Showing papers in "Journal of the Physical Society of Japan in 1994"

Giant Magnetotransport Phenomena in Filling-Controlled Kondo Lattice System: La1-xSrxMnO3

Abstract: Giant magnetotransport phenomena including the field-induced nonmetal-metal transition have been found in single crystals of La 1- x Sr x MnO 3 near the critical composition ( x ≈0.17) for the nonmetal-metal transition and in the temperature region around the magnetic phase transition. Change of the resistivity shows a universal curve as a function of the magnitude of temperature- or field-induced magnetization, the most of which agrees with the prediction by the D =∞ and S =∞ Kondo lattice model with strong ferromagnetic (Hund) coupling.

Spin-Glass Behavior and Magnetic Phase Diagram of La1-xSrxCoO3 ( 0 ≤x ≤0.5) Studied by Magnetization Measurements

Abstract: Magnetic properties of La 1- x Sr x CoO 3 (0 ≤ x ≤0.5) have been studied by magnetization measurements in low magnetic fields. It has been found that there exist spin-glass (0< x ≤0.18) and cluster-glass (0.18 ≤ x ≤0.5) regions. Ferromagnetic long-range order previously reported does not take place. Spin-glass behavior is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co 3+ and Co 4+ and the antiferromagnetic one between like spins.

Transport Properties of the Kondo Lattice Model in the Limit S=∞ and D=∞

Abstract: The Kondo lattice model with Hund's ferromagnetic spin coupling is investigated as a microscopic model of the perovskite-type 3 d transitionmetal oxide La 1- x Sr x MnO 3 . In the classical spin limit S =∞ and the infinite-dimensional limit D =∞, the one-body Green's function is exactly calculated. Transport properties of the system in the presence of magnetic fields are obtained. The giant magnetoresistance of this model, which is in good agreement with the experimental data of La 1- x Sr x MnO 3 , is explained by the spin-disorder scattering process.

Resonant 3d, 3p and 3s Photoemission in Transition Metal Oxides Predicted at 2p Threshold

Abstract: We present calculated 3 d , 3 p and 3 s resonant photoemission (RPE) spectra in the transition metal (TM) compounds CuO, NiO, CoO, FeO, MnO and TiO 2 at TM 2 p core threshold on the basis of a cluster model. Our model includes the intraatomic multipole electron-electron interactions and the hybridization between transition metal ions and ligand. The imaginary part of the self-energy matrix of the intermediate state is calculated by taking into account the 2 p 3 d 3 d , 2 p 3 d 3 p , 2 p 3 d 3 s and 2 p 3 p 3 p Coster-Kronig decay, which guarantees a sum rule between the integrated RPE spectra and absorption spectra. The dependence of RPE on the incident photon energy tuned to various multiplet structures of TM 2 p absorption spectra is shown to be useful to identify the second order optical process, which is argued to include much Auger decay component. It is also shown, by a comparison with experiment, that the present 3 d RPE contributes to a removal of discrepancies of estimated parameter values among ...

A Theory of D-E Hysteresis Loop Based on the Avrami Model

Abstract: The D - E hysteresis loop of ferroelectrics is theoretically studied on the basis of the extended Avrami theory. If the sideway velocity depends only on the instant value of the applied field, the volume fraction of the reversed area is expressed as q ( E )=1-exp (- f - d Φ( E )), where f and d are, respectively, the frequency of the applied field and the growth dimension of domains, and Φ is a function of E . This result is obtained irrespective of the waveform of the applied field and the field dependence of the sideway velocity, if the nucleation event is deterministic. For the stochastic nucleation due to thermal fluctuation, on the other hand, the above result is modified as q ( E )=1-exp (- f -( d +1) Φ( E )). The effect of the delay of wall motion is also discussed.

First Principle Calculation of the Magnetocrystalline Anisotropy Energy of FePt and CoPt Ordered Alloys

Abstract: The spin-polarized band calculations including spin-orbit interaction for L1 0 -FePt and CoPt ordered alloys have been performed with LMTO-ASA method in the frame of local spin density functional approximation. It has been shown that strong uniaxial magnetic anisotropy of both alloys is brought about by a large spin-orbit coupling of Pt atom and a strong hybridization of Pt d bands with highly polarized Fe (Co) d bands. The obtained magnetocrystalline anisotropy energy (MAE) is about 16×10 6 J/m 3 for FePt and 9×10 6 J/m 3 for CoPt. It is also found that both MAE's have a trend of increase with increasing axial ratio c / a in the vicinity of measured c / a . This can be regarded as being associated with the behavior that the MAE's decrease with increasing band filling.

