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S. Sasaki

Publications and source records attributed to S. Sasaki.

At least 55 records · Page 3Linked to original sources
arXiv2005

Summing up Non-anti-commutative Kaehler potential

We offer a simple non-perturbative formula for the component action of a generic N=1/2 supersymmetric chiral model in terms of an arbitrary number of chiral superfields in four dimensions, which is obtained by the Non-Anti-Commutative (NAC) deformation of a generic four-dimensional N=1 supersymmetric non-linear sigma-model described by arbitrary Kaehler superpotential and scalar superpotential. The auxiliary integrations responsible for fuzziness are eliminated in the case of a single chiral superfield. The scalar potential in components is derived by eliminating the auxiliary fields. The NAC-deformation of the CP(1) Kaehler non-linear sigma-model with an arbitrary scalar superpotential is calculated as an example.

hep-th↗
arXiv2005

Electrical pump-and-probe study of spin singlet-triplet relaxation in a quantum dot

Spin relaxation from a triplet excited state to a singlet ground state in a semiconductor quantum dot is studied by employing an electrical pump-and-probe method. Spin relaxation occurs via cotunneling when the tunneling rate is relatively large, confirmed by a characteristic square dependence of the relaxation rate on the tunneling rate. When cotunneling is suppressed by reducing the tunneling rate, the intrinsic spin relaxation is dominated by spin-orbit interaction. We discuss a selection rule of the spin-orbit interaction based on the observed double-exponential decay of the triplet state.

cond-mat.mes-hall↗
arXiv2005

The HST Cosmos Project: Contribution from the Subaru Telescope

The Cosmic Evolution Survey (COSMOS) is a Hubble Space Telescope (HST) treasury project.The COSMOS aims to perform a 2 square degree imaging survey of an equatorial field in $I$(F814W) band, using the Advanced Camera for Surveys (ACS). Such a wide field survey, combined with ground-based photometric and spectroscopic data, is essential to understand the interplay between large scale structure, evolution and formation of galaxies and dark matter. In 2004, we have obtained high-quality, broad band images of the COSMOS field ($B, V, r^\prime, i^\prime,$ and $ z^\prime$) using Suprime-Cam on the Subaru Telescope, and we have started our new optical multi-band program, COSMOS-21 in 2005. Here, we present a brief summary of the current status of the COSMOS project together with contributions from the Subaru Telescope. Our future Subaru program, COSMOS-21, is also discussed briefly.

astro-ph↗
arXiv2005

Non-Anti-Commutative deformation of effective potentials in supersymmetric gauge theories

We studied a nilpotent Non-Anti-Commutative (NAC) deformation of the effective superpotentials in supersymmetric gauge theories, caused by a constant self-dual graviphoton background. We derived the simple non-perturbative formula applicable to any NAC (star) deformed chiral superpotential. It is remarkable that the deformed superpotential is always `Lorentz'-invariant. As an application, we considered the NAC deformation of the pure super-Yang-Mills theory whose IR physics is known to be described by the Veneziano-Yankielowicz superpotential (in the undeformed case). The unbroken gauge invariance of the deformed effective action gives rise to severe restrictions on its form. We found a non-vanishing gluino condensate in vacuum but no further dynamical supersymmetry breaking in the deformed theory.

hep-th↗
arXiv2004

Locating the Warm-Hot Intergalactic Medium in the Simulated Local Universe

We present an analysis of mock spectral observation of warm-hot intergalactic medium (WHIM) using a constrained simulation of the local universe. The simulated map of oxygen emission lines from local WHIM reproduces well the observed structures traced by galaxies in the real local universe. We further attempt to perform mock observations of outer parts of simulated Coma cluster and A3627 adopting the expected performance of DIOS (Diffuse Intergalactic Oxygen Surveyor), which is proposed as a dedicated soft X-ray mission to search for cosmic missing baryons. We find that WHIMs surrounding nearby clusters are detectable with a typical exposure time of a day, and thus constitute realistic and promising targets for DIOS. We also find that an X-ray emitting clump in front of Coma cluster, recently reported in the XMM-Newton observation, has a counterpart in the simulated local universe, and its observed spectrum can be well reproduced in the simulated local universe if the gas temperature is set to the observationally estimated value.

