For a newly discovered iron-based high T_{c} superconductor LaFeAsO1-xFx, we have constructed minimal model, where inclusion of all five Fe d bands is found to be necessary. The random-phase approximation applied the model investigate origin superconductivity. We conclude that multiple spin-fluctuation modes arising from nesting across disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also another candidate.

We study the effect of lattice structure on spin-fluctuation-mediated superconductivity in iron pnictides adopting five-band models several virtual structures LaFeAsO, as well actual materials such NdFeAsO and LaFePO obtained from maximally localized Wannier orbitals. Random phase approximation is applied to solve Eliashberg equation. This reveals that gap function strength superconducting instability are determined by cooperation or competition among multiple spin-fluctuation modes arising...

In iron pnictides, we find that the moderate electron-phonon interaction due to Fe-ion oscillation can induce critical d-orbital fluctuations, without being prohibited by Coulomb interaction. These fluctuations give rise strong pairing for s-wave superconducting (SC) state sign reversal ($s_{++}$-wave state), which is consistent with experimentally observed robustness of superconductivity against impurities. When magnetic are also strong, SC shows a smooth crossover from ($s_\pm$-wave state)...

Exotic quantum phase transitions in metals, such as the electronic nematic state, have been discovered one after another and found to be universal now. The emergence of unconventional density-wave (DW) order frustrated kagome metal AV 3 Sb 5 its interplay with exotic superconductivity attract increasing attention. We find that DW is bond order, because sizable intersite attraction caused by interference among paramagnons. This mechanism important metals geometrical frustration prohibits...

Based on the five-orbital model, we study effect of local impurity in iron pnictides, and find that interband scattering is promoted by $d$-orbital degree freedom. This fact means fully gapped sign-reversing $s$-wave state, which predicted spin fluctuation theories, very fragile against impurities. In BCS theory, only 1% impurities with intermediate strength induce huge pair breaking, resulting large in-gap state prominent reduction ${T}_{c}$, contrary to prediction based simple orbital-less...

We study the mechanism of orbital or spin fluctuations due to multiorbital Coulomb interaction in iron-based superconductors, going beyond random-phase approximation. For this purpose, we develop a self-consistent vertex correction (VC) method, and find that multiple addition are mutually emphasized by "multimode interference effect" described VC. Then, both antiferro-orbital ferro-orbital (=nematic) simultaneously for J/U~0.1, which contribute s-wave superconductivity. Especially, give...

The main features in iron pnictide superconductors are summarized as (i) the orthorhombic transition accompanied by a remarkable softening of shear modulus, (ii) high-${T}_{c}$ superconductivity close to phase, and (iii) stripe-type magnetic order induced orthorhombicity. To present unified explanation for these features, we analyze multi-orbital Hubbard-Holstein model with Fe-ion optical phonons based on orbital fluctuation theory. In random-phase approximation (RPA), small electron-phonon...

Nematicity and magnetism are two key features in Fe-based superconductors, their interplay is one of the most important unsolved problems. In FeSe, magnetic order absent below structural transition temperature $T_{str}=90$K, stark contrast that emerges slightly $T_{str}$ other families. To understand such amazing material dependence, we investigate spin-fluctuation-mediated orbital ($n_{xz}\neq n_{yz}$) by focusing on orbital-spin driven strong-coupling effect, called vertex correction. This...

We study the neutron-scattering spectrum in iron pnictides based on random-phase approximation five-orbital model for fully gapped $s$-wave states with sign reversal $({s}_{\ifmmode\pm\else\textpm\fi{}})$ and without $({s}_{++})$. In ${s}_{++}$-wave state, we find that a prominent hump structure appears just above spectral gap by taking account of quasiparticle damping $\ensuremath{\gamma}$ due to strong electron-electron correlation: as superconductivity develops, reduction gives rise large...

We report superconductivity in the novel 112-type iron-based compound Ca1-xLaxFeAs2. Single-crystal X-ray diffraction analysis revealed that crystallizes a monoclinic structure (space group \(P2_{1}\)), which Fe2As2 layers alternate with Ca2As2 spacer such monovalent arsenic forms zigzag chains. Superconductivity transition temperature (\(T_{\text{c}}\)) of 34 K was observed for \(x = 0.1\) sample, while 0.21\) sample exhibited trace at 45 K. First-principles band calculations demonstrated...

To understand the nematicity in Fe-based superconductors, nontrivial $\mathbit{k}$ dependence of orbital polarization [$\mathrm{\ensuremath{\Delta}}{E}_{xz}(\mathbit{k})$, $\mathrm{\ensuremath{\Delta}}{E}_{yz}(\mathbit{k})$] nematic phase, such as sign reversal splitting between $\mathrm{\ensuremath{\Gamma}}$ and $X$, $Y$ points FeSe, provides significant information. solve this problem, we study spontaneous symmetry breaking with respect to spin susceptibility self-consistently. In due...

