arXiv subjects

T. Kimura

Publications and source records attributed to T. Kimura.

85 records · Page 5Linked to original sources

arXiv2001

Spectacular doping dependence of interlayer exchange and other results on spin waves in bilayer manganites

We report the measurement of spin waves in the bilayer colossal magnetoresistive manganites La(2-2x)Sr(1+2x)Mn2O7 with x=0.30, 0.35 and 0.40. For x=0.35 and 0.40 the entire acoustic and optic dispersion relations are reasonably well described by those for a bilayer Heisenberg Hamiltonian with nearest neighbour exchange only. The in-plane coupling is only weakly dependent on x, but the coupling between the planes of a bilayer changes by a factor of four. The results directly reveal the change from mixed and character of the orbitals to mostly character with increasing hole concentration.

cond-mat.str-el↗

arXiv2000

Fermi surface of Sr2RuO4 from angle resolved photoemission

We investigated the low-energy electronic structure of Sr2RuO4 by angle-resolved photoemission spectroscopy (ARPES) focusing, in particular, on the controversial issues of the Fermi surface (FS) topology and of the extended van Hove singularity (evHs). A detailed study of temperature and photon energy dependence demonstrates the surface-state nature of the electronic band responsible for the evHs, and may suggest the presence of ferromagnetic correlations at the surface. In addition, we detected a replica of the primary FS that indicates a (sqrt2 x sqrt2) surface reconstruction, as confirmed by LEED. In light of these findings, the FS determined by ARPES is consistent with LDA and de Haas-van Alphen (dHvA) results, and it provides additional information on the detailed shape of the alpha, beta and gamma sheets. Also, at this stage, because of the strong sensitivity of the cleaved surface, we could not find any robust signature of the crossover from 2D to 3D Fermi liquid behavior in our ARPES measurements for T<130 K.

cond-mat.str-el↗

arXiv2000

Fermi Surface, Surface States, and Surface Reconstruction in Sr2RuO4

The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and of the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to (sqrt2 x sqrt2) surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the alpha, beta and gamma sheets.

cond-mat.str-el↗

arXiv1999

Temperature-Dependent Pseudogaps in Colossal Magnetoresistive Oxides

Direct electronic structure measurements of a variety of the colossal magnetoresistive oxides show the presence of a pseudogap at the Fermi energy E_F which drastically suppresses the electron spectral function at E_F. The pseudogap is a strong function of the layer number of the samples (sample dimensionality) and is strongly temperature dependent, with the changes beginning at the ferromagnetic transition temperature T_c. These trends are consistent with the major transport trends of the CMR oxides, implying a direct relationship between the pseudogap and transport, including the "colossal" conductivity changes which occur across T_c. The k-dependence of the temperature-dependent effects indicate that the pseudogap observed in these compounds is not due to the extrinsic effects proposed by Joynt.

cond-mat.str-el↗

arXiv1999

Resistive upper critical fields and irreversibility lines of optimally-doped high-T_c cuprates

We present the resistively-determined upper critical field H^ρ_{c2}(T) and the irreversibility lines H^ρ_{irr}(T) of various high-T_c cuprates, deduced from measurements in 61-T pulsed magnetic fields applied parallel to the c-axis. The SHAPE of both H^ρ_{c2}(T) and H^ρ_{irr}(T) depends monotonically on the anisotropy of the material and none of the samples show saturation of H^ρ(T) at low temperatures. The anomalous positive curvature, d^2 H^ρ/dT^2 > 0, is the strongest in materials with the largest normal-state anisotropy, regardless of whether anisotropy is varied by changing the carrier concentration or by comparing a variety of optimally-doped compounds.

cond-mat.supr-con↗

arXiv1999

Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)

We report the results of Raman scattering experiments on single crystals of La(2-x)Sr(x)CuO(4) that span the range from underdoped (x = 0.10) to overdoped (x =0.22). The spectra are consistent with the existence of a strong anisotropic quasiparticle interaction that results in a normal state depletion of spectral weight from regions of the Fermi surface located near the zone axes. The strength of the interaction decreases rapidly with increasing hole concentration and the spectral evidence for the pseudogap vanishes when the optimum doping level is reached. The results suggest that the pseudogap and superconducting gap arise from different mechanisms.

cond-mat.supr-con↗

arXiv1998

Temperature Evolution of the Pseudogap State in the Infra-Red Response of Underdoped La_(2-x)Sr_xCuO_4

The ab-plane optical spectra of two single crystals of underdoped La_(2-x)Sr_xCuO_4 were investigated. The reflectivity of La_(1.87)Sr_(0.13)CuO_4 has been measured in the frequency range 30- 9,000 cm-1 (0.004 -- 1 eV) both parallel and perpendicular to the CuO_2 planes, whereas La_(1.86)Sr_(0.14)CuO_4 was studied only in the ab-plane. The extended Drude model shows that the frequency-dependent effective scattering rate 1/tau(omega)is strongly suppressed below the high-frequency straight-line extrapolation, a signature of the pseudogap state. This suppression can be seen from temperatures below the superconducting transition up to 400 K. In the case of underdoped LSCO the straight-line extrapolation is temperature independent below 200 K, whereas above 200 K there is a strong temperature dependence of the high-frequency 1/tau(omega,T). The out-of-plane direction is also examined for evidence of the pseudogap state.

