37 research outputs found
Coherent quantum phase slip
A hundred years after discovery of superconductivity, one fundamental
prediction of the theory, the coherent quantum phase slip (CQPS), has not been
observed. CQPS is a phenomenon exactly dual to the Josephson effect: whilst the
latter is a coherent transfer of charges between superconducting contacts, the
former is a coherent transfer of vortices or fluxes across a superconducting
wire. In contrast to previously reported observations of incoherent phase slip,
the CQPS has been only a subject of theoretical study. Its experimental
demonstration is made difficult by quasiparticle dissipation due to gapless
excitations in nanowires or in vortex cores. This difficulty might be overcome
by using certain strongly disordered superconductors in the vicinity of the
superconductor-insulator transition (SIT). Here we report the first direct
observation of the CQPS in a strongly disordered indium-oxide (InOx)
superconducting wire inserted in a loop, which is manifested by the
superposition of the quantum states with different number of fluxes. Similarly
to the Josephson effect, our observation is expected to lead to novel
applications in superconducting electronics and quantum metrology.Comment: 14 pages, 3 figure
Superconducting nanowires: quantum confinement and spatially dependent Hartree-Fock potential
It is well-known that in bulk, the solution of the Bogoliubov-de Gennes
equations is the same whether or not the Hartree-Fock term is included. Here
the Hartree-Fock potential is position independent and, so, gives the same
contribution to both the single-electron energies and the Fermi level (the
chemical potential). Thus, the single-electron energies measured from the Fermi
level (they control the solution) stay the same. It is not the case for
nanostructured superconductors, where quantum confinement breaks the
translational symmetry and results in a position dependent Hartree-Fock
potential. In this case its contribution to the single-electron energies
depends on the relevant quantum numbers. We numerically solved the
Bogoliubov-de Gennes equations with the Hartree-Fock term for a clean
superconducting nanocylinder and found a shift of the curve representing the
thickness-dependent oscillations of the critical superconducting temperature to
larger diameters
Metallicities and ages for 35 star clusters and their surrounding fields in the Small Magellanic Cloud
In this work we study 35 stellar clusters in the Small Magellanic Cloud (SMC)
in order to provide their mean metallicities and ages. We also provide mean
metallicities of the fields surrounding the clusters. We used Str\"omgren
photometry obtained with the 4.1 m SOAR telescope and take advantage of and colors for which there is a metallicity calibration presented in
the literature. The spatial metallicity and age distributions of clusters
across the SMC are investigated using the results obtained by Str\"omgren
photometry. We confirm earlier observations that younger, more metal-rich star
clusters are concentrated in the central regions of the galaxy, while older,
more metal-poor clusters are located farther from the SMC center. We construct
the age-metallicity relation for the studied clusters and find good agreement
with theoretical models of chemical enrichment, and with other literature age
and metallicity values for those clusters. We also provide the mean
metallicities for old and young populations of the field stars surrounding the
clusters, and find the latter to be in good agreement with recent studies of
the SMC Cepheid population. Finally, the Str\"omgren photometry obtained for
this study is made publicly available.Comment: 22 pages, 12 figures, 6 tables, Accepted for publication in A&
HARPS-N high spectral resolution observations of Cepheids II. The impact of the surface-brightness color relation on the Baade-Wesselink projection factor of eta Aql
The Baade-Wesselink (BW) method of distance determination of Cepheids is used
to calibrate the distance scale. Various versions of this method are mainly
based on interferometry and/or the surface-brightness color relation (SBCR). We
quantify the impact of the SBCR, its slope, and its zeropoint on the projection
factor. This quantity is used to convert the pulsation velocity into the radial
velocity in the BW method. We also study the impact of extinction and of a
potential circumstellar environment on the projection factor. We analyzed
HARPS-N spectra of eta Aql to derive its radial velocity curve using different
methods. We then applied the inverse BW method using various SBCRs in the
literature in order to derive the BW projection factor. We find that the choice
of the SBCR is critical: a scatter of about 8% is found in the projection
factor for different SBCRs in the literature. The uncertainty on the
coefficients of the SBCR affects the statistical precision of the projection
factor only little (1-2\%). Confirming previous studies, we find that the
method with which the radial velocity curve is derived is also critical, with a
potential difference on the projection factor of 9%. An increase of 0.1 in
