arXiv subjects

Tetsuya Takimoto

Publications and source records attributed to Tetsuya Takimoto.

At least 19 recordsLinked to original sources

arXiv2024

Spin-Triplet Superconductivity Induced by All-In-All-Out Spin Fluctuation in Pyrochlore Electronic System

We study the symmetry of superconductivity around the All-In-All-Out (AIAO) spin ordered phase in a nearly half-filled pyrochlore electronic system. The electronic system is described by the Hubbard model with an anti-symmetric spin-orbit coupling. The AIAO spin ordered state is formed by arranging tetrahedral hedgehog spin-clusters uniformly in the lattice system, where 12 types of spin-clusters are allowed in a unit cell. All spin-cluster susceptibilities are calculated within the random phase approximation with respect to the onsite electron interaction. It is shown that, for the negative spin-orbit coupling coefficient, the AIAO spin-cluster susceptibility diverges at q=0, while other types of spin-cluster susceptibilities are not developed and are structureless. By calculating the superconducting transition mediated by AIAO spin fluctuation, it is found that a spin-triplet superconductivity appears, while there is no signal of superconducting transition for the spin-singlet one. We suggest that the spin-triplet superconductivity appears around the AIAO spin ordered phase in the pyrochlore lattice.

cond-mat.str-el↗

arXiv2015

Charge order induced in an orbital density-wave state

Motivated by the recent ARPES measurements (Evtushinsky et. al., PRL {\bf 105}, 147201 (2010)) and evidence for the density-wave state for the charge and orbital ordering (García et al., PRL {\bf 109}, 107202 (2012)) in La$_{0.5}$Sr$_{1.5}$MnO$_4$, the issue of charge and orbital ordering in a two orbital tight-binding model for layered manganite near half doping is revisited. We find that the charge order with an ordering wavevector 2${\bf Q}$ = $(π, π)$ is induced by the orbital order of $B_{1g}$ representation with a different ordering wavevector ${\bf Q}$, where the primary order parameter results from the strong Fermi-surface nesting. The orbital and charge order parameters develop according to $\sqrt{T_{CO}-T}$ and $T_{CO}-T$, respectively, by decreasing the temperature below the orbital ordering temperature $T_{CO}$. Moreover, the orbital order is found to stabilize the CE-type spin arrangement observed experimentally below $T_{CE} < T_{CO}$.

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arXiv2014

Orbital ordering transition in the single-layer manganites near half doping: a weak-coupling approach

The roles of crystal-field splitting and Jahn-Teller distortions on the orbital ordering transition are investigated in the single-layer manganites near half doping. Crystal-field splitting of energy levels favoring the $d_{3z^2-r^2}$ occupancy provides not only the correct Fermi surface topology for La$_{0.5}$Sr$_{1.5}$MnO$_4$ having a circular electron pocket around the $Γ$ point, but also enhances the flatness of the hole pocket around the M point thereby improving the nesting. In the presence of the circular electron pocket, Jahn-Teller distortions are found to be crucial for the transition to the transverse orbital ordering with ordering wavevector ($0.5π, 0.5π$). In the hole-doping regime $0.5 \leq x \leq 0.7$, the orbital ordering wavevector shows a linear dependence on the hole concentration in accordance with the experiments.

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arXiv2014

On the origin of CE-type orbital fluctuations in the ferromagnetic metallic La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We investigate the orbital fluctuations in the ferromagnetic-metallic phase of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ by considering a two orbital model within a tight-binding description which reproduces the ARPES Fermi surface. We find strong antisymmetric transverse orbital fluctuations at wavevector ($ 0.5 π, 0.5 π$) resulting from the Fermi-surface nesting between the portions of bonding and antibonding bands instead of the widely believed nesting between the portions of bonding band despite their flat segments, which provide an insight into the origin of so called CE-type orbital fluctuations in the ferromagnetic-metallic phase. Subsequent renormalization of the phonons near wavevector ($0.5 π,0.5 π$) and the behavior of the phonon linewidth as a function of momentum are in agreement with the inelastic neutron scattering experiments.

