arXiv subjects

T. Masuda

Publications and source records attributed to T. Masuda.

At least 73 records · Page 4Linked to original sources

arXiv2003

Distribution of exchange energy in a bond-alternating S=1 quantum spin chain

The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP is studied by single crystal inelastic neutron scattering. Parameters of the measured dispersion relation for magnetic excitations are compared to existing numerical results and used to determine the magnitude of bond-strength alternation. The measured neutron scattering intensities are also analyzed using the 1st-moment sum rules for the magnetic dynamic structure factor, to directly determine the modulation of ground state exchange energies. These independently determined modulation parameters characterize the level of spin dimerization in NTENP. First-principle DMRG calculations are used to study the relation between these two quantities.

cond-mat.str-el↗

arXiv2003

Helimagnetism in a disordered and geometrically frustrated quantum spin chain

Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in LiCu$_2$O$_2$, a $S=1/2$ spin-chain compound with competing interactions. The magnetic structure is found to be helimagnetic, with a propagation vector $(0.5,ζ,0)$, $ζ=0.174$. The nearest-neighbor exchange constant and frustration ratio are estimated to be $J=5.8$ meV and $J_2/J_1=0.29$, respectively. For idealized spin chains these parameter values would signify a gapped spin-liquid ground state. Long-range ordering is attributed to intrinsic non-stoichiometry.

cond-mat.str-el↗

arXiv2003

A c*-algebraic framework for quantum groups

We develop a general framework to deal with the unitary representations of quantum groups using the language of C*-algebras. Using this framework, we prove that the duality holds in a general context. This extends the framework of the duality theorem using the language of von Neumann algebras previously developed by Masuda and Nakagami.

math.QA↗

arXiv2003

BiCu$_2$VO$_6$: a new narrow-band spin-gap material

A new spin-ladder family material BiCu$_2$VO$_6$ is studied by means of the magnetic susceptibility, heat capacity and neutron inelastic scattering measurements on powder sample. Singlet ground state and a finite spin gap are confirmed by thermal-activated type susceptibility and by distinct peak at 16 meV in spin excitation. Triple narrow band structure in spin excitation spectrum, probably due to complex crystal structure, is observed and the possibility of weakly-interacting spin-cluster system is discussed.

cond-mat.str-el↗

arXiv2003

Spin waves and the origin of commensurate magnetism in Ba2CoGe2O7

The square-lattice antiferromagnet Ba2CoGe2O7 is studied by means of neutron diffraction and inelastic scattering. This material is isostructural to the well-known Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O7, but exhibits commensurate long-range Neel order at low temperatures. Measurements of the spin wave dispersion relation reveal strong in-plane anisotropy that is the likely reason for the suppression of helimagnetism.

cond-mat.str-el↗

arXiv2002

The Fuzzy S^4 by Quantum Deformation

The fuzzy algebra of S^4 is discussed by quantum deformation. To this end we embed the classical S^4 in the Kaehler coset space SO(5)/U(2). The harmonic functions of S^4 are constructed in terms of the complex coordinates of SO(5)/U(2). Being endowed with the symplectic structure they can be deformed by the Fedosov formalism. We show that they generate the fuzzy algebra \hat A_\infty (S^4) under the * product defined therein, by using the Darboux coordinate system. The fuzzy spheres of higher even dimensions can be discussed similarly. We give basic arguments for the generalization as well.

hep-th↗

arXiv2002

Polarization dependence of spin excitations in BaCu2Si2O7

The polarization dependence of magnetic excitations in the quasi one-dimensional antiferromagnet BaCu2Si2O7 is studied as a function of momentum and energy transfer. The results of inelastic neutron scattering measurements are directly compared to semi-analytical calculations based on the chain-Mean Field and Random Phase approximations. A quantitative agreement between theoretically calculated and experimentally measured dynamic structure factors of transverse spin fluctuations is obtained. In contrast, substantial discrepancies are found for longitudinal polarization. This behavior is attributed to intrinsic limitations of the RPA that ignores correlation effects.

cond-mat.str-el↗

arXiv2002

Dominance of excitation continuum in the longitudinal spectrum of weakly coupled Heisenberg S=1/2 chains

