Searcharxiv⌕ Search

arXiv subjects

H. Tanaka

Publications and source records attributed to H. Tanaka.

At least 127 records · Page 7Linked to original sources
arXiv2013

Ghost modes and continuum scattering in the dimerized distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$

High intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence bond solid state of the distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$. The polarization of the dominant dispersive modes (2 meV $<\hbarω< $7 meV) is determined and found consistent with a dimer series expansion with strong Dzyaloshinskii-Moriya interactions ($D/J=0.18$). A weakly dispersive mode near 5 meV and shifted "ghosts" of the main modes are attributed to the enlarged unit cell below a $T=215$ K structural transition. Continuum scattering between 8 meV and 10 meV might be interpreted as a remnant of the kagome spinon continuum [T.-H. Han et al., Nature 492, 406 (2012)]

cond-mat.str-el↗
arXiv2013

Calibration of the Super-Kamiokande Detector

Procedures and results on hardware level detector calibration in Super-Kamiokande (SK) are presented in this paper. In particular, we report improvements made in our calibration methods for the experimental phase IV in which new readout electronics have been operating since 2008. The topics are separated into two parts. The first part describes the determination of constants needed to interpret the digitized output of our electronics so that we can obtain physical numbers such as photon counts and their arrival times for each photomultiplier tube (PMT). In this context, we developed an in-situ procedure to determine high-voltage settings for PMTs in large detectors like SK, as well as a new method for measuring PMT quantum efficiency and gain in such a detector. The second part describes the modeling of the detector in our Monte Carlo simulation, including in particular the optical properties of its water target and their variability over time. Detailed studies on the water quality are also presented. As a result of this work, we achieved a precision sufficient for physics analysis over a wide energy range (from a few MeV to above a TeV). For example, the charge determination was understood at the 1% level, and the timing resolution was 2.1 nsec at the one-photoelectron charge level and 0.5 nsec at the 100-photoelectron charge level.

physics.ins-det↗
arXiv2013

Microscopic properties of the "pinwheel" kagome compound Rb_2Cu_3SnF_{12}

Using $^{63,65}$Cu nuclear magnetic resonance (NMR) in magnetic fields up to 30 T we study the microscopic properties of the 12-site valence-bond-solid ground state in the "pinwheel" kagome compound Rb$_2$Cu$_3$SnF$_{12}$. We find that the ground state is characterized by a strong transverse staggered spin polarization whose temperature and field dependence points to a mixing of the singlet and triplet states. This is further corroborated by the field dependence of the gap $Δ(H)$, which has a level anticrossing with a large minimum gap value of $\approx Δ(0)/2$, with no evidence of a phase transition down to 1.5\,K. By the exact diagonalization of small clusters, we show that the observed anticrossing is mainly due to staggered tilts of the $g$-tensors defined by the crystal structure, and reveal symmetry properties of the low-energy excitation spectrum compatible with the absence of level crossing.

cond-mat.str-el↗
arXiv2013

Neutrinos from Stored Muons nuSTORM: Expression of Interest

The nuSTORM facility has been designed to deliver beams of electron and muon neutrinos from the decay of a stored muon beam with a central momentum of 3.8 GeV/c and a momentum spread of 10%. The facility is unique in that it will: serve the future long- and short-baseline neutrino-oscillation programmes by providing definitive measurements of electron-neutrino- and muon-neutrino-nucleus cross sections with percent-level precision; allow searches for sterile neutrinos of exquisite sensitivity to be carried out; and constitute the essential first step in the incremental development of muon accelerators as a powerful new technique for particle physics. Of the world's proton-accelerator laboratories, only CERN and FNAL have the infrastructure required to mount nuSTORM. Since no siting decision has yet been taken, the purpose of this Expression of Interest (EoI) is to request the resources required to: investigate in detail how nuSTORM could be implemented at CERN; and develop options for decisive European contributions to the nuSTORM facility and experimental programme wherever the facility is sited. The EoI defines a two-year programme culminating in the delivery of a Technical Design Report.

physics.acc-ph↗
arXiv2012

The magnetization process of the spin-one triangular-lattice Heisenberg antiferromagnet

We apply the coupled cluster method and exact diagonalzation to study the uniform susceptibility and the ground-state magnetization curve of the triangular-lattice spin-1 Heisenberg antiferromagnet. Comparing our theoretical data for the magnetization curve with recent measurements on the s=1 triangular lattice antiferromagnet Ba3NiSb2O9 we find a very good agreement.

cond-mat.str-el↗
arXiv2012

The separation of the chiral and deconfinement phase transitions in the curved space-time

