arXiv subjects

Hiroyuki Yamase

Publications and source records attributed to Hiroyuki Yamase.

At least 37 records · Page 2Linked to original sources

arXiv2021

Anomaly of longitudinal spin susceptibility at superconducting instability inside a magnetic phase

We study the longitudinal spin susceptibility inside a magnetically ordered phase, which exhibits a superconducting instability leading to a coexistence of the two ordered phases. Inside the magnetic phase, the superconducting gap acquires a linear term in a magnetic field applied along the direction of the magnetic moment. We find that such a linear term generates a jump of the longitudinal spin susceptibility when the superconducting instability occurs via a continuous phase transition. This anomaly at the superconducting instability is a thermodynamic signature of the microscopic coexistence of superconductivity and magnetism, and can be a general feature associated with the breaking of spin rotational symmetry inside the magnetic phase.

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arXiv2021

Fermi surface in La-based cuprate superconductors from Compton scattering imaging

Compton scattering provides invaluable information on the underlying Fermi surface (FS) and is a powerful tool complementary to angle-resolved photoemission spectroscopy and quantum oscillation measurements. Here we perform high-resolution Compton scattering measurements for La_{2-x}Sr_{x}CuO_{4} with x=0.08 (Tc=20K) at 300K and 150K, and image the momentum distribution function in the two-dimensional Brillouin zone. We find that the observed images cannot be reconciled with the conventional hole-like FS believed so far. Instead, our data imply that the FS is strongly deformed by the underlying nematicity in each CuO_2 plane, but the bulk FSs recover the fourfold symmetry. We also find an unusually strong temperature dependence of the momentum distribution function, which may originate from the pseudogap formation in the presence of the reconstructed FSs due to the underlying nematicity. Additional measurements for x=0.15 and 0.30 at 300K suggest similar FS deformation with weaker nematicity, which nearly vanishes at x=0.30.

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arXiv2021

Instability toward Formation of Quasi-One-Dimensional Fermi Surface in Two-Dimensional t-J Model

We show within the slave-boson mean field approximation that the two-dimensional t-J model has an intrinsic instability toward forming a quasi-one-dimensional (q-1d) Fermi surface. This q-1d state competes with, and is overcome by, the d-wave pairing state for a realistic parameter choice. However, we find that a small spatial anisotropy in t and J exposes the q-1d instability which has been hidden behind the d-wave pairing state, and brings about the coexistence with the d-wave pairing. We argue that this coexistence can be realized in La_{2-x}Sr_{x}CuO_4 systems

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arXiv2021

Possible Quasi-One-Dimensional Fermi Surface in La_{2-x}Sr_{x}CuO_4

To reconcile the two experimental findings on La_{2-x}Sr_{x}CuO_4, namely, Fermi surface (FS) observed by angle-resolved photoemission spectroscopy and sharp incommensurate magnetic peaks by neutron scattering, we propose a picture that a quasi-one-dimensional FS (q-1dFS) is realized in each CuO_{2} plane whose q-1d direction alternates along the c-axis.

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arXiv2021

Magnetic Excitation of t-J Model with Quasi-One-Dimensional Fermi Surface -- Possible Relevance to LSCO Systems

On the basis of the picture of a quasi-one-dimensional (q-1d) Fermi surface (FS), recently proposed by authors for LSCO systems, spin excitation spectrum, Im chi(q,omega), is calculated in the 'RPA' within the slave-boson mean-field approximation to the t-J model. It is found that Im chi(q,omega) shows both incommensurate (IC) and diagonal IC (DIC) peaks, whose realization does not depend on the existence of the d-wave gap. The peak positions do not change appreciably with omega and the sharp peaks survive down to the low hole doping rate. The d-wave gap suppresses both the IC peak and the DIC peak, but the degree of suppression as a function of omega is different between them. Taking these results together with results for the two-dimensional FS, we argue that essential features of magnetic excitation in LSCO systems can be understood in terms of the q-1d picture of the FS.

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arXiv2020

Detection of Acoustic Plasmons in Hole-Doped Lanthanum and Bismuth Cuprate Superconductors Using Resonant Inelastic X-Ray Scattering

High Tc superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic X-ray scattering (RIXS) to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors [La2-xSrxCuO4 (LSCO) and Bi2Sr1.6La0.4CuO6+d (Bi2201)], crucially completing the picture. Interestingly, in contrast to the quasi-static charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behaviour of valence charges in hole-doped cuprates.

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arXiv2020

Suppression of superconductivity by spin fluctuations in iron-based superconductors

We study the superconducting instability mediated by spin fluctuations in the Eliashberg theory for a minimal two-band model of iron-based superconductors. While antiferromagnetic spin fluctuations can drive superconductivity (SC) as is well established, we find that spin fluctuations necessarily contain a contribution to suppress SC even though SC can eventually occur at lower temperatures. This self-restraint effect stems from a general feature of the spin-fluctuation mechanism, namely the repulsive pairing interaction, which leads to phase frustration of the pairing gap and consequently the suppression of SC.

