13 research outputs found
Thermal Stability of Filtered Vacuum Arc Deposited Er2O3 Coatings
Erbium oxide (Er2O3) coatings were deposited using filtered vacuum arc deposition (FVAD) and their structure and thermal stability were studied as a function of fabrication parameters. The coatings were deposited on silicon wafer and tantalum substrates with an arc current of 50 A and a deposition rate of 1.6 ± 0.4 nm/s. The arc was sustained on truncated cone Er cathodes. The influence of oxygen pressure (P= 0.40-0.93 Pa), bias voltage (Vb= -20, -40 or grounded) and substrate temperature (room temperature (RT) or 673K) on film properties was studied before and after post deposition annealing (1273K for 1 hour, at P~ 1.33 Pa). The coatings were characterized using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Knoop Hardness.
Optical microscope images indicated that the coatings had very low macroparticle concentration on their surface. The macroparticle diameters were less than 2.5 ÎĽm. The coatings were composed of only Er2O3 without any metallic phase under all deposition parameters tested. The coatings deposited on RT substrates were XRD amorphous and had a featureless cross-section microstructure. However, the coatings deposited on 673K heated substrates had a C-Er2O3 structure with (222) preferred orientation and weak columnar microstructure. The coating hardness varied with deposition pressure and substrate bias, and reached a maximum value of 10 GPa at P = 0.4 Pa and Vb = -40 V. The post-deposition annealing caused crystallization, and the coatings hardness dropped to 4 GPa with thermal treatment. However, after post-deposition annealing, no peeling or cracking appeared at the coating surface or the interface with the substrate
Phase slip centers in a two-band superconducting filament: application to MgB2
Within the framework of non-stationary Ginzburg-Landau equations generalized
on a case of two order parameters, we investigated dynamic characteristics of
two-band superconducting MgB2 filaments in a voltage-driven regime. We
calculated current-voltage characteristics of superconducting MgB2 filaments
with different lengths and compared with single-band superconductor of
corresponding sizes. Despite the presence of two interacting bands for small
values of a ratio of effective Cooper pairs weights, phenomenological constants
of diffusion and large lengths of channels the well-known S-form of
current-voltage characteristic for the single-band superconductor with
fluctuations of the current density in a certain interval of the voltage also
takes place in the case of two-band superconductor. Analysis of spectra of the
current density and the time-evolution of order parameters for long channels
points out that such unusual forms of current-voltage characteristics are
caused by the occurrence either chaotic oscillations of order parameters
modules or the interference of some their oscillation modes. The further
increase of the ratio of effective weights of Cooper pairs and phenomenological
constants of diffusion smoothes these oscillations and transforms the system in
an ordered state, where the S-form of current-voltage characteristics becomes
more flattened. The latter can be the feature of two-band superconducting long
channels.Comment: 10 pages, 4 figures, revised versio
Critical Dynamics of Singlet Excitations in a Frustrated Spin System
We construct and analyze a two-dimensional frustrated quantum spin model with
plaquette order, in which the low-energy dynamics is controlled by spin
singlets. At a critical value of frustration the singlet spectrum becomes
gapless, indicating a quantum transition to a phase with dimer order. This T=0
transition belongs to the 3D Ising universality class, while at finite
temperature a 2D Ising critical line separates the plaquette and dimerized
phases.
The magnetic susceptibility has an activated form throughout the phase
diagram, whereas the specific heat exhibits a rich structure and a power law
dependence on temperature at the quantum critical point.
We argue that the novel quantum critical behavior associated with singlet
criticality discussed in this work can be relevant to a wide class of quantum
spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where
the low-energy excitations are known to be spin singlets, as well as to the
CAVO lattice and several recently discovered strongly frustrated square-lattice
antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear
in Phys. Rev.
Chern-Simons Theory for Magnetization Plateaus of Frustrated - Heisenberg model
The magnetization curve of the two-dimensional spin-1/2 -
Heisenberg model is investigated by using the Chern-Simons theory under a
uniform mean-field approximation. We find that the magnetization curve is
monotonically increasing for , where the system under zero
external field is in the antiferromagnetic N\'eel phase. For larger ratios of
, various plateaus will appear in the magnetization curve. In
particular, in the disordered phase, our result supports the existence of the
plateau and predicts a new plateau at .
By identifying the onset ratio for the appearance of the 1/2-plateau
with the boundary between the N\'eel and the spin-disordered phases in zero
field, we can determine this phase boundary accurately by this mean-field
calculation. Verification of these interesting results would indicate a strong
connection between the frustrated antiferromagnetic system and the quantum Hall
system.Comment: RevTeX 4, 4 pages, 3 EPS figure
Quantum magnetism in two dimensions: From semi-classical N\'eel order to magnetic disorder
This is a review of ground-state features of the s=1/2 Heisenberg
antiferromagnet on two-dimensional lattices. A central issue is the interplay
of lattice topology (e.g. coordination number, non-equivalent nearest-neighbor
bonds, geometric frustration) and quantum fluctuations and their impact on
possible long-range order. This article presents a unified summary of all 11
two-dimensional uniform Archimedean lattices which include e.g. the square,
triangular and kagome lattice. We find that the ground state of the spin-1/2
Heisenberg antiferromagnet is likely to be semi-classically ordered in most
cases. However, the interplay of geometric frustration and quantum fluctuations
gives rise to a quantum paramagnetic ground state without semi-classical
long-range order on two lattices which are precisely those among the 11 uniform
Archimedean lattices with a highly degenerate ground state in the classical
limit. The first one is the famous kagome lattice where many low-lying singlet
excitations are known to arise in the spin gap. The second lattice is called
star lattice and has a clear gap to all excitations.