E(B-V) translates into a decrease in the projection factor of 3%. A 0.1
magnitude effect of a circumstellar envelope (CSE) in the visible domain is
rather small on the projection factor, about 1.5%. However, we find that a 0.1
mag infrared excess in the K band due to a CSE can increase the projection
factor by about 6%. The impact of the surface-brightness color relation on the
BW projection factor is found to be critical. Efforts should be devoted in the
future to improve the SBCR of Cepheids empirically, but also theoretically,
taking their CSE into account as well.Comment: Accepted for publication in Astronomy & Astrophysic
Recommended from our members
Surface brightness-colour relations of dwarf stars from detached eclipsing binaries: I. Calibrating sample
Aims. Surface brightness - colour relations (SBCRs) are very useful tools for predicting the angular diameters of stars. They offer the possibility to calculate very precise spectrophotometric distances by the eclipsing binary method or the Baade-Wesselink method. Double-lined Detached Eclipsing Binary stars (SB2 DEBs) with precisely known trigonometric parallaxes allow for a calibration of SBCRs with unprecedented precision. In order to improve such calibrations, it is important to enlarge the calibration sample of suitable eclipsing binaries with very precisely determined physical parameters. Methods. We carefully chose a sample of ten SB2 DEBs in the solar neighbourhood which contain inactive main-sequence components. The components have spectral types from early A to early K. All systems have high-precision parallaxes from the Gaia mission. We analysed high precision ground- and space-based photometry simultaneously with the radial velocity curves derived from HARPS spectra. We used spectral disentangling to obtain the individual spectra of the components and used these to derive precise atmospheric parameters and chemical abundances. For almost all components, we derived precise surface temperatures and metallicities. Results. We derived absolute dimensions for 20 stars with an average precision of 0.2% and 0.5% for masses and radii, respectively. Three systems show slow apsidal motion. One system, HD 32129, is most likely a triple system with a much fainter K6V companion. Also three systems contain metallic-line components and show strong enhancements of barium and ittrium. Conclusions. The components of all systems compare well to the SBCR derived before from the detached eclipsing binary stars. With a possible exception of HD 32129, they can be used to calibrate SBCRs with a precision better than 1% with available Gaia DR3 parallaxes
The Araucaria Project: Improving the cosmic distance scale
The book consists of a number of short articles that present achievements of
the Araucaria members, collaborators, and friends, in various aspects of
distance determinations and related topics. It celebrates the 20-year
anniversary of the Araucaria Project, acknowledges the people who worked for
its success, and popularises our methods and results among broader readership.
This book is a part of a project that has received funding from the European
Union's Horizon 2020 research and innovation programme under grant agreement No
695099.Comment: 114 pages, book published in 2021 on behalf of the Nicolaus
Copernicus Astronomical Center of the Polish Academy of Sciences, to
celebrate 20 years of the Arauria Projec
Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions
The rise of topological phases of matter is strongly connected to their
potential to host Majorana bound states, a powerful ingredient in the search
for a robust, topologically protected, quantum information processing. In order
to produce such states, a method of choice is to induce superconductivity in
topological insulators. The engineering of the interplay between
superconductivity and the electronic properties of a topological insulator is a
challenging task and it is consequently very important to understand the
physics of simple superconducting devices such as Josephson junctions, in which
new topological properties are expected to emerge. In this article, we review
recent experiments investigating topological superconductivity in topological
insulators, using microwave excitation and detection techniques. More
precisely, we have fabricated and studied topological Josephson junctions made
of HgTe weak links in contact with two Al or Nb contacts. In such devices, we
have observed two signatures of the fractional Josephson effect, which is
expected to emerge from topologically-protected gapless Andreev bound states.
We first recall the theoretical background on topological Josephson junctions,
then move to the experimental observations. Then, we assess the topological
origin of the observed features and conclude with an outlook towards more
advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017,
published in "Topological Matter. Springer Series in Solid-State Sciences,
vol 190. Springer
Toward Early-type Eclipsing Binaries as Extragalactic Milestones. I. Physical Parameters of OGLE-LMC-ECL-22270 and OGLE-LMC-ECL-06782
International audienc