astro-ph↗
arXiv2004

Future Japanese missions for the study of warm-hot intergalactic medium

We present our proposal for a small X-ray mission DIOS (Diffuse Intergalactic Oxygen Surveyor) to perform survey observations of warm-hot intergalactic medium using OVII and OVIII emission lines. This will be proposed to a small satellite program planned by ISAS/JAXA in Japan for a launch in 2008. The instrument consists of an array of TES microcalorimeters with an energy resolution 2 eV, cooled by mechanical coolers. The X-ray telescope will employ 4-stage reflection mirrors with a focal length as short as 70 cm and an angular resolution 2'. In addition to DIOS, we briefly describe the NeXT (New X-ray Telescope) mission, which is a larger Japanese X-ray observatory to be launched in 2010 and plans to explore non-thermal processes in the universe.

astro-ph↗
arXiv2003

Excited nucleon spectrum from lattice QCD with maximum entropy method

We study excited states of the nucleon in quenched lattice QCD with the spectral analysis using the maximum entropy method. Our simulations are performed on three lattice sizes $16^3\times 32$, $24^3\times 32$ and $32^3\times 32$, at $β=6.0$ to address the finite volume issue. We find a significant finite volume effect on the mass of the Roper resonance for light quark masses. After removing this systematic error, its mass becomes considerably reduced toward the direction to solve the level order puzzle between the Roper resonance $N'(1440)$ and the negative-parity nucleon $N^*(1535)$.

hep-lat↗
arXiv2002

Bayesian approach to the first excited nucleon state in lattice QCD

We present preliminary results from the first attempt to reconstruct the spectral function in the nucleon and $Δ$ channels from lattice QCD data using the maximum entropy method (MEM). An advantage of the MEM analysis is to enable us to access information of the excited state spectrum. Performing simulations on two lattice volumes, we confirm the large finite size effect on the first excited nucleon state in the lighter quark mass region.

hep-lat↗
arXiv2002

Study of superconducting properties of MgB2

We synthesized single crystalline and policrystalline MgB_2 under ambient pressures. The single crystals of MgB_2 were of good quality, where the crystal structure refinements were successfully converged with R = 0.020. The specific heat of policrystalline MgB_2 samples has been measured in a temperature range between 2 and 60 K in magnetic field up to 6 T. The measurement gave the coefficient of the linear term in the electronic specific heat, gamma = 3.51 mJ/K^2 mol, and the jump of the specific heat, 2.8 mJ/K^2 mol at 38.5 K. It is shown from the analysis of the specific heat that the electronic specific heat in the superconducting state differs largely from the conventional BCS weak coupling theory. From the results of measurements of the magnetic properties on single crystal samples, we found a sharp superconducting transition at 38 K with transition width Delta Tc = 0.8 K and the superconducting anisotropy ratio gamma increasing from about 1 near Tc to 4.0 at 25 K.

cond-mat.supr-con↗
arXiv2002

A vertical diatomic artificial molecule in the intermediate coupling regime in a parallel and perpendicular magnetic field

We present experimental results for the ground state electrochemical potentials of a few electron semiconductor artificial molecule made by vertically coupling two quantum dots, in the intermediate coupling regime, in perpendicular and parallel magnetic fields up to 5 T. We perform a quantitative analysis based on local-spin density functional theory. The agreement between theoretical and experimental results is good, and the phase transitions are well reproduced.

cond-mat↗
arXiv2000

Electron Cotunneling in a Semiconductor Quantum Dot

We report transport measurements on a semiconductor quantum dot with a small number of confined electrons. In the Coulomb blockade regime, conduction is dominated by cotunneling processes. These can be either elastic or inelastic, depending on whether they leave the dot in its ground state or drive it into an excited state, respectively. We are able to discriminate between these two contributions and show that inelastic events can occur only if the applied bias exceeds the lowest excitation energy. Implications to energy-level spectroscopy are discussed.

cond-mat.mes-hall↗
arXiv2000

Kondo effect in an integer-spin quantum dot

The Kondo effect is a key many-body phenomenon in condensed matter physics. It concerns the interaction between a localised spin and free electrons. Discovered in metals containing small amounts of magnetic impurities, it is now a fundamental mechanism in a wide class of correlated electron systems. Control over single, localised spins has become relevant also in fabricated structures due to the rapid developments in nano-electronics. Experiments have already demonstrated artificial realisations of isolated magnetic impurities at metallic surfaces, nanometer-scale magnets, controlled transitions between two-electron singlet and triplet states, and a tunable Kondo effect in semiconductor quantum dots. Here, we report an unexpected Kondo effect realised in a few-electron quantum dot containing singlet and triplet spin states whose energy difference can be tuned with a magnetic field. This effect occurs for an even number of electrons at the degeneracy between singlet and triplet states. The characteristic energy scale is found to be much larger than for the ordinary spin-1/2 case.