We calculated the chiral condensate and the dressed Polyakov loop in the space-time $R\times S^{3}$ and $R\times H^{3}$. The chiral condensate is the order parameter for the chiral phase transition, and the dressed Polyakov loop is the order parameter for the deconfinement phase transition. When there is a current mass, critical points for the chiral and deconfinement phase transitions are different in the crossover region. We show that the difference is changed by the gravitational effect.

hep-ph↗
arXiv2012

Longitudinal magnetic excitation in KCuCl3 studied by Raman scattering under hydrostatic pressures

We measure Raman scattering in an interacting spin-dimer system KCuCl3 under hydrostatic pressures up to 5 GPa mediated by He gas. In the pressure-induced quantum phase, we observe a one-magnon Raman peak, which originates from the longitudinal magnetic excitationand is observable through the second-order exchange interaction Raman process. We report the pressure dependence of the frequency, halfwidth and Raman intensity of this mode.

cond-mat.str-el↗
arXiv2012

GRB 091208B: First Detection of the Optical Polarization in Early Forward Shock Emission of a Gamma-Ray Burst Afterglow

We report that the optical polarization in the afterglow of GRB 091208B is measured at t = 149 - 706 s after the burst trigger, and the polarization degree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a power-law decay with index -0.75 +/- 0.02, which is interpreted as the forward shock synchrotron emission, and thus this is the first detection of the early-time optical polarization in the forward shock (rather than that in the reverse shock reported by Steele et al. (2009). This detection disfavors the afterglow model in which the magnetic fields in the emission region are random on the plasma skin depth scales, such as amplified by the plasma instabilities, e.g., Weibel instability. We suggest that the fields are amplified by the magnetohydrodynamic instabilities, which would be tested by future observations of the temporal changes of the polarization degrees and angles for other bursts.

astro-ph.HE↗
arXiv2012

Towards the Realization of Higher Connectivity in MgB2 Conductors: In-situ or Sintered Ex-situ?

The two most common types of MgB2 conductor fabrication technique - in-situ and ex-situ - show increasing conflicts concerning the connectivity, an effective current-carrying cross-sectional area. An in-situ reaction yields a strong intergrain coupling with a low packing factor, while an ex-situ process using pre-reacted MgB2 yields tightly packed grains, however, their coupling is much weaker. We studied the normal-state resistivity and microstructure of ex-situ MgB2 bulks synthesized with varied heating conditions under ambient pressure. The samples heated at moderately high temperatures of ~900°C for a long period showed an increased packing factor, a larger intergrain contact area and a significantly decreased resistivity, all of which indicate the solid-state self-sintering of MgB2. Consequently the connectivity of the sintered ex-situ samples exceeded the typical connectivity range 5-15% of the in-situ samples. Our results show self-sintering develops the superior connectivity potential of ex-situ MgB2, though its intergrain coupling is not yet fulfilled, to provide a strong possibility of twice or even much higher connectivity in optimally sintered ex-situ MgB2 than in in-situ MgB2.

cond-mat.supr-con↗
arXiv2011

The effect of an imaginary part of the Schwinger-Dyson equation at finite temperature and density

We examined the effect of an imaginary part of the ladder approximation Schwinger-Dyson equation. We show the imaginary part enhances the effect of the first order transition, and affects a tricritical point. In particular, a chemical potential at a tricritical point is moved about 200(MeV). Thus, one should not ignore the imaginary part. On the other hand, since an imaginary part is small away from a tricritical point, one should be able to ignore an imaginary part. In addition, we also examined the contribution of the wave function renormalization constant.

hep-ph↗
arXiv2011

Gauge Parameter Dependence of the 1-loop Fermion Self Energy at Finite Temperature

We show a gauge parameter dependence of the 1-loop fermion self energy at finite temperature before the analytic continuation. We also show a gauge parameter dependence of only the temperature dependence term. The result is the same as the ladder approximation Schwinger-Dyson equation. The wave function renormalization constant approaches 1 by taking a negative gauge parameter. The analogy of the ladder approximation Schwinger-Dyson equation and the 1-loop calculation will help the analysis of the phase transition.

hep-ph↗
arXiv2011

Spectral Functions at finite temperature and chemical potential

There are two formulations at non-zero chemical potential; one is the formulation that a Lagrangian includes a chemical potential, the other is the formulation that a Lagrangian does not include a chemical potential. The existence of two formulations makes a calculation complicated. The results from those formulations are not corresponding directly. This discrepancy exists in the imaginary time formalism and the real time formalism. However, since this is essentially caused by a difference of a frequency, the discrepancy vanishes by modifying the Fourier transform. We show a calculational procedure with a spectral function to understand this.

