The critical Rayleigh number for the onset of thermal convection in a horizontal rectangular parallelepiped cavity heated from below is evaluated numerically, and the three-dimensional flow pattern of the convection and the temperature distribution at the critical state are investigated. The top and bottom boundaries of the cavity are assumed to be perfectly heat-conductive, while the four sidewalls are assumed to be thermally insulating. The critical Rayleigh number is obtained to be 3389 for the case of cubic cavity and the velocity field at the critical state is revealed to have one global circulation. The critical condition and the flow field are obtained also for the cases other than the cubic cavity. It is found that the preferred mode is not always two-dimensional finite rolls with axes parallel to the shorter side, which differs from the conclusion derived for the case of perfectly conducting sidewalls by Davis.

View full abstract

We have studied phase transitions and nuclear magnetic relaxation associated with phase transitions in the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ by means of ¹H nuclear magnetic resonance (NMR). Spin–lattice relaxation was well described by the diffusion limited case, and a strong correlation between the NMR second moment and the spin–lattice relaxation rate, below a high phase transition temperature, provided a clue to its magnetic nature.

View full abstract

The low-temperature lattice effects of the Kondo compounds Ce_xLa_1−xB₆ (x=0.75 and 0.70) with a Γ₈ ground state have been investigated by dilatometric measurements. In an ordered phase IV below T_C=1.6 K (1.4 K) for x=0.75 (0.70), the lattice length along the [001] direction reveals an increase, while the length along the [111] direction shows an appreciable decrease. This lattice distortion is described in terms of the trigonal strain ⟨ε_yz⟩=⟨ε_zx⟩=⟨ε_xy⟩≠0 with Γ₅ symmetry in addition to a volume expansion ε_B=ε_xx+ε_yy+ε_zz with full symmetry Γ₁. The trigonal strain in phase IV indicates a ferro-quadrupole ordering of ⟨O_yz⟩=⟨O_zx⟩=⟨O_xy⟩≠0.

View full abstract

The viscoelastic effect on the hydrodynamic relaxation in semidilute polymer solutions is investigated. From the linearized two-fluid model equations, we predict that the dynamical asymmetry coupling between the velocity fluctuations and the viscoelastic stress influences on the hydrodynamic relaxation process, resulting in a wave-number-dependent shear viscosity.

View full abstract

ESR investigations were performed for a series of quasi-one-dimensional organic conductors, (TMTTF)₂X. The ESR linewidth shows abrupt jumps or humps in the paramagnetic insulating region. The (TMTTF)₂X salts are roughly divided into three groups according to the anisotropy of the ESR linewidth at low temperatures. In this paper, we discuss the possible charge-ordered patterns of three typical salts (X=ReO₄, SbF₆ and Br) from the microscopic point of view.

View full abstract

The electronic and lattice properties of LiBC and related compounds under anisotropic compression conditions are calculated by first-principles methods. The lattice properties are optimized automatically by the first-principles molecular dynamics (FPMD) method. An anomalous lattice behavior of LiBC under a,b-axis compression is found, which implies a kind of negative Poisson ratios.

View full abstract

The upper critical field H_c2 is evaluated for weakly-coupled two-band anisotropic superconductors. By modeling the actual bands and the gap distribution of MgB₂ by two Fermi surface spheroids with average parameters of the real material, we show that H_c2,ab⁄H_c2,c increases with decreasing temperature in agreement with available data.

View full abstract

The electronic properties of quarter-filled organic materials showing spin–Peierls transition are investigated theoretically. By studying the one-dimensional extended Peierls–Hubbard model analytically as well as numerically, we find that there is a competition between two different spin–Peierls states due to the tetramized lattice distortion in the strongly correlated regime. One is accompanied by lattice dimerization which can be interpreted as a spontaneous Mott insulator, while the other shows the existence of charge order of Wigner crystal-type. Results of numerical density matrix renormalization group computations on sufficiently large system sizes show that the latter is stabilized in the ground state when both the on-site and the inter-site Coulomb interactions are large.

