arXiv subjects

K. Yamada

Publications and source records attributed to K. Yamada.

At least 109 records · Page 6Linked to original sources

arXiv2006

Electron-phonon coupling reflecting dynamic charge inhomogeneity in copper-oxide superconductors

The attempt to understand cuprate superconductors is complicated by the presence of multiple strong interactions. While many believe that antiferromagnetism is important for the superconductivity, there has been revived interest in the role of electron-lattice coupling. The recently studied conventional superconductor MgB2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon), that was predicted by standard theory and confirmed quantitatively by experiment. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bond-stretching phonon in the cuprate superconductors La2-xSrxCuO4 (with x=0.07, 0.15). This is in contrast to conventional theory, which does not predict such behavior. The anomaly is strongest in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order. It occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron-phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron-phonon coupling may be important to our understanding of superconductivity, although its contribution to the mechanism is likely to be indirect.

cond-mat.supr-con↗

arXiv2006

Evidence for non-conventional pairing in Na$\_x$CoO$\_2$$\cdot$yH$\_2$O

We report on first investigation of the lattice dynamics in the novel superconducting material Na$\_{0.35}$CoO$\_2$$\cdot$1.3H$\_2$O and the non-hydrated parent compound Na$\_{0.7}$CoO$\_2$ by inelastic x-ray scattering. The measured phonon dispersion along the $Γ-M$ direction show a marked softening with hole doping of two optical phonon branches close to the Brillouin zone boundary. The phonon spectra, dispersion, and softening are well reproduced by first-principle calculations. The calculations indicates that the soft branches are mainly composed of Co-vibration modes. The estimation of the critical temperature based on electron-phonon coupling mechanism undisputedly points to a non-conventional superconducting state in this material.

cond-mat.str-el↗

arXiv2005

Muon Spin Relaxation Studies of Magnetic-Field-Induced Effects in High-$T_{c}$ Superconductors

Muon spin relaxation ($μ$SR) measurements in high transverse magnetic fields ($\parallel \hat c$) revealed strong field-induced quasi-static magnetism in the underdoped and Eu doped (La,Sr)$_{2}$CuO$_{4}$ and La$_{1.875}$Ba$_{0.125}$CuO$_{4}$, existing well above $T_{c}$ and $T_{N}$. The susceptibility-counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards $T \sim 25$ K. No field-induced magnetism was detected in overdoped La$_{1.81}$Sr$_{0.19}$CuO$_{4}$, optimally doped Bi2212, and Zn-doped YBa$_{2}$Cu$_{3}$O$_{7}$.

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.

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arXiv2005

Field-Induced Uniform Antiferromagnetic Order Associated with Superconductivity in Pr$_{1-x}$LaCe$_{x}$CuO$_{4-δ}$

Strong correlation between field-induced antiferromagnetic (AF) order and superconductivity is demonstrated for an electron-doped cuprate superconductor, Pr$_{1-x}$LaCe$_{x}$CuO$_{4-δ}$ (PLCCO). In addition to the specimen with $x=0.11$ (which is close to the AF phase boundary, $x\simeq0.10$), we show that the one with $x=0.15$ ($T_c\simeq16$ K at zero field) also exhibits the field-induced AF order with a reduced magnitude of the induced moment. The uniform muon Knight shift at a low magnetic field ($\sim10^2$ Oe) indicates that the AF order is not localized within the cores of flux lines, which is in a marked contrast with theoretical prediction for hole-doped cuprates. The presence of anomalous non-diagonal hyperfine coupling between muons and Pr ions is also demonstrated in detail.

