arXiv subjects

Yuki Yanagi

Publications and source records attributed to Yuki Yanagi.

33 records · Page 2Linked to original sources

arXiv2018

Electric Toroidal Quadrupoles in Spin-Orbit Coupled Metal Cd$_2$Re$_2$O$_7$

We report our theoretical results on the order parameters for the pyrochlore metal Cd$_2$Re$_2$O$_7$, which undergoes enigmatic phase transitions with inversion symmetry breaking. By carefully examining active electronic degrees of freedom based on the lattice symmetry, we propose that two parity-breaking phases at ambient pressure are described by unconventional multipoles, electric toroidal quadrupoles (ETQs) with different components, $x^2-y^2$ and $3z^2-r^2$, in the pyrochlore tetrahedral unit. We elucidate that the ETQs are activated by bond or spin-current order on Re-Re bonds. Our ETQ scenario provides a key to reconcile the experimental contradictions, by measuring ETQ specific phenomena, such as peculiar spin splittings in the electronic band structure, magneto-current effect, and nonreciprocal transport under a magnetic field.

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arXiv2018

Classification of atomic-scale multipoles under crystallographic point groups and application to linear response tensors

Four types of atomic-scale multipoles, electric, magnetic, magnetic toroidal, and electric toroidal multipoles, give a complete set to describe arbitrary degrees of freedom for coupled charge, spin, and orbital of electrons. We here present a systematic classification of these multipole degrees of freedom towards the application in condensed matter physics. Starting from the multipole description under the rotation group in real space, we generalize the concept of multipoles in momentum space with the spin degree of freedom. We show how multipoles affect the electronic band structures and linear responses, such as the magneto-electric effect, magneto-current (magneto-gyrotropic) effect, spin conductivity, Piezo-electric effect, and so on. Moreover, we exhibit a complete table to represent the active multipoles under 32 crystallographic point groups. Our comprehensive and systematic analyses will give a foundation to identify enigmatic electronic order parameters and a guide to evaluate peculiar cross-correlated phenomena in condensed matter physics from microscopic point of view.

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arXiv2018

Theory of superconductivity in hole-doped monolayer MoS$_{2}$

We theoretically investigate the Cooper-pair symmetry to be realized in hole-doped monolayer MoS$_2$ by solving linearized BCS gap equations on the three-orbital attractive Hubbard-like model in the presence of the atomic spin-orbit coupling. In hole-doped monolayer MoS$_2$, both spin-orbit coupling and the multi-orbital effects are more prominent than those of electron-doped system. Near the valence band edge, the Fermi surfaces are composed of three different types of hole pockets, namely, one mainly consisting of the almost spin-degenerate $|d_{z^{2}}\rangle$ orbital near $Γ$ point, and the others of the spin-split upper and lower bands near ${\rm K}$ and ${\rm K}'$ points arising from the $|d_{x^{2}-y^{2}}\rangle$ and $|d_{xy}\rangle$ orbitals. The number of relevant Fermi pockets increases with increase of the doping. At very low doping, the upper split bands of $|d_{x^{2}-y^{2}}\rangle$ and $|d_{xy}\rangle$ are concerned, yielding extremely low $T_{\rm c}$ due to small density of states of the split bands. For further doping, the conventional spin-singlet state (SS) appears in the $Γ$ pocket, which has a mixture of the spin-triplet (orbital-singlet) (ST-OS) and spin-singlet (orbital-triplet) (SS-OT) states in the K and K$'$ pockets. The ratio of the mixture depends on the relative strength of the interactions, and the sign of the exchange interactions. Moderately strong ferromagnetic exchange interactions even lead to the pairing state with the dominant ST-OS state over the conventional SS one. With these observations, we expect that the fascinating pairing with relatively high $T_{\rm c}$ emerges at high doping that involves all the three Fermi pockets.

cond-mat.supr-con↗

arXiv2017

Fermi surface, pressure-induced antiferromagnetic order, and superconductivity in FeSe

The pressure dependence of the structural ($T_s$), antiferromagnetic ($T_m$), and superconducting ($T_c$) transition temperatures in FeSe is investigated on the basis of the 16-band $d$-$p$ model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe $d$ orbital and Se $p$ orbital is found to drive the ferro-orbital order responsible for the structural transition without accompanying the antiferromagnetic order. The pressure dependence of the Fermi surfaces is derived from the first-principles calculation and is found to well account for the opposite pressure dependences of $T_s$ and $T_m$, around which the enhanced orbital and magnetic fluctuations cause the double-dome structure of the eigenvalue $λ$ in the Eliashberg equation, as consistent with that of $T_c$ in FeSe.

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arXiv2017

Manipulating magnetoelectric effect -- Essence learned from Co$_4$Nb$_2$O$_9$

Recent experiments for linear magnetoelectric (ME) response in honeycomb antiferromagnet Co$_4$Nb$_2$O$_9$ revealed that the electric polarization can be manipulated by the in-plane rotating magnetic field in a systematic way. We propose the minimal model by extracting essential ingredients of Co$_4$Nb$_2$O$_9$ to exhibit such ME response. It is the three-orbital model with $xy$-type atomic spin-orbit coupling (SOC) on the single-layer honeycomb structure, and it is shown to reproduce qualitatively the observed field-angle dependence of the electric polarization. The obtained results can be understood by the perturbative calculation with respect to the atomic SOC. These findings could be useful to explore further ME materials having similar manipulability of the electric polarization.