A low-field spin flop transition in the quasi one-dimensional antiferromagnet \ba is exploited to study the polarization dependence of low-energy magnetic excitations. The measured longitudinal spectrum is best described as single broad continuum, with no sharp ``longitudinal mode'', in apparent contradiction with the commonly used chain-MF/RPA theories. The observed behavior is also quite different than that previously seen in the related \kcuf material, presumably due to a large difference in the relative strength of inter-chain interactions. The results highlight the limitations of the chain-MF/RPA approach.

cond-mat.str-el↗

arXiv2002

X-ray Scattering and Magnetic Susceptibility Study of doped CuGeO_3

We report comprehensive synchrotron x-ray scattering and magnetic susceptibility studies of the doped spin-Peierls materials Cu_{1-x}Zn_xGeO_3 and CuGe_{1-y}Si_yO_3. Temperature versus dopant concentration phase diagrams are mapped out for both Zn and Si dopants. The phase diagrams of both Cu_{1-x}Zn_xGeO_3 and CuGe_{1-y}Si_yO_3 closely resemble that of Cu_{1-x}Mg_xGeO_3, including the observation that the spin gap is established at a much higher temperature than the temperature at which the spin-Peierls dimerization attains long-range order. The spin-Peierls transitions in doped samples exhibit unusual phase transition behavior, characterized by highly rounded phase transitions, Lorentzian squared lineshapes, and very long relaxation times. Phenomenological explanations for these observations are given by considering the effects of competing random bond interactions as well as random fields generated by the dopants. We have also confirmed the reentrance of the spin-Peierls phase when the temperature is lowered through the antiferromagnetic ordering transition. The low temperature re-entrance of the spin-Peierls phase has been explained speculatively using a local phase separation scheme between the spin-Peierls phase and the Neel phase.

cond-mat.str-el↗

arXiv2002

Magnetic anisotropy of BaCu2Si2O7: theory and antiferromagnetic resonance

Antiferromagnetic resonance (AFMR) of BaCu2Si2O7 and a microscopic theory of the magnetic anisotropy of spin 1/2 chain compounds with folded CuO3 geometry being in good agreement with the available data are presented. The AFMR studies at 4.2 K show the existence of two gaps (40 and 76 GHz) at zero magnetic field and of two spin re-orientation transitions for H||c. The microscopic origin of the two gaps is shown to be Hund's rule coupling which leads to a "residual anisotropy" beyond the compensation of the Dzyaloshinskii-Moriya term by the symmetric anisotropy which would be valid without Hund's coupling.

cond-mat.str-el↗

arXiv2001

A Study of a Real-Time Operating System on the Intel-Based Vme Controllers

In this paper, we described the performance differences between VME-based controllers, HP743rt/HP-RT and IA-based platforms. We measured some kind of real-time benchmarks and discussed a relation of architecture dependences.

physics.acc-ph↗

arXiv2001

Upgrade of Linac Control System with New Vme Controllers at Spring-8

We integrated an injector linac control system to the SPring-8 standard system on September 2000. As a result of this integration, the SPring-8 accelerator complex was controlled by one unified system. Because the linac was continuously running as the electron beam injector not only for the SPring-8 storage ring but also for New SUBARU, we had to minimize the hardware modification to reduce the time for the development and testing of the new control system. The integration method was almost the same as that of the integration of the booster synchrotron. We report here on the integration of the linac control system with emphasis on the upgrade of the VMEbus controllers and software involving the operating system Solaris 7 as the real-time OS.

physics.acc-ph↗

arXiv2001

Data Acquisition System with Shared Memory Network

The SPring-8 control system provides data acquisition software called Poller/Collector for periodic data taking. Its polling scheme does not support taking data synchronized with an event, and simultaneity in the data from the different VME systems is not guaranteed. Recently, requirement for fast and event-driven data acquisition has increased, and the synchronization of a set of data from the different VME systems is requested. We added new data-taking software to the standard framework with a shared memory network. We designed the software on the VME by updating the original framework that was already used for feedback control. The data filling process on a PC stores sets of the synchronized data on a shared memory board to the database. The shared memory boards provide 250Mbit/s transmission rate and are linked with optical fiber cables. We will install the system to the linac beam position monitors data acquisition as the first application.

