arXiv subjects

Y. Endoh

Publications and source records attributed to Y. Endoh.

At least 19 recordsLinked to original sources

arXiv2017

Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of Fe$_{1-x}$Co$_x$Si

We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of the helimagnetic transition in Fe$_{1-x}$Co$_x$Si with $x$ = 0.30. In contrast to the sharp transition observed in the archetype chiral magnet MnSi, the transition in Fe$_{1-x}$Co$_x$Si is gradual and long-range helimagnetic ordering coexists with short-range correlations over a wide temperature range. The dynamics are more complex than in MnSi and involve long relaxation times with a stretched exponential relaxation which persists even under magnetic field. These results in conjunction with an analysis of the hierarchy of the relevant length scales show that the helimagnetic transition in Fe$_{1-x}$Co$_x$Si differs substantially from the transition in MnSi and question the validity of a universal approach to the helimagnetic transition in chiral magnets.

cond-mat.mtrl-sci↗

arXiv2016

Damped spin-wave excitations in the itinerant antiferromagnet $\gamma$-Fe$_{0.7}$Mn$_{0.3}$

The collective spin-wave excitations in the antiferromagnetic state of $\gamma$-Fe$_{0.7}$Mn$_{0.3}$ were investigated using the inelastic neutron scattering technique. The spin excitations remain isotropic up to the high excitation energy, ${\hbar\omega}= 78$ meV. The excitations gradually become broad and damped above 40 meV. The damping parameter ${\gamma}$ reaches 110(16) meV at ${\hbar\omega} = 78$ meV, which is much larger than that for other metallic compounds, e.g., CaFe$_2$As$_2$ (24 meV), La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($52-72$ meV), and Mn$_{90}$Cu$_{10}$ (88 meV). In addition, the spin-wave dispersion shows a deviation from the relation $({\hbar\omega})^2 = c^2q^2 + {\Delta}^2$ above 40 meV. The group velocity above this energy increases to 470(40) meV{\AA}, which is higher than that at the low energies, $c = 226(5)$ meV{\AA}. These results could suggest that the spin-wave excitations merge with the continuum of the individual particle-hole excitations at 40 meV.

cond-mat.mtrl-sci↗

arXiv2016

Extended skyrmion lattice scattering and long-time memory in the chiral magnet Fe$_{1-x}$Co$_x$Si

Small angle neutron scattering measurements on a bulk single crystal of the doped chiral magnet Fe$_{1-x}$Co$_x$Si with $x$=0.3 reveal a pronounced effect of the magnetic history and cooling rates on the magnetic phase diagram. The extracted phase diagrams are qualitatively different for zero and field cooling and reveal a metastable skyrmion lattice phase outside the A-phase for the latter case. These thermodynamically metastable skyrmion lattice correlations coexist with the conical phase and can be enhanced by increasing the cooling rate. They appear in a wide region of the phase diagram at temperatures below the $A$-phase but also at fields considerably smaller or higher than the fields required to stabilize the A-phase.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic relaxation phenomena in the Chiral Magnet Fe$_{1-x}$Co$_x$Si: An ac susceptibility study

We present a systematic study of the ac susceptibility of the chiral magnet Fe$_{1-x}$Co$_x$Si with $x$ = 0.30 covering four orders of magnitude in frequencies from 0.1 Hz to 1 kHz, with particular emphasis to the pronounced history dependence. Characteristic relaxation times ranging from a few milliseconds to tens of seconds are observed around the skyrmion lattice A-phase, the helical-to-conical transition and in a region above $T_C$. The distribution of relaxation frequencies around the A-phase is broad, asymmetric and originates from multiple coexisting relaxation processes. The pronounced dependence of the magnetic phase diagram on the magnetic history and cooling rates as well as the asymmetric frequency dependence and slow dynamics suggest more complicated physical phenomena in Fe$_{0.7}$Co$_{0.3}$Si than in other chiral magnets.

cond-mat.mtrl-sci↗

arXiv2012

Neutron Brillouin scattering with pulsed spallation neutron source - spin-wave excitations from ferromagnetic powder samples -

Neutron Brillouin scattering (NBS) method was developed using a pulsed spallation neutron source, and the feasibility of NBS was demonstrated by observing ferromagnetic spin waves in La$_{0.8}$Sr$_{0.2}$MnO$_3$ and SrRuO$_3$ powders. Gapless spin-wave excitations were observed in La$_{0.8}$Sr$_{0.2}$MnO$_3$, which were continuously extended to the lower scattering vector $Q$ from previous results using single crystals. The novel result is a well-defined quadratic $Q$ dependence in the spin-wave dispersion curve with a large energy gap in SrRuO$_3$ indicating robust spin-orbit coupling.

