D. G. Naugle

On-chip manipulation of levitated femtodroplets

We report diamagnetic levitation of droplets and∕or particles of pico–femtoliter volume and demonstrate their on-chip storage and high precision manipulation (translation, merging, assembling and rotation). We also demonstrate a levitation based microfluidic processor to process droplets∕particles with up to a billion times smaller volume than in typical microfluidic devices. The levitated particles can be positioned with up to 300nm accuracy and precisely rotated and assembled, providing a different physical approach for micro-electro-mechanical systems. Force can be applied to the droplets∕particles via magnetic, electric, and gravitational fields with up to femto-Newton accuracy, and potential energy can be controlled with up to 0.2zeptoJ(0.05kBT) precision, thus providing experimental tools for fundamental studies.

FROZEN FLUX SUPERCONDUCTORS

We propose a new class of composite superconductors with embedded magnetic nanoparticles/nanorods. These nanomagnets can create a thermodynamically stable, frozen, dense flux line network. We discuss the methods of fabrication of these novel materials. We show that due to the frozen flux created by magnetic nanorods, the critical current in this composite superconductor can be higher than in the superconductor with a comparable density of columnar defects.

Volume Viscosity in Liquid Argon at High Pressures

The volume viscosity in liquid argon has been calculated from ultrasonic attenuation measurements at temperatures between 85° and 145°K with pressures up to 157 kg/cm2. Measurements taken at densities from 1.06 to 1.42 g/cm3 result in values of 2.3 to 0.8 for the ratio of volume to shear viscosity. These values are compared with theoretical predictions of other investigators. It appears that the volume viscosity increases with increasing density whereas the ratio of volume‐to‐shear viscosity decreases. The ultrasonic attenuation rapidly increases as the temperature approaches the critical temperature. This increase in attenuation is a consequence of the change in the density and compressibility of the medium. Thus no new mechanism of dissipation is required within about 7°K below the critical temperature.

Excess Ultrasonic Attenuation and Intrinsic‐Volume Viscosity in Liquid Argon

The ultrasonic attenuation has been measured in liquid argon by pulse techniques with a single quartz transducer, variable‐path‐length apparatus. Measurements have been made at temperatures between 84° and 112°K with pressures up to 10 atm. The measured attenuation depends linearly on the square of the frequency over the range 30–70 Mc/sec; however, the measured values exceed the values calculated from the classical Stokes—Kirchhoff expression.The excess attenuation has been interpreted as evidence for the existence of the volume viscosity in liquid argon. The volume viscosity coefficient has been calculated and the results compared with those of the current theories of liquid‐transport phenomena.

Surface reactivity of titanium–aluminum alloys: Ti3Al, TiAl, and TiAl3

Titanium‐based alloys, and Ti–Al alloys in particular, are of special interest due to the technological importance of these materials as structural materials in aggressive environments. The surface chemistry of 1000‐A films of Ti3Al, TiAl, and TiAl3, prepared by codeposition in an ultrahigh vacuum system, has been examined by x‐ray photoelectron spectroscopy (XPS). The alloys are examined as deposited and after treatments from room temperature to 600 °C in either vacuum of 5×10−7 Torr or 5×10−3 Torr O2. The alloys are compared with one another with regard to their surface segregation and oxide overlayer formation. Particular attention is given to the nature of the surface oxide that develops under the various conditions, and low take‐off angle XPS is used to determine the depth distribution of the various oxides present. The oxidation process is controlled by temperature, oxygen pressure, the bulk alloy composition and the free energies of formation for the various oxides found in the overlayers.

Angle-dependent van Hove singularities and their breakdown in twisted graphene bilayers

The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. Twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutual misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts as an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles theta less than about 3.5o, corresponding to moire-pattern periods D longer than 4 nm. However, they totally vanish for theta > 5.5o (D < 2.5 nm). Such a behavior indicates that the continuum models, which capture moire-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles.

Giant change in infrared light transmission in La0.67Ca0.33MnO3 film near the Curie temperature

Yu. P. Sukhorukov,, E. A. Gan’shina, B. I. Belevtsev, N. N. Loshkareva, A. N. Vinogradov, K. D. D. Rathnayaka, A. Parasiris, D. G. Naugle Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia Physics Department, Moscow State University, 119899, Moscow, Russia B. Verkin Institute for Low Temperature Physics & Engineering, Ukrainian Academy of Sciences, Kharkov, 61103, Ukraine Department of Physics, Texas A&M University, College Station, TX 77843-4242, USATransport, magnetic, magneto-optical (Kerr effect) and optical (light absorption) properties have been studied in an oriented polycrystalline La0.67Ca0.33MnO3 film which shows colossal magnetoresistance. The correlations between these properties are presented. A giant change in infrared light transmission (more than a thousand-fold decrease) is observed on crossing the Curie temperature (about 270 K) from high to low temperature. Large changes in transmittance in a magnetic field were observed as well. The giant changes in transmittance and the large magnetotransmittance can be used for development of IR optoelectronic devices controlled by thermal and magnetic fields. Required material characteristics of doped manganites for these devices are discussed.

Thickness dependence of the microstructures and magnetic properties of electroplated Co nanowires.

The correlation between the crystal structure and the magnetic properties of Co nanowires of diameter 65 and 200 nm fabricated by electroplating Co into the pores of anodic aluminum oxide membranes has been investigated. Strikingly different microstructures have been observed in these Co nanowires by means of x-ray diffraction and selected area electron diffraction measurements. The 65 nm thick Co nanowires are composed of long and ordered hexagonal close-packed Co grains (>5 microm), while the 200 nm thick Co nanowires are composed of submicron-long hexagonal close-packed and face-centered cubic Co grains. Correspondingly, different magnetic properties have been observed for these Co nanowires. Magnetization measurements have found that the 65 nm thick Co nanowires have a magnetic hysteresis that is significantly larger than that of the 200 nm thick Co nanowires. Spontaneous magnetic moments of the nanowires are parallel to the nanowires in the 65 nm thick Co nanowires, but they are transverse to the nanowires in the 200 nm thick Co nanowires, as observed by the magnetic force microscopy. The correlation between their different magnetic properties and microstructures is discussed.

The surface reactivity of TiCu and TiAl alloys and the ion chemistry of their oxide overlayers

Abstract The surface chemistry of Ti 36 Cu 64 and Ti 36 Al 64 alloys has been examined by X-ray photoelectron spectroscopy (XPS) after: (a) preparation by co-condensation at room temperature, (b) vacuum annealing to pre-selected temperatures, and (c) thermal oxidation in 0.67 Pa(5×10 −3 Torr) O 2 . The respective alloys are compared to one another with respect to their surface segregation and oxide formation properties. Particular attention is given to the nature of the surface film that develops at the various conditions and to the ionic reactions and ion movements that are involved.

Ultrasonic Velocity and Attenuation in Liquid Neon

The velocity and attenuation of 30‐MHz sound waves were measured in liquid neon from 25 to 37°K at pressures to 28 kg/cm2. The values of the velocity, extrapolated to the vapor pressure curve, are from 1.5% higher at 25°K to 0.4% higher at 32°K than corresponding values reported for 2‐GHz sound waves. This negative dispersion is in agreement with recent predictions by Fleury and Boon. Values of the attenuation exceed the classical values calculated from the Navier–Stokes equation. This excess attenuation is attributed to a volume viscosity of the same magnitude as the shear viscosity. No anomaly in the temperature dependence of either velocity or attenuation was observed near the temperature where an anomaly has been reported for thermal conductivity, shear viscosity, and hypersonic velocity.