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arXiv2013

Derivation of the superconducting gap equation for the noncentrosymmetric superconductor Li2Pt3B

We present here the mathematical background of our approach, presented in Phys. Rev. B 86, 134526 (2012) regarding the gap function and symmetry for the noncentrosymmetric (NCS) superconductor $Li_2Pt_3B$. As revealed by the experiment, this NCS superconductor gives rise to line nodes in the superconducting order parameter, which is responsible for many of its experimental behaviors. Owing to the enhanced d-character of the relevant bands that cross the Fermi level,the system gets weakly correlated. The nature and symmetry of this nodal behavior is explained from a microscopic viewpoint. In this article starting with an Hubbard model relevant for this NCS system by considering the effect of the onsite Coulomb repulsion on the pairing potential perturbatively, we extract the superconducting gap equation. Further analysis of this equation predicts a $s_\pm$ wave gap function with line nodes as the most promising candidate in the superconducting state.

cond-mat.supr-con↗

arXiv2012

Low-energy Spin Excitation in Coexistent Phase of Antiferromagnetism and d-wave Superconductivity

Nuclear quadrupole resonance measurements have shown evidences that the heavy fermion compound CeRhIn$_5$ exhibits a coexistent phase with commensurate antiferromagnetism and d-wave superconductivity. In order to clarify the nature of the spin-excitations in the coexistent phase, we have applied the RPA method to an itinerant model, where the effective interaction is given by two mean-field terms of commensurate antiferromagnetism and d-wave superconductivity. It is shown that, around the transition line between the antiferromagnetic and the coexistent states, a low-energy incommensurate spin-excitation is found to develop due to Fermi surface nesting. This feature reminds of the switching of magnetic ordering wave vector observed in the neutron diffraction. Further, we also calculate spin relaxation rate, which gives a reasonable explanation of the temperature dependence of NQR relaxation rate in the system with the coexistent ground state.

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arXiv2011

Nature and symmetry of the order parameter of the noncentrosymmetric superconductor Li2Pt3B

The nature and symmetry of the superconducting gap function in the noncentrosymmetric superconductor (NCS) Li2Pt3B, even many years after its discovery, appears to be full of contradictions. In this letter based on the existing band structure calculations we find that owing to the considerable nesting near the Fermi surface and the enhanced d-character of the relevant bands that cross the Fermi level,the system gets somewhat strongly correlated. Considering the effect of the onsite Coulomb repulsion on the pairing potential perturbatively, we extract possible superconducting transition. The strong normal spin fluctuation gives rise to a singlet dominant gap function with accompanying sign change. Thus our theory predicts a $s_\pm$ wave gap function with line nodes as the most promising candidate in the superconducting state.

cond-mat.supr-con↗

arXiv2011

A phase diagram for a topological Kondo insulating system

The discovery of topological insulators in non-interacting electron systems has motivated the community to search such topological states of matter in correlated electrons both theoretically and experimentally. In this paper we investigate a phase diagram for a topological Kondo insulating system, where an emergent "spin"-dependent Kondo effect gives rise to an inversion for heavy-fermion bands, responsible for a topological Kondo insulator. Resorting to the U(1) slave-boson mean-field analysis, we uncover an additional phase transition inside the Kondo insulating state in two dimensions, which results from the appearance of the topological Kondo insulator. On the other hand, we observe that the Kondo insulating state is distinguished into three insulating phases in three dimensions, identified with the weak topological Kondo insulator, the strong topological Kondo insulator, and the normal Kondo insulator, respectively, and classified by Z$_{2}$ topological indices. We discuss the possibility of novel quantum criticality between the fractionalized Fermi liquid and the topological Kondo insulator, where the band inversion occurs with the formation of the heavy-fermion band at the same time.

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arXiv2011

Signatures of hidden order symmetry in torque oscillations, elastic constant anomalies and field induced moments in URu2Si2

We discuss the conclusions on the symmetry of hidden order (HO) in URu2Si2 that may be drawn from recent torque experiments in rotating magnetic field by Okazaki et al. [1]. They are very sensitive to changes in the magnetic susceptibility induced by HO. We show that the observed twofold angular torque oscillations give evidence that hidden order has degenerate E- type (yz,zx) symmetry where both components are realised. The oscillations have the wrong characteristics or are absent for the 1D nontrivial representations like quadrupolar B1 (x^2-y^2) and B2 (xy) type HO or hexadecapolar A2(xy(x^2-y^2)) type HO. Therefore they may be excluded as candidates for hidden order. We also predict the field-angular variation of possible field-induced Bragg peaks based on underlying E-type order parameter and discuss the expected elastic constant anomalies.

