In laboratory experiments, horizontal translational motion of columnar vortices formed in rotating Rayleigh–Bénard convection was investigated. Two types of measurements, vertical velocity fields and ...horizontal temperature fields, were conducted with water as the test fluid. Using particle image velocimetry, the vertical velocity fields determined the parameter range at which the quasi-two-dimensional columnar vortices emerged. Locally, the duration characteristics of the columns, evaluated with their vertical coherence, indicate the minimum time scale of translational motion of the vortices in the horizontal plane. Vortex tracking of the horizontal temperature fields over long observation periods (
${>}10^{3}~\text{s}$
) was conducted using encapsulated thermochromic liquid crystal visualization. Two cylindrical vessels with different radii showed the emergence of the centrifugal effect in
$O({>}10^{2}~\text{s})$
despite the small Froude number (
$Fr<0.1$
). Further, in the horizontal plane the columnar vortices behaved in a random-walk-like diffusive motion. The statistically calculated mean-squared displacements indicated anomalous diffusive motion of the columns; displacement increasing with time as
$t^{\unicodeSTIX{x1D6FE}}$
with
$\unicodeSTIX{x1D6FE}\neq 1$
. We discuss the causes of this anomaly in both the instantaneous and long-term statistical data gathered from experimental observations over different time scales. The enclosure effect from the repulsion of up-welling and down-welling vortices ensures that vortices diffuse only little, resulting in a sub-diffusive (decelerated) motion
$\unicodeSTIX{x1D6FE}<1$
in
$O(10^{1}~\text{s})$
. With this weak centrifugal contribution, the translational motion of the columns slowly accelerates in the radial direction and thereby yields a super-diffusive (accelerated) motion
$\unicodeSTIX{x1D6FE}>1$
in
$O(10^{2}~\text{s})$
.
Abstract
Transient behaviours during spin-up in rotating Rayleigh–Bénard convection (RBC) with imposed rotation were quantitatively investigated in laboratory experiments. Horizontal and vertical ...velocity fields were measured by particle image velocimetry with water as the test fluid. Varying the aspect ratio, Rayleigh number and Taylor number, a total of twenty parameters were systematically explored. Toroidal and spiral rolls were formed when the flow reached the rigid-body rotation state, and creation of these structures propagated from the rim towards the internal regions together with the development of the spin-up. Alternate alignments of rolls with opposite meridional circulation transported azimuthal momentum in the rigid-body rotation, and a meandering velocity profile in the radial direction, induced Kelvin–Helmholtz (KH) instability generating azimuthally aligned vortices. The vortices progressively decreased in horizontal dimensions with the wall-to-centre propagation of the vortex formation, but the vortical structures remain larger than the columnar vortices formed in the equilibrium state of a rotating RBC. At the intermediate radius of the fluid layer, the wall-to-centre propagation of the roll formation was overtaken by that of the KH vortex formation. Farther into the interior region, thermal plumes forming columnar vortices were generated as separations of the thermal boundary layers, and the system reached an equilibrium state of rotating RBC dominated by columnar vortices. Use of a fluid vessel with a moderate aspect ratio clarified these transitions to occur in a stepwise fashion, and a spin-up time scale unique in the rotating RBC was found to be from a few to 10 times the Ekman time scale.
•A fractional vortex was experimentally formed in a conventional superconductor.•The fractional vortex was realized in a superconducting bi-layer.•Multi-component quantum condensates were generated ...using an s-wave superconductor.
We report the experimental formation of a fractional vortex generated by using a thin superconducting bi-layer in the form of a niobium bi-layer, observed as a magnetic flux distribution image taken by a scanning superconducting quantum interference device (SQUID) microscope. Thus, we demonstrated that multi-component superconductivity can be realized by an s-wave conventional superconductor, because, in these superconductors, the magnetic flux is no longer quantized as it is destroyed by the existence of an inter-component phase soliton (i-soliton).
Quantum mechanics states that hopping integral between local orbitals makes the energy band dispersive. However, in some special cases, there are bands with no dispersion due to quantum interference. ...These bands are called as flat band. Many models having flat band have been proposed, and many interesting physical properties are predicted. However, no real compound having flat band has been found yet despite the 25 years of vigorous researches. We have found that some pyrochlore oxides have quasi-flat band just below the Fermi level by first principles calculation. Moreover, their valence bands are well described by a tight-binding model of pyrochlore lattice with isotropic nearest neighbor hopping integral. This model belongs to a class of Mielke model, whose ground state is known to be ferromagnetic with appropriate carrier doping and on-site repulsive Coulomb interaction. We have also performed a spin-polarized band calculation for the hole-doped system from first principles and found that the ground state is ferromagnetic for some doping region. Interestingly, these compounds do not include magnetic element, such as transition metal and rare-earth elements.
