arXiv subjects

Y. Ueda

Publications and source records attributed to Y. Ueda.

At least 145 records · Page 8Linked to original sources

arXiv2001

A Possible Emission Feature in an X-ray Afterglow of GRB970828 as a Radiative Recombination Edge

A gamma-ray burst of 28 August 1997 was localized by the All-Sky Monitor on the Rossi XTE satellite and its coordinates were promptly disseminated. An ASCA followup started 1.17 days after the burst as a Target of Opportunity Observation and detected an X-ray afterglow. The spectral data displayed a hump around ~5 keV and an absorption column of 7.1 x 10^21 cm^{-2}. This hump structure is likely a recombination edge of iron in the vicinity of the source, taking account of the redshift z = 0.9578 found for the likely host galaxy of the associated radio flare. Radiative Recombination edge and Continuum model can interpret the spectrum from highly ionized plasma in a non equilibrium ionization state. The absorption could be also due to the medium presumably in the vicinity of the GRB.

astro-ph↗

arXiv2001

The "devil's staircase" type phase transition in NaV2O5 under high pressure

The "devil's staircase" type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a * 2b * zc type superstructures. The observed temperature and pressure dependence of modulation wave number qc, defined by 1/z, is well reproduced by the Axial Next Nearest Neighbor Ising (ANNNI) model. The qc is suggested to reflect atomic displacements coupled with charge ordering in this system. The experimental fact implies that two competitive inter-layer interactions between the Ising spins, i.e., the nearest neighbor J1>0 (ferro) and the next nearest neighbor J2<0 (antiferro) along the c-axis, are intrinsic in this compound. A microscopic origin of the inter-layer interaction is not yet known; however, the nearest neighbor interaction J1>0 between the V2O5 layers is especially interesting. It is very surprising that the phase transition of such a complicated charge-lattice-spin coupled system NaV2O5 can be described by the simple ANNNI model.

cond-mat.str-el↗

arXiv2001

Spin fluctuations in a magnetically frustrated metal, LiV2O4

Inelastic neutron scattering is used to characterize spin fluctuations in the $d$-electron heavy fermion spinel $\rm LiV_2O_4$. The spin relaxation rate, $Γ_Q$, for $Q=0.6$ Å$^{-1}$ is 1.5(1) meV at low temperatures and increases linearly with temperature at a rate of 0.41(4) k$_B$. There is antiferromagnetic short range order at low temperatures with a characteristic wave vector $Q_c= 0.64(2)$ Å$^{-1}$ and a correlation length of 6(1) Å. Warming shifts intensity towards lower $Q$ so that magnetic scattering decreases monotonically with $Q>0.3$ Å$^{-1}$ for $T>40$ K. The data are compared with conventional heavy fermion systems, geometrically frustrated insulating magnets, and recent theories for $\rm LiV_2O_4$.

cond-mat.str-el↗

arXiv2001

Low-energy excitations in NaV2O5

In the (ab) polarized Raman scattering spectra of NaV2O5 single crystals, measured with 647.1 nm laser line at T < Tc, we found two modes at 86, and 126 cm-1 not previously reported. These two modes, together with 66, and 106 cm-1 modes, make an array of four low-energy equidistant modes below the energy onset of the continuum at about 132 cm-1. All four modes are strongly suppressed by increasing Na deficiency, indicating their nonvibrational origin and the existence of a quantum phase transition at critical Na deficiency between 3 and 4%. These results question current understanding of NaV2O5 as quasi one-dimensional Heisenberg antiferromagnet.

cond-mat.str-el↗

arXiv2001

Charge-ordering phase transition and order-disorder effects in the Raman spectra of NaV2O5

In the ac polarized Raman spectra of NaV2O5 we have found anomalous phonon broadening, and an energy shift of the low-frequency mode as a function of the temperature. These effects are related to the breaking of translational symmetry, caused by electrical disorder that originates from the fluctuating nature of the V {4.5+} valence state of vanadium. The structural correlation length, obtained from comparisons between the measured and calculated Raman scattering spectra, diverges at T< 5 K, indicating the existence of the long-range charge order at very low temperatures, probably at T=0 K.

cond-mat.str-el↗

arXiv2001

Charge-ordering and optical transitions of LiV2O5 and NaV2O5

We present the measurements of the polarized optical spectra of NaV2O5 and LiV2O5. In an energy range from 0.5 to 5.5 eV we observe similar peaks in the E parallel a spectra of LiV2O5 and NaV2O5, which suggests similar electronic structure along the a axis in both materials. On the other hand, we find an almost complete suppression of the peaks in sigma_b of LiV2O5 around 1 and 5 eV. We attribute this suppression to the charge localization originating from the existence of double-chain charge-ordering patterin in LiV2O5.