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arXiv2020

Elastoresistance measurements on CaKFe$_4$As$_4$ and KCa$_2$Fe$_4$As$_4$F$_2$ with the Fe site of $C_{2v}$ symmetry

We report resistance and elastoresistance measurements on (Ba$_{0.5}$K$_{0.5}$)Fe$_2$As$_2$, CaKFe$_4$As$_4$, and KCa$_2$Fe$_4$As$_4$F$_2$. The Fe-site symmetry is $D_{2d}$ in the first compound but $C_{2v}$ in the latter two, which lifts the degeneracy of the Fe $d_{xz}$ and $d_{yz}$ orbitals. The temperature dependence of the resistance and elastoresistance is similar between the three compounds. Especially, the [110] elastoresistance is enhanced with decreasing temperature irrespective of the Fe-site symmetry. This appears to be in conflict with recent Raman scattering studies on CaKFe$_4$As$_4$, which suggest the absence of nematic fluctuations. We consider possible ways of reconciliation and suggest that the present result is important in elucidating the origin of in-plane resistivity anisotropy in iron-based superconductors.

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arXiv2020

Close inspection of plasmon excitations in cuprate superconductors

Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum ${\bf q}_{\parallel}=(0,0)$ for various doping rates in electron-doped cuprates ${\rm La_{2-x}Ce_xCuO_4}$. We find that those new experimental data are well captured by acoustic-like plasmon excitations in a microscopic study of the layered $t$-$J$ model with the long-range Coulomb interaction. The acoustic-like plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping $t_z$.

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arXiv2019

Phase stiffness in an antiferromagnetic superconductor

We analyze the suppression of the phase stiffness in a superconductor by antiferromagnetic order. The analysis is based on a general expression for the phase stiffness in a mean-field state with coexisting spin-singlet superconductivity and spiral magnetism. Neel order is included as a special case. Close to half-filling, where the pairing gap is much smaller than the magnetic gap, a simple formula for the phase stiffness in terms of magnetic quasi-particle bands is derived. The phase stiffness is determined by charge carriers in small electron or hole pockets in this regime. The general analysis is complemented by a numerical calculation for the two-dimensional Hubbard model with nearest and next-to-nearest neighbor hopping amplitudes at a moderate interaction strength. The resulting phase stiffness exhibits a striking electron-hole asymmetry. In the ground state, it is larger than the pairing gap on the hole-doped side, and smaller for electron doping. Hence, in the hole-doped regime near half-filling the ground state pairing gap sets the scale for the Kosterlitz-Thouless temperature T_c^{KT}, while in the slightly electron-doped regime T_c^{KT} is determined essentially by the ground state phase stiffness.

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arXiv2019

Doping dependence of d-wave bond-charge excitations in electron-doped cuprates

Motivated by the recent experiments reporting the doping dependence of the short-range charge order (CO) in electron-doped cuprates, we study the resonant x-ray scattering spectrum from d-wave bond-charge fluctuations obtained in the two-dimensional t-J model. We find that (i) the CO is short-range, (ii) the CO peak is pronounced at low temperature, (iii) the peak intensity increases with decreasing carrier doping $δ$ down to $δ\approx$ 0.10 and is substantially suppressed below $δ\approx$ 0.10 due to strong damping, and (iv) the momentum of the CO decreases monotonically down to $δ\approx$ 0.10 and goes up below $δ\approx$ 0.10. These results reasonably capture the major features of the experimental data, and the observed short-range CO can be consistently explained in terms of bond-charge fluctuations with an internal d-wave symmetry.

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arXiv2018

Origin of the high-energy charge excitations observed by resonant inelastic x-ray scattering in cuprate superconductors

The recent development of x-ray scattering techniques revealed the charge-excitation spectrum in high-$T_c$ cuprate superconductors. While the presence of a dispersive signal in the high-energy charge-excitation spectrum is well accepted in the electron-doped cuprates, its interpretation and universality are controversial. Since charge fluctuations are observed ubiquitously in cuprate superconductors, the understanding of its origin is a pivotal issue. Here, we employ the layered $t$-$J$ model with the long-range Coulomb interaction and show that an acoustic-like plasmon mode with a gap at in-plane momentum (0,0) captures the major features of the high-energy charge excitations. The high-energy charge excitations, therefore, should be a universal feature in cuprate superconductors and are expected also in the hole-doped cuprates. Acoustic-like plasmons in cuprates have not been recognized yet in experiments. We propose several experimental tests to distinguish different interpretations of the high-energy charge excitations.

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arXiv2018

Dual structure in the charge excitation spectrum of electron-doped cuprates

Motivated by the recent resonant x-ray scattering (RXS) and resonant inelastic x-ray scattering (RIXS) experiments in electron-doped cuprates, we study the charge excitation spectrum in a layered t-J model with the long-range Coulomb interaction. We show that the spectrum is not dominated by a specific type of charge excitations, but by different kinds of charge fluctuations, and is characterized by a dual structure in the energy space. Low-energy charge excitations correspond to various types of bond-charge fluctuations driven by the exchange term (J-term) whereas high-energy charge excitations are due to usual on-site charge fluctuations and correspond to plasmon excitations above the particle-hole continuum. The interlayer coupling, which is frequently neglected in many theoretical studies, is particularly important to the high-energy charge excitations.