Acoustic signatures of the single-site quadrupolar Kondo effect in Y0.966Pr0.034Ir2Zn20 are presented. The elastic constant (C11−C12)/2, corresponding to the Γ3(E)-symmetry electric-quadrupolar ...response, reveals a logarithmic temperature dependence of the quadrupolar susceptibility in the low-magnetic-field region below ∼0.3 K. Furthermore, the Curie-type divergence of the elastic constant down to ∼1 K indicates that the Pr ions in this diluted system have a non-Kramers ground-state doublet. These observations evidence the single-site quadrupolar Kondo effect, as previously suggested based on specific-heat and electrical-resistivity data.
We investigate the two-dimensional (2-D) oscillation of quasi-2-D convection rolls in a liquid metal layer confined by a vessel of aspect ratio five with an imposed horizontal magnetic field. ...Laboratory experiments were performed in the range of Rayleigh $Ra$ and Chandrasekhar $Q$ numbers of $7.9 \times 10^4 \le Ra \le 1.8 \times 10^5$ and $2.5 \times 10^4 \le Q \le 1.9 \times 10^5$ by decreasing $Q$ at set $Ra$-number intervals to elucidate the features and mechanisms of oscillatory convection. Ultrasonic velocity profile measurements and supplemental numerical simulations show that the 2-D oscillations are caused by oscillations of recirculation vortex pairs between the main rolls, which are intensified by periodic vorticity entrainment from the vortex pair by the main rolls. The investigations also suggest that the oscillations occur at sufficiently large Reynolds $Re$ numbers to induce instabilities on the vortex pair. The $Re$ number is smaller for larger $Q/Ra$ in the 2-D oscillation regime and the variations can be approximated by the effective $Ra$ number; namely, the value reduced by the critical value for the onset of convection depending on $Q$. The variations steepen with further large $Q/Ra$ and approach a scaling law of the velocity reduction as $(Ra/Q)^{1/2}$, which is established assuming that viscous dissipation is dominated by Hartmann braking at the walls perpendicular to the magnetic field. The results suggest that these phenomena are organized by the relationship between buoyancy and magnetic damping due to Hartmann braking.
Abstract
An isotropic tight-binding model with the nearest-neighbour hopping on a pyrochlore lattice gives a rich variety of physical properties due to the emergence of the flat-band. Moreover, by ...introducing spin-orbit coupling into this model, the topological properties of the system changes significantly. This model is well applicable to some pyrochlore oxides called s1/s2 pyrochlores. In this paper we apply this model to an s2 pyrochlore oxide Pb
2
Ta
2
O
7
and found a characteristic quadratic touching of the quasi-flat band and dispersive band. Furthermore, when a ferromagnetic order appears due to this quasi-flat band, a pair of Weyl points appears in that direction.
Abstract
In this paper we introduce our method to explore new superconductors: bulk-combinatorial method. Using this method, we have succeeded in finding many new superconductors so far. We also show ...that the problems that arise when identifying new superconducting phases in this way are related to the famous mathematical puzzle, the so-called “1000 poisoned wine bottles”, under some assumptions.
We have derived a tri-axial ellipsoidal model of an LEO object, a Cosmos 2082 rocket body, including its rotational axis direction, rotation period, precession, and a compositional parameter, using ...only light curve data from an optical telescope. The brightness of the object was monitored for two days and least-squares fitting was used to determine these values. The derived axial ratios are 100:18:18, the coordinates of the rotational axis direction on the celestial sphere are R.A.=305.8° and Dec.=2.6°, and its observed average rotation period is 41s. When precession is considered, its amplitude and precession period are 30.5° and 29.4min. These results show that optical light curve data are sufficient to determine the shapes and the motions of LEO objects.
The gap equation for three-gap superconductivity has a chiral solution driven by a repulsive interaction (inter-band exchange repulsion). When a repulsive channel contributes to the ...superconductivity, the emergence of chiral superconductivity should be considered.
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