We have grown epitaxial films of the YBa2Cu307 -,compound on SrTi03 substrates. The superconducting critical current in these films at 77 K is in excess of 10 A/cm and at 4.2 K in excess of 10 A/cm'. PACS numbers: 74.70.Ya, 74.60.Jg Over the last fifteen months the superconducting transition temperature has increased by a factor of 4 to approximately 100 K. This development was initiated by the remarkable observations of Bednorz and Muller, ' who showed that in La2, Ba (Sr,Ca)Cu409, with the K2NiF4 structure, the superconducting transition temperature was in the range of 30-40 K. Shortly after these results were reproduced in a number of laboratories, Chu and his associates showed that the application of pressure increased the transition temperature to 52 K and, furthermore, in the Y-Ba-Cu-0 systems, transition temperatures as high as 100 K could be obtained. Over the last few months a very large number of publications have appeared addressing various aspects of superconductivity in these materials.One of the weak points that has often been cited about this class of superconducting materials is their small critical current. Although a number of publications have appeared on this subject, the value of the critical current is still small relative to the more familiar metallic alloys. For example, the highest published value at 77 K known to these authors is approximately 10 A/cm .The range of potential utility of these materials is determined by the critical current. We have prepared epitaxial films of these materials and find that the critical current at 4.2 K is in excess of 10 A/cm and at 77 K is in excess 10 A/cm . The value of 4.2 K compares favorably with some of the best metallic alloys and we believe it can be increased even further. Epitaxial films with a nominal composition of YBa2CU307, -were prepared by the evaporating of Y, Ba, and Cu from three separate electron-beam sources in a vacuum of approximately 10 to 10 Torr comprised primarily of 02. The substrate was SrTi03 with a [100] direction perpendicular to the plane of deposition. During deposition the substrate was kept atapproximately 400 C. The procedure for our making these films is similar to that described earlier. ' As deposited, the films showed a broad peak in the x ray diAraction pattern characteristic of a highly disordered or an amorphous atomic arrangement. These films were annealed at approximately 900 C in 02 to obtain epitaxial films. This was confirmed both by x-ray diA'raction and electron-microscopy examination. The c axis of the orthorhombic structure was primarily perpendicular to the plane of the film. However, a fraction of the film had crystalline orientation with the c axis in the plane of the film. In addition to the orthorhombic structure there were trace amounts of a second phase present. The details of the film structure will be described in later publications. Figure I shows the typical change in resistance with temperature of our samples. Perhaps the most notable feature of these data is the relatively sharp transiti...

Various pulsed magnetization experiments employing peak fields of up to 2 T and pulse durations of 30 ms and 3 ms were carried out on YBCO samples at temperatures between 20 and 80 K. Trapped magnetic flux profiles were recorded. The highest remanent magnetizations were obtained for a multi-pulse technique with step-wise cooling. The shape and the absolute values of the trapped flux profile are discussed in terms of the dynamics of such pulsed magnetization processes.

We have measured the 1/f noise in polycrystalline films of Al, Al-Si(1'/o), and Al-Cu(4'/o) in the temperature range of 300 to 600 K. The temperature dependence indicated activation energies of 0.69, 0.80, and 0.89 eV, respectively. These energies are similar to the activation energies found for Al diffusion along grain boundaries for films of the same size and composition measured in the same temperature range. Measurements of samples with identical compositions but differing widths and thicknesses revealed significant departures from the usual inverse volume dependence of the 1/f noise. PACS numbers: 72.70.+m, 73.60.Dt The origin of 1/f noise in metal films remains one of the longest-standing unanswered questions of solid-state physics. The details of the microscopic source of the noise remain elusive despite its almost universal presence in all metal films. An empirical expression for the 1/f-noise power, Sz, of a currentbiased metal film obtained by Hooge' in 1969 iswhere V is voltage across the sample, 0 is the sample volume, f is the frequency of measurement, and o. is an empirical parameter. Since the 1/f noise in metal films has been shown to be a fluctuation of the sample resistance, 2 the quantity Sz/V2 is independent of the current bias. Hooge originally postulated that the parameter o. was constant for a11 samples; however, many recent experiments have shown this parameter to be temperature and sample dependent. Detailed measurements of the frequency exponent of the noise have verified that the exact exponent is seldom exactly -1 (or 1/f) but is also a function of temperature and sample.Dutta and Horn proposed that the 1/f noise in a metal film results from thermally activated processes and demonstrated that the noise and frequency exponent when measured as a function of temperature can be used to measure the average activation energy and the width of the distribution of activation energies,The questions of what is activating and where still remain unanswered.We report in this paper the first systematic measurements of the temperature dependence of the 1/f'noise in polycrystalline thin films of Al, A1-Cu, and Al-Si alloy. Additions of Cu or Si to an Al film increase the activation energy for A1 atomic motion along grain boundaries6 in the films but not the activation energy for Al motion in the bulk of a grain. 7 We measured a corresponding increase in the activation energy of the

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.