cond-mat.str-el↗

arXiv2000

The magnetic ordering in the mixed valence compound beta-Na0.33V2O5

The low-temperature electron spin resonance (ESR) spectra and the static magnetization data obtained for the stoichiometric single crystals of $β$-Na$_{0.33}$V$_2$O$_5$ indicate that this quasi-one-dimensional mixed valence (V4+/V5+) compound demonstrates at $T_N=22$ K the phase transition into the canted antiferromagnetically ordered state. The spontaneous magnetization of $3.4\times 10^{-3}$ $μ_B$ per V$^{4+}$ ion was found to be oriented along the two-fold $b$ axis of the monoclinic structure, the vector of antiferromagnetism is aligned with the $a$ axis and the Dzyaloshinsky vector is parallel to the $c$-axis. The experimental data were successfully described in the frame of the macroscopic spin dynamics and the following values for the macroscopic parameters of the spin system were obtained: the Dzyaloshinsky field $H_D=6$ kOe, the energy gaps of two branches of the spin wave spectrum $Δ_1=48$ GHz and $Δ_2=24$ GHz.

cond-mat.str-el↗

arXiv2000

Structural Phase Transition in the 2D Spin Dimer Compound SrCu2(BO3)2

A displacive, 2nd order structural phase transition at Ts=395 K from space group I`4 2 m below Ts to I 4/m c m above Ts has been discovered in the two-dimensional spin dimer compound SrCu2(BO3)2. The temperature evolution of the structure in both phases has been studied by X-ray diffraction and Raman scattering, supplemented by differential scanning calorimetry and SQUID magnetometry. The implications of this transition and of the observed phonon anomalies in Raman scattering for spin-phonon and interlayer coupling in this quantum spin system will be discussed.

cond-mat.str-el↗

arXiv2000

X-ray Anomalous Scattering Study of a Charge-Ordered State in NaV2O5

Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an X-ray diffraction technique using anomalous scattering near a vanadium K-absorption edge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below Tc=35 K. Consequently, the charge ordering pattern is the zigzag-type ladders with the unit cell 2a*2b*4c, but not the chain-type originally proposed for the spin-Peierls state. Charge disproportionation suggested in our model as the average valence V^{4.5+-delta_c/2} is observed below T_C, showing continuous variation of delta_c as a function of temperature.

cond-mat.str-el↗

arXiv2000

Collective Singlet Excitations and Evolution of Raman Spectral Weights in the 2D Spin Dimer Compound SrCu2(BO3)2

We present a Raman light scattering study of the two-dimensional quantum spin system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich structure, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)2 is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound. Based on our analysis of the Heisenberg model relevant for this material, we argue that the collective excitations involving two and three-particle singlet bound states have large binding energies and are observed as well-defined peaks in the Raman spectrum.

cond-mat.str-el↗

arXiv2000

ASCA Observations of the Jet Source XTE J1748-288

XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was observed with ASCA on 1998/09/06 and 1998/09/26, 100 days after the onset of the radio-X-ray outburst. The spectra were fitted with an attenuated power-law model, and the 2-6-keV flux was 4.6 * 10^{-11} erg s^{-1} cm^{-2} and 2.2 * 10^{-12} on 09/06 and 09/26, respectively. The light curve showed that the steady exponential decay with an e-folding time of 14 days lasted over 100 days and 4 orders of magnitude from the peak of the outburst. The celestial region including the source had been observed with ASCA on 1993/10/01 and 1994/09/22, years before the discovery. In those period, the flux was < 10^{-13} erg s^{-1} cm^{-2}, below ASCA's detection limit. The jet blob colliding to the environmental matter was supposedly not the X-ray source, although the emission mechanism has not been determined. A possible detection of a K line from highly ionized iron is discussed.

astro-ph↗

arXiv2000

Raman Scattering from Magnetic Excitations in the Spin Ladder Compounds CaV2O5 and MgV2O5

We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640 K. No magnon bound states are found. In contrast to CaV2O5 the existence of the spin-gap is not confirmed in MgV2O5, since we found no feature in the spectra which could be associated with the onset of the two-magnon continuum. Instead, we observe two-magnon excitation at 340 cm-1, presumably related to the top of the two-magnon brunch.

