained. X‐ray diffraction demonstrated that the best films were nearly single phase (Bi2Sr2Ca1Cu2O8+x) and textured with the c axis perpendicular to the substrate. The effect of composition and annealing on our results is discussed.

The upper critical fields B/sub c//sub 2/(T) of La/sub 2-//sub x/Sr/sub x/CuO/sub 4/(x = 0.15 and x = 0.20) were measured near T/sub c/ in dc fields to 23 T and at 4.2 K with pulsed fields to 45 T. Resistive midpoint transitions for the x = 0.15 sample yield (dB/sub c//sub 2//dT)/sub T //sub = //sub =/ = 2.2 T/K, T/sub c/ = 38.1 K, and B/sub c//sub 2/(0) = 58 T assuming a dirty type-II superconductor with no Pauli paramagnetic limiting. Data for the onset yield 4.5 +- 0.5 K/sup -1/, 40.1 K, and 125 +- 15 T, respectively. Pulsed-field data show that fractions of both materials are superconducting above 45 T at 4.2 K. Calculations of limits of B/sub c//sub 2/(T) are presented and experimental limitations of measurements in these high-field superconductors are indicated.

We have measured the upper critical field ${B}_{c2}$(T) of ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ with x=0.4 near ${T}_{c}$ in dc fields up to 20 T and at 77 K with pulsed fields to 45 T. The data yield (${\mathrm{dB}}_{c2}$/dT${)}_{T={T}_{c}}$=1.3 T/K and ${T}_{c}$=89.5 K for the midpoint of the resistive transitions, and (${\mathrm{dB}}_{c2}$/dT${)}_{T={T}_{c}}$=5\ifmmode\pm\else\textpm\fi{}1 T/K and ${T}_{c}$=95.1 K for the onset. The estimates for ${B}_{c2}$(0) range from 80 to 320 T. Pulsed-field data suggest that a fraction of the material is still superconducting above 45 T at 77 K.

We have reproducibly prepared thin films of Bi‐Sr‐Ca‐Cu‐O with Tc ≥ 105K. Depositions were done at ambient temperature with a subsequent post‐deposition anneal, and did not include lead substitution. X‐ray diffraction data indicates a majority fraction of the 2223 phase. These films possess very large grains of the order of 20‐30 u.m in size. Post‐deposition annealing conditions are a sensitive function of composition. Detailed transport measurements as a function of temperature and magnetic field have been obtained.

The Zeeman energy splitting \ensuremath{\delta} of the superconducting tunneling conductances measured in a magnetic field gives a measure of both the g factor of the conduction electrons and also the l=0 antisymmetric Fermi-liquid parameter ${G}^{0}$. We show how a technique of Fourier analysis, known as cepstral analysis, can be used to measure the Zeeman splitting directly from the total conductance. This technique allows measurement of Fermi-liquid effects which are thought to be of particular importance in high-${T}_{c}$ vanadium-based compounds. We present some results on the recent data of Bending, Beasley, and Tsuei on ${\mathrm{V}}_{3}$Ga in which the g factor is found to be nearly 2.0.

We recast the expression for tunneling current to show that the energy averaging due to a finite voltage is statistically independent from the energy averaging due to finite temperature. The energy averaging takes the mathematical form of convolutions. Convolving the standard Landauer conductance formula in energy with a voltage windowing function averages the conductance over an energy range equal to the applied voltage, just as the independent convolution with the Fermi-Dirac probability density function averages the conductance over an energy range equal to the temperature. We illustrate the effects of voltage broadening versus ordinary thermal broadening of the conductance using a quasi-one-dimensional ballistic conductor as a model system, as in the recent experiments of van Wees et al. [Phys. Rev. Lett. 60, 848 (1988)] and Wharam et al. [J. Phys. C 21, L209 (1988)].

The powder metallurgy (P/M) process and the In Situ process are alternative technologies for producing fine fiber, multifilamentary, A15 superconductors with high overall critical current densities, Jc, at high fields and with good strain tolerance. Several papers were presented at the 1983 Applied Superconductivity Conference by our group.These papers, references cited therein, and related papers should be consulted for progress to that date. Because of space limitations, in this paper we will refer to these papers briefly, examine some fundamental limits to further improved performance, report on new developments in processing, outline recent results, and give some overall assessments of future directions. At this time it is clear that the performance of Nb3Sn can be improved within rather well defined limits, whereas the Jc of Nb3Al, which now surpasses Nb3Sn at high fields, is expected to show further substantial increases in performance. The discussion here is directed toward highest field applications.

We study the dynamics of one-dimensional arrays of Josephson junctions connected in parallel by superconducting wires. These arrays are model systems for the discrete, damped sine-Gordon equation and excellent agreement between theory and experiment is obtained. The influence of boundary conditions and the coupling between two discrete sine-Gordon systems have also been investigated. In Josephson ladders, superconducting islands are connected to other islands by three Josephson junctions. The dynamics of Josephson ladders is more complicated than that of the purely 1D sine-Gordon systems.