Transport Anomalies of High-T c Oxides above Room Temperature

Abstract: Transport properties of La 2- x Sr x CuO 4 , YBa 2 Cu 3 O 6+ x and Bi 1.6 Pb 0.5 Sr 1.9- y La y Cu 1.05 O z have been studied over wide temperature regions. Drastic temperature variations of the Hall coefficients R H found in the underdoped region of the hole carriers suggest that there exists a crossover-like change of the electronic state above room temperature. Temperature dependence of other transport properties studied at high temperatures is also discussed in relation to this anomalous behavior of R H . These results elucidate the formation process of the abnormal metallic state of high- T c oxides at low temperatures, the existence of which characterizes the metal-insulator transition of the Cu-oxide system as a new-type Mott-Hubbard transition.

Exciton Features in 0-, 2-, and 3-Dimensional Networks of [PbI6]4- Octahedra

Abstract: We have investigated reflection spectra at 4 K for single crystals of (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O which consists of isolated [PbI 6 ] 4- octahedra, (C 10 H 21 NH 3 ) 2 PbI 4 and (CH 3 NH 3 )PbI 3 , which consist 2- and 3-dimensional networks of [PbI 6 ] 4- , respectively. The formation energy of the lowest exciton state increases from 1.633 eV to 3.4 eV with decreasing the dimensionality in the crystal structure, from the transfer energy between the adjacent octahedra is estimated to be about 0.5 eV. The on-site Coulomb energy in the octahedron is estimated to be 2.3 eV. We suggest that the large on-site Coulomb energy enhances the exciton binding energy, especially in the 2-dimensional system.

New Coupled Integrable Dispersionless Equations

Abstract: New coupled integrable dispersionless equations are presented and solved by the inverse scattering method.

Structural and magnetic studies of metal-insulator transition in thiospinel CuIr2S4

Abstract: Crystal structure and magnetic susceptibility were investigated for a thiospinel compound CuIr 2 S 4 , which exhibits a transition from metallic to insulating on cooling at the transition temperature T c of 230 K. It was found that the transition is accompanied by a first-order structural transition. As the compound is cooled through T c , the structure transforms from cubic to tetragonal with a volume contraction of 0.7%. The tetragonal phase has an axial ratio of about 1.03. Magnetic susceptibility, which is paramagnetic in the metallic region, decreases sharply and becomes diamagnetic at T c on cooling. The susceptibility is mostly temperature independent in each region above and below T c . Magnetic measurements also revealed that high pressure stabilizes the insulating phase, in contrast with most of the compounds that exhibit metal-insulator transitions. Possible mechanisms of the metal-insulator transition are discussed.

Quadrupole-Strain Interaction in Rare Earth Hexaborides

Abstract: Temperature dependence of the elastic constants in a series of rare earth hexaborides has been investigated by means of the ultrasonic method. The remarkable softening of the transverse C 44 mode in PrB 6 , NdB 6 , DyB 6 and HoB 6 has been discussed in connection with the cooperative Jahn-Teller effect due to the CEF ground state degenerated by the quadrupolar moment. The low temperature behavior of C 44 in Ce x La 1- x B 6 compounds is presented to show the Kondo effect on the quadrupolar moment of Γ 8 ground state. The anomaly of the bulk modulus C B in SmB 6 is related to the valence fluctuation state in the system.

Magnetic Properties of the Spin-1/2 Ferromagnetic-Ferromagnetic-Antiferromagnetic Trimerized Heisenberg Chain

Abstract: The magnetic properties of the ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin-1/2 Heisenberg chain are studied theoretically. The high temperature susceptibilty and the ground state saturation magnetic field are calculated and the exchange energies of the trimer compound 3CuCl 2 ·2dx are determined. The magnetization curve is obtained by numerical diagonalization of finite size systems. The result explains the low temperature magnetization data for 3CuCl 2 ·2dx with the exchange energies obtained as above. It is predicted that the magnetization curve has a plateau at 1/3 of the saturation magnetization if the ferromagnetic exchange energy is comparable to or smaller than the antiferromagnetic exchange energy.

Cu NMR and NQR Studies of High-Tc Superconductor La2-xSrxCuO4.

