arXiv subjects

T. Goto

Publications and source records attributed to T. Goto.

At least 91 records · Page 5Linked to original sources

arXiv2006

Collinear-to-Spiral Spin Transformation without Changing Modulation Wavelength upon Ferroelectric Transition in Tb1-xDyxMnO3

Lattice modulation and magnetic structures in magnetoelectric compounds Tb1-xDyxMnO3 have been studied around the ferroelectric (FE) Curie temperature T_C by x-ray and neutron diffraction. Temperature-independent modulation vectors through T_C are observed for the compounds with 0.50< x < 0.68. This indicates that ferroelectricity with a polarization (P) along the c axis in the RMnO3 series cannot be ascribed to such an incommensurate-commensurate transition of an antiferromagnetic order as was previously anticipated. Neutron diffraction study of a single crystal with x=0.59 shows that the FE transition is accompanied by the transformation of the Mn-spin alignment from sinusoidal (collinear) antiferromagnetism into a transverse spiral structure. The observed spiral structure below T_C is expected to produce P along the c axis with the `inverse' Dzialoshinski-Moriya interaction, which is consistent with the observation.

cond-mat.str-el↗

arXiv2005

An Overdensity of Galaxies near the Most Distant Radio-Loud Quasar

A five square arcminute region around the luminous radio-loud quasar SDSS J0836+0054 (z=5.8) hosts a wealth of associated galaxies, characterized by very red (1.3 < i_775 - z_{850} < 2.0) color. The surface density of these z~5.8 candidates is approximately six times higher than the number expected from deep ACS fields. This is one of the highest galaxy overdensities at high redshifts, which may develop into a group or cluster. We also find evidence for a substructure associated with one of the candidates. It has two very faint companion objects within two arcseconds, which are likely to merge. The finding supports the results of a recent simulation that luminous quasars at high redshifts lie on the most prominent dark-matter filaments and are surrounded by many fainter galaxies. The quasar activity from these regions may signal the buildup of a massive system.

astro-ph↗

arXiv2005

Change in lattice modulation upon gigantic magnetoelectric transition in GdMnO3 and TbMnO3

Single-crystal synchrotron x-ray diffraction measurements in strong magnetic fields have been performed for magnetoelectric compounds GdMnO3 and TbMnO3. It has been found that the P || a ferroelectric phase induced by the application of a magnetic field at low temperatures is characterized by commensurate lattice modulation along the orthorhombic b axis with q = 1/2 and q = 1/4. The lattice modulation is ascribed to antiferromagnetic spin alignment with a modulation vector of (0 1/4 1). The change of the spin structure is directly correlated with the magnetic-field-induced electric phase transition, because any commensurate spin modulation with (0 1/4 1) should break glide planes normal to the a axis of the distorted perovskite with the Pbnm space group.

cond-mat.str-el↗

arXiv2005

The Discovery Potential of a Super B Factory

The Proceedings of the 2003 SLAC Workshops on flavor physics with a high luminosity asymmetric e+e- collider. The sensitivity of flavor physics to physics beyond the Standard Model is addressed in detail, in the context of the improvement of experimental measurements and theoretical calculations.

hep-ph↗

arXiv2005

A Dynamical Simulation of the Debris Disk Around HD 141569A

We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a fly-by encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk of initial radius 400 AU, a gas mass of 50 M_earth, and beta = 4 and suggests that the system is being observed within 4000 yr of the fly-by periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5 M_Jup planet in an eccentric orbit on the planetesimal distribution of HD 141569A.

astro-ph↗

arXiv2005

The Morphology - Density Relation in z ~ 1 Clusters

We measure the morphology--density relation (MDR) and morphology-radius relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed with the Advanced Camera for Surveys on board the Hubble Space Telescope. Simulations and independent comparisons of ourvisually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to zmag = 24, corresponding to L/L* = 0.21 and 0.30 at z = 0.83 and z = 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that observed at z ~ 0, consistent with recent work -- specifically, the growth in the bulge-dominated galaxy fraction, f_E+SO, with increasing density proceeds less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and density <= 500 galaxies/Mpc^2, we find = 0.72 +/- 0.10. At z ~ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities >= 40 galaxies/Mpc^2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Spiral+Irr galaxies relative to the local galaxy population. The Elliptical fraction - density relation exhibits no significant evolution between z = 1 and z = 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

