arXiv subjects

Youichi Yamakawa

Publications and source records attributed to Youichi Yamakawa.

At least 73 records · Page 4Linked to original sources

arXiv2016

Nematicity and magnetism in FeSe and other families of Fe-based superconductors

Nematicity and magnetism are two key features in Fe-based superconductors, and their interplay is one of the most important unsolved problems. In FeSe, the magnetic order is absent below the structural transition temperature $T_{str}=90$K, in stark contrast that the magnetism emerges slightly below $T_{str}$ in other families. To understand such amazing material dependence, we investigate the spin-fluctuation-mediated orbital order ($n_{xz}\neq n_{yz}$) by focusing on the orbital-spin interplay driven by the strong-coupling effect, called the vertex correction. This orbital-spin interplay is very strong in FeSe because of the small ratio between the Hund's and Coulomb interactions ($\bar{J}/\bar{U}$) and large $d_{xz},d_{yz}$-orbitals weight at the Fermi level. For this reason, in the FeSe model, the orbital order is established irrespective that the spin fluctuations are very weak, so the magnetism is absent below $T_{str}$. In contrast, in the LaFeAsO model, the magnetic order appears just below $T_{str}$ both experimentally and theoretically. Thus, the orbital-spin interplay due to the vertex correction is the key ingredient in understanding the rich phase diagram with nematicity and magnetism in Fe-based superconductors in a unified way.

cond-mat.str-el↗

arXiv2016

$p$-orbital density wave with $d$ symmetry in high-$T_c$ cuprate superconductors predicted by the renormalization-group + constrained RPA theory

The discovery of the charge-density-wave formation in the high-$T_c$ cuprate superconductors has activated intensive theoretical studies for the pseudogap states. However, the microscopic origin of the charge-density-wave state has been unknown so far since the many-body effects beyond the mean-field-level approximations, called the vertex corrections, are essential. Toward solving this problem, we employ the recently developed functional renormalization-group method, by which we can calculate the higher-order vertex corrections in a systematic and unbiased way with high numerical accuracy. We discover the critical development of the $p$-orbital-density-wave ($p$-ODW) instability in the strong-spin-fluctuation region. The obtained $p$-ODW state possesses the key characteristics of the charge ordering pattern in Bi- and Y-based superconductors, such as the wave vector parallel to the nearest Cu-Cu direction, and the $d$-symmetry form factor with the antiphase correlation between $p_x$ and $p_y$ orbitals in the same unit cell. In addition, from the observation of the beautiful scaling relation between the spin susceptibility and the $p$-ODW susceptibility, we conclude that the main driving force of the density wave is the Aslamazov-Larkin vertex correction that becomes very singular near the magnetic quantum-critical point.

cond-mat.str-el↗

arXiv2015

Superconductivity in the Hexagonal Ternary Phosphide ScIrP

We report the discovery of a bulk superconducting transition at 3.4 K in the ternary phosphide, ScIrP, which crystallizes in a hexagonal ZrNiAl-type structure without spatial inversion symmetry. On the basis of heat capacity data in a zero magnetic field, ScIrP is suggested to be a weakly-coupled Bardeen-Cooper-Schrieffer superconductor. Alternatively, experimental results under magnetic fields indicate that this material is a type-II superconductor with an upper critical field Hc2 at magnetic fields above 5 T at zero temperature. This moderately high Hc2 does not violate the Pauli limit, but it does imply that there is a significant effect from the strong spin-orbit interaction of Ir 5d electrons in the noncentrosymmetric crystal structure. Electronic structure calculations show an interesting feature of ScIrP, where both the Sc 3d and Ir 5d orbitals contribute to the electronic density of states at the Fermi level.

cond-mat.supr-con↗

arXiv2015

Line-Node Dirac Semimetal and Topological Insulating Phase in Noncentrosymmetric Pnictides CaAgX (X = P, As)

Two noncentrosymmetric ternary pnictides, CaAgP and CaAgAs, are reported as topological line-node semimetals protected solely by mirror-reflection symmetry. The band gap vanishes on a circle in momentum space, and surface states emerge within the circle. Extending this study to spin-orbit coupled systems reveals that, compared with CaAgP, a substantial band gap is induced in CaAgAs by large spin-orbit interaction. The resulting states are a topological insulator, in which the Z2 topological invariant is given by 1; 000. To clarify the Z2 topological invariants for time-reversal-invariant systems without spatial-inversion symmetry, we introduce an alternative way to calculate the invariants characterizing a line node and topological insulator for mirror-reflection-invariant systems.

cond-mat.mes-hall↗

arXiv2015

Revisit of the Orbital-Fluctuation-Mediated Superconductivity in LiFeAs: Nontrivial Spin-Orbit Interaction Effects on the Bandstructure and Superconducting Gap Function

