V. T. Petrashov

Superconducting Phase Coherent Electron Transport in Proximity Conical Ferromagnets

We report superconducting phase-periodic conductance oscillations in ferromagnetic wires with interfaces to conventional superconductors. The ferromagnetic wires were made of Ho, a conical ferromagnet. The distance between the interfaces was much larger than the singlet superconducting penetration depth. We explain the observed oscillations as due to the long-range penetration of an unusual helical triplet component of the order parameter that is generated at the superconductor/ferromagnet interfaces and maintained by the intrinsic rotating magnetization of Ho.

GIANT MUTUAL PROXIMITY EFFECTS IN FERROMAGNETIC/SUPERCONDUCTING NANOSTRUCTURES

takesplace upon an increase in the F S interface barrier resistance. Reentrance of the superconductors to thenormal state reciprocated by changes on the F side has been found in low applied magnetic fields withnew peaks in the differential resistance as an effect of the saturation magnetization. An analysis hasbeen developed providing a base for a numerical description of the system.

Cubic boron nitride films deposited by electron cyclotron resonance plasma

By employing an electron cyclotron resonance plasma‐enhanced chemical vapor deposition technique, we report the successful growth of cubic boron nitride films on single‐crystal (100) silicon wafer without external rf or dc substrate biasing. Ammonia and boron trifluoride gases were used for the deposition of cubic boron nitride. The substrate temperature during deposition was about 675 °C. The films were characterized by infrared spectroscopy and ellipsometry. The existence of cubic boron nitride was identified by the characteristic boron nitride infrared signal at 1110 cm−1. The film thickness was about 1000 A, with a growth rate of 100 A/min.

Electron cyclotron resonance plasma chemical vapor deposition of large area uniform silicon nitride films

Electron cyclotron resonance plasma (f=2.45 GHz, microwave power P=200–800 W) generated in a radially uniform magnetic field (B=875–1000 G) was used to produce a large area (15–20 cm diam) uniform plasma stream at 20–30 cm from the source output. Low temperature (70–300 °C) silicon nitride films with a thickness of 800–3000 A were deposited on 5–20 cm diameter wafers with deposition rates of 100–350 A/min. Film thickness uniformity was ±1% for 7.6–10.0 cm diam wafers, ±3% for 15 cm diam wafers, and ±9% for 20.0 cm diam wafers. It was found that the film deposition rate Wg increased linearly with the silane flow rate, while Wg increased slower than power 1/2 with the nitrogen flow rate. The film refractive index was 1.9–2.0 at a silane/nitrogen flow rate ratio of 0.40–0.6. The effects of plasma density and its profile on the film growth rate and uniformity are discussed.

Antivortex state in crosslike nanomagnets

We report on results of computer micromodelling of anti-vortex states in asymmetrical cross-like ferromagnetic nanostructures and their practical realization. The arrays of cobalt crosses with 1 mkm branches, 100 nm widths of the branches and 40 nm thicknesses were fabricated using e-beam lithography and ion etching. Each branch of the cross was tapered at one end and bulbous at the other. The stable formation of anti-vortex magnetic states in these nanostructures during magnetization reversal was demonstrated experimentally using magnetic force microscopy.

Electrical properties of epitaxial tungsten films grown by laser ablation deposition

An investigation was made of electrical properties of tungsten films of thickness in the range of 30–140 nm, grown by laser ablation deposition in ultrahigh vacuum on [1012] sapphire substrate. From the data on the size effect and the temperature dependence of the resistivity r, supported with reflection high energy electron diffraction measurements, we find that the films, deposited onto clean substrates kept at temperatures higher than 500 °C, grow epitaxially with high quality crystalline structure. The effective electron mean free path changes from 300 to 1400 nm while the residual resistance ratio RRR=r (295 K)/r (4.2 K) changes from 7 to 35. At low temperatures we find the temperature dependent part ρ(T)≊ATn, with A=1.56 mΩ cm K−n, n=3–3.4 for the temperatures T<20 K. From the fit of the data to the equations of the classical size effect theory it was found that the main source of electron scattering at helium temperatures is interface scattering with a specular coefficient q≊0.3 and the bulk electron mean free path l∞ (4.2 K)∼200 μm, which is up to 103 times larger than the film thickness.An investigation was made of electrical properties of tungsten films of thickness in the range of 30–140 nm, grown by laser ablation deposition in ultrahigh vacuum on [1012] sapphire substrate. From the data on the size effect and the temperature dependence of the resistivity r, supported with reflection high energy electron diffraction measurements, we find that the films, deposited onto clean substrates kept at temperatures higher than 500 °C, grow epitaxially with high quality crystalline structure. The effective electron mean free path changes from 300 to 1400 nm while the residual resistance ratio RRR=r (295 K)/r (4.2 K) changes from 7 to 35. At low temperatures we find the temperature dependent part ρ(T)≊ATn, with A=1.56 mΩ cm K−n, n=3–3.4 for the temperatures T<20 K. From the fit of the data to the equations of the classical size effect theory it was found that the main source of electron scattering at helium temperatures is interface scattering with a specular coefficient q≊0.3 and the bulk elect...

