arXiv subjects

Y. Endoh

Publications and source records attributed to Y. Endoh.

At least 55 records · Page 3Linked to original sources

arXiv2000

Systematic Study of Short Range Antiferromagnetic Order and The Spin-Glass State in Lightly Doped La2-xSrxCuO4

Systematic measurements of the magnetic susceptibility were performed on single crystals of lightly doped La2-xSrxCuO4 (x=0.03, 0.04 and 0.05). For all samples the temperature dependence of the in-plane magnetic susceptibility shows typical spin-glass features with spin-glass transition temperatures Tg of 6.3K, 5.5K and 5.0K for x=0.03, 0.04 and 0.05, respectively. The canonical spin-glass order parameter extracted from the in-plane susceptibility of all the samples follows a universal scaling curve. On the other hand, the out-of-plane magnetic susceptibility deviates from Curie law below a temperature Tdv, higher than Tg. Comparing with previous neutron scattering results with an instrumental energy resolution of 0.25 meV from Wakimoto et al., the x-dependence of Tdv is qualitatively the same as that of Tel, the temperature below which the elastic magnetic scattering develops around (pi, pi). Thus, a revised magnetic phase diagram in the lightly doped region of La2-xSrxCuO4 is proposed. The Curie constants calculated from the in-plane susceptibility are independent of the Sr concentration. On the basis of the cluster spin-glass model, this fact might reflect an inhomogeneous distribution of doped holes in the CuO2 plane, such as in a stripe structure.

cond-mat.supr-con↗

arXiv1999

Incommensurate Geometry of the Elastic Magnetic Peaks in Superconducting La1.88Sr0.12CuO4

We report magnetic neutron scattering measurements of incommensurate magnetic order in a superconducting single crystal of La1.88Sr0.12CuO4. We find that the incommensurate wavevectors which describe the static magnetism do not lie along high-symmetry directions of the underlying CuO2 lattice. The positions of the elastic magnetic peaks are consistent with those found in excess-oxygen doped La2CuO4+y. This behavior differs from the precise magnetic order found in the low temperature tetragonal La1.6-xNd0.4SrxCuO4 material for which stripes of spin and charge have been observed. These observations have clear implications for any stripe model proposed to describe the static magnetism in orthorhombic La2CuO4-based superconductors.

cond-mat.supr-con↗

arXiv1999

Incommensurate Spin Dynamics in High Temperature Superconductor of Optimally Doped YBa2Cu3O7

The spin dynamics of an optimally doped YBa2Cu3O7 (Tc = 93 K) crystal array have been investigated in a wide range of momentum and energy (Q - E) space using the time-of-flight neutron scattering method. Incommensurate spin modulation in Q is a characteristic feature, as it is in the under-doped YBa2Cu3O6.7 with a different incommensurability. A linear relationship between the incommensurability and Tc is proposed. Along with the discovery of the same incommensurability in under-doped La2-ySryCuO4, it may be a generic characteristic of the high-Tc oxide superconductor.

cond-mat.supr-con↗

arXiv1999

Direct Observation of a One Dimensional Static Spin Modulation in Insulating La1.95Sr0.05CuO4

We report the results of an extensive elastic neutron scattering study of the incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an insulating spin glass at low temperatures. The present neutron scattering experiments on the same x=0.05 crystal employ a narrower instrumental Q-resolution and thereby have revealed that the crystal has only two orthorhombic twins at low temperatures with relative populations of 2:1. We find that, in a single twin, only two satellites are observed at (1, +/-0.064, L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only along the orthorhombic b*-axis. This demonstrates unambiguously that La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low temperatures, consistent with certain stripe models. We have also reexamined the x=0.04 crystal that previously was reported to show a single commensurate peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the x=0.04 sample in fact has the same IC structure as the $x=0.05$ sample. The incommensurability parameter d for x=0.04 and 0.05, where d is the distance from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear relation d=x. These results demonstrate that the insulator to superconductor transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is coincident with a transition from diagonal to collinear static stripes at low temperatures thereby evincing the intimate coupling between the one dimensional spin density modulation and the superconductivity.

cond-mat.supr-con↗

arXiv1999

Freezing of anisotropic spin clusters in La1.98Sr0.02CuO4

A spin-glass compound, La$_{1.98}$Sr$_{0.02}$CuO$_4$, shows quasi-three-dimensional magnetic ordering below $\sim$40 K. A remarkable feature is that the magnetic correlation length along the orthorhombic $a\rm_{ortho}$ axis is much longer than that along the $b\rm_{ortho}$ axis, suggesting that the spin structure is closely related to the diagonal stripe structure. The spin-glass state can be expressed as a random freezing of quasi-three-dimensional spin clusters with anisotropic spin correlations ($ξ'_a\sim160$ Å, $ξ'_b\sim25$ Å, and $ξ'_c\sim4.7$ Åat 1.6 K). The new magnetic state is important as an intermediate phase between the three-dimensional antiferromagnetic ordered phase in La$_2$CuO$_4$ and the incommensurate phase in La$_{1.95}$Sr$_{0.05}$CuO$_4$ in which the positions of the incommensurate peaks are rotated by 45$^\circ$ in reciprocal space about ($π$,$π$) from those observed in the superconducting La$_2$CuO$_4$ compounds.