View full abstract

When an electron is accelerated by an applied field, there exists phase coherency between Bloch states which the electron goes through. We can interpret this effect as afterimage of electronic motion. Due to this effect, localized eigenstates (Landau states in magnetic fields or Stark states in electric fields) are formed from Bloch states if cyclic motion is completed. However, the phase coherence should exist even when the cyclic motion is incomplete. From this point of view, an electron should be expressed by a superposition of Bloch states even in low fields. We investigate the effect of the field-induced coherency on transport properties by using wavepackets formed as a result of the superposition. It is shown that the wavepackets can be correlated with the Green’s functions. We also show that the present theory is applicable to the Monte Carlo technique which enables a more precise investigation.

View full abstract

The dephasing due to electron–electron Coulomb interactions is studied for open mesoscopic cavities in the diffusive regime where l⁄λ_F>>1 (l: elastic mean free path, λ_F: Fermi wavelength). We consider a square cavity with area L×L, which is connected to reservoirs through a pair of thin leads of width W. It is assumed that L>>W>>l. The system is characterized by the lowest eigenvalue Γ₀ of a diffusion equation. We find that the temperature dependence of the dephasing time τ_φ changes whether temperature T is higher or lower than T_cross≡Γ₀⁄k_B. It is shown that τ_φ behaves as τ_φ⁻¹∝T when T>>T_cross. If T is decreased below T_cross, the T-dependence of τ_φ crosses over to τ_φ⁻¹∝T². It is also shown that in the high-temperature regime of T>>T_cross, the dephasing time is greatly reduced in comparison with that in an ordinary two-dimensional system.

View full abstract

We report comparative substitution effects of nonmagnetic Ti⁴⁺ and magnetic Ir⁴⁺ impurities for Ru⁴⁺ in the spin-triplet superconductor Sr₂RuO₄. We found that both impurities suppress the superconductivity completely at a concentration of approximately 0.15%, reflecting the high sensitivity to translational symmetry breaking in Sr₂RuO₄. In addition, a rapid enhancement of residual resistivity is in quantitative agreement with unitarity-limit scattering. Our result suggests that both nonmagnetic and magnetic impurities in Sr₂RuO₄ act as strong potential scatterers. The behavior observed in Sr₂RuO₄ is similar to that of the nonmagnetic (Zn²⁺) impurity in the high-T_c cuprates, but different from that of the magnetic (Ni²⁺) impurity.

View full abstract

We performed transmission electron microscopy (TEM) and powder neutron diffraction experiments on an A-site ordered manganese perovskite YBaMn₂O₆ which undergoes unusual and multiple phase transitions. In the paramagnetic insulating phase, the so-called CE type of charge and orbital ordering was observed in the monoclinic a–b plane, which has been frequently observed for the ordinary solid solution of A_1−x³⁺A_x^′2+MnO₃ around x=0.5. TEM revealed, however, a fourfold periodicity along the c axis, suggesting a new stacking pattern, in which planes of the CE type are built up according to the sequence [ααββ…]. Interestingly, when the system entered the antiferromagnetic state below 195 K, this stacking pattern changed into [αααα…] or [αβαβ…], suggesting a close interplay between spins and orbitals. The obtained stacking patterns were strongly correlated to the inherent structural alternation, i.e., the Y/Ba order along the c axis.

View full abstract

Element-specific magnetic field hysteresis on a Fe(20 Å)/[Gd(30 Å)/Fe(20 Å)] × 50 multilayer has been measured by means of X-ray magnetic circular dichroism at Gd-L₃ and Fe-K edges at 16 K, 195 K and 300 K, together with bulk magnetization measurements. These results are examined on the basis of Camley’s model, which describes the multilayer as a one-dimensional spin chain. At 195 K, which is near the compensation temperature, the observed hysteresis curve is interpreted as a series of magnetic states: canted Fe-aligned, surface-twisted, and Fe-dominant bulk-twisted states. These unique spin configurations are promoted by the surface magnetization of outermost Fe slabs and the exchange-spring-like behavior within Gd slabs.

