Taturu Yosida

Optical and electron spin resonance spectroscopy of Ti3+ and Ti4+ in Al2O3

The absorption spectrum of Ti3+:Al2O3 crystals grown in a reducing atmosphere consists of the main blue‐green absorption band and a weak infrared band. This infrared absorption decreases in intensity on annealing the as‐grown samples in a reducing atmosphere. The analysis of electron spin resonance spectra of the as‐grown samples, in which the infrared absorption band is observed, indicates that the center associated with the band is a cluster involving Ti3+ and Ti4+ ions with a neighboring, charge compensating Al3+ vacancy. The coexistence of a Ti4+ ion and an Al3+ vacancy in the neighborhood of the Ti3+ ion weakens the crystal field at this ion more than a single Ti4+ ion, giving rise to a red shift of the Ti3+ absorption.

Phase transition in traffic jam experiment on a circuit

The emergence of a traffic jam is considered to be a dynamical phase transition in a physics point of view; traffic flow becomes unstable and changes phase into a traffic jam when the car density exceeds a critical value. In order to verify this view, we have been performing a series of circuit experiments. In our previous work (2008 New J. Phys. 10 033001), we demonstrated that a traffic jam emerges even in the absence of bottlenecks at a certain high density. In this study, we performed a larger indoor circuit experiment in the Nagoya Dome in which the positions of cars were observed using a high-resolution laser scanner. Over a series of sessions at various values of density, we found that jammed flow occurred at high densities, whereas free flow was conserved at low densities. We also found indications of metastability at an intermediate density. The critical density is estimated by analyzing the fluctuations in speed and the density–flow relation. The value of this critical density is consistent with that observed on real expressways. This experiment provides strong support for physical interpretations of the emergence of traffic jams as a dynamical phase transition.

The magnetic and optical properties of in

Electron spin-resonance (ESR) spectra of -doped (LiCAF) crystals measured at low temperature reveal three distinct centres in the crystal. The most intense spectrum is fitted to a spin Hamiltonian with trigonal symmetry and is associated with ions that substitute for ions in the LiCAF structure. Two other spectra have orthorhombic symmetry: they are assigned to ions with charge-compensating -ion vacancies near to the impurity ions. The and components of the polarized optical absorption and luminescence spectra of in LiCAF show there to be at least two centres. The intensities of optical transitions of are calculated using the eigenfunctions of the ground and excited states of in trigonal and orthorhombic symmetry and are compared with the observed polarizations of the absorption and luminescence.

The electron spin resonance and optical spectra of in

The electron spin resonance (ESR) of in was measured with an X-band frequency and low temperatures. The ions in this crystal show tetragonal symmetry with and . The analysis of the g-tensor indicates that the ground-state wavefunctions have dominant components of , resulting in a negative value of the crystal-field parameter . Three optical absorption bands with peaks at , 248 nm, and 298 nm were observed at room temperature. The 298 nm band is more strongly polarized along the a-axis than along the c-axis, whereas the 248 nm band shows the reverse polarization. The properties of the polarization can be explained by using wavefunctions of the lower excited state of , which is split into and in tetragonal symmetry, being lower in energy.

Optical and electron spin resonance spectroscopy of -doped yttrium and gadolinium aluminoborates

Single crystals of -doped aluminoborates (YAB) and (GAB) have been grown by the top-seeded solution growth technique, and the optical absorption, photoluminescence and electron spin resonance (ESR) properties of dopants measured at low temperature. The optical absorption spectrum of comprises two broad bands with peaks at 514 and 576 nm in YAB and at 520 and 586 nm in GAB. The energy separation of the two bands in each crystal is due to the static Jahn - Teller splitting of the excited state of the ions. Photoluminescence from excited ions occurs as a broad band in the near-infrared region with a peak at nm in YAB and 754 nm in GAB when measured at 14 K. Both absorption and photoluminescence spectra are strongly polarized. The spin Hamiltonian parameters for ions substituting at trigonally symmetric sites in YAB have been determined from the orientation dependence of the ESR spectra. The measured shifts in the components of the g-tensor from the free electron g-value of 2.0023 are interpreted in terms of the mixing of the higher component of the ground states and of the excited state into the lowest ground state by spin - orbit interaction.

