arXiv subjects

Rina Tazai

Publications and source records attributed to Rina Tazai.

At least 19 recordsLinked to original sources

arXiv2026

Prominent Intrinsic Orbital Hall Effect in the 135 Kagome Metal Family and Orbital Responses in the Loop-Current Phase

The study of electronic orbital degrees of freedom, including the generation and control of orbital currents and orbital angular momentum, has emerged as a vibrant research field. Here, we study the orbital Hall effect (OHE), one of the key mechanisms for orbital current generation, in transition-metal kagome-lattice metals. We propose a large positive OHE in CsTi$_3$Bi$_5$ and negative OHEs in CsV$_3$Sb$_5$ and CsCr$_3$Sb$_5$ models. Orbital-sector decomposition shows that the $\vert l^z_d \vert=2$ $d$-orbital channel gives a positive contribution, whereas the $\vert l^z \vert=1$ $d$- and $p$-orbital channels can give negative contributions. Control calculations suggest a persistent orbital-sector sign tendency near the actual filling, while strong $p$-$d$ hybridization modulates the quantitative balance and total OHE sign. Thus, the compound and filling dependence of the OHE reflects both the sign tendency of each orbital sector and the hybridization-controlled balance among them. Furthermore, we investigate the loop-current phase of CsV$_3$Sb$_5$ and show that it induces finite local atomic orbital angular momentum. We also show that loop-current-related symmetry lowering allows finite symmetric components of the orbital conductivity tensor. This study provides a basis for exploring orbital currents and local orbital-angular-momentum responses in strongly correlated kagome metals.

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arXiv2026

Nematic and chiral superconductivity emerging within the loop-current phase in kagome metals

The kagome metals $A$V$_3$Sb$_5$ ($A =$ Cs, Rb, K) host multiple symmetry-breaking phases, including charge-density-wave and loop-current orders, and exhibit highly exotic superconductivity with pronounced nematicity and chirality. Remarkably, even dilute impurities transform this exotic superconducting state into an isotropic $s$-wave state. These observations pose a challenge to existing theoretical scenarios. We show that loop-current order induces nematic chiral $d$-wave superconductivity in kagome metals. The loop-current-induced orbital magnetization (OM) stabilizes one chiral superconducting channels. This OM-chirality coupling mechanism is generic and applies to pairing driven by either attractive or repulsive interactions. Furthermore, coexisting loop-current and bond orders give rise to pronounced nematic chiral superconductivity even for an almost $C_6$-symmetric Fermi surface. For attractive pairing, dilute impurities suppress the chiral state and restore a conventional $s$-wave phase, as observed experimentally. The theory further predicts a robust $2\times2$ pair-density modulation. This study provides key insights into time-reversal-symmetry-breaking exotic superconductivity in kagome metals.

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arXiv2026

Impact of multiband effects on non-Fermi-liquid transport phenomena in bilayer nickelates

Recently discovered high-$T_c$ superconductivity in thin-film bilayer nickelates La$_3$Ni$_2$O$_7$ under ambient pressure has attracted great interest. Non-Fermi-liquid transport behaviors, such as $T$-linear resistivity and a positive Hall coefficient that increases at low temperatures, have been reported in this system. In this study, we analyze the non-Fermi-liquid transport phenomena in the thin-film bilayer nickelate La$_3$Ni$_2$O$_7$ using a multiorbital tight-binding model. In La$_3$Ni$_2$O$_7$, the cold spots composed of Ni $d_{x^2-y^2}$ orbital emerge, since the spin fluctuations cause stronger quasiparticle damping $γ$ in the Ni $d_{z^2}$ orbital. Notably, in the present study, we derive a rigorous formula for the Hall coefficient $R_H$ incorporating the $γ$ in the quasi-quantum metric (qQM) term. We find that the $T$ dependence of $γ$ in the qQM term is important in determining $R_H$. In La$_3$Ni$_2$O$_7$, the $T$ dependence of $R_H$ becomes pronounced due to the competition between the positive contribution from the hole band and the negative contribution from the electron band. Moreover, the qQM term plays an important role in describing the Nernst coefficient and other transport phenomena involving the second derivative velocity $v^{μν}$.