View full abstract

It has been clarified that the two excitations, the small energy excitation in the two-level system, and the confined acoustic phonons, determine the low-temperature dependence of the homogeneous linewidth in CuBr and CdSe quantum dots. The former dominates the temperature dependence below 15 K for CuBr quantum dots, causing a difference in temperature dependence between NaBr and glass matrix samples. Above 15 K, the latter makes the same nonlinear temperature dependence. The contribution of confined acoustic phonons is also confirmed in CdSe quantum dots above 10 K as a deviation from the linear temperature dependence. These results closely resemble those previously reported for CuCl quantum dots, \\citerf4 reflecting the universal influence of surface-related effects and confined phonon scattering on the dephasing process of semiconductor quantum dots in matrices.

View full abstract

We have proposed, in our previous papers [Y. Narita et al.: J. Phys. Soc. Jpn. 70 (2001) 1246; S. Saito et al.: J. Phys. Soc. Jpn. 70 (2001) 3517], a method to characterize integrable discrete soliton equations. In this paper we generalize the method further and obtain a q-difference Toda equation, from which we can derive various q-difference soliton equations by reductions.

View full abstract

In this paper, a special integrable differential-difference equation and its related systems are studied. First of all, by using dependent variable transformations, this special lattice is transformed into two bilinear forms. As a result, the corresponding soliton solutions are obtained. A coupled set of bilinear equations is proposed and related to the same special lattice in a certain way. We also derive the t-flow and z-flow of the coupled bilinear equations. Lax pairs for the t-flow and the z-flow are given. Furthermore, a bilinear Bäcklund transformation and the corresponding nonlinear superposition formula for the coupled bilinear equations are presented. Soliton solutions to the coupled bilinear equations are derived.

View full abstract

Periodic waves for evolution equations of the modified Korteweg–de Vries (mKdV) family are expressed as products of elliptic functions. By employing the Hirota bilinear formulation, periodic waves of the single component mKdV with positive cubic nonlinearity are obtained as rational functions of Jacobi elliptic functions. Coupled mKdV systems with two and three components are treated. Dark or kink type solitary waves can exist for systems where the equation for each component is in the bright soliton regime. Finally, a coupled system of (2+1) (2 spatial and 1 temporal) dimensional evolution equations is studied. Periodic waves in terms of products of elliptic functions are also derived. The validity of these new solutions is verified independently by a computer algebra software whenever feasible.

View full abstract

The zero-point energy of a nodispersive but inhomogeneous dielectric sandwiched between perfectly conducting plates is considered. It is assumed that the permittivity increases lineally as separating from one of the plates, and the gradient is small. With the assumption it is shown that the eigenmodes of the electric and magnetic field between plates are determined by an equation including the Airy functions. Using asymptotic expansions of the Airy functions the obtained equation is solved. The Casimir energy is defined as the energy of the radiation at zero temperature, and it is calculated by the mode summation method.

View full abstract

A detailed calculation is presented of the line emission profiles of Li²⁺ ions in a magnetically confined plasma, and applied to interpret the measured spectra from a fusion device [K. Kondo et al.: J. Nucl. Matter 241–243 (1997) 956]. Transition probabilities and wavelengths of Li²⁺ ions are computed by diagonalizing the Hamiltonian including interactions with the magnetic field. The nl-resolved population densities of exited Li²⁺ ions are calculated up to n=20 using a collisional-radiative model including the charge exchange process. The calculations show that 1) spectral profiles emitted by excitation, recombination and charge exchange are quite different, which is due to the different n and l-distributions of these basic atomic rate coefficients; and 2) in order to properly interpret low temperature sepctra the Zeeman effect has to be accounted for.

View full abstract

Subvalence 3s-shell photoionization resonances of Ca were measured with monochromatized synchrotron radiation and photoion time-of-flight spectroscopy method. Charge resolved photoion yield spectra were obtained. Broad peak structures were found in the Ca⁺ spectrum and shallow window structures were found in the Ca²⁺ spectrum. We porformed MCDF calculations to assign the resonance structures. The 3s-shell photoionization of Ar and K were also measured for comparison. A systematic increase was observed in Fano–Beutler parameter and in the resonance width along with the increase of atomic number from Z=18(Ar) to 20(Ca). We discuss also the spectral structures that could be of the 3p double-shake-up satellites, which are observed in the 3s photoionization region.