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arXiv2005

Dispersion of the high-energy phonon modes in Nd$_{1.85}$Ce$_{0.15}$CuO$_4$

The dispersion of the high-energy phonon modes in the electron doped high-temperature superconductor Nd$_{1.85}$Ce$_{0.15}$CuO$_4$ has been studied by inelastic neutron scattering. The frequencies of phonon modes with Cu-O bond-stretching character drop abruptly when going from the Brillouin zone center along the [100]-direction; this dispersion is qualitatively similar to observations in the hole-doped cuprates. We also find a softening of the bond-stretching modes along the [110]-direction but which is weaker and exhibits a sinusoidal dispersion. The phonon anomalies are discussed in comparison to hole-doped cuprate superconductors and other metallic perovskites.

cond-mat.str-el↗

arXiv2005

Field-induced transition between magnetically disordered and ordered phases in underdoped La(2-x)SrxCuO4

We report the observation of a magnetic-field-induced transition between magnetically disordered and ordered phases in slightly under-doped La(2-x)SrxCuO4 with x=0.144. Static incommensurate spin-density-wave order is induced above a critical field of about 3 T, as measured by elastic neutron scattering. Our results allow us to constrain the location of a quantum critical point on the phase diagram.

cond-mat.str-el↗

arXiv2005

Inelastic neutron scattering study of phonons and magnetic excitations in LaCoO$_{3}$

We have investigated the phonon and the magnetic excitations in LaCoO3 by inelastic neutron scattering measurements. The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) for the rhombohedrally distorted perovskite structure containing two chemical formula units of LaCoO3 in the unit cell. We observed two transverse optical (TO) phonon branches along (delta, delta, delta), consistent with previously reported Raman active Eg modes which show remarkable softening associated with the spin-state transition [Ishikawa et al., (Phys. Rev. Lett. 93 (2004) 136401.)]. We found that the softening takes place in the TO mode over the whole BZ. In contrast, the acoustic phonons show no anomalous softening associated with the spin-state transition. The low-energy paramagnetic scattering at 8 K is weak, increasing towards a maximum at E > 15 meV, consistent with excitation of the nonmagnetic low-spin to magnetic intermediate-spin state of Co 3+ ions.

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arXiv2005

Large magnetic field-induced spectral weight enhancement of high-energy spin excitations in $La_{1.88}Sr_{0.12}CuO_{4}$

We report electronic Raman scattering experiments on a superconducting ${\rm La_{1.88}Sr_{0.12}CuO_{4}}$ single crystal in a magnetic field. At low temperatures, the spectral weight of the high-energy two-magnon peak increases linearly with field and is amplified by a factor of more than two at 14 T. The effect disappears at elevated temperatures and is not present in undoped ${\rm La_{2}CuO_{4}}$. This observation is discussed in terms of an electronically inhomogeneous state in which the field enhances the volume fraction of a phase with local antiferromagnetic order at the expense of the superconducting phase.

cond-mat.str-el↗

arXiv2004

Angle-Resolved Photoemission Spectroscopy of the Antiferromagnetic Superconductor Nd1.87Ce0.13CuO4: Anisotropic Spin-Correlation Gap, Pseudogap, and the Induced Quasiparticle Mass Enhancement

We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (pi, 0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to be anisotropic, with the largest value near the intersecting point of the Fermi surface and the AF zone boundary. In addition, we observed that the QP peak disappears around the Neel temperature (TN) while the AF pseudogap is gradually filled up at much higher temperatures, possibly due to the short-range AF correlation.

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

Direct Observation of Non-Monotonic dx2-y2-Wave Superconducting Gap in Electron-Doped High-Tc Superconductor Pr0.89LaCe0.11CuO4

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2-wave symmetry, but obviously deviates from the monotonic dx2-y2 gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results unambiguously indicate the spin-mediated pairing mechanism in electron-doped high-Tc superconductors.

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arXiv2004

Universal magnetic excitation spectrum in cuprates

We have recently used inelastic neutron scattering to measure the magnetic excitation spectrum of La(1.875)Ba(0.125)CuO(4) up to 200 meV. This particular cuprate is of interest because it exhibits static charge and spin stripe order. The observed spectrum is remarkably similar to that found in superconducting YBa(2)Cu(3)O(6+x) and La(2-x)Sr(x)CuO(4); the main differences are associated with the spin gap. We suggest that essentially all observed features of the magnetic scattering from cuprate superconductors can be described by a universal magnetic excitation spectrum multiplied by a spin gap function with a material-dependent spin-gap energy.

cond-mat.supr-con↗

arXiv2004

Square vortex lattice at anomalously low magnetic fields in electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$

We report here on the first direct observations of the vortex lattice in the bulk of electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$ single crystals. Using small angle neutron scattering, we have observed a square vortex lattice with the nearest-neighbors oriented at 45$^{\circ}$ from the Cu-O bond direction, which is consistent with theories based on the d-wave superconducting gap. However, the square symmetry persists down to unusually low magnetic fields. Moreover, the diffracted intensity from the vortex lattice is found to decrease rapidly with increasing magnetic field.

