arXiv subjects

Kazuhiko Kuroki

Publications and source records attributed to Kazuhiko Kuroki.

At least 127 records · Page 7Linked to original sources

arXiv2011

Spin fluctuations and unconventional pairing in KFe$_2$As$_2$

We study the relation between the spin fluctuation and superconductivity in an heavily hole doped end material KFe$_2$As$_2$. We construct a five orbital model by approximately unfolding the Brillouin zone of the three dimensional ten orbital model obtained from first principles calculation. By applying the random phase approximation, we obtain the spin susceptibility and solve the linearized Eliashberg equation. The incommensurate spin fluctuation observed experimentally is understood as originating from interband interactions, where the multiorbital nature of the band structure results in an electron-hole asymmetry of the incommensurability in the whole iron-based superconductor family. As for superconductivity, s-wave and d-wave pairings are found to be in close competition, where the sign change in the gap function in the former is driven by the incommensurate spin fluctuations. We raise several possible explanations for the nodes in the superconducting gap of KFe$_2$As$_2$ observed experimentally.

cond-mat.supr-con↗

arXiv2011

Determination of the pairing state in iron-based superconductors through neutron scattering

We calculate the spin susceptibility in the s_{+-} and s_{++} superconducting states of the iron pnictides using the effective five orbital model and considering the quasiparticle damping. For the experimentally evaluated magnitude of the quasiparticle damping and the superconducting gap, the results at the wave vector ~ (pi,0) show that the s_{+-} state is more consistent with the neutron scattering experiments, while for larger quasiparticle damping and the superconducting gap, the s_{++} state can be more consistent. To distinguish between two cases that reproduce the experiments at the wave vector ~ (pi,0), we propose to investigate experimentally the wave vector ~ (pi,pi).

cond-mat.supr-con↗

arXiv2011

Comment on arXiv:1105.6233 entitled "Neutron-Inelastic-Scattering Peak by Dissipationless Mechanism in the s++-wave State in Iron-based Superconductors" by S. Onari and H. Kontani

Recently, Onari and Kontani submitted a paper [arXiv:1105.6233] which criticizes our recent theoretical study [arXiv:1103.0586] on the neutron scattering experiment as a probe for determining the superconducting gap in the iron pnictides. In their paper, Onari and Kontani have developed a formalism in which the imaginary part of the dynamical spin susceptibility (Im$χ_s(\Vec{q},ω)$) in the superconducting state can be more accurately calculated especially in the $ω<2Δ$ regime, where $Δ$ is the superconducting gap. In section IIIC of their paper, they mention that the conclusions of our paper are "incorrect based on inaccurate numerical calculation". In the present Comment, we show that this in fact is not correct.

cond-mat.supr-con↗

arXiv2011

Maximizing Fermi surface multiplicity optimizes superconductivity in iron pnictides

We study the condition for optimizing superconductivity in the iron pnictides from the lattice structure point of view. Studying the band structure of the hypothetical lattice structure of LaFeAsO, the hole Fermi surface multiplicity is found to be maximized around the Fe-As-Fe bond angle regime where the arsenic atoms form a regular tetrahedron. Superconductivity is optimized within this three hole Fermi surface regime, while the stoner factor of the antiferromagnetism has an overall tendency of increasing upon decreasing the bond angle. Combining also the effect of the varying the Fe-As bond length, we provide a guiding principle for obtaining high $T_c$.

cond-mat.supr-con↗

arXiv2010

Impurity effect in multi-orbital, sign-reversing s-wave superconductors

We study the impurity effects on the transition temperature Tc with use of the T-matrix approximation. We propose a way to visualize the multi-orbital effect by introducing the hybridization function characterizing the multi-orbital effect for the impurity scattering. Characterizing function does not depend on the superconducting pairing symmetry, since this function is defined by the eigenvectors in normal states. The result indicates that an impurity-robust superconductivity does not necessarily imply a sign- preserving pairing. Visualizing the hybridization effect in the effective five-band model for LaFeAsO, we show that the impurity effect on Tc is relatively weaker than that in single-band models.

cond-mat.supr-con↗

arXiv2010

Possible three dimensional nodes in the $s{\pm}$ superconducting gap of BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We examine theoretically the superconducting state of BaFe$_2$(As$_{1-x}$P$_x$)$_2$, an isovalent doping 122 iron pnictide superconductor. We construct a three dimensional ten orbital model from first principles band calculation, and investigate the superconducting gap within the spin fluctuation mediated pairing mechanism. The gap is basically $s\pm$, where the gap changes its sign between electron and hole Fermi surfaces, but three dimensional nodal structures appear in the largely warped hole Fermi surface having strong $Z^2/XZ/YZ$ orbital character. The present result, together with our previous study on 1111 systems, explains the strong material dependence of the superconducting gap in the iron pnictides.

cond-mat.supr-con↗

arXiv2010

Spin-density functional study of the organic polymer dimethylaminopyrrole: A realization of the organic periodic Anderson model