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arXiv2026

Origin of switchable quasiparticle-interference chirality in loop-current phase of kagome metals measured by scanning-tunneling-microscopy

In the kagome superconductors AV3Sb5 (A=Cs,Rb,K), a cascade of correlated electron phases cause exotic symmetry-breaking quantum states. In particular, the dissipationless chiral loop-current phase has been attracting increasing attention. A crucial clue is offered by the chirality of the quasiparticle interference signal observed in scanning tunneling microscopy. However, the connection between loop-current chirality and quasiparticle interference chirality remains poorly understood. Here, we reveal theoretically that a pronounced chiral quasiparticle interference signal emerges in the extremely dilute impurity regime ($lesssim$ 0.1 %). A single impurity at site Z induces a quasiparticle interference chirality $χ_Z=\pm1$, determined by the direction of the Z3 nematicity, itself set by the relative position of the loop-current order in the star-of-David charge-density-wave phase. Notably, even a small magnetic field can smoothly switch the chirality, leading to field-induced shear lattice strain consistent with recent experiments. Our theoretical study provide key insights into the nature of the loop-current-induced symmetry-breaking states in kagome metals.

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arXiv2026

Paramagnon-Interference Mechanism for Three-Dimensional Bond Order in Kagome Metals AV$_3$Sb$_5$ (A=Cs, Rb, K): Analysis by the Density-Wave Equation

The mechanism of CDW and its 3D structure are important fundamental issues in kagome metals. We have previously shown that, based on a 2D model, $2\times 2$ bond order (BO) emerges due to the paramagnon-interference (PMI) mechanism and that its fluctuations lead to $s$-wave superconductivity. This paper studies these issues based on realistic 3D models of kagome metals AV$_3$Sb$_5$ (A=Cs, Rb, K). We reveal that a commensurate 3D $2\times 2\times 2$ BO is caused by the PMI mechanism, by performing the 3D density-wave (DW) equation analysis for all A=Cs, Rb, K models in detail. Our results indicate a BO transition temperature $T_{\rm BO}\sim 100$K within the regime of moderate electron correlation. The 3D structure of BO is attributed to the three-dimensionality of the Fermi surface, while the 3D structure of BO is sensitively changed, since the Fermi surface is quasi-2D. Based on the analysis of the DW equation, by taking into account a finite third-order Ginzburg-Landau (GL) term, (i) shift stacking $2\times 2\times 2$ BO can be realized via a first-order transition below $T_{\rm BO}$. Here, the in-plane BO pattern (tri-hexagonal or star-of-David) is determined by the sign of the third-order GL term, with hole doping tending to favor the tri-hexagonal state. On the other hand, if the third-order GL term is very small, (ii) alternating vertical stacking BO may instead be realized via a second-order transition. The present study enhances our understanding of the rich variety of BOs observed experimentally. It is confirmed that the PMI mechanism is the essential origin of the 3D CDW of kagome metals.

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arXiv2025

Giant critical current peak induced by pressure in kagome superconductor RbV$_{3}$Sb$_{5}$

Superconductivity can coexist or compete with other orders such as magnetism or density waves. Optimizing superconductivity requires identifying competing orders that may disrupt Cooper pair coherence. Here, we use the self-field critical current ($I_{\rm c,sf}$) to probe pressure-tuned superconductivity in the kagome superconductor RbV$_3$Sb$_5$. As pressure destabilizes the charge-density wave (CDW) state, $I_{\rm c,sf}$ drastically enhances, peaking near the critical pressure where the CDW state is completely suppressed at zero temperature. Surprisingly, a weaker $I_{\rm c,sf}$ peak emerges within the CDW phase. Near the pressure of the weaker peak, the superconducting phase transition temperature shifts from an increasing trend with pressure to a near plateau. Our analysis suggests the possibility of a sudden change in the CDW pattern or a Lifshitz transition, highlighting the need for microscopic examinations of the CDW state for understanding the pressure evolution of superconductivity in RbV$_3$Sb$_5$.

