Kaichiro Chiba

139La-NMR Study on La-based High-Tc Cuprates La2-xMxCuO4 (M=Sr, Ba) around x=0.115 under High Magnetic Field.

The effect of the external field on La-NMR spectra has been studied on La-based high- T c cuprates La 2- x Sr x CuO 4 ( x ≈0.115) and La 2- x Ba x CuO 4 ( x =0.125), where the magnetic ordering in Cu-3 d spin exists at low temperature. In the magnetically ordered state, the broadening in the resonance line was observed, reflecting the hyperfine field at the La-site contributed by the Cu-3 d spins. By applying a high magnetic field of 15 T, this broadening in La 2- x Sr x CuO 4 showed a significant shrink, which is interpreted as the spin-flop in the canted-spin system with the Dzyaloshinski-Moriya interaction. A model is proposed to explain both this spin-flop and the previously-reported anomalous field dependence of the structural instability in this system.

139La-NMR study of spin-flop and spin structure in La2-xSrxCuO4 (x = 1/8)

The external field dependence of the hyperfine field at La-site has been studied on La2−xSrxCuO4(x=0.115) and La2−xBaxCuO4(x=0.125), where the magnetic ordering in Cu-3d spins exists at low temperature. In La1.885Sr0.115CuO4, a significant decrease in the resonance line width was observed above a field of Hc≈ 7.5T, which is the direct evidence of the spin-flop in the canted-spin system with Dzyaloshinski-Moriya interaction. The spin-flop field Hcof 7.5T is slightly lower than that in La2CuO4(Hc ≈ 10T), reflecting the decrease of the tilting angle of the CuO6octahedra by the substitution of Sr2+ion for La-site.

Elastic anomalies induced by spin-flop in La1.88Sr0.12CuO4

Abstract Temperature dependence of the velocity V s and the attenuation of the ultrasonic waves are measured to investigate the relation between the spin type of magnetic order and lattice instability of the low-temperature tetragonal phase in La 2− x Sr x CuO 4 ( x∼ 1 8 ). In the field above 8 T along the [0 1 0] ∘ direction of the orthorhombic notation, a significant increase of V s in the c 44 mode is observed below ∼15 K where spin-flop occurs. This gigantic increase of V s suggests that the structural transformation from the orthorhombic to the low-temperature tetragonal phase is induced by the spin-flop.

Magnetic phase transition and electronic anomalies in La{sub 1.96{minus}x}Y{sub 0.04}Sr{sub x}CuO{sub 4} around x{approximately}1/8

La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of Tcand the structural transformation around x∼1/8 in La1.96−xY0.04SrxCuO4. By the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57±14 K for samples around x∼0.115. The. prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x∼0.115 below temperatures Tsand TN, respectively. Tsvs x shows a bell shaped curve with the maximum value of 65 K at x∼00115 where most prominent suppression of Tcappears. TNshows similar x dependence to Tswith the maximum value of 40 K at x∼0.115. The change of the electronic state below Tsand the suppression of Tcbecome more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of {Tc} and the structural transformation around x{approximately}1/8 in La{sub 1.96{minus}x}Y{sub 0.04}Sr{sub x}CuO{sub 4}. by the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57{+-}14 K for samples around x{approximately}0.115. The prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x{approximately}0.115 below temperatures T{sub s} and T{sub N}, respectively. T{sub s} vs x shows a bell shaped curve with the maximum value of 65 K at x{approximately}0.115 where most prominent suppression of {Tc} appears. T{sub N} shows similar x dependence to T{sub s} with the maximum value of 40 K at x{approximately}0.115. The change of the electronic state below T{sub s} and the suppression of {Tc} become more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.

Structural phase transition at low temperature in La2-xSrxCuO4 around x =0.12 under magnetic fields

The effect of magnetic field on the structural phase transitions at low temperature in La2−xSrxCuO4 has been investigated using the ultrasonic sound velocity experiment. An increase in the sound velocityVs is observed below ∼10K aroundx=0.12, which seems to be a precursor of the structural phase transition. This increase ofVs is drastically enhanced by applying a magnetic field. In case ofx=0.120, the temperature whereVs shows the upturn shifts from 5K(H=0T) to 12K(H=15T), whereas, inx=0.160,Vs is not affected by magnetic fields up to 15T. These results suggest that the structure at low temperature (P42/ncm or Pccn) in La2−xSrxCuO4 aroundx=0.12 is stabilized in magnetic fields.

Anomalous broadening of the spin-flop transition in the reentrant spin-glass phase of La2-xSrxCuO4 (x = 0.018)

The magnetization in a lightly doped La 2 - x Sr x CuO 4 (x=0.018) single crystal was measured. Spin-flop transition was clearly observed in the hole-doped antiferromagnetically ordered state under increasing magnetic fields perpendicular to the CuO 2 plane. In the spin-glass phase below 25 K, the spin-flop transition becomes broad but the step in the magnetization curve associated with the transition remains finite at the lowest temperature. We show in this report that, at low temperature, the homogeneous antiferromagnetic order is disturbed by the redistribution of holes, and that the spatial variance of the local hole concentration around x=0.018 increases.

High-field Cu/La-NMR study on high-Tc cuprate La2-xBaxCuO4 (x = 0.125)

Abstract The La-based high-Tc cuprate La2−xBaxCuO4 (LBCO) shows the local suppression of the superconductivity around x= 1 8 . We have studied the temperature dependence of the resonance line width of Cu/La-NMR spectra for LBCO (x=0.125). The line width of La-NMR shows an increase below 40 K, reflecting the magnetic order, while the line width of Cu-NMR does at as high as 160 K. This difference demonstrates that the broadening in Cu-spectra is brought by the inhomogeneity in the electric field gradient, suggesting that CDW exists in this system.

NMR study on magnetic phase transition in electron-doped cuprate (La1−yYy)2−xCexCuO4 with T′-type structure

Abstract The magnetization of 4f-spins in the electron-doped high-Tc cuprate with T′-type structure M2−xCexCuO4 (MNd, Pr, Sm, Eu) has so far prevented NMR studies of the Cu 3d-spin and the conduction carrier. We have successfully synthesized the 4f-spin-free cuprate (La1−yYy)2−xCexCuO4 for Cu-NMR study. The observed sharp spectra of 63/65Cu without quadrupolar splittings similar to other conventional electron-doped cuprates like Nd2−xCexCuO4 indicate that the doped carrier in this system is electron-like. With Ar-annealing treatment, though the electrical resistance showed a drastic reduction, the Cu-NMR Knight shift did not change appreciably, suggesting that the Ar-annealing treatment does not bring any change in the carrier concentration.