Tunneling and conductivity measurements through the metal-insulator transition in amorphous Nb^Si^ are reported. The authors observe the correlation gap A which varies with resistivity and have related this to the metal-insulator transition as observed in the conductivity. The samples were prepared by a process which allows precise control of the Nb concentration. The results as a function of voltage, temperature, and concentration are compared with current theoretical predictions.PACS numbers: 71.30.+h, 71.50.+t, 72.15.Cz, 73.40.Gk The metal-insulator transition in disordered materials has been recently studied in several transport measurements. 1 " 4 All experiments indicate that there is no minimum metallic conductivity. 3 ' 4 In addition, tunneling measurements have shown a strong anomaly in the density of states at the Fermi level indicative of very strong many-body contributions to the metal-insulator transition. 1 ' 2 Altshuler and Aronov 5 had earlier shown that many-body effects were important. In a phenomenological scaling theory McMillan 6 incorporated both localization and correlation effects on equal footing. He predicted a squareroot singularity N(p) = N(0)[l + (E/A) 1/2 ] in the single-particle density of states at the Fermi level on the metallic side of the transition. This was later verified by the tunneling experiments. The parameter A was interpreted as a correlation gap-the precursor of the energy gap in the single-particle density of states which opens up on the insulating side of the metal-insulator transition. At the metal-insulator (M-I) transition, A goes to zero, N(0) goes to zero, and the energy gap begins to open up.In this Letter we report high-precision tunneling and conductivity measurements through the metal-insulator transition in amorphous Nb x Si xx . Our samples were prepared by simultaneously cosputtering Nb and Si onto a substrate producing a shallow concentration gradient. Unlike previous measurements which had difficulty in producing controlled, uniform concentration changes, our method allowed us to measure the properties of the amorphous material in a controlled and systematic way as we moved through the M-I transition in small, uniform increments. As in previous experiments we have observed a correlation gap A which varies with resistivity. However unlike previous experiments we have made low-temperature transport measurements to correlate our tunneling results directly with the metal-insultor transition. In addition we have observed the metal-insulator transition by the onset of the breakdown of the single-step tunneling process when the energy gap appears at the Fermi energy and localized states appear in the Nb^Sii^. By controlling concentration as easily as voltage and temperature we can obtain experimental relationships among them and in this Letter these are presented and compared with current theoretical predictions.Our samples were prepared by cosputtering Nb and Si from separate targets onto sapphire substrates (see inset in Fig. 1). The films had a thickness...

Linear electro-optic coefficients as large as r ~ 4 X 10-5 cm/statvolt have been measured in the system of ferroelectric strontium barium niobates, Sr ... Bal_ ... NbtOs. In the first crystals x varies from 0.75 to 0.25, with Curie temperatures ranging from -600C to 25()OC. At 15 Me, the respective half-wave field distance products range from 48 to 1236 V.The ferroelectric strontium barium niobates, Sr xBal-xNb20s, synthesized recently as well-formed crystals by A. A. Ballman,1 are optically negative, uniaxial cry.stals belonging to the tetragonal point group 4mm. 2 In this series of crystals, x varies from 0.75 to 0.25 with Curie temperatures ranging from -60°C to -250°C. The crystal growth, chemical properties, and ferroelectric characteristics are reported separately. I At room temperature, in poled single crystals we have observed and measured a transverse linear electro-optic effect between one and two orders of magnitude larger than those reported previously in the literature. The crystals are of the tungsten-bronze type with niobium ions octahedrally coordinated with the oxygen ions. 2 Various cation substitutions then allow control of the Curie temperature and other dielectric and optical properties.For a crystal of point group 4mm, the electrooptic matrix has three independent nonzero moduli (r13 = r23, r42 = r51> r33)' In SrxBal-xNb20s the large electro-optic effect was observed for the electric field parallel to the single tetrad symmetry axis (X3), which is also the polar axis, and light propagation normal to the X3 direction.The optical phase retardation in radians, f, is given by f=(21TI/Ao)(n'2-n'3), where n'2 and n'3 are the principal indices of refraction normal to the direction of propagation (XI), Ao is the wavelength of light in vacuum, and I is the optical path length. For point group 4mm, n'2 = no -no3,rI3E/2 and n'3 = ne -ne3r33E/2, where E is the field strength and no and ne are the ordinary and extraordinary indices of refraction respectively. For light parallel to X3 and electric field parallel to XI (a axis), and n'2 = no.Crystals in the series SrxBal-xNb20s which have been investigated are those for which X = 0.25, x = 0.50, and X = 0.75. Measurements were made of the half-wave field distance product [E . l] 11. /2 and the relative dielectric constant, E3' along the X3 or optic axis. The [E . I] 11. /2 at 6328 A with polarization of the incident light at 45° with respect to the principal axes are tabulated in Table I. In Sro.75Bao.25Nb20s for a 1: 1 aspect ratio of electric field path to optical path length the half-wave field distance product [E . I] 11./2 is 48 V at 15 Me. This value is comparable to the value of 28 V (ref.3) for the quadratic effect in potassium tantalate niobate at dc bias fields of 2000 V. The 48 V required is also 60 times smaller than the -2800 V figure given previously for LiTaO:! (ref. 4) and LiNb0 3 . 5The lithium tantalate modulator which was described 4 used a 10 mm light path and a 0.5 mm electric field path and resulted in a reduction of the half-wave re...