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arXiv2025

Microscopic origin of period-four stripe charge-density-wave in kagome metal CsV$_3$Sb$_5$

The interplay between unconventional density waves and exotic superconductivity has attracted growing interest. Kagome superconductors $A\rm{V}_3\rm{Sb}_5$ ($A = \rm{K}, \rm{Rb}, \rm{Cs}$) offer a platform for studying quantum phase transitions and the resulting symmetry breaking. Among these quantum phases, the $4a_0$ stripe charge-density-wave (CDW) has been widely observed for $A=\rm{Rb}$ and $\rm{Cs}$ by scanning tunneling microscopy (STM) and nuclear magnetic resonance (NMR) measurements. However, the microscopic origin of the $4a_0$ stripe CDW remains elusive, and no theoretical studies addressing this phenomenon have been reported so far. In this paper, we propose a microscopic mechanism for the emergence of the $4a_0$ stripe CDW. We analyze the CDW instability in the 12-site kagome lattice Hubbard model with the $2\times2$ bond order driven by the paramagnon-interference mechanism by focusing on the short-range magnetic fluctuations due to the geometrical frustration of kagome lattice. We reveal that the nesting vector of the reconstructed Fermi surface, formed by the $2\times 2$ bond order, gives rise to a $4a_0$-period CDW. Remarkably, the obtained stripe CDW is composed of both the off-site hopping integral modulations and on-site potentials. The real-space structure of the stripe CDW obtained here is in good qualitative agreement with the experimentally observed stripe pattern.

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arXiv2025

Photoinduced non-reciprocal magnetism

Out of equilibrium, the action-reaction symmetry of the interactions is often broken, leading to the emergence of various collective phenomena with no equilibrium counterparts. Although ubiquitous in classical active systems, implementing such non-reciprocal interactions in solid-state systems has remained challenging, as the known quantum schemes require precise control over the system on a single-site level. Here, we propose a novel dissipation-engineering protocol to induce non-reciprocal interactions in solid-state platforms with light, which we expect to be achievable with state-of-the-art experimental techniques. Focusing on magnetic metals for concreteness, we show microscopically that a light injection that introduces the decay channel to a virtually excited state gives rise to non-reciprocal interactions between localized spins. One can even realize a situation where spin A tries to align with spin B but the B tries the opposite, resulting in a chase-and-runaway dynamics. Applying our scheme to layered ferromagnets, we show that a non-reciprocal phase transition from a static to a many-body time-dependent chiral phase emerges. Our work paves the way to bring solid-state systems to the realm of non-reciprocal science, providing yet another possibility to control quantum matter with light.

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arXiv2025

Charge-loop current order and Z3 nematicity mediated by bond-order fluctuations in kagome metal AV3Sb5 (A=Cs,Rb,K)

Recent experiments on geometrically frustrated kagome metal AV3Sb5 (A=K, Rb, Cs) have revealed the emergence of the charge loop current (cLC) order near the bond order (BO) phase. However, the origin of the cLC and its relation to other phases have been uncovered. Here, we discover a novel mechanism of the cLC state, by focusing on the BO phase common in kagome metals. The BO fluctuations in metals mediate the odd-parity particle-hole condensation, which drives the topological charge-current. This state is further stabilized by the finite electron-phonon coupling and the off-site Coulomb interaction. Furthermore, it is worth noting that the predicted cLC+BO phase gives rise to the Z3-nematic state in addition to the giant anomalous Hall effect. The present theory predicts the close relationship between the cLC, the BO, and the nematicity, which is significant to understand the cascade of quantum electron states in kagome metals.