Precise gap structure in LiFeAs (Tc = 18 K) given by ARPES studies offers us significant information to understand the pairing mechanism in iron-based superconductors. The most remarkable characteristics in LiFeAs gap structure would be that "the largest gap emerges on the tiny hole-pockets around Z point". This result had been naturally explained in terms of the orbital-fluctuation scenario (T. Saito et al., Phys. Rev. B 90, 035104 (2014)), whereas an opposite result is obtained by the spin-fluctuation scenario. In this paper, we study the gap structure in LiFeAs by taking the spin-orbit interaction (SOI) into account, motivated by the recent ARPES studies that revealed the significant SOI-induced modification of the Fermi surface topology. For this purpose, we construct the two possible tight-binding models with finite SOI by referring the bandstructures given by different ARPES groups. In addition, we extend the gap equation for multiorbital systems with finite SOI, and calculate the gap functions by applying the orbital-spin fluctuation theory. On the basis of both SOI-induced band structures, main characteristics of the gap structure in LiFeAs are naturally reproduced only in the presence of strong inter-orbital interactions between (xz/yz - xy) orbitals. Thus, the experimental gap structure in LiFeAs is a strong evidence for the orbital-fluctuation pairing mechanism.

cond-mat.supr-con↗

arXiv2015

Quasiparticle Interference in Fe-based Superconductors Based on a Five-Orbital Tight-Binding Model

We investigate the quasiparticle interference (QPI) in Fe-based superconductors in both the $s_{++}$-wave and $s_{\pm}$-wave superconducting states on the basis of the five-orbital model. In the octet model for cuprate superconductors with $d_{x^2 - y^2}$-wave state, the QPI signal due to the impurity scattering at ${q} = {k}_i - {k}_j$ ($E = | Δ( {k}_i ) |$, $i = 1 \sim 8$) disappears when the gap functions at ${k}_i$ and ${k}_j$ have the same sign. However, we show that this extinction rule does not hold in Fe-based superconductors with fully-gapped $s$-wave state. The reason is that the resonance condition $E = | Δ( {k}_i ) |$ is not satisfied under the experimental condition for Fe-based superconductors. We perform the detailed numerical study of the QPI signal using the $T$-matrix approximation, and show that the experimentally observed QPI peak around ${q}_2 = ( π, 0 )$ can be explained on the basis of both the $s_{++}$-wave and $s_{\pm}$-wave states. Furthermore, we discuss the magnetic field dependence of the QPI by considering the Zeeman effect, and find that the field-induced suppression of the peak intensity around ${q}_2$ can also be explained in terms of both the $s_{++}$-wave and $s_{\pm}$-wave states.

cond-mat.supr-con↗

arXiv2015

Spin-Fluctuation-Driven Nematic Charge-Density-Wave in Cuprate Superconductors: Impact of Aslamazov-Larkin-Type Vertex Correction

We present a microscopic derivation of the nematic charge-density-wave (CDW) formation in cuprate superconductors based on the three-orbital d-p Hubbard model, by introducing the vertex correction (VC) into the charge susceptibility. The CDW instability at $q=(Δ_{FS},0)$, $(0,Δ_{FS})$ appears when the spin fluctuations are strong, due to the strong charge-spin interference represented by the VC. Here, $Δ_{FS}$ is the wavenumber between the neighboring hot spots. The obtained spin-fluctuation-driven CDW is expressed as the "intra-unit-cell orbital order" accompanied by the charge transfer between the neighboring atomic orbitals, which is actually observed by the STM measurements. We predict that the cuprate CDW and the nematic orbital order in Fe-based superconductors are closely related spin-fluctuation-driven phenomena.

cond-mat.str-el↗

arXiv2014

Linear Response Theory for Shear Modulus $C_{66}$ and Raman Quadrupole Susceptibility: Significant Evidence for Orbital Nematic Fluctuations in Fe-Based Superconductors

The emergence of the nematic order and fluctuations has been discussed as a central issue in Fe-based superconductors. To clarify the origin of the nematicity, we focus on the shear modulus $C_{66}$ and the Raman quadrupole susceptibility $χ_{x^2-y^2}^{Raman}$. Due to the Aslamazov-Larkin vertex correction, the nematic-type orbital fluctuations are induced, and they enhances both $1/C_{66}$ and $χ_{x^2-y^2}^{Raman}$ strongly. However, $χ_{x^2-y^2}^{Raman}$ remains finite even at the structure transition temperature $T_S$, because of the absence of the band Jahn-Teller effect and the Pauli (=intra-band) contribution, as proved in terms of the linear response theory. The present study clarifies that origin of the nematicity in Fe-based superconductors is the nematic-orbital order/fluctuations.