Resolving thermoelectric “paradox” in superconductors

Researchers bring the theory of thermoelectricity in superconductors and experiment into agreement. For almost a century, thermoelectricity in superconductors has been one of the most intriguing topics in physics. During its early stages in the 1920s, the mere existence of thermoelectric effects in superconductors was questioned. In 1944, it was demonstrated that the effects may occur in inhomogeneous superconductors. Theoretical breakthrough followed in the 1970s, when the generation of a measurable thermoelectric magnetic flux in superconducting loops was predicted; however, a major crisis developed when experiments showed puzzling discrepancies with the theory. Moreover, different experiments were inconsistent with each other. This led to a stalemate in bringing theory and experiment into agreement. With this work, we resolve this stalemate, thus solving this long-standing “paradox,” and open prospects for exploration of novel thermoelectric phenomena predicted recently.

Josephson effects in a superconductor-normal-metal mesoscopic structure with a dangling superconducting arm

Department of Microelectronics and Nanoscience, Chalmers University of TechnologySE-412 96 Goteborg, Sweden(July 11, 2011)We studied a mesoscopic cross-like normal metal structure connected to two superconducting(S) and two normal (N) reservoirs. We observed the Josephson effect under unusual conditionswhen there is no current through one of the two S/N interfaces. The potential difference betweenthe S reservoirs was zero unless the voltage applied between S and N reservoirs exceeded a criticalvalue although the electric potential in the N wire connecting the superconductors varied in anonmonotonic way. The observed effects are discussed theoretically.PACS number: 74.50.+r, 73.23.-b, 85.25.-j

An experimental study of helicon resonance in metals

This review is devoted to helicons-electromagnetic waves propagating in the electron-hole plasma in metals at low temperatures. The review is divided into two parts. In the first part the author presents the theory of, and the experimental data on, helicons in the absence of effects caused by Landau quantisation of conduction electron energy in a strong magnetic field. The author emphasises the peculiar features of helicon propagation which are due to the anisotropy of the electron spectrum of metals, effects not observable in a gaseous plasma. An analysis of data on the anisotropy of the collision damping, the Landau damping and on dopplerons-a new branch of electromagnetic excitations in the vicinity of the Doppler-shifted cyclotron resonance-is given. He also discusses the results of studies of the acoustic satellites of helicon resonance and the peculiarities of helicon propagation in the intermediate state of type-I superconductors. In the second part he presents the results of observations of new effects caused by Landau quantisation, namely oscillations of the phase velocity of helicons, non-linear helicon resonance and the anomalous attenuation of helicons in metals with diamagnetic domains. Examples are discussed in which the helicon resonances are employed to investigate transport relaxation in high magnetic fields, the production of defects by plastic deformation, open orbits and the absolute amplitude of the de Haas-van Alphen effect.

Device fabrication by nanolithography and electroplating for magnetic flux quantization measurements

100‐keV e‐beam lithography and electroplating of gold have been used to fabricate a device for the measurement of quantum changes in magnetic flux. The structure is defined in a 200‐nm‐thick layer of a copolymer of methyl methacrylate and methacrylic acid (PMMA/MAA) on top of a Si wafer covered by 150 nm of Si3N4. The device is a square frame of 2×2 μm2 and 100‐nm linewidth. Gold was electroplated to a thickness of 100 nm. This technique allows the fabrication of high resolution, high aspect ratio gold structures.