View full abstract

The subject of the present paper is a novel electron transfer between two atoms. It takes place irreversibly when, for instance, an atom accommodating an electron slowly approaches to another neutral atom under certain conditions. This irreversible process exhibited by a dynamical system is studied in adiabatic approximation. Possible application to the study of electron transfer in photosynthetic reaction center is also discussed.

View full abstract

The electron-impact transitions of n=2 fine-structure levels in hydrogenic ions are treated employing two kinds of close coupling methods and the Coulomb–Born approximation. By using these methods together, a reduction of calculation labor is attempted. The obtained results considerably differ from those of a previous close coupling calculation by Zygelman and Dalgarno [Phys. Rev. A 35 (1987) 4085] at low energies.

View full abstract

The linear neoclassical tearing mode is investigated using the four-field reduced neoclassical MHD equations, in which the fluctuating ion parallel flow and ion neoclassical viscosity are taken into account. The dependences of the neoclassical tearing mode on collisionality, diamagnetic drift and q profile are investigated. These results are compared with the results from the conventional three-field model. It is shown that the linear neoclassical tearing mode is stabilized by the ion neoclassical viscosity in the banana regime even if Δ′>0.

View full abstract

The temperature dependence of the microscopic structure of the liquid In₂Te₃ mixtures is studied by ab initio molecular-dynamics simulations. The results are discussed in comparison with the experimental results. We show that our simulation can reproduce the characteristic features of the temperature dependence of the observed structure factor. It is found that the ‘InTe₄’ units are relatively stable at lower temperatures while there exist mainly the ‘InTe₃’ units at higher temperatures.

View full abstract

The crystal structures of deuterated glycinium phosphite crystals with various deuterium concentrations x (0.00, 0.25, 0.50, 0.75 and 1.00) in the paraelectric phase were determined using the X-ray imaging plate diffractometer. The hydrogen bond distances increase with increasing deuterium concentration x, especially including the disordered hydrogen atom. There is a linear relation between the transition temperature T_C and the hydrogen bond distance R_O–O. The critical bond length r_c was estimated as 2.415 Å. HPO₃ tetrahedron rotates by duteration.

View full abstract

Pressure–temperature dependence of modulation wavenumber of [N(CH₃)₄]₂CuCl₄ was measured with X-ray diffraction method in the region of 22≤T≤160°C, 0.25≤p≤0.95 GPa. Two commensurate phases with modulation wavenumber q=3⁄8 and 2⁄5 were found within the pressure-induced incommensurate phase. The phase q=3⁄8 is identified as the high-pressure ferroelectric phase F₂ found by Gesi.

View full abstract

The incoherent scattering functions of hopping particles in random systems are theoretically examined in terms of the relaxation mode theory, focusing on the intermediate scattering function and dynamical structure factor. In the former, we discuss how it correlates with the diffusion coefficients in the small q approximation, which reflect the anomalous behavior of the nondiffusive modes. While in the latter, the non-BPP spin–lattice relaxation is investigated where the mechanism is explained by the contribution of the diffusive and extended nondiffusive relaxation modes.

View full abstract

We investigate the effect of the domain wall on the thermal conductivity of solid ³He in U2D2 phase. In our previous work, assuming that the magnetic plane defects are in the single domain crystal, we showed that our model was able to explain the temperature dependence of the thermal conductivity at low temperatures qualitatively. In the U2D2 phase, it is well known that there exist three domains characterized by the anisotropy axes. The static structure of the domain wall separating these domains was investigated by Tsubota et al. and it was reported recently that the structure they had proposed was consistent with the experimental data. In this situation, it is interesting to see whether the predicted domain wall pervents the magnon transfer or not. We apply the same method of our previous work to this system, and calculate the thermal conductivity. We also estimate the effect of the external magnetic field because the strong magnetic field was applied in the experiment. From these investigations, we conclude that the domain wall has little effect on the thermal conductivity.