cond-mat.str-el↗

arXiv2014

Spin Triplet Superconductivity in Sr2RuO4 due to Orbital and Spin Fluctuations: Analysis by Two-Dimensional Renormalization Group Theory

We study the mechanism of the triplet superconductivity in Sr2RuO4 based on the multiorbital Hubbard model. The electronic states are studied using the renormalization group method. Thanks to the vertex correction (VC) for the susceptibility, which is dropped in the mean-field-level approximations, strong orbital and spin fluctuations at $Q=(2π/3,2π/3)$ emerge in the quasi one-dimensional Fermi surfaces composed of $d_{xz}$ and $d_{yz}$ orbitals. Due to the cooperation of both fluctuations, we obtain the triplet superconductivity in the $E_u$ representation, in which the superconducting gap is given by the linear combination of $(Δ_x(k),Δ_y(k))=(\sin 3k_x,\sin 3k_y)$. These results are confirmed by a diagrammatic calculation called the self-consistent VC method.

cond-mat.supr-con↗

arXiv2014

High-Tc Superconductivity near the Anion Height Instability in Fe-based Superconductors: Analysis of LaFeAsO$_{1-x}$H$_x$

The isostructural transition in the tetragonal phase, with sizable change in the anion-height, is realized in heavily H-doped LaFeAsO and (La,P) co-doped CaFe$_2$As$_2$. In these compounds, the superconductivity with higher-Tc (40~50 K) is realized near the isostructural transition. To reveal the origin of the anion-height instability and its important role in realizing the higher-Tc state, we develop the orbital-spin fluctuation theory by including the vertex correction. It is found that the non-nematic orbital fluctuations, which induce the anion-height instability, can develop near the magnetic quantum critical point, in addition to the conventional nematic orbital fluctuations. The non-nematic orbital order triggers the isostructural transition, and its fluctuation would be a key ingredient to realize higher-Tc superconductivity of order 50 K.

cond-mat.str-el↗

arXiv2013

Phase diagram and optical conductivity of La1.8-xEu0.2SrxCuO4

La1.8-xEu0.2SrxCuO4 (LESCO) is the member of the 214 family which exhibits the largest intervals among the structural, charge ordering (CO), magnetic, and superconducting transition temperatures. By using new dc transport measurements and data in the literature we construct the phase diagram of LESCO between x = 0.8 and 0.20. This phase diagram has been further probed in ac, by measuring the optical conductivity σ1(ω) of three single crystals with x = 0.11, 0.125, and 0.16 between 10 and 300 K in order to associate the extra-Drude peaks often observed in the 214 family with a given phase. The far-infrared peak we detect in underdoped LESCO is the hardest among them, survives up to room temperature and is associated with charge localization rather than with ordering. At the CO transition for the commensurate doping x = 0.125 instead the extra-Drude peak hardens and a pseudogap opens in σ1(ω), approximately as wide as the maximum superconducting gap of LSCO.

cond-mat.str-el↗

arXiv2013

Zigzag chain structure transition and orbital fluctuations in Ni-based superconductors

We investigate the electronic state and structure transition of BaNi2As2, which shows a similar superconducting phase diagram as Fe-based superconductors. We construct the ten-orbital tight-binding model for BaNi2As2 by using the maximally localized Wannier function method. The Coulomb and quadrupole-quadrupole interactions are treated within the random-phase approximation. We obtain the strong developments of charge quadrupole susceptibilities driven by the in-plane and out-of-plane oscillations of Ni ions. The largest susceptibility is either O_{X^2-Y^2}-quadrupole susceptibility at q = (pi, 0, pi) or O_{XZ(YZ)}-quadrupole susceptibility at q = (pi, pi, pi), depending on the level splitting between d_{X^2-Y^2} and d_{XZ(YZ)}. These antiferro-quadrupole fluctuations would then be the origin of the strong coupling superconductivity in Ni-based superconductors. Also, we propose that the antiferro-quadrupole O_{X^2-Y^2} order with q = (pi, 0, pi) is the origin of the zigzag chain structure reported in experiments. We identify similarities and differences between Ni- and Fe-based superconductors.

cond-mat.supr-con↗

arXiv2013

Effect of realistic finite-size impurities on Tc in Fe-based superconductors based on the five-orbital tight-binding model

We present a systematic study of the impurity effect on Tc in Fe-based superconductors, assuming that the sign-reversal s-wave state due to inter-pocket repulsion ($s_\pm$-wave state) is realized. For this purpose, we introduce several realistic impurity models with non-local modifications of potentials and hopping integrals around the impurity site. When we use the impurity model parameters for 3d- and 4d-impurity atoms derived from the recent first principle study by Nakamura et al., we find that the $s_\pm$-wave state is very fragile against impurities: The superconductivity with $T_{c0}=30K$ is destroyed by introducing small residual resistivity $ρ_0^{cr} = 5z^{-1} ~ 10z^{-1} [μΩcm]$ ($z^{-1} = m^*/m$ being the mass-enhancement factor), consistently with the previous theoretical study for the on-site impurity model by Onari and Kontani. This result is essentially unchanged for different non-local impurity models with realistic parameters. We also discuss the effect of the impurity-induced non-local orbital order on the superconducting state.