Abstract: Results of extensive Cu NMR and NQR studies in the superconducting La 2- x Sr x CuO 4 with x =0.075∼0.24 have been presented. The rapid decrease of the nuclear spin-lattice relaxation rate, 1/ T 1 , and the spin Knight shift below T c confirm microscopically the existence of the superconducting phase with lower T c than 38 K for 0.10 ≤ x ≤0.24. For x =0.10, magnetic anomalies emerge at low- T in some small parts of the sample, e.g . T -independent T 1 and broadening of spectrum, having some relevance to the magnetic order in La 2- x Ba x CuO 4 with x =0.125. For x >0.15, the T 1 T =const. behavior and the existence of the residual spin Knight shift far below T c have revealed that the superconductivity is of a gapless type with a finite density of states at the Fermi level caused by the mixture of different structures, i.e. tetragonal and orthorhombic.

Photoemission and Bremsstrahlung Isochromat Spectroscopy Studies of TiO2 (Rutile) and SrTiO3

Abstract: Photoemission and bremsstrahlung isochromat spectra have been measured on TiO 2 (rutile) and SrTiO 3 . The magnitudes of the fundamental band gaps estimated from the combined spectra agree well with the results of the photoabsorption and the energy-band calculations. The profiles of the observed spectra do not reveal the fine structure expected to occur in the density-of-states curves obtained by the energy band calculations, suggesting the existence of a mechanism to smear out the fine structure considerably. The combined spectra are used to interpret the charge-transfer-type satellite manifest in the core-level spectra. For 3 s photoemission, satellite features ascribed to the configuration interaction between the 3 s 3 p 6 and 3 s 2 3 p 4 3 d states in the atomic notation are also observed.

Study on D-E Hysteresis Loop of TGS Based on the Avrami-Type Model

Abstract: We measured the D - E hysteresis loop of TGS (triglycine sulphate), and transformed it to the q - E hysteresis loop by a method proposed in this paper, where q is the volume fraction of reversed domains. The experimental result was compared with a theory based on the Avrami model by the present authors [H. Orihara, S. Hashimoto and Y. Ishibashi: J. Phys. Soc. Jpn. 63 (1994) 1031], in which the volume fraction is expressed as q ( E )=1-exp (- f - d Φ( E )), where f and d are, respectively, the frequency of the applied field and the growth dimension of domains, and Φ is a function of E . We obtained a good agreement between them and confirmed the validity of our theory. The frequency dependences of the coercive field and the area inside the loop were measured and power laws were obtained with respect to them, which are discussed on the basis of the theory.

Experimental Investigation on the Validity of Population Dynamics Approach to Bacterial Colony Formation

Abstract: We have investigated the dynamics of a two-dimensional spreading of a bacterial population in a surface environment. After point inoculation of flagellated bacteria ( Bacillus subtilis ) on nutrient-rich semi-solid medium, the bacterial population grew up by multiplication and translocation of cells, and developed a homogeneous round colony. By comparing experimental results with those of numerical simulations of the model equation, we found that this homogeneous population growth of bacteria is an actual manifestation of growth dynamics described by the Fisher's equation.

Magnetic Properties of Extended t-J Model. II. Dynamical Properties

Abstract: The dynamical spin susceptibility, χ( q , ω), of the extended t - J model, which reproduces Fermi surfaces of LSCO and YBCO type of high- T c cuprates, has been evaluated in the slave-boson mean field approximation based on the uniform (u-) and the d -wave singlet (s-) RVB states. The calculated results show reasonable agreement with neutron scattering data for the incommensurate structure in LSCO. For the commensurate structure in 90 K YBCO, the ω-dependence shows general agreement, but the location of the strongest peak at low temperatures does not. The temperature ( T ) dependences of the rates, T 1 -1 and T G -1 , and the shift of NMR are consistent with experimental data in the high-doping region, while they are not in the low-doping (but superconducting) region. We propose that the sharp drop in ( T 1 T ) -1 above T c in the low-doping region may be due to singlet formation.