cond-mat.mes-hall↗
arXiv2000

Polarized structure functions from the lattice

We give a brief sketch of lattice structure function calculations and review previous results for the axial coupling $g_A$. We outline a new technique for treating fermions on the lattice that preserves chiral symmetry, domain wall fermions. Finally, we give preliminary results for the nucleon spectrum using this new technique. Remarkably, a large mass splitting between the $N$ and $N^*$, roughly consistent with experiment, is produced in the calculation. These results are encouraging for proposed calculations of nucleon structure functions.

hep-lat↗
arXiv1999

Ellipsoidal deformation of vertical quantum dots

Addition energy spectra at 0 T of circular and ellipsoidally deformed few-electron vertical quantum dots are measured and compared to results of model calculations within spin-density functional theory. Because of the rotational symmetry of the lateral harmonic confining potential, circular dots show a pronounced shell structure. With the lifting of the single- particle level degeneracies, even a small deformation is found to radically alter the shell structure leading to significant modifications in the addition energy spectra. Breaking the circular symmetry with deformation also induces changes in the total spin. This "piezo-magnetic" behavior of quantum dots is discussed, and the addition energies for a set of realistic deformation parameters are provided. For the case of the four-electron ground state at 0 T, a spin-triplet to spin-singlet transition is predicted, i.e. Hund's first rule no longer applies. Application of a magnetic field parallel to the current confirms that this is the case, and also suggests that the anisotropy of an elliptical dot, in practice, may be higher than that suggested by the geometry of the device mesa in which the dot is located.

cond-mat.mes-hall↗
arXiv1998

Instantons and Monopoles for Nonperturbative QCD

We study the nonperturbative QCD in terms of topological objects, i.e., QCD-monopoles and instantons. In the 't Hooft abelian gauge, QCD is reduced into an abelian gauge theory with QCD-monopoles, and the confinement mechanism can be understood with the dual superconductor picture. Regarding the flux-tube ring as the glueball, we study the glueball properties by combining the dual Ginzburg-Landau theory and the Nambu-Goto action. We find the strong correlations between instantons and monopole world-lines both in the continuum theory and in the lattice QCD. The nonperturbative QCD vacuum is filled with instantons and monopoles, and dense instantons generate a macroscopic network of the monopole world-line, which would be responsible for the confinement force in the infrared region.

hep-ph↗
arXiv1996

Monopole Dominance for Dynamical Chiral-Symmetry Breaking in the Dual Ginzburg-Landau Theory

Using the effective potential formalism, we study dynamical chiral symmetry breaking (D$χ$SB) in the dual Ginzburg-Landau (DGL) theory, where the color confinement is brought by monopole condensation. The effective potential as a function of infrared quark mass is found to have double-well structure, which leads to spontaneous breaking of chiral symmetry. To examine the role of confinement, we divide the effective potential into the confinement part and others, which correspond to the confinement term and other (Yukawa, Coulomb) terms of the gluon propagator in the DGL theory. It is found that the confinement part gives the dominating contribution to the \dcsb, which is regarded as monopole dominance for D$χ$SB.

hep-ph↗
arXiv1996

Confinement and Topological Charge in the Abelian Gauge of QCD

We study the relation between instantons and monopoles in the abelian gauge. First, we investigate the monopole in the multi-instanton solution in the continuum Yang-Mills theory using the Polyakov gauge. At a large instanton density, the monopole trajectory becomes highly complicated, which can be regarded as a signal of monopole condensation. Second, we study instantons and monopoles in the SU(2) lattice gauge theory both in the maximally abelian (MA) gauge and in the Polyakov gauge. Using the $16^3 \times 4$ lattice, we find monopole dominance for instantons in the confinement phase even at finite temperatures. A linear-type correlation is found between the total monopole-loop length and the integral of the absolute value of the topological density (the total number of instantons and anti-instantons) in the MA gauge. We conjecture that instantons enhance the monopole-loop length and promote monopole condensation.

hep-lat↗