astro-ph↗

arXiv2004

The Transformation of Cluster Galaxies at Intermeidate Redshift

We combine imaging data from the Advanced Camera for Surveys (ACS) with VLT/FORS optical spectroscopy to study the properties of star-forming galaxies in the z=0.837 cluster CL0152-1357. We have morphological information for 24 star-forming cluster galaxies, which range in morphology from late-type and irregular to compact early-type galaxies. We find that while most star-forming galaxies have $r_{625}-i_{775}$ colors bluer than 1.0, eight are in the red cluster sequence. Among the star-forming cluster population we find five compact early-type galaxies which have properties consistent with their identification as progenitors of dwarf elliptical galaxies. The spatial distribution of the star-forming cluster members is nonuniform. We find none within $R\sim 500$ Mpc of the cluster center, which is highly suggestive of an intracluster medium interaction. We derive star formation rates from [OII] $λλ3727$ line fluxes, and use these to compare the global star formation rate of CL0152-1357 to other clusters at low and intermediate redshifts. We find a tentative correlation between integrated star formation rates and $T_{X}$, in the sense that hotter clusters have lower integrated star formation rates. Additional data from clusters with low X-ray temperatures is needed to confirm this trend. We do not find a significant correlation with redshift, suggesting that evolution is either weak or absent between z=0.2-0.8.

astro-ph↗

arXiv2004

The Luminosity Function of Early-Type Galaxies at z~0.75

We measure the luminosity function of morphologically selected E/S0 galaxies from $z=0.5$ to $z=1.0$ using deep high resolution Advanced Camera for Surveys imaging data. Our analysis covers an area of $48\Box\arcmin$ (8$\times$ the area of the HDF-N) and extends 2 magnitudes deeper ($I\sim24$ mag) than was possible in the Deep Groth Strip Survey (DGSS). At $0.5 1.7$, E/S0 galaxies at brighter luminosities ($M_B<-20.1$), but are increasingly different at fainter magnitudes where `blue' galaxies are both smaller and have lower Sersic parameters. Fits of the colors to stellar population models suggest that most E/S0 galaxies have short star-formation time scales ($τ<1$ Gyr), and that galaxies have formed at an increasing rate from $z\sim8$ until $z\sim2$ after which there has been a gradual decline.

astro-ph↗

arXiv2004

Physics at Super B Factory

This report presents the results of studies that investigate the physics reach at a Super B factory, an asymmetric-energy e^+e^- collider with a design luminosity of 5 x 10^35 cm^-2s^-1, which is around 40 times as large as the peak luminosity achieved by the KEKB collider. The studies focus on flavor physics and CP violation measurements that could be carried out in the LHC era. The physics motivation, key observables, measurement methods and expected precisions are presented. The sensitivity studies are a part of the activities associated with the preparation of a Letter of Intent for SuperKEKB, which has been submitted recently.

hep-ex↗

arXiv2004

Study of the slepton non-universality at the CERN Large Hadron Collider

In supersymmetric theory, the sfermion-fermion-gaugino interactions conserve the chirality of (s)fermions. The effect appears as the charge asymmetry in $m(jl)$ distributions at the CERN Large Hadron Collider where jets and leptons arise from the cascade decay $\tilde{q} \to q \tildeχ^0_2 \to ql\tilde{l}$. Furthermore, the decay branching ratios and the charge asymmetries in $m(jl)$ distributions are flavor non-universal due to the $\tilde{l}_L$ and $\tilde{l}_R$ mixing. When $\tanβ$ is large, the non-universality between $e$ and $μ$ becomes $O(10)%$ level. We perform a Monte Carlo simulation for some minimal supergravity benchmark points to demonstrate the detectability.

