arXiv subjects

M. Machida

Publications and source records attributed to M. Machida.

At least 19 recordsLinked to original sources

arXiv2024

Observations of spiral and streamer on a candidate proto-brown dwarf

Spirals and streamers are the hallmarks of mass accretion during the early stages of star formation. We present the first observations of a large-scale spiral and a streamer towards a very young brown dwarf candidate in its early formation stages. These observations show, for the first time, the influence of external environment that results in asymmetric mass accretion via feeding filaments onto a candidate proto-brown dwarf in the making. The impact of the streamer has produced emission in warm carbon-chain species close to the candidate proto-brown dwarf. Two contrasting scenarios, a pseudo-disk twisted by core rotation and the collision of dense cores, can both explain these structures. The former argues for the presence of a strong magnetic field in brown dwarf formation while the latter suggests that a minimal magnetic field allows large-scale spirals and clumps to form far from the candidate proto-brown dwarf.

astro-ph.SR↗

arXiv2020

Two-temperature Magnetohydrodynamic simulations for sub-relativistic AGN jets:Dependence on the fraction of the electron heating

We present the results of two-temperature magnetohydrodynamic simulations of the propagation of sub-relativistic jets of active galactic nuclei. The dependence of the electron and ion temperature distributions on the fraction of electron heating fe at the shock front is studied for fe=0, 0.05, and 0.2. Numerical results indicate that in sub-relativistic, rarefied jets, the jet plasma crossing the terminal shock forms a hot, two-temperature plasma in which the ion temperature is higher than the electron temperature. The two-temperature plasma expands and forms a backflow referred to as a cocoon, in which the ion temperature remains higher than the electron temperature for longer than 100 Myr. Electrons in the cocoon are continuously heated by ions through Coulomb collisions, and the electron temperature thus remains at Te > 10^9 K in the cocoon. X-ray emissions from the cocoon are weak because the electron number density is low. Meanwhile, soft X-rays are emitted from the shocked intracluster medium surrounding the cocoon. Mixing of the jet plasma and the shocked intracluster medium through the Kelvin--Helmholtz instability at the interface enhances X-ray emissions around the contact discontinuity between the cocoon and shocked intracluster medium.

astro-ph.HE↗

arXiv2018

Depth of Ultra High Energy Cosmic Ray Induced Air Shower Maxima Measured by the Telescope Array Black Rock and Long Ridge FADC Fluorescence Detectors and Surface Array in Hybrid Mode

The Telescope Array observatory utilizes fluorescence detectors and surface detectors to observe air showers produced by ultra high energy cosmic rays in the Earth's atmosphere. Cosmic ray events observed in this way are termed hybrid data. The depth of air shower maximum is related to the mass of the primary particle that generates the shower. This paper reports on shower maxima data collected over 8.5 years using the Black Rock Mesa and Long Ridge fluorescence detectors in conjunction with the array of surface detectors. We compare the means and standard deviations of the observed $X_{\mathrm{max}}$ distributions with Monte Carlo $X_{\mathrm{max}}$ distributions of unmixed protons, helium, nitrogen, and iron, all generated using the QGSJet~II-04 hadronic model. We also perform an unbinned maximum likelihood test of the observed data, which is subjected to variable systematic shifting of the data $X_{\mathrm{max}}$ distributions to allow us to test the full distributions, and compare them to the Monte Carlo to see which elements are not compatible with the observed data. For all energy bins, QGSJet~II-04 protons are found to be compatible with Telescope Array hybrid data at the 95% confidence level after some systematic $X_{\mathrm{max}}$ shifting of the data. Three other QGSJet~II-04 elements are found to be compatible using the same test procedure in an energy range limited to the highest energies where data statistics are sparse.

astro-ph.HE↗

arXiv2015

Effect of magnetism on lattice dynamics in SrFe$_2$As$_2$ as seen via high resolution inelastic x-ray scattering

Phonon spectra of detwinned {SrFe$_2$As$_2$} crystals, as measured by inelastic x-ray scattering, show clear anisotropy accompanying the magneto-structural transition at 200 K. We model the mode splitting using magnetic DFT calculations, including a phenomenological reduction in force-constant anisotropy that can be attributed to magnetic fluctuations. This serves as a starting point for a general model of phonons in this material applicable to both magnetic and non-magnetic phase. Using this model, the measured splitting in the magnetic phase below $\it T_{N}$, and the measured phonon linewidth, we set a lower bound on the mean magnetic fluctuation frequency above $\it T_{N}$ at 210 K.