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arXiv2025

Odd-parity quadrupole order and induced nonreciprocal transport in the kagome metal CsTi$_3$Bi$_5$ driven by quantum interference

Kagome metals present a fascinating platform of quantum phases thanks to the interplay between the geometric frustration and strong electron correlation. Here, we propose the emergence of the electric odd-parity bond order (BO) that originates from the intra-unit-cell odd-parity configuration in recently discovered kagome metal CsTi$_3$Bi$_5$. The predicted E1u BO is induced by the beyond-mean-field mechanism, that is, the quantum interference among different sublattice spin fluctuations. Importantly, the accompanied nematic deformation of the Fermi surface is just ~1% while the intensity of the quasiparticle interference signal exhibits drastic nematic anisotropy, consistent with the scanning tunneling microscope measurements in CsTi$_3$Bi$_5$. The present odd-parity BO triggers interesting phenomena, such as the non-linear Hall effect and emergent electromagnetism.

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arXiv2025

Giant Impurity Effects on Charge Loop Current Order States in Kagome Metals

The exotic electronic states in the charge loop current (cLC) phase, in which the permanent charge current breaks the time-reversal symmetry, have been attracting increasing attention in recently discovered kagome metals AV3Sb5 (A = Cs, Rb, K). Interestingly, the cLC state is sensitively controlled by applying a small magnetic field as well as a tiny uniaxial strain. In addition, many experiments indicate that the cLC state is sensitive to the small number of impurities. To understand the impurity effects on the cLC electronic states accurately, we analyze the giant unit-cell (up to 1200 sites) kagome lattice model with single impurity potential. The loop current is found to be strongly suppressed within the current correlation length $ξ_J$ centered on the impurity site, where $ξ_J$ increases as the cLC order parameter $η$ decreases. (The cLC order is the pure imaginary hopping integral modulation $δt_{i,j}=\pm iη$.) In addition, both the uniform orbital magnetization $M_{orb}$ and the anomalous Hall conductivity $σ_{xy}$ are drastically suppressed by dilute impurities. Especially, the suppression ratio $R=-ΔM_{orb}/M_{orb}^0$ can exceed 50% with the introduction of 1% impurities. Unexpectedly, the ratio $R$ is qualitatively insensitive to $η$, in highly contrast to a naive expectation that $R$ is proportional to the current suppression area $πξ_J^2$. The resulting giant impurity effect of $M_{orb}$ would originates from the nonlocal contribution of the itinerant circulation of electrons. The present study gives a natural explanation of why the cLC electronic states in kagome metals are sensitive to dilute impurities.

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arXiv2025

Quantum-metric-induced giant and reversible nonreciprocal transport phenomena in chiral loop-current phases of kagome metals

Rich spontaneous symmetry-breaking phenomena with nontrivial quantum geometric properties in metals represent central issues in condensed matter physics. In this context, the emergence of chiral loop-current order, accompanied by time-reversal symmetry-symmetry breaking in various kagome metals, has garnered significant attention. Particularly noteworthy is the giant electrical magnetochiral anisotropy (eMChA) in CsV3Sb5, which provides compelling evidence of time-reversal-symmetry and inversion-symmetry breakings. However, the underlying essence of this observation has remained obscured due to the lack of theoretical understanding. Here, we reveal that the loop-current order causes giant and reversible eMChA coefficient, $γ_{eM}$, is proportional to the loop-current-induced orbital magnetization $M_{orb}$ times the lifetime of conduction electrons $τ$. In kagome metals, the derived $γ_{eM}$ is substantial and reversible by minute magnetic fields, due to the large $τ(\gg a_0/v_{Fermi})$ and the field-induced reversal of $M_{orb}$. By considering the experimentally observed stripe charge-density wave, the loop-current state becomes non-centrosymmetric, thereby giving rise to the eMChA. Surprisingly, the quantum-metric, which defines a fundamental geometric aspect of Bloch wavefunctions, acquires significant momentum dependence in the loop-current phase, resulting in a dramatic enhancement of eMChA by $\sim100$ times. This research not only clarifies the fundamental symmetry-breaking states in kagome metals, but also opens a new path for exploring quantum-metric-induced phenomena arising from exotic quantum phase transitions in strongly correlated metals