cond-mat.str-el↗

arXiv1999

Direct Observation of Antiferro-Quadrupolar Ordering - Resonant X-ray Scattering Study of DyB2C2

Antiferroquadrupolar (AFQ) ordering has been conjectured in several rare-earth compounds to explain their anomalous magnetic properties. No direct evidences for AFQ ordering, however, have been reported. Using the resonant x-ray scattering technique near the Dy L_III absorption edge, we have succeeded in observing the AFQ order parameter in DyB2C2 and analyzing the energy and polarization dependence. Much weaker coupling between orbital degrees of freedom and lattice in 4f electron systems than in 3d compounds provides an ideal platform to study orbital interactions originated from electronic mechanisms.

cond-mat.str-el↗

arXiv1999

Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8)

It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase transition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic insulator (FI) phase. The magnetization shows a sharp jump at the transition field accompanying with a remarkable increase of magnetoresistance. Striction measurements clarified that this transition is associated with the structural change from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic phase. These results evidently show that the FI phase with a pseudo cubic symmetry is more stable in high fields than the FM phase due to the double exchange interaction. The driving force of this transition is explained by the enhancement of the ferromagnetic superexchange interaction induced by an antiferromagnetic type orbital ordering in the pseudo cubic phase, which was recently found in the anomalous X-ray scattering experiments.

cond-mat.str-el↗

arXiv1999

Neutron Scattering Study of Spin Density Wave Order in the Superconducting State of Excess-Oxygen-Doped La2CuO4+y

We report neutron scattering measurements of spin density wave order within the superconducting state of a single crystal of predominately stage-4 La2CuO4+y with a Tc(onset) of 42 K. The low temperature elastic magnetic scattering is incommensurate with the lattice and is characterized by long-range order in the copper-oxide plane with the spin direction identical to that in the insulator. Between neighboring planes, the spins exhibit short-range correlations with a stacking arrangement reminiscent of that in the undoped antiferromagnetic insulator. The elastic magnetic peak intensity appears at the same temperature within the errors as the superconductivity, suggesting that the two phenomena are strongly correlated. These observations directly reveal the persistent influence of the antiferromagnetic order as the doping level increases from the insulator to the superconductor. In addition, our results confirm that spin density wave order for incommensurabilities near 1/8 is a robust feature of the La2CuO4-based superconductors.

cond-mat.supr-con↗

arXiv1999

Observation of New Incommensurate Magnetic Correlations at the Lower Critical Concentration for Superconductivity (x=0.05) in La(2-x)Sr(x)CuO4

Neutron-scattering experiments have been performed on lightly-doped La(2-x)Sr(x)CuO4 single crystals in both the insulating (x=0.03,0.04,0.05) and superconducting (x=0.06) regions. Elastic magnetic peaks are observed at low temperatures in all samples with the maximum peak linewidth occuring at the critical concentration x_c=0.05. New incommensurate peaks are observed only at x=0.05, the positions of which are rotated by 45 degrees in reciprocal space about (pi,pi) from those observed for x>=0.06 in the superconducting phase.

cond-mat.supr-con↗

arXiv1999

Neutron Scattering Study of Elastic Magnetic Signals in Superconducting La1.94Sr0.06Cu4

Recent neutron-scattering experiments on La(2-x)Sr(x)CuO4 single crystals by Wakimoto et al. have revealed that elastic magnetic peaks appear at low temperatures in both insulating (x=0.02-0.05) and superconducting (x=0.06) samples. We have carried out further investigations particularly on the elastic incommensurate peaks for x=0.06, and found that the integrated intensity drastically changes across the low temperature insulator- superconductor boundary; the intensity of x=0.06 is 4 times smaller than that of x=0.05, while the intensity in the insulating region stays constant.

cond-mat.supr-con↗

arXiv1999

Instantaneous Spin Correlations in La2CuO4

We have carried out a neutron scattering study of the instantaneous spin-spin correlations in La2CuO4 (T_N = 325 K) over the temperature range 337 K to 824 K. Incident neutron energies varying from 14.7 meV to 115 meV have been employed in order to guarantee that the energy integration is carried out properly. The results so-obtained for the spin correlation length as a function of temperature when expressed in reduced units agree quantitatively both with previous results for the two dimensional (2D) tetragonal material Sr2CuO2Cl2 and with quantum Monte Carlo results for the nearest neighbor square lattice S=1/2 Heisenberg model. All of the experimental and numerical results for the correlation length are well described without any adjustable parameters by the behavior predicted for the quantum non-linear sigma model in the low temperature renormalized classical regime. The amplitude, on the other hand, deviates subtly from the predicted low temperature behavior. These results are discussed in the context of recent theory for the 2D quantum Heisenberg model.

cond-mat.str-el↗

arXiv1999

Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)

