We investigate numerically the dynamics of traveling clusters in systems of phase oscillators, some of which possess positive couplings and others negative couplings. The phase distribution, speed of traveling, and average separation between clusters
as well as order parameters for positive and negative oscillators are computed, as the ratio of the two coupling constants and/or the fraction of positive oscillators are varied. The traveling speed depending on these parameters is obtained and observed to fit well with the numerical data of the systems. With the help of this, we describe the conditions for the traveling state to appear in the systems with or without periodic driving.
We report the results of a 3D particle-in-cell (PIC) simulation carried out to study the early-stage evolution of the shock formed when an unmagnetized relativistic jet interacts with an ambient electron-ion plasma. Full-shock structures associated w
ith the interaction are observed in the ambient frame. When open boundaries are employed in the direction of the jet; the forward shock is seen as a hybrid structure consisting of an electrostatic shock combined with a double layer, while the reverse shock is seen as a double layer. The ambient ions show two distinct features across the forward shock: a population penetrating into the shocked region from the precursor region and an accelerated population escaping from the shocked region into the precursor region. This behavior is a signature of a combination of an electrostatic shock and a double layer. Jet electrons are seen to be electrostatically trapped between the forward and reverse shock structures showing a ring-like distribution in a phase-space plot, while ambient electrons are thermalized and become essentially isotropic in the shocked region. The magnetic energy density grows to a few percent of the jet kinetic energy density at both the forward and the reverse shock transition layers in a rather short time scale. We see little disturbance of the jet ions over this time scale.
We developed a cryogenic system on a rotating table that achieves sub-Kelvin conditions. The cryogenic system consists of a helium sorption cooler and a pulse tube cooler in a cryostat mounted on a rotating table. Two rotary-joint connectors for elec
tricity and helium gas circulation enable the coolers to be operated and maintained with ease. We performed cool-down tests under a condition of continuous rotation at 20 rpm. We obtained a temperature of 0.23 K with a holding time of more than 24 hours, thus complying with catalog specifications. We monitored the systems performance for four weeks; two weeks with and without rotation. A few-percent difference in conditions was observed between these two states. Most applications can tolerate such a slight difference. The technology developed is useful for various scientific applications requiring sub-Kelvin conditions on rotating platforms.
In the field of radiowave detection, enlarging the receiver aperture to enhance the amount of light detected is essential for greater scientific achievements. One challenge in using radio transmittable apertures is keeping the detectors cool. This is
because transparency to thermal radiation above the radio frequency range increases the thermal load. A technology that maintains cold conditions while allowing larger apertures has been long-awaited. We propose radio-transparent multi-layer insulation (RT-MLI), composed from a set of stacked insulating layers. The insulator is transparent to radio frequencies, but not transparent to infrared radiation. The basic idea for cooling is similar to conventional multi-layer insulation. It leads to a reduction in thermal radiation while maintaining a uniform surface temperature. The advantage of this technique over other filter types is that no thermal links are required. As insulator material, we used foamed polystyrene; its low index of refraction makes an anti-reflection coating unnecessary. We measured the basic performance of RT-MLI to confirm that thermal loads are lowered with more layers. We also confirmed that our RT-MLI has high transmittance to radiowaves, but blocks infrared radiation. For example, RT-MLI with 12 layers has a transmittance greater than 95% (lower than 1%) below 200 GHz (above 4 THz). We demonstrated its effects in a system with absorptive-type filters, where aperture diameters were 200 mm. Low temperatures were successfully maintained for the filters. We conclude that this technology significantly enhances the cooling of radiowave receivers, and is particularly suitable for large-aperture systems. This technology is expected to be applicable to various fields, including radio astronomy, geo-environmental assessment, and radar systems.
Although abundant research has focused recently on the quantum criticality of itinerant magnets, critical phenomena of insulating magnets in the vicinity of critical endpoints (CEPs) have rarely been revealed. Here we observe an emergent CEP at 2.05
T and 2.2 K with a suppressed thermal conductivity and concomitant strong critical fluctuations evident via a divergent magnetic susceptibility (e.g., chi(2.05 T, 2.2 K)/chi(3 T, 2.2 K)=23,500 %, comparable to the critical opalescence in water) in the hexagonal insulating antiferromagnet HoMnO3.
Near-Earth asteroid 162173 (1999 JU3) is a potential target of two asteroid sample return missions, not only because of its accessibility but also because of the first C-type asteroid for exploration missions. The lightcurve-related physical properti
es of this object were investigated during the 2011-2012 apparition. We aim to confirm the physical parameters useful for JAXAs Hayabusa 2 mission, such as rotational period, absolute magnitude, and phase function. Our data complement previous studies that did not cover low phase angles. With optical imagers and 1-2 m class telescopes, we acquired the photometric data at different phase angles. We independently derived the rotational lightcurve and the phase curve of the asteroid. We have analyzed the lightcurve of 162173 (1999 JU3), and derived a synodic rotational period of 7.625 +/- 0.003 h, the axis ratio a/b = 1.12. The absolute magnitude H_R = 18.69 +/- 0.07 mag and the phase slope of G = -0.09 +/- 0.03 were also obtained based on the observations made during the 2011-2012 apparition.
Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate electron tunneling from a C60-terminated tip into a Cu(111) surface. Tunneling between a C60 orbital and the Shockley surface states of copper is shown to prod
uce negative differential conductance (NDC) contrary to conventional expectations. NDC can be tuned through barrier thickness or C60 orientation up to complete extinction. The orientation dependence of NDC is a result of a symmetry matching between the molecular tip and the surface states.
lasma instabilities excited in collisionless shocks are responsible for particle acceleration. We have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an u
nmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. These magnetic fields contribute to the electrons transverse deflection behind the shock. Our initial results of a jet-ambient interaction with anti-parallel magnetic fields show pile-up of magnetic fields at the colliding shock, which may lead to reconnection and associated particle acceleration. We will investigate the radiation in transient stage as a possible generation mechanism of precursors of prompt emission. In our simulations we calculate the radiation from electrons in the shock region. The detailed properties of this radiation are important for understanding the complex time evolution and spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.
We report neutron diffraction and magnetization studies of the magnetic order in multiferroic BiFeO3. In ferroelectric monodomain single crystals, there are three magnetic cycloidal domains with propagation vectors equivalent by crystallographic symm
etry. The cycloid period slowly grows with increasing temperature. The magnetic domain populations do not change with temperature except in the close vicinity of the N{P}eel temperature, at which, in addition, a small jump in magneti- zation is observed. No evidence for the spin-reorientation transitions proposed in previous Raman and dielectric studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The an- harmonicity is much smaller than previously reported in NMR studies. At room temperature, a circular cycloid is observed, within errors. We argue that the observed anharmonicity provides important clues for understanding electromagnons in BiFeO3.
