177 research outputs found
Protected 0-pi states in SIsFS junctions for Josephson memory and logic
We study the peculiarities in current-phase relations (CPR) of the SIsFS
junction in the region of to transition. These CPR consist of two
independent branches corresponding to and states of the contact. We
have found that depending on the transparency of the SIs tunnel barrier the
decrease of the s-layer thickness leads to transformation of the CPR shape
going in the two possible ways: either one of the branches exists only in
discrete intervals of the phase difference or both branches are
sinusoidal but differ in the magnitude of their critical currents. We
demonstrate that the difference can be as large as under maintaining
superconductivity in the s layer. An applicability of these phenomena for
memory and logic application is discussed.Comment: 5 pages, 5 figure
Method for reliable realization of a varphi Josephson junction
We propose a method to realize a Josephson junction by combining
alternating 0 and parts (sub junctions) with an intrinsically
non-sinusoidal current-phase relation (CPR). Conditions for the realization of
the ground state are analyzed. It is shown that taking into account the
non-sinusoidal CPR for a "clean junction with a ferromagnetic (F) barrier, one
can significantly enlarge the domain (regime of suitable F-layer thicknesses)
of the ground state and make the practical realization of
Josephson junctions feasible. Such junctions may also have two different stable
solutions, such as 0 and , 0 and , or and
The effect of normal and insulating layers on 0-π transitions in Josephson junctions with a ferromagnetic barrier
Using the Usadel approach, we provide a formalism that allows us to calculate the critical current density of 21 different types of Josephson junctions (JJs) with a ferromagnetic (F) barrier and additional insulating (I) or/and normal (N) layers inserted between the F layer and superconducting (S) electrodes. In particular, we obtain that in SFS JJs, even a thin additional N layer between the S layer and F layer may noticeably change the thickness dF of the F layer at which the 0-π transitions occur. For certain values of dF, a 0-π transition can even be achieved by changing only the N layer thickness. We use our model to fit experimental data of SIFS and SINFS tunnel junctions
Record electron self-cooling in cold-electron bolometers with a hybrid superconductor-ferromagnetic nanoabsorber and traps
The Cosmic Microwave Background (CMB) radiation is the only observable that allows studying the earliest stage of the Universe. Radioastronomy instruments for CMB investigation require low working temperatures around 100 mK to get the necessary sensitivity. On-chip electron cooling of receivers is a pathway for future space missions due to problems of dilution fridges at low gravity. Here, we demonstrate experimentally that in a Cold-Electron Bolometer (CEB) a theoretical limit of electron cooling down to 65 mK from phonon temperature of 300 mK can be reached. It is possible due to effective withdrawing of hot electrons from the tunnel barrier by double stock, special traps and suppression of Andreev Joule heating in hybrid Al/Fe normal nanoabsorber
Spectropolarimetry of R Coronae Borealis in 1998--2003: Discovery of Transient Polarization at Maximum Brightness
We present an extended optical spectropolarimetry of R CrB from 1998 January
to 2003 September. The polarization was almost constant in the phase of maximum
brightness, being consistent with past observations. We detected, however,
temporal changes of polarization ( %) in 2001 March and August, which
were the first detection of large polarization variability in R CrB near
maximum brightness. The amplitude and the position angle of the `transient
polarization' were almost constant with wavelength in both two events. There
was a difference by about 20 degrees in the position angle between the two
events. Each event could be explained by light scattering due to short-lived
dust puff occasionally ejected off the line of sight. The flatness of the
polarization against the wavelength suggests that the scatterer is a mixture of
dust grains having various sizes. The rapid growth and fading of the transient
polarization favors the phenomenological model of dust formation near the
stellar photosphere (e.g., within two stellar radii) proposed for the time
evolution of brightness and chromospheric emission lines during deeply
declining periods, although the fading timescale can hardly be explained by a
simple dispersal of expanding dust puff with a velocity of km s
. Higher expansion velocity or some mechanism to destroy the dust grains
should be needed.Comment: 22 pages, 10 figures, accepted for publication in A
VLTI observations of the dust geometry around R Coronae Borealis stars
We are investigating the formation and evolution of dust around the
hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars. We aim
to determine the connection between the probable merger past of these stars and
their current dust-production activities. We carried out high-angular
resolution interferometric observations of three RCB stars, namely RY Sgr, V
CrA, and V854 Cen with the mid-IR interferometer, MIDI on the VLTI, using two
telescope pairs. The baselines ranged from 30 to 60 m, allowing us to probe the
dusty environment at very small spatial scales (~ 50 mas or 400 stellar radii).
The observations of the RCB star dust environments were interpreted using both
geometrical models and one-dimensional radiative transfer codes. From our
analysis we find that asymmetric circumstellar material is apparent in RY Sgr,
may also exist in V CrA, and is possible for V854 Cen. Overall, we find that
our observations are consistent with dust forming in clumps ejected randomly
around the RCB star so that over time they create a spherically symmetric
distribution of dust. However, we conclude that the determination of whether
there is a preferred plane of dust ejection must wait until a time series of
observations are obtained.Comment: Accepted for publication in MNRAS; 14 pages, 10 figures, 6 table
Diffraction-limited speckle interferometry and modeling of the circumstellar envelope of R CrB at maximum and minimum light
We present the first speckle interferometric observations of R CrB, the
prototype of a class of peculiar stars which undergo irregular declines in
their visible light curves. The observations were carried out with the 6m
telescope at the Special Astrophysical Observatory near maximum light (V=7,
1996 Oct. 1) and at minimum light (V=10.61, 1999 Sep. 28). A spatial resolution
of 75mas was achieved in the K-band. The dust shell around R CrB is partially
resolved, and the visibility is approximately 0.8 at a spatial frequency of 10
cycles/arcsec. The two-dimensional power spectra obtained at both epochs do not
show any significant deviation from circular symmetry. The visibility function
and spectral energy distribution obtained near maximum light can be
simultaneously fitted with a model consisting of the central star and an
optically thin dust shell with density proportional to r^-2. The inner boundary
of the shell is found to be 82Rstar (19mas) with a temperature of 920K.
However, this simple model fails to simultaneously reproduce the visibility and
spectral energy distribution obtained at minimum light. We show that this
discrepancy can be attributed to thermal emission from a newly formed dust
cloud.Comment: 9 pages including 10 PostScript figures, also available from
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.htm
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