cond-mat.str-el↗

arXiv2011

Nambu-Eliashberg theory for multi-scale quantum criticality : Application to ferromagnetic quantum criticality in the surface of three dimensional topological insulators

We develop an Eliashberg theory for multi-scale quantum criticality, considering ferromagnetic quantum criticality in the surface of three dimensional topological insulators. Although an analysis based on the random phase approximation has been performed for multi-scale quantum criticality, an extension to an Eliashberg framework was claimed to be far from triviality in respect that the self-energy correction beyond the random phase approximation, which originates from scattering with $z = 3$ longitudinal fluctuations, changes the dynamical exponent $z = 2$ in the transverse mode, explicitly demonstrated in nematic quantum criticality. A novel ingredient of the present study is to introduce an anomalous self-energy associated with the spin-flip channel. Such an anomalous self-energy turns out to be essential for self-consistency of the Eliashberg framework in the multi-scale quantum critical point because this off diagonal self-energy cancels the normal self-energy exactly in the low energy limit, preserving the dynamics of both $z = 3$ longitudinal and $z = 2$ transverse modes. This multi-scale quantum criticality is consistent with that in a perturbative analysis for the nematic quantum critical point, where a vertex correction in the fermion bubble diagram cancels a singular contribution due to the self-energy correction, maintaining the $z = 2$ transverse mode. We also claim that this off diagonal self-energy gives rise to an artificial electric field in the energy-momentum space in addition to the Berry curvature. We discuss the role of such an anomalous self-energy in the anomalous Hall conductivity.

cond-mat.str-el↗

arXiv2010

Low-temperature electrical resistivity in paramagnetic spinel LiV2O4

The 3d electron spinel compound LiV2O4 exhibits heavy fermion behaviour below 30K which is related to antiferromagnetic spin fluctuations strongly enhanced in an extended region of momentum space. This mechanism explains enhanced thermodynamic quantities and nearly critical NMR relaxation in the framework of the selfconsistent renormalization (SCR) theory. Here we show that the low-T Fermi liquid behaviour of the resistivity and a deviation from this behavior for higher T may also be understood within that context. We calculate the temperature dependence of the electrical resistivity ρ(T) assuming that two basic mechanisms of the quasiparticle scattering, resulting from impurities and spin-fluctuations, operate simultaneously at low temperature. The calculation is based on the variational principle in the form of a perturbative series expansion for ρ(T). A peculiar behavior of ρ(T) in LiV2O4 is related to properties of low-energy spin fluctuations whose T-dependence is obtained from SCR theory.

cond-mat.str-el↗

arXiv2009

Triplet Cooper Pair Formation by Anomalous Spin Fluctuations in Non-centrosymmetric Superconductors

A microscopic theory for the spin triplet Cooper pairing in non-centrosymmetric superconductors like CePt_3Si and CeTSi_3 (T=Rh, Ir) is presented. The lack of inversion symmetry leads to new anomalous spin fluctuations which stabilize the triplet part in addition to the singlet part originating from the centrosymmetric spin fluctuations. It is shown that both parts have similar nontrivial momentum dependence of A_1 type. Therefore the mixed singlet-triplet gap function has accidental line nodes on both Fermi surface sheets which are stable as function of temperature. This gap function explains the salient features of CePt_3Si and CeTSi_3 superconductors.

cond-mat.supr-con↗

arXiv2008

Anomalous Spin Response in Non-centrosymmetric Compounds

We examine static spin susceptibilities $χ_{αβ}({\bf q})$ of spin components $S_α$ and $S_β$ in the non-centrosymmetric tetragonal system. These show anomalous momentum dependences like $χ_{xx}({\bf q})-χ_{yy}({\bf q})\sim q_x^2-q_y^2$ and $χ_{xy}({\bf q})+χ_{yx}({\bf q})\sim q_x q_y$, which vanish in centrosymmetric systems. The magnitudes of the anomalous spin susceptibilities are enhanced by the on-site Coulomb interaction, especially, around an ordering wave vector. The significant and anomalous momentum dependences of these susceptibilities are explained by a group theoretical analysis. As the direct probe of the anomalous spin susceptibility, we propose a polarized neutron scattering experiment.