hep-ph↗

arXiv2004

Ultra Compact Dwarf galaxies in Abell 1689: a photometric study with the ACS

The properties of Ultra Compact Dwarf (UCD) galaxy candidates in Abell 1689 (z=0.183) are investigated, based on deep high resolution ACS images. A UCD candidate has to be unresolved, have i<28 (M_V<-11.5) mag and satisfy color limits derived from Bayesian photometric redshifts. We find 160 UCD candidates with 22 26.8 mag, the radial and luminosity distribution of the UCD candidates can be explained well by Abell 1689's globular cluster (GC) system. For i<26.8 mag, there is an overpopulation of 15 +/- 5 UCD candidates with respect to the GC luminosity function. For i<26 mag, the radial distribution of UCD candidates is more consistent with the dwarf galaxy population than with the GC system of Abell 1689. The UCD candidates follow a color-magnitude trend with a slope similar to that of Abell 1689's genuine dwarf galaxy population, but shifted fainter by about 2-3 mag. Two of the three brightest UCD candidates (M_V ~ -17 mag) are slightly resolved. At the distance of Abell 1689, these two objects would have King-profile core radii of ~35 pc and r_eff ~300 pc, implying luminosities and sizes 2-3 times those of M32's bulge. Additional photometric redshifts obtained with late type stellar and elliptical galaxy templates support the assignment of these two resolved sources to Abell 1689. Our findings imply that in Abell 1689 there are at least 10 UCDs with M_V<-12.7 mag. Compared to the UCDs in the Fornax cluster they are brighter, larger and have colors closer to normal dwarf galaxies. This suggests that they may be in an intermediate stage of the stripping process. Spectroscopy is needed to definitely confirm the existence of UCDs in Abell 1689.

astro-ph↗

arXiv2004

Magnetization plateaux of S = 1/2 two-dimensional frustrated antiferromagnet Cs$_2$CuBr$_4$

The field induced magnetic phase transitions of Cs$_2$CuBr$_4$ were investigated by means of magnetization process and neutron scattering experiments. This system undergoes magnetic phase transition at Neél temperature $T_\mathrm{N}=1.4$ K at zero field, and exhibits the magnetization plateau at approximately one third of the saturation magnetization for the field directions $H\parallel b$ and $H\parallel c$. In the present study, additional symptom of the two-third magnetization plateau was found in the field derivative of the magnetization process. The magnetic structure was found to be incommensurate with the ordering vector $\boldsymbol{Q}=(0, 0.575, 0)$ at zero field. With increasing magnetic field parallel to the c-axis, the ordering vector increases continuously and is locked at $\boldsymbol{Q}=(0, 0.662, 0)$ in the plateau field range $13.1 \mathrm{T} < H < 14.4 \mathrm{T}$. This indicates that the collinear \textit{up-up-down} spin structure is stabilized by quantum fluctuation at the magnetization plateau.

cond-mat.str-el↗

arXiv2003

Observations and Theoretical Implications of the Large Separation Lensed Quasar SDSS J1004+4112

We study the recently discovered gravitational lens SDSS J1004+4112, the first quasar lensed by a cluster of galaxies. It consists of four images with a maximum separation of 14.62''. The system has been confirmed as a lensed quasar at z=1.734 on the basis of deep imaging and spectroscopic follow-up observations. We present color-magnitude relations for galaxies near the lens plus spectroscopy of three central cluster members, which unambiguously confirm that a cluster at z=0.68 is responsible for the large image separation. We find a wide range of lens models consistent with the data, but they suggest four general conclusions: (1) the brightest cluster galaxy and the center of the cluster potential well appear to be offset by several kpc; (2) the cluster mass distribution must be elongated in the North--South direction, which is consistent with the observed distribution of cluster galaxies; (3) the inference of a large tidal shear (~0.2) suggests significant substructure in the cluster; and (4) enormous uncertainty in the predicted time delays between the images means that measuring the delays would greatly improve constraints on the models. We also compute the probability of such large separation lensing in the SDSS quasar sample, on the basis of the CDM model. The lack of large separation lenses in previous surveys and the discovery of one in SDSS together imply a mass fluctuation normalization σ_8=1.0^{+0.4}_{-0.2} (95% CL), if cluster dark matter halos have an inner slope -1.5. Shallower profiles would require higher values of σ_8. Although the statistical conclusion might be somewhat dependent on the degree of the complexity of the lens potential, the discovery is consistent with the predictions of the abundance of cluster-scale halos in the CDM scenario. (Abridged)