View full abstract

We study the persistent current (PC) in one-dimensional (1D) magnetic-flux threaded mesoscopic rings, which is constructed according to the random-dimer (RD) model. It is found that the PC varies significantly when the Fermi energy is changed in the system. The PC can approach the behaviour of free electrons regardless of the disorder if there is the extended electronic state at the Fermi level; while the PC can be depressed dramatically if the highest-occupied electronic state is localized or in the intermediate case between the extended state and localized one. This property provides a possible explanation to the anomalously large PC observed in some experiments. Furthermore, it is demonstrated that the electronic delocalization leads to unsuppressed persistent currents and \\sqrtN unscattered states exist around the resonant energy in the RD model from the viewpoint of the PC. The possibility to use 1D random-dimer mesoscopic rings as quantum-switch devices is also discussed.

View full abstract

The electrical resistivity and the thermoelectric power of liquid Au–Si alloys have been measured as a function of temperature. The electrical resistivity increases rapidly with the addition for Si to liquid Au, and the composition dependence of the electrical resistivity of liquid Au_1−cSi_c alloys exhibits a maximum at the composition c=0.4. A negative temperature coefficient of electrical resistivity was confirmed for liquid Si and also found for liquid Au_1−cSi_c alloys with 0.15≤c≤0.8. The composition dependence of the thermoelectric power for liquid Au_1−cSi_c alloys shows a rapid change from a positive value in the Au-rich region to a negative value in the composition range c≥0.15 and reaches a minimum value of −6 μV/K at the composition c=0.8. The electrical resistivity and the thermoelectric power of liquid Au–Si alloys have been estimated by the Faber–Ziman theory. The lower eutectic region in the system will be also studied by using the thermodynamic equation.

View full abstract

Transport and magnetic studies on polycrystalline samples of SrFe_1−xCo_xO_3−δ have been carried out to investigate the relationship between the magnetic structure and the anomalous Hall resistivity ρ_H. The hysteretic behavior of the magnetization observed in the measurements with varying temperature T up and then down after zero field cooling indicates that the system has the reentrant spin-glass phase, which is supported by the increasing width of the magnetic reflections observed by neutron diffraction with decreasing T below the Curie temperature T_C. Detailed analyses of the observed Hall resistivity ρ_H indicate that the anomalous Hall coefficient exhibits unusual behavior in the reentrant spin-glass phase. The magnetic field (H)- and T-dependence of the magnetoresistance of the present system can be understood by a spin dependent tunneling model.

View full abstract

Josephson critical current density J_c between a single crystal URu₂Si₂ and an s-wave superconductor has been investigated for URu₂Si₂–Cu–Nb(SNS′) and URu₂Si₂–Nb(SS′) junctions. In contrast to the previous result for point-contacts, J_c is observed along the c-axis as well as the a-axis direction of URu₂Si₂. The J_c value in SS′ junctions is of the same order of magnitude independent of current direction or surface roughness of the (001) plane, suggesting that c-axis Josephson coupling is intrinsic.

View full abstract

Organic alloys λ-(BETS)₂Fe_xGa_1−xCl₄ show superconductivity only under very high magnetic fields parallel to the conducting layer for x≥0.47. As x decreases, the field induced superconducting phase shifts towards lower fields and a striking field-induced insulator to superconductor transition is observed below 4 T for x=0.45. We show that the overall features of the global phase diagram are well understood in terms of Jaccarino-Peter compensation mechanism. These results provide a new road map for the future design of high magnetic field superconductors.

View full abstract

The ground state of two-dimensional t–J model including superconductivity and antiferromagnetism is studied using a new type of analytic estimation of the effect of strong correlation. The method is an extension of the Gutzwiller approximation which gives renormalization of parameters, t and J, due to the Gutzwiller projection operator excluding double occupancies. It is shown that the essential point of the renormalization is its dependence on the expectation values of superconductivity and antiferromagnetic moment. In special the renormalization factor is anisotropic in spin space, which leads to a reliable estimate of antiferromagnetic order parameters in the phase diagram, in contrast to the conventional slave-boson mean-field theory. The obtained phase diagram shows the coexistence of antiferromagnetism and d_x²−y²-wave superconductivity for the doping rate, δ, less than 10%. At half-filling, the renormalization of the J-term represents some aspects of quantum spin nature. For the finite doping case, there are two kinds of superconducting order parameters. The variational order parameter of the d-wave superconductivity increases as the doping rate decreases, which will correspond to the experimentally observed superconducting gap. In contrast, the actual expectation value of superconducting order parameter is proportional to δ in underdoped region and has a maximum near δ=0.11, which probably determines T_c. We also speculate that the ground-state antiferromagnetic order, being destroyed at finite temperatures, gives the energy and temperature scale of the pseudogap. Applications to the inhomogeneous systems such as the vortex state and stripe state are discussed.