cond-mat.supr-con↗

arXiv2015

Phonon and Magnetic Excitations in Superconducting Ca$_{10}$Pt$_4$As$_8$(Fe$_{1-x}$Pt$_x$As)$_{10}$

Inelastic neutron scattering studies have been carried out on selected phonons and magnetic excitations of a crystal of superconducting (SC) Ca$_{10}$Pt$_4$As$_8$(Fe$_{1-x}$Pt$_x$As)$_{10}$ with the onset transition temperature $T_{\rm c}^{\rm onset} \sim$ 33 K to investigate the role that orbital fluctuations play in the Cooper pairing. The spectral weight of the magnetic excitations, $\chi ''({\bm Q}, \omega)$ at ${\bm Q} = {\bm Q}_{\rm M}$ (magnetic $\Gamma$ points) is suppressed (enhanced) in the relatively low (high) $\omega$ region. The maximum of the enhancement is located at $\omega = \omega_{\rm p} \sim$ 18 meV at temperature $T = 3$ K corresponding to the $\omega_{\rm p}/k_{\rm B}T_{\rm c}^{\rm onset} \sim$ 6.3. This large value is rather favorable to the orbital-fluctuation mechanism of the superconductivity with the so-called $S_{++}$ symmetry. In the phonon measurements, we observed slight softening of the in-plane transverse acoustic mode corresponding to the elastic constant $C_{66}$. This softening starts at $T$ well above the superconducting $T_{\rm c}$, as $T$ decreases. An anomalously large increase in the phonon spectral weight of in-plane optical modes was observed in the range of $35 < \omega < 40$ meV with decreasing $T$ from far above $T_{\rm c}$. Because this $\omega$ region mainly corresponds to the in-plane vibrations of Fe atoms, the finding presents information on the coupling between the orbital fluctuation of the Fe 3$d$ electrons and lattice system, useful for studying possible roles of the orbital fluctuation in the pairing mechanism and appearance of the so-called nematic phase.

cond-mat.supr-con↗

arXiv2011

Soft and Isotropic Phonons in PrFeAsO_{1-y}

Phonons in single crystals of PrFeAsO_{1-y} are investigated using high-resolution inelastic x-ray scattering and ab initio pseudopotential calculations. Extensive measurements of several samples at temperatures spanning the magnetic ordering temperature and the superconducting transition temperature show that there are some changes in phonon spectra with temperature and/or doping. We compare our measurements with several ab initio pseudopotential models (nonmagnetic tetragonal, oxygen-deficient O_{7/8} supercell, magnetic orthorhombic, and magnetic tetragonal) and find that the experimentally observed changes are much smaller than the differences between the experimental data and the calculations. Agreement is improved if magnetism is included in the calculations via the local spin density approximation, as the Fe atomic motions parallel to the ferromagnetic ordering direction are softened. However, the antiferromagnetically polarized modes remain hard, and in disagreement with the experimental data. In fact, given the increasing evidence for anisotropy in the iron pnictide materials, the phonon response is surprisingly isotropic. We consider several modifications of the ab initio calculations to improve the agreement with the experimental data. Improved agreement is found by setting the matrix to zero (clipping the bond) between nearest-neighbor antiferromagnetically aligned Fe atoms in the magnetic calculation, or by softening only the in-plane nearest-neighbor Fe-As force constant in the nonmagnetic calculation. We discuss these results in the context of other measurements, especially of phonons, for several FeAs systems. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated here including those known to have static magnetism.

cond-mat.str-el↗

arXiv2010

Strong spin resonance on BaFe2(As0.65P0.35)2 with Tc = 30 K

We performed inelastic neutron scattering on powder sample of the P-doped iron-based superconductor BaFe2(As0.65P0.35)2 with Tc = 30K, whose superconducting (SC) order parameter is expected to have line nodes. We have observed spin resonance at Q $\sim$ 1.2{\AA}^{-1} and E=12 meV in the SC state. The resonance enhancement, which can be a measure of the area of sign reversal between the hole and electron Fermi surfaces (FSs), is comparable to those of other iron-based superconductors without line nodes. This fact indicates that the sign reversal between the FSs is still dominant in this system, and the line nodes should create only limited area of sign-reversal on a single FS. Hence the system can hold relatively high-Tc. Comparison with theoretical calculation indicates horizontal line nodes may be a candidate to reproduce the observation.

