294 research outputs found
Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet
In (S/F) hybrids the suppression of superconductivity by the exchange field
h_ex of the ferromagnet can be partially lifted when different directions of
h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer
geometries where the magnetization directions of the two F-layers can be
controlled separately, this leads to the so-called spin switch. Here we show
that domain walls in a single F-layer yield a similar effect. We study the
transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of
different sizes. For large samples a clear enhancement of superconductivity
takes place in the resistive transition, in the very narrow field range (order
of 0.5 mT) where the magnetization of the Py layer switches and many domains
are present. This effect is absent in microstructured samples. Comparison of
domain wall width \delta_w to the temperature dependent superconductor
coherence length \xi_S(T) shows that \delta_w ~ \xi_S(T), which means that the
Cooper pairs sample a large range of different magnetization directions.Comment: 4 pages, 5 figure
Depairing currents in superconducting films of Nb and amorphous MoGe
We report on measuring the depairing current J_{dp} in thin superconducting
films as a function of temperature. The main difficulties in such measurements
are that heating has to be avoided, either due to contacts, or to vortex flow.
The latter is almost unavoidable since the sample cross-section is usually
larger than the superconducting coherence length \xi_s and the magnetic field
penetration depth \lambda_s. On the other hand, vortex flow is helpful since it
homogenizes the distribution of the current across the sample. We used a pulsed
current method, which allows to overcome the difficulties caused by dissipation
and measured the depairing current in films of thin polycrystalline Nb (low
\lambda_s, low specific resistance \rho) and amorphous Mo_{0.7}Ge_{0.3} (high
\lambda_s, high \rho), structured in the shape of bridges of various width. The
experimental values of J_{dp} for different bridge dimensions are compared with
theoretical predictions by Kupriyanov and Lukichev for dirty limit
superconductors. For the smallest samples we find a very good agreement with
theory, over essentially the whole temperature interval below the
superconducting critical temperature.Comment: 5 pages, 6 figure
Observation of the magnetic domain structures in CuNi thin films at low temperatures
We report on the first experimental visualization of domain structure in
films of weakly ferromagnetic CuNi alloy with different
thickness at liquid helium temperatures. Improved high-resolution Bitter
decoration technique was used to map the magnetic contrast on the top of the
films well below the Curie temperature T ( 60 K). In contrast
to magnetic force microscopy, this technique allowed visualization of the
domain structure without its disturbance while the larger areas of the sample
were probed. Maze-like domain patterns, typical for perpendicular magnetic
anisotropy, were observed. The average domain width was found to be about 100
nm.Comment: 4 pages, 5 figures, will be published in JETP Let
Observation of the Triplet Spin-Valve Effect in a Superconductor-Ferromagnet Heterostructure
The theory of superconductor-ferromagnet (S-F) heterostructures with two
ferromagnetic layers predicts the generation of a long-range, odd-in-frequency
triplet pairing at non-collinear alignment (NCA) of the magnetizations of the
F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx
spin-valve type proximity effect heterostructure, in which a very thin Nb film
between the F-layers serves as a normal conducting (nc) spacer. The resistance
of the sample as a function of an external magnetic field shows that for not
too high fields the system is superconducting at a collinear alignment of the
Cu41Ni59 and Co layer magnetic moments, but switches to the normal conducting
state at a NCA configuration. This indicates that the superconducting
transition temperature Tc for NCA is lower than the fixed measuring
temperature. The existence of a minimum Tc, at the NCA regime below that one
for parallel or antiparallel alignments of the F-layer magnetic moments, is
consistent with the theoretical prediction of a singlet superconductivity
suppression by the long-range triplet pairing generation.Comment: 7 pages, 4 fgures, Submitted to Physical Review Letter
УСТАНОВЛЕНИЕ ПРИЧИН РАЗРУШЕНИЯ ПАКЕТОВ ЛОПАТОК ТУРБИН ПРИ ВЫПОЛНЕНИИ СУДЕБНЫХ ЭКСПЕРТИЗ
The technique ofdetermination of rupture sources of the turbine blades packets, which can be used at performance of forensic expertise is suggested. Its distinctive feature is completeness of the account of the various factors concerning different areas of the applied physics, such as gas dynamics, heat conductivity, thermoelasticity, static and dynamic strength. The following series of calculations was made for acknowledgement of rupture sources of bladed disk. I. Calculation of a viscous compressed subsonic current. II. Calculation periodic gas-dynamic loads operating on the blades. III. Calculation of a non-stationary stress-strain state of the bladed disk under the action of periodic forces. The technique is based on use of the numerical methods of gas dynamics and dynamic strength. The steam current through a turbomachine step calculated with the help of model of the viscous, compressedfluid. The forced vibrations of the bladed disk under the action of the aerodynamic loads are analyzed by the numerical integration of the nonlinearfinite elements design model. Four cases are considered for the numerical calculations of the stress-strain state. I. The clearance in the disk slots are not accounted and the vibrations under the action ofgas-dynamic loads from ideal directing device are investigated. II. The clearance in the disk slots are not accounted and the vibrations under the action of gas- dynamic loads from defective directing device are investigated. III. The clearance in the disk slots are taken into account and the vibrations under the action of gas-dynamic loads from ideal directing device are investigated. IV. The clearance in the disk slots are taken into account and the vibrations under the action of gas-dynamic loads from defective directing device are investigated. It’s established that a source of rupture ofworking blades of a regulating step of a compartment of a high pressure of the steam turbine is damage of blades of the directing device.Запропоновано методику встановлення причин руйнування робочих лопаток, яка може використовуватися при виконанні судових інженерно-технічних експертиз. Її відмітною рисою є повнота урахування різних факторів, що належать до різних галузей прикладної фізики, таким як газодинаміка, теплопровідність, термопружність, статична й динамічна міцності. Методика ґрунтується на використанні числових методів газодинаміки й динамічної міцності. Протікання пари через ступені турбомашини розраховувалося на підставі моделі в’язкого середовища, що стискається. Для дослідження вимушених коливань облопаченого диска під дією аеродинамічних навантажень застосовувалося чисельне інтегрування нелінійної скінченно-елементної моделі конструкції
Transition Radiation Spectra of Electrons from 1 to 10 GeV/c in Regular and Irregular Radiators
We present measurements of the spectral distribution of transition radiation
generated by electrons of momentum 1 to 10 GeV/c in different radiator types.
We investigate periodic foil radiators and irregular foam and fiber materials.
The transition radiation photons are detected by prototypes of the drift
chambers to be used in the Transition Radiation Detector (TRD) of the ALICE
experiment at CERN, which are filled with a Xe, CO2 (15 %) mixture. The
measurements are compared to simulations in order to enhance the quantitative
understanding of transition radiation production, in particular the momentum
dependence of the transition radiation yield.Comment: 18 pages, 15 figures, submitted to Nucl. Instr. Meth. Phys. Res.
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