View full abstract

We investigate a one-dimensional S=1⁄2 antiferromagnetic Heisenberg model coupled to quantum lattice vibration using a quantum Monte Carlo method. We study the ground-state lattice fluctuation where the system shows a characteristic structure factor. We also study the mass dependence of magnetic properties such as the magnetic susceptibility and the magnetic excitation spectrum. For heavy mass, the system shows the same behavior as the case of classical lattice vibration. On the other hand, for light mass, magnetic properties coincide with those of the static uniform chain. We investigate the physical mechanism of this behavior and propose the picture of quantum narrowing.

View full abstract

The pressure effects on an S=1 Haldane compound Ni(C₅H₁₄N₂)₂N₃(PF₆) (NDMAP) have been investigated through the magnetic susceptibility (χ) measurements using single crystalline samples and the powder x-ray structural analysis experiment under pressure up to 11.5 kbar. The crystal-axis dependence of χ has been analyzed with the results of quantum Monte Carlo simulation. The results suggest that both the intrachain interaction (J) and single-ion anisotropy (D) are enhanced by pressure: J=31.5 K (P=0 kbar) → 90.0 K (10.0 kbar) and D⁄J=0.3 (P=0 kbar) → 0.4 (9.5 kbar). As for J, there is the linear increase against pressure as dJ⁄dP=2.0 K/kbar in the pressure region below P=8.0 kbar, at around which dJ⁄dP rapidly changes to dJ⁄dP=2.1×10¹ K/kbar. The ratio of D⁄J changes within 0.3–0.4, and this result suggests that D as well as J drastically change at around P=8.0 kbar. Furthermore, at P=9.5 kbar, the development of paramagnetic moment on the easy plane is detected, and it is assumed that the interchain interaction may be also enhanced under pressure. The structural analysis shows that the interchain shrinkage is larger than the intrachain one. However, a result concerning the rapid changes of J and D at around P=8.0 kbar has been not observed, and we suppose that the changes of bond-angles such as –N–Ni–N– and –N–N–N– as well as interatomic shrinkage probably change under pressure. We assume that those bond-angles of NDMAP at around P=9.0 kbar (J\\simeq70 K) may become close to those of Ni(C₅H₁₄N₂)₂N₃(ClO₄) (NDMAZ), where the value of J is 71.0 K.

View full abstract

Hole-doping effects are investigated on the t–J ladder model with the linked-tetrahedra structure. We discuss how a metal–insulator transition occurs upon hole doping with particular emphasis on the effects of geometrical frustration. By computing the electron density and the spin correlation function by the density matrix renormalization group, we show that strong frustration triggers a first-order transition to a metallic phase, when holes are doped into the plaquette-singlet phase. By examining spin excitations in a metallic case in detail, we discuss whether the spin-gap phase persists upon hole doping according to the strength of frustration. It is further shown that the lowest excited state in a spin-gap metallic phase can be described in two independent quasiparticles.

View full abstract

A novel macroscopically degenerate state called kagomé ice, which was recently found in a spin ice compound Dy₂Ti₂O₇ in a magnetic field applied along the [111] direction of the cubic unit cell, is studied by specific heat measurements. The residual entropy of the kagomé ice is estimated to be 0.65 J/K mol Dy, which is nearly 40% of that for the tetrahedral spin ice obtained in a zero field (1.68 J/K mol Dy) and is in good agreement with a theoretical prediction. It is also reported that the kagomé ice state, which is stabilized at a range of magnetic field of 0.3–0.6 T, is a “gas” phase and condenses into a “liquid” phase with nearly zero entropy at a critical field of 1 T.