cond-mat.str-el↗

arXiv2010

Polarization-analyzed resonant inelastic x-ray scattering of the orbital excitations in KCuF3

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of orbital excitations in KCuF3 . By performing the polarization analysis of the scattered photons, we disclose that the excitation between the eg orbitals and the excitations from t2g to eg exhibit distinct polarization dependence. The polarization dependence of the respective excitations is interpreted based on a phenomenological consideration of the symmetry of the RIXS process that yields a necessary condition for observing the excitations. In addition, we show that the orbital excitations are dispersionless within our experimental resolution.

cond-mat.str-el↗

arXiv2008

Local moment fluctuations in an optimally-doped high Tc superconductor

We present results of neutron scattering experiments on YBa2Cu3O6.95 (Tc=93K). Our results indicate that magnetic collective modes due to correlated local moments are present both above and below Tc in optimally doped YBCO. The magnon-like modes are robust and not overdamped by itinerant particle-hole excitations, which may point at a substantial static or slowly fluctuating charge inhomogeneity. We compare the experimental results to predictions of the Fermi liquid (FL) theory in the Random Phase Approximation (RPA).

cond-mat.supr-con↗

arXiv2007

Momentum-resolved charge excitations in high-Tc cuprates studied by resonant inelastic x-ray scattering

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of high-Tc cuprates. Momentum-resolved charge excitations in the CuO2 plane are examined from parent Mott insulators to carrier-doped superconductors. The Mott gap excitation in undoped insulators is found to commonly show a larger dispersion along the [pi,pi] direction than the [pi,0] direction. On the other hand, the resonance condition displays material dependence. Upon hole doping, the dispersion of the Mott gap excitation becomes weaker and an intraband excitation appears as a continuum intensity below the gap at the same time. In the case of electron doping, the Mott gap excitation is prominent at the zone center and a dispersive intraband excitation is observed at finite momentum transfer.

cond-mat.str-el↗

arXiv2007

Momentum Dependent Charge Excitations of Two-Leg Ladder: Resonant Inelastic X-ray Scattering of (La,Sr,Ca)14Cu24O41

Momentum dependent charge excitations of a two-leg ladder are investigated by resonant inelastic x-ray scattering of (La,Sr,Ca)14Cu24O41. In contrast to the case of a square lattice, momentum dependence of the Mott gap excitation of the ladder exhibits little change upon hole-doping, indicating the formation of hole pairs. Theoretical calculation based on a Hubbard model qualitatively explains this feature. In addition, experimental data shows intraband excitation as continuum intensity below the Mott gap and it appears at all the momentum transfers simultaneously. The intensity of the intraband excitation is proportional to the hole concentration of the ladder, which is consistent with optical conductivity measurements.

cond-mat.str-el↗

arXiv2006

Spin Gap in Optimally-doped YBCO

Spin gap in optimally-doped YBCO was investigated by inelastic neutron scattering. We found that q-integrated spectral weight in the vicinity of the spin gap has a shape of a step function at 10K with sharp resolution-limited onset at 33 meV, i.e. there is no observable spectral weight at 31 meV but full spectral weight at 35 meV. The spin gap does not soften or "fill in" as the temperature is increased to Tc. Above Tc we find no evidence for q-dependent magnetic signal at 35 meV.

cond-mat.supr-con↗

arXiv2004

Momentum Dependence of Charge Excitations in the Electron-Doped Superconductor Nd1.85Ce0.15CuO4: a RIXS Study

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-Tc superconductor Nd1.85Ce0.15CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.

cond-mat.str-el↗

arXiv2004

Mott gap excitations in twin-free YBa2Cu3O7-d (Tc = 93 K) studied by RIXS

Mott gap excitations in the high-Tc superconductor of the optimal doped YBa2Cu3O7-d (Tc = 93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab-plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculation based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra by taking the different parameters of the on-site Coulomb energy. The fact of the Mott gap of the CuO chain site is much smaller than that of CuO2 plane site is observed for the first time.

cond-mat.str-el↗

arXiv2004

Resonant inelastic x-ray scattering study of hole-doped manganites La1-xSrxMnO3 (x=0.2 and 0.4)