cond-mat.supr-con↗

arXiv2012

Impurity-Induced Electronic Nematic State in Iron-Pnictide Superconductors

We propose that impurity-induced electronic nematic state is realized above the orthorhombic structure transition temperature $T_S$ in iron-pnictide superconductors. In the presence of strong orbital fluctuations near $T_S$, it is theoretically revealed that a single impurity induces non-local orbital order with $C_2$-symmetry, consistently with recent STM/STS measurements. Each impurity-induced $C_2$ orbital order aligns along a-axis by applying tiny uniaxial pressure along b-axis. In this impurity-induced nematic phase, the resistivity shows sizable in-plane anisotropy ($ρ_b/ρ_a \sim 2$) even above $T_S$, actually observed in various "detwinned" samples. The present study indicates the existence of strong orbital fluctuations in iron-pnictide superconductors.

cond-mat.str-el↗

arXiv2012

Effect of inelastic scattering on the nuclear magnetic relaxation rate 1/T1T in iron-based superconductors

We present a microscopic study of the nuclear magnetic relaxation rate 1/T1 based on the five-orbital model for iron-based superconductors. We mainly discuss the effect of the "inelastic" quasi-particle damping rate γ due to many-body interaction on the size of the coherence peak, for both s++ and s+- wave superconducting states. We focus on Ba(Fe1-xCox)2As2, and systematically evaluate γ in the normal state from the experimental resistivity, from optimally to over-doped compounds. Next, γ in the superconducting state is calculated microscopically based on the second order perturbation theory. In optimally doped compounds (Tc ~ 30 K), it is revealed that the coherence peak on 1/T1T is completely suppressed due to large γ for both s++ and s+- wave states. On the other hand, in heavily over doped compounds with Tc < 10 K, the coherence peak could appear for both pairing states, since γ at Tc is quickly suppressed in proportion to Tc^2. By making careful comparison between theoretical and experimental results, we conclude that it is difficult to discriminate between s++ and s+- wave states from the present experimental results.

cond-mat.supr-con↗

arXiv2010

Orbital Order, Structural Transition and Superconductivity in Iron Pnictides

We investigate the 16-band d-p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below TQ which induces the tetragonal-orthorhombic structural transition at Ts = TQ, together with the stripe-type antiferromagnetic order below TN. Near the phase transitions, the system shows the s++ wave superconductivity due to the orbital fluctuation for a large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation for a small g case with TQ < TN. The former case is consistent with the phase diagram of doped iron pnictides with Ts > TN.

cond-mat.supr-con↗

arXiv2010

Effect of the spin-orbit interaction and the electron phonon coupling on the electronic state in a silicon vacancy

The electronic state around a single vacancy in silicon crystal is investigated by using the Green's function approach. The triply degenerate charge states are found to be widely extended and account for extremely large elastic softening at low temperature as observed in recent ultrasonic experiments. When we include the LS coupling $λ_{\rm Si}$ on each Si atom, the 6-fold spin-orbital degeneracy for the $V^{+}$ state with the valence +1 and spin 1/2 splits into $Γ_{7}$ doublet groundstates and $Γ_{8}$ quartet excited states with a reduced excited energy of $O(λ_{\rm Si}/10)$. We also consider the effect of couplings between electrons and Jahn-Teller phonons in the dangling bonds within the second order perturbation and find that the groundstate becomes $Γ_{8}$ quartet which is responsible for the magnetic-field suppression of the softening in B-doped silicon.

cond-mat.str-el↗

arXiv2010

Cooperative effects of Coulomb and electron-phonon interactions in the two-dimensional 16-band d-p model for iron-based superconductors

We study the electronic states and the superconductivity in the two-dimensional 16-band d-p model coupled with A1g, B1g and Eg local phonons and obtain the rich phase diagram including the magnetic, charge and orbital ordered phases on the parameter plane of the Coulomb and electron-phonon interactions. When the electron-phonon interaction is dominant, the charge fluctuations induce the s++ wave superconductivity, while when the Coulomb interaction is dominant, the magnetic fluctuations induce the s+- wave superconductivity. Remarkably, the orbital fluctuations are enhanced due to the cooperative effects of the Coulomb and electron-phonon interactions and induce the s++ wave and the nodal s+- wave superconductivities.

cond-mat.supr-con↗