cond-mat.supr-con↗

arXiv2010

Phase-separated Ferromagnetism in Spin-imbalanced Fermi Atoms Loaded on an Optical Ladder: a DMRG study

We consider repulsively-interacting cold fermionic atoms loaded on an optical ladder lattice in a trapping potential. The density-matrix renormalization-group method is used to numerically calculate the ground state for systematically varied values of interaction U and spin imbalance p in the Hubbard model on the ladder. The system exhibits rich structures, where a fully spin polarized phase, spatially separated from other domains in the trapping potential, appears for large enough U and p. The phase-separated ferromagnetism can be captured as a real-space image of the energy gap between the ferromagnetic and other states arising from a combined effect of Nagaoka's ferromagnetism extended to the ladder and the density dependence of the energy separation between competing states. We also predict how to maximize the ferromagnetic region.

cond-mat.quant-gas↗

arXiv2009

First-principles-based $\pm s$-wave modelling for iron-based superconductors:Studies for specific heat and nuclear magnetic relaxation rate

In order to consistently explain controversial experimental results on superconducting states observed by different probes in typical iron-based superconductors, we construct a realistic multi-band $\pm s$-wave pairing model by combining the quasiclassical formalism with the first-principles calculation. The model successfully resolves the controversies in contrast to the fact that simplified models such as two-band $\pm s$-wave one fail to do. A key in the model is the existence of relatively small gaps which leads to material-dependent peculiarities.

cond-mat.supr-con↗

arXiv2009

Direct Numerical Confirmation of Pinning Induced Sign Change in Superconducting Hall Effect in Type-II Superconductors

Using the time-dependent Ginzburg-Landau equation with the complex relaxation time and the Maxwell equation, we systematically examine transverse motion of vortex dynamics in the presence of pinning disorders. Consequently, in a plastic flow phase in which moving and pinned vortices coexist, we find that the Hall voltage generally changes its sign. The origin of the sign change is ascribed to a fact that moving vortices are strongly drifted by circular current of pinned vortices and the enforced transverse moving direction becomes opposite to that by transport current. This suggests that the Hall sign change is a behavior common in all disordered type-II superconductors.

cond-mat.supr-con↗

arXiv2009

Thermal Equilibria of Magnetically Supported, Black Hole Accretion Disks

We present new thermal equilibrium solutions for optically thin and thick disks incorporating magnetic fields. The purpose of this paper is to explain the bright hard state and the bright/slow transition observed in the rising phases of outbursts in BHCs. On the basis of the results of 3D MHD simulations, we assume that magnetic fields inside the disk are turbulent and dominated by the azimuthal component and that the azimuthally averaged Maxwell stress is proportional to the total pressure. We prescribe the magnetic flux advection rate to determine the azimuthal magnetic flux at a given radius. We find magnetically supported, thermally stable solutions for both optically thin and thick disks, in which the heating enhanced by the strong magnetic field balances the radiative cooling. The temperature in a low-$\beta$ disk is lower than that in an ADAF/RIAF but higher than that in a standard disk. We also study the radial dependence of the thermal equilibrium solutions. The optically thin, low-$\beta$ branch extends to $ \dot M \gtrsim 0.1 {\dot M}_{\rm Edd}$, in which the temperature anti-correlates with the mass accretion rate. Thus optically thin low-$\beta$ disks can explain the bright hard state. Optically thick, low-$\beta$ disks have the radial dependence of the effective temperature $T_{\rm eff} \propto \varpi^{-3/4}$. Such disks will be observed as staying in a high/soft state. Furthermore, limit cycle oscillations between an optically thick low-$\beta$ disk and a slim disk will occur because the optically thick low-$\beta$ branch intersects with the radiation pressure dominated standard disk branch. These limit cycle oscillations will show a smaller luminosity variation than that between a standard disk and a slim disk.

astro-ph.HE↗

arXiv2009

Polarization Plateau in Atomic Fermi Gas Loaded on Triangular Optical Lattice

In order to demonstrate that atomic Fermi gas is a good experimental reality in studying unsolved problems in frustrated interacting-spin systems, we numerically examine the Mott core state emerged by loading two-component atomic Fermi gases on triangular optical lattices. Consequently, we find that plateau like structures are observable in the Mott core polarization as a function of the population imbalance. These plateau states are caused by a flexibility that the surrounding metallic region absorbs the excess imbalance to keep the plateau states inside the Mott core. We also find spin patterns peculiar to the plateau states inside the Mott core.