physics.acc-ph↗

arXiv2001

Toward a Reliable Gigabit Network - an Upgrade of the Spring-8 Network

The SPring-8 controls network has maintained accelerator operations in high reliability and shown good performance during the past years. To cope with the increase of loads on the network due to faster data acquisition and the addition of equipment data, networking hardware has been installed in the last few years. The upgraded network replaces the original FDDI backbone and switches with mixed FDDI/gigabit ethernet and Layer-3 switches. It is necessary to keep the double ring topology for the FDDI and introduce link aggregation technology for the gigabit ethernet to maintain the full redundancy and bandwidth of the system. This paper will discuss the network performance of the gigabit ethernet including its latency and redundancy. We also discuss a future plan for the network including Quality-of-Service over the gigabit ethernet.

physics.acc-ph↗

arXiv2001

Cu Nuclear Quadrupole Resonance Study of the Spin-Peierls Compound Cu1-xMgxGeO3: A Possibility of Precursory Dimerization

We report on a zero-field 63Cu nuclear quadrupole resonance (NQR) study of nonmagnetic Mg impurity substituted Cu1-xMgxGeO3 (single crystals; the spin-Peierls transition temperature Tsp~14, 13.5, and 11 K for x=0, 0.0043, and 0.020) in a temperature range from 4.2 K to 250 K. We found that below T*~77 K, Cu NQR spectra are broadened and nonexponential Cu nuclear spin-lattice relaxation increases for undoped and more remarkably for Mg-doped samples. The results indicate that random lattice distortion and impurity-induced spins appear below T*, which we associate with a precursor of the spin-Peierls transition. Conventional magnetic critical slowing down does not appear down to 4.2 K below Tsp.

cond-mat.str-el↗

arXiv2001

Two-stage spin-flop transitions in S = 1/2 antiferromagnetic spin chain BaCu_2Si_2O_7

Two-stage spin-flop transitions are observed the in quasi-one-dimensional antiferromagnet, BaCu${}_2$Si${}_2$O${}_7$. A magnetic field applied along the easy axis induces a spin-flop transition at 2.0 T followed by a second transition at 4.9 T. The magnetic susceptibility indicates the presence of Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain neighboring spins. We discuss a possible mechanism whereby the geometrical competition between DM and interchain interactions, as discussed for the two-dimensional antiferromagnet La${}_2$CuO${}_4$, causes the two-stage spin-flop transitions.

cond-mat.str-el↗

arXiv2001

Separation of the magnetic phases at the Néel point in the diluted spin-Peierls magnet CuGeO3

The impurity induced antiferromagnetic ordering of the doped spin-Peierls magnet Cu(1-x)Mg(x)GeO(3) was studied by ESR technique. Crystals with the Mg concentration x<4% demonstrate a coexistence of paramagnetic and antiferromagnetic ESR modes. This coexistence indicates the separation of a macroscopically uniform sample in the paramagnetic and antiferromagnetic phases. In the presence of the long-range spin-Peierls order (in a sample with x=1.71%) the volume of the antiferromagnetic phase immediately below the Néel point T_N is much smaller than the volume of the paramagnetic phase. In the presence of the short-range spin-Peierls order (in samples with x=2.88%, x= 3.2%) there are comparable volumes of paramagnetic and antiferromagnetic phases at T=T_N. The fraction of the antiferromagnetic phase increases with lowering temperature. In the absence of the spin-Peierls dimerization (at x=4.57%)the whole sample exhibits the transition into the antiferromagnetic state and there is no phase separation. The phase separation is explained by the consideration of clusters of staggered magnetization located near impurity atoms. In this model the areas occupied by coherently correlated spins expand with decreasing temperature and the percolation of the ordered area through a macroscopic distance occurs.

cond-mat.str-el↗

arXiv2001

Spin dynamics in the $S=1/2$ weakly-coupled chains antiferromagnet BaCu$_2$Si$_2$O$_7$

Inelastic neutron scattering is used to identify single-particle and continuum spin excitations in the quasi-one-dimensional $S=1/2$ antiferromagnet \ba. In the data analysis, close attention is given to resolution effects. A gap in the continuum spectrum at $Δ_c=4.74.8$ meV is directly observed. The gap energy is in excellent agreement with existing theoretical predictions. Below the gap, the spectrum is dominated by sharp accoustic spin wave excitations.

cond-mat.str-el↗