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arXiv2024

Drastic magnetic-field-induced chiral current order and emergent current-bond-field interplay in kagome metals

In kagome metals, the chiral current order $η$ with time-reversal-symmetry-breaking is the source of various exotic electronic states, while the method of controlling the current order and its interplay with the star-of-David bond order $ϕ$ are still unsolved. Here, we reveal that tiny uniform orbital magnetization $M[η,ϕ]$ is induced by the chiral current order, and its magnitude is prominently enlarged under the presence of the bond order. Importantly, we derive the magnetic-field ($h$)-induced Ginzburg-Landau free energy expression $ΔF[h,η,ϕ]$, which enables us to elucidate the field-induced current-bond phase transitions in kagome metals. The emergent current-bond-$h$ trilinear coupling term in the free energy, $-3m_1 hηϕ$, naturally explains the characteristic magnetic field sensitive electronic states in kagome metals, such as the field-induced current order and the strong interplay between the bond and current orders. Furthermore, we present a natural explanation for the drastic strain-induced increment of the current order transition temperature T_{TRSB} reported by a recent experiment.

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arXiv2024

Low temperature phase transitions under CDW state in kagome metals AV3Sb5 (A=Cs,Rb,K): Significance of mix-type Fermi surface electron correlations

To understand the multistage phase transitions in V-based kagome metals inside the charge-density-wave (CDW) phase, we focus on the impact of the "mix-type" Fermi surface because it is intact in the CDW state on the "pure-type" Fermi surface. On the mixed-type Fermi surface, moderate spin correlations develop, and we reveal that uniform (q=0) bond order is caused by the paramagnon interference mechanism, which is described by the Aslamazov-Larkin vertex correction. A dominant solution is the E2g-symmetry nematic order, in which the director can be rotated arbitrarily. In addition, we obtain the A1g-symmetry non-nematic order, which leads to the change in the lattice constants without symmetry breaking. The predicted E2g and A1g channel fluctuations at q=0 can be observed by the elastoresistance measurements. These results are useful to understand the multistage phase transitions inside the 2 $\times$ 2 CDW phase. The present theory has a general significance because mix-type band structure universally exists in various kagome lattice models.

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arXiv2024

Real-space Loop Current Pattern in Time-reversal-symmetry Breaking Phase in Kagome Metals

The charge loop current (cLC) state has attracted increasing attention in kagome metals. Here, we calculate the spontaneous currents along the nearest sites $i$ and $j$, $J_{i,j}$, induced by the cLC order that is the imaginary and odd-parity hopping integral modulation $δt_{i,j}$. We reveal that the magnitude of $J_{i,j}$ strongly depends on the nearest sites $i$ and $j$ in the $2\times2$ cLC state, where $η\equiv |δt_{i,j}|$ is equivalent for all nearest sites. The obtained $J_{i,j}$ becomes large near the van-Hove singularity (vHS) filling ($n\sim n_{vHS}$) even when $η$ is fixed. Interestingly, the obtained $J_{i,j}$ exhibits the logarithmic divergence behavior at low temperatures for $n\sim n_{vHS}$ with a fixed $η$, by reflecting the vHS points that are the characteristic of kagome metals. The present study provides useful information for local electronic state measurements, such as the site-selective NMR and STM experiments.

