arXiv subjects

T. Tsuji

Publications and source records attributed to T. Tsuji.

9 recordsLinked to original sources

arXiv2023

Extending spin dephasing time of perfectly aligned Nitrogen-Vacancy centers by mitigating stress distribution on highly misoriented chemical-vapor-deposition diamond

Extending the spin-dephasing time (T2*) of perfectly aligned nitrogen-vacancy (NV) centers in large-volume chemical vapor deposition (CVD) diamonds leads to enhanced DC magnetic sensitivity. However, T2* of the NV centers is significantly reduced by the stress distribution in the diamond film as its thickness increases. To overcome this issue, we developed a method to mitigate the stress distribution in the CVD diamond films, leading to a T2* extension of the ensemble NV centers. CVD diamond films of approximately 50 μm thickness with perfectly aligned NV centers were formed on (111) diamond substrates with misorientation angles of 2.0, 3.7, 5.0, and 10°. We found that T2* of the ensemble of NV centers increased to approach the value limited only by the electron and nuclear spin bath with increasing the misorientation angle. Microscopic stress measurements revealed that the stress distribution was highly inhomogeneous along the depth direction in the CVD diamond film at low misorientation angles, whereas the inhomogeneity was largely suppressed on highly misoriented substrates. The reduced stress distribution possibly originates from the reduction of the dislocation density in the CVD diamond. This study provides an important method for synthesizing high-quality diamond materials for use in highly sensitive quantum sensors.

physics.app-ph↗

arXiv2021

Formation of orbital molecules on a pyrochlore lattice induced by A-O bond covalency

The pyrochlore ruthenate In$_2$Ru$_2$O$_7$ displays a subtle competition between spin-orbital entanglement and molecular orbital formation. At room temperature, a spin-orbit-entangled singlet state was identified. With decreasing temperature, In$_2$Ru$_2$O$_7$ undergoes multiple structural transitions and eventually forms a nonmagnetic ground state with semi-isolated Ru$_2$O units on the pyrochlore lattice. The dominant hopping through the Ru-O-Ru linkage leads to molecular orbital formation within the Ru$_2$O units. This molecular orbital formation is unique in that it involves the O$^{2-}$ anions, unlike the transition-metal dimers observed in systems with edge-sharing octahedra. We argue that the covalent character of In-O bonds plays a pivotal role in the structural transitions and molecular orbital formation and such bonding character of "$A$-site" ions is an important ingredient for electronic phase competition in complex transition metal oxides.

cond-mat.str-el↗

arXiv2014

Backward Clusters, Hierarchy and Wild Sums for a Hard Sphere System in a Low-Density Regime

We study the statistics of backward clusters in a gas of hard spheres at low density. A backward cluster is defined as the group of particles involved directly or indirectly in the backwards-in-time dynamics of a given tagged sphere. We derive upper and lower bounds on the average size of clusters by using the theory of the homogeneous Boltzmann equation combined with suitable hierarchical expansions. These representations are known in the easier context of Maxwellian molecules (Wild sums). We test our results with a numerical experiment based on molecular dynamics simulations.

math-ph↗

arXiv2012

Comparative Modelling of the Spectra of Cool Giants

Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.

astro-ph.SR↗

arXiv2009

The K giant star Arcturus: the hybrid nature of its infrared spectrum

We study infrared spectrum of Arcturus to clarify the nature of the cool component of its atmosphere, referred to as the CO-mosphere, with the use of the IR spectral atlas by Hinkle et al.(1995). The nature of CO lines shows an abrupt change at logW/nu = -4.75, and the lines stronger than this limit can no longer be analyzed by the classical line-formation theory. A more simple manifestation of this fact is that the curves-of-growth (CG) of CO lines show an unpredictable upturn at logW/nu = -4.75. Similar unusual behaviors of empirical CG are confirmed in other red (super)giant stars, and it looks as if the CG is a hybrid of at least two components of different origins. Although strong lines of the CO fundamentals observed in Arcturus show strengthening compared with the predicted photospheric spectrum, the weaker lines show slight weakening, and we interpret these results as due to absorption/emission by the molecular clouds formed in the extended atmosphere. Now do clouds exist in stellar atmospheres? It is by no means easy to answer this question by spectroscopic observations alone, but we find several arguments in favor of such a possibility in Arcturus by analyzing the CO lines. In cooler (super)giant stars in which CO lines show similar unusual behaviors as in Arcturus, the presence of molecular clouds in the outer atmospheres was demonstrated by direct observations with spatial interferometry. We suggest that the formation of molecular clouds is a general feature in cool luminous stars from early K to late M (super)giant stars.