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arXiv2004

Soft phonons and structural phase transitions in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$

Soft phonon behavior associated with a structural phase transition from the low-temperature-orthorhombic (LTO) phase ($Bmab$ symmetry) to the low-temperature-tetragonal (LTT) phase ($P4_{2}/ncm$ symmetry) was investigated in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ using neutron scattering. As temperature decreases, the TO-mode at $Z$-point softens and approaches to zero energy around $T_{\rm d2}=62$ K, where the LTO -- LTT transition occurs. Below $T_{\rm d2}$, the phonon hardens quite rapidly and it's energy almost saturates below 50 K. At $T_{\rm d2}$, the energy dispersion of the soft phonon along in-plane direction significantly changes while the dispersion along out-of-plane direction is almost temperature independent. Coexistence between the LTO phase and the LTT phase, seen in both the soft phonon spectra and the peak profiles of Bragg reflection, is discussed in context of the order of structural phase transitions.

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arXiv2004

Relationship between charge stripe order and structural phase transitions in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$

The nature of charge stripe order and its relationship with structural phase transitions were studied using synchrotron X-ray diffraction in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ ($0.05\leq x\leq 0.10$). For $x=0.05$, as temperature increased, incommensurate superlattice peaks associated with the charge order disappeared just at the structural phase transition temperature, $T_{\rm d2}$. However, for $x=0.075$ and 0.09, the superlattice peaks still existed as a short range correlation even above $T_{\rm d2}$, indicating a precursor of charge ordering. Furthermore, temperature dependences of the superlattice peak intensity, correlation length, and incommensurability for $x=0.05$ are different from those for $x=0.075$ and 0.09. These results suggest that the transition process into the charge stripe order strongly correlates with the order of the structural phase transitions. A quantitative comparison of the structure factor associated with the charge order have been also made for all the samples.

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arXiv2004

Reflectivity measurements in uniaxial superconductors: a methodological discussion applied to the case of La(2-x)Sr(x)CuO(4)

Most of the novel superconductors are uniaxial crystals, with metallic planes ($ab$) orthogonal to an insulating axis ($c$). Far-infrared measurements of the reflectivity $R_{ab} (ω)$ provide valuable information on their low-energy electrodynamics, but involve delicate experimental issues. Two of them are a possible contamination of $R_{ab} (ω)$ from the c axis and the extrapolation of the $R_{ab}$ data to $ω$ =0, both above and below $T_c$. Here we discuss quantitatively these issues with particular regard to La$_{2-x}$Sr$_x$CuO$_{4+y}$, one of the most studied high-$T_c$ materials.

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arXiv2004

Strong correlation between field-induced magnetism and superconductivity in Pr$_{0.89}$LaCe$_{0.11}$CuO$_{4}$

We report the local magnetic response of an electron-doped cuprate superconductor, Pr$_{0.89}$LaCe$_{0.11}$CuO$_{4}$ ($T_c\simeq26$ K), studied by muon spin rotation (\msr) over a field range from 200 Oe to 50 kOe. In the paramagnetic state, the muon Knight shift along the crystal c-axis ($K_μ^z$) is proportional to the in-plane susceptibility $χ_{ab}$. More surprisingly, $K_μ^z$ is strongly enhanced by the occurrence of superconductivity (below $\sim$40 kOe), changing its sign from positive to negative around $\sim1$ kOe with increasing field. This finding can be understood by considering the weak polarization of Pr ions due to superexchange interaction with the antiferromagnetic Cu sublattice, which coexists with superconductivity.

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