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arXiv2023

A Rigorous Formalism of Unconventional Symmetry Breaking in Fermi Liquid Theory and Its Application to Nematicity in FeSe

Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic order in Fe-based superconductors and the star-of-David charge density order in kagome metals. Such exotic symmetry breaking in metals is a central issue of modern condensed matter physics, while its theoretical foundation is still unclear in comparison with the well-established theory of superconductivity. To overcome this difficulty, here we introduce the "form factor" that generalizes the nonlocal order parameter into the Luttinger-Ward (LW) Fermi liquid theory. We then construct a rigorous formalism of the "density-wave equation" that gives the thermodynamically stable form factor, similarly to the superconducting-gap equation. In addition, a rigorous expression of the Ginzburg-Landau free-energy for the unconventional order is presented to calculate various thermodynamic properties. In the next stage, we apply the derived formalism to a typical Fe-based superconductor FeSe, by using the one-loop LW function that represents the free-energy gain due to the interference among paramagnons. The following key experiments are naturally explained: (i) Lifshitz transition (=disappearance of an electron-pocket) due to the bond+orbital order below $T_c$. (ii) Curie-Weiss behavior of the nematic susceptibility at higher T, and the deviation from the Curie-Weiss behavior at lower T near the nematic quantum-critical-point. (iii) Scaling relation of the specific heat jump at $T_c$, $ΔC/T_c \propto T_c^b$ with $b \sim 3$. (Note that b=0 in the BCS theory.) These results lead to a conclusion that the nematicity in FeSe is the bond+orbital order due to the "paramagnon interference mechanism". The present theory paves the way for solving various unconventional phase transition systems.

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arXiv2023

Unconventional density waves and superconductivities in Fe-based superconductors and other strongly correlated electron systems

To seek high-$T_c$ pairing mechanism, many scientists have focused on the mysterious spontaneous rotational symmetry breaking above Tc, such as nematic order at $q=0$ and smectic order at $q\ne0$. Such exotic correlation-driven symmetry breaking in metals has become a central issue in condensed matter physics. We demonstrate the emergence of the nematic and smectic orders due to orbital polarization ($n_{xz}\ne n_{yz}$) and the symmetry breaking in the correlated intersite hopping (= bond order $δt_{i,j}$) in Fe-based and cuprate superconductors. In addition, we discuss exotic spontaneous loop current orders driven by the pure imaginary $δt_{i,j}$. These interesting ``unconventional density-waves'' originate from the quantum interference between different spin fluctuations that is described by the vertex correction (VC) in the field theory. In the next stage, we discuss electron-correlation driven superconductivity due to the fluctuations of unconventional density-waves. For this purpose, we suggest the beyond-Migdal-Eliashberg gap equation by including the VCs into the equation. In Fe-based superconductors, high-$T_c$ $s$-wave superconductivity can be mediated by nematic and smectic fluctuations because the pairing interaction is magnified by the VCs. We also discuss the multipolar fluctuation pairing mechanism in heavy fermion systems, owing to the cooperation between the strong spin-orbit interaction and the strong electron correlation. To summarize, we suggest that the quantum interference mechanism described by the VCs is the key ingredients to explain not only various unconventional density-waves, but also exotic superconducting states in many strongly correlated metals. We finally discuss some interesting future issues with respect to the quantum interference mechanism.

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arXiv2022

Mechanism of exotic density-wave and beyond-Migdal unconventional superconductivity in kagome metal AV3Sb5 (A=K, Rb, Cs)

Exotic quantum phase transitions in metals, such as the nematic and smectic states, were discovered one after another and found to be universal now. The emergence of unconventional density-wave order in frustrated kagome metal AV$_3$Sb$_5$ and its interplay with exotic superconductivity attract increasing attention. We reveal that the smectic bond-density-wave is naturally caused by the paramagnon interference mechanism, because strong scatterings among different van-Hove singularity points are induced. In addition, the fluctuations of the bond-order induce sizable "beyond-Migdal" pairing glue, and therefore both singlet nodal $s$-wave pairing and triplet $p$-wave pairing states are expected to emerge. The coexistence of both states would explain exotic superconducting states. Unexpected similarities between kagome metal and some Fe-based superconductors are discussed. This study enables us to understand the exotic density wave, superconductivity and their interplay in kagome metals based on the interference mechanism.

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