Modification of certain bonds leads to quantum phase transitions which are
also discussed briefly. Furthermore, we discuss the magnetization process of
the Heisenberg antiferromagnet on the 11 Archimedean lattices, focusing on
anomalies like plateaus and a magnetization jump just below the saturation
field. As an illustration we discuss the two-dimensional Shastry-Sutherland
model which is used to describe SrCu2(BO3)2.Comment: This is now the complete 72-page preprint version of the 2004 review
article. This version corrects two further typographic errors (three total
with respect to the published version), see page 2 for detail
Properties of SnO2 films fabricated using a rectangular filtered vacuum arc plasma source
Transparent and conducting SnO2 films of 57-200nm thickness were deposited on microscope glass slide substrates, using a rectangular filtered vacuum arc deposition system. The 40 glass slides were equally distributed on a 400 × 420mm substrate carriage, and were exposed to a Sn plasma beam, produced by a rectangular vacuum arc plasma gun with a Sn cathode, and passed through a rectangular magnetic macroparticle filter towards the substrates. The carriage with the substrates was transported past the 94 × 494mm filter outlet. The SnO2 films were fabricated on the glass substrates at room temperature by maintaining the chamber oxygen background pressure at 0.52Pa. The film composition, and electrical and optical properties were studied as a function of the film thickness. The films were stored under ambient air conditions, and their electrical resistance was measured as a function of storage time over a period of several months. The average resistivity of films was 10-17m? cm for films with thickness (t) less than 100nm, but that of t > 100nm it was 5-9m? cm. The resistivity of the films with t > 100nm did not change significantly after 8months of storage in ambient air. The optical transmittance of the films in the visible spectrum was in the range of 75-90%. The optical constants, i.e., the refractive index and the extinction coefficient of the films at wavelength ? = 550nm were in the range of 2.02-2.09 and 0.013-0.023, respectively, and the optical band gap energy was 4.15-4.21eV. Unlike the electrical resistivity, the optical parameters weakly depended on t. © 2008 Elsevier B.V. All rights reserved.Ministry of Education, Science and TechnologyThe authors gratefully acknowledge financial support from the Israeli Ministry of Science and Technology, Mr. M. Govberg for his engineering assistance, Mr. D. Abunie and Mr. Y. Epstein for their assistance in conducting the experiments, Mr. O. Margulis for his assistance in figure preparation, Dr. L. Burshtein for XPS studies, and Dr. Y. Rosenberg for the XRD analysis
Optical characterization of filtered vacuum arc deposited zinc oxide thin films
ZnO thin films, 100-250 nm thick, were deposited on microscope glass slides and UV fused silica (UVFS) substrates using a filtered vacuum arc deposition (FVAD) system, operating at room temperature (RT) and 200 A for 60 s. The cathode was prepared from 99.9% pure Zn metal and the oxygen background pressure during deposition was in the range 0.67-0.93 Pa. The films were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), optical transmission and spectroscopic ellipsometry. As-deposited ZnO films were found to be polycrystalline with c-axis orientation. The atomic concentration ratio of Zn to O in the film as determined by XPS analysis was stoichiometric. Film transmission in the visible was 70-90%. The maximum and minimum values of the refractive index n and the extinction coefficient k in the visible, for all samples, were 2.23 to 1.90 and 0.6 to approximately 0, respectively. The type of inter-band electron transition was found to be direct transition with optical band gap in the range of 3.25-3.42 eV. © 2006 IOP Publishing Ltd
Filtered vacuum arc deposition of transparent conducting Al-doped ZnO films
Transparent conducting ZnO:Al and ZnO films of 380-800 nm thickness were deposited on glass substrates by filtered vacuum arc deposition (FVAD), using a cylindrical Zn cathode doped with 5-6 at.% Al or a pure Zn cathode in oxygen background gas with pressure P = 0.4-0.93 Pa. The crystalline structure, composition and electrical and optical properties of the films were studied as functions of P. The films were stored under ambient air conditions and the variation of their resistance as function of storage time was monitored over a period of several months. The Al concentration in the film was found to be 0.006-0.008 at.%, i.e., a few orders of magnitude lower than that in the cathode material. However, this low Al content influenced the film resistivity, ?, and its stability. The resistivity of as-deposited ZnO:Al films, ? = (6-8) × 10- 3 ? cm, was independent of P and lower by a factor of 2 in comparison to that of the ZnO films deposited by the same FVAD system. The ? of ZnO films 60 days after deposition increased by a factor of ~ 7 with respect to as-deposited films. The ZnO:Al films deposited with P = 0.47-0.6 Pa were more stable, their ? first slowly increased during the storage time (1.1-1.4 times with respect to as-deposited films), and then stabilized after 30-45 days. © 2006 Elsevier B.V. All rights reserved