Single Uranium-Site Properties of the Dilute Heavy Electron System UxTh1-xRu2Si2 (x≤0.07)

Abstract: Specific heat C , electrical resistivity ρ , magnetization M and magnetic susceptibility χ have been measured for the dilute tetragonal heavy-electron compounds U x Th 1- x Ru 2 Si 2 ( x ≤0.07), in the temperature range 100 mK≤ T ≤360 K and in the magnetic fields H up to 55 kOe. Non-Fermi-liquid behavior with unusual lnT dependence of C , ρ and χ has been observed at the low temperatures: as the temperature is lowered below about 10 K, C / T and χ increase logarithmically, and ρ decreases logarithmically. Systematical variations of these quantities with uranium concentration indicate that the observed non-Fermi-liquid behavior is ascribed to the properties of a single U ion embedded in the Th site of the ThRu 2 Si 2 metal. The magnetization curves M vs H / T indicate a strong uniaxial magnetic anisotropy along the tetragonal c -axis as well as a strong reduction of the 5 f moment of U ions. A possible interpretation based on the recent developments of the S =1/2 two-channel Kondo model is given.

Optical Stark Spectroscopy of a Brownian Oscillator in Intense Fields

Abstract: The optical Stark effect of a two-level system coupled to a Brownian oscillator (i.e. a harmonic mode which in turn is coupled to a heat bath) is studied using equations of motion for a reduced density matrix. These equations, derived using path integral techniques, can be used to study the combined effects of strong fields and dephasing processes at finite temperature, and interpolate continuously from the coherent to the overdamped limits where they reduce to the stochastic Gaussian-Markovian equation. Numerical calculations of probe absorption spectra for various pump intensities are presented, and show dynamical Stark splitting. In contrast to the Bloch equations which contain an infinite-temperature dephasing, we find that at finite temperature, the Stark peaks may have different heights even when the pump pulse is on resonance.

Spin Fluctuation Spectra and High Temperature Superconductivity

Abstract: The spin fluctuation mechanism for the high temperature superconductivity in cuprates is investigated with a parametrization of the spin fluctuation spectra in terms of the self-consistent renormalization theory of spin fluctuations. The doping concentration dependence of the transition temperature in the best studied cuprates is explained in terms of the parameter values estimated from the normal state experiments. We find that the larger the amplitude and the energy spread of spin fluctuations, the higher T c . In this context the cuprates have much favorable properties compared with some other antiferromagnetic spin fluctuation systems whose parameter values are estimated for comparison.

Dynamical Phase Transition in One Dimensional Traffic Flow Model with Blockage

Abstract: Effects of a bottleneck in a linear trafficway is investigated using a simple cellular automaton model. Introducing a blockage site which transmit cars at some transmission probability into the rule-184 cellular automaton, we observe three different phases with increasing car concentration: Besides the free phase and the jam phase, which exist already in the pure rule-184 model, the mixed phase of these two appears at intermediate concentration with well-defined phase boundaries. This mixed phase, where cars pile up behind the blockage to form a jam region, is characterized by a constant flow. In the thermodynamic limit, we obtain the exact expressions for several characteristic quantities in terms of the car density and the transmission rate. These quantities depend strongly on the system size at the phase boundaries; We analyse these finite size effects based on the finite-size scaling.

Electronic and Lattice Properties of Carbon Nanotubes

Abstract: The symmetry properties of chiral monolayered carbon nanotubes, along with the method of zone-folding, are used to study their electronic and lattice properties. The energy bands and phonon modes are classified according to the irreducible representations of the 1D periodicity of the chiral nanotubes.

Electronic Structure and Magnetocrystalline Anisotropy Energy of MnAl

Abstract: The spin-polarized band calculations including spin-orbit interaction for τ -phase (L1 0 ) MnAl have been performed with LMTO-ASA method in the frame of LSD. It has been predicted that the MnAl has a magnetic moment of 2.4 µ B with a help of the tetragonal distortion from cubic B2 to L1 0 structure. The magnetocrystalline anisotropy energy (MAE) is obtained as (0.26 ± 0.01) meV/f.u. This leads to 1.5 ×10 6 J/m 3 for magnetic anisotropy constant which is comparable to the measured value ∼10 6 J/m 3 . The dependence of the MAE on axial ratio c / a also reflects the influence of the tetragonal distortion, that is, the MAE shows drastic change from negative to positive at the vicinity of c / a =0.7071 which corresponds to cubic B2 lattice. For larger value of c / a , MAE decreases with increasing c / a and goes into negative again for c / a >1.2.

Numerical Study of the S=1 Antiferromagnetic Spin Chain with Bond Alternation.