cond-mat.str-el↗

arXiv2008

Theory of induced quadrupolar order in tetragonal YbRu_{2}Ge_{2}

The tetragonal compound YbRu$_{2}$Ge$_{2}$ exhibits a non-magnetic transition at $T_0$=10.2K and a magnetic transition at $T_1$=6.5K in zero magnetic field. We present a model for this material based on a quasi-quartet of Yb$^{3+}$ crystalline electric field (CEF) states and discuss its mean field solution. Taking into account the broadening of the specific heat jump at $T_0$ for magnetic field perpendicular to [001] and the decrease of $T_0$ with magnetic field parallel to [001], it is shown that ferro-quadrupole order of either O$_{2}^{2}$ or O$_{\rm xy}$ - type are prime candidates for the non-magnetic transition. Considering the matrix element of these quadrupole moments, we show that the lower CEF states of the level scheme consist of a $Γ_{6}$ and a $Γ_{7}$ doublet. This leads to induced type of O$_{2}^{2}$ and O$_{\rm xy}$ quadrupolar order parameters. The quadrupolar order introduces exchange anisotropy for planar magnetic moments. This causes a spin flop transition at low fields perpendicular [001] which explains the observed metamagnetism. We also obtain a good explanation for the temperature dependence of magnetic susceptibility and specific heat for fields both parallel and perpendicular to the [001] direction.

cond-mat.str-el↗

arXiv2003

Strong-coupling theory of superconductivity in a degenerate Hubbard model

In order to discuss superconductivity in orbital degenerate systems, a microscopic Hamiltonian is introduced. Based on the degenerate model, a strong-coupling theory of superconductivity is developed within the fluctuation exchange (FLEX) approximation where spin and orbital fluctuations, spectra of electron, and superconducting gap function are self-consistently determined. Applying the FLEX approximation to the orbital degenerate model, it is shown that the $d_{x^2-y^2}$-wave superconducting phase is induced by increasing the orbital splitting energy which leads to the development and suppression of the spin and orbital fluctuations, respectively. It is proposed that the orbital splitting energy is a controlling parameter changing from the paramagnetic to the antiferromagnetic phase with the $d_{x^2-y^2}$-wave superconducting phase in between.

cond-mat.supr-con↗

arXiv2002

Superconductivity in the Orbital Degenerate Model for Heavy Fermion Systems

Magnetism and superconductivity of new heavy fermion compounds CeTIn$_5$ (T=Co, Rh and Ir) are investigated by applying fluctuation exchange approximation to an orbital degenerate Hubbard model. The superconducting phase with $d_{x^2-y^2}$-symmetry is found to appear next to the antiferromagnetic phase with increasing the orbital splitting energy. The present theory suggests that the orbital splitting energy plays a key role of controlling parameter for the quantum phase transitions in the heavy fermion system.

cond-mat.supr-con↗

arXiv2002

Spin Fluctuation Induced Superconductivity Controlled by Orbital Fluctuation

A microscopic Hamiltonian reflecting the correct symmetry of $f$-orbitals is proposed to discuss superconductivity in heavy fermion systems. In the orbitally degenerate region in which not only spin fluctuations but also orbital fluctuations develop considerably, cancellation between spin and orbital fluctuations destabilizes $d_{x^{2}-y^{2}}$-wave superconductivity. Entering the non-degenerate region by increasing the crystalline electric field, $d_{x^{2}-y^{2}}$-wave superconductivity mediated by antiferromagnetic spin fluctuations emerges out of the suppression of orbital fluctuations. We argue that the present scenario can be applied to recently discovered superconductors CeTIn$_{5}$ (T=Ir, Rh, and Co).

cond-mat.supr-con↗

arXiv2002

Orbital Fluctuation-Induced Triplet Superconductivity : Mechanism of Superconductivity in ${\rm Sr}_{2}{\rm RuO}_{4}$

The mechanism of superconductivity in ${\rm Sr}_{2}{\rm RuO}_{4}$ is studied using a degenerate Hubbard model within the weak coupling theory. When the system approaches the orbital instability which is realized due to increasing the on-site Coulomb interaction between the electrons in the different orbitals, it is shown that the triplet superconductivity appears. This superconducting mechanism is only available in orbitally degenerate systems with multiple Fermi surfaces.

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