cond-mat.str-el↗

arXiv2000

Direct Evidence for the Localized Single-Triplet Excitations and the Dispersive Multi-Triplets Excitations in SrCu2(BO3)2

We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5 and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplets excitations at 5 meV exhibit a more dispersive behavior.

cond-mat.str-el↗

arXiv2000

Soundwave Anomalies in SrCu_{2}(BO_{3})_{2}

The temperature and high magnetic field dependence of the longitudinal soundwave mode c_{11} in the two dimensional dimer system SrCu_{2}(BO_{3})_{2} is presented. c_{11}(T) shows anomalies due to strong interdimer spin-strain coupling. We can quantitatively interpret the temperature dependence of c_{11}(T) together with the magnetic susceptibility $χ_{m}(T)$ with a molecular field approximation of coupled dimer triplets. The sound velocity up to 50T shows very sharp softening between the magnetization plateaus at low temperature. We argue that these pronounced effects arise from a resonant interaction between the phonons and the magnetic excitations which show softening between the plateaus.

cond-mat.str-el↗

arXiv2000

Investigation of thermal and magnetic properties of defects in a spin-gap compound NaV2O5

The specific heat, magnetic susceptibility and ESR signals of a Na-deficient vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07 - 10 K, well below the transition point to a spin-gap state. The contribution of defects provided by sodium vacancies to the specific heat was observed. It has a low temperature part which does not tend to zero till at least 0.3 K and a high temperature power-like tail appears above 2 K. Such dependence may correspond to the existence of local modes and correlations between defects in V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of freedom for the defects, with their effective number increasing in temperature and under magnetic field. The latter results in the nonsaturating magnetization at low temperature. No long-range magnetic ordering in the system of defects was found. A model for the defects based on electron jumps near vacancies is proposed to explain the observed effects. The concept of a frustrated two-dimensional correlated magnet induced by the defects is considered to be responsible for the absence of magnetic ordering.

cond-mat.str-el↗

arXiv2000

Modeling of the Magnetic Susceptibilities of the Ambient- and High-Pressure Phases of (VO)_{2}P_{2}O_{7}

The magnetic susceptibilities chi versus temperature T of powders and single crystals of the ambient-pressure (AP) and high-pressure (HP) phases of (VO)_{2}P_{2}O_{7} are analyzed using an accurate theoretical prediction of chi(T, J1, J2) for the spin-1/2 antiferromagnetic alternating-exchange (J1, J2) Heisenberg chain. The results are consistent with recent models with two distinct types of alternating-exchange chains in the AP phase and a single type in the HP phase. The spin gap for each type of chain is derived from the respective set of two fitted alternating exchange constants and the one-magnon dispersion relation for each of the two types of chains in the AP phase is predicted. The influences of interchain coupling on the derived intrachain exchange constants, spin gaps, and dispersion relations are estimated using a mean-field approximation for the interchain coupling. The accuracies of the spin gaps obtained using fits to the low-T chi(T) data by theoretical low-T approximations are determined. The results of these studies are compared with previously reported estimates of the exchange couplings and spin gaps in the AP and HP phases and with the magnon dispersion relations in the AP phase measured previously using inelastic neutron scattering.

cond-mat.str-el↗

arXiv2000

Far infrared study of the two dimensional dimer spin system SrCu_2(BO_3)_2

Using far-infrared spectroscopy in magnetic fields up to 12T we have studied a two-dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several infrared active modes in the dimerized state (below 10K) in the frequency range from 3 to 100cm^-1. The measured splitting from the ground state to the excited triplet M_S=0 sublevel is Delta_1=24.2cm^-1 and the other two triplet state sublevels in zero magnetic field are 1.4cm^-1 below and above the M_S=0 sublevel. Another multiplet is at Delta_2=37.6cm^-1 from the ground state. A strong electric dipole active transition polarized in the (ab)-plane is activated in the dimer spin system below 15K at 52cm^-1.

cond-mat.str-el↗

arXiv2000

High frequency ESR investigation on dynamical charge disproportionation and spin gap excitation in NaV_2O_5

A significant frequency dependence of the ESR line width is found in NaV_2O_5 between 34-100 K and the line width increases as the resonance frequency is increased from 95 GHz to 760 GHz. The observed frequency dependence is qualitatively explained in terms of the dynamical charge disproportionation. The present results show the essential role of the internal charge degree of freedom in a V-O-V bond. We have also proposed the existence of the Dzyaloshinsky-Moriya interaction in the low temperature charge ordered phase considering the breaking of the selection rule of ESR realized as the direct observation of the spin gap excitation.

cond-mat.str-el↗