We study the pairing mechanism in iron pnictide superconductors based on five-orbital Hubbard-Holstein model. Due to Fe-ion oscillations, $s$-wave superconducting (SC) state without sign reversal (${s}_{++}$-wave state) is induced by orbital fluctuations using realistic model parameters. The virtue of present theory that famous empirical relation between ${T}_{c}$ and As-Fe-As bond angle automatically explained since electron-phonon coupling creates strongest when ${\text{As}}_{4}$...

Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic Fe-based superconductors and star-of-David charge density kagome Such exotic metals is a central issue of modern condensed matter physics, while its theoretical foundation still unclear comparison with well-established theory superconductivity. To overcome this difficulty, here we introduce "form factor" that generalizes...

We study the superconducting state in recently discovered high-${T}_{\mathrm{c}}$ superconductor ${\mathrm{K}}_{x}$Fe${}_{2}$Se${}_{2}$ based on ten-orbital Hubbard-Holstein model without hole pockets. When Coulomb interaction is large, a spin-fluctuation-mediated $d$-wave appears due to nesting between electron Interestingly, symmetry of body-centered tetragonal structure requires existence nodes gap, although fully gapped realized case simple structure. In presence moderate electron-phonon...

The isostructural transition in the tetragonal phase with a sizable change anion height, is realized heavily H-doped LaFeAsO and (La,P) codoped CaFe2As2. In these compounds, superconductivity higher Tc (40-50 K) near transition. To find origin of anion-height instability role realizing higher-Tc state, we develop orbital-spin fluctuation theory by including vertex correction. We analyze LaFeAsO(1-x)H(x) that non-nematic orbital fluctuations, which induce instability, are automatically...

We study the mechanism of triplet superconductivity (TSC) in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ based on multiorbital Hubbard model. The electronic states are studied using recently developed renormalization group method combined with constrained random-phase approximation, called RG + cRPA method. Thanks to vertex correction (VC) for susceptibility, which is dropped mean-field-level approximations, strong orbital and spin fluctuations at...

In order to explore how superconductivity arises when charge fluctuations and spin coexist, we have obtained a phase diagram against the off-site repulsion $V$ band filling $n$ for extended, repulsive Hubbard model on square lattice with fluctuation exchange approximation. We found existence of (i) transition between ${d}_{xy}$ ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ pairing symmetries, (ii) $f$-pairing in CDW phases intermediate $0.5<n<1.0$ large $V$, (iii) anisotropic cases symmetry changing,...

Abstract Optical absorption, photoluminescence and Raman scattering of lead‐halide based inorganic–organic perovskite semiconductors were measured under quasi‐hydrostatic pressure at room temperature. For the 3D semiconductor, (CH 3 NH )PbBr , free exciton band exhibits red‐shifts with pressure, jumps to a higher energy by 0.07 eV 0.8 GPa, which is associated phase transition from cubic an orthorhombic structure confirmed scattering. Above shows blue‐shifts further increasing eventually...

We investigate the neutron scattering spectrum in iron pnictides based on random-phase approximation five-orbital model with a realistic superconducting (SC) gap, Delta=5meV. In normal state, is suppressed by large inelastic quasi-particle (QP) rate gamma* ~ Delta. fully-gapped s-wave state without sign reversal (s++), hump-shaped enhancement appears just above 2Delta, since QP prohibited SC gap. That is, hump structure produced dissipationless QPs for Ek<3Delta. The obtained result more...

Motivated by the nematic electronic fluid phase in Sr(3)Ru(2)O(7), we develop a combined scheme of renormalization-group method and random-phase-approximation-type method, analyze orbital susceptibilities (d(xz), d(yz))-orbital Hubbard model with high accuracy. It is confirmed that present exhibits ferro-orbital instability near magnetic or superconducting quantum criticality, due to Aslamazov-Larkin-type vertex corrections. This mechanism order presents natural explanation for expected be...

To understand the origin of nodal gap structure realized in BaFe${}_{2}$(As,P)${}_{2}$, we study three-dimensional based on ten-orbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop when random-phase approximation is used. By solving Eliashberg equation, obtain fully gapped $s$-wave state (without) sign reversal between holelike electronlike Fermi surfaces due to (orbital) fluctuations, so-called...

Optical conductivity spectra of single crystals the perovskite-type ${3d}^{1}$ metallic alloy system ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{VO}}_{3}$ have been studied to elucidate how electronic behavior depends on strength electron correlation without changing nominal number electrons. The reflectivity measurements were made at room temperature between 0.05 eV and 40 eV. effective mass deduced by analysis Drude-like contribution optical plasma frequency do not show...

In order to clarify whether the odd-frequency superconductivity can be realized or not, we study a quasi-one-dimensional triangular lattice in Hubbard model using random-phase approximation and fluctuation exchange approximation. We find that spin-singlet $p$-wave pairing stabilized on isosceles lattice.