While the periodic Anderson model (PAM) has been recognized as a good model for various heavy f-electron systems, here we design a purely organic polymer whose low-energy physics can be captured by PAM. By means of the spin density functional calculation, we show that polymer of dimethylaminopyrrole is a candidate, where its ground state can indeed be magnetic depending on the doping. We discuss the factors favoring ferromagnetic ground state.

cond-mat.mtrl-sci↗

arXiv2010

Anion height as a controlling parameter for the superconductivity in iron pnictides and cuprates

Both families of high $T_c$ superconductors, iron pnictides and cuprates, exhibit material dependence of superconductivity. Here, we study its origin within the spin fluctuation pairing theory based on multiorbital models that take into account realistic band structures. For pnictides, we show that the presence and absence of Fermi surface pockets is sensitive to the pnictogen height measured from the iron plane due to the multiorbital nature of the system, which is reflected to the nodeless/nodal form of the superconducting gap and $T_c$. Surprisingly, even for the cuprates, which is conventionally modeled by a single orbital model, the multiorbital band structure is shown to play a crucial role in the material dependence of superconductivity. In fact, by adopting a two orbital model that considers the $d_{z^2}$ orbital on top of the $d_{x^2-y^2}$ orbital, we can resolve a long standing puzzle of why the single layered Hg cuprate have much higher $T_c$ than the La cuprate. Interestingly, here again the apical oxygen height measured from the CuO$_2$ plane plays an important role in determining the relative energy difference between $d_{x^2-y^2}$ and $d_{z^2}$ orbitals, thereby strongly affecting the superconductivity.

cond-mat.supr-con↗

arXiv2010

Two-Orbital Model Explains the Higher Transition Temperature of the Single-Layer Hg-Cuprate Superconductor Compared to That of the La-Cuprate Superconductor

In order to explore the reason why the single-layered cuprates, La$_{2-x}$(Sr/Ba)$_x$CuO$_4$ ($T_c\simeq$ 40K) and HgBa$_2$CuO$_{4+δ}$ ($T_c\simeq$ 90K), have such a significant difference in $T_c$, we study a two-orbital model that incorporates the $d_{z^2}$ orbital on top of the $d_{x^2-y^2}$ orbital. It is found, with the fluctuation exchange approximation, that the $d_{z^2}$ orbital contribution to the Fermi surface, which is stronger in the La system, works against d-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of $T_c$ in the cuprates.

cond-mat.supr-con↗

arXiv2010

Realization of Berezinskii's superconductivity in quasi-one-dimensional systems

We revisit the pairing symmetry competition in quasi-one-dimensional systems. We show that spin-triplet s-wave pairing, where the pair is formed by electrons with different times and has an odd-frequency symmetry, can be realized in systems with strong one-dimensionality when the strength of charge fluctuation dominates over spin fluctuation. The present study provides a novel microscopic mechanism for this exotic pairing originally proposed by Berezinskii in 1974.

cond-mat.supr-con↗

arXiv2010

"Pudding Mold"-type Band as an Origin of Large Thermopower in tau-type Organic Conductors

We study the origin of the large thermopower in quasi-two-dimensional a $τ$-type organic conductor, $τ-(EDO-S,S-DMEDT-TTF)_2(AuBr_2)_{1+y}$ ($y \le 0.875$), from the view point of a "pudding mold"-type band structure. We calculate the electronic band structure using an \textit{ab initio} band calculation package, and obtain a tight binding model fit to the \textit{ab initio} band structure. Using the model and the Boltzmann's equation approach, we calculate the temperature dependence of the Seebeck coefficient. We conclude that the peculiar band structure is the origin of the large Seebeck coefficient and the appearance of the maximum value at a certain temperature.

cond-mat.mtrl-sci↗

arXiv2010

Origin of coexisting large Seebeck coefficient and metallic conductivity in the electron doped SrTiO$_3$ and KTaO$_3$

We study the origin of the large Seebeck coefficient despite the metallic conductivity in the La-doped SrTiO$_3$ and Ba-doped KTaO$_3$. We calculate the band structure of SrTiO$_3$ and KTaO$_3$, from which the Seebeck coefficient is obtained using the Boltzmann's equation. We conclude that the multiplicity of the $t_{2g}$ bands in these materials is one major origin of the good thermoelectric property in that when compared at a fixed total number of doped electrons, the Seebeck coefficient and thus the power factor are larger in multiple band systems than in single band ones because the number of doped electron bands {\it per band} is smaller in the former. We also find that the second nearest neighbor hopping integral, which generally has negative values in these materials and works destructively against the Seebeck effect, is nearly similar between KTaO$_3$ and SrTiO$_3$ despite the larger band width in the former. This can be another factor favorable for thermopower in the Ba-doped KTaO$_3$.

cond-mat.mtrl-sci↗

arXiv2010

Possibility of spin fluctuation mediated $d+id'$ pairing in a doped band insulator $β$-MNCl (M=Hf,Zr) superconductors