cond-mat.supr-con↗

arXiv1998

An ordered stack of spin valves in a layered magnetoresistive perovskite

The layered compound La2-2xSr1+2xMn2O7 (x=0.3) consists of bilayers of metallic MnO2 sheets separated by insulating material. The compound exhibits markedly anisotropic magnetoresistance at temperatures well below the three-dimensional magnetic ordering temperature TC=90 K in addition to colossal magnetoresistance around TC. We present neutron diffraction data which show that the magnetic structure of this material switches from antiferromagnetic stacking of the (ferromagnetically ordered) sheets in zero field to ferromagnetic stacking in a field of 1.5 Tesla. The data are the first to be collected on any manganite as a function of applied field, exactly as the magnetoresistance data themselves are collected. They provide a natural explanation of the low-field magnetoresistance in the ordered phase in terms of spin-polarised tunnelling between the magnetic layers and suggest that the material is a bulk stack of spin-valve devices.

cond-mat.str-el↗

arXiv1998

Doping Dependent Density of States and Pseudogap Behavior in La2-xSrxCuO4

We have made a high-resolution photoemission study of La2-xSrxCuO4 in a wide hole concentration (x) range from a heavily overdoped metal to an undoped insulator. As x decreases, the spectral density of states at the chemical potential (μ) is suppressed with an x-dependence similar to the suppression of the electronic specific heat coefficient. In the underdoped region, the spectra show a pseudogap structure on the energy scale of 0.1 eV. The width of the pseudogap increases with decreasing x following the x-dependence of the characteristic temperatures of the magnetic susceptibility and the Hall coefficient.

cond-mat.supr-con↗

arXiv1998

Evidence for Magnetic Pseudoscaling in Overdoped La(2-x)Sr(x)CuO(4)

We report the results of electronic Raman scattering experiments on an overdoped La(1.78)Sr(0.22)CuO(4) single crystal as a function of temperature. The scattering rate Gamma(w->0,T) has been determined from the normal state B(1g) spectra in the range 50 K < T < 300 K. Gamma(T) decreases linearly from 300 K to about 175 K and then undergoes a reduction with respect to the expected mean-field behavior. This trend suggests a crossover to pseudoscaling regime at about T(cr)=160 K. The results are in good agreement with the prediction of the nearly antiferromagnetic Fermi liquid model. There is no evidence of a pseudogap in the spectra obtained from this overdoped sample.

cond-mat.supr-con↗

arXiv1997

Doping Dependence of the Pseudogap State in the ab plane IR responce of La(2-x)Sr(x)CuO(4)

The ab plane optical spectra of two single crystals of La(2-x)Sr(x)Cu(4), one overdoped and one underdoped, were investigated. We observe a gap-like depression of the effective scattering rate below 700 cm(-1) in both systems. This feature persists up to 300 K in the underdoped sample with the concentration of Sr x=0.14 but loses prominance at temperatures above 300 K in the overdoped regime (x=0.22). Below 700 cm(-1) the scattering rate is temperature dependent and superlinear in frequency for both samples. Above this frequency the effective scattering rate becomes linear in frequency and is temperature independent in the case of the underdoped La(1.86)Sr(0.14)CuO(4) up to 300 K. On the other hand, the overdoped La(1.76)Sr(0.22)CuO(4) shows a scattering rate temperature dependence above 700 cm(-1) at all temperatures. This behaviour of the frequency and temperature dependent scattering rates is a signature of a pseudogap state in other HTSC materials and suggests that both the under and overdoped single-layer HTSC systems Ls(2-x)Sr(x)CuO(4) have a pseudogap at temperatures exceeding 300 K.

cond-mat.supr-con↗

arXiv1994

On operad structures of moduli spaces and string theory

Recent algebraic structures of string theory, including homotopy Lie algebras, gravity algebras and Batalin-Vilkovisky algebras, are deduced from the topology of the moduli spaces of punctured Riemann spheres. The principal reason for these structures to appear is as simple as the following. A conformal field theory is an algebra over the operad of punctured Riemann surfaces, this operad gives rise to certain standard operads governing the three kinds of algebras, and that yields the structures of such algebras on the (physical) state space naturally.

hep-th↗

arXiv1992

Treatment of Constraints in Stochastic Quantization Method and Covariantized Langevin Equation

We study the treatment of the constraints in stochastic quantization method. We improve the treatment of the stochastic consistency condition proposed by Namiki et al. by suitably taking account of the Ito calculus. Then we obtain an improved Langevin equation and the Fokker-Planck equation which naturally leads to the correct path integral quantization of the constrained system as the stochastic equilibrium state. This treatment is applied to $O(N)$ non-linear $σ$ model and it is shown that singular terms appearing in the improved Langevin equation cancel out the $δ^n(0)$ divergences in one loop order. We also ascertain that the above Langevin equation, rewritten in terms of independent variablesis, actually equivalent to the one in the general-coordinate-transformation-covariant and vielbein-rotation-invariant formalism.

hep-th↗