astro-ph.SR↗

arXiv2008

A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres

The atmospheres of substellar objects contain clouds of oxides, iron, silicates, and other refractory condensates. Water clouds are expected in the coolest objects. The opacity of these `dust' clouds strongly affects both the atmospheric temperature-pressure profile and the emergent flux. Thus any attempt to model the spectra of these atmospheres must incorporate a cloud model. However the diversity of cloud models in atmospheric simulations is large and it is not always clear how the underlying physics of the various models compare. Likewise the observational consequences of different modeling approaches can be masked by other model differences, making objective comparisons challenging. In order to clarify the current state of the modeling approaches, this paper compares five different cloud models in two sets of tests. Test case 1 tests the dust cloud models for a prescribed L, L--T, and T-dwarf atmospheric (temperature T, pressure p, convective velocity vconv)-structures. Test case 2 compares complete model atmosphere results for given (effective temperature Teff, surface gravity log g). All models agree on the global cloud structure but differ in opacity-relevant details like grain size, amount of dust, dust and gas-phase composition. Comparisons of synthetic photometric fluxes translate into an modelling uncertainty in apparent magnitudes for our L-dwarf (T-dwarf) test case of 0.25 < Δm < 0.875 (0.1 < Δm M 1.375) taking into account the 2MASS, the UKIRT WFCAM, the Spitzer IRAC, and VLT VISIR filters with UKIRT WFCAM being the most challenging for the models. (abr.)

astro-ph↗

arXiv2007

Comparison of cloud models for Brown Dwarfs

A test case comparison is presented for different dust cloud model approaches applied in brown dwarfs and giant gas planets. We aim to achieve more transparency in evaluating the uncertainty inherent to theoretical modelling. We show in how far model results for characteristic dust quantities vary due to different assumptions. We also demonstrate differences in the spectral energy distributions resulting from our individual cloud modelling in 1D substellar atmosphere simulations

astro-ph↗

arXiv2004

Spectral Classification Beyond M

Significant populations of field L and T dwarfs are now known, and we anticipate the discovery of even cooler dwarfs by Spitzer and ground-based infrared surveys. However, as the number of known L and T dwarfs increases so does the range in their observational properties, and difficulties have arisen in interpreting the observations. Although modellers have made significant advances, the complexity of the very low temperature, high pressure, photospheres means that problems remain such as the treatment of grain condensation as well as incomplete and non-equilibrium molecular chemistry. Also, there are several parameters which control the observed spectral energy distribution - effective temperature, grain sedimentation efficiency, metallicity and gravity - and their effects are not well understood. In this paper, based on a splinter session, we discuss classification schemes for L and T dwarfs, their dependency on wavelength, and the effects of the parameters T_eff, f_sed, [m/H] and log g on optical and infrared spectra. We will discuss the various hypotheses that have been presented for the transition from the dusty L types to the clear atmosphere T types. We conclude with a brief discussion of the spectral class beyond T. Authors of each Section are identified by their initials.

astro-ph↗

arXiv2000

Water in Emission in the ISO Spectrum of the Early M Supergiant Star mu Cephei

We report a detection of water in emission in the spectrum of the M2 supergiant atar mu Cep (M2Ia) observed by the Short Wavelength Spectrometer (SWS) aboard Infrared Space Observatory (ISO) and now released as the ISO Archives. The emission first appears in the 6 micron region (nu2 fundamental) and then in the 40 micron region (pure rotation lines) despite the rather strong dust emission. The intensity ratios of the emission features are far from those of the optically thin gaseous emission. Instead, we could reproduce the major observed emission features by an optically thick water sphere of the inner radius about two stellar radii (1300Rsun), Tex = 1500K, and Ncol (H2O) = 3.0E+20/cm2. This model also accounts for the H2O absorption bands in the near infrared (1.4, 1.9, and 2.7 micron) as well. The detection of water in emission provides strong constraints on the nature of water in the early M supergiant stars, and especially its origin in the outer atmosphere is confirmed against other models such as the large convective cell model. We finally confirm that the early M supergiant star is surrounded by a huge optically thick sphere of the warm water vapor, which may be referred to as MOLsphere for simplicity. Thus, the outer atmosphere of M supergiant stars should have a complicated hierarchical and/or hybrid structure with at least three major constituents including the warm MOLsphere (T about 1.0E+3K) together with the previously known hot chromosphere (T about 1.0E+4K) and cool expanding gas-dust envelope (T about 1.0E+2K).

astro-ph↗