Yasumasa Takayanagi

Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2

Raman selection rules for electronic and magnetic excitations in BaFe 2 As 2 were theoretically investigated and applied them to the separate detection of the nodal and anti-nodal gap excitations at the spin density wave (SDW) transition and the separate detection of the nearest and the next nearest neighbor exchange interaction energies. Raman spectra are composed of magnetic excitations with gradually decreasing intensity toward far above the SDW transition temperature ( T SDW ) and electronic excitations induced by the Brillouin zone folding below T SDW . The SDW gap has Dirac nodes, because many orbitals participate in the electronic states near the Fermi energy. Using a two-orbital band model the electronic excitations near the Dirac node and the anti-node are found to have different symmetries. Applying the symmetry difference to Raman scattering the nodal and anti-nodal electronic excitations are separately obtained. The low-energy spectra from the anti-nodal region have critical fluctuation just a...

Correlation between Raman sum and optical conductivity sum in La2−xSrxCuO4

In a strongly correlated electron system, the single-particle spectral function changes into a coherent peak and incoherent humps which extend over 1 eV. The incoherent parts lose the symmetry and k dependence, so that the Raman spectra with different symmetries become identical and they are expressed by the optical conductivity. We found that the B1g and B2g spectra in La(2-x)Sr(x)CuO4 become identical above 2000 cm(-1) in the underdoped phase, if Fleury-Loudon type B1g two-magnon scattering is removed. The first Raman susceptibility moment correlates with the generalized optical conductivity moment. The good correlation arises from the incoherent states of a hump from 1000 to 4000 cm(-1). The hump is the only structure of the incoherent electronic states in the mid-infrared absorption spectra below 1.4 eV at low carrier densities. The energy is twice the separated dispersion segments of the spin wave in the k(perpendicular) stripe direction. The incoherent state is formed by the magnetic excitations created by the hole hopping in the antiferromagnetic spin stripes in the real space picture.

s or d(xy) Wave Superconducting Gap Created from the Electron-Phonon Coupled States in Underdoped La2-xSrxCuO4

In La 2- x Sr x CuO 4 the superconducting gap energy in the B 2g Raman spectra is almost constant from the optimum doping to near the insulator–metal (I–M) transition, indicating that the gap symmetry is s or d ( x y ), because the Fermi arc shrinks into the nodal direction of the d ( x 2 - y 2 ) gap. The reason of the different result from other experiments comes from the dual components of the low-energy electronic states around (π/2, π/2), the resonant peak relating to the I–M transition and the polaron states. The superconducting coherent peak is found to be created only in the polaron states in the underdoped phase.

Electric resistivity and phase diagram viewed from charges in La2- xSrxCuO4

The electric transport changes from the power-law temperature dependent resistivity ρ= a + b T c to the logarithmic temperature dependent conductivity σ= d + e ln T as temperature decreases across 90 K in the underdoped phase. The crossover temperature is independent of the carrier density differently from the pseudo-spin-gap. The crossover is not caused by the simple relaxation time, but by the change of the density of states near the Fermi energy in the quasiparticle (resonant) peak at (π/2,π/2). The exponent c decreases from 2 at x =0.03 to 1 at x =0.16 and then increases to 1.4 at x =0.25.

s or d(xy) wave superconducting gap created from the electron-phonon coupled states in underdoped La2-xSrxCuO4

The present Raman scattering experiment in La2-xSrxCuO4 disclosed that the low-energy conductive electronic states at (π/2, π/2) in the underdoped phase are composed of dual electronic states of the central resonant peak and the electron-phonon coupled polaron states. Both states develop at the lowest-energy end of the B2g electron - two-magnon coupled high-energy peak. The narrow resonant peak is enhanced at the insulator-metal transition, but it is insensitive to the superconducting transition indicating no relation to the superconductivity. On the other hand the polaron peak changes into the superconducting coherent peak (SCP) exactly below Tc. The gap energy determined from the SCP is almost constant from the optimum doping to near the insulator-metal transition, indicating that the gap symmetry is s or d(xy), because the Fermi arc with the large electronic density of states shrinks into the nodal direction of the d(x2 - y2) gap. In the overdoped phase the low-energy conductive electronic states are created at (π, 0) by decreasing the peak energy of the B1g electron - two-magnon coupled states. The gap symmetry in the overdoped phase is consistent with d(x2 - y2).

Raman and Optical Reflection Studies of Electronic States in La2−xSrxCuO4

The electronic states in La2−xSrxCuO4 are systematically investigated by Raman scattering, infrared‐ultraviolet reflection spectroscopy, and electric resistivity. A narrow quasi‐particle band is created at EF and sharply grows up by collecting the density of states from the high energy region at 1–1.5 eV as temperature decreases near the insulator‐metal transition. The relaxation time of conducting carriers is limited by the quasi‐particle band width near the insulator‐metal transition, but it drastically increases in the overdoped phase. The quasi‐particle band gives the anomalous electronic properties.The electronic states in La2−xSrxCuO4 are systematically investigated by Raman scattering, infrared‐ultraviolet reflection spectroscopy, and electric resistivity. A narrow quasi‐particle band is created at EF and sharply grows up by collecting the density of states from the high energy region at 1–1.5 eV as temperature decreases near the insulator‐metal transition. The relaxation time of conducting carriers is limited by the quasi‐particle band width near the insulator‐metal transition, but it drastically increases in the overdoped phase. The quasi‐particle band gives the anomalous electronic properties.

Symmetry crossover of the superconducting coherent peak and the quasi-particle band in high Tc superconductors

Abstract The carrier density dependence of superconducting coherent peaks (pair-breaking peaks), two-phonon peaks, and two-magnon peaks was systematically investigated by Raman scattering on cleaved surfaces of YBa 2 Cu 3 O y (YBCO), La 2− x Sr x CuO 4 (LSCO), Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8+ δ (Bi2212), and Bi 2− x Pb x + y Sr 2− y − z La z CuO 6+ δ (Bi2201). The coherent peak is observed generally around ( π /2, π /2) at low carrier densities and ( π ,0) at high carrier densities. The clear crossovers are observed at y =6.63 ( p =0.1) in YBCO and at x =0.15 in LSCO. These crossovers are related to the change from ( π /2, π /2) to ( π ,0) of the low-energy density of states of the quasi-particle band. However, the rapid change in the coherent peak cannot be interpreted only by the smooth change of the low-energy density of states, but needs an additional anisotropic pair-breaking mechanism. We found that the momentum of the mode giving the strongest two-phonon peak changes from ( π , π ) to ( π ,0) with the increase of carrier density in good correlation with the coherent peak. It suggests that this electron–phonon interaction determines the superconducting region in the momentum space.