Abstract: We study the S =1 quantum spin chain with bond alternation H =∑ i (1-(-1) i δ) S i · S i +1 by the density matrix renormalization group method recently proposed by White (Phys. Rev. Lett. 69 (1992) 2863). We find a massless point at δ c =0.25 ±0.01. We also find the edge states in the region δ δ c . At the massless point, the spin wave velocity v s is 3.66±0.10 and the central charge c is 1.0±0.15. Our results indicate that a continuous phase transition occurs at the massless point δ=δ c accompanying breaking of the hidden Z 2 × Z 2 symmetry.

Lattice Instability in Metallic Carbon Nanotubes

Abstract: A lattice instability toward in-plane Kekule and out-of-plane distortions is studied in metallic carbon nanotubes in both k · p approximation and tight-binding model. The k · p model Hamiltonian is given by Dirac's relativistic equation in the presence of distortion. The resulting gap equation is solved analytically in the presence of an Aharonov-Bohm magnetic flux through the cross section of nanotube, leading to the conclusion that the in-plane and out-of-plane distortions cannot coexist. A tight-binding model gives results in excellent agreement with k · p results and justifies the k · p model Hamiltonian.

Itinerant-localized duality picture of spin fluctuations in high-Tc cuprates. I: Nested spin-fluctuation theory

Abstract: The mode-mode coupling theory of spin fluctuations in the normal state of high- T c cuprates is developed on the basis of the itinerant-localized duality picture of technically nested two-dimensional metals. This offers a unified description of the anomalies observed in the normal state of high- T c cuprates. The so-called “spin-gap” behaviors of the resistivity, the specific heat, the uniform spin susceptibility, and the optical conductivity are characterized by the energy scale T * of the crossover between the itinerant and the localized spin fluctuations, while those of the Hall coefficient, the thermo-electric power, the longitudinal NMR relaxation rate, and the neutron scattering intensity, are characterized by different energy scales less than T * . The integrated weight of the nested spin fluctuation decreases when decreasing the temperature or approaching the perfect nesting, leading to the “spin-gap” behavior.

Spin gap behavior in ladder-type of quasi-one-dimensional spin (S=1/2) system SrCu2O3

Abstract: Low-energy spin excitation in a spin S =1/2 two-leg ladder system SrCu 2 O 3 has been investigated by Cu NMR. From an activated decrease of the nuclear-spin lattice relaxation rate, 1/ T 1 , in a T -range of 100–300 K, a spin gap, Δ, is demonstrated to open with a value of 680 K, which is in quantitative agreement with the theoretical value of Δ∼0.5 J =650 K , if the same value of 1300 K as in the square lattice system is used as the antiferromagnetic exchange coupling constant, J . It is furthermore deduced from the measurement of the spin-echo decay rate, 1/ T 2G , that a magnetic coherence length, ξ/ a ( a : lattice spacing between Cu atoms along the chain), remains finite with (ξ/ a )∼13, comparable to a value in La 2 CuO 4 near T N . This class of cuprate is thus a frustrated quntum antiferromagnet (AF) with a spin gap.

Simultaneous Measurement of Thermal Diffusivity and Conductivity Applied to Bi-2223 Ceramic Superconductors

Abstract: A method to measure the thermal diffusivity α and the conductivity κ under an identical experimental setup has been developed and α and κ of Bi-2223 oxide superconductor have been measured quasi-simultaneously. The results are analyzed on the basis of the BRT and Tewordt-Wolkhausen theory. The simultaneous measurement makes it possible to estimate the specific heat C and the Debye temperature Θ D , as well as to separate the electron and phonon contributions to the diffusivity. The simultaneous measurement also provides a useful check on the reliability and the consistency of the analyses.

On the Magnetism and Electronic Conduction of Itinerant Magnetic System Ca1-xSrxRuO3

Abstract: Electrical resistivity ρ and magnetic susceptibility of sintered Ca 1- x Sr x RuO 3 have been measured. The Weiss temperature changes sign at about x =0.5 in consistent with other studies. Temperature dependence of normalized ρ is almost identical for all the materials in the paramagnetic temperature range irrespective of the sign of magnetic interaction. The Rhodes-Wolfarth ratio is 1.3 for x =1. This ratio and the temperature dependence of ρ suggest that 4 d -electrons in this system are in the intermediately localized state as in Ni. The sign of magnetic interaction is discussed in connection with the orthorhombic lattice distortion, and the antiferromagnetic interaction is attributed to this distortion.