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arXiv2017

Optical Selection Rules in Spin-Orbit Coupled Systems on Honeycomb Lattice

We investigate the optical properties in a minimal model of spin-orbit coupled systems on honeycomb lattice at half-filling. The absorption of the circularly polarized light in the charge and antiferromagnetic ordered states is particularly studied, and the spin-valley selective excitation depending on the handedness of the light is found. It is shown that the optical selection rules plainly differ with the type of broken symmetries and topological properties of the electronic states. Consequently, the abrupt change of spin-valley selectivity in the optical absorption occurs at the topological transition caused by changing the magnitude of the order parameters.

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arXiv2015

Hole-$s_{\pm}$ State Induced by Coexisting Ferro- and Aniferromagnetic and Aniferro-orbital Fluctuations in the Iron Pnictides

The multi-orbital Hubbard model is investigated in order to clarify the electron correlation effects and the superconductivity in the iron-based superconductor. The renormalization effects on the self-energy and the two-particle irreducible vertex function are calculated on the basis of the dynamical mean field theory. We find that the vertex function exhibits a strong renormalization with the significant orbital dependence as compared with the renormalization factor, when the antiferromagnetic and the antiferro-orbital fluctuations are comparably enhanced due to the electron and the hole Fermi surfaces nesting effects. The orbital-dependent vertex function together with the $q\sim0$ nesting between the two-hole Fermi surfaces results in the inter-orbital ferromagnetic fluctuation gradually enhanced which is expected to be observed in LiFeAs. We show that the hole-$s_{\pm}$-wave superconductivity with the sign change of the two-hole Fermi surfaces is realized by the enhanced ferromagnetic fluctuation accompanied by the antiferromagnetic fluctuation and the antiferro-orbital fluctuation.

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arXiv2014

Continuous Mott Transition in a Two-Dimensional Hubbard Model

We investigate nonmagnetic metal-insulator transition in the 1/5-depleted square lattice Hubbard model at half-filling within the 8-site cellular dynamical mean field theory. We find that a metal-insulator transition without any signatures of the first order transition, a continuous Mott transition, takes place in a certain range of parameters. The nature of the continuous Mott transition is nothing but a Lifshitz transition driven by the on-site Coulomb interaction. The renormalized matrix elements of hoppings and the spin-spin correlation functions reveal that physics of this transition is strong enhancement of the dimerization due to the non-local effects of electron-electron interaction.

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arXiv2013

Local correlation effects on the s+-- and s++-wave superconductivities mediated by magnetic and orbital fluctuations in the 5-orbital Hubbard model for iron pnictides

We investigate the electronic state and the superconductivity in the 5-orbital Hubbard model for iron pnictides by using the dynamical mean-field theory in conjunction with the Eliashberg equation. The renormalization factor exhibits significant orbital dependence resulting in the large change in the band dispersion as observed in recent ARPES experiments. The critical interactions towards the magnetic, orbital and superconducting instabilities are suppressed as compared with those from the random phase approximation (RPA) due to local correlation effects. Remarkably, the s++-pairing phase due to the orbital fluctuation is largely expanded relative to the RPA result, while the s+--pairing phase due to the magnetic fluctuation is reduced.

cond-mat.supr-con↗

arXiv2013

SU(3) Dirac electrons in the 1/5-depleted square-lattice Hubbard model at 1/4 filling

We investigate the magnetic and metal-insulator (M-I) phase diagram of the 1/5-depleted square-lattice Hubbard model at 1/4 filling by the mean-field approximation. There exist three magnetic phases of nonmagnetic (N), antiferromagnetic (AF), and ferromagnetic (F) types, each realized for the large intrasquare hopping t1, intersquare hopping t2, and Coulomb interaction U, respectively. Within each magnetic phase, the M-I transition of Lifshitz type emerges and, finally, six kind of phases are identified in the U-t1/t2 plane. When t1=t2, we find that the Dirac cone and nearly flat band around the Gamma point form the SU(3) multiplet. The SU(3) effective theory well describes the phase transitions between NI, paramagnetic-metal (PM), and AF phases. The NI and AFI phases are characterized by different Berry phases as in polyacetylene or graphene.

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arXiv2012

Dimensional Reduction and Odd-Frequency Pairing of the Checkerboard-Lattice Hubbard Model at 1/4-Filling

The ferromagnetism of the checkerboard lattice Hubbard model at quarter filling is one of the few exact ferromagnetic ground states known in the family of Hubbard models. When the nearest neighbor hopping, t1, is negligible compared with the second neighbor one, t2, the system reduces to a collection of Hubbard chains. We find that the 1D character is surprisingly robust as long as t1 < t2. This phenomenon of dimensional reduction due to the geometrical frustration leads to peculiar magnetic orders with 1D character for intermediate U and is responsible for odd-frequency superconducting states close to the magnetic boundary.

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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.

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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↗

arXiv2010

Two types of s-wave pairing due to magnetic and orbital fluctuations in the two-dimensional 16-band d-p model for iron-based superconductors

We study superconductivity in the two-dimensional 16-band d-p model extracted from a tight-binding fit to the band structure of LaFeAsO, using the random phase approximation. When the intraorbital repulsion U is larger than the interorbital one U', an extended s-wave (s+--wave) pairing with sign reversal of order parameter is mediated by antiferromagnetic spin fluctuations, while when U U'.

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arXiv2008

Magnetism and Superconductivity in the Two-Dimensional 16 Band d-p Model for Iron-Based Superconductors

The electronic states of the Fe2As2 plane in iron-based superconductors are investigated on the basis of the two-dimensional 16-band d-p model which includes the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms U and U', the Hund's coupling J and the pair-transfer J'. Using the random phase approximation (RPA), we obtain the magnetic phase diagram including the stripe and the incommensurate order on the U'-J plane. We also solve the superconducting gap equation within the RPA and find that, for large J, the most favorable pairing symmetry is extended s-wave whose order parameter changes its sign between the hole pockets and the electron pockets, while it is dxy-wave for small J.

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