cond-mat.str-el↗

arXiv2008

The Polish doughnuts revisited I. The angular momentum distribution and equipressure surfaces

We construct a new family of analytic models of black hole accretion disks in dynamical equilibria. Our construction is based on assuming distributions of angular momentum and entropy. For a particular choice of the distribution of angular momentum, we calculate the shapes of equipressure surfaces. The equipressure surfaces we find are similar to those in thick, slim and thin disks, and to those in ADAFs.

astro-ph↗

arXiv2008

Mott Phase in Polarized Two-component Atomic Fermi Lattice Gas:A Playground for S=1/2 Heisenberg Model in Magnetic Field

We investigate effects of pseudo-spin population imbalance on Mott phases in 1D trapped two-component atomic Fermi gases loaded on optical lattices based on the repulsive Hubbard model in harmonic traps. By using the density matrix renormalization group method, we numerically calculate density profiles of each component and clarify the pseudo-spin magnetism. Consequently, we find that all the features from weakly imbalance to fully polarized cases are well described by S=1/2 antiferromagnetic Heisenberg chain under magnetic field. These results indicate that the Mott phases offer experimental stages for studying various interacting spin systems.

cond-mat.str-el↗

arXiv2008

Hole Localization in One-Dimensional Doped Anderson-Hubbard Model

We study the interplay of disorder and correlation in the one-dimensional hole-doped Hubbard-model with disorder (Anderson-Hubbard model) by using the density-matrix renormalization group method. Concentrating on the doped-hole density profile, we find in a large $U/t$ regime that the clean system exhibits a simple fluid-like behavior whereas finite disorders create locally Mott regions which expand their area with increasing the disorder strength contrary to the ordinary sense. We propose that such an anomalous Mott phase formation assisted by disorder is observable in atomic Fermi gases by setup of the box shape trap.

cond-mat.str-el↗

arXiv2008

Magnetism Localization in Spin-Polarized One-Dimensional Anderson-Hubbard Model

In order to study an interplay of disorder, correlation, and spin imbalance on antiferromagnetism, we systematically explore the ground state of one-dimensional spin-imbalanced Anderson-Hubbard model by using the density-matrix renormalization group method. We find that disorders localize the antiferromagnetic spin density wave induced by imbalanced fermions and the increase of the disorder magnitude shrinks the areas of the localized antiferromagnetized regions. Moreover, the antiferromagnetism finally disappears above a large disorder. These behaviors are observable in atomic Fermi gases loaded on optical lattices and disordered strongly-correlated chains under magnetic field.

cond-mat.str-el↗

arXiv2008

Lattice Dynamics of LaFeAsO_{1-x}F_{x} and PrFeAsO_{1-y} via Inelastic X-Ray Scattering and First-Principles Calculation

The lattice dynamics of LaFeAsO_{1-x}F_{x} (x=0, 0.1) and PrFeAsO_{1-y} (y~0.1) are investigated using inelastic x-ray scattering and ab-initio calculation. Measurements of powder samples provide an approximation to the phonon DOS, while dispersion is measured from a single crystal of PrFeAsO_{1-y}. A model that agrees reasonably well with all of the data at room temperature is built from results of ab-initio calculations by softening the strength of the Fe-As bond by 30%.

cond-mat.supr-con↗

arXiv2008

LDA + Negative U Solves a Puzzle of too Large Calculated Magnetic Moment in Iron-based Superconductor LaFeAsO_{1-x}F_x

A puzzle in the iron-based superconductor LaFeAsO_{1-x}F_x is that the magnetic moment obtained by first-principle electronic structure calculations is unexpectedly much larger than the experimentally observed one. For example, the calculated value is ~ 2.0 μ_B in the mother compound, while it is ~ 0.3 μ_B in experiments. We find that the puzzle is solved within the framework LDA + U by expanding the U value into a slightly negative range. We show U dependence of the obtained magnetic moment in both the undoped x=0.0 and doped x = 0.125. These results reveal that the magnetic moment is drastically reduced when entering to the slightly negative range of U. Moreover, the negative U well explains other measurement data, e.g., lattice constants and electronic DOS at the Fermi level. We discuss possible origins of the negative U in these compounds.

cond-mat.supr-con↗