[Show abstract][Hide abstract] ABSTRACT: Nontrivial k-dependence of the orbital polarization ($\Delta E_{xz}(k)$,
$\Delta E_{yz}(k)$) in the orthorhombic phase, such as the sign-reversal of the
orbital splitting between $\Gamma$- and X-points in FeSe, provides significant
information to understand the nematicity in Fe-based superconductors. To solve
this problem, we analyze the multiorbital Hubbard models in the orbital-ordered
state by extending the orbital-spin fluctuation theory. The present theory
describes the spontaneous symmetry breaking with respect to the orbital
polarization and spin susceptibility self-consistently. In the orbital-ordered
state in FeSe, we obtain the two Dirac cone Fermi pockets in addition to the
sign reversal of the orbital polarization, consistently with experiments. The
orbital-order in Fe-based superconductors originates from the strong
orbital-spin interplay due to the Aslamazov-Larkin processes.
[Show abstract][Hide abstract] ABSTRACT: 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 emergence of the nematic orbital-order ($n_{xz} \neq n_{yz}$)
based on various first-principles Hubbard models. In Fe-based superconductors,
spin-fluctuation-mediated large orbital-fluctuations appear because of the
strong orbital-spin interplay due to the many-body effect. This effect is very
significant in FeSe due to the small ratio between the Hund's and Coulomb
interactions ($J/U$) and large $d_{xz},d_{yz}$-orbitals weight at the Fermi
level. For this reason, in FeSe, orbital order is established by weak spin
fluctuations, so the magnetism is absent below $T_{\rm str}$. In contrast, in
LaFeAsO, the magnetic order appears just below $T_{str}$ both experimentally
and theoretically. Thus, the orbital-spin interplay is the key ingredient of
the wide variety of the normal-state phase diagram in Fe-based superconductors.
[Show abstract][Hide abstract] ABSTRACT: A ternary compound, MgPtSi, was synthesized by solid-state reaction. An
examination of the compound by powder X-ray diffraction revealed that it
crystallizes in the orthorhombic TiNiSi-type structure with the Pnma space
group. The structure comprises alternately stacked layers of Mg and PtSi
honeycomb network, which is reminiscent of MgB2, and the buckling of the
honeycomb network causes orthorhombic distortion. Electrical and magnetic
studies revealed that MgPtSi exhibited superconductivity with a transition
temperature of 2.5 K. However, its isostructural compounds, namely, MgRhSi and
MgIrSi, were not found to exhibit superconductivity.
Physical Review B 05/2015; 91(17). DOI:10.1103/PhysRevB.91.174514 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We study the mechanism of the triplet superconductivity (TSC) in Sr2RuO4
based on the multiorbital Hubbard model. The electronic states are studied
using the recently developed renormalization group method combined with the
constrained random-phase-approximation, called the RG+cRPA 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 $Q
=(2\pi/3,2\pi/3)$ emerge in the quasi one-dimensional Fermi surfaces (FSs)
composed of xz- and 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
$(\Delta_x(k),\Delta_y(k))\sim (\sin 3k_x,\sin 3k_y)$. Very similar results are
obtained by applying the diagrammatic calculation called the self-consistent VC
method. Thus, the idea of "orbital+spin fluctuation mediated TSC" is confirmed
by both RG+cRPA method and the self-consistent VC method. We also reveal that a
substantial superconducting gap on the xy-orbital FS is induced from the gaps
on the quasi one-dimensional FSs, in consequence of the large orbital-mixture
due to the 4d spin-orbit interaction.
Physical Review B 03/2015; 91(15). DOI:10.1103/PhysRevB.91.155103 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: On the basis of the Eilenberger theory, spatial variation of the local NMR relaxation rate ${T}_{1}^{$-${}1}$ is quantitatively estimated in the vortex lattice state, to clarify the differences between the $s$-wave and the $d$-wave superconductors. We study the temperature and the magnetic field dependencies of ${T}_{1}^{$-${}1}$ inside and outside of the vortex core, including influences of nonmagnetic impurity scatterings in the Born limit and in the unitary limit. These results are helpful to detect detailed characters of local electronic structures in the vortex lattice states via site-selective NMR experiments.
Physical Review B 01/2015; 91(1). DOI:10.1103/PhysRevB.91.014509 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As1-xPx)2 (x = 0.30, Tc = 30 K). Strongly anisotropic gap has been found only in the electron Fermi surface while the gap on the entire hole Fermi surfaces are nearly isotropic. These results are inconsistent with horizontal nodes but are consistent with modified s± gap with nodal loops. We have shown that the complicated gap modulation can be theoretically reproduced by considering both spin and orbital fluctuations.
