arXiv subjects

M. Isobe

Publications and source records attributed to M. Isobe.

61 records · Page 4Linked to original sources

arXiv1998

Lattice vibrations of alpha'-NaV2O5

We have measured far infrared reflectance and transmittance spectra as well as Raman scattering spectra of α'-NaV2O5 single crystals for all the principal polarizations. The temperature range above the phase transition temperature T_c=35 K was investigated, mainly. On the basis of this experimental study and of the lattice dynamics calculations we conclude that the symmetry of NaV2O5 in the high temperature phase is described by the centrosymmetric D_{2h}^{13} space group. The assignment of the observed phonons is given. Values of dielectric constants are obtained from the infrared data. Asymmetric shapes of several infrared lines as well as higher order infrared vibrational spectra are discussed. The crystal field energy levels of the 3d electron localized at the V^{4+} site have been calculated in the framework of the exchange charge model using the values of effective charges obtained from the lattice dynamics calculations. According to the results of these calculations, the earlier observed broad optical bands in the region of 1 eV can be interpreted as phonon assisted d-d transitions.

cond-mat.str-el↗

arXiv1998

Magnetic excitation spectra of $NaV_2O_5$

We present low temperature Raman spectra of $NaV_2O_5$ single crystal. The assignation of the observed modes is done on the basis of Monte Carlo calculation of excited states of Heisenberg dimerized Hamiltonian, without frustration. We found a good agreement between measured and calculated magnetic excitations for $δ=0.048$ and $J=452 K$. The singlet-triplet, singlet-singlet excitations as well as two-magnon mode are observed to be at 66, 127 and 132 cm^{-1}.

cond-mat↗

arXiv1998

Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3

A huge increase of thermal conductivity k is observed at the phase transition in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04). This behavior is compared with that of pure and Zn-doped CuGeO3 where only modest kinks in the k(T) curves are observed at the spin-Peierls transition. The change of k at critical temperature Tc could be partially attributed to the opening of an energy gap in the magnetic excitation spectrum excluding the scattering of thermal phonons on spin fluctuations. However, the reason for such a strong anomaly in the k(T) may lie not only in the different energy scales of CuGeO3 and NaV2O5, but also in the different character of the phase transition in NaV2O5 which can have largely a structural origin, e.g. connected with the charge ordering.

cond-mat.str-el↗

arXiv1998

Raman Scattering in the Inorganic Spin-Peierls System alpha'-Na_{1-delta}V_2O_5

We have studied the spin-Peierls (SP) transition in alpha'-Na_{1-delta}V_2O_5 (delta = 0, 0.01 and 0.1) by means of Raman scattering. At room temperature, we observe six A_1 and three A_2 phonon modes and a broad Raman band. Below T_{SP}several new peaks and a new broad band appear. The new peak at 62 cm^{-1} originates from the SP-gap excitation. The new peak at 128 cm^{-1} and the new broad band between 130 and 400 cm^{-1} come from two magnetic excitations. The new peaks at 102, 646 and 944 cm^{-1} are assigned to the folded phonon modes and their Na^+-ion deficiency dependence shows that the defect of Na^+ ion suppresses the SPtransition. The polarized Raman spectra below T_{SP} suggest that the possible crystal symmetry is C_s^2(Pn) or C_1^1(P1). The asymmetric lineshape of the 531-cm^{-1} peak superimposed on the electronic Raman band from the d-d transition around 600 cm^{-1} is interpreted in terms of the Fano resonance between the electronic continuous band and the phonon with a finite lifetime. The defects of the Na^+ ions reduce the Fano effect because the life time of the phonon and the phonon-continuum interaction are decreased.

cond-mat.str-el↗

arXiv1997

Appearance of new lines and change in line shape in the IR spectrum of a NaV_2O_5 single crystal at the spin-Peierls transition

We report the first observation of new lines in the infrared spectrum of a crystal due to unit-cell doubling at the spin-Peierls transition (in NaV_2O_5). The change in the shape of the spectral lines at the spin-Peierls transition is recorded for the first time. A contour characteristic of a Fano resonance is observed above the transition temperature Tsp and the standard symmetric contour is observed below Tsp. We attribute this effect to the opening of a gap in the magnetic-excitation spectrum at the spin-Peierls transition.

cond-mat.str-el↗

arXiv1997

Raman, infrared and optical spectra of the spin-Peierls compound NaV_2O_5

We have measured polarized spectra of Raman scattering, infrared and optical transmission of NaV_2O_5 single crystals above the temperature of the spin-Peierls transition Tsp=35 K. Some of the far-infrared (FIR) phonon lines are strongly asymmetric, due to the spin-phonon interaction. In addition to the phonon lines, a broad band was observed in the c(aa)c Raman spectrum and in the E||a FIR transmission spectrum. A possible origin of these bands is discussed. The absorption band at 10000 cm-1 1.25 eV is attributed to vanadium d-d electronic transitions while the absorption edge above 3 eV is supposed to correspond to the onset of charge-transfer transitions.

cond-mat.str-el↗

arXiv1997

Observation of Spin Freezing in CaV4O9 and CaV2O5 by uSR

We have performed muon spin relaxation and susceptibility measurements of CaV_4O_9 and CaV_2O_5, systems in which spin gap-like behavior has been found. We observe spin freezing in the two compounds below 12K and 50K, respectively. Our results indicate the possibility that a substantial fraction of the of the vanadium magnetic moment is not in a spin singlet sate at low temperatures in either material.

cond-mat.str-el↗