hep-ph↗
arXiv2011

Real-time terahertz near-field microscope

Terahertz (THz) waves have been significantly developed in the last fifteen years because of their great potential for applications in industrial and scientific communities1,2. The unique properties of THz waves as transparency for numerous materials and strong absorption for water-based materials are expected to broadly impact biosensing3 such as medical imaging4, chemical identifications5, and DNA recognition6. In particular, for accessing information within a scale comparable to the cell size where interactions between cell membrane and other organelle structures occur, micron size spatial resolution is required. Due to the large wavelength, however, the joint capability of THz near-field imaging with real-time acquisition, which is a highly desirable ability for observing real-time changes of in vivo sample, remains undone. Here, we report a real-time THz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second. We achieve high spatial resolution on a large area by combining two novel techniques: THz pulse generation by tilted-pulse-front excitation7 and electro-optic (EO) balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a THz pulse. Our results are the first demonstration of a direct quantification of a 2-dimensional subwavelength THz electric field taken in real-time.

physics.optics↗
arXiv2010

Pinwheel VBS state and triplet excitations in the two-dimensional deformed kagome lattice

Determining ground states of correlated electron systems is fundamental to understanding novel phenomena in condensed matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global topology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states, potentially prompting the emergence of quantum spin states, such as resonating valence bond (RVB) and valence bond solid (VBS). Although theoretically proposed to exist in a kagome lattice -- one of the most highly frustrated lattices in two dimensions (2D) being comprised of corner-sharing triangles -- such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the "pinwheel" VBS ground state in the S=1/2 deformed kagome lattice antiferromagnet Rb2Cu3SnF12. In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.

cond-mat.str-el↗
arXiv2010

Elementary excitations and specific heat in quantum sine-Gordon spin chain KCuGaF6

Elementary excitations of an S = 1/2 antiferromagnetic Heisenberg chain KCuGaF6 were investigated through specific heat and electron spin resonance (ESR) measurements. In this compound, a staggered field is induced perpendicular to the external field because of the alternating g tensor and the Dzyaloshinsky-Moriya interaction with an alternating D vector. Such a spin system can be mapped onto the quantum sine-Gordon (SG) model, when subjected to the external magnetic field. Specific heat shows clear evidence of the field-induced gap, which is related to the elementary excitations, solitons and breathers, characteristic of the quantum SG model. q = 0 excitations originated from solitons and breathers were directly observed by high-frequency high-field ESR. These experimental results are well described by the quantum SG field theory.

cond-mat.str-el↗
arXiv2009

Electrical Detection and Magnetic-Field Control of Spin States in Phosphorus-Doped Silicon

Electron paramagnetic resonance of ensembles of phosphorus donors in silicon has been detected electrically with externally applied magnetic fields lower than 200 G. Because the spin Hamiltonian was dominated by the contact hyperfine term rather than by the Zeeman terms at such low magnetic fields, superposition states $ α| \uparrow \downarrow >+β| \downarrow \uparrow >$ and $-β| \uparrow \downarrow > + α| \downarrow \uparrow >$ were formed between phosphorus electron and nuclear spins, and electron paramagnetic resonance transitions between these superposition states and $| \uparrow \uparrow >$ or $| \downarrow \downarrow >$ states are observed clearly. A continuous change of $α$ and $β$ with the magnetic field was observed with a behavior fully consistent with theory of phosphorus donors in silicon.

cond-mat.other↗
arXiv2009

Early phase observations of extremely luminous Type Ia Supernova 2009dc

We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is $Δm_{15}(B)=0.65\pm 0.03$, which is one of the slowest among SNe Ia. The peak $V$-band absolute magnitude is $M_{V}=-19.90\pm 0.15$ mag even if the host extinction is $A_{V}=0$ mag. It reaches $M_{V}=-20.19\pm 0.19$ mag for the host extinction of $A_{V}=0.29$ mag as inferred from the observed Na {\sc i} D line absorption in the host. Our $JHK_{s}$-band photometry shows that the SN is one of the most luminous SNe Ia also in near-infrared wavelengths. These results indicate that SN 2009dc belongs to the most luminous class of SNe Ia, like SN 2003fg and SN 2006gz. We estimate the ejected $^{56}$Ni mass of $1.2\pm 0.3$ $\Msun$ for no host extinction case (or 1.6$\pm$ 0.4 M$_{\odot}$ for the host extinction of $A_{V}=0.29$ mag). The C {\sc ii} $λ$6580 absorption line keeps visible until a week after maximum, which diminished in SN 2006gz before its maximum brightness. The line velocity of Si {\sc ii} $λ$6355 is about 8000 km s$^{-1}$ around the maximum, being considerably slower than that of SN 2006gz, while comparable to that of SN 2003fg. The velocity of the C {\sc ii} line is almost comparable to that of the Si {\sc ii}. The presence of the carbon line suggests that thick unburned C+O layers remain after the explosion. SN 2009dc is a plausible candidate of the super-Chandrasekhar mass SNe Ia.

astro-ph.HE↗