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arXiv2017

Static spin susceptibility in magnetically ordered states

We report that special care is needed when longitudinal magnetic susceptibility is computed in a magnetically ordered phase, especially in metals. We demonstrate this by studying static susceptibility in both a ferromagnetic and an antiferromagnetic state in the random phase approximation to the two-dimensional Hubbard model on a square lattice. In contrast to the case in the disordered phase, a first derivative of the chemical potential (or the density) with respect to a magnetic field does not vanish in a magnetically ordered phase when the field is applied parallel to the magnetic moment. This effect is crucial and should be included when computing magnetic susceptibility in the ordered phase, otherwise an unphysical result would be obtained. In addition, consequently the magnetic susceptibility becomes different when computed at a fixed density and a fixed chemical potential in the ordered phase. In particular, we cannot employ magnetic susceptibility at a fixed chemical potential to describe a system with a fixed density even if the chemical potential is tuned to reproduce the correct density.

cond-mat.str-el↗

arXiv2017

Charge-Density-Excitation Spectrum in the t-t'-J-V Model

We study the density-density correlation function in a large-N scheme of the t-t'-J-V model. When the nearest-neighbor Coulomb interaction V is zero, our model exhibits phase separation in a wide doping region and we obtain large spectral weight near momentum q=(0,0) at low energy, which originates from the proximity to phase separation. These features are much stronger for electron doping than for hole doping. However, once phase separation is suppressed by including a finite V, the low-energy spectral weight around q=(0,0) is substantially suppressed. Instead a sharp zero-sound mode is stabilized above the particle-hole continuum. We discuss that the presence of a moderate value of V, which is frequently neglected in the t-J model, is important to understand low-energy charge excitations especially close to q=(0,0) for electron doping. This insight should be taken into account in a future study of x-ray scattering measurements.

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arXiv2016

Fermi surface reconstruction and drop of Hall number due to spiral antiferromagnetism in high-$T_c$ cuprates

We show that a Fermi surface reconstruction due to spiral antiferromagnetic order may explain the rapid change in the Hall number as recently observed near optimal doping in cuprate superconductors [Badoux~\textit{et. al.}, Nature \textbf{531}, 210 (2016)]. The single-particle spectral function in the spiral state exhibits hole pockets which look like Fermi arcs due to a strong momentum dependence of the spectral weight. Adding charge-density wave order further reduces the Fermi surface to a single electron pocket. We propose quantum oscillation measurements to distinguish between commensurate and spiral antiferromagnetic order. Similar results apply to certain metals in which topological order replaces antiferromagnetic order.

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arXiv2016

Structure of the pairing gap from orbital nematic fluctuations

We study superconducting instability from orbital nematic fluctuations in a minimal model consisting of the $d_{xz}$ and $d_{yz}$ orbitals, and choose model parameters which capture the typical Fermi surface geometry observed in iron-based superconductors. We solve the Eliashberg equations down to low temperatures with keeping the renormalization function and a full momentum dependence of the pairing gap. When superconductivity occurs in the tetragonal phase, we find that the pairing gap exhibits a weak momentum dependence over the Fermi surfaces. The superconducting instability occurs also inside the nematic phase. When the $d_{xz}$ orbital is occupied more than the $d_{yz}$ orbital in the nematic phase, a larger (smaller) gap is realized on the Fermi-surface parts, where the $d_{xz}$ ($d_{yz}$) orbital component is dominant, leading to a substantial momentum dependence of the pairing gap on the hole Fermi surfaces. On the other hand, the momentum dependence of the gap is weak on the electron Fermi surfaces. We also find that while the leading instability is the so-called $s_{++}$-wave symmetry, the second leading one is $d_{x^2-y^2}$-wave symmetry. In particular, these two states are nearly degenerate in the tetragonal phase whereas such quasi-degeneracy is lifted in the nematic phase and the $d_{x^2-y^2}$-wave symmetry changes to highly anisotropic $s$-wave symmetry.

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arXiv2016

Plasmon excitations in layered high-Tc cuprates

Motivated by the recent resonant inelastic x-ray scattering (RIXS) experiment for the electron-doped cuprates Nd_{2-x}Ce_{x}CuO_{4} with x around 0.15, we compute the density-density correlation function in the t-J model on a square lattice by including interlayer hopping and the long-range Coulomb interaction. We find that collective charge excitations are realized not inside the particle-hole continuum, but above the continuum as plasmons. The plasmon mode has a rather flat dispersion near the in-plane momentum q=(0,0) with a typical excitation energy of the order of the intralayer hopping t when the out-of-plane momentum qz is zero. However, when qz becomes finite, the plasmon dispersion changes drastically near q=(0,0), leading to a strong dispersive feature with an excitation gap scaled by the interlayer hopping tz. We discuss that the mode recently observed by RIXS near q=(0,0) can be associated with the plasmon mode with a finite qz.

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