113 research outputs found
Recent Progress with bioSFQ Circuit Family for Neuromorphic Computing
Superconductor single flux quantum (SFQ) technology is attractive for
neuromorphic computing due to low energy dissipation and high, potentially up
to 100 GHz, clock rates. We have recently suggested a new family of bioSFQ
circuits (V.K. Semenov et al., IEEE TAS, vol. 32, no. 4, 1400105, 2022) where
information is stored as a value of current in a superconducting loop and
transferred as a rate of SFQ pulses propagating between the loops. This
approach in the simplest case dealing with positive numbers, requires
single-line transfer channels. In the more general case of bipolar numbers, it
requires dual-rail transfer channels. For this need, a new comparator with
dual-rail output has been developed and is presented. This comparator is an
essential part of a bipolar multiplier that has also been designed, fabricated,
and tested. We discuss strategic advantages of the suggested bioSFQ approach,
e.g., an inherently asynchronous character of bioSFQ cells which do not require
explicit clock signals. As a result, bioSFQ circuits are free of racing errors
and tolerant to occasional collision of propagating SFQ pulses. This tolerance
is due to stochastic nature of data signals generated by comparators operating
within their gray zone. The circuits were fabricated in the eight-niobium-layer
fabrication process SFQ5ee developed for superconductor electronics at MIT
Lincoln Laboratory.Comment: 5 pages, 7 figures, 12 references. This paper was presented at
Applied Superconductivity Conference, ASC 2022, October 23-28, 2022,
Honolulu, Hawai
Multi-layered Ruthenium-modified Bond Coats for Thermal Barrier Coatings
Diffusional approaches for fabrication of multi-layered Ru-modified bond coats for thermal
barrier coatings have been developed via low activity chemical vapor deposition and high activity
pack aluminization. Both processes yield bond coats comprising two distinct B2 layers, based on
NiAl and RuAl, however, the position of these layers relative to the bond coat surface is reversed
when switching processes. The structural evolution of each coating at various stages of the
fabrication process has been and subsequent cyclic oxidation is presented, and the relevant
interdiffusion and phase equilibria issues in are discussed. Evaluation of the oxidation behavior of
these Ru-modified bond coat structures reveals that each B2 interlayer arrangement leads to the
formation of α-Al 2 O 3 TGO at 1100°C, but the durability of the TGO is somewhat different and in
need of further improvement in both cases
Progress toward superconductor electronics fabrication process with planarized NbN and NbN/Nb layers
To increase density of superconductor digital and neuromorphic circuits by
10x and reach integration scale of Josephson junctions (JJs) per chip,
we developed a new fabrication process on 200-mm wafers, using self-shunted
Nb/Al-AlOx/Nb JJs and kinetic inductors. The process has a layer of JJs, a
layer of resistors, and 10 fully planarized superconducting layers: 8 Nb layers
and 2 layers of high kinetic inductance materials, MoN and NbN, with sheet
inductance of 8 pH/sq and 3 pH/sq, respectively. NbN films were deposited by
two methods: with =15.5 K by reactive sputtering of a Nb target in
Ar+N mixture; with in the range from 9 K to 13 K by plasma-enhanced
chemical vapor deposition (PECVD) using
Tris(diethylamido)(tert-butylimido)niobium(V) metalorganic precursor. PECVD of
NbN was investigated to obtain conformal deposition and filling narrow trenches
and vias with high depth-to-width ratios, which was not possible to achieve
using sputtering and other physical vapor deposition (PVD) methods at
temperatures below required to prevent degradation of Nb/Al-AlOx/Nb
junctions. Nb layers with 200 nm thickness are used in the process layer stack
as ground planes to maintain a high level of interlayer shielding and low
intralayer mutual coupling, for passive transmission lines with wave impedances
matching impedances of JJs, typically <=50 , and for low-value
inductors. NbN and NbN/Nb bilayer are used for cell inductors. Using NbN/Nb
bilayers and individual pattering of both layers to form inductors allowed us
to minimize parasitic kinetic inductance associated with interlayer vias and
connections to JJs as well as to increase critical currents of the vias.
