arXiv subjects

S. Wakimoto

Publications and source records attributed to S. Wakimoto.

At least 37 records · Page 2Linked to original sources

arXiv2010

Magnetic nature of the 500 meV peak in $\rm La_{2-x}Sr_{x}CuO_4$ observed with resonant inelastic x-ray scattering at the Cu $K$-edge

We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at ${\bf q}=(π,0)$ in resonant inelastic x-ray scattering (RIXS) experiments on $\rm La_2CuO_4$. The intensity of this peak persists above the Néel temperature (T$_{N}$=320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon $\rm B_{1g}$ Raman peak in $\rm La_2CuO_4$, and with suitable rescaling, agrees with the Raman peak shifts in $\rm EuBa_2Cu_3O_6$ and $\rm K_2NiF_4$. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

cond-mat.str-el↗

arXiv2009

Muon Spin Rotation Measurements of Heterogeneous Field Response in Overdoped La2-xSrxCuO4

Transverse-field muon spin rotation measurements of overdoped La2-xSrxCuO4 reveal a large broadening of the local magnetic field distribution in response to applied field, persisting to high temperatures. The field-response is approximately Curie-Weiss like in temperature and is largest for the highest doping investigated. Such behaviour is contrary to the canonical Fermi-liquid picture commonly associated with the overdoped cuprates and implies extensive heterogeneity in this region of the phase diagram. A possible explanation for the result lies in regions of staggered magnetization about dopant cations, analogous to what is argued to exist in underdoped systems.

cond-mat.str-el↗

arXiv2009

Polarized neutron scattering studies of the kagome lattice antiferromagnet KFe3(OH)6(SO4)2

We report polarized neutron scattering studies of spin-wave excitations and spin fluctuations in the S=5/2 kagome lattice antiferromagnet KFe3(OH)6(SO4)2 (jarosite). Inelastic polarized neutron scattering measurements at 10 K on a single crystal sample reveal two spin gaps, associated with in-plane and out-of-plane excitations. The polarization analysis of quasi-elastic scattering at 67 K shows in-plane spin fluctuations with XY symmetry, consistent with the disappearance of the in-plane gap above the Neel temperature T_N = 65 K. Our results suggest that jarosite is a promising candidate for studying the 2D XY universality class in magnetic systems.

cond-mat.str-el↗

arXiv2009

Coherent quasi-particles-to-incoherent hole-carriers crossover in underdoped cuprates

In underdoped cuprates, only a portion of the Fermi surface survives as Fermi arcs due to pseudogap opening. In hole-doped La$_{2}$CuO$_4$, we have deduced the "coherence temperature" $T_{coh}$ of quasi-particles on the Fermi arc above which the broadened leading edge position in angle-integrated photoemission spectra is shifted away from the Fermi level and the quasi-particle concept starts to lose its meaning. $T_{coh}$ is found to rapidly increase with hole doping, an opposite behavior to the pseudogap temperature $T^*$. The superconducting dome is thus located below both $T^*$ and $T_{coh}$, indicating that the superconductivity emerges out of the coherent Fermionic quasi-particles on the Fermi arc. $T_{coh}$ remains small in the underdoped region, indicating that incoherent charge carriers originating from the Fermi arc are responsible for the apparently metallic transport at high temperatures.

cond-mat.supr-con↗

arXiv2009

Spin chirality and electric polarization in multiferroic compounds $R$Mn$_2$O$_5$ ($R=$Ho, Er)

Polarized neutron diffraction experiments have been performed on multiferroic materials $R$Mn$_{2}$O$_{5}$ ($R=$Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the former has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.

cond-mat.str-el↗

arXiv2008

Absence of time reversal symmetry breaking in La_{2-x}Sr_{x}CuO_{4}

We have performed zero-field muon spin rotation measurements on single crystals of La_{2-x}Sr_{x}CuO_{4} to search for spontaneous currents in the pseudo-gap state. By comparing measurements on materials across the phase diagram, we put strict upper limits on any possible time-reversal symmetry breaking fields that could be associated with the pseudo-gap. Comparison between experimental limits and proposed circulating current states effectively eliminates the possibility that such states exist in this family of materials.

cond-mat.supr-con↗

arXiv2008

Charge excitations associated with charge stripe order in the 214-type nickelate and superconducting cuprate

Charge excitations were studied for stipe-ordered 214 compounds, La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ and 1/8-doped La$_{2}$(Ba, Sr)$_{x}$CuO$_{4}$ using resonant inelastic x-ray scattering in hard x-ray regime. We have observed charge excitations at the energy transfer of 1 eV with the momentum transfer corresponding to the charge stripe spatial period both for the diagonal (nikelate) and parallel (cuprates) stripes. These new excitations can be interpreted as a collective stripe excitation or charge excitonic mode to a stripe-related in-gap state.