Substitutional disorder and the ground state spectroscopy of gallogermanate crystals

Electron spin resonance (ESR) spectra at X-band microwave frequencies are reported for dopant ions in (CGGO), (SGGO), (LGGO) and (LGS). The ESR measurements identify two distinct sites for in CGGO and SGGO and a single site in LGGO and LGS characterized by spin S = 3/2 for ions in the ground state. In CGGO a sharp line ESR spectrum, fitted to an axially symmetric spin Hamiltonian with , and , is assigned to substituents at trigonally distorted octahedral sites with a regular array of and ions in second-nearest-neighbour sites. A second spectrum of broader lines, due to ions in orthorhombically distorted sites associated with random occupation of the second-nearest-neighbour sites by and ions, has slightly different spin Hamiltonian parameters; , and . In this spectrum the principal axis is rotated by about from the c-axis towards the a-axis. In SGGO there are also ESR spectra from ions in trigonal and orthorhombic symmetry sites with almost identical spin Hamiltonian parameters. The ground state splittings of the trigonal centres measured optically by fluorescence line narrowing gives for both crystals. In contrast, only the trigonal centres are identified in LGGO and LGS, their ESR spectra being very similar to those of the trigonal centres in CGGO and SGGO. These results are discussed in terms of the substitutional disorder in these crystals and the nature of the site environment of substituents. The spectra correspond to ions in strong and weak crystal field sites, respectively, in agreement with earlier optical studies.

An electron-spin-resonance study of substitutional disorder in -doped

Electron spin-resonance (ESR) spectra of ions in (CYA) crystals have been measured at X-band microwave frequency. The spectra are only observed below 50 K: they are very broad and easily saturated by low microwave power at 4.2 K. The ESR lines, which vary in position with orientation of the magnetic field relative to the crystalline axes, have been fitted to a tetragonal spin Hamiltonian with the principal components observed to be close to those of ions in and . The associated microwave absorption is due to the lowest crystal-field level of the ground state of the ion in CYA. The measured g-tensor components and for in CYA can be explained in terms of the ground-state wavefunction composed of and components of mixed by the crystal-field terms and . The inhomogeneous broadening of the asymmetric ESR lines reflects a distribution of g-values, which is produced by tilting the principal z-axis of the effective spin away from the crystal c-axis and a distribution of the ground-state spin levels in of the ions through the random occupation of ions at the appropriate site in the disordered CYA lattice.

Electron spin-resonance study on Ce3+ in BaLiF3

Three distinct Ce3+ sites in BaLiF3 crystals estimated from the optical spectra are associated with configurations of Ce3+ accompanied by different charge compensators. This assignment is consistent with the electron spin-resonance (ESR) result that there are two tetragonal Ce3+ centres distorted along the [001] axis and two orthorhombic Ce3+ centres distorted along the [110] axis in the absence of the cubic centre. The configurations of the Ce3+ centres correspond to the substitution for Ba2+ ions along the [001] and [110] axes with Li+ ions and the Ba2+-ion vacancies along the [001] and [110] axes. The dominant component of the Ce3+ luminescence spectrum with the peak at ~320 nm and the large Stokes shift energy (~8300 cm-1) is assigned as due to the substitution for a Ba2+ ion along the [001] axis with a Li+ ion. As the ionic radius (0.74 A) of Li+ is much smaller than that (1.60 A) of Ba2+, the Li substitution produces more space, resulting in the large lattice relaxation in the 5d excited state of Ce3+.

An electron spin-resonance study of Cr3+ in LiSrxCa1-xAlF6 with x = 0, 0.5, 0.8, and 1

Electron spin-resonance (ESR) spectra of Cr3+ ions in LiSrxCa1-xAlF6 crystals with x = 0, 0.5, 0.8, and 1 have been measured at X-band microwave frequencies in the temperature range 1.3-300 K. The intense ESR lines from Cr3+ ions in these crystals have been fitted to a trigonal spin Hamiltonian to determine values of the g-tensor (g,g) and zero-field splitting, 2D. The principal g-values (g g1.974) are almost the same in all cases whereas 2D increases sequentially for x = 0, 0.5, 0.8, and 1. The value of |2D| for Cr3+:LiCaAlF6 decreases gradually as the temperature is increased from 1.3 K to 300 K. Over this same temperature range the value of |2D| for Cr3+:LiSrAlF6 increases. The ESR spectra for Cr3+ in LiSr0.5Ca0.5AlF6 and LiSr0.8Ca0.2AlF6 are inhomogeneously broadened by discrete centres with pseudo-trigonal symmetry, different configurations of which correspond to the random occupation of the second-nearest-neighbour cation sites by Ca2+ and Sr2+ ions.

Czochralski growth and IR-to-visible upconversion of Ho3+- and Er3+-doped SrLaAlO4

Abstract We report on the Czochralski growth and optical properties of Ho3+- and Er3+-doped strontium lanthanum aluminate (SrLaAlO4) perovskite. Ti-Sapphire laser excitation of the 4I15/2→4I9/2(Er3+) and 5I8→5I4(Ho3+) transitions in Ho3+- and Er3+-doped SrLaAlO4 yields intense green upconversion fluorescence which persists from 10xa0K through to room temperature. No upconversion fluorescence could be observed for 800 nm excitation of the 3H6→3H4 transitions in Tm3+-doped SrLaAlO4.