[Show abstract][Hide abstract] ABSTRACT: The absence of nesting between electron and hole pockets in LiFeAs with ${T}_{\mathrm{c}}=18$ K attracts great attention, as an important hint to understand the pairing mechanism of Fe-based superconductors. Here, we study the five-orbital model of LiFeAs based on the recently developed orbital-spin fluctuation theories. It is found that the experimentally observed gap structure of LiFeAs, which is a ``fingerprint'' of the pairing mechanism, is quantitatively reproduced in terms of the orbital-fluctuation-mediated ${s}_{++}$-wave state. Specifically, the largest gap observed on the two small hole pockets composed of (${d}_{xz},{d}_{yz}$) orbitals can be explained, and this is a hallmark of the orbital-fluctuation-mediated superconductivity. The ${s}_{++}$-wave gap structure becomes more anisotropic in the presence of weak spin fluctuations. As the spin fluctuations increase, we obtain the ``hole-${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave state,'' in which only the gap of the large hole pocket made of the ${d}_{xy}$ orbital is sign reversed, due to the cooperation of orbital and spin fluctuations. This gap structure with ``sign reversal between hole pockets'' is similar to that recently reported in $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$.
Physical Review B 07/2014; 90(3). DOI:10.1103/PhysRevB.90.035104 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The isostructural transition in the tetragonal phase with a sizable change in the anion height, is realized in heavily H-doped LaFeAsO and (La,P) codoped CaFe_{2}As_{2}. In these compounds, the superconductivity with higher T_{c} (40-50 K) is realized near the isostructural transition. To find the origin of the anion-height instability and the role in realizing the higher-T_{c} state, we develop the orbital-spin fluctuation theory by including the vertex correction. We analyze LaFeAsO_{1-x}H_{x} and find that the non-nematic orbital fluctuations, which induce the anion-height instability, are automatically obtained at x∼0.5, in addition to the conventional nematic orbital fluctuations at x∼0. The non-nematic orbital order triggers the isostructural transition, and its fluctuation would be a key ingredient to realize higher-T_{c} superconductivity of order 50 K.
[Show abstract][Hide abstract] ABSTRACT: 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 $Q=(2\pi/3,2\pi/3)$ 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 $(\Delta_x(k),\Delta_y(k))=(\sin
3k_x,\sin 3k_y)$. These results are confirmed by a diagrammatic calculation
called the self-consistent VC method.
[Show abstract][Hide abstract] ABSTRACT: The absence of nesting between electron and hole-pockets in LiFeAs with
$T_c=18$K attracts great attention, as an important hint to understand the
pairing mechanism of Fe-based superconductors. Here, we study the five-orbital
model of LiFeAs based on the recently-developed orbital-spin fluctuation
theories. It is found that the experimentally observed gap structure of LiFeAs
is quantitatively reproduced in terms of the orbital-fluctuation-mediated
$s_{++}$-wave state without sign-reversal. Especially, the largest gap observed
on the small two hole-pockets composed of ($d_{xz}, d_{yz}$) orbitals can be
explained, and this is a hallmark of the orbital-fluctuation-mediated
superconductivity. The $s_{++}$-wave gap structure becomes more anisotropic in
the presence of weak spin fluctuations. As the spin fluctuations increase, we
obtain the ``$s_\pm^{h}$-wave state'', in which only the gap of the large
hole-pocket made of $d_{xy}$-orbital is sign-reversed, due to the cooperation
of orbital and spin fluctuations. This gap structure with ``sign-reversal
between hole-pockets'' is similar to that recently reported in
(Ba,K)Fe$_2$As$_2$.
[Show abstract][Hide abstract] ABSTRACT: The isostructural transition in the tetragonal ($C_4$) phase, with sizable
change in the As-height, is realized in heavily H-doped LaFeAsO, Pr-doped
CaFe$_2$As$_2$, and Na-doped BaFe$_2$As$_2$. Here, we explain the overall phase
diagram of LaFeAsO$_{1-x}$H$_x$ by considering the vertex correction (VC) due
to spin fluctuations. In heavily-doped case ($x\sim0.5$), the non-nematic
orbital order is caused by the VC due to $d_{xy}$-orbital spin fluctuations,
and triggers the $C_4$ isostructural transition. In lightly-doped case
($x\sim0$), the orthorhombic phase is realized by the orbital-nematic order,
which originates from the VC due to ($d_{xz}$, $d_{yz}$)-orbital spin
fluctuations. The non-nematic orbital fluctuations that couple to the As-height
change would be essential for the second-Tc dome in LaFeAsO$_{1-x}$H$_x$.
[Show abstract][Hide abstract] ABSTRACT: We report superconductivity in the novel 112-type iron-based compound
Ca1-xLaxFeAs2. Single-crystal X-ray diffraction analysis revealed that the
compound crystallizes in a monoclinic structure (space group P21), in which
Fe2As2 layers alternate with Ca2As2 spacer layers such that monovalent arsenic
forms zigzag chains. Superconductivity with a transition temperature (Tc) of 34
K was observed for the x = 0.1 sample, while the x = 0.21 sample exhibited
trace superconductivity at 45 K. First-principles band calculations
demonstrated the presence of almost cylindrical Fermi surfaces, favorable for
the high Tc in La-doped CaFeAs2.