cond-mat.str-el↗

arXiv2008

Complex Spin Structures and Origin of the Magneto-Electric Coupling in YMn2O5

By combining neutron four-circle diffraction and polarized neutron diffraction techniques we have determined the complex spin structures of a multiferroic, YMn2O5, that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magneto-electric coupling, our results are consistent with both the spin-current model that requires a magnetic transverse (cycloidal) spiral structure in order to induce a spontaneous electric polarization and the magneto-restriction model. Our results also explain why the electric polarization changes at the commensurate-to-incommensurate phase transition.

cond-mat.str-el↗

arXiv2008

Magnetic Excitations of the Diagonal Incommensurate Phase in Lightly-Doped La2-xSrxCuO4

We present inelastic neutron scattering experiments on a single-domain crystal of lightly-doped La1.96Sr0.04CuO4. We find that the magnetic excitation spectrum in this insulating phase with a diagonal incommensurate spin modulation is remarkably similar to that in the superconducting regime, where the spin modulation is bond parallel. In particular, we find that the dispersion slope at low energy is essentially independent of doping and temperature over a significant range. The energy at which the excitations cross the commensurate antiferromagnetic wave vector increases roughly linearly with doping through the underdoped regime.

cond-mat.str-el↗

arXiv2007

Observation of a 500meV Collective Mode in La$_{2-x}$Sr$_x$CuO$_4$ and Nd$_2$CuO$_4$

Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La$_{2-x}$Sr$_x$CuO$_4$ at 500 meV. The mode is peaked around the ($π$,0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with x=0, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr content and is absent at $x$=0.17, where it is replaced by a continuum of excitations. The peak is only observed when the incident x-ray polarization is normal to the CuO planes and is also present in Nd$_2$CuO$_4$. We suggest possible explanations for this excitation.

cond-mat.str-el↗

arXiv2007

Charge-transfer exciton in La2CuO4 probed with resonant inelastic x-ray scattering

We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the dispersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features was also examined. The observed charge transfer edge shift, along with the low dispersion of the first charge transfer excitation are attributed to the lattice motion being coupled to the electronic system. In addition, we observe a dispersionless feature at 1.8 eV, which is associated with a d-d crystal field excitation.

cond-mat.str-el↗

arXiv2007

Disappearance of antiferromagnetic spin excitations in over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$

Magnetic excitations in the energy range up to 100 meV are studied for over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ and 0.30, using time-of-flight neutron spectroscopy. Comparison of spectra integrated over the width of an antiferromagnetic Brillouin zone demonstrates that the magnetic scattering at intermediate energies, $20 \lesssim ω\lesssim 100$ meV, progressively decreases with over-doping. This strongly suggests that the magnetism is not related to Fermi surface nesting, but rather is associated with a decreasing volume fraction of (probably fluctuating) antiferromagnetic bubbles.

cond-mat.supr-con↗

arXiv2007

Distinct doping dependences of the pseudogap and superconducting gap La$_{2-x}$Sr$_{x}$CuO$_4$ cuprate superconductors

We have performed a temperature-dependent angle-integrated photoemission study of lightly-doped to heavily-overdoped La$_{2-x}$Sr$_{x}$CuO$_4$ and oxygen-doped La$_2$CuO$_{4.10}$. We found that both the magnitude $Δ$* of the (small) pseudogap and the temperature \textit{T}* at which the pseudogap is opened increases with decreasing hole concentration, consistent with previous studies. On the other hand, the superconducting gap $Δ_{sc}$ was found to remain small for decreasing hole concentration. The results can be explained if the superconducting gap opens only on the Fermi arc around the nodal (0,0)-($π,π$) direction while the pseudogap opens around $\sim$($π$, 0).

cond-mat.str-el↗

arXiv2006

Ni-Impurity Effects on Incommensurate Spin Correlations in Superconducting La2-xSrxCuO4 (x=0.06 and 0.07)

Neutron scattering experiments have been carried out to explore Ni-impurity effects on static spin correlations in La2-xSrxCuO4 (LSCO) in the vicinity of the superconductor-insulator boundary where both parallel and diagonal spin-density modulations (SDM) coexist at low temperature. Upon dilute Ni substitution the incommensurability decreases for both types of SDM, while the volume fraction of the diagonal (parallel) SDM increases (decreases). Subsequent Ni doping induces a bulk three-dimensional antiferromagnetic (AF) order when x ~ Ni concentration. TN of such the AF order depends on x and seems to disappear at x ~ 0.1. These effects are approximately ascribed by a reduction of mobile holes, and by a transition from the parallel to the diagonal SDM induced by Ni.

cond-mat.supr-con↗

arXiv2006

Dielectric properties and lattice dynamics of Ca-doped K$_{0.95}$Li$_{0.05}$TaO$_{3}$