New resonance steps are found in the experimental current-voltage characteristics of long, discrete, one-dimensional Josephson junction arrays with open boundaries and in an external magnetic field. The junctions are underdamped, connected in parallel, and DC biased. Numerical simulations based on the discrete sine-Gordon model are carried out, and show that the solutions on the steps are periodic trains of fluxons, phase-locked by a finite amplitude radiation. Power spectra of the voltages consist of a small number of harmonic peaks, which may be exploited for possible oscillator applications. The steps form a family that can be numbered by the harmonic content of the radiation, the first member corresponding to the Eck step. Discreteness of the arrays is shown to be essential for appearance of the higher order steps. We use a multi-mode extension of the harmonic balance analysis, and estimate the resonance frequencies, the AC voltage amplitudes, and the theoretical limit on the output power on the first two steps., Comment: REVTeX, 17 pages, 7 figures, psfig; to appear in J. Applied Physics

We study the dynamics of fully frustrated, underdamped Josephson arrays. Experiments reveal remarkable similarities among the dc current-voltage characteristics of several kinds of square and triangular arrays, where two resonant voltages are observed. Simulations indicate that a dynamical checkerboard solution underlies these similarities. By assuming such a solution, we reduce the governing equations to three coupled pendulum equations, and thereby calculate the voltages of the intrinsic resonances analytically., 5 pages, 3 figures included by psfig, to appear in Phys.Rev.B, Fig.1 corrected

Two discrete rings of nonlinear oscillators with topologically trapped kinks exhibit features due to coupling interactions between the rings. These interaction effects include phase locking between kinks in different rings, precession of the kink/antikink collision region, excitation of kink/antikink pairs, and time-dependent switching. We study these phenomena in simulations of two coupled discrete sine-Gordon equations, and in experiments on two inductively coupled rings of niobium Josephson junctions. @S0163-1829~98!06237-7#

We have prepared highly oriented thin films of Bi Sr Ca Cu Oxide by reactive magnetron sputtering on MgO substrates. Deposition of films with a Bi:Sr:Ca:Cu ratio of 2:2:2:4.5 together with long anneals in low oxygen partial pressures at ≈880°C yields films with R=0 as high as 106 K and a large fraction of the 110 K superconducting phase (2223) relative to the 85 K phase (2212) as judged by x-ray diffraction. A dc SQUID was was patterned based on the large (10–20 μm) grains in these films and operated up to 75 K. The detailed noise measurements which we report for this SQUID are the first for any Bi Sr Ca Cu Oxide based SQUID.

Resistive transitions of single crystal Ba 2 YCu 3− χ Ni χ O 7 were studied in magnetic field up to 20 tesla. For dop concentrations of χ = 0.01 and χ 0.03, the transition temperature in zero field decreased from the undoped value of 88K to 82K and 76K respectively, while the transition width †T (10%–90%) is still less than 1K. Moreover, Ni-doping has a dramatic effect on the resistive transition, tending to suppress the field dependent broadening.

We have measured the temperature and magnetic field dependence of the critical current densities in highly textured BiSrCaCuOxide thin films. The samples measured were either nearly single phase Bi 2 Sr 2 CaCu 2 O 8 or a mixture of Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 Ca 2 Cu 3 O 10 . The zero-field critical current density increased almost linearly with decreasing temperature for both types of samples, reaching approximately 5×10 4 A/cm 2 at 4.2K. At 4.2K the critical current density was > 10 4 A/cm 2 even in fields of 6.7 kOe. However, the current carrying capacity of both types of samples was easily destroyed at 77K by application of fields of only a few hundred Oe perpendicular to the copper-oxide planes. Critical current densities were much less sensitive to field at both 4.2K and 77K when the field was applied parallel to the copper-oxide planes.

Summary form only given. The authors discuss evidence of (1) electron standing waves forming underneath a 2000-AA period Ti/Au Schottky grid or grating incorporated as a gate into a GaAs HEMT (high-electron-mobility transistor) structure, and (2) modulation of the scattering time due to intersubband scattering in an array of approximately 100 parallel wires of 400-AA width formed by laterally patterning a GaAs/GaAlAs heterostructure. The authors show how knowledge of the electron group velocity along with a scattering rate derived from Fermi's golden rule can explain the observed effects. They compare the mobility modulation from intersubband scattering to the well-known prediction of Sakaki for increased mobility in quasi-one-dimensional wire due to the finite range of the scatterers. By first solving for the density of states and conductivity in a one-dimensional periodic potential, the authors show how to incorporate energy level broadening due to elastic and inelastic scattering, temperature broadening, and an increase in the dimensionality of the device by convolving with various known functions. >