Journal of the Physical Society of Japan 11/2013; 82(12). DOI:10.7566/JPSJ.82.123702 · 1.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to clarify a pairing symmetry in quasi-one-dimensional organic superconductors (TMTSF)(2)X, we investigate the superconducting state in the extended Hubbard model on a quarter-filled square lattice within the random phase approximation. We find that a competition among four pairing states including odd-frequency pairing occurs in the quasi-one-dimensional system where the 2k(F) spin fluctuation coexists with the 2k(F) charge fluctuation. We suggest that the odd-frequency pairing can appear in a realistic parameter.
Journal of the Physical Society of Japan 10/2013; 82(10):104702. DOI:10.7566/JPSJ.82.104702 · 1.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Motivated by the nematic electronic fluid phase in Sr_{3}Ru_{2}O_{7}, we develop a combined scheme of the renormalization-group method and the random-phase-approximation-type method, and analyze orbital susceptibilities of the (d_{xz}, d_{yz})-orbital Hubbard model with high accuracy. It is confirmed that the present model exhibits a ferro-orbital instability near the magnetic or superconducting quantum criticality, due to the Aslamazov-Larkin-type vertex corrections. This mechanism of orbital nematic order presents a natural explanation for the nematic order in Sr_{3}Ru_{2}O_{7}, and is expected to be realized in various multiorbital systems, such as Fe-based superconductors.
[Show abstract][Hide abstract] ABSTRACT: To understand the amazing variety of the superconducting states of Fe-based
superconductors, we analyze the multiorbital Hubbard models for LaFeAsO and
LiFeAs going beyond the random-phase approximation (RPA), by calculating the
vertex correction (VC) and self-energy correction. Due to the spin+orbital mode
coupling described by the VC, both orbital and spin fluctuations mutually
develop, consistently with the experimental phase diagram with the orbital and
magnetic orders. Due to both fluctuations, the s-wave gap function with
sign-reversal ($s_{\pm}$-wave), without sign-reversal ($s_{++}$-wave), and
nodal s-wave states are obtained, compatible with the experimental wide variety
of the gap structure. Thus, the present theory provides a microscopic
explanation of the normal and superconducting phase diagram based on the
realistic Hubbard model.
[Show abstract][Hide abstract] ABSTRACT: To understand the recently established unique magnetic and superconducting
phase diagram of LaFeAsO$_{1-x}$H$_x$, we analyze the realistic multiorbital
tight-binding model for $x=0 \sim 0.4$ beyond the rigid band approximation.
Both the spin and orbital susceptibilities are calculated in the presence of
the Coulomb and charge quadrupole interactions. It is found that both orbital
and spin fluctuations strongly develop at both $x \sim 0$ and 0.4, due to the
strong violation of the rigid band picture in LaFeAsO$_{1-x}$H$_x$. Based on
this result, we discuss the experimental phase diagram, especially the
double-dome superconducting phase. Moreover, we show that the quadrupole
interaction is effectively produced by the vertex correction due to Coulomb
interaction, resulting in the mutual development of spin and orbital
fluctuations.
Physical Review B 04/2013; 88(4). DOI:10.1103/PhysRevB.88.041106 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To understand the origin of the nodal gap structure realized in
BaFe$_2$(As,P)$_2$, we study the three-dimensional gap structure based on the
three-dimensional ten-orbital Hubbard model with quadrupole interaction. In
this model, strong spin and orbital fluctuations develop by using the
random-phase-approximation. By solving the Eliashberg gap equation, we obtain
the fully-gapped s-wave state with (without) sign reversal between hole-like
and electron-like Fermi surfaces due to strong spin (orbital) fluctuations, so
called the $s_\pm$-wave ($s_{++}$-wave) state. When both spin and orbital
fluctuations strongly develop, which will be realized near the orthorhombic
phase, we obtain the nodal s-wave state in the crossover region between
$s_{++}$-wave and $s_\pm$-wave states. The obtained nodal s-wave state
possesses the loop-shape nodes on electron-like Fermi surfaces, due to the
competition between attractive and repulsive interactions in k-space. In
contrast, the SC gaps on the hole-like Fermi surfaces are fully-gapped due to
orbital fluctuations. The present study explains the main characters of the
anisotropic gap structure in BaFe$_2$(As,P)$_2$ observed experimentally.
[Show abstract][Hide abstract] ABSTRACT: 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 $\rho_0^{cr} = 5z^{-1} ~
10z^{-1} [\mu\Omega cm]$ ($z^{-1} = m^*/m$ 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.