Relaxor behavior and lattice dynamics have been studied for a single crystal of K$_{1-x}$Li$_x$TaO$_3$ $(x=0.05)$, where a small amount of a Ca impurity ($\sim 15$~ppm) was incorporated. The dielectric measurements revealed Debye-like relaxations with Arrhenius activation energies of 80 and 240 meV that are assigned to Li$^+$ dipoles and the Li$^+$-Li$^+$ dipolar pairs, respectively. In the neutron scattering results, diffuse scattering ridges appear around the nuclear Bragg peaks below $\sim 150$ K and phonon line broadening features start to appear at even higher temperatures suggesting that polar nano-regions (PNR's) start to form at these temperatures. These results are supported by the dielectric data that reveal relaxor behavior starting at $\sim 200$ K on cooling. From analyses of the diffuse intensities, the displacements include a uniform phase shift of all of the atoms in addition to the atomic displacements corresponding to a polarization vector of the transverse optic soft ferroelectric mode, a finding that is analogous to that in the prototypical relaxor material Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.

cond-mat.mtrl-sci↗

arXiv2006

Neutron Scattering and magnetization studies of Ba$_2$Cu$_{2.95}$Co$_{0.05}$O$_4$Cl$_2$: A decorated two-dimensional antiferromagnet

Ba$_2$Cu$_3$O$_4$Cl$_2$ has two inter-penetrating square Cu sublattices, one with square root 2 times the in-plane spacing of the other. Isotropic magnetic interactions between the two sublattices are completely frustrated. Quantum fluctuations resolve the intrinsic degeneracy in the ordering direction of the more weakly coupled sublattice in favor of collinear ordering. We present neutron scattering and magnetization studies of the magnetic structure when the Cu ions are substituted with Co. The Co spins create new magnetic interactions between the two sublattices. The ordering behavior of both Cu sublattices is retained largely unmodified. Between the phase transitions of the two sublattices spin-glass behavior is observed. Magnetization results show a strong enhancement to the ferromagnetic aspect of the magnetic structure. The combination of glassy behavior and large moments strongly suggest that the Co moments induce the formation of local canted states.

cond-mat.str-el↗

arXiv2005

Resonant inelastic X-ray scattering study of overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$

Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge has been performed for heavily overdoped samples of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x= 0.25$ and 0.30. We have observed the charge transfer and molecular-orbital excitations which exhibit resonances at incident energies of $E_i= 8.992$ and 8.998 keV, respectively. From a comparison with previous results on undoped and optimally-doped samples, we determine that the charge-transfer excitation energy increases monotonically as doping increases. In addition, the $E_i$-dependences of the RIXS spectral weight and absorption spectrum exhibit no clear peak at $E_i = 8.998$ keV in contrast to results in the underdoped samples. The low-energy ($\leq 3$ eV) continuum excitation intensity has been studied utilizing the high energy resolution of 0.13 eV (FWHM). A comparison of the RIXS profiles at $(π~0)$ and $(π~π)$ indicates that the continuum intensity exists even at $(π~π)$ in the overdoped samples, whereas it has been reported only at $(0 ~0)$ and $(π~0)$ for the $x=0.17$ sample. Furthermore, we also found an additional excitation on top of the continuum intensity at the $(π~π)$ and $(π~0)$ positions.

cond-mat.supr-con↗

arXiv2005

Magnetic properties of the overdoped superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ with and without Zn impurities

The magnetic properties of the Zn-substituted overdoped high-$T_c$ superconductor La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_{y}$O$_{4}$ have been studied by magnetization measurements and neutron scattering. Magnetization measurements reveal that for Zn-free samples with $x \geq 0.22$ a Curie term is induced in the temperature dependence of the magnetic susceptibility implying the existence of local paramagnetic moments. The induced Curie constant corresponds to a moment of 0.5 $μ_B$ per additional Sr$^{2+}$ ion that exceeds $x=0.22$. Zn-substitution in the overdoped \lsco also induces a Curie term that corresponds to 1.2 $μ_B$ per Zn$^{2+}$ ion, simultaneously suppressing $T_c$. The relationship between $T_c$ and the magnitude of the Curie term for Zn-free \lsco with $x \geq 0.22$ and for Zn-substituted \lsco with $x = 0.22$ are closely similar. This signifies a general competitive relationship between the superconductivity and the induced paramagnetic moment. Neutron scattering measurements show that Zn-substitution in overdoped \lsco anomalously enhances the inelastic magnetic scattering spectra around the $(π, π)$ position, peaking at $ω\sim 7$ meV. These facts are discussed on the basis of a "swiss-cheese" model of Zn-substituted systems as well as a microscopic phase separation scenario in the overdoped region indicated by muon-spin-relaxation measurements.

cond-mat.supr-con↗