We study two types of models for the superconducting layered nitride $β$-MNCl(M=Hf,Zr); a single band model on a triangular lattice, and a two band model on a honeycomb lattice. We find that the former model does not suffice as an effecitive model, while the latter one can be a good candidate. We propose from the study on the two band model a possibility of spin fluctuation mediated $d+id'$-wave superconductivity in the doped $β$-MNCl. We show that the relatively high $T_c$ obtained in the doped band insulator is a characteristic feature of the spin fluctuation mediated superconductivity on a honeycomb lattice. We also find that the gap anisotropy on the Fermi surface strongly increases upon increasing the doping concentration, and the inter layer hopping suppresses superconductivity. These results are in qualitative agreement with the experimental findings.

cond-mat.supr-con↗

arXiv2009

Pairing competition in a quasi-one-dimensional model of organic superconductors (TMTSF)$_{2}X$ in magnetic field

We microscopically study the effect of the magnetic field (Zeeman splitting) on the superconducting state in a model for quasi-one-dimensional organic superconductors (TMTSF)$_{2}X$. We investigate the competition between spin singlet and spin triplet pairings and the Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state by random phase approximation. While we studied the competition by comparison with the eigenvalue of the gap equation at a fixed temperature in our previous study (Phys. Rev. Lett. \textbf{102} (2009) 016403), here we obtain both the $T_c$ for each pairing state and a phase diagram in the $T$(temperature)-$h_z$(field)-$V_y$(strength of the charge fluctuation) space. The phase diagram shows that consecutive transitions from singlet pairing to the FFLO state and further to $S_z=1$ triplet pairing can occur upon increasing the magnetic field when $2k_{F}$ charge fluctuations coexist with $2k_{F}$ spin fluctuations. In the FFLO state, the singlet d-wave and $S_{z}=0$ triplet $f$-wave components are strongly mixed especially when the charge fluctuations are strong.

cond-mat.supr-con↗

arXiv2009

Pnictogen height as a possible switch between high-T_c nodeless and low-T_c nodal pairings in the iron based superconductors

We study the effect of the lattice structure on the spin-fluctuation mediated superconductivity in the iron pnictides adopting the five-band models of several virtual lattice structures of LaFeAsO as well as actual materials such as NdFeAsO and LaFePO obtained from the maximally-localized Wannier orbitals. Random phase approximation is applied to the models to solve the Eliashberg equation. This reveals that the gap function and the strength of the superconducting instability are determined by the cooperation or competition among multiple spin fluctuation modes arising from several nestings among disconnected pieces of the Fermi surface, which is affected by the lattice structure. Specifically, the appearance of the Fermi surface $γ$ around $(π,π)$ in the unfolded Brillouin zone is sensitive to the pnictogen height $h_{\rm Pn}$ measured from the Fe plane, where $h_{\rm Pn}$ is shown to act as a switch between high-$T_c$ nodeless and low-$T_c$ nodal pairings. We also find that reduction of the lattice constants generally suppresses superconductivity. We can then combine these to obtain a generic superconducting phase diagram against the pnictogen height and lattice constant. This suggests that NdFeAsO is expected to exhibit a fully-gapped, sign-reversing s-wave superconductivity with a higher $T_c$ than in LaFeAsO, while a nodal pairing with a low $T_c$ is expected for LaFePO, which is consistent with experiments.

cond-mat.supr-con↗

arXiv2009

Finite temperature semimetal-insulator transition on the honeycomb lattice

A semimetal-insulator transition in the Hubbard model on the honeycomb lattice is studied by using the dynamical mean field theory. Electrons in the honeycomb lattice resemble the Dirac electron liquid and for weak interactions the system is semimetal. With increasing the local interaction a semimetal-insulator transition occurs. We find a nonanalytical structure of the phase transition which consists of a first-order transition line ending in a second-order transition point and high-temperature crossover line. A phase separation of semimetal and insulator occurs at low temperatures. Maxwell construction is performed to determine the first order transition line. The phase diagram is also presented.

cond-mat.str-el↗

arXiv2009

Unconventional Pairing in Doped Band Insulators on a Honeycomb Lattice : the Role of the Disconnected Fermi Surface and a Possible Application to Superconducting $β$-MNCl (M=Hf,Zr)

We investigate the possibility of realizing unconventional superconductivity in doped band insulators on the square lattice and the honeycomb lattice, where the latter is found to be a good candidate due to the disconnectivity of the Fermi surface. We propose applying the theory to the superconductivity in doped layered nitride $β$-$M$NCl ($M=$Hf, Zr). Finally, we compare two groups of superconductors with disconnected Fermi surface, $β$-$M$NCl and the iron pnictides, which have high $T_c$ despite some faults against superconductivity are present.

cond-mat.supr-con↗

arXiv2009

Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor

For the iron-based high $T_c$ superconductor LaFeAsO$_{1-x}$F$_x$, we construct a minimal model, where all of the five Fe $d$ bands turn out to be involved. We then investigate the origin of superconductivity with a five-band random-phase approximation by solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realise, basically, an extended s-wave pairing, where the gap changes sign across the nesting vector.

cond-mat.supr-con↗