Electronic excitations near the Fermi energy in the hole doped manganese oxides (La1-xSrxMnO3, x=0.2 and 0.4) have been elucidated by using the resonant inelastic x-ray scattering (RIXS) method. A doping effect in the strongly correlated electron systems has been observed for the first time. The scattering spectra show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the energy of the spectral weight shifts toward lower energies. The excitation spectra show little change in the momentum space as is in undoped LaMnO3, but the scattering intensities in the low energy excitations of x=0.2 are anisotropic as well as temperature dependent, which indicates a reminiscence of the orbital nature.

cond-mat.str-el↗

arXiv2004

Softening of Cu-O bond stretching phonon in tetragonal HgBa$_2$CuO$_{4+δ}$

Phonons in nearly optimally doped HgBa$_2$CuO$_{4+δ}$ were studied by inelastic X-ray scattering. The dispersion of the low energy modes is well described by a shell model, while the Cu-O bond stretching mode at high energy shows strong softening towards the zone boundary, which deviates strongly from the model. This seems to be common in the hole-doped high-$T_\mathrm{c}$ superconducting cuprates, and, based on this work, not related to a lattice distortion specific to each material.

cond-mat.supr-con↗

arXiv2004

Dispersion of Magnetic Excitations in Superconducting Optimally Doped YBa_2Cu_3O_6.95

Detailed neutron scattering measurements of YBa_2Cu_3O_6.95 found that the resonance peak and incommensurate magnetic scattering induced by superconductivity represent the same physical phenomenon: two dispersive branches that converge near 41 meV and the in-plane wave-vector q_af=(pi/a, pi/a) to form the resonance peak. One branch has a circular symmetry around q_af and quadratic downward dispersion from ~41 meV to the spin gap of 33+-1meV. The other, of lower intensity, disperses from ~41 meV to at least 55 meV. Our results exclude a quartet of vertical incommensurate rods in q-w space expected from spin waves produced by dynamical charge stripes as an origin of the observed incommensurate scattering in optimally-doped YBCO.

cond-mat.str-el↗

arXiv2003

Oxygen Phonon Branches in YBa2Cu3O7

We report results of inelastic neutron scattering measurements of phonon dispersions in optimally doped YBa2Cu3O6.95 and compare them with model calculations. The focus is on the in-plane oxygen bond-stretching phonon branches. The study of these modes is complicated by anticrossings with c-axis-polarized branches; such effects are interpreted through lattice-dynamical shell-model calculations. The in-plane anisotropy of the bond-stretching phonons was firmly ascertained from measurements on a detwinned sample. Studying the in-plane modes involving out-of-phase motion for the two Cu-O layers within a unit cell as well as those with in-phase motion was of great help for establishing a clear experimental picture. The measurements confirm that the in-plane oxygen bond-stretching phonon branches disperse steeply downwards from the zone center in both the a and the b directions indicating a strong electron-phonon coupling. For the b-axis-polarized bond-stretching phonons, there is an additional feature of considerable interest: a sharp local frequency minimum was found to develop on cooling from room temperature to T = 10 K at the wave vector q = 0.27 r.l.u..

cond-mat.str-el↗

arXiv2003

Evidence for Dynamic Charge Stripes in the Phonons of Optimally Doped YBCO

Inelastic neutron scattering investigations on optimally doped YBCO revealed a very pronounced temperature dependence in the Cu-O in-plane bond-stretching vibrations along the (010) direction: a shift of spectral weight by at least 10 meV has been observed in a narrow range of wave vectors halfway to the zone boundary. The temperature evolution starts at about 200 K, well above the superconducting transition temperature. The displacement pattern of the anomalous phonons indicates a dynamic one-dimensional charge-density modulation within the planes, but excludes a direct contribution from the Cu-O chains.

cond-mat.str-el↗

arXiv2003

Spin Pseudo Gap in La2-xSrxCuO4 Studied by Neutron Scattering

Spin excitations of La2-xSrxCuO have been studied using inelastic neutron scattering techniques in the energy range of 2 meV =< w =< 12 meV and the temperature range of 8 K =< T =< 150 K. We observed a signature of a spin pseudo gap in the excitation spectrum above Tc for the slightly overdoped sample with x = 0.18. On heating, the spin pseudo gap gradually collapses between T = 80 K and 150 K. For the x = 0.15 and 0.20, although the visibility of gap-like structure at T ~ Tc is lower compared to the x = 0.18 sample, the broad bump of kai"(w) appears at w ~ 5 meV,close to the spin-gap energy at base temperature, suggests the existence of the spin pseudo gap in the normal state.

cond-mat.supr-con↗