Comprehensive neutron-powder diffraction and Rietveld analyses were carried out to clarify the relation between the crystal and magnetic structures of La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6 octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic easy-axis at low temperature is parallel to the c axis. As the JT distortion becomes suppressed with increasing x, a planar ferromagnetic structure appears at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure at x =< 0.39. The canting angle between neighboring planes continuously increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with decrease of JT distortion can be ascribed to the change in the eg orbital state from d3z^2-r^2 to dx^2-y^2.

cond-mat.str-el↗

arXiv1999

Spin Dynamical Properties of the Layered Perovskite La1.2Sr1.8Mn2O7

Inelastic neutron-scattering measurements were performed on a single crystal of the layered colossal magnetoresistance (CMR) material La1.2Sr1.8Mn2O7 (Tc ~ 120K). We found that the spin wave dispersion is almost perfectly two-dimensional with the in-plane spin stiffness constant D ~ 151meVA. The value is similar to that of similarly doped La1-xSrxMnO3 though its Tc is three times higher, indicating a large renormalization due to low dimensionality. There exist two branches due to a coupling between layers within a double-layer. The out-of-plane coupling is about 30% of the in-plane coupling though the Mn-O bond lengths are similar.

cond-mat.str-el↗

arXiv1999

Quantum Monte Carlo Study of Weakly Coupled Spin Ladders

We report a quantum Monte Carlo study of the thermodynamic properties of arrays of spin ladders with various widths ($n$), coupled via a weak inter-ladder exchange coupling $αJ$, where $J$ is the intra-ladder coupling both along and between the chains. This coupled ladder system serves as a simplified model for the magnetism of presumed ordered spin and charge stripes in the two-dimensional CuO$_2$ planes of hole-doped copper oxides. Our results for $n=3$ with weak inter-ladder coupling $α=0.05$, estimated from the $t-t'-t''-J$ model, show good agreement with the ordering temperature of the recently observed spin density wave condensation in La$_2$CuO$_{4+y}$. We show that there exists a quantum critical point at $α_c \simeq 0.07$ for $n=4$, and determine the phase diagram. Our data at this quantum critical point agree quantitatively with the universal scaling predicted by the quantum nonlinear $σ$ model. We also report results on random mixtures of $n=2$ and $n=3$ ladders, which correspond to the doping region near but above 1/8. Our study on the magnetic static structure factor reveals a saturation of the incommensurability of the spin correlations around 1/8, while the incommensurability of the charge stripes grows linearly with hole concentration. The implications of this result for the interpretation of neutron scattering experiments on the dynamic spin fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ are discussed.

cond-mat.str-el↗

arXiv1999

Neutron Scattering Studies on Magnetic Structure of the Double-Layered Manganite La_{2-2x}Sr_{1+2x}Mn_2O_7 ($0.30 \leq x \leq 0.50$)

Systematic powder diffraction studies have been carried out to establish the magnetic phase diagram of La_{2-2x}Sr_{1+2x}Mn_2O_7 (LSMO327) in a wide hall concentration range ($0.30 \leq x \leq 0.50$), using the HERMES diffractometer. LSMO327 exhibits a planar ferromagnetic structure for $0.32\leq x\leq 0.38$ at low temperatures. A finite canting angle between planar magnetic moments on neighboring planes starts appearing around $x\sim 0.40$ and reaches 180$^{\circ}$ (A-type antiferromagnet) at x=0.48. At x=0.30, on the other hand, the magnetic moments are aligned parallel to the c-axis.

cond-mat.str-el↗

arXiv1999

Transition between two ferromagnetic states driven by orbital ordering in La_{0.88}Sr_{0.12}MnO_3

A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a novel ferromagnetic insulator.We identify that the key parameter in the transition is the orbital degree of freedom in e_g electrons. By utilizing the resonant x-ray scattering technique, orbital ordering is directly detected below T_{OO}, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites.

cond-mat.str-el↗

arXiv1998

Field Induced Transition from Metal to Insulator in the CMR Manganites

The gigantic reduction of the electric resistivity under the applied magnetic field, CMR effect, is now widely accepted to appear in the vicinity of the insulator to metal transition of the perovskite manganites. Recently, we have discovered the first order transition from ferromagnetic metal to insulator in $\rm La_{0.88}Sr_{0.12}MnO_3$ of the CMR manganite. This phase transition induces the tremendous increase of the resistivity under the external magnetic field just near above the phase transition temperature. We report here fairly detailed results from the systematic experiments including neutron and synchrotron X-ray scattering studies.

cond-mat.str-el↗

arXiv1998

Interplay of the CE-type charge ordering and the A-type spin ordering in a half-doped bilayer manganite La{1}Sr{2}Mn{2}O{7}

We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7} exhibits CE-type charge-ordered and spin-ordered states below $T_{N, CO}^A = 210$ K and below $T_{N}^{CE} \sim 145$ K, respectively. However, the volume fraction of the CE-type ordering is relatively small, and the system is dominated by the A-type spin ordering. The coexistence of the two types of ordering is essential to understand its transport properties, and we argue that it can be viewed as an effective phase separation between the metallic $d(x^{2}-y^{2})$ orbital ordering and the charge-localized $d(3x^{2}-r^{2})/d(3y^{2}-r^{2})$ orbital ordering.

cond-mat.str-el↗