View full abstract

The rotation patterns of the nuclear magnetic resonances of ⁷Li (I=3⁄2) and ⁵¹V (I=7⁄2) in the nonlinear optical Li₃VO₄ single crystal were measured in two mutually perpendicular crystal planes. From these results, the quadrupole coupling constants and the asymmetry parameters at room temperature were determined for the first time as follows: e²qQ⁄h=53.68±2 kHz and η=0.169 for ⁷Li, and e²qQ⁄h=1.44±2 MHz and η=0 for ⁵¹V. The symmetry for the electric field gradient (EFG) of V ions surrounded by four oxygen atoms is higher than that of Li ions surrounded by four oxygen atoms. The principal axes of the EFG tensor are a=Y, b=Z, c=X for ⁷Li, and a=b=X, c=Z for ⁵¹V nucleus. Also, the ⁷Li and the ⁵¹V spin–lattice relaxation rates were measured, and the behavior is quite similar for both nuclei. The measured relaxation rate was found to be linear in the square of the temperature. The Raman process with a T² dependence is considered to be more effective than the direct process for nuclear quadrupole relaxation.

View full abstract

⁵⁷Fe Mössbauer study on the ordered double perovskite A₂FeReO₆ (A = Ca, Sr) has been conducted. Fe ions are in 2.5+ states that correspond to the hybridized state of 3d t_2g down-spin electrons with Re⁵⁺ 5d t_2g down-spin electrons. The orbital effect due to this seems to be absent. On the metal–insulator transition of Ca₂FeReO₆ specimen, slight change of the electronic state toward Fe³⁺ state is recognized. The quadrupole interaction is well explained by a point charge model, which may suggest that while the electrons of Re are itinerant, those of Fe stay rather localized.

View full abstract

The nature of the inhomogeneous broadening of optical spectra has been investigated for Eu³⁺ ions embedded in pure and impurity ion (Sc³⁺ or Zr⁴⁺) doped Y₂O₃ crystals. High-resolution absorption spectra of the ⁷F₀–⁵D₀ transitions have been taken as excitation spectra of luminescence. Using optical-radio-frequency (rf) double resonance technique, the distribution of two hyperfine transitions (ω₁:I_z=±1⁄2 to ±3⁄2 and ω₂:I_z=±3⁄2 to ±5⁄2) in the ⁷F₀ ground state were measured as a function of the optical excitation energy and displayed as a 2-dimensional map on the optical and rf axes. Thus the ionic sites are labeled by three parameters and some new sites were distinguished by using both ω₁ and ω₂. The single crystals with a disorder systematically controlled by doping of various ions (Sc³⁺ and Zr⁴⁺) were investigated and the character of the inhomogeneous distribution was found to be strongly dependent on the doped ions. By applying the mapping method to the Sc³⁺ doped Y₂O₃, the individual sites are discriminated inside the inhomogeneously broadened optical spectra, while the discrete peaks coalesce into a single broad peak in the optical spectrum at higher concentration.

View full abstract

We have intensively studied the optical absorption spectra of Cu₂O at 4.2 K and 1.5 K in an extended range of magnetic fields up to 25 T as image maps. Quasi-continuous data of the magnetic field dependence of the absorption peaks and their interaction properties reveal that the magnetic-field-driven mixing of the energy levels occurs intensively even in the lower range of magnetic fields.

View full abstract

Synchronization is studied in a spatially-distributed network of weekly-coupled, excitatory neurons of Hodgkin–Huxley type. All neurons are coupled to each other synaptically with a fixed time delay and a coupling strength inversely proportional to the distance between two neurons. We found that a robust, noise-resistant phase clustering state occurred regardless of the initial phase distribution. This has not been shown in previous studies where similar clustering states were found only when the coupling was inhibitory. The spatial distribution of neurons in each synchronous cluster is determined by the spatial distribution of the coupling strength. Phase-interaction properties of the model neurons in the network are used to explain why such a clustering state can be robust.

View full abstract