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X-ray and electron beam diffraction analyses have been carried out on thin films deposited from a beam of carbon ions. Results show that the films consist of a polycrystalline background of cubic diamond with a particle size of 50–100 Å with single-crystal regions up to 5 μm in diameter.

Measurements of the ferromagnetic resonance line-width maximum in single-crystal yttrium iron garnet (YIG) were previously reported by Dillon. 1 His data show that the line width increases with decreasing temperature, reaches a maximum below liquid nitrogen temperature, and decreases again to approximately the roomtemperature value at liquid helium temperatures. We have been investigating the nature of this line-width maximum at low temperatures to determine if the effect is intrinsic to ferromagnetic resonance and to determine its origin. The line widths reported in this paper are determined from the absolute absorption of the sample and the saturation magnetization vs T curve measured on single-crystal YIG. The large effects on the line width AH at room temperature due to inhomogeneous broadening by scattering from pits on the surface of a YIG sphere were evaluated previously. 2 Consequently, we measured AH vs T for a series of three spheres prepared using polishing papers having mean grit sizes of 15, 5, and 0.3 microns, respectively. These measurements were made at 9300 Mc/sec. Figure 1 shows that the ratios of maximum line widths to room temperature line widths are 2.5, 4, and 13, respectively, the larger ratios occurring for the better polished spheres. It is also clearly shown that the contribution to the line width due to surface preparation is essentially additive over the temperature range. We conclude therefore that the low-temperature line-width maximum is an intrinsic property of the material and not of the surface. Further, if perfect polishing were attained the line-width maximum would still be approximately 6 oersteds in this sample.Earlier investigations on the same samples 3 have shown a small but repeatable frequency dependence of line width at room temperature. The line width AH (full width between half maximum values) is 0.44 oersted at 3000 Mc/sec and 0.52 oersted at 9300 Mc/sec along the [111] axis. Taking these measurements over the temperature range, there are three essential features. See Fig. 2. The line width maximum at 3000 Mc/sec is reduced by 40 % in magnitude and is shifted from 40°K to 30°K. Also it is seen that the room-temperature frequency dependence of 300 FIG. 1. AH vs_ T for single-crystal spheres of YIG 0.014 to 0.017 inch in diameter. H^c oriented along the [111] axis. The labels on the curves indicate the mean grit size of the final polishing paper used to prepare the spheres. The lowest curve is Fig. 3 repeated here for comparison. 300 FIG. 2. AH vs T measured at 9300 and 3000 Mc/sec. Hfc oriented along the [111] axis. 32