astro-ph↗

arXiv2003

Star formation rate indicators in the Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) first data release provides a database of 106000 unique galaxies in the main galaxy sample with measured spectra. A sample of star-forming (SF) galaxies are identified from among the 3079 of these having 1.4 GHz luminosities from FIRST, by using optical spectral diagnostics. Using 1.4 GHz luminosities as a reference star formation rate (SFR) estimator insensitive to obscuration effects, the SFRs derived from the measured SDSS Halpha, [OII] and u-band luminosities, as well as far-infrared luminosities from IRAS, are compared. It is established that straightforward corrections for obscuration and aperture effects reliably bring the SDSS emission line and photometric SFR estimates into agreement with those at 1.4 GHz, although considerable scatter (~60%) remains in the relations. It thus appears feasible to perform detailed investigations of star formation for large and varied samples of SF galaxies through the available spectroscopic and photometric measurements from the SDSS. We provide herein exact prescriptions for determining the SFR for SDSS galaxies. The expected strong correlation between [OII] and Halpha line fluxes for SF galaxies is seen, but with a median line flux ratio F_[OII]/F_Halpha=0.23, about a factor of two smaller than that found in the sample of Kennicutt (1992). This correlation, used in deriving the [OII] SFRs, is consistent with the luminosity-dependent relation found by Jansen et al. (2001). The median obscuration for the SDSS SF systems is found to be A_Halpha=1.2 mag, while for the radio detected sample the median obscuration is notably higher, 1.6 mag, and with a broader distribution.

astro-ph↗

arXiv2003

Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality

We present a comprehensive study of the coupled tetrahedra-compound Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T_N. This energy is determined experimentally. The ordering temperature T_N has an inverse-log dependence on the coupling parameters near quantum criticality.

cond-mat.str-el↗

arXiv2002

High-field magnetization of the 3d heavy-fermion system LiV$_2$O$_{4-d}$ (d = 0, 0.08)

Metamagnetic behavior has been observed in LiV2O4 powder sample around 38 T at 4.2 K. On the other hand, magnetization for oxygen deficient LiV2O3.92 shows no indication of metamagnetism up to 40 T, and shows substantially reduced magnetic moment compared to that of LiV2O4. These results suggest that ferromagnetic interaction is strongly enhanced by magnetic fields in LiV2O4, whereas antiferromagnetic interaction is dominant in LiV2O3.92.

cond-mat.str-el↗

arXiv2002

Studies of transverse and longitudinal relaxations of $^{55}$Mn in molecular cluster magnet Mn$_{12}$Ac

The transverse and longitudinal relaxation rates 1/$T_2$ and 1/$T_1$ of $^{55}$Mn in molecular cluster magnet Mn$_{12}$Ac have been measured al low temperatures down to 200mK and in the fields upto 9T. Both of 1/$T_2$ and 1/$T_1$ exhibit remarkable decreases with decreasing temperature and with increasing field, with the relative relation $T_1/T_2 \approx 200$. In the analysis, we adopt a simple model that the thermal fluctuation of the cluster spin $S$=10 associated with the spin-phonon interactionis, is only due to the excitation to the first excited state from the ground state with the average life-times $τ_ 1$ and $τ_0$ ($τ_ 0$$\gg$$τ_1$). We show that 1/$T_2$ is interpreted in terms of the strong collision regime as given by 1/$τ_ 0$, and that 1/$T_1$ is understood by the high-frequency limit based on standard perturbation treatment for the step-wise fluctuating field, thus being proportional to 1/$τ_0ω_N^2$.

cond-mat↗

arXiv2002

Anomalous broadening of the spin-flop transition in the reentrant spin-glass phase of La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.018$)

The magnetization in a lightly doped La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.018$) single crystal was measured. Spin-flop transition was clearly observed in the hole doped antiferromagnetically ordered state under increasing magnetic fields perpendicular to the CuO$_2$ plane. In the spin-glass phase below 25K, the spin-flop transition becomes broad but the step in the magnetization curve associated with the transition remains finite at the lowest temperature. We show in this report that, at low temperature, the homogeneous antiferromagnetic order is disturbed by the re-distribution of holes, and that the spatial variance of the local hole concentration around $x=0.018$ increases.

cond-mat.str-el↗