Fabrication details and results of electrical characterization of NbN films,
wires, and vias, and comparison with Nb properties are given.Comment: 12 pages, 16 figures, 4 tables, 49 references. Submitted to IEEE TAS
on Nov. 10, 202
The Influence of Specimen Thickness on the High Temperature Corrosion Behavior of CMSX-4 during Thermal-Cycling Exposure
CMSX-4 is a single-crystalline Ni-base superalloy designed to be used at very high temperatures and high mechanical loadings. Its excellent corrosion resistance is due to external alumina-scale formation, which however can become less protective under thermal-cycling conditions. The metallic substrate in combination with its superficial oxide scale has to be considered as a composite suffering high stresses. Factors like different coefficients of thermal expansion between oxide and substrate during temperature changes or growing stresses affect the integrity of the oxide scale. This must also be strongly influenced by the thickness of the oxide scale and the substrate as well as the ability to relief such stresses, e.g., by creep deformation. In order to quantify these effects, thin-walled specimens of different thickness (t = 100500 lm) were prepared. Discontinuous measurements of their mass changes were carried out under thermal-cycling conditions at a hot dwell temperature of 1100 C up to 300 thermal cycles. Thin-walled specimens revealed a much lower oxide-spallation rate compared to thick-walled specimens, while thinwalled specimens might show a premature depletion of scale-forming elements. In order to determine which of these competetive factor is more detrimental in terms of a component’s lifetime, the degradation by internal precipitation was studied using scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS). Additionally, a recently developed statistical spallation model was applied to experimental data [D. Poquillon and D. Monceau, Oxidation of Metals, 59, 409–431 (2003)]. The model describes the overall mass change by oxide scale spallation during thermal cycling exposure and is a useful simulation tool for oxide scale spallation processes accounting for variations in the specimen geometry. The evolution of the net-mass change vs. the number of thermal cycles seems to be strongly dependent on the sample thickness
Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 C
This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclicoxidation tests at 1,050 C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 C for 100 h and cyclicoxidation tests at 900 C which were reported in part I [N. Vialas and D. Monceau,
Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural
features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial
voids was always observed when the b to c0 transformation leads to the formation of a two-phase b/c0 layer in contact with the alumina scale. On the contrary, no voids
were observed in RT22/IN792, since this b to c0 transformation occurs gradually by an inward transformation of b leading to the formation of a continuous layer of c0
phase, parallel to the metal/scale interface
Effect of anisotropic impurity scattering in superconductors
We discuss the weak-coupling BCS theory of a superconductor with the
impurities, accounting for their anisotropic momentum-dependent potential. The
impurity scattering process is considered in the t-matrix approximation and its
influence on the superconducting critical temperature is studied in the Born
and unitary limit for a d- and (d+s)-wave superconductors. We observe a
significant dependence of the pair-breaking strength on the symmetry of the
scattering potential and classify the impurity potentials according to their
ability to alter T_c. A good agreement with the experimental data for Zn doping
and oxygen irradiation in the overdoped cuprates is found.Comment: 31 pages, RevTex, 15 PostScript figure
Bose-Einstein Condensation in the Relativistic Pion Gas: Thermodynamic Limit and Finite Size Effects
We consider the Bose-Einstein condensation (BEC) in a relativistic pion gas.
The thermodynamic limit when the system volume goes to infinity as well as
the role of finite size effects are studied. At the scaled
variance for particle number fluctuations, ,
converges to finite values in the normal phase above the BEC temperature,
. It diverges as at the BEC line , and
at in a phase with the BE condensate. Possible
experimental signals of the pion BEC in finite systems created in high energy
proton-proton collisions are discussed
Supergroup approach to the Hubbard model
Based on the revealed hidden supergroup structure, we develop a new approach
to the Hubbard model. We reveal a relation of even Hubbard operators to the
spinor representation of the generators of the rotation group of
four-dimensional spaces. We propose a procedure for constructing a matrix
representation of translation generators, yielding a curved space on which
dynamic superfields are defined. We construct a new deformed nonlinear
superalgebra for the regime of spinless Hubbard model fermions in the case of
large on-site repulsion and evaluate the effective functional for spinless
fermions.Comment: 17 pages, Theoretical and Mathematical Physics, V.166, n.2,
p.209-222,201
Critical temperature of an anisotropic superconductor containing both nonmagnetic and magnetic impurities
The combined effect of both nonmagnetic and magnetic impurities on the
superconducting transition temperature is studied theoretically within the BCS
model. An expression for the critical temperature as a function of potential
and spin-flip scattering rates is derived for a two-dimensional superconductor
with arbitrary in-plane anisotropy of the superconducting order parameter,
ranging from isotropic s-wave to d-wave (or any pairing state with nonzero
angular momentum) and including anisotropic s-wave and mixed (d+s)-wave as
particular cases. This expression generalizes the well-known Abrikosov-Gor'kov
formula for the critical temperature of impure superconductors. The effect of
defects and impurities in high temperature superconductors is discussed.Comment: 4 eps figure
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