We report a direct observation, via electron energy spectroscopy, of lateral tunneling and lateral ballistic electron transport in a two-dimensional electron gas (2D EG). This was accomplished through the use of a novel transistor structure employing two potential barriers, induced by 50-nm-wide metal gates deposited on a GaAs/AlGaAs selectively doped heterostructure. Hot electrons with very narrow energy distributions ( -5 meV wide) have been observed to ballistically traverse 2D EG regions ==170 nm wide with a mean free path of about 480 nm. PACS numbers: 73.40.Gk, 73.20.Dx Ballistic transport of hot electrons was established recently in n "'"-type GaAs by the use of energy spectroscopy in a hot-electron structure. l These experiments utilized an injector at one end of a transport region and a spectrometer at the other end, with the electrons moving normal to the plane of the layers (vertical transport). This technique proved to be very powerful since it permitted the energy distribution and the mean free path of the ballistic electrons to be determined. The very recent demonstration of quantized resistance in a confined quasi-two-dimensional electron gas (2D EG) 2,3 strongly suggests ballistic transport of electrons near equilibrium parallel to the interface between the layers (lateral transport). We report here the first utilization of an energy spectroscopy technique to establish directly lateral tunneling through an induced potential barrier and the existence of lateral ballistic transport in a 2D EG. This was done by inducing two closely spaced potential barriers in the 2D EG via two narrow Schottky metal gates deposited on the surface of the structure. One barrier was employed as a tunnel injector and the second as a spectrometer. We have measured narrow hot-electron distributions ballistically traversing lateral 2D EG regions 170 nm wide, and have estimated their mean free path.Several structures were made on a selectively doped GaAs/AlGaAs heterostructure grown by molecularbeam epitaxy. On top of an undoped GaAs buffer layer, an undoped AlGaAs layer (50 nm thick, AlAs mole fraction x=34%) was grown, followed by a thin heavily doped GaAs cap layer (15 nm thick). A sheet of Si atoms, with an areal density of -2x 10 12 cm ~2, was deposited under overpressure of As when growth of the AlGaAs was interrupted (planar doping), 30 nm away from the GaAs buffer layer; these supply the electrons in the 2D EG [ Fig. 1(a)]. The 2D EG had a carrier density of 3x10 ll cm -2 and a mobility of 3xl0 5 cm 2 /Vsec at 4.2 K. Two parallel AuPd gates, each 52 nm wide and 0.5 jam long, were patterned 93 nm apart using electron-beam nanolithography, on a 5-^m-wide isolated 2D EG line [ Fig. 1(b)]. Ohmic contacts were made to the three regions defined by both gates. Biasing the gates negatively with respect to the central region between them (called the base) depleted the 2D EG underneath the gates and prevented the free motion of the equilibrium electrons among the three regions [emitter (E), base (B), and collector (C)]. The...

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This letter discusses the effects of a moderate magnetic field applied to a SQUID magnetometer and introduces the invention of the ‘‘flux dam.’’ A flux dam is a weak link or Josephson junction in series with the pickup loop which allows magnetic flux to enter and leave the inside of the superconducting loop easily for moderate changes in magnetic field. This largely prevents the flux lines from entering and being pinned in the superconducting parts of the magnetometer which would create extra low frequency noise. SQUID magnetometers with flux dams in the pickup loop can be operated in the Earth’s magnetic field without a significant increase in low frequency flux noise.

Individual electron trapping sites in thermally oxidized silicon substrates have been observed using a scanning tunneling microscope (STM). The excellent spatial resolution of the STM allows single defects to be identified and located on the surface. The tunneling current near single trapping sites was observed to switch between two well defined values as the site occupation changed. The magnitude of the change in the current and spatial extent of the current fluctuations, typically 3 nm, is consistent with the calculated change in tunneling current from a single electron being trapped in the oxide.